Physics Test 1 Outline (Class 1-7) PDF

!1 PHYSICS TEST 1 OUTLINE (CLASS 1-7) CLASS 1 INTRODUCTION TO ULTRASOUND PHYSICS A. Reasons for Studying the Physics and Principles Governing Ultrasound 1. Learn how ultrasound works 2. Understanding artifacts of sound 3. Preparation for instrument performance measurements 4. Awa...

!1 PHYSICS TEST 1 OUTLINE (CLASS 1-7) CLASS 1 INTRODUCTION TO ULTRASOUND PHYSICS A. Reasons for Studying the Physics and Principles Governing Ultrasound 1. Learn how ultrasound works 2. Understanding artifacts of sound 3. Preparation for instrument performance measurements 4. Awareness of safety and risk factors 5. Preparation for the certifying examination B. Brief History of the Development of Ultrasound 1. Lazzaro Spallanzani – experimentation with bats, navigates with echo reflection 2. Francis Galton – silent dog whistle generating high frequency sound waves 3. Christian Doppler – the Doppler effect, change in frequency due to motion 4. Pierre and Jacques Currie – piezoelectric effect: Sound – Electricity (Direct) Electricity – Sound (Indirect) 5. Paul Langevin – invention of the hydrophone 6. Karl Dussik – ultrasound detection of brain tumors 7. George Ludwig – ultrasound detection of gallstones 8. Holmes / Howry – Pan scanner – utilized the cattle tank – produced bistable (black/white, 1 bit system) images C. Pulse-Echo Technique in Ultrasound Imaging 1. Sound pulse generated 2. Sound travels into soft tissue 3. Echoes produced at organ boundaries and within tissues return to source 4. Echoes detected, processed, and imaged 5. Location of the dots relates to anatomical location 6. The machine knows where to place the dots by knowing a. The direction of the beam entering the patient b. The distance D =1/2 (R x T) – placing the dots axially (vertically) on the screen c. The lateral registration (left to right)– by the scan lines going down the screen 7. Brightness of the dots are related to the strength of the reflection – the greater the difference in impedance, the stronger the reflection, the brighter the dot D. 13 microsecond rule  It takes 13us to travel 1cm and back (2cm total distance) in soft tissue E. Scanning Formats 1. Linear scan – rectangular display format a. Different starting points, parallel directions 2. Sector scan – pie or wedge shaped display format a. Same starting point, different directions 3. Blunted sector (rocker top) – curvilinear probe, sector with “bite” taken out b. Different starting points, different directions F. Doppler Ultrasound 1. Doppler effect – change in frequency related to reflector motion 2. Doppler use in blood flow imaging - Checks velocities and flow characteristics 3. Types of Doppler ultrasound – Continuous wave, Spectral analysis (Pulse Wave), Color Doppler, Power Doppler 4. Echoes detected, processed, and imaged 5. Brightness of the dots related to reflectors strength 6. Location of dots relates to anatomical location 7. Doppler information is conveyed by: Waveform, Audio, or Color 8. Doppler shift equation: Doppler shift = Reflected Frequency - Incident Frequency G. Mathematical Principles Review 1. Sequence of operations in algebraic equations: Parentheses, Exponents, Multiplication, Division, Addition, Subtraction 2. Direct and indirect proportionality in working with equations a. Direct – an increase in one variable results in an increase in another. % changes are proportional too. Numerator – Numerator b. Indirect – an increase in one variable results in a decrease in another. % changes are proportional too. Numerator - Denominator 3. Distance Equation: Distance = Rate x Time (D = R x T) 4. Imaging Depth Equation: Imaging Depth = (Rate x Time) / 2 H. Scientific Notation in Expression of Numbers 1. Shorthand method of writing very large or small numbers a. Large numbers – move the decimal point to the place after the first number. Count the number of places to the decimal points’ original location. Multiply the number by ten raised to the number of places that the decimal point moved. b. Ex. 2549 = 2.549 x 10(3rd)  c. Small numbers – move the decimal point to the place after the first non-zero number. Count the number of places to the decimal points’ original location. Multiply the number by ten raised to the (-) number of places that the decimal point moved d. Ex..005109 = 5.109 x 10(-3rd) I. Engineering Prefixes for Metric Measurements 1. Giga - billion 2. Mega - million 3. Kilo – thousand 4. Hecto – hundred 5. Deca – tens 6. Deci - tenths 7. Centi - hundredth 8. Milli - thousandth 9. Micro - millionth 10. Nano - billionth J. Most Commonly Used Metric Units in Ultrasound 1. Units of length – mm, cm 2. Units of area – mm(squared), cm(squared) 3. Units of volume – mm(cubed), cm(cubed) K. Decibels and Intensity 1. Not an exact number. It is used to express a ratio between the reflected and source signal 2. Based on a logarithmic scale (non-linear) 3. Used in relation to intensities, amplitudes, and power +3db = 2x -3db = ½ +6db = 4x -6db = ¼ +9db = 8x -9db = 1/8 +10db = 10x -10db = 1/10 +20db = 100x -20db = 1/100 +30db = 1000x -30db = 1/1000 L. Logarithms 1. Mathematical numerical rating system 2. Number of 10 s that are multiplied to equal a specific number 3. Ex. Log of 100 = 10 x 10 = 10(2nd) = 2 (the exponent) Ex. Log of 1000 = 10 x 10 x 10 = 10(3rd) CLASS 2 INTRODUCTION TO THE PRINCIPLES OF ACOUSTIC WAVES A. Definition of Sound and Ultrasound – sensation produced by vibrations reaching the organs of hearing through a gas, liquid, or solid medium 1. Infrasound - < 20 Hertz 2. Audible sound – 20-20,000 Hertz (20khz) 3. Ultrasound - > 20,000 Hertz B. Ultrasound Use 1. Bats and dolphins - ultrasonic 2. Elephants - infrasound 3. Military applications (sonar) C. Sound Travel in a Medium 1. Sound requires a medium to travel (propagate) 2. The medium can be any material such as air, water, tissue, or metal 3. Sound cannot travel in a vacuum!!! (because it propagates by physical contact) D. Sound as a Mechanical Longitudinal Wave 1. A wave is a disturbance that travels through a medium and moves its energy from one location to another 2. A wave is mechanical, it propagates by a molecule hitting another molecule, then returning back to its original location. 3. A wave transports energy, not matter! 4. A wave is longitudinal because the particle motion is parallel to the direction of travel 5. Two main zones of a wave a. Compression – area of increased particle density / pressure and density b. Rarefaction – area of decreased particle density / pressure and density E. Acoustic Variables – traveling variations in certain quantities 1. Pressure – concentration of force. Main variable. Pascals, lbs/sq in, atmospheres 2. Density – concentration of mass or weight. Kilo per cubic cm 3. Distance (particle motion)– measurement of length. Meters, cm, mm F. Descriptive Terminology of Sound 1. Period – time required for one cycle to occur, microseconds in ultrasound. Typically.77-.12us 2. Frequency – number of cycles per second of time, units are Hertz. 3. Frequency is perceived as pitch. High frequency = high pitch  4. Range of frequencies for ultrasound imaging – from 2.0 through 12.0 MHz 4. There is an inverse relationship between penetration and image resolution a. Low frequency = good penetration, poor images b. High frequency = good images, poor penetration 5. Frequency and period are inversely related and are reciprocals (a number multiplied by its reciprocal = 1) 6. Frequency = 1/Period Period = 1/Frequency CLASS 3 GENERAL CHARACTERISTICS OF SOUND A. Wavelength – length of space that one wave occupies 1. Units – millimeters, meters, inches, etc 2. Ultrasound units – millimeters (mm), centimeters (cm) 3. Typical ultrasound wavelength:.1 -.8mm 4. Mathematical equation: Wavelength = Speed / Frequency 5. 6. Wavelength’s effect on image quality – higher frequencies produce shorter wavelengths which result in better resolution, but with less penetration. Lower frequencies result in poorer image quality, but has better penetration 7. Factors affecting wavelength – sound source (sets the frequency, therefore sets wavelength) and medium in which sound travels (which determines the distance the wave travels) B. Propagation Speed – speed at which sound travels in a medium 1. Units – millimeters per microsecond (mm/us) or meters per second (m/s) 2. Propagation speed of soft tissue: 1540 m/s, or 1.54 mm/us 3. Only the medium can change the propagation speed of sound. 4. The speed of sound is affected by stiffness and density a. Stiffness – with increase, sound speed increases; with decrease, sound speed decreases (antonym – compressibility and elasticity) b. Density – with increase, sound speed decreases; with decrease, sound speed increases 5. Of the two factors, stiffness has the dominant effect C. Speed of Sound in Different Body Tissues Bone – 4080 meters per second Tendon – 1700 meters per second Muscle – 1600 meters per second  Blood – 1555 meters per second Liver – 1550 meters per second Soft Tissue – 1540 meters per second Fat – 1460 meters per second Lung - 460 meters per second Air – 330 meters per second 1. Average speed in soft tissue – 1540 meters per second (1.54 millimeters per microsecond) B. Impedance – resistance to the propagation of the sound wave. 1. Units – Rayls 2. Mathematical equation: Impedance = Density x Propagation Speed (Z = P x c) 3. Impedance is directly related to Density and Speed 4. Therefore, if Speed or Density increases, Impedance increases 5. Impedance of sound in different mediums a. Air 0.0004 x 10(6th) b. Lung 0.18 x 10(6th) c. Fat 1.34 x 10(6th) d. Water 1.48 x 10(6th) e. Liver 1.65 x 10(6th) f. Blood 1.65 x 10(6th) g. Muscle 1.71 x 10(6th) h. Bone 7.80 x 10(6th) 6. Average impedance for soft tissue – 1,630,000 Rayls 1.63 x 10(6th) Rayls CLASS 4 PULSED ULTRASOUND PRINCIPLES A. Pulsed Ultrasound Process 1. Send a pulse of sound 2. Listen for echo to return to transducer 3. The echo is changed into an electrical signal that can be processed 4. The information is displayed on the monitor B. Placement of Echo Information on Display 1. Strength or amplitude of echo must be known (brightness)  2. Anatomic location of reflector a. Direction of the sound entering the patient b. Propagation speed of sound c. Time of flight to the reflector d. Distance to the reflector 3. Range Equation allows ultrasound unit to calculate the depth of the echo producing structure a. Distance = Rate x Time (ROUND TRIP) 4. Distance to the reflector Distance = (Rate x Time) / 2 (DISTANCE TO REFLECTOR) C. Pulse Anatomy 1. Collection of cycles 2. Cycles travel as a group 3. Has a beginning, middle, and end D. Pulse Duration – duration of the pulse 1. Units related to time – microseconds 2. Typical pulse duration in diagnostic imaging -.5-3 microseconds 3. Mathematical equation - pulse duration equals period time the number of cycles in a pulse, or pulse duration equals the number of cycles divided by frequency 4. PD = PERIOD x # OF CYCLES 5. PD = # OF CYCLES / FREQUENCY 6. PD decreases if: a. Frequency increases – (period decreases) b. Damping increases – (# of cycles decreases) 7. Pulse duration not under control of operator – determined by the source E. Pulse Repetition Period – time from the beginning of one pulse to the beginning of the next pulse 1. Units – microseconds 2. PRF and PRP are inversely related and reciprocals 3. Operator has some control of PRP, and therefore may change PRF 4. Operator changes PRP by changing DEPTH (idle time) F. Pulse Repetition Frequency – number of pulses emitted in one second of time 1. Units – Hertz (Hz)or Kilohertz 2. PRF in diagnostic ultrasound – 1 to 10 Kilohertz 3. Limitations on PRF (why we get aliasing) – due to imaging depth (idle time). a. The echo must be received before the next pulse is sent  b. The deeper the target, the longer it takes to come back to the probe 4. When depth decreases (shallower), PRP decreases, so PRF increases, and vice versa G. Duty Factor – fraction (percentage) of time that the pulse is being transmitted 1. Units – UNITLESS, expressed as a decimal or fraction 2. Ranges from 0% - 100% 3. Mathematical equation – duty factor equals pulse duration divided by pulse repetition period 4. DF = PD / PRP 5. Duty factor low in diagnostic ultrasound: (.001 -.01) (less than 1%) 6. Duty factor is set by the source (manufacturer) 7. But can be changed by the sonographer by changing DEPTH 8. If depth decreases, IDLE TIME decreases, PRP decreases, DUTY FACTOR INCREASES H. Spatial Pulse Length – length or distance of an ultrasound pulse in space 1. Units – millimeters 2. SPL determines Axial Resolution. AR = ½ SPL 3. Mathematical equation – Spatial pulse length equals the wavelength time the number of cycles in the pulse, or spatial pulse length equals propagation speed divided by frequency time the number of cycles in the pulse 4. SPL = WAVELENGTH x # OF CYCLES 5. SPL = (SPEED / FREQUENCY) x # OF CYCLES 6. SPL is set by the source (wavelength and frequency), but may be changed by the SPEED OF THE MEDIUM CLASS 5 ATTENUATION AND INTENSITIES A. Amplitude – maximum variation in an acoustic variable 1. Definition – measure of difference between the average and maximum (peak) values of sound 2. Units – dependent upon acoustic variable, most often decibels 3. Decreases (attenuates) as the wave travels 4. Adjusted by the Power control key B. Power – rate of energy transference 1. Definition – rate at which energy is transmitted to body 2. Units – Watts(W) or milliwatts (mW) 3. Determined by the source 4. Operator can adjust the power by the Power control key 5. Increasing power increases the voltage that drives the crystal  6. Effect of increased power on monitor – increased overall echogenicity 7. Effect of increased power related to effects – increased bioeffects (stronger waves transmitted) C. Intensity – concentration of power in an area 1. Definition – amount of power in a specific physical space 2. Units – watts per centimeter squared (Watts/cm(2) 3. Mathematical formula – intensity equals power divided by area (relationship between variables) 4. INTENSITY = POWER / AREA 5. Bioeffects – increased intensity increases bioeffects 6. Can be adjusted by changing the Power control key 7. Focusing the beam also increases intensity (smaller area) 8. Relationship between power, intensity, and amplitude a. INTENSITY = POWER / AREA b. INTENSITY (is proportional to) POWER c. INTENSITY (is proportional to) AMPLITUDE(squared) d. POWER (is proportional to) ) AMPLITUDE(squared) 9. Intensities vary in time and space a. Spatial – Distance or Space b. Temporal – Time c. Peak – Maximum d. Average – Mean e. SP – Spatial Peak (maximum value) f. SA – Spatial Average (average value over time) g. TP – Temporal Peak (maximum value over time) h. TA – Temporal Average (average value over time) i. PA – Pulse Average (average value over the pulse) 10. Types of intensities and values a. SPTP – Mechanical Index (MI) – Cavitation (stable / transient) b. SPPA c. SPTA – Thermal Index (TI) – Heating (TIS, TIB, TIC) d. SATP e. SAPA f. SATA – lowest value 11. With CW. TP and TA are equal (machine is always transmitting) a. SPTA = SPPA  b. SPTP = SPTA c. SATP = SATA 12. Beam Uniformity Ratio - SP / SA D. Attenuation – weakening of sound as it travels 1. Definition – decrease of sound in amplitude and intensity as it propagates 2. Units - decibels 3. Three components (causes) of attenuation d. Absorption – change of sound energy to heat energy (main factor) e. Reflection – redirection of sound to transducer f. Scattering – sound directed in all directions 4. Attenuation ranges in soft tissue – air has the highest attenuation a. Water.0002 b. Blood.18 c. Muscle 1.2 d. Fat.6 e. Liver.9 f. Kidney 1.0 g. Bone 20.0 h. Lung 41.0 i. Soft Tissue.5 - 1.2 E. Attenuation Coefficient 1. Definition – the amount of sound attenuated (lost) per centimeter of travel (rate of loss) 2. Units – decibels per centimeter 3. Mathematical formula – attenuation coefficient equals frequency divided by two 4. AC = ½ FREQUENCY 5. The higher the frequency, the greater the rate of loss of energy Ex. 2Mhz pulse: 2Mhz / 2 = 1 db/cm rate of loss of energy 10Mhz pulse: 10Mhz / 2 = 5 db/cm rate of loss of energy F. Total Attenuation – the total loss of energy (in decibels) lost after traveling a specific distance 1. Mathematical formula – Attenuation equals one half Frequency times Path Length 2. ATTENUATION = ½ FREQUENCY x PATH LENGTH 3. Attenuation increases with increases in Frequency or Path Length (both directly related) I. Half Value Layer Thickness – half boundary layer, penetration depth, depth of penetration 1. Definition – depth at which original intensity is reduced to half of the original value  2. HVL = 3 / AC 3. If AC increases, HVL decreases!11 4. Ex. 6Mhz pulse. AC = 6Mhz / 2 AC = 3 HVL = 3 / AC HVL = 3 / 3 HVL = 1 cm depth CLASS 6 ECHOES AND REFLECTIONS (PART 1) A. Acoustic Interfaces and Sound 1. Reflected back to transducer 2. Scattered in different directions a. Backscatter – big, rough (non-specular) boundaries b. Rayleigh Scatter – due to the small size of the reflector (RBC’s) 3. Sound may be reflected, transmitted, or scattered at a boundary B. Requirements for Reflection 1. Perpendicular incidence – ninety degrees, right angle, orthogonal, normal 2. Different acoustic impedances – greater reflection with greater impedance mismatch C. Specular Reflections 1. Arise at large smooth boundaries 2. Mirror – like Ex. Diaphragm 3. Ninety degree incidence required D. Sound Scattering 1. Occurs when sound encounters irregular, rough, heterogeneous, or reflectors smaller than a wavelength of sound 2. Types – backscatter and Rayleigh scatter a. Backscatter: Non-specular surface (large and rough). i. Reflections can generally come from most angles ii. Reflections are weaker than those that come from a specular reflector b. Rayleigh: Caused by reflector smaller than one wavelength (red blood cells). i. Sound is scattered in all directions 3. Scattering increases with higher frequencies E. Interaction with Sound Interface 1. Perpendicular incidence – the reflector is 90 degrees to the beam. 2. Oblique incidence – the reflector angle is anything other than 90 degrees 3. Acute – less than 90 degrees  4. Obtuse – greater than 90 degrees F. Incident Sound Intensity Equal to Transmitted and Reflected Sound Intensities INCIDENT INTENSITY = TRANSMITTED INTENSITY + REFLECTED INTENSITY REFLECTED INTENSITY = INCIDENT INTENSITY – TRANSMITTED INTENSITY TRANSMITTED INTENSITY = INCIDENT INTENSITY – REFLECTED INTENSITY a. Incident Intensity – the intensity of sound just before it encounters a boundary (initial) b. Reflected Intensity – the intensity of sound just after it encounters a boundary and is reflected back to the probe c. Transmitted Intensity – the intensity of sound after it encounters a boundary and continues to travel in the same direction G. Quantification of Transmission and Reflection 1. Intensity reflection coefficient (IRC) – percentage of intensity reflected at a boundary or tissue interface 2. Intensity transmission coefficient (ITC) – percentage of intensity transmitted at a boundary or tissue interface H. Requirements for Reflection 3. Perpendicular incidence – ninety degrees 4. Different acoustic impedances – the greater the impedance mismatch, the stronger the reflection, the brighter the dot I. Equations Relating To Normal Incidence (90 degrees) 1. Intensity Reflection Coefficient – the percentage of the sound’s intensity that is reflected when sound hits a boundary between two media 2. Intensity Transmission Coefficient - the percentage of the sound’s intensity that is transmitted when sound hits a boundary between two media 3. INCIDENT INTENSITY (100%) = IRC + ITC 4. Also ITC = 1 – IRC 5. Also IRC = 1 - ITC 6. IRC = [Z(2) – Z(1)](squared) [Z(1) + Z(2)](squared) IRC = ( _____ - ______)(squared) (_____ + ______)(squared) 6. The greater the impedance difference, the greater the reflection, the brighter the dot 7. High reflection (IRC) occurs between air / soft tissue interfaces 8. Less reflection (IRC) occurs between bone / soft tissue interfaces 9. Least reflection between 2 similar soft tissue interfaces CLASS 7 ECHOES AND REFLECTIONS (PART 2) A. Oblique Incidence – angle of incidence is other than ninety degrees 1. Luez’s Law – angle of incidence equals angle of reflection 2. Oblique incidence (not 90 degrees) a. Reflection and Transmission uncertain (because it’s sound) B. Refraction – change in sound direction as it passes from one medium to another 1. Refraction as a cause of imaging artifacts – improper lateral placement on display, edge shadowing 2. Factors required for refraction a. Oblique incidence b. Different propagation speeds of sound between media 3. Snell’s Law – used in calculation of angle of refraction a. Sine Refracted Angle = Speed Medium 2 Sine Incident Angle Speed Medium 1 b. If going from SLOW to FAST media, the refracted angle will be GREATER (POSITIVE – bend up) c. The greater the difference in speed between mediums, the greater the angle 4. Sine – ratio of two sides of a right triangle a. Sine = Opposite / Hypotenuse C. Harmonics – multiples of the fundamental (incident) frequency 1. Caused by the distortion of the wave as it travels 2. Harmonics are generated non-linearly (they get stronger as they travel initially) 3. Begins with low intensity, then increases (non-linear) 4. The fundamental (first harmonic) wave is removed, only the second harmonic wave is processed for imaging 5. How harmonics improves image quality: Better images (higher frequencies), Less distortion (cleaner images) and less side or grating lobes a. Much of the distortion of the sound wave occurs within the first few centimeters of the surface of the skin b. Harmonic waves are generated non-linearly. Their strength increases as they travel c. Therefore, the second harmonic wave picks up less distortion in the first few centimeters than the fundamental (1st harmonic) wave would d. The second harmonic wave is 2x the fundamental frequency (higher frequency = better images) d. The first harmonic wave is filtered out, and the SECOND harmonic wave is used for imaging  e. The second harmonic wave has a better “signal to noise ratio” than the fundamental (1st harmonic) wave f. The result is improved image quality with less distortion g. Harmonic waves generate less side and grating lobes 1. Harmonic waves are non-linear and are only generated by strong waves, not weak ones 2. As the sound beam passes through the tissue and distorts, it generates side and grating lobes 3. Because the main beam is strong, strong harmonic waves are generated by the non-distorted main beam 4. Very few harmonics are generated by the side and grating lobes because they are weak 6. ADVANTAGES i. Better resolution (higher frequencies processed) ii. Cleaner images (less distortion from the first few centimeters of travel – harmonics are weak initially) iii.A better signal to noise ratio (SNR) (cleaner signal) iv. Fewer side and grating lobes v. Narrower beams (higher frequencies = longer NZL’s, more gradual beam tapering) vi. Less divergence in the far zone 7. DISADVANTAGES i. Lower power output (harmonic waves are weaker than fundamental waves) 8. APPLICATIONS a. Uses – good acoustic windows, shallow depths, due to low power (penetration) b. Not advised for poor acoustic windows or deep depths. Turn harmonics off. D. SPI – Pulse Inversion Harmonics 1. A method of removing the fundamental waves from the reflected signal 2. Two consecutive pulses are sent down the same scan line. The first is the normal pulse, the second is an inverted (180 degrees out of phase) copy of the first pulse. 3. The two pulses are combined in the receiver. Because the fundamental waves exhibit linear behavior, the out of phase pulse cancels out the fundamental waves (destructive interference) 4. Harmonic waves exhibit non-linear behavior, so the harmonic waves are in phase with each other and interfere constructively 5. The result is a cleaner signal 6. The disadvantage is reduced temporal resolution due to the extra pulses sent E. SPI – Power Modulation Harmonic Imaging 1. A method of removing the fundamental waves from a reflected signal  2. Two consecutive pulses are sent down the scan line, the first is half the strength of the second 3. Because harmonic waves exhibit non-linear behavior, the first weaker reflection does not contain harmonics, the second reflection is strong and contains the harmonic waves 4. During reception, the first reflection is doubled, then subtracted from the second reflection leaving only the harmonic waves with less distortion 5. The disadvantage is reduced temporal resolution due to the extra pulses sent 9. Contrast Media – substances introduced by injection 1. Microbubbles that can pass through capillary walls 2. Purpose of contrast media a. Provide image enhancement in gray scale and Doppler images b. Improve gray scale differentiation between lesions with similar echogenicity c. Improvement of weak Doppler signals 3. Working principles of contrast media a. There is a great impedance difference between tissue and gas – improving reflection, brighter dots b. Microbubbles also expand and contract at harmonic frequencies c. Microbubbles are constructed of an outer shell and inner gas d. The shells and gasses are changed depending upon longevity and resonance characteristics that are needed e. The shells dictate microbubble duration – how long the bubbles remain intact f. The inner gas dictates expansion characteristics (inert gas) 4. Generation of harmonic signals by microbubbles in contrast media yields improvement in spatial resolution and fewer artifacts – microbubbles work well with harmonics because the bubbles oscillate at harmonic frequencies F. Contrast Media and the Mechanical Index 1. The mechanical index is an estimation of the amount of contrast harmonics produced and cavitation 2. MI = (peak RAREFACTIONAL pressure) / (square root of frequency) 3. Therefore, higher power waves and lower frequencies increase MI, which increases harmonics and cavitation 4. The relationship between harmonics and the mechanical index is non-linear 5. Low mechanical index beams do not generate harmonics because the bubbles expand and contract uniformly (linearly) 6. High mechanical index (greater than 1.0) beams generate substantial harmonics Bubble disruption (implosion) also creates strong harmonics G. Constructive Interference of Sound Waves (in phase) 1. Waves are at the same place at the same time  2. Result is cumulative – waves reinforce one another H. Destructive Interference (180 degrees out of phase) 1. Waves are not in the same place at the same time 2. Result is negative – cancel each other out I. Beats – a general rise and fall in tone when cancellation is incomplete a. Due to differences in frequencies b. Due to differences in phasing (other than 180 degrees) PHYSICS TEST 2 OUTLINE (CLASS 8-10) CLASS 8 ULTRASOUND TRANSDUCERS (PART 1) A. Transducer Definition – device capable of changing one form of energy into another B. Examples of Different Types of Transducers 1. Light bulb – electrical energy to light and heat energy 2. Ear – sound energy to nerve impulses 3. Motor – electrical energy to motion 4. Battery – chemical energy to electrical energy 5. Ultrasound transducer (probe) – ultrasound energy to electrical energy and vice versa C. The Piezoelectric Effect (press amber) – conversion of acoustic pressure energy into electric energy or vice versa. 1. Discovered by Jacques and Pierre Curie 2. Direct Piezoelectric Effect (reception) - conversion of ultrasound energy into electric energy 3. Indirect (reverse) Piezoelectric Effect (transmission) – conversion of electric energy into ultrasound energy D. Piezoelectric (ferroelectric) Materials – known as: element, crystal, ceramic, active elenent 1. Natural - Q-RAT a. Quartz b. Rochelle Salt c. Amber d. Tourmaline e. Sucrose f. Topaz g. Berlinite h. Lead Titanate  2. Man-made (ceramics) – placed in a strong electric field, heated to the curie point (point of depolarization, about 360 degrees Celsius (680 degrees Fahrenheit), and allowed to cool in the electric field (preserving polarization). a. The crystal is heated to the curie point (about 360 degrees Celsius) – depolarization occurs (poles not aligned) b. The crystal is placed in a strong electrical field to align the positive and negative poles c. The crystal is allowed to cool in the electrical field, polarizing the crystal d. Some examples of man-made crystals: 1. Lead zirconate titanate 5 (PZT, most common) 2. Barium titanate 3. Lead metaiobate 4. Polyvinylidene E. Transducer Construction 1. Piezoelectric element (crystal, ceramic)– converts electrical voltage to sound and vice versa 2. Matching layer – increases sound transmission into tissue (reduces impedance differences between crystal and tissue) 3. Damping Layer - (backing material) – improves detail resolution by reducing ringing (pulse length) 4. Plastic case – encloses components and protects from damage 5. Electrical wire – transmits voltage to and from piezoelectric element(s) F. Matching Layer 1. Placed in front of the crystal 2. Has an intermediate impedance between the crystal and tissue 3. Helps transmission – the lower the difference in impedance, the less reflection, the more sound transmitted into the body 4. May have multiple layers 5. Thickness – ¼ wavelength 6. Made of epoxy resin with aluminum powder G. Crystal – Element, Ceramic 1. Fo = SPEED IN CRYSTAL / 2x THICKNESS 2. Thickness – ½ wavelength (¼ to ½ wavelength for SPI) H. Damping layer 1. Attached to the back of the crystal 2. Limits the ringing of the crystal - reduces the number of cycles in the pulse 3. Improves spatial (axial) resolution by: a. Decreasing the number of cycles in the pulse b. Which decreases the pulse duration  c. Which also decreases the spatial pulse length d. (Axial resolution = ½ SPL, anything that shortens the SPL improves axial resolution) 4. Made of epoxy resin with tungsten, or other metal powders I. Bandwidth 1. Definition – range of frequencies present in a pulse of sound 2. Center frequency – the operating frequency of the transducer 3. Broad bandwidth – pulse with many frequencies present (damped – good images) 4. Narrow bandwidth – pulse with fewer frequencies present (undamped – poor imges) 5. Bandwidth is dependent on damping (directly related) 6. The more damping, the wider the bandwidth (more frequencies present in the pulse) 7. The more damping, the shorter the pulse, the better the axial (spatial) resolution J. Effects of Damping: 1. INCREASED frequencies in a pulse (bandwidth) 2. IMPROVED Axial Resolution (shorter pulse, better resolution) 3. DECREASED Q Factor 4. DECREASED Sensitivity (ability to emit a clean frequency & receive weak echoes) 5. DECREASED Pulse Amplitude K. Q Factor (previously known as QUALITY FACTOR) 1. Definition – number relating to the transducers ability to operate at center frequency 2. The more damping: the shorter the pulse, the better the axial resolution, the wider the bandwidth, the lower the Q Factor 3. Mathematical equation – Q Factor equals operating frequency divided by the bandwidth 4. Q Factor = Fo / Bandwidth 5. Q Factor estimation - # cycles in the pulse 6. Q Factor is unitless 7. The lower the Q Factor, the better the axial resolution L. Multihertz Transducers – can operate at more than one center frequency due to broad bandwidth a. Can select different operating frequencies within the bandwidth with the push of a button b. Makes changing the transducer frequency “USER ADJUSTABLE” M. Operating Frequency – AKA: N. resonance frequency, preferred frequency, natural frequency 1. Factors that determine operating frequency of transducer a. Propagation speed in piezoelectric material b. Thickness of the piezoelectric element  2. Mathematical equation – operating frequency equals propagation speed in the piezoelectric material divided by two times the thickness of the piezoelectric element 3. Fo = SPEED IN CRYSTAL / (2X THICKNESS) 4. When Speed increases, Operating Frequency INCREASES 5. When Thickness increases, Operating Frequency DECREASES 6. Typical thickness of the piezoelectric crystal:.2 - 1mm 7. Typically ½ wavelength (¼ - ½ wavelength for SPI) CLASS 9 ULTRASOUND TRANSDUCERS (PART 2) A. Diffraction of Sound 1. Definition – spreading out of sound in all directions when produced by a small source 2. Huygen’s Principle – explains away diffraction a. Each point on transducer is a separate source of sound b. Sound wavelets overlap and interface c. Interference (destructive) of wavelets causes ultrasound beam to assume hourglass shape B. Anatomy of a Beam of Sound 1. Hourglass shape 2. Focus – narrowest point of the sound beam, optimum lateral resolution 3. Focal Zone – L 4. Near Zone (Near Field, Fresnel Zone) – area between transducer face and focus 5. Near Zone (Focal) Length (NZL) – distance between the transducer face and focus 6. Far Zone (Far Field, Fraunhofer Zone) – area beyond focus where the beam diverges C. Near Zone Length 1. Factors affecting near zone length a. NZL = [(DIA)(squared) x Frequency] / 6 b. Transducer diameter – if increased – longer (deeper) NZL c. Frequency – if increased – longer (deeper) NZL 2. Longer NZL’s are better because resolution is better in the near zone due to converging scan lines 3. Intensity varies greatly in the near zone, diffuses in the far zone 4. Beam diameter is ½ the transducer’s diameter at 1 NZL 5. Beam diameter is the same as original diameter at 2 NZL’s D. Far (Fraunhofer) Zone – The area beyond the focus where the beam begins to widen (diverge) 1. Factors affecting far zone a. Frequency – higher frequencies have narrower beams and diverge less in far zone (longer NZL)  b. Crystal diameter – larger crystals produce beams that diverge less in far zone (longer NZL) E. Detail (Spatial) Resolution – ability to distinguish structures as separate in space 1. Axial resolution – (LARRD - longitudinal, axial, range, radial, depth) – ability to visualize two structures as separate and distinct when located along (parallel to) the beam’s path a. Units – distance – smaller numbers mean better resolution b. Mathematical equation – axial resolution equals spatial pulse length divided by two. AR = ½ SPL c. Improvement of axial resolution: 1. Increase frequency – shortens the period 2. Increase damping – reduces the of number of cycles in pulse (shortens SPL) d. Typical values for AR = 1mm e. The sonographer can change the axial resolution by changing frequency (transducer) f. AXIAL RESOLUTION REMAINS THE SAME AT ALL DEPTHS g. Poor axial resolution – vertical jelly bean 2. Lateral resolution – (LATA – lateral, angular, transverse, azimuthal) – ability to distinguish two structures as separate and distinct when located perpendicular to beam’s path a. Units – distance (the smaller, the better) b. Mathematical equation: Lateral Resolution = Beam Diameter c. Narrower beam widths produce improved lateral resolution d. Lateral resolution (beam diameter) changes in relation to distance from transducer face – LATERAL RESOLUTION CHANGES WITH DEPTH. IT IS ALWAYS CHANGING e. Lateral resolution is best at the narrowest point of the beam (focus) f. Improvement of lateral resolution – a. Focus the beam b. Use a higher frequency probe c. Change the size of the transducer diameter g. Poor lateral resolution – horizontal jelly bean 3. Slice Thickness Resolution – (AKA elevational resolution, partial volume, elevational axis, section thickness, Z axis) – ability to distinguish two structures as separate and distinct at ninety degrees to a two dimensional scan a. Units – distance b. Varies with distance from transducer face c. Best in near zone d. Conventionally focused (pre 1.5D) e. Improved by focusing, 1.5 D transducer f. Poor slice thickness resolution – cyst fill-in  g. Ghost wall in the bladder 4. Spatial Resolution a. Resolution in three dimensions b. Ratings from best to worst: 1. Axial 2. Lateral 3. Elevational CLASS 10 ULTRASOUND TRANSDUCERS (PART 3) A. Ultrasound Beam Focusing – conventional and electronic 1. Purpose – improvement of resolution in near zone a. Improves lateral resolution b. Narrows beam in the near zone, but widens the beam in the far zone 2. Conventional Focusing Techniques – (single element transducers) a. Acoustic lens - external b. Curved piezoelectric element - internal c. Acoustic mirror - internal 3. Disadvantages of conventional focusing a. Focal Depth cannot be changed (fixed) – not user adjustable b. The transducer must be changed to change the depth of focus 4. Electronic focusing (multi-element transducers) – electronic curvature a. Changes the time delays between the crystals b. Makes the focal depth USER ADJUSTABLE B. Operation of Array Transducers 1. Electronic focusing (electronic curvature) a. The beam former adjusts the firing sequence with tiny time delays to fire the outer pairs of crystals before the inner pairs b. The firing sequence curves the wavefront c. The focus will be the point of intersection d. The greater the delays, the greater the curvature, the shallower the focus 2. Beam steering (electronic slope) – firing from left to right or right to left by application of voltage pulses with tiny time delays a. Steering beam (electronic slope) - sending beam through different paths to create complete 2D image (frame) b. The beam former adjusts the firing patterns and timing to go from left to right, or right to left. c. The firing sequence puts the wavefront on a slope  d. The wave propagates perpendicular to the wavefront C. Dynamic Receive Focusing – (smiley hot dog) 1. Employs time delays during reception of echoes – because of the delays used during transmission (curvature) 2. Improvement of resolution from a range of imaging depths 3. Tiny time delays used in transmission are added back to the received pulse so that all echoes arrive back at the transducer at the same time. D. Aperture / Dynamic Aperture 1. Aperture – size of piezoelectric element or groups of elements that produce ultrasound 2. Dynamic aperture - changing the size of the group of crystals to match the depth according to the NZL equation a. NZL = [(DIA)squared x FREQ] / 6 b. Functions to maintain beam diameter at different depths c. Improves lateral resolution d. The larger the group, the deeper the focus (larger diameters = longer NZL) e. The higher the frequency, the deeper the longer the NZL (higher frequency = longer NZL) E. Accessory lobes a. Low power beams off the main axis b. Source of artifacts c. 2 types i. Side lobes 1. Caused by physically large crystals (early probes) 2. Can only be reduced by harmonics ii. Grating lobes 1. Occurs with array transducers 2. Can be reduced by apodization, subdicing, and harmonics F. Apodization 1. Changing amplitude of voltages with focusing and steering 2. Reduces grating lobes (artifacts) 3. Drives the outer crystals with less voltage 4. Less voltage means less energy at the ends of the transducer, reducing the strength of the grating lobes G. Subdicing 1. Used to reduce grating lobes 2. The crystal is cut into multiple pieces, then wired back together to act as one 3. More wavelets are formed, increasing destructive interference, which helps cancel out grating lobes H. Multiple Transmit Focus – multiple focal zones 1. Multiple pulses sent down each line 2. Echo information taken only from focus of each pulse 3. Information is compounded to form image 4. Advantage – improved lateral resolution from multiple distances from transducer 5. Disadvantage – slows frame rate, which results in poorer temporal resolution I. Care of the Transducer 1. Danger of damaging piezoelectric elements if dropped 2. Electrical cords should not be compressed or crushed 3. Transducers are never autoclaved (high heat depolarizes the crystals) 4. Transducers must be cleaned according to manufacturer’s instructions 5. Transducers should be inspected for damage, or wear and tear on a regular basis J. Different Types of Ultrasound Transducers 1. Continuous Wave a. No pulsing, sends a continuous beam of sound b. 2 crystals: 1 transmitting, 1 receiving c. Displays a waveform and audio d. NO IMAGE e. Focusing - conventional f. Steering - human g. Crystal damage – no waveform or audio h. Advantage – ability to measure high velocities without aliasing i. Disadvantage – range (depth) ambiguity 2. Mechanical transducer a. Single element b. Element oscillates or rotates c. Image format - sector d. Focusing – conventional – fixed focus e. Steering - mechanical f. Conventional focusing employed g. Crystal damage – no image h. Transducer must be changed to change focus i. Advantage – range (depth) resolution j. Disadvantage - aliasing 3. Array transducers (generally) a. Multiple elements and channels b. Channel – piezoelectric element and it’s electrical connection c. Produces a well-defined beam d. Scan format dependent upon the arrangement of the piezoelectric elements (general statement, early probes) 4. Annular array transducer a. Elements arranged in the form of concentric rings b. Image format – sector c. Different starting point, parallel horizontal lines d. Focusing - conventional e. Steering - mechanical f. Crystal damage – horizontal banding 5. Annular PHASED array transducer g. Elements arranged in the form of concentric rings h. Image format – sector a. Different starting point, parallel horizontal lines b. Focusing - ELECTRONIC c. Steering – MECHANICAL (same for BOTH regular and phased) d. Crystal damage – horizontal banding 6. Linear sequential (switched array) a. Elements arranged in a line b. Image format – rectangular c. Working principle – groups of elements switched rapidly to create image d. Focusing achieved through short delay times (electronic focusing) e. Different starting points – parallel directions f. Focusing – electronic g. Steering – electronic h. Crystal damage – single vertical dropout i. Rectangular image format j. Good resolution – higher frequencies k. Superficial depths 7. Curvilinear, convex array transducer a. Same operating principle as the linear sequential  b. Except the elements are arranged in curved line c. Image format – blunted sector, curved top, rocker top d. Different starting points – different directions e. Focusing - electronic f. Steering – NONE g. Crystal damage – single vertical dropout h. Blunted sector image format i. Good penetration j. Deep depths 8. Linear Phased array transducer – (echo probe) a. Electronic phasing – tiny time delays used for electronic focusing and steering b. Firing pattern of elements may constantly change a. Image format – sector shaped b. 1 starting point – different directions c. Focusing – electronic d. Steering – electronic e. Crystal damage – erratic output f. Focusing - the greater the delay, the greater the curvature, the shallower the focus g. Steering – the greater the delay, the greater the angle 9. Vector transducer a. Combination of linear and phased technologies b. Firing patterns and timing may change each time c. Image format – trapezoid d. Different starting points – different directions e. Focusing – electronic f. Steering – electronic g. Crystal damage – erratic output 10. 1.5D a. Adds electronic focusing in the elevational plane (Z plane) b. Improves resolution in the elevational plane by narrowing the beam 11. 2.0 a. Adds electronic steering to the elevational plane (Z plane) 12. 3D a. Uses volumetric rendering to produce a 3D model  b. Advantage - provides surface details to images c. Disadvantage - computer dependent – lower frame rate (temporal resolution) 13. 4D a. Real time 3D (live) PHYSICS TEST 3 OUTLINE (CLASS 11-17) LECTURE CONTENT: CLASS 11 ULTRASOUND TRANSDUCERS (PART 4) A. Presentation of Transducer Reports B. Transducer Overview 1. Function of a transducer 2. Ultrasound transducer function 3. The piezoelectric principle 4. Factors affecting operating frequency of the transducer 5. Transducer construction 6. Function of transducer components 7. Function of damping material and effect on image resolution 8. Function of the matching layer and effect on sound transmission 9. Sound beam anatomy – near zone, focus, far zone 10. Spatial resolution – axial, lateral, slice thickness 11. Focusing and steering of the beam C. Transducer Types 1. Mechanical transducers 2. Linear switched transducers 3. Convex sequential transducers 4. Phased array transducers 5. Convex phased array transducers 6. Annular array transducers 7. Vector transducers CLASS 12 IMAGING INSTRUMENTS (PULSER & RECEIVER) A. The Seven Components of the Imaging System – Pulser, Beam Former, Transducer, Receiver, Memory, Display, Master Synchronizer  1. Purpose of the imaging system – to produce ultrasound image on the display 2. How information is derived from echoes 3. Master synchronizer sometimes listed as the sixth component B. Beam Former – handles all of the timing delays for electronic focusing / steering (phasing)/composite imaging and tells the pulser when to fire the pulses. C. The Pulser 1. Function of pulser – produces electrical voltage that drives transducer to produce a sound pulse 2. Relationships of the pulser a. Pulser tells the scan converter that contact has been made (starts timer to calculate the time of flight) b. Continuous Wave – pulser sets the frequency for continuous wave. Frequency is equal to the electrical frequency c. (Pulse Wave – frequency is dependent on the thickness and speed of the crystal – Fo=SPEED / 2x THICKNESS) d. Controls acoustic power of ultrasound unit by changing the voltage that drives the crystals e. Controls PRP, PRF, amplitude of the pulse, and frequency for continuous wave ultrasound D. Acoustic Power (AKA: output power, transmit, output, energy output, output gain) 1. Greater voltage yields greater acoustic power 2. Set power levels as low as possible 3. Increased power yields increased overall echogenicity (brightness) on the display 4. Power controls the maximum depth of penetration 5. Minimize power and maximize gain - ALARA principle E. The Receiver – 5 Functions of the receiver 1. Amplification a. Small voltages boosted to make them stronger b. Units of amplification – decibels c. Operator adjustable through overall gain control key d. Function – increases overall echogenicity e. DOES NOT INCREASE DEPTH OF PENETRATION f. Preamplifier – clips large voltage spikes or amplifies very weak signals prior to amplification 2. Compensation (AKA time gain compensation, swept gain, depth gain compensation) a. Purpose – to compensate for attenuation due to depth b. Operator adjustment of TGC c. Goal – ultrasound image of UNIFORM echogenicity (brightness) from top to bottom  d. The deeper the target, the more compensation needed (slider to the right) 3. Compression, Log Compression, Dynamic Range a. Purpose – to squeeze signal amplitudes into a narrower range b. Definition – ratio of the greatest to smallest amplitude that ultrasound instrument can handle c. Dynamic range (log compression, compression) relationships i. Lower dynamic range – less shades of grey – greater contrast ii. Higher dynamic range – more shades of grey – less contrast iii. THERE IS AN INVERSE RELATIONSHIP BETWEEN THE TERMS “DYNAMIC RANGE” AND “COMPRESSION” iiii. Dynamic range of components: Transducer/receiver – 100-200 Memory (scan converter) – 40-45 Display/printer – 20-30 Compression of the electrical signal is necessary to prevent data loss (out of range) 4. Demodulation a. Purpose – change voltages into another form b. NOT USER ADJUSTABLE!!! c. Rectification – negative signals changed to positive (for image generation) d. Smoothing – signal averaged to reduce spikes or fill voids 5. Rejection (AKA Suppression, Threshold, Filter, Wall Filter) f. Purpose – suppressing or eliminating small voltage amplitudes to reduce noise (spectral Doppler) g. Reducing clutter (reflector motion obscuring color Doppler flow information) h. Sonographer controlled through filter control key CLASS 13 IMAGING INSTRUMENTS (MEMORY) A. Definition of Scan Converter – AKA Memory 1. Function - storage of information coming from receiver 2. Methods of storage / types of imaging memories a. Analog – infinitely variable b. Digital – discreet binary numbers B. Analog Scan Converter – electric charges stored on silicon wafer 1. Advantage – provides excellent resolution (infinitely variable) 2. Disadvantage – unstable image C. Digital Scan Converter - signals stored as binary numbers in random access memory 1. Function – converts analog signals into digital format, then back for output devices 2. Numbers are stored as binary numbers 3. Advantage – provides stable storage D. Binary Number System 1. Bit – a binary number. It has two states: on and off 2. Eight bits – one byte 3. Specific sequence of zeros and ones are combined to represent any base 10 number E. Pixel – Picture Element – a dot on the screen 1. Information stored as a bit within a pixel 2. Increased pixel density improves spatial resolution 3. Decreased pixel density lowers spatial resolution 4. Location of pixel relationship with echo arrival time (depth) and beam orientation (laterally) 5. Voxel - picture element in three-dimensional imaging F. Relationship of Number of Bits and Contrast Resolution 1. Addition of bits (bit depth) increases the number of shades of gray the ultrasound unit can display 2. Calculation of shades of gray – raise number 2 to the exponential power of bits present in system 3. Ex. 3 bit resolution = 2 (raised to the 3rd power) = 2x2x2 = 8 shades of gray G. Preprocessing Function – only available while the image is live 1. Preprocessor allows change, alteration, or manipulation of echoes prior to storage in computer memory 2. Not available after the image is frozen 3. Examples – TGC, dynamic range, RES or Write Zoom 4. RES (Regional Expansion Selection) or Write Zoom rescans the specified area (ROI – region of interest) with more scan lines, improving spatial resolution H. Other Pre Processing Function 1. Persistence – type of frame averaging, smoother image, lowers frame rate 2. Edge Enhancement – increases contrast (sharpness) with small lesions (cyst vs solid) 3. Smoothing – filtering technique to reduce noise, uses averaging 4. Fill in Interpolation – fills in missing PIXELS due to diverging scan lines. Seen in sector and blunted sector I. Postprocessing Function – functions available after freezing the image 1. Postprocessing allows change, manipulation, or alteration of echoes after storage in computer memory (after freezing the image) 2. Changes image appearance after frame is frozen  3. Examples – Caliper placement, Black/white inversion (changes the background from black to white), Read magnification (regular zoom), Postprocessing curve (changes the greyscale map – brightness of pixels), cineloop CLASS 14 IMAGING INSTRUMENTS (DISPLAY AND STORAGE) A. Define the Display Component of the Ultrasound Imaging System 1. Most common display device – TV monitor (CRT) 2. Working principles of monitor display – electrons directed at light emitting phosphorescent screen causing screen to glow; change in electron beam intensity changes brightness of light 3. Raster scan format – beam directed in pattern of horizontal sweeps from left to right and top to bottom 4. How the display is written a. Interlaced display (old) – 525 horizontal closely spaced lines with odd lines written first and even lines second; odd lines take 1/60th of a second, even lines take 1/60th of a second, so 1/30th of a second to write one frame with 30 frames per second b. Non-interlaced display (new) – progressive scan with lines written in sequence. Each frame takes 1/30th of a second, yielding 30 frames per second. 5. High resolution monitor (HD) – increased number of horizontal lines (over 1,000) with more pixels, yielding better spatial resolution. Standard resolution 525 lines. 6. Monitor controls a. Brightness – adjusts brightness level of screen b. Contrast – adjusts range of echo brightness to darkness on screen B. Hard Copy Storage Devices – Capture and Preserve Images – Advantages and Disadvantages of Each Type of Device 1. Multi image camera – uses photographic emulsion film with standard developer 2. Laser printer – uses photographic emulsion film and develops film with laser technology 3. Thermal printer – uses heat sensitive paper to record gray scale echo information 4. Color thermal printer – uses heat sensitive paper; heat stimulates the release of color dyes onto paper 5. Strip charts – uses strips of paper to record M Mode or spectral Doppler information 6. Videotape cassette – uses polyester based strip coated with dipole magnets to record real time information – magnetic media 7. Fiber optics – uses paper or dry silver paper to record M Mode image information – magnetic media 8. Magneto-optical disc – used for digital archiving; combines read and write capabilities of magnetic technology with storage capabilities of optical media 9. PACS (Picture Archiving and Communications System), DIN (Digital Imaging Network), IMACS (Image Management Acquisition Control System) – digital storage and transmission of images linked by a network; makes teleradiology (ability to transfer images remotely) possible C. Display Modes in Imaging 1. A Mode – amplitude mode – Depth on X axis, Amplitude on Y axis, 1 DIMENSIONAL, NO 2D IMAGE, NO DUPLEXING 2. M Mode – motion mode – Time on X axis, Motion on Y axis, good temporal resolution 3. B Mode – brightness mode – Depth on X axis (dots represent reflector position) and Amplitude (brightness) on the Z axis 4. 3D and 4D – method of combining many 2D images to form 3D image. More surface detail, lowered temporal resolution, difficult to image deeper structures. 4D is 3D in real time. D. Temporal Resolution 1. Definition – resolution relating to time 2. Temporal resolution is directly related to frame rate (FR, FPS) 3. The higher the frame rate, the better the temporal resolution 4. Full motion video – 30 frames per second (FPS) 5. Flicker – occurs below 16 frames per second (FPS) 6. Factors affecting temporal resolution a. Number of pulses per scan line – more pulses, lower frame rate (worse temporal resolution) b. Maximum imaging depth – deeper depth, lower frame rate c. Lines per image/ line density/sector angle – more lines, lower frame rate d. Number of focal zones – more focal zones, more pulses, lower frame rate 7. Mathematical equation: Penetration depth (cm) X number of focal zones X Lines per frame < 77,000 If the number exceeds 77,000 the frame rate drops CLASS 15 DOPPLER PRINCIPLES AND EFFECT (HEMODYNAMICS OF BLOOD FLOW) A. Operating Principles of Doppler Ultrasound B. Use of Doppler Ultrasound in Diagnostic ultrasound 1. Assessment of blood flow 2. Qualitative assessment - color 3. Quantitative assessment – spectral analysis C. Hemodynamics – Principles that Govern Blood Flow 4. Required for blood flow: a. Pressure Gradient (energy gradient, pressure differential) – generated by the heart and gravity. The heart is the greatest source of energy. Blood always flows from high pressure to low pressure, until it equalizes. The greater the difference in pressure, the greater the volume of flow.  b. Pathway for travel – connects the two pressure points (vessel) 5. Factors that influence resistance to flow – affected by viscosity, friction, inertia a. Viscosity – fluid’s thickness or stickiness. Units – poise. Affected by anemia (decrease), polycythemia vera (increase) b. Friction – Resistance to motion because of objects touching each other. Vessel walls and blood. c. Inertia – Tendency of an object to maintain its “status quo”. Pressure must overcome inertia to get flow. 6. Poiseuille’s equation – mathematical equation that predicts volume flow rate; d. Assumes laminar flow in long straight tubes e. Q = volume flow, P1 P2 = pressure gradient, r = radius of vessel, L = length of vessel, n = viscosity of fluid f. Q = P/R Q = Pressure / Resistance g. Q = (P1- P2) (pi) r (fourth) / 8nL h. Direct relationship between quantity of f veins under ultrasound so I know the walls wouldn't be echogenic but they would be compressible OK which means they're going to be thin right right so think about it well like when you're when you're doing the liver right you see our you see the arteries you see the portal system do you see the veins when you're looking at the petticoat hepatic veins what do you see you should just see black area and then meet right you don't actually see the walls right if you get them like a 90° you're actually really thin thing but the reason why you don't seem is why because earth made to what made to distand it's the same 3 layers are just thinner that's why you don't see him right because they're made there that's why when you're looking at the aorta you see thing pop pop pop when you switch over to IVC it's soft and floppy right it's low pressure is low pressure high pressure side low pressure side right it's just like just like water coming into your house high pressure and low pressure side high pressure you've got the deposits just like stenosis and you got high pressure leaks on the drain so you get what blood clots and we got bad valves low, and pressure gradient and radius of the vessel i. Quantity increases if: Pressure gradient or radius (size of vessel) increases i. Indirect relationship between quantity of flow and viscosity, length of vessel i. Quantity decreases if: Viscosity or Length of vessel increases j. The radius has the greatest effect on flow (raised to the fourth power) D. Bernouli’s principle – There is an inverse relationship between pressure and velocity. 1. Energy is the ability to do work 2. PRESSURE – potential (stored) energy 3. VELOCITY – kinetic (energy in motion) energy 4. The pre-stenotic area has high PRESSURE (potential energy) and low VELOCITY (kinetic energy) 5. AT THE STENOSIS there is a DROP in PRESSURE and an increase in VELOCITY.  6. The post-stenotic area sees a PRESSURE INCREASE and a VELOCITY DROP, but pressures and velocities will be below their maximums. 7. Blood flows from a high pressure area to a low pressure area until it can no longer overcome resistance. 8. The greater the difference in pressure, the greater the flow 9. Greater flow through smaller openings results in higher velocities to move the same amount of blood with each stroke of the heart (velocities increase with stenosis). E. Blood Flow Patterns 1. Types of blood flow a. Plug – uniform velocity across vessel lumen. Exhibits a blunted profile. Occurs at the entrance of the great vessels (aorta, IVC), or exiting the heart. b. Laminar – blood flows in concentric layers, with faster velocities at the center of the vessel due to friction from the vessel walls. Exhibits a parabolic or bullet shaped profile=. The ideal flow state because increases in pressure will result in an increase in flow volume. c. Disturbed - occurs at bifurcations and changes in vessel size from SMALL TO BIG. Minor laminar separation and spectral broadening; may be normal or abnormal. Occurs at a Reynolds number of aprox 1500 d. Turbulent – abnormal flow pattern in which flow patterns are chaotic and erratic; caused by stenosis or abrupt changes in vessel lumen. Complete separation of the laminations (layers). Exhibits significant spectral broadening under spectral analysis and a mosaic color tile pattern under color Doppler. Occurs at a Reynolds number of aprox 2000. Will show turbulent areas as the color green when using the variance map under color Doppler. Has reached the physical maximum flow limit. Arterial system – pulsatile. Purpose – deliver high pressure blood from the heart to the capillary beds. Appearance – thick and echogenic (to handle the high pressure) Arteries are composed of three layers: Tunica Intima – inner layer that lines the vessel wall Tunica Media – middle (muscle) layer that regulates flow (resistance) Tunica Externa (Adventitia) – provides the vascular network that supports the vessel cells and structural support. Flow is controlled by changing resistance through vasodilation and vasoconstriction. Vasodilation – relaxes the media layer which increases flow (increases radius). Vasoconstriction – muscle contraction of the media layer decreases flow (decreases radius). Arterial blood flows from the deep system to the superficial system The deep system is the primary circulatory system that has paired arteries and veins The superficial system provides flow to the superficial layers of the body. May NOT have paired arteries and veins. Sources of pressure – heart (main source) and gravity (hydrostatic pressure) Velocity and pressure rises and falls with contractions of the heart Systole - contraction phase of the heart Arteries expand under heart contractions (systole) Arteries act as a blood reservoir  Provide forward flow of blood during Diastole – as it comes back to normal size High, or increasing Peak Systolic Velocity (PSV) – indicates stenosis Stenosis is a narrowing of the lumen (open part of the vessel) Typically due to plaque buildup or kinking of the vessel (congenital) As stenosis increases, velocities increase, then finally decrease just before occlusion (completely blocked) Dicrotic notch – indicates the closing of the aortic valve and separates systole from diastole Diastole - relaxation phase of the heart Lower pressure Flow in diastole is dependent upon resistance in the capillary beds Resistance is controlled by the size of the open lumen (radius) in the capillary beds Flow in diastole is indicated by the End Diastolic Velocity (EDV) The lower the resistance, the higher the flow in diastole (high EDV) Vasodilation causes relaxation of the media (muscle) layer which increases the radius of the open lumen, increasing flow The higher the resistance, the lower the flow in diastole (low EDV) Vasoconstriction causes contraction of the media (muscle) layer which reduces the radius of the open lumen, reducing flow Venous system Purpose - to handle varying amounts of blood without raising pressure and returns venous blood back to the heart. Must fight gravity to return venous blood from the lower extremities back to the heart Phasic – depends upon the phases of respiration for venous flow Inspiration – diaphragm descends (down), increasing intra-abdominal pressure which stops the flow of blood from the lower extremities back to the heart. At the same time the intra-thoracic pressure decreases, allowing blood flow from the upper extremities back to the heart. Expiration – diaphragm ascends (up), decreasing intra-abdominal pressure which allows the flow of blood from the lower extremities back to the heart. At the same time the intra-thoracic pressure increases, stopping the blood flow from the upper extremities back to the heart. Blood pressure is regulated by blood volume. Greater blood volume = higher blood pressure Blood pressure rises with ingestion of salt (“water follows salt”, which raises blood volume and pressure) Venous walls are thin and elastic The vein walls distend (dilate) to offset the increased volume of blood, maintaining blood pressure at a safe level. Veins have valves to assure unidirectional flow Valves are an “elaboration of the intimal layer” Veins are composed of three layers: Tunica Intima – inner layer. Lines the wall of the vessel and has outgrowths which are the leaflets of the valves Tunica Media – middle (muscle) layer. Provides strength but is thin and elastic to allow for distention (dilation) to handle the varying amounts of blood. Tunica Externa (Adventitia) – provides the vascular network to nourish the vessel cells and increases structural strength Shape of the veins changes from dog bone to round Shape of the veins is dependent upon the transmural pressure (difference in pressure between the inside and outside of the vessel) The higher the transmural pressure inside the vein, the rounder the shape of the vein Calf Muscle Pump – provides increased venous pressure in the legs when the calf muscles contract Venous blood drains from the superficial system to the deep system. Venous blood in the calf drains into venous sinuses (cavities) which are located between the soleal and gastrocnemius muscle groups Contraction of the calf muscles squeezes the venous sinuses, increasing the blood pressure in the deep veins of the calf, which returns blood back to the heart. The valves keep the blood flowing towards the heart (provides unidirectional flow) Hydrostatic Pressure - Weight of a column of blood from the heart to the point where the pressure is measured 1 Venous pressure changes between supine and standing 2. Supine - 15 mm/Hg 3. Standing - 102 mm/Hg 4. Hydrostatic pressure is mainly affected by body position CLASS 16 DOPPLER SHIFT SPECTRAL ANALYSIS A. The Doppler Shift – change in frequency of sound as a result of motion between the source or reflector 1. Doppler shift equation – Doppler shift equals reflected frequency minus incident frequency DS = REFLECTED FREQUENCY – INCIDENT FREQUENCY Also, DS = 2 Fo V COS (theta) C 2 = two Doppler shifts (trip down and back) Fo = Operating frequency. The higher the frequency, the greater the Doppler shift V = Velocity (what the machine does not know). Doppler shift frequency is proportional to velocity of flow COS (theta) = Angle Correction. COS = ADJACENT / HYPOTENEUSE Accounts for differences in angles to the beam COS of 0 degrees = 1 (true velocity) COS of 60 degrees = 1/2 COS of 90 degrees = 0 (no doppler shift) COS range from 0 – 1 c = speed of sound in soft tissue (1.54mm/us) 2. Positive Doppler Shift – reflector and source are moving toward each other 3. Negative Doppler Shift – reflector and source are moving away from each other 4. Even though we use ultrasonic frequencies, the Doppler shift is (difference between the frequencies) within the audible range B. Components of the Continuous Wave Instrument (Doppler instrument) 1. Voltage generator (pulser) 2. Transducers; source and receiving 3. Loudspeaker 4. Memory and display C. Continuous Wave Operating Principles 1. Sends and receives sound continuously 2. Contains two elements within transducer 3. Presents information as a waveform and audibly (no image) 4. Quadrature detection – determines direction of Doppler signal 5. Bi-directional device – can differentiate between a positive and negative Doppler shift (has quadrature detection) 6. Uni-directional device – cannot differentiate between positive and negative Doppler shift (no quadrature detection) 7. Advantages of CW Doppler – measures high velocities accurately, no aliasing, probe is small and easy to manipulate 8. Disadvantage of CW Doppler – range ambiguity (unable to determine depth), no image D. Components of the Pulsed Wave Instrument (Doppler instrument) 1. Voltage generator and gate (pulser) 2. Transducer 3. Receiver and gate 4. Loudspeaker 5. Memory and display E. Pulsed Wave Doppler Operating Principles 1. One element in transducer that sends and receives sound 2. Presents information as a spectral waveform and audible (image is present) 3. Advantages of Pulsed Doppler – ability to perform duplex scanning (two functions at the same time), range resolution (ability to determine depth) 4. Disadvantage of Pulsed Doppler – aliasing F. Information gained through Spectral Analysis a. Velocity of flow information (precise) b. Flow direction c. Range of frequencies d. Duration of signal  e. Magnitude of signal G. Aliasing – Improper Representation of Positive Shift as a Negative 1. Occurs due to undersampling 2. Relationship to Nyquist limit – aliasing occurs when the Doppler shift frequency exceeds one half the PRF (Nyquist Limit = 1/2PRF) 3. Therefore, the maximum Doppler shift that can be displayed without aliasing will be ½ the pulse repetition frequency (Ex. If PRF is 8,000 the maximum shift displayed will be 4,000 Hz 4. Techniques to eliminate aliasing – a. Baseline – cosmetic adjustment. Adjusts the wave up or down to fit b. Scale – increase velocity scale (increases the range so the wave can fit w/o aliasing) c. Depth – image from a shallower depth (reduces idle time, reduces PRP, increases PRF) d. Lower Transducer Frequency – the lower the frequency, the smaller the Doppler shift e. CW Doppler –continuous wave does not pulse, cannot exceed nyquist limit, no aliasing 5. Information obtained through pulsed wave Doppler technique – a. Direction of flow b. Velocity of flow c. Presence of flow d. Range of Frequencies present e. Signal Duration f. Signal Magnitude (strength) 6. Arterial waveforms – The arteries display different waveforms depending upon the priority of blood flow. a. Low resistance capillary beds - Result in high end diastolic velocities (EDV). Found where blood is required throughout the entire cardiac stroke (organs). May also be caused by vasodilation (enlargement of the vessel lumen due to relaxation of the media layer) due to physical activity b. High resistance capillary beds - Result in low end diastolic velocities (EDV) due to the high resistance to flow. This is due to the small radius of the vessel capillaries in order to stop end diastolic flow. These waveforms are found where constant blood flow is not required (peripheral vessels – arms/legs). May also be caused by vasoconstriction (reduction of the vessel lumen due to constriction of the media layer) to divert blood to muscles undergoing physical activity. c. High systolic velocities – Due to stenosis: indicated by an increase in systolic velocity, rather than the typical gradual decline in velocities as the blood loses energy as it flows further down the vessel. d. Low end diastolic velocities – When seen in vessels that should have HIGH end diastolic velocities, this indicates an increase in resistance in the capillary beds which may be a sign of rejection. 7. Venous waveforms – gradual rise and fall, surges  8. Affected by: a. Phasicity – phases of respiration. Flow alternates between the upper and lower halves of the body b. Hydrostatic pressure – gravity – weight of a column of blood from heart to measured point. c. Calf muscle pump – contraction of the calf muscles increases venous pressure in the lower extremities veins. 9. Augmentation – compression of the calf muscles, increases venous flow towards the heart (proximal), checks for incompetent valves. Faulty valves are indicated by retrograde flow after augmentation (compression of the calf muscles) Visualized as flow on the opposite side of the baseline. H. Spectral Waveform Analysis (PW) – Separation of Frequencies in the Signal Into Individual Frequencies 1. Purpose – provides quantitative data on reflector velocities within the signal 2. Fast Fourier Transform (FFT) – mathematical process used to analyze the signal and present information in real time I. OLD methods of Spectral Waveform Analysis a. Zero Crossing Detector = Analog. Counts the number of times the voltage crosses the baseline. More crossings = higher frequencies b. Time Interval Histogram = Analog. Counts the number of times the voltage crosses the baseline within a specific period of time c. Chirp Z Transform = Increased flexibility. Can specify both spectral analysis bandwidth and the resolution within that bandwidth J. Quantitative Doppler Measurements 1. Pulsatility Index – maximum velocity minus minimum velocity divided by mean velocity PI = MAX – MIN / MEAN 2. Resistive Index – maximum velocity minus minimum velocity divided by maximum velocity RI = MAX – MIN / MAX CLASS 17 COLOR DOPPLER A. Applications of Doppler Color flow imaging 1. Color Doppler information is superimposed over the greyscale image 2. Used to facilitate imaging 3. Determines: a. Presence of flow b. Direction of flow c. Qualitative data on reflector velocity,  d. Reflector Variance – differences in speed and direction between reflectors B. Operating Principles of Color Doppler Imaging 1. Presentation of flow information in color, superimposed on a 2D gray scale image 2. Information from multiple locations along each scan line is composited to yield a color image of area of interest 3. Packet Size – Number of Pulses per Line of Color. Typically 8 – 30 pulses per packet 4. Larger packet size yields better velocity measurements, color sensitivity, worse temporal resolution 5. Smaller packet size yields worse velocity estimates, lessened color sensitivity, improved temporal resolution C. Advantages of Color Doppler Imaging 1. Provides two dimensional information 2. Facilitates the examination 3. Provides information relating to flow direction, character of flow, relative (average) velocities, flow amplitude D. Disadvantages of Color Doppler Imaging 1. Poorer temporal resolution (lower frame rate) 2. Subject to aliasing – it is a form of pulsed Doppler, so the color wraps around the color bar and positive shift information is inaccurately portrayed as a negative (red shows as blue, and vice versa. a. Reasons: i. High velocities ii. Scale set too low 3. Unable to measure exact velocity of flow (color information is qualitative, not quantitative) F. Color Assignment Maps – Methods of Displaying Colors as Related to Velocities 1. Velocity mode a. Darker colors represent slower velocities, lighter colors represent faster velocities. b. The color at the top half of the bar represents flow towards the transducer. c. The color at the bottom half of the bar represents flow away from the transducer d. The black stripe in the center of the bar represents no Doppler shift (no color) 2. Variance mode – addition of another color (green) that indicates the degree of velocity variance *(differences between the various frequencies) within the signal, used in cardiac imaging. The additional color is used to indicate disturbed or turbulent flow. 3. Significance of colors – BART – normal color assignment. Blue Away, Red Towards. Red indicates positive shift, blue indicates negative shift, black indicates no shift. G. Color Components 1. Hue – color perceived by the eye 2. Saturation – intensity of color in an image (more saturation, less dilution with the color white)  3. Luminance – brightness of the hue and saturation of the color H. Estimation of Velocities in Color Flow Imaging 1. Autocorrelation – mathematical (digital) calculation of average shifts from echoes in same packet a. Used for color Doppler b. Calculates the average Doppler shift c. Faster than FFT, but not as accurate (cannot measure exact velocities) d. Qualitative, not quantitative 2. Time domain processing – pinpoints specific echo out of many in relation to time a. Newer method b. Faster than FFT I. Wall Filter, Thump Filter, High-Pass Filter a. Used to suppress color from tissue movement, while still allowing color from blood flow (clutter) b. Blood flow emits a high frequency signal c. Tissue movement emits a low frequency signal d. The Wall Filter suppresses the low frequency (tissue movement) part of the signal, allowing the high frequency (blood flow) signal to pass through, hence, it called a High-Pass Filter J. Color Power Doppler (Color Doppler Energy, Ultrasound Angio, Color Power Angio) 1. Operating principles – flow information based upon amplitude (strength) of blood cell motion 2. The power (strength) of the Doppler shift is related to the concentration of RBC’s (the higher the concentration, the higher the strength of the signal, the brighter the color) 3. Based on the power (strength) of the reflectors, not velocities 4. Benefits – useful in determining if flow is present (more sensitive than regular color Doppler) 5. Differences from Pulsed wave techniques – NO ALIASING 6. Advantages of Power Doppler – not angle dependent, no aliasing, greater sensitivity to flow 7. Disadvantages of Power Doppler – not sensitive to direction of flow, cannot provide velocity information, cannot depict flow characteristics, poorer temporal resolution, flash artifacts CLASS 18 ARTIFACTS IN IMAGING (PART 1) A. Artifact Definition – Anything on the Display that does Not Represent Real Echo Information a. Echoes present on the image that should not be there b. Echoes NOT present on the image should be there c. Some artifacts are helpful, others detrimental B. Reasons for Artifacts 1. Physical laws governing ultrasound are broken  2. Image information is misinterpreted 3. Faulty performance of ultrasound unit 4. Failure to operate equipment properly 5. Physical nature of sound C. Presumptions Related to Sound (faulty assumptions) 1. Sound travels in a straight line from transducer and back 2. Echo information comes from reflectors along beam path 3. Echo strength is directly related to reflector’s strength 4. Ultrasound travels at a speed of 1540 meters per second 5. The imaging plane is very thin D. Elimination of Artifacts 6. Adjust the instrument controls 7. Change the imaging plane 8. Change the acoustic window 9. Change the patient position E. Categories of Artifacts – Based Upon Cause 1. Resolution 2. Propagation speed 3. Side lobes and grating lobes 4. Refraction 5. Attenuation 6. Doppler artifacts F. Resolution Artifacts 1. Axial (LARRD) and Lateral (LATA) resolution – size and shape misrepresentation, missing information (blank spaces). If the resolution is too low, two reflectors will be seen as one (cannot detect the space between the reflectors) 2. Section Thickness (Slice thickness, Partial volume)– thicker imaging plane than expected, fill-in of cystic structures (Cyst Fill-In) with false echoes, or Ghost Walls showing in the imaging plane 3. Speckle – interference effects from sound scattering, grainy appearance of ultrasound image. The scattering causes constructive and destructive interference resulting in black dots. Reduced by Frequency Compounding. G. Reflection Artifacts 1. Reverberation – sound bounces back and forth between the reflector and transducer, multiple equally spaced false echoes are displayed distal (deeper). 2. Comet Tail – a form of reverberation. A series of closely spaced false tapered echoes along beam’s axis  3. Twinkle – Color Doppler version of comet tail. Commonly caused by a trapped stone in the distal ureter (UPJ - ureteropelvic junction) 4. Ring Down – a form of reverberation. A series of closely spaced false echoes along beam path, associated with resonance from gas bubbles. Straight vertical line (NOT tapered) 5. Mirror Image – sound bounces from reflected structure to mirror reflector and back, false image displayed distal (deeper) to the real reflector (on the other side of the mirror reflector). 6. Multi-Path – sound travels different pathways to and from transducer, reflector placed at improper depth with inaccurate amplitude H. Speed Error Artifacts – sound propagates at a speed other than 1540 M/s – results in inaccurate depth and amplitude of reflector. Fast medium = falsely low time of flight = reflector placed shallower. Slow medium = artificially high time of flight = reflector placed deeper. CLASS 19 ARTIFACTS IN IMAGING (PART 2) A. Side / grating lobe Artifacts 1. Side lobes – accessory beams off the main axis of beam, false image information portrayed lateral to real image, occurs with single element transducers 2. Grating lobes - accessory beams off the main axis of beam, false image information portrayed lateral to real image, occurs with multi-element (array) transducers B. Refraction - occurs when sound encounters a curved reflector, or at oblique incidence and differences is speed a. Reflectors may be falsely portrayed laterally b. Reflector may display edge shadowing c. Reflector information may be missing d. Reflector amplitude may be falsely low B. Attenuation Artifacts 1. Shadowing (AKA: posterior shadowing, poor through transmission) a. Inability of sound beam to pass through an object (highly attenuating structure - proximal) b. Echoes deep to structure (distal) are of lower amplitude (darker) c. Black shadow displayed posterior to structure d. May result in valuable diagnostic information 2. Enhancement (AKA: posterior enhancement, good through transmission) a. Occurs when sound passes through a weakly attenuating structure (fluid) b. Echoes distally are falsely enhanced in amplitude c. Echoes posterior to structure are brighter (more echogenic) d. May result in valuable diagnostic information 3. Focal enhancement a. Horizontal area of increased echo amplitude (brighter)  b. Related to increase in sound intensity within area c. Caused by improper setting of TGC or Focal Zones d. Related to Banding – Banding include BOTH bright and dark bands C. Doppler Artifacts 1. Aliasing a. Due to undersampling b. Doppler shift exceeds the Nyquist limit c. Positives displayed as negatives on spectral display – wrap around d. Exceeds the Nyquist limit (1/2 PRF) e. Correct by increasing PRF (Scale), lowering baseline, shallower depths, lower frequencies, CW 2. Mirror image Doppler - crosstalk a. Duplication of waveform on opposite side of strong reflector b. May be caused by Doppler gain that is set too high c. May be caused by Doppler angle that is too close to ninety degrees 3. Flash artifact a. False color flashes not related to reflector motion b. May be caused by motion, respiration, talking, Color Doppler gain too high, persistence too high, wide color box, filter setting too low c. Power Doppler especially prone to this artifact 4. Clutter a. Caused by reflector motion obscuring color flow characteristics b. May be corrected by changing Wall Filter setting c. The wall filter suppresses the low frequency tissue movement signal, but lets the high frequency flow signal through. 5. Electronic noise a. Appears as “snow” or vertical squiggly lines on display b. Caused by operation of other electronic equipment nearby CLASS 20 MAINTENANCE AND QUALITY CONTROL A. Quality Assurance 1. Evaluation of the ultrasound unit to guarantee optimal imaging a. Inspection b. Evaluation c. Record keeping  2. Preventative measures performed by sonographer one to two times per week a. Clean filters b. Check electrical connections and cords c. Clean unit d. Clean transducers (before and after every patient) 3. Evaluation of the ultrasound unit every four to six months by ultrasound operator or service engineer a. 24/7 – 4 times per year, clinic (daytime hours) – 2 times per year b. Use of AIUM test objects c. Use of AIUM phantoms 4. Record keeping techniques a. Documentation of inspection of ultrasound machine b. Documentation of equipment evaluation testing c. Documentation of machine repairs 5. Importance of Quality Assurance testing a. Minimizes misdiagnosis b. Minimizes machine downtime c. Minimizes number of scans that must be repeated d. Detects gradual deterioration of the ultrasound equipment e. MINIMIZES LAWSUITS B. Imaging Performance Testing 1. Use of the AIUM Phantom and Test Object a. Testing for system sensitivity b. Testing for dynamic range c. Testing for detail resolution d. Testing for dead zone e. Testing for axial and lateral resolution f. Testing for depth calibration accuracy g. Testing for TGC characteristics h. Testing for contrast resolution through mock cystic and solid structures (PHANTOM ONLY) 2. Use of the Doppler Phantom Test Object – evaluates performance of the Doppler instrument a. Contains simulated vessels with fluid forced through at pre-determined velocities b. Simulated stenosis with known velocities c. Can simulate different cardiac states  d. Alternate type uses a moving string to scatter sound to evaluate flow e. Capabilities – evaluates Color and Spectral flow patterns, discriminates between direction of flow, determines accuracy of flow velocity detection, evaluates accuracy of gate position CLASS 21 BIOEFFECTS AND SAFETY A. Definition of Dosimetry – Study of Characteristics of Ultrasound That Have the Potential to Cause Biological Effects and Damage B. Clarification of the Findings of Research as Related to Possible Bioeffects C. Acoustic Exposure and Mechanisms of Action 1. Thermal effects 2. Cavitation 3. Mechanical effects D. The AIUM Statement pertaining to ultrasound safety 1. In vivo mammalian bioeffects – Heating studies 2. In vitro (test tubes) biological effects – Cavitation studies 3. Epidemiology – Survey, used for people 4. Safety in research and training 5. The AIUM'S intensity for safety -