Physics Chapters 13-24 Review

• Chapters covered in this physics final review: 13-24
• There should be a review of all PowerPoint slides and the textbook.
• Some exam content may not be covered in these slides.

Temporal Resolution and Real Time Imaging (Chapter 13)

• Temporal resolution is affected by speed of sound in the medium and imaging depth.

Factors Affecting Frame Rate

• Imaging depth is a systems setting that affects frame rate.
• The number of pulses per frame also affects frame rate.

Shallow Imaging vs Deep Imaging

• Shallow imaging means a short go-return time.
• Shallow imaging means a shorter T frame.
• Shallow imaging means a higher frame rate.
• Shallow imaging gives superior temporal resolution.
• Deep imaging means a long go-return time.
• Deep imaging means a longer T frame.
• Deep imaging means a lower frame rate.
• Deep imaging gives inferior temporal resolution.
• Deeper imaging requires more listening time.
• Shallower imaging requires less listening time.

Factors Determining Number of Pulses per Frame

• The number of focal points determines pulses per frame.
• The sector size is a determinant of pulses per frame.
• The line density determines pulses per frame.

Single Focus vs. Multi-Focus

• Single focus uses one pulse per scan line.
• With single focus, a shorter T frame is achieved.
• Single focus achieves a higher frame rate.
• Single focus gives superior temporal resolution.
• Single focus means inferior lateral resolution
• Multi-focus uses many pulses per scan lines
• With multi-focus, a longer T frame is achieved.
• Multi-focus results in a lower frame rate.
• Multi-focus causes inferior temporal resolution.
• Multi-focus achieves superior lateral resolution.

Narrow Sector vs Wide Sector

• Narrow sector means fewer pulses per frame
• Narrow sector gives a shorter T frame.
• Narrow sector results in a higher frame rate.
• Narrow sector allows for superior temporal resolution.
• Wide sector means more pulses per frame
• Wide sector gives a longer T frame
• Wide sector results in a lower frame rate.
• Wide sector results in inferior temporal resolution

Low Line Density vs Wide Sector

• Low line density means widely spaced lines.
• Low line density results in fewer pulses per frame.
• Low line density provides a shorter T frame
• Low line density attains a higher frame rate.
• Low line density means superior temporal resolution.
• Low line density means inferior spatial resolution
• Wide sector utilizes tightly packed lines.
• Wide sector includes more pulses per frame.
• Wide sector needs a longer T frame.
• Wide sector leads to a lower frame rate.
• Wide sector results in inferior temporal resolution.
• Wide sector achieves superior spatial resolution.

Temporal Resolution

• Worse temporal resolution includes greater depth, more pulses, a wide sector, multi-focus and high line density
• Better temporal resolution includes less depth, fewer pulses, a narrow sector, single focus and low line density

Pulse Echo Instrumentation (Chapter 14)

• Ultrasound systems contain major components.
• The transducer transforms electrical energy into acoustic energy during transmission.
• The transducer converts returning acoustic energy into electrical energy during reception.
• The pulser creates the pulse and adjusts the voltage during transmission
• The beam former adjusts electrical spike voltage to reduce lobe artifacts and distributes the delay patterns
• The receiver amplifies returning echo's evenly throughout the image.
• Order of receiver functions: Amplification, compensation, compression, Demodulation & Reject.
• The display presents processed data using a cathode ray tube (CRT or television)
• The storage archives ultrasound studies and typical storage devices or media.
• The synchronizer maintains and organizes the proper timing and interaction system's components.

Function Adjustment

• Amplification treats all signals identically, entire image gets brighter or darker, and this is adjustable.
• Compensation treats signals differently based on reflector depth, uniform image from top to bottom, and this is adjustable.
• Compression treats signals differently depending on strength to change grayscal mapping, and this is adjustable.
• Demodulation prepares electrical signals to be suitable for display CRTs and this is not adjustable
• Reject only affects weak signals, strong signals remain unchanged leaving weak echoes or eliminated from the image, and this is adjustable

Output Power vs Receiver Gain

• Output power changes brightness of entire image, alters signal to noise ratio, alters patient exposure, and carries Bioeffect concerns to decrease if image is too bright.
• Receiver gain changes the brightness of entire image, does not affect signal to noise ratio, does not change patient exposure, and carries no bioeffect concerns to increase this first if image is too dark.
• To scan, use the ALARA principle to determine which to adjust.
• ALARA is "As Low As Reasonably Achievable"
• Always choose the option that minimizes patient exposure while still obtaining a clinically relevant image.

Display and Image Storage (Chapter 15)

• Two user controls that alter characteristics of the image are contrast and brightness.
• Contrast controls the amount of black & white or gray scale on the monitor.
• Brightness controls how bright or dark the image is on the monitor.

Bistable vs. Gray Scale

• Bistable images are composed of only 2 shades: black & white, high contrast and narrow range.
• Gray scale displays multiple levels of brightness, such as: white, light gray and medium gray and it offers LOW contrast and wide range

Scan Converters

• The scan converter translates the information from the spoke format into the video format.
• Scan converters made gray scale displays possible by storing the image information and then displaying it on a cathode ray tube
• Storage of the image information in the scan converter is called "writing".
• The image data is then "read" from the scan converter for display on the CRT.
• Scan Converters allows the image to be manipulated/altered between storing and displaying, such as Black and white inversion.
• In early days of two dimensional imaging, scan converters were constructed from analog technology.
• Today, they use computer technology and are called digital scan converters.

Analog vs Digital Scan Converter

• An analog signal display is from the transducer in the real world.
• A digital scan converter occurs in the computer world.
• An analog signal display takes place in the real world.

Analog vs Digital Numbers

• Analog numbers is in the real world with unlimited number of choices, and are continuous value.
• Digital numbers is in the computer world with limited choices, and are discrete data.

Pixel Density

• Low pixel density has few pixels per inch with larger pixels leading to less detailed image with lower spatial resolution.
• High pixel density has many pixels per inch with smaller pixels leading to more detail image with higher spatial resolution

Bits Per Pixel

• Fewer bits per pixel means fewer shades of gray and degraded contrast resolution
• More bits per pixel means more shades of gray and improved contrast resolution

Elements Of Digital Images

• A pixel is the smallest element of a digital picture.
• A bit is the smallest amount of computer memory.

Shades

• An image with 1 bit has 2 shades.
• An image with 2 bits has 4 shades.
• An image with 3 bits has 8 shades.
• An image with 4 bits has 16 shades.
• An image with 5 bits has 32 shades
• An image with 6 bits has 64 shades.
• An image with 7 bits has 128 shades.
• An image with 8 bits has 256 shades.
• With 8 bits, the number 2 is multiplied by itself 8 times resulting 256
• Thus, 256 different gray shades can be stored with 8 bits.
• To store from 9 to 16 shades of gray 4 bits are required.
• Three bits can store up to 8 shades of gray, since a bit is the smallest amount of computer memory, a total of 4 bits are needed to store more than 8 different shades of gray.
• Four bits will actually store a maximum of 16 different shades of gray, but 4 bits are required to display more 9 different shades.

Preprocessing vs Postprocessing

• Preprocessing includes: Time gain compensation, Log compensation Write magnification And fill in interpolation.
• Postprocessing involves any change after freeze frame such as Black or white inversion, Read magnification, and Contrast variation.

Read Magnification vs Write Magnification

• Read magnification uses old data, it is postprocessing, uses larger pixel size, and has Same # of pixels as in the original ROI retaining Unchanged spatial resolution and Unchanged temporal resolution.
• Write magnification acquires new data, it is preprocessing, identical pixel size with more pixels than in the original ROI, improved spatial resolution and possible improved temporal resolution

Dynamic Range(Chapter 16)

• Received Signal losses 82 dB in 3 mm Al, has 16 dB SNR, and + 5 dB loss in 0.6 cm air
• 103 dB System Dynamic Range is achieved

Dynamic Range of Components

• The transducer's Dynamic Range is 120 dB.
• The receiver's Dynamic Range is 100 to 120 dB.
• Scan Converter's Dynamic Range is 40 to 50 dB.
• The display has a Dynamic Range of 20 to 30 dB.
• The archive's Dynamic Range is 10 to 30 dB.

Fewer Shades vs More Shades

• Fewer Shades (Fig. A) has Few choices, Black and white (bistables), Narrow dynamic range, and High contrast.
• More Shades (Fig. B) has Many choices Gray scale, wide dynamic range and Low contrast

Mathematics of Compression

• Mathematics of compression using decibels are straightforward: ADD or SUBTRACT.
• If an uncompressed signal has a range of 85 dB and undergoes 30 dB COMPRESSION resulting signal is 55 dB. 85 dB – 30 dB = 55 dB
• If AFTER Compression, the dynamic range is 70 dB, as the original signal was compressed by 40 dB, resulting in range of 110 dB.

Harmonics And Contrast Agents(Chapter 17)

• Harmonics can be created during transmission and are primarily created along the beam's main axis.
• Harmonic imaging is the creation of an image from sound reflections at twice the frequency of the transmitted sound.
• Harmonic frequency sound waves arise from nonlinear behavior.
• Tissue harmonics are created in the tissues during transmission and they are most likely created along the beam's main axis
• With tissue harmonics sound travels faster in compressions
• With contrast agents, stronger harmonics generates, and the microbubbles act in a nonlinear manner when struck by sound waves.
• Microbubbles expands to a greater extent than they are compressed and contract evenly in a linear fashion.

Mechanical Index

• The amount of contrast harmonics produced may be estimated by a number called mechanical index (MI).
• The mechanical index depends on the frequency of the transmitted sound and the pressure of the sound wave.
• The numerical value of the mechanical index, and therefore, harmonic production, increases with lower frequency sound, and stronger sound waves (high pressure variation)
• Low mechanical index sound beams do not create harmonics because the microbubbles expand and contract evenly in a linear fashion.
• When the MI is less than 0.1, bubbles create only backscatter.

Mechanical Index Chart

• Lower MI gives Less cavitation, Less pressure and Higher frequency.
• Higher MI gives More cavitation, More pressure and Lower frequency
• Low MI: Less Than 0.1 causes No harmonic and Backscatter behavior with Linear behavior coming from Higher frequency sound with Low beam strength.
• Higher MI: 0.1 Το 1.0 achieves Strong harmonic with Resonance due to Non - linear behavior from Lower frequency sound with Higher beam strength
• Highest MI: Greater Than 1 causes Stronger harmonic with Bubble disruption with Extreme non – linear behavior due to Lowest frequency sound with Highest beam strength.

Harmonics Category

• TISSUE HARMONICS is Created during transmission in tissue and Occurs as sound propagates in tissue and Result from non – linear behavior of transmitted sound beam giving Weaker harmonic signal
• CONTRAST HARMONICS is Created during reflection off of microbubble where Occurs only when contrast agents are present with MI greater than 0.1 and Results from non linear behavior of microbubble and gives Stronger harmonic signal

Pulse Inversion Harmonics

• In Pulse Inversion harmonic imaging, instead of only a single pulse, there are two pulses transmitted down each ray line.
• With pulse inversion harmonics, two consecutive ultrasound pulses are transmitted down each scan line.
• A type of sonographic imaging that transmits two pulses of opposite phase in rapid succession so they can be cancelled out upon reception is termed Pulse inversion harmonics

Hemodynamics(Chapter 18)

• Pulsatile flow has variably velocity. Blood accelerates and decelerates from cardiac contraction; pulsatile flow commonly appears in arterial circulation.
• Phasic flow has variable velocity. Blood accelerates and decelerates from respiration with phasic flow often appearing in the venous circulation..
• Steady flow, a fluid moves at constant speed or velocity, such as water in a garden hose. it is present in the venous circulation when individuals stop breathing for a moment..

Laminar Flow

• Plug flow is when all layers and blood cells travel at the same velocity.
• Parabolic flow has a bullet shaped profile. Velocity is highest in the center and gradually decreases to its minimum at the vessel wall

Turbulent Flow

• Turbulent flow is characterized by chaotic flow patterns in many different directions and at many speeds causing streamlines to be obliterated, as Turbulent flow converts flow energy into other forms such as sound and vibration.
• Sound associated with turbulence is called a murmur or a bruit.
• The Reynolds number predicts whether flow is laminar or turbulent.
• With turbulent flow
• Reynolds number is >2,000
• With laminar flow
• Reynolds number is <1,500

Effects of Stenosis

• Stenosis causes a change in flow direction downstream with increased velocity, and turbulence decreasing pulsitivity
• A force that OPPOSES THE FLOW OF A FLUID (BLOOD) is called RESISTANCE and it is due to vessel length and diameter.
• As a vessel diameter decreases, the resistance increases or as a vessel length increases, the resistance increases and the flow decreases

Ohm's Law

• Ohm's Law deals with the relationship between voltage and current in an ideal conductor.
• The movement of fluid through a tube and the movement of electricity through a wire are similar.
• For a fluid: Pressure gradient = flow x resistance
• An electrical system: Voltage = current x resistance
• Fluids: PRESSURE and FLOW face ELECTRICITY: VOLTAGE and CURRENT

Hydrostatic Pressure

• Measurement on Supine, the arterial blood pressure measured in an artery of arm is 140 mmHg.
• In supine individuals, all arteries are at the same level as the heart.
• Thus, the hydrostatic pressure in the arterial circulation is 0 mmHg because The arterial pressure at knee equals the pressure at heart is 140mmHg with same in carotid artery.
• Hydrostatic pressure during standing has varied pressure depending on location.

Hydrostatic Pressure In Standing Patient

• Hydrostatic measurements depends on the location of the vessel.
• With the vessels at heart level will will have hydrostatic pressure of 0 mmHg • The ankle is 240 mmHg (140 + 100) and Is Far below heart level,
• The Knee is 215 mmHg (140 + 75) and is Somewhat below heart level,
• The Waist is 190 mmHg (140 + 50 and Slightly below heart, with Mid chest set at 140 mmHgHeart level, where Top of head set at 110 mmHg (140 – 30) is Above heart

Breathing and venous flow

• Diaphragm moves downward toward the abdominal cavity.
• Thoracic pressure decreases.
• Flow in the arms increases
• Flow in the vena cava increases and the abdominal pressure increases.
• Flow in legs decreases during INSPIRATION
• Diaphragm moves upward into thorax and Thoracic pressure increases.
• Flow in legs increases during EXPIRATION • Venous flow in the legs correlates with movement of the diaphragm.
• Downward movement of the diaphragm (inspiration) decreases venous flow in the legs.
• Upward movement of the diaphragm (expiration) increases venous flow in the legs
• When venous flow in the leg decreases, venous return to the heart increases and opposite in decreasing

Doppler (Chapters 19 & 20)

• Doppler Shift Measured in Hertz.
• Directly proportional to velocity
• Directly proportional to transducer frequency
• A dedicated continuous wave Doppler transducer exhibits range ambiguity, higher sensitivity, and high quality factor

Aliasing

• Aliasing is Less with Slower blood velocity, Lower frequency transducer and Shallow gate (high PRF)..
• Aliasing is More pronounced with Faster blood velocity, Higher frequency transducer and Deep gate (low PRF)
• Aliasing will occur at any Doppler shift higher than the Nyquist frequency and it is equal to 1/2 the PRF.

Aliasing Frequency

• Nyquist frequency is equal to : PRF(Hz)/2 and it is avoided by adjusting scales

Pulsed vs. Continuous Wave (CW) Doppler

• Pulsed Doppler is range resolution and limited to maximum velocity with Aliasing requiring At least one crystal with Damped PZT, Low Q factor, Wide bandwidth , and Lower sensitivity.
• CW Doppler is Range ambiguity with Unlimited maximum velocity offering No aliasing that needs At least two crystal with Undampened PZT achieving High Q-factor, Narrow bandwidth, and Higher sensitivity.

Modality

• Continuous Wave identifies the highest velocity jets anywhere along the length of ultrasound beams that is Most sensitive with Very good temporal resolution, having No, offering Peak velocity measurements.
• Pulsed Wave accurately identifies location of flow with Moderate sensitivity and has Very good temporal resolution showing Peak velocity measurements.
• Power Doppler Uses with low velocity and Small Volume vessels but it gives Poorest temporal resolution rejecting to aliasing to has No velocity measurements.
• Doppler has range resolution offers color jets while it is Subject to flash artifact