Ultrasound Lecture Notes PDF

EQUIPP REVIEW CENTER AUDIBLE SOUND LECTURE  16 Hz to 20,000 Hz Mam Irene Alerna, RRT INFRASOUND HISTORY OF ULTRASOUND  Below 16 Hz  1918 - Sound Navigation and Ranging was used...

EQUIPP REVIEW CENTER AUDIBLE SOUND LECTURE  16 Hz to 20,000 Hz Mam Irene Alerna, RRT INFRASOUND HISTORY OF ULTRASOUND  Below 16 Hz  1918 - Sound Navigation and Ranging was used ULTRASOUND GENERATORS  Early 1950’s –Water bath immersion technique TYPES OF ULTRASOUND  Late 1950’s – First contact WAVES compound B-scanner  Longitudinal/Compression  1970’s – Gray scale imaging Waves  Mid 1970’s – Real time  Transverse/Shear Waves scanning systems  Surface/Rayleigh Waves  1980’s – Doppler technique ACOUSTIC VARIABLES SOUND  Period (T): the time taken for  A mechanical energy one complete cycle to occur (s  Requires a vibrating object to or µs) produced  Wavelength (λ): length of  Cannot travel through a space over which one cycle vacuum occurs (m or mm)  Amplitude (Depth): the ULTRASOUND maximum displacement that  High frequency sound waves occurs in an acoustic variable  Above 20,000 cycles per  Frequency: cycle per second second (20 kHz) (Hz)  Inaudible to humans  Velocity: frequency times  Used to scan tissues of the body wavelength  Ultrasound Pulse: 2-10 MHz  Pulse Duration: 1 microsecond PIEZOELECTRIC CRYSTALS  Pulse Repetition: 1000  Generate ultrasound waves times/second   Capable of changing electrical  Two-dimensional images in signals into mechanical which the echo amplitude is (ultrasound) waves depicted as dots of different brightness PIEZOELECTRIC CRYSTAL AS REAL-TIME TRANSMITTER OF SOUND  Shows movement as it occurs  Converting electrical energy into mechanical energy (sound) M-MODE  Shows movement as a function PIEZOELECTRIC CRYSTAL AS of time TRANSMITTER OF SOUND  Used in cardiac scanning  Converting mechanical energy (sound) into electrical energy DOPPLER ULTRASOUND  Demonstrates and measures NOTE: blood flow  Small crystal diameter o Increased beam DOPPLER EFFECT divergence  The change in apparent  Larger crystal diameter frequency of a wave as a result o Decreased beam of relative motion between the divergence observer and the source  Stationary Reflector: reflected DIFFERENT MODES OF echoes are the same as the ULTRASOUND transmitted waves  Reflector that Moves Closer: A-MODE reflected echoes are higher than  Echoes are shows as peaks the transmitted echoes  Distance between various  Reflector that Moves Away: structures can be measured reflected echoes are lower than  Used to build two-dimensional the transmitted echoes B-mode image BASIC TYPES OF DOPPLER B-MODE ULTRASOUND UNIT   Average Propagation for Soft 1.) Continuous Wave Doppler Unit Tissues: 1540 m/s  Ultrasound is continuous  Average Propagation for Soft  Measures high velocities Tissues: 4620 m/s accurately  No depth resolution WAVELENGTH 2.) Pulsed Wave Doppler Unit  The length of a single cycle of  Ultrasound is transmitted in the ultrasound wave pulses  Inversely proportional to the  With good depth resolution frequency  Measures the speed of the  Determines the resolution of blood in a particular vessel the scanner  Cannot measure high blood  Higher the frequency, the velocities in deep vessels shorter the wavelength  High velocities may be wrongly displayed as low velocities FOCUSING  Adjustment of the ultrasound 3.) Colour Doppler Unit beam  Shows different flow-velocities  To improve resolution in different colours  May be electronic or by a lens attached to the transducer 4.) Duplex Doppler System  Combination of a B-mode and AMPLIFICATION Doppler system  Done by the time-gain-  Allows the Doppler beam to be compensation (TGC) amplier directed accurately at any  To compensate for ultrasound particular blood vessels attenuation in any part of the body WAVE PROPAGATION  To improve the quality of the  The transmission and spread of final image ultrasound waves to different tissues BOUNDARIES   The line at the periphery of two  Property of a substance tissues which propagate  Describes how the particles of ultrasound differently that substance behave when  The zone of echoes at the subjected to pressure wave interface  High density substance – high acoustic impedance  Low density substance – low PIEZOELECTRIC EFFECT acoustic impedance  Piezein – “press or pressure”  Formula: Z=pc  Ability of a material to generate o p = density of material an electrical charge un response (kg/m3) to applied pressure o c = speed of sound (m/s) o Z = acoustic impedance PIEZOELECTRIC MATERIALS (rayls)  Crystalline materials composed of dipolar molecules SPEED SUBSTANCE Z  Quartz – naturally occurring (m/s) crystals Air 0.0004 330 Fat 1.38 1450  Lead zirconate titanate – man Water 1.48 1480 made ceramic Blood 1.61 1570  Natural Materials: Kidney 1.62 1560 o Quartz Soft Tissue 1.63 1540 o Tourmaline Liver 1.65 1550 o Rochelle Salt Muscle 1.70 1580  Synthetic Materials: Bone 7.80 3500 o Lead zirconate titanate PZT (crystal) 30 3870 (PZT) ACOUSTIC IMPEDANCE AND o Barium titanate REFLECTION o Lead metaniobate  Substances with same o Ammonium dihydrogen acoustic impedance: phosphate o 100% energy o Lithium sulphate transmission o No reflection ACOUSTIC IMPEDANCE   Substances with a small 2.) Sector/Curvilinear Array difference in acoustic Transducer impedance:  Provides wide field of view o 95% energy transmission  Most useful in abdominal and o 5% reflection obstetric scanning  Substances with a large  Best suited to image deep lying difference in acoustic structures impedance:  3.5 MHz o 1% energy transmission o 99% reflection 3.) Convex Transducer  Wide fan-shaped TRANSDUCER/PROBE  Useful for all parts of the body  A device which converts one  Except for specialized form of energy to another echocardiography  Converts electrical energy into ultrasound waves and vice 4.) Phased Array Transducer versa  Flat faced transducer  Contains piezoelectric crystals  Wide field of view o Transmit ultrasound  useful in cardiac and cranial beam ultrasound o Receive reflected echoes COMPONENTS AND TRANSDUCERS/SCANNING CONSTUCTION OF A TYPICAL PROBES TRANSDUCER  The most expensive part of any ultrasound unit 1.) PHYSICAL HOUSING  Contains all individual 1.) Linear Array Transducer components  Parallel scan lines  Provides the necessary  Rectangular field of view structural support  Vascular, small parts and  Acts as an electrical and musculoskeletal applications acoustic insulator  Above 4 MHz 2.) ELECTRICAL 4.) BACKING/DAMPING CONNECTIONS MATERIALS  Formed in front and back of the  Shortens the ultrasound pulse crystal length  Made of thin film of gold or  Eliminates the vibrations from silver the back face 3.) PIEZOELECTRIC  Controls the length of ELEMENTS vibrations from the front face  Crystalline minerals that  Improves axial resolution generate voltages when  Materials: subjected to a mechanical force o Plastic or epoxy resin  Piezein – “to press or squeeze” o Cork  Piezoelectric Effect – o Rubber discovered by Jacques and o Araldite loaded with Pierre Curie tungsten powder  Thinner Piezoelectric Materials o Higher resonant 5.) ACOUSTIC LENS frequencies  Reduce the beam width of the transducer FREQUENCY  Improve image resolution  Affects the quality the  Width of the Beam: ultrasound image determines lateral resolution  Higher Frequency  Lateral Resolution: the ability o Shorter wavelength to resolve structure across or o Better Resolution perpendicular to the beam axis o Lower Penetration  Materials: o Higher Absorption o Aluminum  Lower Frequency o Perspex o Longer wavelength o Polystyrene o Poor Resolution o Higher Penetration 6.) IMPEDANCE MATCHING o Lower Absorption LAYER  o Sandwich between the piezoelectric crystal and the 2.) General Purpose Ultrasound patient  Sector or convex transducer o Chosen to improved  3.5 MHz transmission into the body  Focused at 7-9 cm BANDWIDTH 3.) Pediatric Ultrasound o The range of frequencies  5.0 MHz transducer: for contained within an ultrasound children pulse  Focused at 5-7 cm o Wide Bandwidth:  Sector transducer of 7 MHz: o Shorter spatial pulse o Neonatal brain scans length o For adult testis and neck o Wider range of  Focused at 4-5 cm frequency o Narrow Bandwidth: ULTRASOUND BEAM o Longer spatial pulse  Area through which the sound length energy emitted from the o Narrower range of ultrasound transducer frequency  Three dimensional and symmetrical around its central CHOOSING THE APPOPRIATE axis TRANSDUCER TWO REGIONS OF 1.) Obstetric Ultrasound ULTRASOUND BEAM  Linear or convex transducer 1.) Near Field/Fresnel zone  3.5 MHz: better in later 2.) Far Field/Fraunhofer zone pregnancy  Increasing Frequency  5.0 MHz: best during early o Longer near field pregnancy o Less far field divergence  Focused at 7-9 cm  Narrow Crystal Diameter o Narrower near field  o More far field divergence  The ability of an imaging  Thin Crystal system to differentiate between o Decreased near field structures o Increased far field  Spatial Resolution: resolution  Thick Crystal in space o Increased near field  Contrast Resolution: o Decreased far field resolution of gray shades  Temporal Resolution: BEAM INTENSITY resolution in time  The power (measured in watts) SPATIAL RESOLUTION flowing through a unit area  Detail Resolution  The ability to display two SIDE LOBES/GRATING LOBES structures situated close  Lobes at various angles to the together as separate images main beam  Higher Frequency:  Approximately 15% of the o Better resolution energy in the beam o Lower penetrability  Cause a degradation of lateral o Higher absorption resolution  Lower Frequency: o Poor resolution BEAM WIDTH o Higher penetrability  The dimension of the beam in o Lower absorption the scan plane  Affects the spatial resolution TWO COMPONENTS OF  Narrow Beam Width SPATIAL RESOLUTION o Better spatial resolution 1.) AXIAL RESOLUTION SLICE THICKNESS  Longitudinal, Linear, Depth or  Three dimensional volume Range displayed as a two dimensional  The ability to distinguish two image objects parallel to the ultrasound beam RESOLUTION  Depends upon the spatial pulse length and wavelength   Short Spatial Pulse Length: good axial resolution ULTRASOUND INTERACTIONS  Longer Spatial Pulse Length: AND ATTENUATIONS poor axial resolution ATTENUATION 2.) LATERAL RESOLUTION  Decrease in the intensity and  Azimuthal, Transverse, amplitude of the ultrasound Angular or Horizontal waves as they pass through  The ability to distinguish two tissues objects perpendicular to the  Unit: decibels per centimeter ultrasound beam  Depends upon the beam FIVE MAIN PROCESSES THAT diameter CAUSE ATTENUATIONS  Smaller Beam Width: better lateral resolution 1.) ABSORPTION  Larger Beam Width: poor  Occurs when ultrasound energy lateral resolution is lost to tissues by its conversion to heat CONTRAST RESOLUTION  Main factor causing attenuation  The ability of the imaging  Higher Frequency: system to differentiate between o Greater amount of body tissue and display them as absorption different shades of gray  Bone: higher absorption  Optimized by using the correct coefficient overall gain  Increasing protein content gives increasing absorption TEMPORAL RESOLUTION o Blood –> Fat –> Nerve –  Frame Rate > Muscle –> Skin –>  The ability of the imaging Tendon system to display events which –> Cartilage –> Bone occurs at different times as  Best Absorption: tendon, separated images ligament, fascia, joint capsule  Higher Frame Rate: better & scar tissue temporal resolution 2.) REFLECTION  Occurs when two large ULTRASOUND ARTIFACTS structure of significantly  A structure in an image which different acoustic impedance does not directly correlate with form an interface actual tissue being scanned  Occurs when a sound wave 1.) REVERBERATION strikes an object that is larger  Comet tail than the wavelength  The production of spurious or false echoes due to repeated 3.) SCATTERING reflections between two  Occurs when an ultrasound interfaces with a high acoustic wave strikes a boundary or impedance mismatch interface between two small  The presence of two or more structures strong reflecting surfaces  Occurs when a sound wave  Often occur at: strikes an object that is equal to o Skin-transducer interface or smaller than the wavelength o Gas surface and transducer 4.) REFRACTION  Prevention/Elimination:  Occurs when the beam o Increase the amount of encounters an interface between gel used two different tissues at an o Used a stand-off gel pad oblique angle o Reduce the gain  The beam will be deviated as it o Move the position of the travels through the tissue transducer  Occurs due to difference in wave velocity across an 2.) ACOUSTIC SHADOWING interface between two materials  Caused by highly attenuating structure 5.) DIVERGENCE  Often occur at:  Occurs when the beam travels o Soft tissue and gas through tissue and it will o Soft tissue and bone or diverge due to diffraction calculus effects o Calcified mass 3.) ACOUSTIC ENHANCEMENT 7.) SIDE LOBE ARTIFACT  Caused by weakly attenuating  Echoes generated by side lobes structures assumed by the transducer to have arisen form the central  Often occur at: axis of the main lobe o Distal to fluid-filled  Appearance can give rise to a urinary bladder, false diagnosis gallbladder or cyst  Inherent characteristic of the o Fluid-filled mass transducer 4.) EDGE SHADOWING 8.) MIRROR IMAGE ARTIFACT  Combination of refraction and  Caused by specular reflection reflection occurring at the of the beam at a large smooth edges of rounded structures interface  Often seen in: 5.) BEAM WIDTH ARTIFACT o Fluid-air interface  Variations of all echoes o Diaphragm returning to the transducer  Prevention/Elimination: 9.) DOUBLE IMAGE ARTIFACT o Correct positioning of  Caused by refraction of the the focal zone beam  Often occur at: 6.) SLICE THICKNESS o Rectus abdominis muscle ARTIFACT  Prevention/Elimination:  Occurs due to the thickness of o Move the transducer the beam slightly to one side to  Dependent upon beam avoid the junction of angulation rectus abdominis muscle  Often seen in: o Transverse view of the 10.) EQUIPMENT-GENERATED urinary bladder ARTIFACT  Inherent characteristic of the  Caused by incorrect use of the transducer equipment control “Passing the Board Exam is your main purpose to learn” 05/13/14