Understanding Ultrasound Physics Fundamentals and Exam Review PDF

wna: if aad ' 4aU est) bei ~> WiL> Understanding Ultrasound Physics  Digitized by the Internet Archive in 2021 with funding from Kahle/Austin Foundation https://archive.org/details/understandingultO000edel ...

wna: if aad ' 4aU est) bei ~> WiL> Understanding Ultrasound Physics  Digitized by the Internet Archive in 2021 with funding from Kahle/Austin Foundation https://archive.org/details/understandingultO000edel  Understanding Ultrasound Physics Fundamentals and Exam Review Sidney K. Edelman, Ph.D. Research Scientist Texas Heart Institute Houston, Texas Clinical Instructor Section of Cardiology Department of Medicine Baylor College of Medicine Houston, Texas E.S.P. Publishers Houston E.S.P. Inc. 9903 Cedarhurst Houston, Texas 77096 U.S.A. (713) 728-1280 First Edition, 1990 Second Printing, 1991 Library of Congress Catalog Card Number: 90-081503 ISBN 0-9626444-0-4 Copyright © 1990 by Sidney K. Edelman. All rights reserved. This book is protected by copyright. No part of it may be reproduced in any manner or by any means without permission in writing from the publisher. Printed in the United States of America by: D. Armstrong Co., Inc. Houston, Texas  For my wife, Diane, and my daughters, Jennifer Ruth and Lauren Alexis  PREFACE In many ways, diagnostic ultrasonography is a reflection of society. In the past twenty years, dramatic and revolutionary technologies have been applied to solve many problems and to increase the utility of our tools. To use these techniques in a responsible manner requires skillful and talented caretakers; sonographers, physicians, and other ultrasound professionals. Diagnostic ultrasound has evolved from the relatively simple noninvasive era of m-mode and compound scanning into the current sophisticated era of "more- invasive" endo-cavitary and catheter-based techniques. Of course, computer technologies have significantly impacted our imaging systems. Although our profession is expanding and rapidly changing, the following axiom has held true throughout the history of diagnostic ultrasound: Those individuals who understand the fundamental physical principles and instrumentation of their trade are better equipped to deal with clinical and technical challenges that arise. This book is designed to help teach those fundamental principles of ultrasound physics and instrumentation. It derives from lecture notes and manuscripts that I have used to teach ultrasound physics during the past eight years. The book has two major divisions. The first is a review of the fundamentals while the second is a selection of multiple choice questions with detailed answers. This material must be reviewed carefully and cannot be scanned rapidly. Although we have stressed brevity, each statement is important and should be thoroughly understood. The review questions serve to reinforce and restate these concepts. We hope that presenting the information in a variety of formats will clarify some of the difficult to understand (and difficult to teach) concepts. In addition, the text was designed to assist sonographers in preparation for certification. The fundamentals and exam review format should help guide most sonographers along the path of professional registration. You will note that there are few numerical examples in the text. Solving numerical problems is not a required skill. Rather, devote your energies to understanding concepts and relationships. Try to apply the same skills to the study of ultrasound physics that you use each day as a clinical sonographer. I hope that your chosen profession will be more enjoyable and fulfilling with this knowledge. SKE, Houston, Texas  — | ae ( i nseaes oN ~ —n tases li ou } ° an, Poste ages oe a! bow lenge 204] vi er 7) ~ 39 bep-ak t wag § a ‘ h !. mee! & =m C= of ine a bel ; ? \ebtid an rae = ; ue ari ie Joe8 oy — +f 2 eis Zé tiie? 66) ote) Geeter> ict Soe “7 As 4 ’ a he af: Gh Pae bu PS ew tae ar & ie } @ 7 _ tiny =e ~2-oieee * ae | Pa. aw one @ : 7 bis pe. ey Te i get 9 1 On oer 2S Ur ys cued tae Ge er ry (eamtiaiesd at) inert @ iy) rik. wu | AMP? 15/0 @@ Hiv bh ante = ‘ ric sul) equewine lee wae ii deny, » = _ ea as - @ a o@ a =) uv aaa >. S667) Gres (rid ae ja 1w~0 Frist!) iw wewaeriies! Ao av ag! a i 62S Goes, tee Ss Fas oj to ee By Both Sai(1 TICS eer 6 Hie aT = AS toga Ge Eas ogy peer @ 2A A Gini op" wi tribes ¢ i bee a Pe Wriu4 sd Coro IV Chins @oew Ut ua phe CRU) ay! Pavesi) Gee vam gad (\ seeeie= am (ad baw Gal pri) om +> Sen 0 Sheiq ae oem aliens ne? fT oe itie Cae? Wi ai » © >) €) Cora 44 ig ayy igre “pre ey : ire é “> 2u ) twrear reise z 0 ai a gm 7 ey wi whem beam focusing Electronic slope -> beam steering If we imagine the time delays to represent the surface of a reflecting mirror, the direction and the shape of the wave become apparent, I-76 Understanding Ultrasound Physics piezoelectric crystals sound beam no slope no steering no curve no focusing electronic pulses upwards slope = upwards steering no curve = no focusing downwards slope = downwards steering no curve = no focusing Fundamentals 1-77 piezoelectric crystals sound beam no slope = no steering curved pattern = focused beam electronic pulses upwards slope = upwards steering curved pattern = focused beam downwards slope = downwards steering curved pattern = focused beam I-78 Understanding Ultrasound Physics _, ANNULAR PHASED ARRAYS Concentric rings (donut shaped) cut from the same circular slab of piezoelectric material. Small diameter rings have a shallow focus but diverge rapidly. Large diameter rings have a deep focal length. Strategy - selected focal zones, use inner crystals for shallow regions and outer crystals for deep regions. Phasing provides focusing in all planes at all depths; a core sample. Steering is performed mechanically and creates a sector-shaped image. Annular phased array beam profile ringed a) : elements \Y = Identical dynamic extended focus in vertical and horizontal planes for tight focus and thin tomographic slice (Credit: Advanced Technology Laboratories) WATER PATH SCANNERS Scanners have a water bath, acoustic standoff or offset that is built into the system. The bath is placed between the sweeping or steering mechanism of the transducer and the patient. Large transducer face is presented onto the patient. Permits superficial structures with significant anatomic information to be imaged more clearly. Fundamentals I-79 A mechanical scanner produced this image of a neonatal brain. Note the sector-shaped image. Steering is achieved by moving the active element mechanically while focusing is obtained by using a lens or by using an internal curved crystal. (Credit: Corometrics Medical Systems) Of-Mar-89. $8. 84.45 SPA‘3.S$730 eat gaan a ees ee ed ec ee a S i : An image of the liver and kidney is produced with a phased array system. Note that the sector-shaped image is similar to the mechanical scanner. Both beam steering and focusing are achieved using electronic means by using tiny time delays in the pulses delivered to the array elements. (Credit: Diasonics, Inc.) I- 80 Understanding Ultrasound Physics “tala A scan of the fetal abdomen displaying bilateral fetal kidneys with multiple cysts. This scan was produced with an annular phased array transducer. Note that the sector shape of the image is identical to the mechanical and phased array systems. The beam is steered using a mechanical method. However, the beam is focused with an electronic, phased array approach. (Credit: Advanced Technology Laboratories) FOU 100 A scan of an adult gallbladder with small stones. The image was produced by a curved array transducer. Note that a sector image is obtained but this sector differs from the previous three examples. The region near the transducer is much wider than the other examples because of the large size of the transducer. The sector shape is produced without steering due to the curvature of the active elements in the array. Focusing is achieved using conventional methods (lens or curved crystals). (Credit: Acoustic Imaging) Fundamentals I-81 Se a A transverse scan of a fetal abdomen obtained with a linear switched or sequential array transducer. Note the rectangular image shape. The width of the image is determined by the width of the crystal array. There is no beam steering and focusing is achieved using conventional means (lens or curved crystal). (Credit: Acoustic Imaging) SUMMARY Transducer Image Steering Focusing Type Shape Technique Technique mechanical sector mechanical conventional phased array sector electronic electronic annular sector mechanical electronic curved linear blunted sector none conventional linear switched rectangular none conventional I - 82 Understanding Ultrasound Physics TIMING and REAL-TIME IMAGING In one second, sound travels 1.54 km in soft tissue. this is: 0.77 km round trip. 77,000 cm round trip. TIME: Imaging with a single pulse to a specific depth requires time. Creating a single frame with a large number of pulses requires time. Presenting many frames in rapid sequence requires time. Therefore, imaging depth, lines per frame and frame rate duel for time. A compromise must be met to balance these factors. Imaging depth (cm) = round trip depth sound travels per pulse. Lines/Frame: number of trips per single image. number of pulses per image. Frame/Second: number of images per second, frame rate. typically 10 -60 per second. Pulse Repetition Frequency (PRF): number of pulses per second. PRF = lines/frame x frame rate = pulses per picture x pictures per second Imaging depth x frame rate x lines per frame < 77,000 Fundamentals REVIEW - DISPLAYS AND IMAGING 1 The upward deflection of a dot on a screen is characteristic of mode ultrasound displays. Jes The only "mode" display that relates to time as well as position is mode. 3. Which of the following are the good features of B-scanning? A) easy to perform B) insensitive to motion c) good image quality D) can make large images 4. Tor F. Mechanical scanning produces pictures that are similar to phased array images. Ds T or F. There are many active elements firing at almost the same time in a mechanical scanner. 6. T or F. There are many active elements firing at almost the same time in a phased array scanner. th The firing pattern that steers a beam up or down relates to 8. The firing pattern that focuses an ultrasound beam relates to 9. T or F. There are large time delays in the firing pattern of a phased array transducer. 10. TorF. A machine that displays both A-mode and two-dimensional imaging is called a duplex scanner. Le T or F. The critical factor in determining frame rate, line density and imaging depth is the frequency of the ultrasound. 12. TorF. The number of lines per frame and the frame rate determine the frequency. 13. TorF. The number of lines per frame and the frame rate determine the pulse repetition frequency. 14. TorF. Ifthe imaging depth of a scan in 15 cm and there are 100 lines in the image, the number of pulses making up the scan is 1500. 15. TorF. Ifthe imaging depth of a scan in 15 cm and there are 100 lines in the image, the number of pulses making up the scan is 100. I - 84 Understanding Ultrasound Physics ANSWERS - DISPLAYS AND IMAGING 1. The upward deflection of a dot on a screen is characteristic of A-MODE ultrasound displays. Ps, The only "mode" display that relates to time as well as position is M- MODE OR MOTION mode. I, Which of the following are the good features of B-scanning? C) good image quality D) can make large images 4. True. Mechanical scanning produces pictures that are similar to phased array images. 2: False. There are many active elements firing at almost the same time in a phased array scanner. 6. True. There are many active elements firing at almost the same time in a phased array scanner. tk The firing pattern that steers a beam up or down relates to ELECTRONIC SLOPE. 8. The firing pattern that focuses an ultrasound beam relates to ELECTRONIC CURVATURE. 2) False. There are SMALL time delays in the firing pattern of a phased array transducer. 10. False. A machine that displays both DOPPLER and two-dimensional imaging is called a duplex scanner. 11. False. The critical factor in determining frame rate, line density and imaging depth is TIME. 12. False. The number of lines per frame and the frame rate determine the PULSE REPETITION FREQUENCY. 13. | True. The number of lines per frame and the frame rate determine the pulse repetition frequency. 14. False. If the imaging depth of a scan in 15 cm and there are 100 lines in the image, the number of pulses making up the scan is 100. 15. True. If the imaging depth of a scan in 15 cm and there are 100 lines in the image, the number of pulses making up the scan is 100. Fundamentals I-85 INSTRUMENTATION Information- __ time of flight. strength. direction (for compound scanning). frequency (for Doppler). Ultrasound System - the entire device that produces US beams, retrieves the echoes and produces visual and audio images. 6 components - connected together so that information can be transferred to and from each individual part. Master Synchronizer - the electronics that coordinate all of the functions of the other components of the ultrasound system. Transducer - turns electrical into acoustic energy during transmission and turns returning acoustic into electrical energy during reception. Pulser (transmitter) - the electronics that control the electrical signals sent to the transducer for pulse generation. Determines the PRF, pulse amplitude, and pulse repetition period. Determines the firing pattern for phased array systems. Receiver and Image Processor - the electronics associated with taking the electronic signal produced by the transducer from the returning echoes and producing a picture on an appropriate display. Display - the device associated with the presentation of processed data. It can be a CRT (television), a transparency, a spectral plot or a number of other formats or devices. Storage - any number of devices and "media" that are used to permanently store the US studies or data. Includes: video tape, paper, film, transparent film and computer discs. I - 86 Understanding Ultrasound Physics se a mel (\, Synchronizer , a ‘\ ina se fe[Reser Information flow between the components of an ultrasound system. PULSER Receives timing signal from synchronizer. Produces electrical voltage that excites piezoelectric crystal. Electrical voltage 10 - 500 volts. Greater electrical voltages, greater ultrasonic intensity. Pulser signals depend upon system and transducer. PULSER MODES A. Continuous wave: Constant electrical signal in the form of a sine wave. electrical frequency = ultrasound frequency B. Pulsed wave, single crystal: Short duration electrical "spike". One spike per ultrasound pulse. C. Pulse wave, arrays: Short duration electrical "spike". Many spikes per ultrasound pulse, one for each crystal in the array. TRANSDUCER OUTPUT Depends upon the signal from the pulser. Also called: output gain, acoustic power, pulser power, energy output, transmitter output. Depends upon excitation voltage from pulser. Piezoelectric crystal vibrates with a magnitude related to pulser voltage. Controlled by sonographer. Fundamentals I-87 RECEIVER FUNCTIONS Amplification - making the signals returning from the transducer larger so that they may be processed by the rest of the electronics. Units: dB. Input: micro - or millivolts. Typically, power increases 50 to 100 decibels. Also called: receiver gain. Can be controlled by operator. imeem Compensation - since attenuation is strongly related to the path length, the echo returning from a great depth has a lower amplitude than one returning from a shallow depth. Compensation makes all echoes reflected from similar interfaces appear at the same magnitude regardless of the depth from which they return. Objective: Display identical reflectors similarly. Obstacle: Equal reflectors produce different signals because of different path lengths. Longer paths - more attenuation. Solution: Vary amplification to adjust for attenuation. Controlled by the sonographer. Synonyms: Time gain compensation (TGC). Depth compensation (DGC). Swept gain compensation. = a I - 88 Understanding Ultrasound Physics TGC Curve Near gain: Minimum amplification level. Delay: Echoes from shallow depth do not require extra amplification. Regulates at what depth TGC begins. Amount of amplification called near gain. Slope: Additional amplification is granted here. Knee: At this depth and greater, maximum and constant amplification of signals. Far gain: Maximum amplification level. Amplification far gain knee near gain Depth Reject - there are very low level signals that are associated only with noise. Rejection eliminates all signals that are not of a minimum value SO as to eliminate this noise. Synonyms: threshold, suppression. Controlled by the sonographer. haha AM Fundamentals I-89 Compression - the process of reducing the total range from the smallest to the largest signal. Done without altering the relationships between voltages. Dynamic range - the measure of the signal magnitude handled by the various components of an ultrasound system. Reflected echoes have a large dynamic range, much larger than the components of an ultrasound system. Goal: Make electronics happy. Solution: Reduce the dynamic range. Requirement: Maintain relationships: largest is still largest smallest is still smallest. ma nh Demodulation - changing the shape of the output of the piezoeleciric crystal into something easier to process by the rest of the ultrasound system electronics. Has two components, rectification and smoothing. Rectification - turning all of the negative voltages into positive ones. = Smoothing - putting an envelope around the "bumps" to even out the rough edges. I-90 Understanding Ultrasound Physics Order of 1. Amplification operations: 2. Compensation 3. Compression Reject and Demodulation then follow with no specific order. NOTE: These functions must be performed in the appropriate order for the system to function properly. Controlled by 1. Amplification Sonographer: 2. Compensation 3. Reject NOTE: The functions of a receiver are performed by all ultrasound systems however, they are named differently by the various manufacturers. Understand these functions and relate them to the system that you use. Try to learn these functions without reference to a particular manufacturer or system type. Fundamentals EA 1-91 eeee ee CRT (cathode ray tube) - television screens; an electron beam strikes phosphor coating on screen and it glows. Defecting Coil Focus Coil Fluorescent Electron Gun Screen Anode A television display is a result of a collection of 525 closely spaced horizontal lines. StartofFleld1 Start Of Field2 First, lines 1,3,5,7,...,525 are written by the election beam. This is called the odd-field. Then the remaining lines 2,4,6,8,....524 are written. This is called the even-field. The process of odd and even field image presentation is called an interlaced display. On normal televisions, each field requires 1/60th of a second and a complete image requires 1/30th of a second. End Of Field2 pee Bistable - either on or off, white or black. Gray scale display - can have a number of different levels of brightness (white, dark gray, light gray, black). The assignment of a different shade of gray for each echo amplitude. Also, different colors can represent different signal strengths. Controls for display Brightness - controls the brilliance of the signals displayed. Contrast - controls the range of brilliance from the weakest to the strongest that are displayed. I-92 Understanding Ultrasound Physics SCAN CONVERTERS Scan converter - made gray scaling displays possible by storing the image and then displaying it on a CRT. The image can be manipulated or altered in between the "storing" and the "displaying" of the data. Ex: A black on white image can be turned into a white on black. Pre processing - manipulating the digital data BEFORE it is stored in the scan converter but after it is in digital format. Post processing - manipulating the data AFTER is has been stored in the scan converter memory but prior to display. Increases versatility of display process, but whatever data isn’t stored is gone forever. The determination of whether processing is “pre” or "post" is made by whether data has been stored in the scan converter. ANALOG & DIGITAL ANALOG: a variable that can attain a continuum of values. weight of an individual length of a piece of string DIGITAL: a variable than can attain only discrete values. number of children in a family number of words in an article ANALOG SCAN CONVERTER Made gray scale imaging possible originally. Divides picture into a matrix (up to 1000 x 1000 picture element s) with electrical storage elements (silicon wafer) at each location. Electrons from the CRT gun strike these elements and the "charge" is stored. Stored charges are then read to retrieve information. Fundamentals DISADVANTAGES OF ANALOG SCAN CONVERTERS Image Fade charges on silicon wafer dissipate. Image Flicker _ constant switching between read and write modes. Drift inconsistent pictures from day to day. Deterioration _ tubes age and image degrade. DIGITAL SCAN CONVERTERS Uses a computer and computer memory to digitize images. This converts the image into numbers and stores the numbers in memory. The numbers can be processed or manipulated and then re-translated and displayed on the ultrasound machine as an image. PIXEL - the smallest element of a digital picture. If we divide the picture into a grid (like a checkerboard), each individual box is a pixel. Each pixel can have only one color or shade of gray. The more pixels per inch (called: pixel density), the better the picture looks. The image on the right has a greater pixel density than the image on the left. BIT (binary digit) - the smallest amount of digital storage. A bit is bistable; it has a value of either zero or one. A group of "bits" is assigned to each pixel to store the grey scale color assigned to that pixel. The more bits per pixel, the more gray shades can be assigned. The more bits per pixel, the more shades of gray that can be represented. The group of bits represents a binary number. DIGITAL TO ANALOG CONVERSION.- in order for a digital image to appear on a CRT, the signal must be re-translated into analog form. This is achieved with the use of a digital to analog converter which acts after the image processing but before image display. I-94 Understanding Ultrasound Physics HOW MANY GRAY SHADES CAN A COLLECTION OF BITS REPRESENT 1. Find out how many bits are assigned to each pixel. 2. Multiply the number 2 by itself the same number of times as there are bits. That’s the answer. Example: What is the number of shades that can be represented by 4 bits? We have 4 bits, so take the number 2 and multiply it by itself 4 times! Zeke ceXe eX —2 16 The largest number of shades represented by four bits is 16. Example: What is the number of shades represented by one bit? the answer is 2. (off or on, black or white) Example: What is the largest number of shades represented by 7 bits? We multiply the number 2 by itself 7 times. 2, XPAEXEOU IOr=al 28 EXE LEX DeXXE Seven bits can represent 128 shades of gray. Fundamentals I-95 Paper media: charts from pen writers Magnetic media: computer discs computer memory magnetic tape video tape Chemically derived photographs media: transparent film Matrix camera film Optical media: laser discs compact discs  I-96 Understanding Ultrasound Physics | ARTIFACTS ARTIFACT - an error in imaging. TYPES OF ARTIFACTS - images that are: not real, not present, incorrect size or shape, improper location or brightness. CAUSES OF ARTIFACTS Violation of assumptions that are incorporated into the design of ultrasound systems. Violation of assumptions of viewer. Equipment malfunction. Poor ultrasound system design. Improper use of ultrasound systems. Physics of ultrasound. Operator error. Viewer error. BASIC ASSUMPTIONS OF ACOUSTIC IMAGING SYSTEMS Sound beams travels in a straight line. Reflections are produced by structures that are located along the main axis of the sound beam. Intensity of a reflection corresponds to a reflector’s scattering strength. Sound travels exactly 1,540 meters/second. The imaging plane is extremely thin. Sound travels directly to a reflector and back. TYPES OF ARTIFACTS AXIAL AND LATERAL RESOLUTION Causes: ultrasound interacting with tissues. physics of ultrasound. Presentation: closely spaced objects appear as one. very small reflectors appear larger on image. Fundamentals I - 97 AXIAL RESOLUTION Structures are along main axis of ultrasound beam. Units: mm (any unit of distance). Smaller values produce better picture. Synonyms: longitudinal, range, radial, depth. Approximately equal to half of the length of the ultrasound pulse. Typical value: 0.3 to 1.0 mm. LATERAL RESOLUTION Structures are perpendicular to long axis of sound beam. Units: mm (any units of distance). Smaller values produce better pictures. Synonyms: azimuthal, transverse, angular. Approximately equal to beam diameter, varies with depth. Typical value: 2-10 mm (improved by focusing). ACOUSTIC SPECKLE Appears as tissue texture close to the transducer but does not correspond to scatterers in tissue. Produced by ultrasound wavelet interference. Results in general image degradation. Tissue texture identified by hollow black arrows may be caused by beam interference rather than by reflectors in the tissue. This may represent acoustic speckle. (Credit: Denis Gratton, Health Sciences Center, Winnipeg, Canada) I-98 Understanding Ultrasound Physics SLICE OR SECTION THICKNESS Ultrasound beam has measurable thickness. We assume that the imaging plane is razor thin. Therefore, the reflections produced by structures above or below the beam’s main axis appear in the image. In addition, hollow structures such as cysts may also be filled in. REFRACTION Waves can change direction (refract) when traveling from one media to another. We assume that sound travels in a straight line. Therefore, images on the screen are placed in improper locations. The anatomic structure at position A is artifactually placed at B on the image. REVERBERATION Appears as multiple, equally spaced reflections on the image. When two or more strong reflectors lie in the path of a pulse, the sound "ping-pongs" back and forth between the reflectors. We assume that sound travels directly to the reflector and back to the transducer, It is easy to identify reverberations since they are equally spaced on the display. a The horizontal linear echoes identified by the white arrows are reverberations from a Structure within the cardiac chamber. The true location of | the structure is marked by the solid white arrow. (Credit: Leonard Pechacek, Houston, TX) Fundamentals I-99 COMET TAIL A form of reverberation produced when two or more strong reflectors are close together and have a high propagation speed. In this case, sound does not travel directly to a reflector and back to the transducer and a strong linear echo appears at the reflector and extends deeper than it. MIRROR IMAGE Sound can bounce off of a strong reflector in its path, such as the diaphragm. The structure can act as a mirror and reflects the pulse towards another reflector. Ultrasound systems believe that sound travels directly to the second reflector and back to the transducer. Therefore, a second image of the reflector can be incorrectly placed on the scan. A longitudinal scan of the liver and kidney displays a mirror image artifact. The artifact identified by the white arrows was produced by sound waves reflecting off of the strong diagonal structure and directed to the kidney/liver interface. (Credit: Denis Gratton, Health Sciences Center, Winnipeg, Canada) MULTIPATH Appears when the path lengths that the pulse travels to and from a reflector are of different lengths. The pulse glances off of a second structure on the way to or from the primary reflector. Multipath is not explicitly seen on a scan but rather, it generally degrades image quality and diminishes longitudinal resolution. I- 100 Understanding Ultrasound Physics SIDE LOBES The acoustic pulse produced by a transducer does not have ideal geometry. Significant acoustic energy may be emitted by the transducer ina direction different than the main axis of the sound beam. This results in reflectors appearing in improper, off-axis locations on the image. In addition, reflectors can appear in multiple locations on the image. The horizontal echo identified by the white arrow and labeled | "SL" is a side lobe artifact. It } does not represent anatomy but rather is produced by the strong horizontal reflector that appears to its left. (Credit: Leonard Pechacek, Houston, TX) GRATING LOBES Linear array transducers produce off-axis acoustic waves as a result of the regular spacing of active elements. The transdcuer may emit acoustic energy in a direction other than the beam’s main axis. Reflectors may appear in improper, off-axis locations on theiimage. This artifact is not commonly seen because of a corrective process called sub-dicing. Transverse scan of the liver produced with an "old technology" linear array scanner produced a grating lobe artifact identified by the white arrows. (Credit: Denis Gratton, Health Sciences Center, | Winnipeg, Canada) Fundamentals I- 101 Side Lobes Grating Lobes SHADOWING by attenuation: Abnormally weak or absent reflections on an image when ultrasound pulses travel through structures with abnormally high attenuation (such as bone). The acoustic energy and reflecting echoes distal to the structure are greatly diminished. However, we interpret scans thinking that the intensity of a reflection is directly related to the scattering strength of the object that is imaged. by refraction: Reflections from a defocused beam have reduced intensity and are processed as weak or absent on image. This occurs when ultrasound beams change direction and diverge (defocusing). Therefore, reflectors are absent on image and areas distal to the refractor are abnormally anechoic. meu Tw RANSVERSE PLAC FUNDUS Shadowing behind the mandible (solid black arrows) and the fist (solid white arrows) | of the fetus results from ' attenuation of the sound beam at the bone/tissue boundary. (Credit: ACUSON) I- 102 Understanding Ultrasound Physics | ENHANCEMENT by attenuation: Enhancement occurs when a structure appears to have an abnormally high brightness. This occurs when sound travels through a medium with an attenuation rate lower than surrounding tissue. Reflectors at depths greater than the weak attenuator are abnormally bright in comparison to neighboring tissues. by focusing: Reflections from a highly focused pulse may have abnormally Strong intensities and can be displayed with abnormal brightness. This occurs when a beam is focused and the concentration of energy within the beam is increased. Therefore, certain images appear brighter or stronger than others as a result of reflector positioning near the focus. Enhancement of tissues (white | arrow) deeper than the breast cyst are displayed. The attenuation of the sound through the cyst is less than that of the surrounding tissues and results in this abnormal brightness. (Credit: ACUSON) Punktionsnadel in der Custe Transverse scan of the liver showing enhancement (black arrow) at depths greater than the grossly dilated common hepatic bile duct. (Credit: Denis Gratton, Health Sciences Center, Winnipeg, Canada) Fundamentals I- 103 PROPAGATION SPEED ERRORS Reflectors can be placed in improper positions on an image if the sound travels at a speed other than 1,540 m/s. This occurs when sound travels through media with speeds not equal to speed of soft tissue. Reflectors appear in improper locations on scans. In addition, measurements made on these images are inaccurate. When medium speed is greater than soft tissue: Sound travels faster than the ultrasound system expects. Returns from trip in body - fast, Fast, FAST!! Machine assumes reflectors are close to transducer. All images are too shallow. All distances are underestimated (reported too small). When the medium speed is less than soft tissue: Sound travels slower than the ultrasound system expects. Returns from trip in body - slow, Slow, SLOW!! Machine assumes reflectors are far from transducer. All images are placed too deep. All distances are overestimated (reported too large). Scan of the liver. Note that the diaphragm directly below the circular mass within the liver appears to have a discontinuity (white arrow). This propagation speed artifact, the apparent interruption of the diaphragm, may be a result from sound’s speed difference in the mass and the surrounding tissues. (Credit: Denis Gratton, Health Sciences Center, Winnipeg, Canada) I - 104 Understanding Ultrasound Physics QUALITY ASSURANCE The routine periodic evaluation of the US system to guarantee optimal image quality. Requires: a number of different tests to evaluate the system’s components. repairs. preventative maintenance. record keeping. Goals: proper equipment operation. detect gradual changes. minimize downtime. reduce number of repeat scans. Medical/Legal: a must for every laboratory (in theory). An ultrasound system is only as strong as its weakest component. NOTE: Various aspects of all instrumentation should be routinely inspected to ensure consistency of its performance. It is important to validate the reliability of the images produced and the measurements made on each ultrasound machine and each transducer. Fundamentals 1-105 AIUM 100 mm TEST OBJECT A commercially available object with an array of strategically located pins placed in it. By scanning the object, the accuracy and some of the performance characteristics of the US machine may be evaluated. It has a propagation speed similar to that of soft tissue: 1,540 m/s. On the other hand, it DOES NOT have attenuation properties of soft tissue. Ultrasound systems can be evaluated for depth accuracy, dead zone, linearity, axial and lateral resolution and position registration. This device is important in the evaluation of B-scanners. (Credit: Nuclear Associates) = Ring Down em aor | ° Side view of target groups showing function of each. a Lateral Vertical e Resolution Distance s Calibration, :. & Linearity g Axial Resolution eee SS Horizontal Distance peer | Calibration & Linearity ot I - 106 Understanding Ultrasound Physics TISSUE EQUIVALENT PHANTOM A commercially available object which contains a medium that mimics soft tissue. It has a conduction velocity of 1,540 m/s and an attenuation coefficient similar to soft tissue. Imbedded in the phantom are strategically located pins, structures that mimic cysts and structures that mimic solid masses. This is a useful device in the evaluation of tissue texture images. A tissue-mimicking device is appropriate for assessing image quality from current real-time scanners. (Credit: Radiation Measurements, Inc.) GAINS Scan of a tissue equivalent phantom used to assess quality assurance. (Credit: ACUSON) Fundamentals I - 107 DOPPLER PHANTOM A commercially available system which contains static simulated vessels as well as regular simulated vessels at a variety of angles to the imaging surface. A pumping system forces echogenic fluid through the vessels at know velocities, pulse rates and durations. In addition, a constriction is found in one of the vessels. With these features, the phantom can be used to assess the accuracy of pulsed, continuous wave and color systems. (Credit: Radiation Measurements, Inc.) I - 108 Understanding Ultrasound Physics BEAM PROFILE/SLICE THICKNESS PHANTOM A commercially available object which contains a diffuse scattering plane at an angle to the incident sound beam. The phantom’s medium mimics soft tissue. The slice thickness is a measure of the beam geometry in the z- plane, which is perpendicular to the imaging plane. This important measurement is not obtained with other, more traditional test objects. Increased slice thickness will diminish spatial resolution and reduce the ability to distinguish small, low contrast lesions. (Credit: ATS Laboratories) beam profile scan window scattering plane slice thickness scan window Fundamentals I- 109 PERFORMANCE MEASURES Minimum Sensitivity - With the TGC set flat, increasing the gain from the its minimum value to the point when an echo is displayed in the CRT determines the minimum sensitivity. The same rod in the AIUM 100 mm test object should always be imaged. In the tissue equivalent phantom, tissuelike texture should be observed at the same depth. Normal Sensitivity - The gain setting where all of the rods in the AIUM test object are displayed on the CRT. Normal sensitivity is, of course, found at a higher gain than the minimum sensitivity. All other measurements of performance are made at this gain setting. Dead Zone - Using the series of pins located near the top of the AIUM test object, we find the dead zone. It is the distance close to the transducer that cannot produce an accurate image on the display, and it results from the time that it takes for the piezoelectric crystal and the receiver electronics to switch from the transmit to the receive mode. In the tissue equivalent phantom, the depth at which the tissuelike texture finally appears. An acoustic standoff can be used to eliminate the dead zone from interfering with imaging important structures. Registration Accuracy - The machine’s ability to place echoes in proper positions while imaging from different orientations is called registration accuracy. Range Accuracy or Vertical Depth Calibration - Using pins that are positioned at ever increasing depths, we assess the machine’s ability to display echoes in the proper depth. This is called the range accuracy. If there is an error, it may be due to 1) machine malfunction or 2) the speed of sound in the phantom is not 1.54 km/sec. Horizontal Calibration - The machine’s ability to position echoes in their correct position along a distance perpendicular to the US beam. I- 110 Understanding Ultrasound Physics Focal Zone - The distance along the US beam where the intensity is the highest and the width of the beam is the narrowest. Another device called a beam profiler can also be used by looking for the maximum peak in the A-mode tracing. Longitudinal Resolution - Using the set of pins that are positioned at ever- increasing distances front-to-back, the smallest distance at which two pins are displayed as two separate echoes is recorded. This is the longitudinal resolution. Lateral Resolution - Using the set of pins that are positioned close together side-to-side, the minimum distance that two rods are displayed as two separate images can be determined. This is the lateral resolution at that depth. Compensation Operation or Uniformity - Using a group of pins at ever- increasing depths while scanning from the top of the test object, the echoes are displayed with TGC off and then with TGC on. With the TGC off, the echoes should be displayed with reduced amplitude as depth increases. With the TGC on, all of the echoes should have the Same amplitude, regardless of the depth from which they return. Mock Cysts and Tumors - Using the tissue equivalent phantom to check dimensions of cysts. Also, note texture and fill-in of both hollow and solid masses. Display and Gray Scale Dynamic Range - Using any group of pins, adjusting the gain of the system should produce changes in the gray scale or brightness of the echoes. Also, monitor machine at power-up to see if the initializing sequence is performed correctly. Look at the relationship between the screen of the machine and all hard copy. Fundamentals I-111 MEASURING THE OUTPUT OF ULTRASOUND MACHINES Hydrophone - a small probe with a piezoelectric crystal at its end (a small ultrasound transducer). It attaches to an oscilloscope and displays acoustic signals as received by the crystal. (Credit: Nuclear Associates) SS Acousto-optics - based on the interaction of two types of waves, sound and light. It allows quantitation of sound beam’s characteristics. A shadowing system, called a Schlieren, uses this principle to measure beam profiles. Radiation force - an incident sound wave can exert a small but measurable force on the object that it strikes. If the object is a balance or a float (acting as a tiny scale), we can measure the beam intensity. Calorimeter - a transducer which turns acoustic energy into heat. When the total heat gain is measured along with the time that it took to obtain the heat, the total power of the US beam can be calculated. Thermocouple - a small device imbedded in an absorbing material. The sound is absorbed, turned into heat, and the thermocouple measures the change in temperature. The US intensity at specific points in space are measured by a thermocouple. Crystals - Cholesteric liquid crystals or starch/iodine blue, when struck by different US intensities, turn different colors. The shape and colors of the crystals give us insight into the shape and strength of the beam. I- 112 Understanding Ultrasound Physics BIOLOGIC EFFECTS & SAFETY Dosimetry - the science and practice of identifying and measuring those characteristics of an US field which are especially relevant to its potential for producing biological effects. What are these characteristics? WE AREN’T SURE! Very high intensities of US cause damage to biologic tissues. There are no known cases of diagnostic imaging at standard intensities resulting in biological effects and tissue injury. Reference: Bioeffects Considerations for the Safety of Diagnostic Ultrasound. Journal of Ultrasound in Medicine. Volume 7, (supp), 1-38, Sept. 1988. Note: AIUM statements and conclusions found in this section were published in the above reference. TECHNIQUES TO STUDY BIOEFFECTS MECHANISTIC APPROACH: Propose a specific physical mechanism that could produce bioeffects. Theoretical analysis to estimate scope of bioeffects at various exposures. Identify "cause - effect" relationship. EMPIRICAL APPROACH: Acquire or review data from patients or animals. Correlate exposure and effects. Identify "exposure - response" relationship. TECHNIQUES - STRENGTHS (+) & WEAKNESSES (-) MECHANISTIC: _ broad variety of exposure conditions. (+) are assumptions valid? (-) are there other mechanisms? (-) Fundamentals Tes EMPIRICAL: No need to understand mechanism. (+ and -) Offers insight into biological significance. (+) Species differences. (-) Statistical problems. (-) Best Case: mechanistic & empirical studies with consistent results. MECHANISMS OF BIOEFFECTS THERMAL MECHANISM As US propagates in body, acoustic energy is converted into heat and tissue temperature elevation. Body temperature is regulated at 37° C core. Life processes do not proceed normally at other temperatures. However, it is common to experience a 2° increase in tissue temperature during fever, exercise or sunbathing. EMPIRICAL DATA Serious damage from prolonged elevation of the body temperature by 2.59 C or 4.59 F. A 2° to 4° degree rise in testicular temperature can cause infertility. A 1° C (2° F) rise in temperature appears safe. Maximal heating is related to beam intensity, SPTA. MECHANISTIC DATA Analytical model agrees with experimental data even though: Ultrasound beam structure is complex. Diagnostic equipment is diverse. Tissue characteristics are different. ne Eee CONCLUSIONS REGARDING A THERMAL BIOEFFECTS MECHANISM Approved by AIUM, October 1987 1. A thermal criterion is one reasonable approach to specifying potentially hazardous exposures for diagnostic ultrasound. I- 114 Understanding Ultrasound Physics 2. Based solely on a thermal criterion, a diagnostic exposure that produces a maximum temperature rise of 19 C above normal physiological levels may be used in clinical examinations without reservation. 3. An in situ temperature rise to or above 41° C is considered hazardous in fetal exposures; the longer this temperature elevation is maintained, the greater is the likelihood for damage to occur. 4. Analytical models of ultrasonically induced heating have been applied successfully to in vivo mammalian situations. In those clinical situations where local tissue temperatures are not measured, estimates of temperature elevations can be made by employing such analytical models. 5. Calculations of ultrasonically induced temperature elevation, based on a simplified tissue model and a simplified model of stationary beams, Suggest the following: For examinations in fetal soft tissues with typical perfusion rates, employing center frequencies between 2 and 10 MHz and beam widths less than 11 wavelengths, the computed temperature rise will not be significantly above 1° C if the in situ SATA intensity does not exceed 200 mW/cm2. If the beam width does not exceed eight wavelengths the corresponding intensity is 300 mW/cm2. However, if the same beam impinges on fetal bone, the local temperature rise may be higher. CAVITATION Interaction between sound waves and microscopic, stabilized gas bodies in the medium. This is how lithotripsy works. Lithotripsy is the shattering of kidney stones and other rigid structures in the body using ultrasonic energy. Uncertainties regarding _location, size, chemical composition, the gaseous nuclei: conditions under which they occur, materials in which they exist. STABLE CAVITATION Bubbles tend to oscillate when exposed to acoustic waves of small amplitude. Bubbles a few micrometers in diameter double in size. Bubbles intercept and absorb much of the acoustic energy. Shear stresses and microstreaming are produced in surrounding fluid. Fundamentals I- 115 TRANSIENT CAVITATION Bubbles expand and collapse violently. Depends upon the pressure of ultrasound pulses. (Pressure units: MPa, mega pascals). Threshold for transient cavitation is only 10% greater than the pressure for stable cavitation. Highly localized violent effects: Enormous pressures - shock waves - mechanical stress, potential for colossal temperatures. CONCLUSIONS REGARDING CAVITATION Approved by the AIUM, October 1987 1. Acoustic cavitation may occur with short pulses and has the potential for producing deleterious biological effects. 2. Currently available information nS that pulses with peak pressures greater than 10 MPa (3300 W/cm 2) can induce cavitation in mammals. 3. With the limited data available, it is not possible to specify threshold pressure amplitudes at which acoustic cavitation will occur in mammals, with diagnostically relevant pulse lengths and repetition rates. EPIDEMIOLOGY Branch of medicine dealing with the prevalence of disease. Empirical, performed using a clinical survey. Most bioeffects studies deal with in utero fetal exposures to ultrasound. IN UTERO EXPOSURE Why concerns exist: Used often during normal pregnancies; skews risk-benefit ratio Half of pregnancies in U.S. are scanned; major public health implications. Has potential to effect the fetus for decades. Variables: birthweight, structural anomalies, neurologic development of child, cancer, hearing. I- 116 Understanding Ultrasound Physics LIMITATIONS OF EPIDEMIOLOGIC STUDIES Often retrospective. Ambiguities: justification for exam, gestational age, number of exams, exposure, and mode. Other risk factors: maternal age, nutrition, smoking, alcohol, drugs. STATISTICAL CONSIDERATIONS Smaller the effect, the harder it is to detect. Requires large number of patients. STATISTICAL POWER: How many patients are required to detect a harmful effect in a Statistically valid manner? For example: If a 5% event rate occurs naturally and if ultrasound exposure increases this rate by 10%, then 5,200 patients are required to confirm this increase. CONCLUSIONS REGARDING EPIDEMIOLOGY Approved by the AIUM, October 1987 1. Widespread clinical use over 25 years has not established any adverse effect arising from exposure to diagnostic ultrasound. 2. Randomized clinical studies are the most rigorous method for assessing potential adverse effects of diagnostic ultrasound. Studies using this methodology show no evidence of an effect on birthweight in humans. 3. Other epidemiologic studies have shown no causal association of diagnostic ultrasound with any of the adverse fetal outcomes studied. Fundamentals I- 117 IN VIVO AND IN VITRO STUDIES IN VIVO BIOEFFECTS In vivo - observed in living tissues. New data: Higher intensities needed to produce bioeffects with focused beams. Smaller beam area means less thermal buildup and less interaction. with cavitation nuclei. Maximum Intensities (SPTA): 100 mW /cm - unfocused. 1 W/ cm? - focused. CONCLUSIONS REGARDING IN VIVO MAMMALIAN BIOEFFECTS Approved by the AIUM, October 1987 In the low megahertz frequency range there have been (as of this date) no independently confirmed significant biological effects in mammalian tissues exposed in vivo to unfocused ultrasound with intensities below 100 mW/cm*, or to focused ultrasound with intensities below 1 W/cm. Furthermore, for exposure times greater than 1 second and less than 500 seconds (for unfocused ultrasound) or 50 seconds (for focused ultrasound), such effects have not been demonstrated even at higher intensities, when the product of intensity and exposure time is less than 50 joules/cm*. IN VITRO STUDIES Observations made "in test-tubes" with a controlled environment. Careful measurements can be made under rigorous experimental techniques. May not pertain to in vivo bioeffects but still must be considered as real. I- 118 Understanding Ultrasound Physics AIUM STATEMENT ON IN VITRO BIOLOGICAL EFFECTS Approved March 1988 It is difficult to evaluate reports of ultrasonically induced in vitro biological effects with respect to their clinical significance. The predominant physical and biological interactions and mechanisms involved in an in vitro effect may not pertain to the in vivo situation. Nevertheless, an in vitro effect must be regarded as a real biological effect. Results from in vitro experiments suggest new endpoints and serve as basis for design of in vivo experiments. In vitro studies provide the capability to control experimental variables and thus offer a means to explore and evaluate specific mechanisms. Although they may have limited applicability to in vivo biological effects, such studies can disclose fundamental intercellular or intracellular interactions. While it is valid for authors to place their results in context and to suggest further relevant investigations, reports of in vitro studies which claim direct clinical significance should be viewed with caution. AIUM STATEMENT ON CLINICAL SAFETY Approved March 1988 Diagnostic ultrasound has been in use since the late 1950s. Given its known benefits and recognized efficacy for medical diagnosis, including use during human pregnancy, the American Institute of Ultrasound in Medicine herein addresses the clinical safety of such use: No confirmed biological effects on patients or instrument Operators caused by exposure at intensities typical of present diagnostic ultrasound instrume nts have ever been reported. Although the possibility exists that such biologic al effects may be identified in the future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the risks, if any, that may by present. arenes Fundamentals I- 119 AIUM STATEMENT ON SAFETY IN TRAINING AND RESEARCH Approved March 1988 Diagnostic ultrasound has been in use since the late 1950s. No confirmed adverse biological effects on patients resulting from this usage have ever been reported. Although no hazard has been identified that would preclude the prudent and conservative use of diagnostic ultrasound in education and research, experience form normal diagnostic practice may or may not be relevant to extended exposure times and altered exposure conditions. It is therefore considered appropriate to make the following recommendation: In those special situations in which examinations are to be carried out for purposes other than direct medical benefit to the individual being examined, the subject should be informed of the anticipated exposure conditions, and of how these compare with conditions for normal diagnostic practice. The AIUM suggests: do not perform studies without reason do not prolong studies without reason use minimum output power and maximum amplification to optimize image quality — ELECTRICAL AND MECHANICAL HAZARDS Several different instruments as well as the individual components of the ultrasound system may be linked to a patient at any given time. Precautions such as proper electrical grounding should always be taken to avoid electrical hazard. Instruments should be routinely checked for proper condition. However, ultrasound systems present no special electrical safety hazards. Mechanically, the machine should be inspected to assure proper physical status. Since the transducer is in direct patient contact, it may be considered the component most likely to pose a threat, albeit small, to a patient. In summary: be prudent, use your head, be careful, be judicious BE AN ULTRASOUND PROFESSIONAL I- 120 Understanding Ultrasound Physics DECIBELS AND INTENSITY RATIOS DECIBEL INTENSITY RATIO DECIBEL INTENSITY RATIO 1 1.26 -1 0.79 2 1.58 -2 0.63 ~ 4) 2.00 -3 0.50 5 3.16 -5 0.32 6 4.00 -6 0.25 10 10.0 -10 0.10 LS 31.6 “Ale 0.032 20 100. -20 0.01 25 316. -25 0.0032 30 1000. -30 0.001 40 10000. -40 0.0001 The intensity ratio is the final intensity divided by the original intensity. SINES AND COSINES OF CERTAIN ANGLES ANGLE SINE COSINE 0 0.00 1.00 5 087 996 10 173 985 15 258 966 20 342 940 25 422 934 30 500 366 35 573 819 40 642 766 45 707 707 50 166 642 55 '819 573 60 866 500 65 934 422 T 10 940 342 A 15 966 258 80 ‘985 173 85 996 087 90 1.00 0.00 II. Exam Review II - 122 Understanding Ultrasound Physics il What are the effects of an ultrasound wave on living tissues known as? a) toxic effects b) acoustic propagation properties c) biological effects d) transmission properties As sound travels through a medium, what are the effects of the medium on the wave called? a) toxic effects b) acoustic propagation properties c) bioeffects d) transmission properties Select the sequence that appears in increasing order. a) mega, kilo, hecto, milli, giga b) nano, milli, micro, deci, deca, mega c) centi, deci, deca, hecto d) milli, hecto, centi, deci, nano, giga The letters below represent the abbreviations for the prefixes of the metric system. Select the sequence that appears in decreasing order. a) m, k, M, g, da b) g,m,k,d,u c) g, k, di, m, u,n d) M, k, da, d, c Match the following prefixes with their meanings. a) mega 1) hundreds b) hecto 2) thousands c) milli 3) thousandths d) kilo 4) millions €) nano 5) billions f) giga 6) billionths Which of the following is not a measure of area? a) square cm b) meters squared c) cubic meters d) feetx feet The perimeter of an anatomical structure is measured by a sonographer. Which of the following choices is a reasonable value for the measurement? a) 6 cm bjpoce c) 15mm d) 18 dB What are the units associated with the circumference of a circle? a) mm b) mm? c) cm? d) m4 Exam Review = 123 1. C. The effects of ultrasound on the human body are called biological effects or bioeffects. There have been no confirmed bioeffects on humans with acoustic intensities typical of those used in diagnostic imaging. 2. B. Acoustic propagation properties are the effects of the medium on the wave propagating through it. Acoustic means sound. Propagation means to travel. Therefore, this term means the properties of traveling sound. 3. C. Centi means one-hundredth, deci means one-tenth, deca means ten and hecto means hundred. Thus, this sequence is increasing, starting from the smallest and ending with the largest. 4. D. M stands for mega and means millions. k stands for kilo and means thousands. da stands for deca and means tens. d stands for deci and means one-tenth. c stands for centi and means one-hundredth. Thus, this sequence starts with the largest value and continually gets smaller. 5. The correct matches are as follows: a& 4, b&1,c&3, d&2, e&6,andf & 5. You should be familiar with the abbreviations, prefixes and meanings for all of the terms associated with the metric system. 6. C. An area is measured in units of distance squared. For example, a rectangle’s area is its length multiplied by its width, and can be reported in square feet, in2, or square miles. Selection C is the only answer that is not in units of area. 7. C. The term perimeter means the length of the outer boundary of a structure. For example, a square with an edge that is 5 inches long will have a perimeter measuring 20 inches (four sides, each with a length of 5 inches). The only choice with units of length is choice C. Choice A is in units of area. Choice B has units of volume. Choice D has units of decibels and is not applicable to a measurement of length. 8. A. Similar to the question above, the circumference of a circle is the length of the circle’s outer boundary. We must select an answer with units of length, and the only choice with those units is selection A. Selection B has units of area and choice C has units of volume. Choice D, with units of length to the fourth power, has no meaning in geometry. II - 124 Understanding Ultrasound Physics 9 The volume of a cystic structure is estimated from sonographic data. Which of the following is an acceptable measurement of this volume? a) 6 b) 6cm c) 6 cm2 d) 6 cm3 10. The speed of red blood cells traveling through a blood vessel is 750 cm/sec. You are asked to measure this speed in miles per hour. What information would be sufficient for you to complete your task? a) how many seconds in a minute and how many blood cells in the vessel b) the number of miles in a meter c) how many seconds per hour and the number of miles in a centimeter d) the direction of red blood cell motion and the Doppler shift frequency if Sound can be characterized as: a) energy flowing through a vacuum b) a variable c) cyclical oscillations in certain variables __d) a principle of acoustics 12: Which of the following is true of all waves? a) they travel through a medium b) all carry energy from one site to another c) their amplitudes do not change d) they travel in a straight line 13. A longitudinal wave is propagating from the East toward the West at a speed of 2 miles per hour. What is the direction of motion of the particles within the wave? a) from the East to the West only b) alternately from East to West and then from West to East c) from North to South only d) alternately from South to North and then from North to South 14. A particle within a transverse wave is traveling vertically. What is the direction of the wave’s propagation? a) horizontal b) vertical c) diagonal; both horizontal and vertical d) cannot be determined Exam Review II - 125 a 9. D. The units of volume are length cubed such as ft3 or cubic centimeters. Another way to say this is length to the third power. The only option with these units is choice D. Choice A has no units while selection B has units of length and choice C has units of area. 10. C. To convert one unit to another requires a factor that relates the two terms. For example, to change a measurement of length from feet into inches, we must know how many inches there are in a foot. In this case, we are asked to convert cm per sec into miles per hour. Therefore, we require two pieces of information: the relationship between seconds and hours (relating one unit of time to another) and the relationship between miles and centimeters (relating one unit of distance to another). When units are changed, the actual amount does not change. For example, we have the same purchasing power with ten dimes as we do with four quarters. The units may be different, but the "total picture" remains the same. 11. C. Sound is a wave. A wave is the rhythmical variation throughout time, choice C. 12. B. Waves carry energy from one place to another. Choice A is incorrect because some waves, such as light, can travel through a vacuum. Choice C is incorrect because many waves get weaker as they travel. Certain waves do not travel in a straight line, thus making choice D incorrect. 13. B. A longitudinal wave is defined as a wave whose particles vibrate back and forth in the same direction that the wave is propagating. Therefore, since the wave is traveling from the East to the West, the particles in this wave will vibrate repeatedly from East to West and then from West to East. 14. A. The particles within a transverse wave travel in a direction that is perpendicular to the direction of propagation of the wave itself. Ifa transverse wave is traveling vertically, the particles in the wave are traveling horizontally. A water wave is a primarily a transverse wave. The wave propagates sideways along the surface of the water, while a ball floating on top of the water moves up and down as the wave passes. II - 126 Understanding Ultrasound Physics 15. Which of the following types of waves do not require a medium in order to propagate? (More than one answer may be correct) a) light b) heat c) sound d) television 16. Which of the following describes the characteristics of a sound wave? a) longitudinal, non-mechanical b) mechanical, transverse c) transverse, acoustic d) mechanical, longitudinal True or False. Which of the following 8 selections are acoustic variables? 17. frequency 18. pressure 19. propagation speed 20. wavelength 21. temperature 22. intensity 23. motion of particles in the wave 24. density 25. When the pressure of an acoustic wave is measured, it can be reported with which units? (More than one answer may be correct) a) atmospheres, (atm) b) Pascals, (Pa) c) millimeters of mercury,(mm Hg) d) pounds/square inch, (Ib / in’) 26. A force is applied to a surface. If the force is tripled and, at the same time, the surface area over which the force is applied is also tripled, what is the new pressure? a) three times larger than the original b) one third of the original c) six times more than the original d) unchanged Exam Review Wh 127 15. A, B, and D. Sound cannot travel through a vacuum; it requires a medium in order to propagate. Other waveforms such as light, heat and TV waves are capable of traveling through a vacuum. 16. D. Sound is both a mechanical wave and a longitudinal wave. A mechanical wave such as sound actually imparts energy to the molecules of the medium that it travels through. The molecules of the medium vibrate, striking their neighbors, which in turn vibrate. This chain reaction results in the acoustic energy traveling through the medium. 17. False. Acoustic variables are measured quantities whose values change as a sound wave propagates. These quantities are pressure, temperature, density and the motion of particles in the wave. 18. True. 19. False. 20. False. 21. True. 22. False. 23. True. 24. True. 25. A, B, C and D. All of these terms are appropriate to represent pressure (just as weight can be reported with units of pounds, ounces, tons, or grams). 26. D. Pressure is defined as an amount of force divided by the area it is applied to. If the applied force is tripled and, at the same time, the area over which it is applied is tripled, then the pressure remains unchanged. II - 128 Understanding Ultrasound Physics 27. A sound wave propagating in air can be considered to be a rhythmical compression and rarefaction of air molecules as depicted below. Where is the location of highest density? 3 SiSane 3 3 3 3 3 3 3 HH ; 3 3 3 a b c d @ Temperature is an acoustic variable. Are the next 3 statements true or false? 28. Temperature is a measure of the energy in a wave. 29. Heat flows from areas of low temperature to areas of high temperature. 30. The units of "degrees" are acceptable as an acoustic variable. 31. The sketch below is an example of a(n) wave. a) sound b) electro-mechanical c) transverse d) longitudinal Match the 4 acoustic variables with their correct units. (More than one answer may be correct) ame ELessire 1. Ib/in3 33. Temperature 2. Fahrenheit degrees 34. Density 3. miles 35. Particle motion 4. Pa 5. cm 6. degrees centigrade 7. Ib/in2 8. kg/m3 36. What is the number of cycles an acoustic variable completes in a second? a) period b) frequency c) pulse repetition period d) variable rate Exam Review II - 129 Die D. The highest density is located where a volume contains the largest number of molecules. The location where this occurs in the above figure is at letter D.. True. Temperature is a measure of the total energy of an object. 29; False. Heat flows from areas of higher temperatures to areas of lower temperature. That is why we feel warmth when sitting near a fire. The heat moves from the fire towards us.. True. Temperature is an acoustic variable. Since temperature is measured in degrees, these units are appropriate as an acoustic variable. aie C. The sketch in the question is a transverse wave. The particles in the wave move up and down, while the direction of propagation of the wave is sideways. 20s: 4 and 7. Units of pressure include Pascals (Pa) and pounds per square inch (Ib/in2). Dos 2 and 6. Units of temperature include both Fahrenheit and centigrade degrees.. Land 8. Density is the concentration of mass per unit of volume. Mass is measured in pounds (lbs) or kilograms (kg). Volume has units of length cubed, such as cubic inches (in>) or cubic meters (m3). Therefore, the correct answers are 1 and 8, Ib/in? and kg/m?. 3); 3 and 5. Particle motion is measured in units of distance, such as miles or cm. These are the only choices available that appear simply as distance. 36. B. This is the definition of the term frequency. Frequency can be thought of as the rate of recurrence or the number of regularly occurring events in one second. II - 130 Understanding Ultrasound Physics Sis Which of the following cannot be considered a unit of frequency? a) per day b) cycles/sec C\—rIZ d) Hertz e) cycles What is the range of frequencies emitted by transducers used in ultrasonic imaging? a) 1to3 Mhz b) 1 to 1,000 kHz c) -10,000 to + 10,000 Hz d) 2,000,000 to 10,000,000 Hz Oo) True or False. A sonographer can change the frequency of sound emitted by the ultrasonic crystal of the transducer typical of current diagnostic imaging instruments. What establishes the frequency of an ultrasound wave? a) the transducer b) the medium that the sound travels in c) both a and b d) neither choice a nor b 41. True or False. With standard ultrasound pulses, the frequency of the ultrasound normally changes as the wave propagates through the body. 42. Ultrasound is defined as a sound with a frequency of a) greater than 20,000 kHz b) less than 1 kHz c) greater than 10 MHz d) greater than 0.02 MHz 43. True or False. Waves in the ultrasound range behave in the same general manner as sound waves that are audible. Exam Review a II - 131 a 37. E. The term cycles informs us of how many events took place, but does not inform us of the duration of time that it took for those events to occur. Therefore, choice E is incomplete, and is not a unit frequency. All of the other choices alert us to the fact that there were a number of events that took place in a specific time span. 38. D. Frequencies commonly used in diagnostic imaging range from approximately 2 to 10 megahertz. Another way to describe this is 2 to 10 million cycles per sec. 39. False. Imaging transducers used most commonly with today’s ultrasound systems emit acoustic energy at one primary frequency. In order to alter the primary frequency used in an exam, the sonographer must select a different transducer. Some systems appear to allow the sonographer to change the frequency of the transducer, but this is not the case. These systems have multiple ultrasound crystals located in the transducer housing. When the sonographer changes the frequency, they are actually selecting a different crystal within the assembly. 40. A. When created by a transducer, an ultrasound pulse has a certain, specific frequency. The frequency of the pulse is not determined by the medium that the sound travels through. Only the sound source (the transducer) establishes the wave’s frequency. 41. False. In diagnostic imaging, the frequency of the sound wave remains constant and does not routinely change as the sound propagates through the body. 42. D. Ultrasound is defined as an acoustic wave that is not audible to humans. An inaudible wave has a frequency of at least 20,000 Hertz or 0.02 MHz. 43. True. As stated above, the primary difference between audible and ultrasonic waves is that humans can hear audible waves. A wave’s behavior or adherence to physical laws and principles is the same regardless of whether it can be heard by humans or not. II - 132 Understanding Ultrasound Physics ° 44. What is characteristic of acoustic waves with frequencies exceeding 20,000 Hz when compared with waves having frequencies of less than 20,000 Hz? a) they travel more effectively in soft tissue b) they travel more rapidly c) they attenuate less when traveling in soft tissue d) humans can’t hear them 45. Which of the following units are appropriate to describe the period of an acoustic wave? (More than one answer may be correct) a) minutes b) microseconds c) meters d) mm/us e) deciliters 46. is the reciprocal of period. a) inverse period b) pulse repetition period c) frequency d) propagation period 47. What is the range of periods commonly found in waves produced by ultrasound systems? a) 0.001 to 1 sec b) 0.1 to 1 usec c) 0.1 to 1 msec d) 10 to 100 nsec 48. Let us compare two acoustic waves, A and B. The frequency of wave A is one-third the frequency of wave B. How does the period of wave A compare with the period of wave B? a) A is one-third as long as wave B_ b) A is the same as wave B c) Ais three times as big as wave B d) cannot be determined 49. With standard ultrasonic imaging, what happens to the period of a wave as it propagates? a) increases b) decreases c) remains the same 50. True or False. If the periods of two waves are the same, then the frequencies of the waves must also be the same. 51. True or False. The sonographer has the ability to alter the period of an ultrasound wave that is produced by a transducer typically used in diagnostic imaging. Exam Review II - 133 a 44. D. Waves with frequencies exceeding 20 kHz are inaudible to humans. They travel at the same speed as waves with lower frequencies. They attenuate at a faster rate than waves at lower frequencies. However, they cannot be heard by humans. 45. Aand B. The period of a wave is defined as the time that elapses as a wave oscillates through a single cycle. The units of period must be a measure of time, such as minutes, seconds, etc. Choices A and B are units of time. The incorrect selections C, D and E are units of distance, speed and volume, respectively. 46. C. Frequency is the reciprocal of period. Mathematical reciprocals are related in the following manner: first, as one increases, the other decreases; and secondly, when the two numbers are multiplied together, the result is unity. For example, a wave with a period of one-hundredth of a second has a frequency of 100 per second. 47. B. Ultrasonic imaging waves have a period in the range of 0.1 to 1.0 usec.