Basic physics of ultrasound imaging

John E. Aldrich, PhD, FCCPM

T o accurately interpret ultra-sound images, a basic under-standing of the physical prin-ciples involved in ultrasound

image generation is essential. Althoughoften considered a simple bedside tech-nology, these principles can be somewhatcomplicated. A comparison with thosepertaining to radiographic imaging illus-trates this. Most clinicians are familiarwith radiographic imaging. In this tech-nique, the x-rays travel in straight linesbetween the x-ray tube and the detector.The intensity of the x-ray beam at thedetector is largely dependent on the tis-sue through which the beam passes. Al-though radiation is scattered as it passesthrough the body, this scatter adds only ageneral increased background to the im-age. Ultrasound behaves very differently.Many of the objects and artifacts seen inultrasound images are due to the physicalproperties of ultrasonic beams, such asreflection, refraction, and attenuation.Indeed, physical artifacts are an impor-tant element in clinical diagnosis. Thus,although these principles may be some-what complicated to the initial user, ap-preciating them may greatly benefit thepatient in terms of increased accuracy ofinterpretation and diagnosis (1, 2).

ULTRASOUND QUANTITIESAND UNITS

Unlike x-rays, sound waves constitutea mechanical longitudinal wave, whichcan be described in terms of particle dis-placement or pressure changes. Some ofthe more important quantities that are de-scribed in ultrasound imaging consist of:frequency, propagation speed, pulsed ultra-sound, interaction of ultrasound with tis-sue, angle of incidence, and attenuation.

Frequency. The frequency of an ultra-sound wave consists of the number of cy-cles or pressure changes that occur in 1sec. The units are cycles per second orhertz. Frequency is determined by thesound source only and not by the mediumin which the sound is traveling. Ultrasoundis actually sound with a frequency in excessof 20 kHz, which is the upper limit of hu-man hearing. Typical ultrasound frequen-cies used for clinical purposes are in the 2MHz to 10 MHz range.

Propagation Speed. Propagation speed isthe speed at which sound can travel througha medium and is typically considered 1540m/sec for soft tissue. The speed is determinedsolely by the medium characteristics, espe-cially those of density and stiffness (Fig. 1).

Pulsed Ultrasound. Pulsed ultrasounddescribes a means of emitting ultrasoundwaves from a source. To achieve thedepth of resolution required for clinicaluses, pulsed beams are used. Typically, thepulses are a millisecond or so long andseveral thousand are emitted per second.

Ultrasound Interaction with Tissue. Asa beam of ultrasound travels through amaterial, various things happen to it. Areflection of the beam is called an echo, acritical concept in all diagnostic imaging.The production and detection of echoesform the basis of the technique that is usedin all diagnostic instruments. A reflectionoccurs at the boundary between two mate-

rials provided that a certain property of thematerials is different. This property isknown as the acoustic impedance and is theproduct of the density and propagationspeed (Fig. 2). If two materials have thesame acoustic impedance, their boundarywill not produce an echo. If the differencein acoustic impedance is small, a weak echowill be produced, and most of the ultra-sound will carry on through the secondmedium. If the difference in acoustic im-pedance is large although, a strong echowill be produced. If the difference in acous-tic impedance is very large, all the ultra-sound will be totally reflected. Typically insoft tissues, the amplitude of an echo pro-duced at a boundary is only a small per-centage of the incident amplitudes, as listedin Table 1. Areas containing bone or air canproduce such large echoes that not enoughultrasound remains to image beyond thetissue interface. This is where the tech-nique of the operator becomes important,as the operator must avoid such areas andutilize specific windows that avoid them toimage deeper structures. Strong reflectionsor echos show on the ultrasound image aswhite and weaker reflections as gray. Fig-ure 3 illustrates these properties of strongand weak ultrasound beam reflectors.

Angle of Incidence. The percentage re-flections given in Table 1 are applicablefor a normal angle of incidence. If a beamof ultrasound strikes a boundary ob-liquely, however, then the interactionsare more complex than for normal inci-dence. The echo will return from theboundary at an angle equal to the angle ofincidence, as shown in Figure 4. Thetransmitted beam will be deviated from astraight line by an amount that dependson the difference in the velocity of ultra-sound at either side of the boundary. Thisprocess is known as refraction, and theamount of deviation is given by the rela-

From the Department of Radiology, VancouverHospital and the University of British Columbia, Van-couver, British Columbia, Canada.

The author has not disclosed any potential con-flicts of interest.

For information regarding this article, E-mail:John.aldrich@vch.ca.

Copyright © 2007 by the Society of Critical CareMedicine and Lippincott Williams & Wilkins

DOI: 10.1097/01.CCM.0000260624.99430.22

The appearance of ultrasound images depends critically onthe physical interactions of sound with the tissues in the body.The basic principles of ultrasound imaging and the physical

reasons for many common artifacts are described. (Crit CareMed 2007; 35[Suppl.]:S131–S137)

KEY WORDS: ultrasound; basic principles; physics; artifacts

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tionship known as Snells law, which re-lates the angle of refraction to the speedof sound in that tissue.

At a tissue–air interface, �99.9% ofthe beam is reflected, so none is availablefor further imaging. Transducers, there-fore, must be directly coupled to the pa-tient’s skin without an air gap. Couplingis accomplished by use of gel or oil be-tween the transducer and the patient.

As discussed previously, reflection ofultrasound beams forms the basis for alldiagnostic instrumentation. Refraction,however, only serves to confuse the issuebecause the machine always assumes thatthe ultrasound travels in a straight line.It will not know if any pulse has beendeviated from its path. However, the pre-vious arguments refer to a somewhat ide-alized situation in which the boundariesare smooth and the reflections are calledspecular. In practice, we have rough sur-faces that produce echoes in many direc-

tions, and this actually helps to produceimages of irregularly shaped objects.Rough surfaces have features that are thesame sort of size as the wavelength of theultrasound and produce echoes in manydirections. In practice, this enables soundto get back to the transducer fromoblique surfaces.

Attenuation. Although the challengeof overcoming the problem of weakly re-flected pulses is significant in itself, theintensity of the ultrasound beam is fur-ther reduced by attenuation due to vari-ous processes such as reflection, refrac-tion, scattering, and absorption. All theseprocesses divert energy from the mainbeam. Reflection and refraction occur atsurfaces that are large compared with thewavelength of the ultrasound. For objectsthat are small in comparison with thewavelength, energy is scattered in manydirections, and the eventual fate of theultrasound is to be absorbed as particlevibration and the production of heat. Theamount of attenuation varies with thefrequency of ultrasound. A high-fre-quency beam will be attenuated morethan a lower frequency. This means thatif the examiner wants to penetrate andsubsequently image deep into the body,he or she will, in general, have to use alower-frequency transducer. Unfortu-nately, because higher frequencies enablefiner detail to be resolved, there must

therefore be a compromise between res-olution and penetration for different im-aging applications.

In most situations involving diagnos-tic ultrasound, the absolute intensity ofthe beam is not important. What is im-portant, however, is the relative intensi-ties of the initial pulse and its subsequentreturned echo. Because the change inintensity is so large, relative intensitychange is measured in decibels, wherethe intensity in decibels � 10 log(trans-mitted intensity/incident intensity) (Ta-ble 2). The variation of the attenuation ofultrasound varies with frequency, and itis approximately true that attenuation(decibels) per centimeter is the same asthe ultrasound frequency (megahertz).Therefore a 3-MHz beam will be attenu-ated 3 dB in the first centimeter (de-creased to 50%), another 3 dB in the nextcentimeter, and so on. The depth of pen-etration is defined as the depth of tissuerequired to reduce the intensity by 50%and is given by the simple relationship3/frequency (megahertz). For example, a6-MHz beam will be reduced in intensityby 75% in 1 cm of tissue. After passingthrough 20 cm of tissue and being re-flected back to the transducer, the inten-sity of the ultrasound beam will be re-duced by a factor of more than a million.

TRANSDUCERS

The beam of ultrasound comes from asmall device called a transducer. Ultra-sound transducers convert electrical tomechanical energy and work on the pi-ezoelectric principle. Piezoelectric mate-rials have the property that if they aremechanically deformed, they produce anelectric field that tries to restore theshape. Conversely, if an electric field isapplied to a piezoelectric material, in theform of a voltage pulse (typically a fewhundred volts), then the material willphysically deform.

CONSTRUCTION OF ANULTRASOUND PROBE

The transducer element of a simpleprobe is usually a disk-shaped crystal sev-eral millimeters in diameter and having athickness ranging from 1.8 mm for a1-MHz transducer to 0.18 mm for a 10-MHz transducer. The thickness deter-mines the frequency, which is discussedbelow. After the transducer has been ma-chined into the correct shape and size, athin metallic film is placed on the two flat

Figure 1. Speed of ultrasound in different materials.

Figure 2. If two materials have the same acoustic impedance, their boundary will not produce an echo.If the difference in acoustic impedance is small, a weak echo will be produced and most of theultrasound will carry straight on. If the difference in acoustic impedance is large, a strong echo willbe produced. If the difference in acoustic impedance is very large, all the ultrasound will be totallyreflected.

Table 1. Percentage reflection of ultrasound atboundaries

Boundary % Reflected

Fat/muscle 1.08Fat/kidney 0.6Soft tissue/water 0.2Bone/fat 49Soft tissue/air 99

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surfaces so that electrical contact can bemade (Fig. 5). Hence, the crystal can bemade to vibrate or will produce electricalsignals in response to picking up a vibra-tion. Behind the transducer is what iscalled backing or damping material. Thisabsorbs the backward-directed energyand also stops the vibrating transducer sothat only a short, sharp pulse of ultra-sound (typically 2–3 cycles) is produced

in response to the electrical pulse. Ashort pulse is a requirement for a goodpulse echo device. On the face of thetransducer is a plastic film, which helpsprotect the active element. It also hasanother function, which is to assist trans-missions of sound into the patient. Asdescribed above, ultrasound will be re-flected nearly completely at a tissue-airinterface. The ultrasound beam from thetransducer would therefore be reflectedwithout this plastic film and the couplinggel, which is used in all ultrasound ex-aminations.

When the electrical pulse is applied tothe transducer element, it is rather likestriking a bell with a hammer. Just as thebell produces a sound that is determinedby its size, the transducer will resonate(i.e., vibrate at one particular frequencythat is determined by its size). The criti-cal factor here is the width of the trans-ducer. After the pulse is applied, the two

opposing faces will move and send outpressure waves.

PULSE ECHO SYSTEMS

With the rapid improvements in com-puter technology, ultrasound instrumen-tation is undergoing considerable changeat the present time. However, the basicprinciples of image production remainthe same as when the early models wereintroduced. Figure 6 illustrates the im-portant sections of a pulse echo instru-ment. The basic pulse echo device detectsthree things: 1) echo strength, 2) echodirection, and 3) time of arrival of echoesfrom tissue boundaries.

At each interface between tissues ofdifferent acoustic density, there may bereflections that will be received at a cer-tain time after the pulse leaves the trans-mitter, as shown in Figure 7. Differentialamplification can be applied to take ac-count of the weaker reflections that come

0

-20

-40

-120

-80

-60

-100

Intensitydecibels

Soft tissue - air

Soft tissue - bone

Brain – blood

PlacentaLiverBrain

Skin-waterFat - muscle

BloodElectronic noise

Largeboundaryinterface

Smallboundaryinterface

Figure 3. Range of reflected intensities.

Figure 4. Reflection and refraction of ultrasound beams. When ultrasound is incident obliquely on aboundary of two materials of different acoustic impedance, the beam will be reflected and transmittedas shown. i, angle of incidence; r, angle of reflection; t, angle of refraction; c1, speeed of sound inmedium 1; c2, speed of sound in medium 2.

Table 2. Relative sound intensities in decibels

Intensity Change Relative Value

�30 dB 1000�20 dB 100�10 dB 10�3 dB 20 dB 1�3 dB 0.5�10 dB 0.1�20 dB 0.01�30 dB 0.001

Figure 5. When the electrical pulse is applied tothe transducer element, it is rather like striking abell with a hammer: just as the bell produces asound that is determined by its size, the trans-ducer will resonate (i.e., vibrate at one particularfrequency, which is determined by its size). Thecritical factor here is the width of the transducer.After the pulse is applied, the two opposing faceswill move and send out pressure waves.

Figure 6. Block diagram of a ultrasound system.The transmitter produces the voltage spikes thatdrive the transducer, (typically 20–600 V at 1kHz). It also informs the display electronics ofthe time of each pulse. The receiver takes thereturning echoes and performs various functionson them to compensate for the change in inten-sity and to reduce noise.

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from boundaries deep in the body, thusallowing a trained examiner to adjust theimage to focus on structures of particularclinical interest.

ULTRASOUND BEAMS

Transducer arrays are probe assem-blies with more than one transducer ele-ment. The arrangement can be a line, amatrix, or an annular array, dependingon application.

Arrays

The linear array is operated by apply-ing voltage pulses to elements in turn,producing the effect of a simple scanningmotion, as shown in Figure 8. The lineararray typically consists of �256 elementsmeasuring 1 mm � 10 mm each. Typi-cally, the image is rectangular. Focusingof the beam can be achieved by a concavelens in one direction or by electronic“phasing” in the other.

Phased Array

By using delays in the pulsing of ele-ments, the beam from an array can besteered (i.e., made to sweep as in a scan-ning motion) (Figs. 9 and 10). This typeof arrangement is called a phased arrayor steered array, and it produces a sectortype of image in real-time machines. Allelements of the array are pulsed in thismode of operation as opposed to the lin-ear array, in which only small groups arepulsed together at any particular time.

Figure 7. Ultrasound (US) reflections and gaincompensation. At the top of the figure, the ultra-sound pulse is seen passing through the body. Ateach interface, there may be reflections, whichwill be received at a certain time after the pulseleaves the transmitter, as shown in the seconddiagram. Differential amplification can be ap-plied to take account for the weaker reflectionsthat come from boundaries deep in the body.

Figure 8. Linear phased array.

Figure 9. Electronic steering and focusing of ultrasound beam.

Figure 10. Beam focusing in a phased array.

Figure 11. Example of a reverberation artifact.

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Real-Time Ultrasound

At present, the majority of ultrasoundmachines are what is known as real-timedevices. The term real-time simply refersto the ability of the machine to display arapid sequence of B-scan images suchthat any motion is visualized as it actuallyoccurs. Typically between 15 and 60 im-age frames per second can be shown inmodern equipment. It is worth notingthat as the number of frames shown persecond (the framing rate) is increased,the smoother is the appearance of anymotion. To produce the effect of contin-uous movement, �16 frames/sec need tobe displayed.

Doppler Ultrasound

So far, this review has dealt with re-flections from stationary boundaries. Ifthe boundary is moving with respect tothe source of sound, the frequency of theecho will be changed. Readers will befamiliar with this in their daily lives. Forexample, when an ambulance is heardapproaching, the pitch of the siren seemsto increase. In pulse echo ultrasound, thesource of ultrasound (the transducer) isstationary, and the reflectors such as tis-sue interfaces or blood in the body maybe moving. If the boundary is movingtoward the transducer, the frequencypicked up will be higher than the origi-nal; if the boundary is moving away fromthe transducer, the frequency picked upwill be lower than the original. In a su-perficial artery, the frequency change isthe order of a few kilohertz; for the wallof the heart, the frequency change is afew hundred hertz.

Instrumentation. Modern pulsed-waveDoppler systems produce a regular B-scan type of image so that structures canbe clearly identified. Thereafter, a regioncan be selected for interrogation where itis desired to measure movement. Theechoes from this region are separatedfrom all the rest by an “electronic gate”that only accepts signals from a specifieddepth. These signals are again passedthrough a demodulator to detect any fre-quency shift. The Doppler informationcan be presented in two ways. In colorDoppler, the direction of blood flow ormovement is mainly shown, with flowaway from the probe colored red and flowtoward the probe colored blue. PowerDoppler, on the other hand, shows mag-nitude of flow. Power Doppler is oftenmore sensitive to flow; blue shows thegreatest flow and red the least flow.

Figure 12. Example of a ring-down artifact.

Figure 13. Example of a mirror-image artifact.

Figure 14. Example of a reflection artifact.

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Artifacts

Artifacts are errors in images. They arenormally caused by physical processesthat affect the ultrasound beam and thatin some way alter the basic assumptionsthe operator makes about the beam. Tounderstand them, one needs to considerthe basic assumptions made in producingan ultrasound image:

● Sound waves travel in straight lines.

● Reflections occur from structuresalong the central axis of the beam.

● Intensity of reflection correspondsto the reflector scattering strength

● Sound travels at exactly 1540 m/sec.

● Sound travels directly to the reflec-tor and back.

These assumptions do not always holdtrue. There are numerous cases of excep-tions to these assumptions. Althoughsome of these artifacts may actually pro-vide useful information or allow for novelinterpretations, the majority are poten-tial pitfalls that may confuse the exam-iner if not considered. The major artifacts

encountered in critical care ultrasoundare outlined below.

Reverberation. Reverberation artifactsappear as multiple equally spaced linesalong a ray line. Reverberation is causedby the sound bouncing back and forthbetween tissue boundaries and then re-turning to the receiver (Fig. 11).

Ring Down. Ring-down artifacts areproduced when small crystals such ascholesterol or air bubbles resonate at theultrasound frequency and emit sound.Because the sound is emitted after theinitial reflection is received by the trans-ducer, the system thinks the emittedsound is coming from structures deeperin the body. Air bubbles in the abdomenare shown in Fig. 12.

Mirror Images. Sound can bounce off astrong, smooth reflector such as the dia-phragm. The surface acts as mirror andreflects the pulse to another tissue inter-face. The ultrasound system believes thesecond interface is beyond the first surface,and this is where it appears on the scan. InFigure 13, the arrow shows the real object,which appears as if reflected in a mirror.

Reflections. Reflection is somewhatsimilar to the mirror image describedabove but has a very different appearanceand is caused by multiple reflections.Sound can bounce off a strong, smoothreflector, such as the posterior bladderwall, and be reflected back to the trans-ducer, giving the appearance of the struc-ture deep to the bladder wall as would beseen with fluid collection (Fig. 14).

Enhancement. Enhancement is seen asan abnormally high brightness. This occurswhen sound travels through a mediumwith an attenuation rate lower than sur-rounding tissue. Reflectors at depthsgreater than the weak attenuation are ab-normally bright in comparison with neigh-boring tissues. Enhancement of tissues(Fig. 15, arrow) deeper than cysts or ductsis common. The attenuation of the soundthrough the fluid in these tissues is lessthan that of the surrounding tissues andresults in this abnormal brightness. Thetissues deeper than the gallbladder showabnormal brightness in Figure 15.

Attenuation. Tissue deeper thanstrongly attenuating objects, such as calci-fication, appear darker because the inten-sity of the transmitted beam is lower. In thescan of the gallbladder in Figure 16, the leftside shows enhancement as describedabove; the right side shows decreased beamintensity because of attenuation in calcifiedgallstones (arrow).

Figure 15. Example of an enhancement artifact.

Figure 16. In this scan of the gallbladder, the left side shows enhancement described above; the rightside shows a decreased beam intensity because of attenuation in calcified gallstones (arrow).

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CONCLUSIONS

Unlike other imaging technologiesused in critical care medicine, such ascomputed tomography, magnetic reso-nance, or even radiography, the acquisi-tion of ultrasound images is a very user-dependent process. Further, the inter-pretation of ultrasound images involves

the recognition of many circumstanceswherein information may be hidden ordistorted by the physical interaction ofultrasound waves with tissue. For thesereasons, it behooves ultrasound examin-ers to familiarize themselves with the ba-sic physical principles of ultrasound, es-pecially those concerning ultrasoundartifacts.

REFERENCES

1. Categorical Course in Diagnostic RadiologyPhysics: CT and US Cross-Sectional Imaging.Oak Brook, IL, Radiological Society of NorthAmerica, 2000

2. Edelman SK: Understanding Ultrasound Phys-ics. Houston, TX, D. Armstrong and Company,1996

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