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RICHARD L. MORIN, PHDTHE MEDICAL PHYSICS CONSULT

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tate-of-the-Art Ultrasound Imagingechnology

icholas J. Hangiandreou, PhD

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ltrasound imaging systems haveeen used in medicine for over 50ears [1]. Modern scanners havehe characteristics of being inex-ensive, portable, safe, and realime in nature, all of which makeltrasound one of the most widelysed medical imaging modalities.o a great degree, current ultra-

ound scanners are based on theame fundamental principles as therst devices used for human imag-

ng. Although often referred to as aature technology, ultrasound im-

ging continues to evolve in capa-ility, and there seems to be no rea-on to believe that this evolutionill slow in the near future. Theurpose of this article is to providen overview of the technology andeatures commonly found in cur-ent state-of-the-art ultrasound im-ging equipment.

One major point of differenceetween seminal ultrasound imag-ng equipment and modern scan-ers is the use of tissue harmonic

maging. Although it was intro-uced relatively recently, it has al-eady earned a role as an indispens-ble ultrasound imaging mode.he benefits of tissue harmonic im-

ging were first observed in workeared toward the imaging of ultra-ound contrast materials. The termarmonic refers to frequencies thatre integral multiples of the fre-uency of the transmitted pulsecalled the fundamental frequencyr first harmonic). The second har-onic has a frequency of twice the

undamental. Ultrasound wavesropagate through tissue in a non-

inear fashion. The wave velocity is

lightly greater for higher pressure d

004 American College of Radiology1-2182/04/$30.00 ● DOI 10.1016/j.jacr.2004.04.011

ave phases than for lower pressurehases [2]. The net result of thisonlinear propagation is a distor-ion of the ultrasound wave from aerfect sinusoid to a more peaked,awtooth shape. The distorted waveontains frequency componentsrom many higher order harmonicse.g. the second, third, fourth, etc).he intensity decreases as the orderf the harmonic increases, and theigher frequency harmonic compo-ents are attenuated to a greater de-ree, so most harmonic imaging isurrently performed using the sec-nd harmonic component. Stron-er harmonic components are gen-rated as a ultrasound wave travelshrough greater tissue path lengths,o little harmonic signal originatesn the patient’s body wall close tohe transducer. Harmonic imagesherefore demonstrate good rejec-ion of artifacts and clutter arisingrom multiple pulse reflections inhese near-surface tissues. Har-onic imaging is generally consid-

red to be most useful for “techni-ally difficult” patients with thicknd complicated body wall struc-ures. Because the strongest har-onic signals originate in regions

f ultrasound pulses having thereatest pressure amplitudes (i.e.,ear the beam axis), harmonic im-ges demonstrate superior lateralnd elevational spatial resolutionompared with fundamental im-ges.

Early static B-mode ultrasoundcanners allowed sonographers thebility to move and rock the trans-ucer in the image plane, resulting

n the collection of multiple echo

ata sets from the same regions in t

he patient but from ultrasoundeams oriented along different di-ections. Because the appearance ofpeckle varies according to theeam line direction, averaging ech-es acquired from different direc-ions tended to average out and fil-er the speckle pattern, making themages look “smoother.” This abil-ty was lost with the transition toutomatic scanning mechanicalector and array transducers, but acanner mode called spatial com-ound imaging restores this speckleeduction capability. Here, elec-ronic pulse steering is used to im-ge the same tissues multiple timessing sets of parallel ultrasoundeams oriented along different di-ections. The echoes from theseultiple acquisitions are averaged

ogether into a single compositemage. Because multiple sets of ul-rasound beams are used, moreime is required for data acquisitionnd the imaging frame rate is gen-rally reduced compared with con-entional B-mode imaging. Spatialompound images often show re-uced levels of speckle, noise, clut-er, and refractive shadows and im-roved lesion contrast and marginefinition [3]. Enhancement andhadowing artifacts may also be re-uced. Because these artifacts areften useful, the ability to quicklywitch between conventional-mode and compound mode is

mportant.Another benefit of early static B-ode scanners that was lost with

he introduction of mechanical andlectronic automatic scanning wasarge imaging fields-of-view. Ex-

ended field-of-view imaging with

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692 The Medical Physics Consult

odern scanners has sought to re-tore this capability when imagingsing array transducers [4]. Theransducer is slowly translated later-lly across the large anatomic re-ion of interest while multiple ul-rasound image data sets arecquired. The proper relative posi-ions of the multiple images are de-ermined in the scanner by compar-ng image features in the regions ofverlap between successive images.his process registers the imagesith respect to one another. Nohysical position sensors of anyype are required. The registeredmage data are accumulated in aarge image buffer and then com-ined to form the complete, largeeld-of-view image.A fundamental tradeoff in ultra-

ound imaging is that between im-ging depth and spatial resolution.hort, highly focused ultrasoundulses that optimize spatial resolu-ion are generally obtained withigher ultrasound frequencies, buthe attenuation of pulse and echontensity also increases with fre-uency. It is often the case that sub-ptimal spatial resolution must beccepted to image with a lownough frequency to produce de-ected echoes of adequate intensity.maging with coded ultrasoundulses is a potential approach forvercoming this limitation, provid-ng good penetration (associatedith low-frequency pulses) andood spatial resolution (associatedith high-frequency pulses) [5]. In

his imaging mode, long ultra-ound pulses are used (i.e., eachulse consists of many ultrasoundycles) instead of the very shortulses typical of conventional-mode imaging. These long pulsesarry greater ultrasonic energywithout exceeding peak intensityonstraints), increasing the energynd signal-to-noise ratio of echoes

hat return from large depths in the s

atient. These long pulses arecoded” or shaped with a very spe-ific, characteristic profile, and theesulting echoes will have a similarhape. Many different shapes areossible, including digitally codedulses and “chirp” pulses. A pro-essing step called “pulse compres-ion” is applied to the detected echoignals, in which the locations ofhe characteristic pulse shapes aredentified. These pulse shape loca-ions may be determined with aight spatial tolerance and corre-pond with the locations of echo-enic structures in the body. Theocations of reflectors may thus bedentified with good spatial resolu-ion. The longer pulses and pulseompression allow images withood signal-to-noise ratio and spa-ial resolution to be acquired atreater depths than are possible foronventional B-mode imaging.

One specific application ofoded pulse excitation is B-modeow imaging. Here, coded pulseechniques are used to obtain ech-es from blood with an improvedignal-to-noise ratio. (Echoes fromlood are normally extremely weak,ence the black appearance of vas-ulature in conventional B-modemages.) The blood echoes are se-ectively enhanced with respect tohe stronger echoes from surround-ng static tissues, resulting in a B-

ode image in which the echoesrom blood are directly visualizedn grayscale. When a dynamic se-uence of images is displayed, theotion of the blood can be dis-

erned.Conventional ultrasound array

ransducers are constructed of a sin-le row of very narrow piezoelectriclements. The “height” of the ele-ents is correlated with the ultra-

ound slice thickness, which is usu-lly focused by an acoustic lens.his is referred to as a one-dimen-

ional array because all of the ele- t

ents are in a single row. Recentlyome two-dimensional transducerrrays have become commerciallyvailable, constructed of a two-di-ensional matrix of piezoelectric

lements. Most often, the matrix issymmetric in the sense that therere significantly more elements inach row than there are rows. Theseew arrays begin to make electronicocusing of the image slice possible.lectronic slice focusing can lever-ge many of the same techniquessed currently for focusing of the

ateral ultrasound beam width, pro-iding multiple, user-selectable el-vational focal depths and muchore uniform slice profiles. Thisill significantly improve the abil-

ty to detect and accurately charac-erize very small structures.

Often the term 2-dimensional ar-ay is reserved for transducers hav-ng similar numbers of rows andolumns (or similar element di-ensions along each direction).

Asymmetric” higher dimensionalrrays may be referred to as 1.25-imensional, 1.5-dimensional, or.75-dimensional arrays, depend-ng on the number of rows and theevel of independence with whichhe individual rows may be excited6]. The greater the number ofows, the greater the capability forlectronic focusing. Full 2-dimen-ional arrays will be capable of rapidlectronic steering of the ultra-ound image plane through a 3-di-ensional volume, in addition to

lectronic slice focusing. This elec-ronic slice steering capability willaturally allow the real-time acqui-ition of 3-dimensional and 4-di-ensional ultrasound data sets.reehand approaches to 3-dimen-ional ultrasound imaging usingonventional 1-dimensional trans-ucer arrays are currently commer-ially available, but these typicallyequire slow, manual scanning of

he ultrasound image plane

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The Medical Physics Consult 693

hrough a 3-dimensional volume inhe patient, followed by the recon-truction and rendering of 3-di-ensional images in a postprocess-

ng step. Systems that mechanicallyweep a conventional 1-dimen-ional transducer array through a-dimensional volume are nowommercially available and do pro-uce real-time 4-dimensional ultra-ound data. However, these areedicated 4-dimensional systemsith limited conventional imaging

apabilities.Many features of modern ultra-

ound scanners are geared towardmproving the overall usability ofhe devices. Automatic image opti-ization allows sonographers to ac-

ivate a single control causing thecanner to vary a number of param-ters, such as overall gain and time-ain compensation, to improve theverall appearance of the acquiredmages. This same functionality islso available for optimizing Dopp-er spectra. In both cases, significantcan time may be saved, and theniformity of acquired data qualityhould be improved. Ergonomic as-ects of the equipment have under-one significant recent improve-ent, with fine adjustment of

osition and orientation of scanneronitors and keyboards becoming

ommonplace. Voice control ofcanners is also becoming available,

ith the intent to allow sonogra-hers to adjust imaging parametershile scanning, without awkward

eaching and twisting toward thecanner keyboard. Digital Imagingnd Communications in MedicineICOM connectivity features, in-

luding acquisition work list, store,nd storage commitment services,lso improve system usability byinimizing the need for the man-

al hand entry of data at the scan-er keyboard. Scanner size andortability are also improving.omplete scanners weighing less

han about 15 kg are available, asre other systems that provide ul-rasound scanning capability withransducer probes (approximately00 g each) and proprietary soft-are used in conjunction with a

aptop computer.Fundamental and practical de-

elopments such as those discussedbove are driving a rapid advance inhe state of the art of ultrasoundmaging technology. On the hori-on are even more exciting develop-ents in the emerging areas of im-

ging the mechanical properties ofissues [7], contrast-enhanced im-ging [8], molecular imaging [9],nd gene therapy [9,10].

EFERENCES

1. Goldberg B, Kimmelman B. Medical diag-nostic ultrasound: a retrospective on its 40th

anniversary. Washington (DC) andRochester (NY): American Institute of Ul-trasound in Medicine and Eastman Kodak;1988.

2. Fowlkes JB, Averkiou M. Contrast and tis-sue harmonic imaging. In: Goldman LW,Fowlkes JB, editors. Categorical course indiagnostic radiology physics: CT and ultra-sound cross-sectional imaging. Oak Brook(IL): Radiological Society of North Amer-ica; 2000. p. 77-96.

3. Jespersen SK, Wilhjelm JE, Sillesen H:Multi-angle compound imaging. UltrasonImaging 1998;20:81-102.

4. Weng L, Tirumalai AP, Lowery CM, et al.ultrasound extended-field-of-view imagingtechnology. Radiology 1997;203:877-80.

5. Pedersen MH, Misaridis TX, Jensen JA.Clinical evaluation of chirp-coded excita-tion in medical ultrasound. UltrasoundMed Biol 2003;29:895-905.

6. Wildes DG, Chiao RY, Daft CMW, RigbyKW, Smith LS, Thomenius KE. Elevationperformance of 1.25D and 1.5D transducerarrays. IEEE Trans Ultrason Ferroelect FreqContr 1997;44:1027-37.

7. Hall TJ. AAPM/RSNA physics tutorial forresidents: topics in ultrasound: beyond thebasics: elasticity imaging with ultrasound.Radiographics 2003;23:1657-71.

8. Burns PN. Instrumentation for contrastechocardiography. Echocardiography 2002;19:241-58.

9. Dayton PA, Ferrara KW. Targeted imagingusing ultrasound. J Magn Reson Imaging2002;16:362-77.

0. Pislaru SV, Pislaru C, Kinnick RR, et al.Optimization of ultrasound-mediated genetransfer: comparison of contrast agents andultrasound modalities. Eur Heart J 2003;24:1690-8.

icholas J. Hangiandreou, PhD, Department of Radiology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905; e-mail:angiandreou@mayo.edu.