211-10 Ultrasound Imaging

Ultrasound Imaging 211

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Suzanne Amador Kane Physics

9.UltrasoundImagingPhysics Suzanne Amador Kane, Physics Department, 370 Lancaster Avenue, Haverford College, HaverfordPA19041.ThesecurricularmaterialsweredevelopedwithfundingfromtheHoward HughesMedicalInstituteandAndrewW.MellonFoundation. AllphotographsarecourtesyAmerican3BScientificUltrasonicEchoscopemanual;alllinedrawings SuzanneAmadorKane,2003,IntroductiontoPhysicsinModernMedicine,Taylor&Francis,Inc.) AdditionalReadings:American3BScientificUltrasonicEchoscopemanual(inbinderinthelab); SuzanneAmadorKane,2003,IntroductiontoPhysicsinModernMedicine,Taylor&Francis,Inc.

1.Introduction We usually think of images as being formed from light (or other invisible forms of electromagnetic radiation such as xrays.) In fact, many animals use sound to image their environments.Forexample,batsusesonarrangingtodetectobstaclesandpotentialprey(such asmoths)orpredators(suchasowls).Dolphinsuseaspecialorganintheirhead(themelon)to performsimilar sonarfunctions underwater. The principle ofsonar workswhenthe animal emitsapulseofsound,whichthentravelsoutwardatthespeedofsound,v s,ofthesurrounding medium.Ifthesoundwaveisreflectedfromanobject,itthentravelsbacktothesourceandis detectedasanechoatthesource. Thus,thesoundwavetravelsadistanceequaltotwicethe distancefromthesourcetoreflectingobject,L,inatimeT,relatedbytheequation: L=vsT/2Eq(1) Consequently,iftheanimalmeasuresthetimeelapsedbetweentheemissionandreceivingofthe soundpulse,T,itcancomputehowfarawaythereflectingobjectis,L,ifitknowsthespeedof sound.(Recallthatthefactorof2accountsfortheroundtriptakenbythesoundpulse.)

Figure 1 Schematic depiction of a bats sonar ranging method for finding prey, such as the moth shown here. Bats emit clicks 3 to 20 milliseconds long, with frequencies in a range that depends upon species. Some bats use audible sound, others ultrasound (25-90+ kHz)


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Thebatemitshighpitchedclickswithfrequencieslargelyabovethehumanaudiblerangeof20 20,000Hz.Suchhighfrequenciesarecalledultrasound,buttheydonotdifferfromaudiblesound inanyotheraspect.Humanshaveadaptedsonarrangingfornumerousreasons.Weuseitin ourintroductorylabsindistancesensors.Sonarrangingisusedtodetectsubmarines,obstacles andschoolsoffishunderwater.Mostrecently,medicalphysicistshavediscoveredhowtoturn ultrasoundimagingintoapowerfultoolforstudyingbodystructureandfunction.Forexample, medicalultrasoundimagingisanimportantimagingmethodforobstetrics(imagingthefetusin theuterusduringpregnancy),internalmedicine(abdominalimaging,amongothers),cardiology (imagingtheheartandcirculatorysystem),andcancerimaging(e.g.,distinguishingtumorsfrom fluidfilledcystsinthebreastandabdomen).Inthislaboratory,youwillexploretheacousticsof ultrasound and understand how one can make sophisticated imaging devices using these phenomena. We suggestthat youcheck out thisamazingwebsite tolearnmoreaboutone applicationof medicalultrasoundimaginginmoredetail:http://www.obultrasound.net/

WARNING: The device you will use today is test equipment designed to teach physicists and doctors about ultrasound imaging. It does not use the same intensities as a diagnostic imaging device and you should not use it on your own body. (One of your professors already tried it out to see what would happen, and the high intensity sound produced a highly painful sensation!) However, regular diagnostic ultrasound imaging is painless because of the lower ultrasound intensities used.2.PrinciplesofUltrasoundImaging Letusreviewthebasicsofsoundwavesbeforeproceedingtoamoredetailedexplanationoftheir useinimaging.Soundwavesarelongitudinalwavesinvolvingthealternatingcompressionand rarefactionofamedium, withapower determinedbythe wavesamplitude. 1 The speedof soundwavesisaconstantindependentoffrequency,wavelengthanddirectioninfluidsand manymaterials. Sometypicalvaluesofthespeedofsoundforrelevantmaterialsareshown below.Wewillusethefirstthreevalues;valuesofinterestformedicalimagingarealsonoted.

MaterialAir Acrylic (polyacrylate) plastic Water Soft tissue (average value) Muscle Fat Bone1

Speed of sound (m/s)344 2690 1500 1540 1570 1440 3500

This is true only for isotropic (spatially uniform) media. If you have time, you can actually learn how to also excite transverse sound waves in solids that support such oscillations. Consult the manufacturers lab manual for more information.


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Today,wewillbeusinga3BScientificUltrasonicEchoscopemodeledonactualmedicalunits. (AllequipmentforthislabcanbeorderedfromAmerican3BScientific;seetheendofthelab manualfororderinginformationandadetailedequipmentlist.)First,examinethefrontpanelof theEchoscope,shownbelowinFig.2. Thisdeviceprovidestheelectricalsignalstoemitand receiveultrasoundpulses. Youcancontrolthe intensity ofultrasoundemittedandthe gain (amplification) of the echo signal received. You are also provided with two ultrasound transducers(Fig.3). Thesearedevicesthatconvertelectricalenergyintosoundwavesusing piezoelectricity.Theblackworkingendofthetransducerisapiezoelectriccrystalthatundergoes mechanicaldeformationswhenavoltageisappliedacrossit. IftheappliedvoltageisAC,the piezoelectric vibrates in response. Very short AC voltage pulses are applied in our setup, mimicking the bats pulses and the actual ultrasound pulses used in medical imaging. The ultrasoundproducedhasacentralfrequencyof1MHz(MegaHertz)(blue transducer)and4 MHz (greentransducer). To begin, connect the 1MHztransducer into the Reflectionprobe socket,item(12)inFig.2. Makesureswitch(11) isonReflect.NOTTrans.; thisenablesthe sonarranging(pulse echo) mode. Inthismode,thesametransduceremitsbriefultrasound pulses,thenactsasareceivertodetectthereturningechoes.(Fig.4C)

DONT DROP THE TRANSDUCERS! THEY ARE FRAGILE! BECAUSE THEY ARE ROUND AND WILL ROLL, PUT THEM IN A BOX WHEN NOT IN USE! THESE ARE HIGH VOLTAGE DEVICES. TREAT THEM WITH RESPECT! The ultrasonic gel you are using is nontoxicthis is the same water soluble gel they use in actual medical exams. You clean up your hands with soap and water after using it.

Locatethe~1cmthickacrylicblock.Usethecaliperssuppliedtomeasureitsthicknessinmm andrecordyourvalue.Now,usetheUltrasoniccouplinggel(whitesqueezebottle)tosquirta littlegel(aboutasmuchgelasyouputtoothpasteonyourbrush)ontothetransducerhead. Pressthetransducerheadagainstthetopsurfaceoftheacrylicblockandmoveitaroundtoform agoodsealwiththe gel(Fig.4A). Youwill nowsee howtouse thissetuptomeasure the distancefromthefronttobackoftheplasticplateusingsonarranging. Next,turnonthecontrolunitusingthepowerswitchatthebottomrightcorner(Fig.2). The Receiverpowerswitch(5)abovethetransducer socketallowsyoutoselect thepowerofthe ultrasoundpulseemittedfromthetransducer.Thiscanbeadjustedin5dBsteps,from0to35 dB.(Recallthatadecibelisalogarithmicscaleforpower;eachincreaseof10dBcorrespondsto anincreaseinpowerofafactorof10:dBvalue=10log(P2/P1),wherethelogistobase10.)Set


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thepowerknob(5)to15dBfornow.Knobs(1)through(4)controlthegainbywhichtheechois amplified.TurntheseinthefullCCWpositionfornow.

Figure2A)UltrasonicEchoscopeunitwithfrontpanellabels.B)Typicalstartscreen.


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AB

Figure3A)Ultrasoundtransducer.B)Schematicoftransducerconstruction.C)Depictionof pulseecho/sonarrangingmodeoperation. You are finally ready to start the software (it should be an icon on the desktop named:

Ash_eng.exe

;doubleclickonittobegin.)YoushouldseeascreensomethinglikeFig.2B.Note

thatyoucanatanytimeselectfromthetopmenuFile/PrintScreen. Thiswillallowyouto recordplotsofyourmostimportantdatawhenrequested;youshouldalsoanswerallquestions andperformallcalculationscontainedinthefollowingexercises. Youshouldbeobservingonthetopofyourscreenaplotofsoundintensityvs.Time(in s). SelectfromthetopmenuDisplayandselectAscanandHFData. Youwillseeasignalwith


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rapidly varying amplitude with time; this is called the HF (high frequency) data and it correspondstoaplotoftheactualpiezoelectricvoltagereceivedvs.time.Thevariationofthis signalreflectsthatoftheactualoriginalultrasoundpulse(atfarleftstartingatzerotime)andthe reflectedechoes(atgreatertimes). TheenvelopemodulatingthissignaliscalledAscan(for amplitude)anditcanbedisplayedalone,ortheHFdataonlycanbedisplayed.Thevariationof the HF data shows everything you want to know about the ultrasound pulses. Take a few minutesnowandplayaroundwithwaysofusingthetoolbarbuttonsforZoomIn,ZoomOut andthebuttonlabeled100or200toseehowtovarythexaxisscale.Then,usethearrowsatthe leftmostsideofthetopplottovarytheyaxisscale. (ThebottomplotshowstheGainofthe devicevs.time.Itshouldcorrespondtoaflatlineat0dB,indicatingthatthereceivedechoesare beingmultipliedbyaconstantfactorof1atalltimes.Thiswillbecomemoreinterestinglateron whenyouadjusttheGain.) Usingtheredandgreencursorsprovided,measurethetimeseparationbetweentwoadjacent closelyspacedpeaksoftheHFdataandconfirmyourtransducersfrequency value. (Todo cursormeasurementsinx,clickanddragthecursorswhereyouwantthem.Theirvaluesandthe separation are automatically displayed at the bottom of the display.) Because the HF data consistsofbriefpulses,itcannotbeentirelyatonefrequency,however.SelecttheSTOPbutton fromthetoolbar,thenhittherightmostbuttonlabeledFFT. Anewwindowwillpopupthat displaysthefrequencyFouriertransformofyoursignal.WhereistheFouriertransformpeaked? WhatititswidthinHz?(Noteyoucanusethecursorsonthisplot;thevaluesaredisplayedat thebottomoftheFFTwindow.) Whataboutthe originalsignaldeterminesthe widthofits Fourierspectrum? OnceyouhaveaplotthatlooksapproximatelylikethatinFig.4B,dothefollowingexercises: 1) Theplotyouseedisplaystheactualvoltagereceivedbythetransducerasitemitsan ultrasoundpulse(leftmostpulse)andreceivesechoes(secondandhigherorderpulsesto the right).; compare Fig. 3C. The separation between the first and second pulse correspondstothetimeinEq.1;measurethetimeusingtheamplitudemaximaofboth pulsesusingthecursorsprovided.Usethistimeandyourmeasureddistancetocheck thespeedofsound,usingEq.(1). Inputthecorrectspeedofsound(topmenuitem Setup/SoundVelocity);thenominalvalueforacrylicisgivenintheTableabove.Then, selecttheDepthmodesoyouaredirectlydisplayingplotsvs.Depth.(Youcandothis fromthetoolbar,ortheXaxistopmenuitem.)


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2) Next,usingthesecond,thickeracrylicblockprovided,measureitsthicknessusingthe calipers.Nowcheckitsthicknessusingthetransducer(remembertousecouplinggel). RecordyournewplotofAvs.Depthandyourresults. 3) Trychangingthepowersettingfrom0to35dBwhilewatchingtheAmodeplot.Record thechangesinAatafixedDepthwhenyouvarytheoutputPower;howdotheseagree withyourexpectations?

Figure4A)UltrasoundtransducerinpositionabovethinacrylicplateandB)resultingpulse echoesmeasuredinAmode. 3.Whatdeterminestheintensityofultrasoundechoes? Just astheindexofrefractiongovernstheintensityofreflectionsinoptics,thereisasimilar quantityforsoundwavescalledtheacousticimpedance,z.Theacousticimpedanceistheproduct ofthedensityandspeedofsoundofamedium: Z= vsEq. (2) Eq3) Theintensityofsoundreflectedfromaninterfacebetweentwomaterialswithdifferentvaluesof acousticimpedanceisgovernedbytheequation(Fig.5A): Itotal=totalintensityofincidentsoundwave Irefl=intensityofreflectedwave(echo) Itrans=intensityoftransmittedwaveEq.(3)


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AB Figure5A)SchematicillustrationofthegeometryusedindefiningEq.3B)Setupfor measuringechoesfrommultipleinterfaces. Thus, mismatches in acoustic impedance are what create sound reflections. The greater the acoustic impedance mismatch, the larger the intensity of the reflected sound wave or echo. Conversely, if two different materials have an interface, but they have the same acoustic impedance,noechowillresult,animportantphenomenoncalledimpedancematching. Withinthebody,differentorgansandtissueshavedifferingacousticimpedances.Thefollowing tablegivesafewimportantvalues:

Material Fat Water Muscle Bone

z [kg/(m2s] 1.30 106 1.50 106 1.65 106 7.80 106

Asapracticalmatter,thismeansthatwhensoundwavestravelthroughthebody,theywill reflectfrominterfacesbetweenbonesandskin,ormuscleandfat.Ifwecanusesonarrangingto measure the distances between these interfaces, then we can map out the positions of all reflectingsurfaces. Bymeasuringhowintensethereflectionsare,wecanalsodeterminehow greatanacousticimpedancemismatchcausedthem. You have already used impedance matching in the first exercise. Since air has almost zero densitycomparedtoplastic,itsacousticimpedance isalsoclosetozerocomparedtothat of plastic.Asaresult,ifyoudidnotusethecouplinggeltoformasealbetweenyourtransducer and plastic block, 100% of the ultrasound intensity would be reflected back, and none transmitted.Couplinggelsareusedinthisfashioninmedicalimagingaswell.Besuretousegel ineachofthefollowingexercises.


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Nowyouwillrecordthereflectionsbetweentwoblocksofplastic.First,takeyoursetupfromthe previousexercise,andnowplaceanotherplasticblockunderthefirst.Doesthesignalchange? Doestheultrasoundpulsetravelfromthefirstblockintothesecond?(Whatdoesthatinterface actually look like?) What would you expect to see if you saw additional echoes from the additionalinterfaces?Nowremovethesecondblockandusealittlecouplinggeltoformaseal betweenitandthesecondblock;repeatthemeasurement.Nowwhatdoesyoursignallooklike? Whyisitdifferentfromthefirstexercise? Nowyouwillintroduceultrasoundintoawaterbath,asshowninFig.5B.(Remembertosetthe speedofsoundtothatforwater!) Usingtheblack plasticholderprovided,placethe1MHz transduceragainstthewaterbathsoitseals(usegel!) PlacetheacrylicorblackplasticPOM (polyoxymethylene)blockattachedtoacrossplateanddialontop;setthedialsotheultrasound hitstheblackblockatnormalincidence(90degreesonthedial.)Now,collectanAscanofthe resultingechoes.Trymovingtheplasticblock(keepingitsanglefixed)toseehowthischanges theplot. Thislast scanisactually more similar toamedicalimaging setupthantheearlier exercises,sinceyouarenowsendingultrasoundpulsesthroughmediawithfairlysimilarvalues of acoustic impedance. This means that there is enough ultrasound intensity after the first interfacetopenetrateintotheplasticblockandallowimagingofbothinterfaces. Similarly,in medicalimaging,ultrasoundcanpenetratefairlyfarintothebody,allowingonetomeasurethe locationsandacousticimpedancemismatchesofmanydifferentinterfaces. 5.UltrasoundabsorptionandTimeGainCompensation(TGC) Assound(orultrasound)travelsthroughamedium,itisabsorbedbythemoleculeswithin.This absorptionisproportionaltotheincidentultrasoundintensity,sotheresultingabsorptionresults inanintensitythatdependsexponentiallyupondistancewithinthemediumthrough:2 Itrans=Itotal exp(x/D)Eq(4) Here the relevant intensities are the total incident pulse intensity, I total , and the transmitted intensityremainingafterabsorption,Itrans.Hereweareusingxtorefertothedepthwithinthe sample,andDisaquantitywithunitsofdistancethatdeterminesthedegreeofabsorption.Asa result,ifoneplotssuccessiveechoesasafunctionofTime/Depth,youwillseearapidfalloffin intensityduetotwoeffects:theabsorptionfactorfromEq(4)andthereflectedlossesfromEq.(3); seeFig.6Aforillustration.

2

Here we only consider attenuation due to absorption and specular reflection. In an actual ultrasound imaging experiment, one might also encounter losses due to scattering from small objects and diffuse reflections from rough surfaces. The ultrasound pulse also spreads out as it travels through the medium, so there is a falloff in intensity due to this geometrical effect.


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Figure6.A)IllustrationofaflatGainprofileasafunctionoftime.Inthiscase,theechoes receivedaremultipliedbythesamefactor(0dBoroneinthiscase).Asaresult,thefalloffin intensityofsuccessiveechoesduetoabsorptionandmultiplereflectionsareevident.B)Use oftimegaincompensation(TGC)toprovideagainprofilethatincreaseswithtimeoverthe echopulsetraintocorrectforabsorption.Asaresult,thepulsesreceivedareapproximately comparableinintensity. We can measure the absorption falloff using the following experiment. Set up the 1 MHz transducer with the phantom (acrylic block drilled with holes) as shown in Fig. 7. In this orientationyoucangenerateechoesfromequallyspacedholes~1cmapart,butslightlydisplaced sidetoside.Thismeansyoucancomparetheintensitiesfromsuccessiveholesatdepthsequalto 1cm,2cm,3cm,etc.withlittleeffectduetomultiplereflections. Recordtheechoesfromeach hole,startingatthemostshallow,whilesteadilyslidingthetransducertotheleft.Youshouldbe abletovisualizetheexponentialfalloffinechointensityreadily. Measuretheintensityofthe first echo and make a quick plot using your favorite spreadsheet program to verify the exponentialfalloff.(Youcandothismosteasilybyplottingtheintensityonalogscaleandthe depthonalinearscale,yieldingalinearplot.) Repeat your qualitative measurements with the 4MHz transducer to see how the absorption varies with frequency. Based on this criterion alone, would a higher frequency or a lower frequencytransducerbeappropriateforabdominalimaging?Forimagingtheeye?


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Figure 7 Setup for observing absorption as a function of depthandfrequency. Since the decrease due to absorption follows a general exponentiallaw,wecancorrectforitbymultiplyingthesignal by a gain exponentially increasing with Time/Depth. This is showninFig.6B,wheretheincreasingGainisshownatbottom, andthepulseechoesarecloserinintensitytoeachotherasa result.NotethatsincetheGainisplottedasalogarithmonthe yaxis, this exponential increase becomes a straight line. (ComparetheflatGainplotinFig.6A.) Since thiscorrection requiresthe Gaintoincrease inTime toCompensate forthedecrease inintensity caused by attenuation,itiscalledTimeGainCompensation,orTGCforshort.Thisisacommonfeatureof medicalultrasoundimagingscanners,whichneedtoremovetheuninterestingabsorptioneffects to reveal the actual information due to body structure or function. Otherwise, the medical imageswouldalwaysfadeoutwithdepth!TryoutthisTGCtechniquewithyourexperiment abovetoseeifyoucanreducetheeffectofdepthdependentabsorption. Setupyour1MHztransducersoitiscoupledtothe1cmthickacrylicblockagain.Youshould besetuptoseemultipleechoesonyourscreen.Now,adjustyourcontrolunitsGainsettingsas follows:(ConsultFig.2A):1)LeavetheStartdialfullyCCW;2)TurnSlopefullCW;3)Turn WidthfullCW;4)TurnThresholdfullCW.NowyoushouldseeaplotsimilartothatinFig.6B. TryadjustingeachoftheTGCsettingsindividuallyandexamineeachresultingGainplottosee whatchanges.YoucanconsultthelabelsinFig.6Btoseewhichfeaturestowatchoutfor.Tryto find the best TGC settings for all four parameters that gives you the best correction (most uniform echoes) for all the data visible. Remember how to do these adjustments for later exercises. Wheredoestheultrasoundenergylosttoabsorptiongo?Mostlyheatinginthislaboratory,butin medicinehighintensityultrasoundcanalsocausecelldisruption(duetotheshearforcesfrom the associated compressionrarefaction) and cavitation (the creation of bubbles due to low pressuresattherarefactions). Thus,medicalultrasoundusedincontinuouswave(CW)mode, withoutpulses,isusedasatherapeuticagent. Itcanbeusedatveryhighlevelstodestroy tumors, and at lower intensities to relieve pain and inflammation. You may have received ultrasoundtherapyinsportsmedicineusingtheseideas.


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6.Makinganultrasoundimage:Bscans AnAmodeplotofamplitudevs.Time/Depthisnotveryrevealingofanatomy.However,ifone movesthetransducerbackandforth,onecanmeasurethetwodimensional(2D)locationsof objects:thedepth(y)comesfromsonarrangingandthetransducerspositionsuppliesasecond (x)dimension. Inthisfashion,onecancollect 2Dmapsofbody anatomy. This istypically displayedbymakinga2Dmapofreflectedintensityasafunctionofx(transducerposition)and y (depth). The echo intensity is indicated by actually plotting a variety of intensities on a computer screen, and assigning a grayscale of white to gray to black depending upon how intense theactualechowas. Thismapofechointensityvs.positioniscalled aBscan. Itis illustratedinFig.8C.

ABC Figure8A)Acrylicimagingphantomwithholesdrilledatvaryingdepthsandseparations.B) Using the transducer with the phantom. C) Sample Bmode scan made by dragging the transducerinBtowardtheright. YouwillnowmakeaBscanbymovingyourtransduceralongasamplewithdifferentfeaturesat differentlocations. Findtheplasticblockwithholesdrilledatdifferentdepths.(Fig.8A&B) Suchobjectsarecalled phantoms inmedicine; theyareusedtocalibrateandcheckoutactual medicalultrasounddevices.Makeplotsofthereflectedechoprofilefortheshallowestandthe deepestholesyoucanimage(recordingwhichisthedeepestholeforwhichyoucanseeechoes withthe1MHztransducer)andadjustyourTGCsoyougetthenicestamplitudecurve. (You willprobablyhavethestartvalue beforethefirstecho,andtheslopeandwidthadjustedto encompassthislargestreflection.)Now,lookatthetopmenuandselectB(rightness)modeform the Imaging menu. You will get a new window that allows you to accumulate a Bscan. However, you will have to move the transducer by hand so it passes over several holes at differentdepths. TrythisafterfirstclickingontheStartbutton. ClickonStopafteryouhave finishedyourScan,andexaminetheimage.Makesureyouunderstandhowtheresultingplot agreeswithinformationaboutthedepthsoftheholesyouscannedover.(Note:wefindthatthe


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transducercanbedifficulttomovebecausethegelsealssowellagainsttheblocks,soyoumay notbeabletoimagealloftheholes.)Printoutandannotateyourbestimage. Actual medical ultrasound scanners actually have builtin mechanisms for sweeping the ultrasound beam so as to automatically accumulate Bscans. One method involves have a transducer head that rapidly rotates to scan different angles; another involves using many adjacenttransducerelementstofireultrasoundpulsesinmanyadjacent,parallelpaths,justas youdidbyhand. 7.Ultrasoundimagingresolutiondependsonfrequency Thefrequencyofasoundwavedeterminesitswavelength, ,throughtheequation: =vs/fEq(5) Thewavelengthofasoundwavealsodetermineshowsmallafeaturewillproduceadistinct reflection.Inotherwords,thehigherthefrequency,thebetterthespatialresolutionthesmaller theseparationatwhichonecandistinguishechoesfromtwodistinct,nearbyobjects.Compute thewavelengthofyour1MHzand4MHztransducersnowforacrylicandsofttissue,andrecord theminyournotebook.Writedownafewexamplesofwhatfeaturesinthebodycouldusefully beimagedusingeachtransducer. Now,pickupthephantomagainandfindthetwosmallholeslocatedveryclosetogethernear oneoftheends.Positionyour4MHztransducerabovethetwoholes,locatingyourtransducer alongthenarrowerendofthephantom.Slidethetransduceracrossthetwoholesfromsideto side.Whatdoyousee?Whatdoyouseeifyoupositionthetransducerexactlyinthecenterof the two holes? Why? Now, do the same test with the 1MHz transducer. Do you see any differenceasyouscanacrossthetwoholesnow?Whyorwhynot?Whathaschanged? 7.Ultrasoundimagingabsorptiondependsonfrequency Youmightthinkafterthelastexercisethatmedicalultrasoundshouldalwaysusethehighest frequencyattainable,sincethatwillgivethebestspatialresolution.However,thatassumption leavesout one importantfact: the absorptionofultrasound increaseswithfrequency. Asa result, ultrasound imaging is a tradeoff between spatial resolution and achievable depth of imaging. Toseethatthisisso,now,useyour4MHztransducertoimagethephantomblock drilledwithmanyholes.Recordwhichisthedeepestholeyoucandetectanechofor.Howdoes itcomparetowhatyoufoundforthe1MHztransducer? 9.ImagingmotionwithMmodeScans


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Forourlastexercise,wewillseehowM(otion)modeultrasoundscanscanbeusedtoaccumulate informationaboutpulsatilemotion,suchasthatfoundincardiologyimaging.ThisMmodeis usedinimagingtheheartandcirculatorysystemtoshowtheperiodicmotionofbloodflowor heartvalveaction. An Mmode display consists of an Amode scan repeated as a function of time, and then displayedusingamapofgrayscaleintensity.LikeaBscan,thegrayscaleintensityindicatesthe echo intensitiesmeasuredandthe ydirectionindicatesdepth. UnlikeaBmode scan, thex directionisTime.Whatresultsisadisplaywhichdisplayshowthedepthofareflectingobject variesasafunctionoftime. Wewillsimulatethemotionofavalvewithintheheartusinga modelprovidedbythemanufacturer(Fig.9A).UsingFig.9Basaguide,orienttheheartvalve modelsothesideofthecylinderwithanopentopisup.Fillthischamber(butnotthebottom chamber)withwatertowithin1cmofthetop.Useathreefingerclampwithalabstandtokeep itfromspilling.Now,mountthe1MHztransduceronanotherthreefingerclamp.Loweritinto thewaterslowlysoatmostonly3mmisimmersed.SetupanAmodescanandadjustthegain soyougetagoodplot.NowselecttheMmodefromthetopmenuImaging/MModeoption. Pressstartwhileyoupumptheheartvalvemodeltosimulatethemotionoftheheartvalve.Your Mmode scan should resemble that from Fig. 9C. Make a printout and annotate what is happeningduringthescaninyourcase.

ABC Figure9A)ModelheartvalveforMmodeanalysis.B)Howtosetuptheheartvalvemodel withthetransducer.C)SampleMmodescanmadebypumpingtheheartvalvemodelduring thescan.

Please clean all the gel off your transducers and empty the trough when you are finished. Please place the transducers in a box so they will not roll of the table.Optional:UltrasoundImagingofamodelBreastTumor


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Your setup includes a model of a female breast made of plastic filled with silicone oil to approximatethepropertiesofactualbreasttissue (mostlyfatandglands). Siliconeoilhasa speed of sound of ~1000m/s. Located in the model are two lumps that approximate the propertiesofbenigntumors.Eachisroughly1cmindiameterandcanbelocatedbypalpating (pressingyourhandagainst)thesample,asisdoneinmanualbreastselfexaminations. These modelsareusedbothtoteachwomenhowtoperformbreastselfexams,toseewhatatumor feelslike,butalsotoallowphysicianstotryoutultrasoundimagingofsuspiciouslumps.You canrepeatyourBscansabove,orjustcollectanAscanvs.Depth,toseeifyoucanlocatethe tumor. In real medical applications, ultrasound imaging is a fast, noninvasive method for distinguishingbetweenfluidfilledcysts(whichfeelliketumors,buthavegeneratenoechoes withintheirfluidcenters)andsolidtumors(whicharefilledwithsolidtissuethatcangenerate internalechoes). FinalWriteup Youmayenjoyansweringthesechallengingquestionsinyourfinalwriteup: 1) Bearing in mind the lessons learned from your experiments, what would be the advantages and disadvantages of using the two different frequency ultrasound transducerinmedicalimaging.Whatfrequencymightoneusetoimagefeatureswithin theeye?Majororganslocateddeepwithintheabdomen? 2) HowcouldtheideaofBscansbeimprovedontoallow3Dultrasoundimaging?(That is,threespatialdimensions.) 3) Manybodytissueshavesuchsimilarvaluesofacousticimpedancethatonlyweakechoes aregeneratedattheirinterfaces.Acontrastagentisamaterialintroducedintothebodyto enhance the ultrasound reflections, and thus enable imaging of a greater variety of anatomicalfeatures.Whatwouldyouneedasapropertyofanidealultrasoundcontrast agent?Canyouthinkofanywaystoenhancethecontrastofbodytissues? EquipmentList AllequipmentforthislabcanbeorderedfromAmerican3BScientific;American 3B Scientific 2189 Flintstone Drive, Unit O Tucker, GA 30084 Tel.: 770.492.9111 Fax: 770.492.0111 [email protected]

Acompletelistingofnecessaryequipmentfollows. Theonlypreparationrequiredistomake sure that the computer used has the English version of the software and all related drivers installedandworkingwiththeLPT1interface,aswellasmakingsuretheEchoscopeisconnected tothatinterface. 1. UltrasonicEchoscope(CPU10010) 2. 1MHztransducer(CPU10015)

Ultrasound Imaging 211 3. 4MHztransducer(CPU10017)

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4. 1cmand2cmthickacrylicblocks(CPU10025andhomemadeblock) 5. Acrylicblockdrilledwithholesatdifferentdepths(CPU10027) 6. Plastictankfilledwithwaterforabsorptionstudies(CPU10020) 7. Ultrasoniccouplinggel(CPXP999) 8. HeartValvemodel(CPU10029) 9. PolyoxymethyleneTestBlock(CPU10023) 10. Modelofsinglebreastwithbenigntumor(L55/1)

211-10 Ultrasound Imaging

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