Build This Doppler Ultrasound Monitor

7/28/2019 Build This Doppler Ultrasound Monitor

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JOE JAFFETH E AVERAGE HUMAN HEA RT CAR-ries out its pumping action over100,000 times every day. Gener-ating its own electric signals toactuate the heart muscles, theheart contracts and relaxes dur-ing each bea t. We will show youhow you c an convert the hearts'motion into audio sounds usin gultrasound electronics with ourDoppler ultrasonic stethoscope.For less than $150 you c an buildt h i s e d u ca t io n a l i n s t r u m e n twhich will h e l ~ ou learn moreabou t hu ma n ch>siology.

In 1957, a n article in The Jour-nal o f the Acoustical Society ofAmerica described how cardiacfunctions could be inspected bythe use of Doppler ultrasoundusing a frequency of about 2MHz. The Doppler effect is thechange in frequency of sound,light, or radio waves tha t occurswhen a transmitter and receiverare in motion relative to eachother. When a transducer sendsa n ultrasonic beam into th e body,a port ion of the energy is reflectedback by internal body structu res.If th e st ruc ture moves, the fre-

quency of the reflected beam ischanged in proportion to t he ve-locity of the movement.Almost thirty years ago thistechnology was developed into av a l u a b le a n d c o m p l e t e l yharmless tool for non-invasiveexamination of movements in-side th e body by the medical pro-f e s si o n. E x p e r i m e n t s h a v eshown tha t beaming very low-en-ergy high-frequency sound intothe body is not harmful . Thetechniaue is used all over the

world to listen to t he he art beat ofunbo rn babies in a mother 'swomb. Now you can listen to th echaracteristic Doppler sou nd sfrom your own heart which canbe heard with a n easily built Dop-pler ultrasonic stethoscope. It isimportant to note that this in-strument is for experimentationand entertainment.Piezoelectric backgroundTransducers are devices whichchange one form of enerm intoanothver form. Some transz ucersare reversible, meaning they ca nchange energy forms in either di-rect ion . P iezoelect r ic t rans-ducers are reversible. They canchange electric energy into me-chanical energy an d mechanicalenergy back into electric energy.The quartz-crystal oscillator is afamiliar piezoelectric transducer,which is used as a highly stablean d accurate frequency source. 9Ear ly phonograph pickups rnIused piezoelectric Rochelle-saltcrystals. Both quartz crystalsand Rochelle-salt crystals are ;;;naturally occu rring materials. 2


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FIG. 1-THE TRANSM ITTER CIRCUIT.Q1 is an RF oscillator whose 2.25-MHzfre quenc y sdetermined by C4 and TI. A secondary tap on T2 provides a low-impedance output todrive XTAL l in the transducer.When either of those materialsare excited by an applied voltage,they change in dimension or ex-ert pressure if they are con-strained from movement. Whenpressure is applied to these mate-rials, they generate voltage. Oneof the f i rs t appl icat ions ofpiezoelectricity was developed byProfessor M.l? Langevin duringWorld War I when he was com-missioned by the French to find away to locate enemy submarines.He solved the problem by develop-ing a n underwater piezoelectricmicrophone.About 5 0 years ago the firstsynthetic piezoelectric materialswere developed. Today, commonlyused synthet ic piezoelectric ma-terials include barium titanate,lithium sulfate, lead niobate, an d

lead zirconate-titanate. Evenquartz crystals can now be man-made.The stethoscopeThe basic component of thestethoscope is the transducer,which contains two lead zirco-nate-titanate piezoelectric crys-ta ls . One of the crystals i senergized by the ou tpu t of a 2.25-MHz oscillator/amplifier so tha t i texpands an d contracts at thatfrequency, setting up pressure orsound waves t hat are transmit-ted into the body. When thatwave, which is very directional,passes from one medium to an-other i n the body, a portion is re-flected back to the second crystal,which generates a voltage. If thereflecting surface i s stationary,

the voltage generated by the re-ceiving crystal has the same fre-quency as the transmitted wave.If the reflecting surface is movingaway from the transducer, th e re-flected frequency is lower thanthe transmit ted wave. Similarly,if the reflecting surface is movingtoward the transducer, the re-flected frequency is higher. Bymixing a portion of the transmit-ted frequency with the receivedfrequency, the received frequencyis modulated in both frequencyand amplitude. Using a n ampli-tude-mo dulated (AM) detector,we can obtain an audio signalwhose frequency is proportionalto the velocity of the movingstructure within the body.Circuit operationT h e t r a n s m i t t e r c i r c u i t i sshown in Fig. 1. An RF-oscillatorbuilt around Q1operates at about2.25 MHz. Positive feedback isprovided from a secondary tap inT1 to the emitter of Q1. The fre-quency is determined by C 3 andthe inductive tuning of T1. Theoscillators' output is coupledthrough C 5 to Q2, an inductively-tuned FW amplifier. A secondarytap on T2 provides a low-imped-ance output to drive the trans-mitt er crystal XTALl in thetransducer. The ultrasonic powergenerated is less than 15 milli-watts per s quare centimeter oftransducer surface.The receiver and audio circuitsare shown i n Fig. 2. The receiveruses two identical stages of in-ductively-tuned RF amplifica-tion. The voltage generated in the


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is cou-Q3 through C8, and theQ3 is coupled to Q4RF

Hz range.A low-power audio amplifier,s. It has a gain of 100,which ist by C17-R16 with some basest determined by C18-R17, asof the so unds generated bye Doppler effect are in the lowio range. The volume may bejusted by potentiometer ~ 2 5t FIG. 3--TRANSDUCER CONSTRUCTION.Silver-bearing solder is used to avoid lifting theof IC1. The output of silver electrode from the ceramic crystal surface. Energy conversion is most efficientgoes to JI where the when crystals are "air-backed" resulting in energy being radiated from the front of thead se t is plugged in to. If two crystal.ish to listen at th e same

sroom demon strations, a nd in.

transducerThe construction of the trans-shown in Fig. 3. The twotals of lead zirconate-titanateor Channel Industries?kt- '/&inch rec-les approximately %-inch. Silver electrodes are depos-h crystal surface, an d

nd from one side to the othere same side of th e crystal. Fine, numb er 36 AWG or smaller,to each of the elec-using a silver-bearing sol-

cr-ys-ace. Those wires are con-ed to the terminals of XTALlnd XTAL2 on the c ircui t board.a minimu m of solder to avoide resonance charac-tics of the crystal.When dealing with ultrasoun d,uant ity of charac terist ic

various problems dealingwaveform genera tion, prop-

'

p is the density of th e me-in kg/m3 an d c is the so und

FIG. 4-THE AUTHORS' PROTOTYPE.Note that LED1 and S1 are mounted on thefoil side of the PC board. The transducer ismounted on the end plate of the enclosurewith its leads close to their solder pads.

kglrn3 x mls = kglrnzs.To obt ain ma xim um energyconversion efficiency, th e crystalsshould be acoustically matchedwith t he plastic panel. When twomediums are closely matched,most of the energy will be trans-mitted through t he materials .When a n ultrasonic beam meetsan interface of dissimilar materi-als , more of the energy is reflectedwhere there is a large differencein the acoustic impedance be-tween t he two materials.The acoustic impedance of the

crystals is about 30 million andthat of th e body i s 1.5 million,with air being less th an 50 , all inun its of kg/m2s. Because the den-sity of air is so mu ch lower tha nthat of the crystal, and the ve-locity of sound in air is muchslower than in the crystal, almostall the energy is reflected at thatinterface when the back-side ofthe crystals are in contact withair. Tha t difference in impedanceresu lts i n most of the energybeing radiated from the f ront ofthe crystal, and improved sen-sitivity of the receiving crystal.Ju s t as vou want most of th eenergy to be reflected at the rearof the crystal, it is desirable tha tmost of the energy be transmit-ted a t th e front surfa ce of th ecrystal and into the body. Be-cause the crystals are too fragileto be placed in direct contactwith the body, they are cementedwith epoxy to a sheet of plastica b o u t % e -in ch t h i c k , w h ic hshould have a n acoustic imped-ance between t ha t of th e crystaland the body. This results in


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-FIG.&PARTS PLACEMENT. N l o ~ ~ n tnd solder all components as show n here.more energy being tra nsmitte dinto the body instead of being re-flected at t he sk in surface. Whengluing th e crystals to the plastic,be sure to exclude any air fromthe interface and use a minimumamount of glue. Sheet acrylic orfiberglass such as that used forPC boards, or a rigid vinyl sheetall have suitable acoustic imped-ances and provide the requiredprotection for the crystals.When more sensitivity is re-qui red , a dab of ultrasound gel isplaced on the transducer face toimprove the impedance matchan d exclude any air tha t may betrapped between the transducerface an d t he skin. Water or miner-al oil will also work.ConstkuctionThe authors' completed pro-totype is shown in Fig. 4. All thecomponents, except the trans-ducer, are mounted on a single-sided PC board a s shown in theparts placement diagram in Fig.5. An etched, drilled, and platedthrough PC board is availablefrom th e source mentioned in thepar ts list, or you can make yourown board us ing the patte rn pro-vided. Note th at LED1 an d S1 aremounted on the foil side of the PCboard. The volume control ismounted on the component sidewith the shaft going through theboard. Use two 3/s-inch long re-sistor cutoffs and solder them to

TP1 and TP2. After soldering thecomponents on the PC board, thetransducer is connected.The transducer is mounted onthe end plate of the enclosurewith its leads close to the ir solderpads. Insert the end plate andtransducer into the slot on thetop half of the enclosure an d sol-der the transducer leads to theirappropriate terminals. Now in-s ta l l t he 9-volt ba t te ry. T hestethoscope is now ready for tun -ing after you plug in the head-phone.Connect a frequency counterfrom the emitter of Q1 to ground.Then connect a DMM, set on the10-mA range, between TP1 andTP2 and tur n the instrument on.Your current meter should readless than 10 rnA. n n e T1 to 2.3MHz, then alternately tune T2and T1 to reduce the current to aminimum. If you don't have a fre-quency counter, tune T1 for aminimum current between TP1and TP2 and then alternatelytune T1 and T2 for a lower mini-mum current. As the final cur-rent will be between l and 2 rnA,use a lower 5- or 2-mA rangewhen possible.After you have correctly tunedT1 and T2, turn off the instru-ment, remove the DMM and sol-der the leads of TP1 and TP2together. Connect the DMM be-tween the cathode of Dl andground, using the 5- or 10-volt


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6 INCHESTHIS IS THE SOLDER side of the PC board.

maximum voltage, which1 and 2 volts.If you don't have a frequencyDMM available, you

r he art . With theand headphones con-ed to the circuit board, pu t al or ultraso und gele face of the transdu cer andest near your h eart. Try toaof rib s rather t ha n directlyver a r ib. I r n the volume upti l you h ear some' Doppler

ly tune T1-T4, star tingT3 and T4, to increase the

loading and distortion.If you don't hear any soundsa

If tha t doesn't produce anyds, check t he circuit boardges and cold solder

and useAs mentioned earlier, max-n there is a good impedance

ce and the skin, with n o air issu ch as Aquasonic is specifi-

Apply a small amou nt of liquidgel to the transduc er surface andp la ce t h e t r a n s d u c e r f i rm lyagainst the bare chest, severalinches to the left of the centerand about 10 inches below theshoulder. Place the transduc er s othe ultrasonic beam passes be-tween two ribs for best transmis-sion. You will hear the soundsassociated with the movement ofthe heart . Keeping the trans-ducer firmly against the chestand chang ing the direction of theultrasonic beam you will hea r dif-fe ren t soun ds dep ending onwhat surfaces are in th e path ofthe beam. When you take a deepbreath th e soun ds may disappearbecause the lung s fill with air,covering a portion of the heart.As previously noted, a ir is a poorconductor of high-frequencysound.There are many aspects ofheart ac t ion. Firs t , re turnin gblood from the venous systemfills the right atr ium . Avalve con-necting this atrium to the rightventricle then opens and con-traction of the a tri um forces theblood into the ventricle. Th e valvethen closes and another valveconnecting the ventricle to th epulmonary artery opens. Theright ventricle con trac ts, forcingblood in to the pu lmonary systemto return carbon dioxide to thelungs to be exhaled and to picku p o x yg en f r o m t h e a i r w ebreathe in. The blood then re-turn s to th e left atri um where it ispumped into the left ventriclethrough another valve. Finallyth e l e ft v e n tr i c le c o n t r a c t s ,

pumping blood into the arierialsystem to feed the body an d th eheart itself.Each of the four cham bers ofthe heart contract and relax atdifferent times of the hear t cycle.Their associated valves open a ndclose synchronously The move-me nt of all those struc tures an dthe movement of blood throughthem provide th e Doppler sou ndswhich you hear with the Dopplerultrasonic stethoscope.When you move the tra nsdu ceracross t he skin you'll hea r somescra tching sounds. To avoid th is,tu rn th e volume down while youmove the transducer.Because there is attenuatio n ofthe so und wave as i t passesthrough the body, those with aheavy build may have to try alter-nate body positions to bring t hehea rt closer to th e chest wall. ?tYosuggested positions are lying onthe left side or leaning forward ina sitt ing position.When listening to the heartw i t h D o p p le r u l t r a s o u n d anumber of different sounds areheard, one after the other, inrapid succession as the heartchambers and valves move andthe blood flows throug h t hem .One can listen to blood flow sepa-rately from other sou nds by plac-ing the transducer on the neckwhere you feel the pulsation ofth e carotid artery. Because the ar-tery is small compared to theheart, it will take some time tolearn how to orient the trans-ducer in the direction of bloodflow through th e artery. You m us tuse the gel for that experiment.You may be able to hear a slightchange i n blood flow correspond-ing to the dicrotic notch in thepulse wave.Blood flow so un ds may also beheard from th e brachial artery inthe arm on the inside of theelbow. That is the location wheret h e p h y s i c i a n p l a ce s t h es te thoscope when m easur ingblood pressure. The transduceris again oriented in the directionof blood flow and gel must beused . When listening to t he bloodflow in the brachial artery, youmay want to try an experiment.Clench your fist to s top th e flow of 2blood i n the h and for about 5 or 6 2seconds. When the fist is un- mrnclenched the blood flows again nan d you will hear some interest- ;ing wind-like sounds . R-E !?

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Build This Doppler Ultrasound Monitor