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2 7 1 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-27 NO 6 NOVEMBER 1980

An Ultrasonic Catheter for Intravascular Measurement of Blood Flow

Technical Details ROY WMARTm MEMBER IEEE AND DENNIS WWATKINS

Abstract-This unit features a cylindrical ultrasonic transducer array of six elements for detecting lumen area along with a single element for detecting blood velocity both located at the tip of a 2-mm diameter catheter Flow is derived from the area and velocity information Numerous technical problems associated with the ultrasonic physio-logical clinical and sue considerations have been overcome Included was the developmentof a widely applicable method to obtain small radiating surfaces (075 mm2) which transceive with no pattern dis- tortion or damping due to the electrical connections or mechanical mounting Radiation sidelobes averaged -23 dB Signal transfer for the velocity and array units of -24 dB and -32 dB respectively were measured Distinct wall echoes have been obtained from pulmonary arteries aswell as ample scattering signals from the blood of dogs with the catheter all information of use in flow measurements

INTRODUCTION

THE VALUE of measuring the volume of blood ejected by the heart per minute (cardiac output) or per beat (stroke

volume) in the critically ill or in patients undergoing major surgery is well recognized [ l ] 121 At present only intermittent measurement has been available and logistical problems associated with performing the measurement have restricted wide- spread clinical use

However introduction of a catheter into the cardiovascular system in these patients for the purpose of monitoring pressures performing blood analysis and fluid transfusion has been routine Our objective in constructing the ultrasonic unit was to develop a system which allows continuous measure-ment of cardiac blood flow as well The expectation is that the ultrasonic catheter will prove simpler to use than the inter-mittent techniques now in practice

The key measurement principle appears in Fig 1 Earlier work demonstrated that the technique is independent of cath-eter tip movement tilt rotation and radial and axial translation [3] Flow measurements were found to correlate with a standard intermittent technique (dye dilution) with a coeffi- cient equal to 099 variability at the 95-percent confidence level was found to be within+ 5 percent and the slope of a linear regression ranged between 089 and 079 This paper discusses catheter requirements provides details gives fabrication information and reports performance characteristics

Fig 1 Blood flow measurement method Stroke volume and cardiac output are measured by integrating pulmonary artery blood flow In-stantaneous pulmonary artery flow (Q) is measured by 1) determining vessel sectional area A (area of the wall outlined by a plane which passes through the vessel at any arbitrary angle) and 2) multiplying it by blood velocity V which flows perpendicular to the plane hence Q = A V In order to accomplish this measurement an ultrasonic catheter is positioned so the tip is in the pulmonary artery Vessel wall to catheter distances di (where i = 1-6) are detected by scanning radially transceiving transducer array Sectional vessel area is computed from this information by 1) solving for the constants of an equation which describe an ellipse which best fits the wall contour detected and 2) computing the area of the ellipse using these con-stants Blood velocity is detected via the tip transducer from the ultrasound backscattered energy from the blood cells using pulsed Doppler techniques The balloon is inflated only during catheter insertion so as to allow the blood flow to assist in directing the catheter tip to the pulmonary artery Pressure lumens are used in part for guidance to position the tip just distal to the pulmonary artery valve When pressures are obtained at the two ports as indicated the desired position is confirmed

DESIGNCONSIDERATIONS Three concerns are of major importance First the catheter

must meet the clinical requirements necessary for catheter-ization allow pressure measurement and fluid transfusion Second flow must be accurately measured Gird the ultra-sonic sensing system must be biocompatible for use in humans and overall clinically acceptable

Catheterization Requirements The physical characteristics of the catheter (size length

Manuscript received December 10 1979 revised July 10 1980 This flexibility form and material type) are important considera-work was supported by National Institute of Health Grant HL 14645 tions in pressure measurement and fluid transmission The The authors are with the Department of Anesthesiology University of Washington Seattle WA 98195 catheter should be small enough to allow catheter insertion

0018-9537801100-0277$0075 0 1980 IEEE

278 IEEETRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-27 NO 6NOVEMBER 1980

into a vein by percutaneous puncture [4] (to avoid trauma and morbidity associated with surgically exposing the vessel) but large enough to allow pressure measurement and fluid transfusion with ease The largest available catheter ldquointro-ducerrdquo kit of which we are aware accepts a 3-mm outside di-ameter (OD) catheter (USCI OOSS9) A l-mm inner diameter (ID) catheter or equivalent area is generally acceptable for pressure monitoring and fluid movement (The flow resistance for blood for a l-mm ID Teflon catheter is approximately 01 5 mm Hg mincm3 per 1 -cm length ) A catheter length of at least l-m is necessary to catheterize the pulmonary artery from a vein in the extremities

The flexibility and form of the catheter are very important considerations in positioning the catheter tip at the desired point Since the tip must be located in the pulmonary artery in order to measure right heart volume blood flow the posi-tioning and verification of this position are crucial We employ a technique called the Swan-Ganz method [S] which has gained wide clinical acceptance for pulmonary artery cathe-terizations This method employs a flexible catheter with a balloon located near the tip The balloon is partially inflated during insertion and the flow tends to direct the tip into and through the heart to the out-flow tractWith the commercial Swan-Ganz catheters monitoring thewaveform of the pres-sures during catheter insertion usually provides information enough to guide the positioning of the tip in the pulmonary artery and fluoroscope is not required

Measurement Accuracy The second consideration is the accuracy of flow measure-

ment Although present intermittent measurement techniques are no more accurate than +15-20 percentthis information is of use clinically [ 6 ] [7] We hope to achieve accuracies in the range of + l0 percent or better An exhaustive treatment of all the errors and potential errorsis beyond the scope of this paper but major errorsof importance are highlighted

The technique discussed in Fig 1 assumes the velocity profile to be constant at each point across the vessel Deviations from such a flat profile will automatically introduce errorsof which the magnitude is dependent upon catheter radial position and the amount of deviation Justdistal to the pulmonary valve (within 3 cm) the profiles have been found to be flat [S] however the amount of deviation which occurs in the population at large is yet unknown

The flow measurement method is theoretically insensitive to catheter tilt since the area increases in proportion to the amount that the velocity decreases due to the tilt [3] Conse-quently the product remains constant Errors due to motion or catheter translation have been found to have little effect on the measurementof stroke volume and cardiac output however these errors are a consideration when the waveform of the flow is of interest

The maximum physiological values expected for an adult human are as follows a pulmonary artery ID of 3 cm a wall thickness of 2 mm blood velocities of less than 200 cms and vessel cross sectional areas equal to 7 cmz The magnitude of flow is expected to be no greater than 10 lmin

Errors in the measurement of area and velocity produce a total error which is primarily the sumof each error (see (1) in the Appendix) If errors are opposite in sign they tend to cancel if errors are of the same sign they add and increase Errors in area determination are approximately twice the error of a single radius measurement (see (2) in the Appendix) However in practice since area is not calculated from a single radius errors from all six array elements tend to average to one smaller than that for a single radial determination There-fore error should not be as severe as (2) indicates Further some errors due to alignment or other reasons can be corrected for in the data processing if they are identified

Additional sources of error associated with the measurement of catheter-to-wall distances can arise from echo artifact and false echo detection The presence of false echoes due to side-lobes or mutual coupling of energy among array elementsor connecting leads can contribute to artificial detection as well as a low signal-to-noise ratio (SNR) Mutual coupling of en-ergy is particularly detrimental

For example a strong specular reflecting structure may be located in front of transducer A with the angle of incidence equal to zero whereas received echoes from transducerB may be weaker due to encountering structure at angles of incidence not equal to zero and due only to diffuse scattering Because of mutual coupling spurious echoes from transducerA then may appear in channel B comparable in magnitude to the echoes received from transducer3 This action gives the false impression of structure location in the radiation field of trans-ducer B A measure of mutual coupling is provided by deter-mining the ratio of the received energy in B with respect to the received energy A when A is activated Sidelobe transception has a similar effect Both forms of interference should be less than -20 dB but even -20 dB may not be adequatein some instances Certainly the greatest difficulty occurs in this regard when the ultrasonic catheter is close to the wall of the vessel

Weak wall echoes can result when the sound impinges on the wall at large angles of incidence and the sound must travel an increased distance through the blood This condition is primarily a function of the size of the vessel the distance the catheter tip is located from the center of thevessel and the angle that the catheter is tilted with respect to the axis of the vessel Since these variables are not easily controlled low signal levels can be frequently encountered This occurrence results in a reduced SNR which can affect distance determination accuracy To minimize the effect of this action trans-ducers must be designed for maximum sensitivity and receivers for a low noise figure Further ability to adjust rotateor re-strict the angle of the catheter with respect to the axis of the vessel andor its distance from the axis would also be most helpful

Care should be taken in aligning the transducers so their beam patterns fall on a plane perpendicular to the axisof the catheter Alignment errors can result in errors in measurement of the vessel lumen area

Echo-ranging errors will be affected by the spacial resolution of the transducer system Spacial resolution involves both lat-

279 MARTIN AND WATKINS MEASUREMENT FLOWOF BLOODINTRAVASCULAR

era1 and longitudinal resolution of the beam and both are important when the beam is at other than normal incidents The effect is the production of a region of uncertainty in the range as to where the front of the vessel wall is located in addition to the normal uncertainty due torange resolution (ie the backscattered tone burst is time spread due to encountering the wall at a nonzero angle of incidence) This additional range of uncertainty will be referred to in this paper as range increment (AR) In the Appendix an approximate evaluation of this factor is made and the following equation was derived

+W tan B(cos a - 4)AR1 N-

2 cos a

where a = tan ( w l w z ) w l W are the lateral and longitudinal beamwidths respectively and 8 and 4 describe the angles at which the radiating beam impinges on a plane tangent to the wall The range resolution (range uncertainty with no tilt) is a function of the fractional bandwidth of the system and the SNR of the wall echo We define this range increment of un-certainty as AR and it has been shown that(AR) is related to the fractional bandwidth (fb) by the equation

CA R = -

2f0 fb

where fo is the frequency of the carrier used and C is the speed of sound [ 9 ] The total range increment of uncertainty as to wall location then is approximately equal to the sum ofAR and A R

Using the equations above w l W 8 and have been computed for values of R (range distance to the wall) and fb that would allow measurement within + I O percent accuracy (Table I) The importance of these variables becomes apparent when observing them under various conditions as illustrated in the table

Finally errors in velocity determination are a lesser function of the catheter design and more a function of the signal processing technique and the overall measurement concept Ade- quate signal transfer is required Further the radiation pattern should be normal to the area measurement plane and the radiation pattern should be highly directional In order to accurately represent the velocity of the flow normal to theplane area over which the area is measured the fractional bandwidth must be high enough to allow velocity measurements close to the catheter tip We believe measurement within 5 mm of the tip is acceptable in this regard

Human Usabirity In order for the ultrasonicsystem to be clinically usable in

humans it must be reliable biocompatible and produce min-imal additional risk to the patient than would a normal cath-eter Reliability in performance is particularly important since the information measured may be used in making life preserving decisions A high degree of durability is necessary to insure reliable performance such devices often are subjected to stress during insertion transportation cleaning sterilization and storage Such stress is often exaggerated by various demands and changing schedules of the hospital environment

TABLE I EFFECTOF SEVERAL FACTORS OF AREAMEASUREMENTO N ACCURACY A

BASEDON SINGLE RADIUSCALCULATIONS

Beam Widths (mm) Y v l l Impingement

Minimum Radius Fractional Longitudinal Lateral Angles (degrees) bandwidth

R(m) for -10 i C 10 W e Bfb W 2

48 l 0

2 6 z x 0

6 4 l 3 3 2 0 20

4 2 3 3 20 20

7 l 5 5 20 2 0

5 2 5 5 2 0 20

106 l 5 5 40 40

85 2 S 5 40 4 0

6 5 10 5 5 40 4 0

18 l 2 8 20 20

10 l 2 8 35 35

8 2 2 8 35 35

10 2 2 8 45 45

10 l 2 14 20 20

10 2 2 14 30 30

105 10 2 14 40 40

10 2 2 14 4 3 0

x = any value

See (2) in the Appendix

To preserve reliable performance the device must be pro-tected from the deteriorating effects of blooda harsh solution for many materials However the ideal coating or protecting mechanism must be completely compatible with the blood avoiding the activation of clotting andcoagulationmechanisms Smooth surfaces also are important in minimizing this effect The amount of ultrasonic andelectrical energy emitted must be at a level and of a form not capable of producing detrimen-tal biological effects (Ultrasonic energy levels less than 004 Wcm-rsquo have been reported to be ldquosaferdquo for use for an un-limited length of time [IO] ) Finally electrical circuits associated with the catheter must be as isolated from the body and earth ground to avoid the introduction of an electrical path- way for current flow from other patient-attached electrical devices (the electrical cautery for example)

DEVELOPEDCATHETERDESCRIPTION General

The main body of the device we have developed consists of a commercially available 3-lm catheter madeof polyvinyl chlor- ide (23805 Edwards Laboratories Santa Ana CA) One lumen contains the wires for connection to the transducers and provides an air channel to inflate the Swan-Ganz balloon The other two lumens (ports located 1 cm apart) allow measurement of right ventricular and pulmonary arterial pressures and provide for fluid transfusion As illustrated in Fig 1 the two ports aid in confirming proper catheter positioning (see Fig 2 for picturesof an assembled catheter)

280 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-27 NO 6 NOVEMBER 1980

Micro-coax O-rings

Fig 3 Pressurizable catheter connector Cross sectional view of elec-trical connector and balloon inflation inlet Inflation chamber is sealed with O-rings in several places and with the soldering around the microcoax shield that is fed through printed circuit board Note Loop in microcoax allows catheter flexing without strain on the coax connection Subminiature coax socket 51565-1 manufactured by American PamCor Inc Valley Forge PA

(b)

Fig 2 (a) Tip of the ultrasonic catheter with the balloon inflated (b) Total catheter Catheter is 12 mm in length and has 2-mm OD Note Balloon inflation connection lumen is via the electrical connector

Seven microcoax wires connect the seven transducers to the electrical connector (UT-8S Uniform Tubes Inc 200 W 7th Ave Collegeville PA 19426) Each coax 02 mm in diameter with a characteristic impedance of SO s1 consists of a 0005-mm ID copper wire a surrounding Teflon insulator of 01 mm diameter and an outer solid-conducting silver cylinder (Note Although we have used copper silver outer conductors are more flexible and retain their original shape more readily) These seven cables are the primary determinant of flexibility of the catheter Using the coax instead of single wires mini- mizes electrical mutual couplingof energy among array elements by connecting the shields to the transceivers common connection also the coax provides a more consistent charac-teristic impedance than would wires randomly routed down the catheter The latter is important for electrical matching purposes

The catheter connector assembly provides a disengageable electrical connection between the tiny microcoaxin the cath-eter and the larger RG-174 coax linked to the transceiver Disengagement is necessary for catheter cleaning and a number of other practical reasons The micro-coax cables are connected in a manner to permit slack minimizing strain in the wires and terminations when the catheter is flexed impor-tant in preventing connection failures These wires share the same lumen used as an air passages for inflating the balloon therefore the connector assembly includes a leakproof inter-connecting chamber and pressure connection (See Fig 3 for a cross sectional view of the connector) Because the micro-coax is small and delicate the use of several sections in the connector allows assembly steps which avoid the production of excessive strain in the wires or their connections

(b)

Fig 4 Magnified views of assembled tip (a) Note catheter-to-wall d ~ s -tance detection transducers (b) Note velocity transducer at tip

Tip Assembly (Photographsof a completed tip appear in Fig 4 an exploded

view of the tip appears in Fig 5(a)) Three separate inserts are used in the tip assembly Insert A provides a rigid and precise base for linking insert R to the flexible catheter body Insert B is a mounting fixture for the array of transducers used for detecting the catheter to vessel wall distances Insert Cis a mounting fixture for the blood velocity detection transducer These inserts are used instead of a single piece in order to facilitate soldering alignment fabrication inspection and overall convenience in assembly Although molding could be used each insert presently is individually milled using highly precise machmes Photographs of a set of finished inserts appear in Fig S(b) and (c)

Insert B the most complicated of the three inserts is milled so as to provide a hexagonal support for mounting the ele-ments of the six-transducer array This hexagonal cut provides for alignment Transducers transceive nominally at angles of 60 around the circumference of the catheter (see Figs 1 and 5) In addition each mounting support is milled to provide spacing behind each transducer (Fig S(a)) Spacing 1) allows

MARTIN AND WATKINS INTRAVASCULAR OF BLOOD 281MEASUREMENT FLOW

One 01 SIX catheter to wall dlstance measurement transducers

Mlcro-coax A B C

2mm bore soldered to Transducer Insert4 4

Catheter body TIP Inserts (brass)(polyvmyl clorlde)

(C)

Fig 5 Tip assembly is composed of three separate inserts to allow assembly in a series of steps Composed of brass the inserts provide common electrical connection for one side of ultrasonic transducers Microcoax has 02-mm OD Of seven used two appear for illustra- tion purposes Transducers are air backed by milling a space in insert behind the transducer (a) Exploded view of the catheter tip (b) Brass inserts disassembled (c) Assembled

the transducers tobe backed with a low acoustic impedance material thereby enhancing front-face transception and pro-viding acoustic isolation among array elements and2) insures that transducers are supported onlynear their edges Because the elements are designed with acoustically inactive edges the effective area of the transducer is almost entirely free from damping Although fixture damping increases the fractional bandwidth fb it decreases the transfer of the transducer con-siderably [l l ] Diagonal holes drilled into the insert (Fig 5) allow the microcoax to pass from the central passage to each transducer element The angle facilitates the introduction of the coax

All six transducer elements are soldered to the insert simul-taneously by 1) coating the contact area with a soldering cream (Multicore Solders Westburg NY Type XM 2733) 2) fixing each element to the mounting support with a temporary fix-ture 3) heating the total insert to a temperature at which the soldering cream flows freely and4) allowing the fixture to cool thus leaving intact the solder connection The outer con-ductors of the six microcoaxcables are soldered with soldering

cream to the insert at the same time This soldering action pro-vides the connection between the outer conductorsof the coax and the back plating of the transducers via the brass insert This connection also provides mechanical support be-tween the microcoax and the insert which relieves strain from the electrical connection of the center conductor (Later the center conductor is attached to the frontface of the transducer to complete the circuit using a conductive epoxy to avoid re- heating the element) This support protects from strain during assembly and later during normal catheter flexing

It is worth noting that all surfaces to be soldered or epoxied are carefully cleaned Each part is immersed in a solution of acetone andor copper cleaner (the copper cleaner is made by Multicore Solders) and suspended in a beaker in an ultrasonic cleaner for 5 min After soldering and testing we inject liquid Styrofoam behind each transducer element Once dried styro-foam has an impedance similar to that of air thus the trans-ducer is backed with a low impedance material which later prevents epoxy from flowing behind the element during the final coating of the tip

Insert C has three cylindrical milled recesses The outer recess allows the front face of the velocity transducer t o be mounted flush with the edge of the outer rim of the insert the outer rim protects the element and provides a rigid structure for fixing it to the insert The back face of the element is soldered to the insert along the second rim bordering the second recess Since the second recess provides a space behind the transducer most of itcan be air backed The third recess provides a passage for the microcoax Once the outer conduc-tor of the microcoax is soldered to it the electric connection is completed between the transducer back face and the coax via the insert This action also mechanically fixes the coax to the insert and relieves strain on the connection of the inner conductor to the transducer In order t o provide this later connection the inner conductor is routed with a section of electrical insulation left on it through a channel milled as indicated in Fig 5(a) The conductor is epoxied (conducive epoxy 410 Epo-Tek Watertown MA) or soldered to the front face plating Soldering the outer conductor of the microcoax and the back face of the transducer is usually done simultaneously using the soldering cream as discussed previously

Final assembly requires several steps After all transducer elements have been secured to the inserts and electrically tested insert B is engaged to insert A and glued in place (Per-matex lock nut compound) similarly insert Cis secured to insert B Next the assembly receives a thin coat of epoxy to protect and electrically isolate it from the blood We use a two-part epoxy (Scotchcast Electrical Resin 3M Company) which has a low viscosity when heated shortly after mixing The low viscosity insures a thin and smooth coat thickness over the transducer elements is approximately 0025 mm-005 mm If necessary we buff the assembly further to re-move thick deposits In the future we hope to incorporate heparin doping into the epoxy to produce asurface which is free from blood coagulation problems Finally we check for electrical isolation by submerging the tip in a liquid conducting media and testing for leakage between the coax and the solution

282 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-27NO 6 NOVEMBER 1980

~ONDUCTIVE$LATINO

Fig 6 Transducer element for measuring catheter-to-wall distance One of elements used in radial array Material PZTS Element operates in thickness mode at frequency of 147 MHzNote Plating par-tially covers both faces overlapping region constitutes active area of transducer

Transducers The transducer element used in the array meets several de-

sign requirements (Fig 6) The operating frequencyof 147 MHi provides favorable scattering from the vessel wall and insures a radiation angle that remains fairly narrow (4at - 3 -dB points) for the small width of the element The width of the element is closely limited to the maximum value shown in order for six elements to be located on a 2-mm OD catheter However since the minimum number of unknowns that canbe used to describe an ellipse with offset and rotated axes is five a minimum of five transducer elements could be used to ob-tain the five measurements describing the ellipse A sixth transducer element is added to reduce uncertainty in wall lo-cation which may result for example froma tilted catheter tip within the vessel

We have obtained uniquely several important transducer fea-tures by partially removing the plating from the transducer faces as illustrated in Fig 6 This method provides a smaller radiating surface for decreasing the longitudinal beamwidth an active radiating surface free from any surface distortions due to the electrical connection and an active acoustic area free from damping due to the mounting fixture Electro-acoustic transduction occurs primarily in the region between the overlapping plating There is some minor spreading beyond the region due to electrical field fringing and other sec-ondary effects Further since the electrical activity is confined to the central region of the element the unavoidable mechanical mass of the electrical connections are near the edge and therefore produce no distortionor damping to the radiation field Certainly if the total area of the element were active inherent distortions would result Although in a large transducer such distortions may be insignificant in our small transducer such distortions are important Similarly supporting the transducer at the edges avoids introducing damping effects to the active area of the transducer The elements we use are cut from a 1 -cm ceramic disk with a diamond cutting tool using the milling machine to be mentioned later

Our velocity transducer is a disk of 185-mm diameter of PZT5 material It also operates at 147 MHz a convenient

frequency since it is already used in the system for the array and it is fairly optimal backscatter frequency for blood at a distance of 1 cm [l21 The velocity transducer is time multi-plexed with the scanning of the array We use a pulsed Doppler mode of operation with a range gate set to sample a 4-5-mm distance from the transducer The pulsed mode permits use of the same transducer for transmission and reception without any special separating circuitry as would be necessary in continuous wave operation for a single transceiving element [l 31 ~ 4 1

PERFORMANCEAND SPECIFICATIONS This catheter is durable the wires withstand considerable

flexing (but not sharp bends) The actual flexibility of the assembled catheter is such that when a weight of 20 g is hung 1 cm from the tip of the catheter (with it supported 35 cm from the tip) the catheter bends at anangle of 30 from hori-zontal (temperature = 37OC) When the weight is removed the catheter recovers to within 4 of its original horizontal position For the purposes of comparison a regular Swan- Ganz catheter (Edwards Laboratory 23805) subjected to the same test bent to an angle of 39 and recovered to within 8 of horizontal

Radiation patterns from one of thecircumferential array elements appear in Fig 7 These patterns were obtained by moving the transducer so that a glass spherical target 03 mm in diameter passed through the radiation field The levels of echoes from the target were then recorded and plotted Fig 7(a) shows the longitudinal patterns to be fairly well confined to the active regions of the overlapping plating of the trans-ducer At 1 cm distance the 6-dB beamwidths are02 mm for the longitudinal direction and 14 mm for the lateral direction Fig 8 shows the radiation pattern at 1 cm distance as recorded for the array and the velocity transducer The higher sidelobe measured was 20 dB below themain lobe with the average of -27 dB k 3 dB The angles of radiation between the main lobes are close to 60 2 3 except for the lobe shown at0 (Fig 8) which is t6and -5 from the main lobes on each side (The shift of at least 5 from the desired 60 in this case is the result of misalignment during the substitution ofa second transducer for the original element fractured accidentally In future cath-eters we feel all units will be within k3 of 60) Certainly the radiation pattern for the velocity transducer has a very narrow beam (-6 dB width is 3 wide) with no detectable side lobes

The signal transfer obtained from a flat target at several distances in water is shown in Fig 9 Transfer was calculated by dividing the voltage received at the receiver input by half the open circuit voltage of the transmitter (This procedure nor-malizes the transfer calculations with respect to an optimal transmitter match) A passive matching network was included between the transceiver and the catheter for these measure-ments The signal transfer was approximately 10 dB higher for the velocity transducer possibly because the larger area of the element provided less impedance mismatch between it and the coax Consequently less power was lost in the microcoax

The 3-dB bandwidthof the transducer-catheter system without matching was tested by connectinga 50-2 generator di-

2 83MARTIN AND WATKINS INTRAVASCULAR MEASUREMENT OF BLOOD FLOW

Longitudinal distance

+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance

-from the transducer face

(a)

-6001 Transducer width (6rnm)

I2omm mm

dostance

(b) Fig 7 Contour plots of radiation patterns of one array element (a) Longitudinal (b) Lateral Measurement was obtained

by moving 03-mm diameter glass sphere through field and recording echo amplitude Transducer excitation was tone burst of three cycles at 147 MHz

i-

2 84 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-27NO 6 NOVEMBER 1980

t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~

4 5 - Transducer lo ielleclor ogtlance Itm1

Fig 9 Signal transfer versus distance to a flat reflector obtz lined from flat polished brass target at various distances from transduc er elements in catheter tip

Fig 10 Signal processed dog pulmonary artery wall echoes recorded withwith our intravascular catheter (Fig 2) Each trace represents echoes from one of six radially transceiving elements (Fig 1) with vertical separation introduced for display purposes Horizontal axis distance from catheter (25 mmmajor division) The signals were recorded from our processor output where preprocessing included full wave rectification ensemble integration and a form of matched filtering [ 161 Brightened portion of trace indicates point at which processor detected these echoes Effect of ensemble integration at increasing the amplitude of the wall echo with each new ensemble addition is apparent in several channels Matched filtering produced smoothing and enhancement of signals of large enough duration to be wall echo

I

0 -

ig 11 ECG and several physiological variables measured from dogs pulmonary artery (PA) using catheter of Fig 2 our transceiver system and our on-line processing system Greater motion artifact oc- curred in PA pressure waveform than usually encountered We believe this was Primarily due to catheter lateral whip Blood velocity flowand stroke volume measurement were influenced little by this motion Respiratory effects on velocity and flow are illustrated over several heart beats

t

at

rectly to the catheter connector The frequencyof a tone burst was varied and the amplitudeof the echo from the flatreflec-tor was recorded The electrical driving point impedance was also measured at the connector The 3-dB fraction bandwidth for a representative array element was found to be 01 1 (The electrical matching used in the system improves the bandwidth to approximately 02) The average standard deviation of the driving point electrical impedance over this bandwidth mea-sured at the connector was magnitude = 335 43 S2 phase = -297 182

We measured mutual couplings of energy between the seven individual transducer systems in the catheter by 1) positioning a flat reflector in front of one transducer element and 2 ) mea-suring the signal level from this target present at each transducers particular receiver This measurement was performed during the respective listening period of each receiver (ie time immediately after that transducer had been activated by its respective transmitter) Then the ratio of signal amplitudes between the transducer located in front of thetarget and the other transducers were computed Mutual coupling was on the average of -28 dB between the array elements and -20 dB between an array element and the velocity element

The catheter shown in Fig 2 has been used in over four dogs o date In each case clear and distinct vessel wall echoes were

obtained A representative example is shown in Fig 10 In ddition good backscattering from blood cells appeared via he velocity transducer Velocity flow stroke volume and

other physiological parameters from an experiment using the catheter is shown in Fig 1l Using this and other identical catheters a series of intensive testing with a number of dogs is planned in the near future evaluate the accuracy of our system

DISCUSSION The catheter design reported here i s a marked improvement

over the earlier units However some areas need further re-fining For example we still have some difficulty in positioning the catheter tip in the pulmonary artery The difficulty arises in passing the tip from the right ventricle into the pul-monary artery In order to negotiate this passage the tip must take a turnof approximately 160 after entering the ventricu-lar chamber from the right atrium Too often the tip after encountering the far wall of the ventricle during insertion turns toward the apex of the ventricle instead of toward the pulmonary artery In a standard Swan-Ganz catheter the balloon is located adjacent to the tip(our location is more proximal) and the flow tends to direct the tip more easily in the desired direction Also since it approximates a sphere at the tip in the standard catheter theballoon is more easily directed by the wall In our caselocating the balloon at the tip

- - -

- - - -

MARTIN AND INTRAVASCULAR MEASUREMENT OF BLOOD FLOWWATKINS 285

would interfere with our measurementshowever we are presently exploring several possibilities for improving the catheter in this regard

Further we are considering the advantages of a new ultrasonic frequency for the circumferential array The amplitude of the wall echoes varies a great deal (as large as SO dB in some cases) due to the range of distances between the catheter and the wall and the angles that sound may impinge on the wall [1S] This wide range results from the various positions and angles in the vessel that the catheter can assume In other

ment Other workers in this and related fields should find similar assembly stations useful

Although some problems remain we have made progress towards a practical and useful clinical tool We hope in the near future to prove our design thoroughly accurate and then we will investigate the human usability of the device

APPENDIX FURTHER ERROR CONSIDERATIONSAND RESOLUTION

Error words the longer the distance the more the blood absorption Flow (Q) is expressed by the following equation Q = A V the larger the angle the less backscattering from the wall Another complicating factor is that scattering from the blood appears to be higher during systole just distal to the valve than in other locations This result may be due to scattering from boundary layers or turbulent regions This scattering some-times has the false appearance of echoes originating from the wall In any event we expect that a lower frequency would offer less blood absorption and scattering howeverless wall backscattering and a wider radiation angle would also result We are uncertain at what frequency these counteracting fac-tors are optimal Clearly geometry dynamics and differences between intact and removed tissue make it difficult to assess the in vivo situation by in vitro means However an i n vivo analysis is equally difficult since it would require a special multifrequency catheter in order to make a meaningful frequency comparison

As indicated by the calculations in Table I reduction in the lateral width of radiation beam could improve accuracy con- siderably when large angles of tilt or rotation are present For example the reduction of the 6-dB lateral width from14 to 08 are shown in Table I Possibilities for improvement in this regard are the use of a larger catheter allowing larger element width the use of only five elements instead of six or the use of some type offocusing

The technique we have described for plating a rectangular element should be of utility in other types of geometrics and arrays For example in disks plating may be deposited or removed on the opposing faces so the overlapping region is free from damping of the mounting assembly andor distur-bances from the electrical connection Although we have not yet employed this idea in our velocity unit this method remains a future consideration In linear arrays the method can also be used to produce aperture shading by varying the area of the overlapping plating on each element across the array

Our catheter fabrication ability has been enhanced im-mensely by the developmentof special alignment fixtures and a microassembly station In this station we have incor- porated a microprecision mill (7406 Servo Products Co Altadena CA) a microscope (Bausch and Lomb Sterozoom S) micromanipulators and probes (US-l Nanishaga Scien- tific Instruments Tokyo Japan) andMTK Series Probes (Cir- con CorpSanta Barbara CA) local heating devices (Heat Torch Master Appliance Co Madison WI) and a miniature tilting indexing table (800 Master-craft Engineering Co Riverside CA) These tools proved invaluable for the special- ized work required for fabricating miniature cylindrical assem- blies and have increased reproducibility precision and align-

where A is the sectional area of the vessel (enscribed area of a plane which transects the vessel at any angle) and V is the velocity of the blood flow If the velocity component measured is not normal to the plane in which the area is measured error arises which is equal to the cosine of the angle of this depar- ture The fractional expression of error resulting from indi-vidual components of the measurement are expressed by the following equation

AQ - AV AA(AA)(AV)t-+-- (11Q V A A V

where AQ AV and AA are the amountof errors in the mea-surement of QA and V respectively

Insight into the error in the measurement of area may be obtained by considering a cross section through the vessel In this case area with error can be expressed as AA tA = n(AR t R)rsquo where the radius is R and AR is the error in the measure-ment of the radius The fractional measurement error of the area then is expressed

AA R(AR +R)lsquo - RRrsquo A RR

which simplifies to

AA - 2AR ARrsquo+- ( 2 )A R Rrsquo lsquo

When the catheter is located at the center and aligned with the axis of the vessel the radius R is determined from a single wall echo transit time measurement by R = CT2 where C i s the velocity of sound and T i s the transit time Then the frac-tional error in radius can be expressed as

AR ATAC (AC) AT t-t--R C T C T

Resolution The lateral and longitudinal resolution of an ultrasonic

beam becomes a factor in the range resolution when the beam impinges on a ldquoroughrdquo scattering surface nonperpendicularly due to time spreading of the burstof energy For simplicity assume a propagating rectangular beam composed of a single plane wave with lateral width equal to Wand longitudinal width equal t o W where w 1 is distance OB =FA and w 2is distance BA = OF Fig 12(a) The scattering surface encountered by the beam represents the Y-Z plane The angles 0 and 4 describe the tilt and rotation of the beam withrespect to this plane The distance along the column represents range distance from the transceiver The range increment over which

286 IEEE TRANSACTIONS ON SONICSAND ULTRASONICS VOL SU-27 NO 6 NOVEMBER 1980

Substituting (7) into (g) (8) into (S) and (5) into (4) gives the t lsquo rsquo

Z

I

(b)

Fig 12 (a) Rectangular column rotated by angle 0 with respect to Z-Y plane and tilted by angle 0 with respect to Z axis alongZ-W plane formed by surface ABUF (bj End view of rectangular column Dashed lines represent rectangle ABUF before rotation by the angle 4 Note W axis lies in X-Y plane

relationship for range increment as

scattering from the Y-2surface will occur due to impinge-ment of a plane wave front ABOF is represented by the distance CA That is scattering will begin as the first wavefront of a burst of energy encounters the surface at point 0and it will continue scattering from the first wavefront until the point in the wavefront which was initially furthest from the surface pointA reaches the surface at point C Hence the total uncertainty as to where the surface is located due to this factor only( A R ) is equal tof distance CA divided by two

In order to derive the relations between AR and W w 2 0 and Jit is helpful to observe from Fig 12(a) that distances DE = CA and that the angles OEA and OED are right angles Therefore

DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where

W1a = tan-rsquo - W 2

ACKNOWLEDGMENT We thank James B Phillips who made many contributions in

earlier versions of the catheter design described here Through some of that early work we were able to identify trouble areas and avoid them in this design We also thank Gerald H Pollack for his helpful consultation on the project and David Frsquohillips for his helpful suggestions on the manuscript

REFERENCES

AA Afifi et alldquoPrognostic indexes in acute myocardial infarc-tion complicated by shockrdquoAmer J Cardiol vol 33 pp 826-8321974 F Wiberg-Jorgensen ldquoHemodynamic measurements related to mitral valve replacementrdquo Vascular Surgery vol 6 pp 43-54 1972 R W Martin G H Pollack and J B Phillips ldquoStroke volume measurement with an ultrasonic catheter tip systemrdquo in Ultra-sound in Medicine vol 3A D White and R E Beacon Eds New York Plenum 1977pp 23-39 P 0Daily R B Griepp and N E Shurnway ldquoPercutaneous internal jugular vein cannulationrdquo Arch Surg vol 101 pp 534-536 1970 H J C Swan et alldquoCatheterization of the heart in man with the use of a flow-directed balloon tipped catheterrdquo New England J Medvol 283pp447-451 1970 H Smulyan ldquoReliability of the indicator-dilution techniquerdquo Amer Heart J vol 62 pp 140-141 1961 H Smulyan R P Cuddy and R Eich ldquoAn evaluation of indi-cator-dilution technique in the dogrdquoJ Appl Physiol vol 17 pp 729-7341962 S R Reuben J P Swadling and G de J Lee ldquoVelocity pro-files in the main pulmonary artery of dogs and man measured with a thin-film resistance aneometerrdquo Circ Res vol 27 pp 994-10011970 R W Martin ldquoUltrasonic system for measurement of vessel cross-sectionrdquo PhD dissertation Univ of Washington Seattle 1975 pp 115-1 16 Available from Univ Microfilm 300 No Zeeb Rd Ann Arbor MI P N T Wells ldquoThe possibility of harmful biological effects in ultrasonic diagnosisrdquo in Cardiovascular Applications in Ultra- sound R S RenemanEdLondonNorth-Holland 1974 ch 1 p 13 -Physical Principles of Ultrasonic Diagnosis London Aca- demic 1969 p 48 F D McLeod ldquoMultichannel pulse Doppler techniquesrdquo ch 7 in Cardiovascular Applications in Ultrasound R S Reneman Ed London North-Holland 1974 ch 7 p 96 J M Reid D L Davis H J Rickets and M P Spencer ldquoA new Doppler flow meter system and its operation with catheter mounted transducersrdquo in Cardiovascular Applications ofUltra-sound R S Reneman Ed London North-Holland 1974 p 189 E M Poore ldquo1 mm catheter tip Doppler probe using a single crystal and bridgerdquo Ultrasonic Zmaging vol l pp 101-103 Jan 1979 RWMartin G H Pollack and J Phillips ldquoAn ultrasonic cath-eter tip instrument for measuring volume blood flowrdquo inhoc 1975 IEEE Ultrasonic Symp IEEE 75CH0994-4SU pp 13-16 R W Martin G H Pollack J B Phillips and D W Watkins ldquoSignal enhancement for automatic identificationof arterial wall echoes from an intravessel scannerrdquo in Ultrasound in Medicine vol 4 D White and E A Lyons Eds New York Plenum 1978 p 420

278 IEEETRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-27 NO 6NOVEMBER 1980

into a vein by percutaneous puncture [4] (to avoid trauma and morbidity associated with surgically exposing the vessel) but large enough to allow pressure measurement and fluid transfusion with ease The largest available catheter ldquointro-ducerrdquo kit of which we are aware accepts a 3-mm outside di-ameter (OD) catheter (USCI OOSS9) A l-mm inner diameter (ID) catheter or equivalent area is generally acceptable for pressure monitoring and fluid movement (The flow resistance for blood for a l-mm ID Teflon catheter is approximately 01 5 mm Hg mincm3 per 1 -cm length ) A catheter length of at least l-m is necessary to catheterize the pulmonary artery from a vein in the extremities

The flexibility and form of the catheter are very important considerations in positioning the catheter tip at the desired point Since the tip must be located in the pulmonary artery in order to measure right heart volume blood flow the posi-tioning and verification of this position are crucial We employ a technique called the Swan-Ganz method [S] which has gained wide clinical acceptance for pulmonary artery cathe-terizations This method employs a flexible catheter with a balloon located near the tip The balloon is partially inflated during insertion and the flow tends to direct the tip into and through the heart to the out-flow tractWith the commercial Swan-Ganz catheters monitoring thewaveform of the pres-sures during catheter insertion usually provides information enough to guide the positioning of the tip in the pulmonary artery and fluoroscope is not required

Measurement Accuracy The second consideration is the accuracy of flow measure-

ment Although present intermittent measurement techniques are no more accurate than +15-20 percentthis information is of use clinically [ 6 ] [7] We hope to achieve accuracies in the range of + l0 percent or better An exhaustive treatment of all the errors and potential errorsis beyond the scope of this paper but major errorsof importance are highlighted

The technique discussed in Fig 1 assumes the velocity profile to be constant at each point across the vessel Deviations from such a flat profile will automatically introduce errorsof which the magnitude is dependent upon catheter radial position and the amount of deviation Justdistal to the pulmonary valve (within 3 cm) the profiles have been found to be flat [S] however the amount of deviation which occurs in the population at large is yet unknown

The flow measurement method is theoretically insensitive to catheter tilt since the area increases in proportion to the amount that the velocity decreases due to the tilt [3] Conse-quently the product remains constant Errors due to motion or catheter translation have been found to have little effect on the measurementof stroke volume and cardiac output however these errors are a consideration when the waveform of the flow is of interest

The maximum physiological values expected for an adult human are as follows a pulmonary artery ID of 3 cm a wall thickness of 2 mm blood velocities of less than 200 cms and vessel cross sectional areas equal to 7 cmz The magnitude of flow is expected to be no greater than 10 lmin

Errors in the measurement of area and velocity produce a total error which is primarily the sumof each error (see (1) in the Appendix) If errors are opposite in sign they tend to cancel if errors are of the same sign they add and increase Errors in area determination are approximately twice the error of a single radius measurement (see (2) in the Appendix) However in practice since area is not calculated from a single radius errors from all six array elements tend to average to one smaller than that for a single radial determination There-fore error should not be as severe as (2) indicates Further some errors due to alignment or other reasons can be corrected for in the data processing if they are identified

Additional sources of error associated with the measurement of catheter-to-wall distances can arise from echo artifact and false echo detection The presence of false echoes due to side-lobes or mutual coupling of energy among array elementsor connecting leads can contribute to artificial detection as well as a low signal-to-noise ratio (SNR) Mutual coupling of en-ergy is particularly detrimental

For example a strong specular reflecting structure may be located in front of transducer A with the angle of incidence equal to zero whereas received echoes from transducerB may be weaker due to encountering structure at angles of incidence not equal to zero and due only to diffuse scattering Because of mutual coupling spurious echoes from transducerA then may appear in channel B comparable in magnitude to the echoes received from transducer3 This action gives the false impression of structure location in the radiation field of trans-ducer B A measure of mutual coupling is provided by deter-mining the ratio of the received energy in B with respect to the received energy A when A is activated Sidelobe transception has a similar effect Both forms of interference should be less than -20 dB but even -20 dB may not be adequatein some instances Certainly the greatest difficulty occurs in this regard when the ultrasonic catheter is close to the wall of the vessel

Weak wall echoes can result when the sound impinges on the wall at large angles of incidence and the sound must travel an increased distance through the blood This condition is primarily a function of the size of the vessel the distance the catheter tip is located from the center of thevessel and the angle that the catheter is tilted with respect to the axis of the vessel Since these variables are not easily controlled low signal levels can be frequently encountered This occurrence results in a reduced SNR which can affect distance determination accuracy To minimize the effect of this action trans-ducers must be designed for maximum sensitivity and receivers for a low noise figure Further ability to adjust rotateor re-strict the angle of the catheter with respect to the axis of the vessel andor its distance from the axis would also be most helpful

Care should be taken in aligning the transducers so their beam patterns fall on a plane perpendicular to the axisof the catheter Alignment errors can result in errors in measurement of the vessel lumen area

Echo-ranging errors will be affected by the spacial resolution of the transducer system Spacial resolution involves both lat-




2 cos a


CA R = -

2f0 fb











48 l 0

2 6 z x 0

6 4 l 3 3 2 0 20

4 2 3 3 20 20

7 l 5 5 20 2 0

5 2 5 5 2 0 20

106 l 5 5 40 40

85 2 S 5 40 4 0

6 5 10 5 5 40 4 0

18 l 2 8 20 20

10 l 2 8 35 35

8 2 2 8 35 35

10 2 2 8 45 45

10 l 2 14 20 20

10 2 2 14 30 30

105 10 2 14 40 40

10 2 2 14 4 3 0

x = any value






Micro-coax O-rings


(b)




(b)







Mlcro-coax A B C



(C)









~ONDUCTIVE$LATINO














+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES





2 cos a


CA R = -

2f0 fb











48 l 0

2 6 z x 0

6 4 l 3 3 2 0 20

4 2 3 3 20 20

7 l 5 5 20 2 0

5 2 5 5 2 0 20

106 l 5 5 40 40

85 2 S 5 40 4 0

6 5 10 5 5 40 4 0

18 l 2 8 20 20

10 l 2 8 35 35

8 2 2 8 35 35

10 2 2 8 45 45

10 l 2 14 20 20

10 2 2 14 30 30

105 10 2 14 40 40

10 2 2 14 4 3 0

x = any value






Micro-coax O-rings


(b)




(b)







Mlcro-coax A B C



(C)









~ONDUCTIVE$LATINO














+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES



Micro-coax O-rings


(b)




(b)







Mlcro-coax A B C



(C)









~ONDUCTIVE$LATINO














+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES




Mlcro-coax A B C



(C)









~ONDUCTIVE$LATINO














+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES



~ONDUCTIVE$LATINO














+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES




+lmm]

+05mm-

Active section

Omm- of transducer

-05mm-

lmm1i ~---2 0 D g 7

1Omm 20mm 30mm

Dlstance


(a)


I2omm mm

dostance



i-


t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES



t

I 60- ~-

1~~

2 -~ ~ ~~

3 ~~




I

0 -


t

at








- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES


- - -

- - - -
















which simplifies to








Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES




Z

I

(b)





DEA R = +-

2

DE = OE tan 0 (5)

and from Fig 12(b)

+W tan 0 (cos (a- 0))AR1 =

2 cos ff where




REFERENCES


Documents

An Ultrasonic Catheter for Intravascular Measurement of Blood Flow…ieee-uffc.org/wp-content/uploads/2017/02/t8060277.pdf · 2018-03-07 · IEEE TRANSACTIONS ON SONICS AND ULTRASONICS,