Description CHAREALEDS LANCEAE, · PDF filetransducer may be recorded in the conventional time-motion display, ... and a block diagram is shown in figure 2. All ... backing slab which

Multiscan Echocardiography

I. Technical Description

By NICOLAS BOM, PH.D., CHAREALEDS T. LANCEAE, I., CERD VN ZWIETEN,

FRANx E. KLOSTER, M.D., AND Jos ROELANDT, M.D.

SUMMARYThe principle of a prototype multiple element echocardiographic instrument is described. The

major goal of this system is instantaneous visualization of moving cross-sections of the heart.As a result a direct impression of cardiac anatomy and dimensions may be obtained. The trans-ducer contains 20 small acoustic elements positioned in a row. System characteristics and trans-ducer details are given. A flexible five lever time-gain compensation unit which permits independentgain setting for specific depth ranges is described. Recording modes include video tape, photo-graphic, film and line scan techniques. The signal from any selected element of the Multiscantransducer may be recorded in the conventional time-motion display, thus combining Multiscancapabilities with conventional echocardiographic techniques.

Additional Indexing Words:Ultrasound Scanning technique

SINGLE ELEMENT echocardiography is nowgenerally accepted as a useful clinical tool.

However, since only echoes from structures "seen"by the narrow sound beam are obtained, simulta-neous infornation about adjacent cardiac structuresis not available. This has led to difficulties inrecognizing the echo origin and limits the capabili-ties of such systems. Therefore, the need exists fornew techniques which allow easier aiming of thetransducer as well as improved orientation and echoidentification.

Visualization of moving cross-sections of theheart should overcome several of the drawbacks ofsingle element echocardiography. However, sincethe heart is a moving structure, the construction ofa real time two dimensional echo image poses adifficult technical problem. Various mirror systemshave been developed as initial attempts. A recentpublication by Hertz and Lindstroml describes twodimensional visualization of the heart at 16

From the Thoraxcenter, Medical Faculty and UniversityHospital, Erasmus University, Rotterdam, The Nether-lands.

This work was completed while Dr. Kloster was recipientof a Special Research Fellowship Award of the NationalHeart and Lung Institute, 1 F03 HE52660-01.

Address for reprints: Dr. ir. N. Bom, Department ofExperimental Echocardiography, P. 0. Box 1738, ErasmusUniversity, Rotterdam, The Netherlands.

Received May 7, 1973; revision accepted for publicationJune 27, 1973.

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frames/sec. The same image rate was obtained witha mirror system developed by Patzold et al.2 for usein obstetrics. A limited scanning rate and bulkyprobes proved to be some of the problemsassociated with these systems.The first description of an application of element

phasing with variable delay lines and with elementgrouping to "steer" the acoustic beam was given bySomer.3 His system was primarily designed for usein neurology and requires a high degree ofelectronic sophistication.With the use of a large number of elements

positioned in a row and functioning one at a time inrapid sequence, it should be possible to obtaininstantaneous cross-sectional visualization of theheart in a relatively simple way. Rapid electronicswitching allows a high frame rate and thetransducer can remain small so that aiming is lessdifficult. A comparable system has been suggestedfor use in ophthalmology by Buschmann.4 Hedescribed a ten element system wherein theindividual elements are mounted in an arc to matchthe shape of the eyeball. Uchida, Hagiwara andIrie5 described a system based on 200 closelyinterspaced transducers. Electronic switching anduse of overlapping groups of 20 small elementsyielded two dimensional echograms at a rate of 17frames/sec. No clinical evaluation of these twosystems has been presented in the literature todate.

Circulation, Volume XLVIII, November 1973

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MULTISCAN ECHOCARDIOGRAPHY

transducer piezo-electricelements

echo transmissiondirec

FigureSchematic drawing of a multi-element transducer andpermission of Bio-Medical Engineering).

The Multiscan SystemThe multiscan principle is best summarized as n-

parallel single element brightness modulated lines,or "B-modes," activated at almost the same instant.The multi-element transducer principle, which hasbeen described previously,6' 7 is shown in figure 1,and a block diagram is shown in figure 2. Allacoustic elements have a separate excitation stageand reception circuit. Each element in sequencetransmits a short acoustic pulse into the tissues andreceives the returning echoes. Echoes are displayedon an oscilloscope. The depth of structures isindicated on the horizontal axis, while the verticalposition of each line corresponds to the position ofthe respective element in the transducer. Fastelectronic scanning results in a repetition rate ofapproximately 150 frames/ sec.

A picture of the prototype apparatus is shown infigure 3. The dimensions of the present prototypecabinet are 42 x 45 x 17 cm and its weight is 15kg.

In figure 4 the system is shown in an in vitrodemonstration. The transducer was positionedbefore a cylinder gauze suspended in a water tank.The circular cross-sectional echogram can berecognized readily on the display.

System CharacteristicsDisplayAn echogram from one frame of a 16 mm motion

picture obtained in vivo is shown in figure 5. Theechogram represents a cross-sectional image of


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:ton line height correspondingto element position

echo return time

oscilloscope display

corresponding oscilloscope display (reproduced by

8 X 16 cm. The transducer is held in the oblique

position on the precordium, as described in clinical

reports.8,' The case number, date of investigation

and calibration scale (for video recording) are

presented at the top. These data may be selected

using the thumbwheels shown on the frontpanel in

figure 3 (upper right corner). The electrocardio-

gram during the preceding three seconds, is

displayed at the bottom, with the timing of the

echogram frame in the cardiac cycle indicated by

the right hand end of the tracing. These data have

been incorporated to facilitate later study of video

or film recordings.

Time Diagram

The time sequence of a Multiscan frame is shown

in figure 6. The duration of a complete echogram

frame is 6560 gsec. This corresponds to a repetition

rate of approximately 150 frames/sec. Excitation of

element 1, and the transmission of the soundwave is

followed by a reception period of 200 psec.

Element 1 then becomes silent and after a 30 gsectotally inactive period the same sequence is

repeated by elements 2, 3, etc. Upon completion of

the activation of all 20 echolines, the electrocardio-

gram is sampled and displayed during 460 jusec.This is followed by a 1500 ,gsec period during

which the identification characters are displayed.

Transducer

The overall size of the transducer is dictated by

cardiac geometry and also by the need for easy



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BOM ET AL.

controls

Figure 2

Block diagram of a 20-element system.

handling. This has resulted in a transducer 8 cm inlength and 1 cm in width. The number of elements

on the linear array has been chosen as acompromise between optimal resolution near the

Figure 3

Multiscan cabinet, display monitor and transducer (reproduced by permission of Organon Teknika, Inc.).Circulation, Volume XLVIII November 1973

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Figure 4

In vitro demonstration of transducer and a cylindrical targetin a water tank with the resulting echogram. The truediameter of the cylinder is 3.5 cm.

posterior wall and the line density on the display.Increasing the number of elements would decreasethe single element width and thereby degradelateral resolution, especially near the posterior wall.As a compromise 20 elements were chosen, eachwith a length of 1 cm and a width of 4 mm.The transducer is shown in figure 7. The

transducer frequencies are the same as used inconventional echocardiography, 2.25 MHz and 4.5MHz. The apparatus automatically switches to thecorresponding frequency when the transducer isconnected to the instrument.The transducers were produced by bonding a

piezo-electric ceramic plate (P1-60) onto a 4 cm

backing slab which consists of 92% tungsten and 8%araldite (percentage by weight). This backingshortens the acoustic impulse necessary for gooddepth resolution. Further pulse shortening is

Circulation, Volume XLVIlI, November 1973

Figure 5

Multiscan echocardiogram frame of a sagittal cross-sectionof the heart. Patient identification data are printed at thetop) and the electrocardiogram is recorded at the bottom ofthe frame. Anterior chest wall echoes are to the left and thebrighit dots on the left indicate the transducer position.

accomplished by a quarter wave length matchinglayer which is applied to the transducer face. Thislayer also increases the efficienicy of electrical toacoustical power conversion, covers the electrodes,and protects the patient against any electricalcontact. During examination a sinusoidal voltage of250 volt peak-to-peak and 1 gcsec in duration isapplied to each element, which results in a trans-mitted pulse of 2 jasec duration.

ResolutionDepth resolution, defined as the distance in depth

at which two structures may be separated, dependsprimarily on pulse length. In figure 8 typical echoesat 2.25, 4.5 MHz and at an intermediate frequencyof 3.5 MHz are shown as obtained from awater/perspex interface. For the echoes shownhere, with their width taken between the 10%6detection levels, a depth resolution of.the order of1.25 mm results.

Lateral resolution may be definied in variousways, and depends on element size, transmissionfrequency, depth of structures and dynamic rangeof echo intensity. The lateral resolution of a single 4mm wide elemenit from the Multiscan transducer atthe intermediate frequency (3.5 MHz) at distancesof 6 and 10 cm is shown in figure 9. The illustrationdemonstrates th-at lateral resolution decreases withincreasing depth as a function of dynamic range.

These d.efinitions of lateral resolution are basedon use of a single element, and apply to any singleelement application. A more pertinent question for

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BOM ET AL.

EXUITAT ION TIME DIAGRAM

RECEPTION INACTIVE TIME

1E.C.G. SAMPLING

-.'1 h----200 u sec- 30 1-

E.C.G. DISPLAY

2 3 20

-4600 ,usec 460 1500

ECHO-LINES E.C.G. CHARACTERS

6560 asec.

Figure 6

Time diagram showing the sequence and duration of events during one Multiscan frame.

the Multiscan system was to establish the extent to

which an echo from a point reflector positioned infront of a given element was erroneously displayedby adjacent elements. A thin wire target was placedat 6 and 10 cm in front of the transducer in a

watertank; the findings are demonstrated in figure10. Because of limitations in lateral resolution a

point reflector tends to show as a curved verticalline displayed over a number of adjacent elementsif the gain setting is adjusted improperly.

Lateral resolution poses a serious limitation in all

Figure 7

A 4.5 MHz transducer (reproduced by permission ofOrganon Teknika, Inc.).

echo systems. The rapid deterioration as depthincreases is especially bothersome. In the Multiscansystem the dynamic range in echo brightness of thedisplay on the oscilloscope is in the order of 20 dB.This two-dimensional echo system therefore re-quires a precise matching of the available 20 dBdynamic range as a function of echo strength toobtain optimal resolution.

Time Gain Compensation

A particularly easy and flexible method wasdeveloped for gain compensation (fig. 11). Thegain for returning echoes from any depth rangemay be determined by the lever controlling thatparticular range only. As each lever may be set toselect any gain between 0 and 80 dB, optimal use ofgain control in individual situations is assured.

Acoustic Output and SafetyA small sensor was calibrated at 2.25 and 4.5

MHz on the basis of the effect of radiation pressure.This sensor was used to measure intensity levels atvarious positions in front of the transducer face. Allmeasurements were carried out in a watertank. Sixcm in front of the center of the transducer the peakintensity during a transmission pulse of 2 ,usecduration was measured to be 0.7 W/cm2 and 3.6W/cm2 for the 2.25 MHz and the 4.5 MHztransducers, respectively. Average intensity was alsomeasured, taking into account the acoustic energy

Circulation, Volume XLVlIJ, November 1973

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NIULTISCAN ECHOCARDIOGRAPHY

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Figure 9

Figure 8

Typical echoes at 2.25 MHz (A), 3.5 MHz (B) and 4.5 MHz(C) as received from a water/perspex interface at 10 cm

distance (scale 0. 5 usee/div).

radiated by adjacent elements also. At 6 cm

distance the average acoustic intensity was

measured to be 0.6 mW/cm2 for the 2.25 MHz and2 mW/cm2 for the 4.5 MHz transducer. This is wellbelow the maximum intensity allowed for prolong-ed diagnostic procedures as presented by Ulrich.'0Circulation, Volume XLVIII, November 1973

Lateral resolution of a single 4 mm wide element from theMultiscan transducer with the intermediate frequency (3.5MHz) and measured at 6 and 10 cm distance. When theecho from a point reflector along the main axis of the ele-ment is just visible at 0 dB, it will no longer be visible ifmoved laterally over any distance. If the echo strength isincreased 10 dB, a reflector at 6 cm depth may be moved3.7 mm in either direction and will still be visible (dottedline), while a reflector at 10 cm depth can be moved 6 mmlaterally and still be visible. The limits of lateral resolutionfor a range of intensities can be determined in this way.

Recording MethodsThe instrument was designed for oscilloscope

display and is optimally used when operated duringdirect vision from the oscilloscope. However,attempts have also been made to record the movingimages for later viewing or for "hard-copy" records.All patient data are recorded on video tape using aslave oscilloscope and video camera. With such arecording method appreciable loss in definitionoccurs but gross anatomy and motion are preserved.These records are satisfactory for subsequentreview of overall function and anatomy. As polaroidphotographs may be recorded directly from theoscilloscope via an ECG-activated trigger device,direct single frames can be obtained at selected

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BOM ET AL.

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ELEMENT NUMBERFigure 10

Erroneous display on adjacent elements of echoes from a

point reflector in front of a given element as a function ofdynamic range. The echo from a point reflector 10 cm infront of element N (top panel) is just visible at d!B(hatched area). If the echo strength is increased by 5 dB,the echo will become just visible at the elemknts imnwdi-ately adjacent (N + 1 and N-1, stippled area). When theecho strength increases to 17 dB, echoes from a source di-rectly in front of element N will be detected by elements 2away (N + 2 and N - 2, vertical lines). For a reflector at6 cm distance (bottom panel), distortion caused by thiseffect is less serious.

times in the cardiac cycle. However, this methodlimits the potential of the Multiscan system sincemotion cannot be preserved.

Preliminary investigations have been carried outto record the echo images on 16 and 35 mm film for

12 4 8

B

16 cmFigure 11

Time gain compensation may be controlled by individuallevers.

subsequent analysis of moving structures. These areof acceptable quality for viewing in motion, butmore satisfactory film, developing and oscilloscopecharacteristics must be worked out before they canbe used for single frame study. The presentprototype is now used in conjunction with acontinuous hardcopy machine (Honeywell 1856).This recorder is also used for the conventional M-mode recording if a single element transducer isoperated. In addition any individual element fromthe multi-element transducer may be used selective-ly for M-mode recording. As this combines the twodimensional orientation facility of the Multiscanwith single element recording, more exact knowl-edge of the echo pathway is provided.

Presently the most promising option seems to bethe recording of long sequences of complete framesdirectly on the photosensitive paper of the hard-copy unit. This results in a series of 19 x 40 mmimages recorded at 25 frames/sec (see fig. 12). The


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ECGFigure 12

Direct line-scan recording of sequential cardiac images at 25 frames/sec. For cross-sectional orientationsee figure 2 in Part II.9 The closed aortic valve cusps (small arrow) can be seen centered in the aorticroot inl frames 1 and 2 at end-diastole. The closed mitral valve (asterisk) is visible in frames 4-6 after theonset of ventricular systole (QRS in frame 3). Correct two-dimensional geometry can be preserved onlyby combining a high recording paper speed (500 mm/sec), a small image size (19 X 14 mm) and a reducedframe rate.

most obvious advantage is the absence of any inter-mediate video or film camera.

Conclusions

One of the major advantages of the Multiscansystem is the capability for direct viewing ofmoving cardiac structures. This has been theprimary aim of the present system. The recording ofthe images has as yet been incompletely solved, andfurther research needs to be done in this area. At

the present time, the sequenitial "stamp-size" imagesobtained with the linescan recorder seem mostpromising.

Lateral resolution remains one of the mainlimitations in diagnostic ultrasound. For optimalresults with the Multiscan system a time gaincompensation technique to permit incdependentgaini control in five differenit depth ranges wasdeveloped. This system proved also to be veryvltaluable when applied to single element recording.

Circulation, Volume XLVIII, Novermber 1973

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Presently under development are a smaller trans-ducer for pediatric purposes as well as a 40 lineoscilloscope display to provide a more acceptableimage.Although the Multiscan technique is still in its

initial stage, its application in conjunction with theconventional single element recording methodopens a new area in echocardiography.

AcknowledgmentWe wish to thank the head of our department, Prof. P. G.

Hugenholtz, for his encouragement and helpful suggestionsand we gratefully acknowledge the assistance of the researchworkshop under the guidance of ir. H. Bak.

References1. HERTZ CH, LINDSTR6M K: A fast ultrasonic scanning

system for heart investigation. The third internationalconference on medical physics, including medicalengineering, Chalmers university of technologyGoteborg Sweden: 35.6, August 1972

2. PxTzoLD J, KRAUSE W, KRESSE H, SOLDNER R: Presentstate of an ultrasonic cross-section procedure withrapid image rate. IEEE Trans Biomed Eng 263:1970

3. SOMER JC: Ultrasound diagnosis. Progress report.Institute of Medical Physics TNO: 37, August 1968

4. BUSCHMANN W: New equipment and transducers forophthalmic diagnosis. Ultrasonics 18: January-March1965

5. UCHIDA R, HAGIWARA Y, IRIE T: Electro-scanningultrasonic diagnostic equipment. Jap Med El 58:1971/1972.

6. BoM N, LANCEE CT, HONKOOP J, HUGENHOLTZ PG:Ultrasonic viewer for cross-sectional analyses ofmoving cardiac structures. Biomed Eng 6: 500,1971

7.- BOM N: New concepts in echocardiography. Deptof Cardiology, University of Rotterdam ISBN90.207.0346.3, June 1972

8. ROELANDT J, KLOSTER FE, TEN CATE FJ, BOM N,LANCEE CT, HUGENHOLIZ PG: Multiscan echo-cardiography. Hartbulletin 4: 51, 1973

9. KLOSTER FE, ROELANDT J, TEN CATE FJ, BoM N,HUGENHOLTZ PG: Multiscan echocardiography. PartII: Technique and initial results. Circulation 48:1075, 1973

10. ULRICH WD: Ultrasound dosage for experimental useon human beings. Naval Medical Research Institute,Report no 2: 19 August 1971


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and JOS ROELANDTNICOLAAS BOM, CHARLES T. LANCÉE, IR., GERARD VAN ZWIETEN, FRANK E. KLOSTER

Multiscan Echocardiography: I. Technical Description

Print ISSN: 0009-7322. Online ISSN: 1524-4539 Copyright © 1973 American Heart Association, Inc. All rights reserved.

is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Circulation doi: 10.1161/01.CIR.48.5.1066

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Description CHAREALEDS LANCEAE, · PDF filetransducer may be recorded in the conventional time-motion display, ... and a block diagram is shown in figure 2. All ... backing slab which