Echocardiography—where are we now and where are we going?

MARCH 1976

The American Journal of Medicine VOLUME 60

NUMBER 3

EDITORIAL

Echocardiography-Where Are We Now and Where

Are We Going?

ARTHUR E. WEYMAN, M.D.

HARVEY FEIGENBAUM. M.D.

Indianapolis, Indiana

M-mode echocardiography has rapidly become an established diagnostic technic in cardiology. Because of its noninvasive nature, high resolution and rapid sampling rate, this method of examination has proved particularly valuable in determining cardiac chamber size, patterns of valve motion, structural abnormalities, and direc- tion, velocity and amplitude of left ventricular wall motion [I]. The increased interest in and use of echocardiography have stimulated a desire to obtain more diagnostic information from the heart using reflected ultrasound. This has led to the development of improved technics of performing the M-mode examination and approaches to interpreting the data derived. In addition, it has stimulated an interest in (1) developing a method for imaging the heart in a spatially oriented, cross-sectional or twodimens/onal dynamic display, (2) determining patterns and quantitating velocity of intracardiac blood flow using the doppler-shift principle, and (3) attempting to determine properties of cardiac tissue by their effect on the amplitude and frequency spectrum of the ultrasonic pulse. We will re- view briefly the present status of clinical echocardiography and indicate some of the directions being taken in ultrasonic research.

The use of ultrasound in cardiac diagnosis is based on the principle that a pulse of ultrasound transmitted into the heart will be reflected in part by each separate structure it encounters. The

From the Department of Medicine, Cardiology amount ,_of reflected energy or echo strength can be quantitated; Division, Indiana University School of Medicine, and the distance of the reflecting structure from the transmitting Indianapolis, Indiana. Requests for reprints source or transducer can be determined if the speed of ultrasound should be addressed to Dr. Arthur E. Weyman. Indiana, University School of Medicine, 1100 West Michigan Street, Indianapolis, Indiana 46202. Manuscript accepted September 2, 1975.

in tissue and the time required for the pulse to transit the tissue, strike the reflecting surface, and return as an echo are known. By converting time to distance the standard echograph displays these echoes at a distance from a reference line proportional to the dis-

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ECHOCARDKSRAPHY-WEYMAN, FEKXNBAUM

tance from the transducer to the reflecting structure. Depending on the type of display, the intensity of the returning echo can be represented by amplitude (A- mode) or brightness (B-mode). Echoes from stationary structures remain fixed relative to the transducer or reference line whereas those from moving structures vary in position. To better analyze the pattern of echo motion produced by moving structures, the echoes, displayed as a line of brightness-modulated dots (B-mode), can be swept across an oscilloscope (M-mode). In this form of display, echoes produced by stationary structures are represented as straight lines whereas those from moving structures appear as wavy lines. By examining the pattern and locations of the moving echoes, a great deal of information concerning the structures producing them can be derived.

The original and principle use of echocardiography to date is to direct the ultrasonic beam toward a spe- cific structure or group of structures and, in this manner, to record a so-called “icepick” view of the motion pattern exhibited by these structures. The mitral valve, because it exhibits the most dynamic motion of any of the cardiac structures and also because it is the most readily accessible to the ultrasonic beam has been the most extensively studied. Motion of the anterior mitral leaflet toward the transducer at the onset of the rapid ventricular filling, following atrial systole and away from the transducer at the comple- tion of rapid ventricular filling, and with ventricular systole, inscribes the classic “M” shaped pattern of anterior mitral leaflet motion on the M-mode recording. The posterior mitral leaflet, which moves in a mirror image of the anterior leaflet, appears as a “W.” The normal “M” shaped pattern of mitral valve motion is altered in a characteristic manner in a number of disease states. Fortunately, most of the cardiac structures have slightly different motion patterns which aid in their identification and in the differentia- tion of normal from abnormal motion. Much of echocardiography, therefore, is pattern recognition.

The isolated “icepick” view of the various parts of the heart is still the major method of making echocardiographic diagnoses in patients with valvular heart disease. With the expanded use of echocardiography in congenital and ischemic heart disease, it became apparent that not only the motion pattern of isolated structures and areas but also the interrela- tionships of these areas were important. This led to the development of the concept of M-mode scanning. A scan merely refers to moving the ultrasonic beam. With an M-mode scan the ultrasonic beam is moved while the strip chart is constantly recording. The ultrasonic beam is usually moved in a sector scan, which means that the point of the transducer is

stationary and only the angle changes. Such a record shows the interrelationship between various parts of the heart. This longitudinal relationship has become very important in a variety of cardiologic abnormalities. In patients with congenital heart disease, for example, visualization of the continuity between the mitral valve and the aortic valve as well as the interventricular septum and anterior aortic root is frequently essential. Scanning is also valuable in patients with coronary artery disease and segmental dysfunction of the ventricle, since it allows wall motion from several areas to be compared and a qualitative estimate of the extent of abnormal wall motion derived. By scanning from several different transducer locations a major portion of the left ventricle can be examined. If. in addition to the routine sector scan from the mitral area, the transducer is moved to a mid and low position, relative to the body of the left ventricle, and the scans are repeated, a more extensive area of the interventricular septum and posterior wall along with the normal apical tapering can be ap- preciated. Sliding the transducer laterally across the precordium (longitudinal scan) permits the anterior left ventricular wall to be visualized. By placing the transducer in the subxyphoid area and scanning through the left ventricle, the medial portion of the inter-ventricular septum and lateral wall of the ventricle can be recorded. Thus, by scanning the left ventricle from various transducer positions, wall motion from a large segment of the chamber becomes available for analysis.

Along with improvements and innovations in the technic of performing the M-mode examination, improved methods of analysis have increased the amount of diagnostic information which can be derived from the M-mode record. In pediatric echocardiography the deductive approach to analyzing the M-mode record has been emphasized [2]. This approach combines knowledge of the embryologic development of the heart, with the echocardiographi- tally determined position in space and interrelationship of the atrioventricular and semilunar valves to determine the relationship of the cardiac chambers and great vessels, and thus diagnose complex congenital lesions. This type of reasoning can also be applied to adult echocardiography. For example, the isolated “icepick” view of the mitral valve in a pa- tient with mitral stenosis may provide only a part of the cardiac diagnosis. If pulmonary hypertension is present, changes in pulmonic valve echo motion would be expected. If the hypertension is severe and pulmonary insufficiency results, the valvular insufficiency should be reflected by fluttering of the anteri- or tricuspid leaflet, while the resulting volume overload of the right ventricle will cause dilation of this

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chamber, as well as paradoxical motion of the interventricular septum. If the right ventricular volume overload pattern is present in the absence of pulmonary insufficiency and fluttering of the anterior tricuspid leaflet, tricuspid insufficiency would be the most likely cause. By analyzing the motion patterns of a number of different cardiac structures, a more com- plete cardiac diagnosis can be developed. Thus, at present, by combining (1) the basic “icepick” view of isolated intracardiac structures; (2) the technics of M-mode scanning to demonstrate the interrelation- ships of these structures; (3) examination from a number of different transducer locations to increase the area of the heart that can be recorded; and (4) in- terpretation of the M-mode echogram based on a consideration of the principles of cardiac embryolo- gy, anatomy and the pathophysiology of cardiac disease, a wide range of important and frequently complex cardiac diagnoses can be recognized.

WHERE ARE ‘WE GOING?

Despite the advances in M-mode echocardiography, it appears that a great deal more information can be obtained from the heart using pulsed reflected ultrasound. This can be achieved by variations in the method of echo display, utilizing the frequency shift in the ultrasonic pulse produced by flowing blood, and analyzing changes in the amplitude and frequency spectrum of returning signals produced by alter- ations in cardiac structure.

The major interest in echocardiographic research at present is directed toward developing a system which will provide a spatially oriented, cross-sectional or two-dimensional dynamic image of the heart.

Although M-mode scanning provides some spatial orientation, it is at best qualitative. The apparent lateral distances between intracardiac structures reflect the speed of transducer motion rather than correct spatial relationships. There have been a number of approaches to the development of a cross-sectional display. The major efforts currently undergoing clinical evaluation include (1) B-mode scanning, (2) the multiple crystal or “multiscan system,” (3) mechanical sector scanning, and (4) the multiple crystal with electronic beam steering or phased array technic. Each of these systems approaches the problem of compiling a cross-sectional or two-dimensional image of the heart in a slightly different manner.

B-mode scanning was the first attempt at developing a two-dimensional image of the heart. This system utilizes a standard transducer combined with a position sensitive arm and a storage oscilloscope for image assembly. The transducer may be moved

manually [3] or mechanically [4] across the precordium producing a longitudinal scan. An image of the

ECHOCARDIOGRAPHY-WEYMAN, FEIGENBAUM

underlying heart is developed by placing each of the recorded B-mode lines of information on the raster of the storage oscilloscope according to their relative position in space as determined by the position-sensitive arm. By gating the oscilloscope with an electro- cardiogram only pulses transmitted during a particular segment of the cardiac cycle can be recorded and in this manner individual systolic or diastolic frames compiled. By assembling frames at multiple points in the cardiac cycle (multiple electrocardio- graphic gates) and rapidly sequencing the individual frames, a moving picture of an individual cardiac cycle can be produced. Just as B-mode scanning permits a cross-sectional image of an extensive area of the heart to be developed, it is dependent on a large area of the heart being accessible for examination. Since a major portion of the left ventricle is frequently obscured by intervening lung, rib or sternum, recording this type of image can be extremely diffi- cult. In addition, distortion of the image may be produced by arrhythmias during the period of assembly or by respiratory variation in cardiac position. Al- though an appreciation of cardiac motion can be obtained by analyzing systolic and diastolic frames, B- mode scanning does not represent a dynamic or real time method of display. Attempts to introduce dynamic motion by computer storage and reassembly of multiple gated frames are presently under way. Al- though this method of cross-sectional imaging has been in clinical use and commercially available for a number of years and several clinical applications have been suggested [ 5,6], it has not as yet attract- ed widespread interest or use.

The first attempt at real time cardiac imaging using reflected ultrasound involved the multiple crystal or “multiscan” technic [7,8]. In this system, a longitudinal scan of the heart is assembled by align- ing a number of transducers next to one another and activating them in sequence rather than sliding an individual transducer across a given cardiac area. In the original system twenty (4 mm) transducers were used to produce a multielement transducer 8 cm in width. By activating the transducers in rapid sequence, a real time image of an 8 by 16 cm area of the heart was achieved. Since each transducer provides only one line of information, a resulting line density of 2.5 lines of information per centimeter was obtained. This low density combined with a wide beam width resulting from the necessary use of small transducers resulted in poor over-all resolution. In addition, in clinical practice, alignment of the large multi-element transducer parallel to the cardiac axis frequently results in portions of the transducer lying outside an echocardiographic window. When this occurs, gaps in the display may result.

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Initial Clinical studies attempting to use the multiscan system to determine ventricular volumes have been somewhat disappointing [9]. Clinical applications of this system in pediatrics, particularly in infants in whom the ribs and sternum are less of an ob- stacle to transmission of the ultrasonic beam, have been much more encouraging [lo]. Despite the limitations of the early multi-element systems, they have been extremely valuable in stimulating a widespread interest in dynamic cross-sectional imaging of the heart.

beam it is also the most complicated. The quality of the initial images, however, has been excellent.

The third type of cross-sectional cardiac imaging involves the use of a mechanical sector scanner [ 1 l-131. In this type of display a single transducer is mechanically angled through an arc of from 30 to 45 degrees. By limiting the area of the scan and in- creasing the frequency of pulse generation by the echograph, a high line density per frame and rapid frame rate are achieved. This results in a high resolution display which is ideal for visualizing localized areas of the heart. Further, the lightweight handheld transducer utilized in this system permits the echo- cardiographer to take advantage of any available ultrasonic window to examine the heart. In addition to the normal echocardiographic window along the left sternal border, the transducer can be placed at the cardiac apex, in the suprasternal notch or the epi- gastrium to permit visualization of the heart from a number of different locations. The high resolution achieved by limiting the area of the scan, however, necessitates large areas such as the left ventricle being viewed in sections, resulting in less than opti- mal spatial orientation.

Evaluation of these systems to date has been mainly based on a comparison of their engineering characteristics [ 151. Very preliminary clinical studies suggest that the high resolution and rapid frame rate of the mechanical sector scanners makes them a valuable approach for examining small rapidly moving cardiac structures such as the aortic valve [ 161. In this issue of the “Green Journal,” we have described our experience using a mechanical sector scanner to locate the level of left ventricular outflow tract obstruction in a large number of consecutive patients. Henry et al. [ 17,181 have previously demonstrated the ability of a similar system to visualize the relationship of the great arteries in patients with transposition complexes and the mitral valve orifice in patients with mitral stenosis. These investigators demonstrated an excellent correlation between the surgically measured mitral valve orifice and the mitral valve orifice demonstrated on cross-sectional scanning. This is an area in which diagnostic ultrasound may offer an improvement over cardiac catheteriza- tion and angiography since these invasive technics permit only indirect calculation of the mitral valve area based on hydraulic formulas rather than true visualization of the mitral valve orifice itself.

In pediatric cardiology, when one is interested in the interrelationship of large structures such as the ventricular chambers and great arteries, it is possible that the wide area of visualization provided by the multi-element technic using high frequency, high resolution transducers may prove superior.

The final approach to the two-dimensional imaging In addition to its use in imaging intracardiac struc- of the heart involves the use of a multiple element tures pulse reflected ultrasound can also be utilized transducer with electronic steering of the beam [ 141. to examine patterns of intracardiac blood flow [19]. This so-called phased array technic functions in the This is based on the principle that the frequency of a following manner: If a number of parallel crystals are pulse of ultrasound striking a moving column of activated simultaneously, a single beam similar to blood will be shifted in proportion to the velocity of that produced by a single crystal will occur. If these flow (doppler shift principle). Comparing the frequen- crystals are activated in series, then a longitudinal cy of the transmitted and returning pulses permits scan of a rectangular area of underlying heart will be the frequency shift to be quantitated. If the frequency developed. If, however, these multiple crystals are shift of a pulse of ultrasound randomly transmitted energized almost simultaneously, but slightly out of into the heart is sampled, the shift measured will re- phase, such that the ultrasonic beam is initiated at flect the mean velocities of all structures through one end of the transducer slightly before the other which the beam moves. Examination of flow in a lo- end, then the beam will be transmitted at an angle to calized area of the heart can be achieved by sam- the transducer rather than straight downward into the pling only those signals which return to the echo- tissue. By altering the phasing at which the transduc- graph during a time frame proportional to the depth ers are energized, the beam can be directed or of the desired area of study (range gating). The mag- steered across the precordium. This type of electron- nitude of the velocity vector measured with this type ic beam steering represents the most sophisticated of system will be related both to the velocity of the of the two-dimensional systems. Because of the flow itself and to the angle between the incident complex computer control required to direct the beam and moving column of blood. Since to date it

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has not been possible to measure this angle accu- rately, quantitation of blood flow velocity has not been possible. In addition, to convert flow velocity measurements to volume flow, a determination of cross-sectional area is required. Because of these limitations, the application of the doppler flow method to the heart has been mainly limited to detection of cardiac lesions by recording the localized areas of turbulence they create [20]. Using range gated doppler flow meters to determine only the presence or absence of flow it has been possible to demonstrate the patency of such structures as saphenous vein bypass grafts [21]. More extensive studies of the intracardiac patterns of blood flow have been obtained by placing a doppler flow probe at the tip of an intracardiac catheter [22]. Although this has certainly provided interesting and valuable information, it is somewhat at variance with the basic noninvasive nature of the echocardiographic technic.

Combining a (range gated) doppler flow system with one of the previously described cross-sectional cardiac imaging systems should permit both the cross-sectional area of intracardiac structures as well as the angle of incidence between the sampling doppler beam and the particular cardiac structure to be determined. In this manner, hopefully, quantitation of cardiac blood flow will by possible. Barber et al. [23] have already developed a combined two-dimensional echo-pulsed doppler system for simultaneous visualization and blood flow measurement in periph-

1.

2.

3.

4.

5.

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eral vessels. Although the depth of the field of this system is too shallow for cardiac evaluation, it does demonstrate the feasibility of such an approach. There are a number of difficulties encountered in attempting to measure pulsatile flow in a moving ves- sel, but this type of combined system appears to have great clinical promise. By the time this report appears in press systems similar to this should be undergoing clinical evaluation in several laboratories.

In addition to using reflected ultrasound to visualize intracardiac structures and to determine patterns of intracardiac blood flow, the reflected pulse may provide the raw acoustic data for more extensive analy- ses directed toward determining the properties of the tissue reflecting the ultrasound. Lele and Namery [24] have demonstrated a change in the frequency and amplitude spectrum of echoes from heart muscle as a result of occluded blood flow. Acoustic re- flectance of infarcted myocardium was found to be considerably lower than that of the control specimen, indicating that its characteristics of acoustic impe- dence were lower. Studies are under way to determine whether extensive computer analysis of this raw acoustic data can provide basic information concerning change in properties of cardiac muscle. For instance, this might be valuable in differentiating normal from infarcted or ischemic myocardium. Studies of this sort are at a very embryonic level, but they do indicate some of the other potential uses of diagnostic ultrasound.

Feigenbaum H: Echocardiography. Philadelphia, Lea & Fe- biger, 1972.

REFERENCES

Solinger R, Elbl F, Minhas K: Deductive echocardiographic analysis in infants with congenital heart disease. Circu- lation 60: 1072, 1974.

King DL: Cardiac ultrasonography: a stop-action technique for imaging intracardiac anatomy. Radiology 103: 367, 1972.

Ebina T, Oka S, Tanaka M, Kosaka S, et al: The ultrasono- tomography for the heart and great vessels in living human’ subjects by means of the ultrasonic reflection technique. Jap Heart J 6: 331, 1967.

King DL, Steeg CN, Ellis K: Visualization of ventricular sep- tal defects by cardiac ultrasonography. Circulation 46: 1215, 1973.

tricular volume by multiscan echocardiography (abstract 103). Circulation (suppl Ill) 49-50: 28, 1974.

10. Sahn DJ. Terry R, O’Rourke R, Leopold G, Friedman WF: Multiple crystal cross-sectional echocardiography in the diagnosis of cyanotic congenital heart disease. Circula- tion 50: 230, 1974.

11. Griffith JM, Henry WL: A sector scanner for real-time two- dimensional echocardiography. Circulation 49: 1147, 1974.

Teichholz LE, Cohen MV, Sonnenblick EH, Gorlin R: Study of left ventricular geometry and function by B-scan ultrasonography in patients with and without asynergy. N Engl J Med 291: 1220, 1974.

Born N, Lancee CT, VanZwieten G, Kloster FE, Roelandt J: Multiscan echocardiography. I. Technical description. Circulation 48: 1066, 1973.

Kloster FE, Roelandt J, TenCate FJ, Born N, Hugenholtz PG: Multiscan echocardiography. II. Technique and initial clinical results. Circulation 48: 1075, 1973.

Roelandt J, TenCate F, VanDorp W, Born N, Hugenholtz PG: Limitations of quantitative determination of left ven-

12. Eggleton RC, Dillon JC, Feigenbaum H, Johnston KW, Chang S: Visualization of cardiac dynamics with real- time B-mode ultrasonic scanner. Circulation (suppl Ill) 49-50: 27, 1974.

13. Eggleton RC, Feigenbaum H, Johnston KW, Weyman AE, Dillon JC, Chang S: Visualization of cardiac dynamics with real-time B-mode ultrasonic scanner. Ultrasound in Medicine, vol 1 (White D, ed), New York, Plenum Press, 1975, p 385.

14. Thurstone FL, VonRamm OT: Electronic beam steering for ultrasonic imaging. Ultrasound in Medicine (de Vlieger M, White DN, McCready VR, eds), New York, American Elsevier Publishing Co., 1974, p 304.

15. Eggleton RC. Johnston KW: Real-time mechanical scanning system compared with array techniques. Institute of Electrical and Electronics Engineers Proceedings In Sonics and Ultrasonics, Milwaukee, November 1974.

16. Weyman AE, Dillon JC, Chang S, Feigenbaum H: Cross-

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sectional echocardiogram in assessing the severity of valvular aortic stenosis. Am J Cardiol 37: 358, 1978.

17. Henry WL, Maron BJ, Griffith JM, Redwood DR, Epstein SE: Differential diagnosis of anomalies of the great arteries by real-time, two-dimensional echocardiography. Circu- lation 51: 283, 1975.

18. Henry WL, Griffith JM, Michaelis LL, McIntosh CL, Morrow AG, Epstein SE: Measurement of mitral orifice area in patients with mitral valve disease by real-time, twodi- mensional echocardiography. Circulation 51: 827, 1975.

19. Baker DW: Pulsed ultrasonic doppler blood-flow sensing.

culation 48: 810, 1973. 21. Gould KL, Mozersky DJ, Hokanson DE, Baker DW, Kennedy

JW, Summer DS, Strandness DE Jr: A non-invasive technique for determining patency of saphenous vein coronary bypass grafts. Circulation 86: 595, 1972.

22. Benchimol A, Desser KB, Gartlan JL Jr: Left ventricular blood-flow velocity in man studied with the doppler ultrasonic flowmeter. Am Heart J 85: 3. 1973.

23. Barber FE, Baker DW. Nation AW, Strandnese DE Jr, Reid JM: Ultrasonic duplex echodoppler scanner. IEEE Trans Biomed Eng 21: 109, 1974.

24. Lele PP, Namery J: Detection of myocardial infarction by IEEE Trans Sonics Ultrasonics, SU-17, July 1970. _ ultrasound. 25th Annual Conference on Engineering in

20. Johnson SL, Baker DW, Lute RA, Dodge HT: Doppler echo- Medicine and Biology, Bal Harbour, Florida, October cardiography. The localization of cardiac murmurs. Cir- 1972.

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Documents

Echocardiography—where are we now and where are we going?