Real-time 3-dimensional echocardiography (RT3D)

Transcription

1 ORIGINAL ARTICLES Feasibility of Real-Time 3-Dimensional Treadmill Stress Echocardiography Donna R. Zwas, MD, Shin Takuma, MD, Samantha Mullis-Jansson, MD, Ali Fard, MD, Hina Chaudhry, MD, Henry Wu, MD, Marco R. Di Tullio, MD, and Shunichi Homma, MD, New York, New York Rapid acquisition of echocardiographic images is critical for the predictive accuracy of stress echocardiography. Real-time 3-dimensional echocardiography (RT3D) allows review of several standard 2- dimensional images from a single volumetric data set. To assess the feasibility of RT3D for treadmill stress echocardiography, we performed treadmill stress RT3D on 20 volunteers (10 men and 10 women; mean age 32 ± 6 years) with a device that uses a matrix phased-array transducer in a 60- degree pyramidal volume. Images are displayed as 2 steerable, intersecting B-scan sectors with adjustable C-scan planes parallel to the transducer face. At preexercise and immediate postexercise assessment, the volumetric data were obtained from apical and parasternal windows, respectively. Left ventricular segments were divided into 16 standard segments according to criteria defined by the American Society of Echocardiography. The use of both volume sets resulted in visualization of 98% of the segments at peak exercise. Even with only an apical volume set, 89% of the segments were adequately visualized. Image optimization and acquisition time at peak exercise was 35 ± 18 seconds from the apical window and 50 ± 28 seconds from the parasternal window. This preliminary study indicates that RT3D treadmill stress echocardiography is feasible and may be an important application of this new 3- dimensional device. (J Am Soc Echocardiogr 1999;12: ) Real-time 3-dimensional echocardiography (RT3D) permits the rapid acquisition of a volumetric data set containing the entire left ventricle, thereby allowing for subsequent viewing of any 2-dimensional (2D) slice within the volume at the discretion of the interpreting physician.this study was designed to test the feasibility of using an RT3D device for treadmill stress echocardiography. METHODS Subjects We studied 20 healthy volunteers (10 men and 10 women; mean age 32 ± 6 years,mean height ± 12.4 cm,mean weight 70.5 ± 15.3 kg) by RT3D (VOLUMETRICS Medical Imaging, Durham, NC). All were asymptomatic and without known coronary or structural heart disease, and none From the Division of Cardiology, Department of Medicine, Columbia Presbyterian Medical Center, New York, New York. Reprint requests: Dr Shunichi Homma, Division of Cardiology, Columbia Presbyterian Medical Center, PH 3-342, 630 West 168th Street, New York, NY Copyright 1999 by the American Society of Echocardiography /99 $ /1/96954 were taking medications. No subject was excluded on the basis of pre-exercise images. Real-Time 3D Equipment The RT3D system was developed at the Duke University Center for Emerging Cardiovascular Technology. 1,2 The device is unique because the image of an obtained volume can be manipulated to show various sectors within the volume (Figure 1). We used the 2.5-MHz matrix-array transducer that scans 3D volume electronically. Because this matrix-array transducer can transmit and receive acoustic signals throughout entire pyramidal volume in real time, it is not necessary to establish temporal gating, such as electrocardiographic (ECG) or respiratory gating. The matrix array consists of 2D phased arrays,instead of the linear arrays, and offers steering in both the azimuth and elevation of the beam, permitting interrogation of an entire pyramidal volume. In 2D arrays, 512 elements connect to the system; 256 of the 512 elements are used for transmission, and 256 are used for reception. Because RT3D has 16:1 parallel processing, the volumetric scanner would have a 4096 channel-receive system (256 elements 16 parallel processing). RT3D can accomplish a typical cardiac scan at 20 frames/s at a depth setting of 15 cm. The data are displayed as 2 adjustable orthogonal B-scans, 285

2 286 Zwas et al May 1999 Figure 1 A volumetric scanning and steerable sectors. Demonstrated are 2 adjustable orthogonal B-scans perpendicular to the transducer and 3 adjustable C-scans parallel to the face of the transducer. which are perpendicular to the transducer, and 2 or 3 C- scans, which are parallel to the face of the transducer (Figure 1). All scan angles are steerable so that various planes within the volume set can be reviewed on screen (Figure 2) or off, with the stored data set on optical disk (3M, Imation, Oakdale, Minn). Exercise Protocol After standard ECG leads were placed on a subject, preexercise images were obtained in a left lateral decubitus position. Both apical and parasternal volume sets were obtained. Exercise testing was performed according to the standard Bruce protocol. Exercise end points were the attainment of target heart rate (HR) (>85% of predicted maximum HR), development of dyspnea, or leg fatigue necessitating the cessation of exercise. The electrocardiogram was monitored continuously. Immediately after exercise a parasternal and an apical volume data set were obtained in left lateral decubitus position. The time required to obtain data sets from apical and parasternal windows before and after exercise were recorded. Image Review and Analysis In this study the standard 16-segment model of the American Society of Echocardiography (ASE) 3 was evaluated from the apical volume set. Cross-sectional C-scans were steered into short-axis views at the basal, mid, and apical slices, and evaluated for the presence of visible and interpretable myocardial thickening. Several slices at each level were viewed to maximize the number of segments seen. In addition, the 16-segment model of ASE was also evaluated by combining the slices from both the apical and the parasternal data sets. Long-axis B-scan images also were obtained and evaluated to confirm the presence of visible myocardial thickening. All segments were coded as either acceptable for interpretation or unacceptable by the consensus of 2 experienced observers. RESULTS Volunteers exercised for a mean of 12 minutes 42 seconds ± 2 minutes 45 seconds (range 9 minutes 52 seconds to 19 minutes) of the Bruce protocol to a mean peak HR of 188 ± 6 bpm and mean peak blood pressure of 157/65 mm Hg (range 144/56 to 186/90 mm Hg). All subjects completed the exercise protocol without chest discomfort, and no subject demonstrated ECG changes suggestive of ischemia. A right bundle branch block developed in 1 volunteer but resolved rapidly during recovery. Blood pressure, HR, and the acquisition times are

3 Volume 12 Number 5 Zwas et al 287 Figure 2 Initial views of real-time 3-dimensional echocardiography. Shown are 2 orthogonal B-scan images obtained from an apical window (right) and 3 parallel C-scan images (left). Figure 3 The number of interpretable segments. RT3D, Real-time 3-dimensional echocardiography. Pre, Pre-exercise scanning; Post, postexercise scanning. shown on Table 1. A total of 320 myocardial segments were obtained for analysis.with the apical volume set, 89% of all segments were visualized after exercise. With both apical and parasternal sets, 98% of all segments could be visualized (Figure 3). DISCUSSION Stress echocardiography is one of the more difficult techniques used in cardiac ultrasonography laboratories, both from the standpoints of the sonographer and the physician interpreter. Unlike pharmacologic stress tests, imaging must occur immediately after the cessation of exercise. Treadmill stress echocardiography has been shown to be an accurate diagnostic method for the detection of coronary artery disease. 4-6 However, the recovery of wall motion is time dependent after peak stress. 7,8 Because myocardial ischemia rapidly resolves after cessation of exercise, the amount of time required to complete image acquisition is crucial to achieve high sensitivity of the test. For this reason, special care has been taken in the ergonomic design of

4 288 Zwas et al May 1999 Table 1 Hemodynamic parameters at pre-exercise and postexercise scanning Pre-exercise scanning Postexercise scanning AT (s) HR (bpm) BP (mm Hg) AT-post (sec) HR (bpm) BP (mm Hg) RT3D api 108 ± ± /60 35 ± ± /62 RT3D para 126 ± ± ± /60 AT, Acquisition time; AT-post, time from cessation of exercise to completion of acquisition; HR, heart rate; BP, blood pressure; RT3D, real-time 3-dimensional echocardiography; api, apical volume set; para, parasternal volume set. exercise echocardiography laboratories to minimize the time required for the patient to move from the treadmill to the left lateral decubitus image position. 7 Any maneuver that decreases the time until imaging is completed presumably adds to the sensitivity of treadmill stress echocardiography for the detection of subtle, short-lived wall motion abnormalities.ase recommends that postexercise imaging be accomplished within 60 to 90 seconds of termination of exercise. 9,10 In this study, the postexercise imaging was accomplished in 30 seconds with a single apical data set. In addition, by combining the apical and the parasternal data, the number of adequately visualized segments was 98%, which meets the rate suggested by ASE (success rates higher than 95% should be attained). 10 The imaging times shown on Table 1 include both image optimization and data acquisition times; the actual data acquisition time to capture 2 heart beats is 1 second at a HR of 120 bpm.therefore, as operator experience increases, real-time 3D echocardiography offers great potential for shortening the total imaging time. Another advantage of RT3D is its potentially increased accuracy as a result of additional viewing planes. Once the image of the heart is within the pyramidal volume, the sonographer needs only to record the volume set. Later, the physician can position the multiple scan plans to review the precise views that are required to evaluate wall motion. The mean length of the left ventricle in this RT3D study was 71 ± 9 mm, and the machine is steerable in 1-mm intervals of cross-sectional short-axis planes; therefore 70 short-axis viewing planes were available for evaluating wall motion. To be compatible with the established analysis method, the standard 16-segment model was used. However, if each plane is divided into 4 parts, 280 segments are available for analysis. If segments adjacent to the nonvisualized segments are used for analysis, of 35 unsatisfactory segments in postexercise apical data set, 10 addi- tional segments could be analyzed. This additional information should prove useful for the analysis of wall motion. In addition, all images within the volume set are obtained simultaneously, therefore corresponding segments in different views can be reviewed without the time variation that exists in a conventional 2D stress test. However, the potential limitations of this equipment should be noted. ASE recommends that at a normal HR, 20 frames/s for the majority of the examination are needed for digital capture and playback.and with HR >140 bpm, it is recommended that frame rate should be increased to 30 frames/s. 10 RT3D scans at a speed of 18 to 40 frames/s as determined by the depth settings. The frame rate at the 15-cm depth setting was 20 frames/s. Thus the low frame rate may limit the accuracy of the test. In addition, the image quality at this point lags behind that of the best commercially available digital machines. In conclusion, although continued development of both hardware and software is necessary, RT3D treadmill stress echocardiography is feasible and will very likely be an important application of this new 3D device. REFERENCES 1. Hsieh A, Collins M, Ota T, et al. Real-time 3D echocardiography: description of a new technique [abstract]. Circulation 1996;94:I Shiota T, Jones M, Chikada M, et al. Real-time three-dimensional echocardiography for detecting right ventricular stroke volume in an animal model of chronic right ventricular volume overload. Circulation 1998;97: Schiller NB, Shah PM, Crawford M, et al. Recommendations for quantitation of the left ventricle by two-dimensional echocardiography. American Society of Echocardiography Committee on Standards, Subcommittee on Quantitation of Two-Dimensional Echocardiograms. J Am Soc Echocardiogr 1989;2: Pellikka PA. Stress echocardiography in the evaluation of chest pain and accuracy in the diagnosis of coronary artery disease. Prog Cardiovasc Dis 1997;39: Berberich SN, Zager JR, Plotnick GD, et al. A practical

5 Volume 12 Number 5 Zwas et al 289 approach to exercise echocardiography: immediate post exercise echocardiography. J Am Coll Cardiol 1984;3: Sheihkh KH, Bengston JR, Helmy S, et al. Relation of quantitative coronary lesion measurements to the development of exercise-induced ischemia assessed by exercise echocardiography. J Am Coll Cardiol 1990;15: Homans DC, Laxson DD, Sublett E, et al. Effect of exercise intensity and duration on regional function during and after exercise-induced ischemia. Circulation 1991;83: Kloner RA, Allen J, Cox TA, et al. Stunned left ventricular myocardium after exercise treadmill testing in coronary artery disease. Am J Cardiol 1991;68: Armstrong WF. Stress echocardiography: introduction, history, and methods. Prog Cardiovasc Dis 1997;39: Armstrong WF, Pellikka PA, Ryan T, et al. Stress echocardiography: recommendations for performance and interpretation of stress echocardiography. Stress Echocardiography Task Force of the Nomenclature and Standard Committee of the American Society of Echocardiography. J Am Soc Echocardiogr 1998;11: