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Comparison of planar and tomographic exercise thallium-201 imaging methods for the evaluation of coronary artery disease
JACC Vol. 13, No. 3 March 1. 1989:613-6
613
Editorial Comment
Comparison of Planar and Tomographic Exercise Thallium-201 Imaging Methods for the Evaluation of Coronary Artery Disease* HOSEN KIAT, MBBS, FRACP, DANIEL
S. BERMAN,
JAMSHID
MADDAHI,
Los Angeles,
Calijkornia
MD, FACC, MD, FACC
Thallium-201 myocardial imaging is the technique of choice for assessing regional myocardial perfusion and is the most widely applied radionuclide technique for detecting and evaluating coronary artery disease (1). Conventional planar thallium-201 imaging, however, is limited by its twodimensional nature, which results in substantial myocardial tissue overlap. The introduction of single photon emission computer tomography (SPECT) (2) offers promise to circumvent the limitations of planar imaging by providing a threedimensional view of the myocardium with reduction in myocardial overlap and enhancement of lesion contrast. With multiple commercial SPECT camera computer systems now available, SPECT is rapidly becoming widely used in clinical practice. A growing body of information (3-7) suggests that it is superior to planar imaging for thallium-201, with the largest comprehensive comparison now being reported by Fintel et al. (3) in this issue of the Journal. Planar versus tomographic imaging by visual analysis. Fintel and associates (3) studied 136 patients by randomly sequenced planar and tomographic imaging and compared the visual diagnostic performance of the two imaging methods. By receiver operating characteristic (ROC) analysis (8) the study reported improved diagnostic performance for
*Editorials published in Journal of the American Colleae of Cardioloav reflect the views of the authors and do not necessarily represent-the viewsof JACC or the American College of Cardiology. From the Departments of Medicine (Division of Cardiology) and Nuclear Medicine, Cedars-Sinai Medical Center and the Department of Medicine, University of California School of Medicine, Los Angeles, California. This work was supported in part by Specialized Center of Research Grant 7651 from the National Institutes of Health, Bethesda, Maryland and a grant from the American Heart Association, Greater Los Angeles Affiliate, Los Angeles, California. Address for reerints: Daniel S. Berman, MD, Nuclear Cardiology, Cedars-Sinai Medical Center, 8700 Beverly Boulevard, Los Angeles, California 90048. 01989 by
the American
College
of Cardiology
detecting and localizing coronary artery disease by tomography over the entire range of clinically relevant specificity values (ie., specificities of ~50%). In subgroup analyses, they found that thallium SPECT was superior in men and in patients with “milder” disease (those with no prior myocardial infarction, single vessel disease and 50 to 69% coronary stenoses). Single photon emission computed tomography was also superior for identifying left anterior descending and left circumflex coronary artery disease. The improved performance by SPECT in localizing disease is of particular clinical importance in the era of coronary angioplasty. Tomographic imaging allows perfusion defects to be more accurately attributed to a branch of a coronary vessel and has been shown to be useful in identifying the “culprit” lesion in myocardial ischemia (9). With respect to correct identijication of the number of d&used vessels, Fintel et al. (3) showed that SPECT was
significantly better than the planar method for identifying patients with single or double vessel diseases. They also demonstrated the superiority of tomographic imaging in detecting multivessel disease in patients with prior myocardial infarction (3) an ability that is of major potential importance in management decisions. Although not shown in this study, others (4,6) have also found tomography to be superior in correct identification of triple vessel disease. In general, therefore, current studies (3-7) are in agreement that SPECT is better than the planar method for assessing disease extent. Order of imaging. Fintel et al. (3) showed that the order of imaging influenced the diagnostic performance of both imaging methods. Each method performed better as the first than as the second test although, even as a second test, imaging by tomography proved to be more accurate than the planar method. The better performance of the first versus the second test is most likely due to early redistribution of thallium-201, which could potentially occur during the 235 min that pass between tracer injection and completion of the initial poststress imaging sequences (10-13). In this respect, radiopharmaceuticals that do not redistribute, such as the technetium-99m isonitriles (14-17), have a potential advantage over thallium-201 for combined planar and tomographic imaging. Optimalthallium-201dose and acquisition protocols. Fintel et al. (3) report that the use of 1.5 to 2 mCi of thallium-201 generally produced “satisfactory” initial poststress and 4 h redistribution images for visual analysis. However, total imaging time was >30 min for tomographic acquisition (30 min of imaging time plus time for camera motion). Because of the relatively uncomfortable position required for tomography (supine with arms extended over the head), long acquisition times increase the possibility of patient motion 0735-1097/89/$3.50
614
KIAT ET AL. EDITORIAL COMMENT
during imaging. We and others (18) have found it preferable to increase the injected dose to 3 to 4 mCi and to reduce the imaging time to about 20 min. This combination results in improved count statistics, enhancing image quality for the initial and 4 h redistribution images. In addition, it allows the acquisition of late (18 to 72 h) redistribution images, which may be necessary to detect late reversibility in patients with nonreversible defects at 4 h (19-21). Visual versus quantitative analysis of thallium-201tomog raphy. Although the results of visual analysis by trained observers as reported by Fintel et al. (3) are likely to remain pivotal in the overall evaluation of tomographic thallium-201 images, objective quantitative analysis methods for SPECT have been developed and validated (18,22-24). These quantitative approaches circumvent the problem of intra- and interobserver variability, help in training of inexperienced readers and assist even experienced observers by providing an objective “second opinion.” Those approaches that utilize a polar display (1822) also provide a method for immediate appreciation of the presence, size and location of perfusion abnormality. Beyond objectifying interpretation, quantitative tomography offers major improvement in diagnostic performance over quantitative planar imaging, with (7,25) and without (26) addition of washout analysis, in the detection of left circumflex artery disease, which is concordant with the visual results of Fintel et al. (3). Additionally, quantitative SPECT thallium-201 imaging is emerging as an important technique for assessing the size of myocardial perfusion defect (27,28) and has been applied clinically to compare the efficacy of various treatment regimens after myocardial infarction (29). Widespread clinical utilization of these objective quantitative approaches is likely in the near future. Quality control with SPECTimaging. Although the available published reports (3-7) demonstrate the improved accuracy of SPECT over planar imaging, it is essential to recognize that tomographic imaging provides many more sources of potential false positive studies. Before image acquisition, SPECT requires greater attention than does planar imaging to collimator integrity, detector uniformity and proper center of rotation. During image acquisition, SPECT is subject to problems related to patient motion (30) and upward motion of the heart (“upward creep”) (31). Even with the shortened (20 min) tomographic imaging time, we (30,31) have found that a substantial proportion of patients demonstrate motion or “upward creep,” which results in artifactual perfusion defects. Thus, a limitation of the study by Fintel et al. (3) is the absence of mention of quality control, specifically as to what extent, if any, patients were excluded because of motion or other artifacts on tomographic images. The problems are not insurmountable. Patient motion can be minimized by keeping the patient awake during acquisition, comfortably restraining the patient’s arms out of the field of view and instructing patients
JACC Vol. 13, No. 3 March 1, 1989:6134
to refrain from talking. Upward creep can be reduced by interposing a 5 min anterior view planar acquisition between the end of exercise and the beginning of tomographic acquisition. As added benefits, addition of this anterior planar view at stress and 4 h allows assessment of the lung uptake of thallium (32), an important marker of disease extent (32,33), as well as evaluation of prognosis (34,35) and transient dilation of the left ventricle, a specific marker of severe and extensive coronary artery disease (36). Quality control for motion and upward creep is achieved by direct evaluation of the projection images in a tine display and by evaluation of added projection images. After acquisition, tomography is more subject than planar imaging to errors in selection of myocardial slices for visual display and quantitation and in alignment of slices for comparison of stress and redistribution images. With its inherently greater complexity and larger number of sources for false positive studies, thallium-201 tomogra-
phy is likely to have a lower specificity than planar imaging whether this is measured by findings in patients with normal coronary arteriograms or patients with a low likelihood of coronary artery disease (37,38). With proper attention to quality control, however, we believe that SPECT is the thallium-201 imaging method of choice. Future directions of tomographic myocardial perfusion imaging. There are many areas in which SPECT requires further development. In the clinical realm, it must be recognized that a large body of data has been collected by planar imaging with respect to the prognostic content of thallium201 studies (35,39-51). By visual analysis (39-45), normal studies have been associated with an excellent prognosis and abnormal studies with a high risk for subsequent cardiac events. This risk has been demonstrated to be a complex function of both the severity and the extent of the perfusion defect (40). Furthermore, the risk stratification by planar thallium-201 scintigraphy was shown to be superior to that of coronary angiography (45). The prognostic importance of pulmonary thallium uptake (34,35) was mentioned earlier. Quantitative analysis of thallium-201 planar studies has also been demonstrated to improve identification of patients with high risk coronary angiographic findings (52), to provide important prognostic information (35,46-51) and to add prognostically to information provided by coronary angiography (51). Studies assessing whether thallium-201 SPECT has similar prognostic power are needed. With respect to technical development, correction for tissue attenuation and scatter could improve the diagnostic accuracy of thallium201 SPECT by minimizing frequently seen basal inferior and basal septal a&factual defects due to variable attenuation as well as problems associated with breast artifact. Image quality may be further enhanced through the employment of an elliptical detector orbiting for acquisition. With the development of the technetium-99m isonitriles and their imminent clinical availability, SPECT may become
JACC Vol. 13, No. 3 March 1. 1989:613-6
KIAT ET AL. EDITORIAL COMMENT
even more important for myocardial perfusion imaging. We have preliminarily observed (53) that image defect contrast is lower by planar imaging with isonitriles than by planar thallium-201 imaging, whereas with SPECT, the isonitrile defect contrast was equal to that observed with thallium-201. Furthermore, isonitrile imaging results in much higher count rates due to the ability to use far higher doses than thallium201, thereby making it possible to perform gated tomographic studies (54), which are likely to improve perfusion defect detection and allow simultaneous assessment of perfusion and function. The work by Fintel et al. (3) has shown the superiority of tomographic over planar thallium-201 imaging at the early stage of visual image assessment. The promise of widespread use of objective quantitative analysis methods, and the forthcoming developments noted, indicate a bright future for single photon tomographic myocardial perfusion imaging.
References
61.5
13 Belier GA, Watson DD, Ackell P. Time course of thallium-201redistribution after transient myocardial ischemia. Circulation 1980;61:791-7. 14 Jones AG, Davison A, Abrams MJ, et al. Biological studies of a new class of technetium complexes: the hexakis(alkylisonitrile) technetium(l) cations. Int J Nucl Med Biol 1984:11:225-34. 15 Okada RD. Clover D, Gaffney T, Williams S. Myocardial kinetics of technetium-99m-hexakis-2-methoxy-2-methylpropyl-isonit~le. Circulation 1988:77:491-8. 16 Stirner H. Buell U, Kleinhans E, Bares R, Grosse W. Myocardial kinetics of 99T’“hexakis-(2-methoxy-isobutyl-isonitrile)(HMIBI) in patients with coronary heart disease: a comparative study versus 2OITlwith SPECT. Nucl Med Commun 1988~9: 15-23. 17 Kiat H, Maddahi J, Roy LT, Van Train K, Friedman J, Berman DS. Comparison of technetium-99m methoxy isobutyl isonitrile and thallium201for evaluation of coronary artery disease by planar and tomographic methods. Am Heart J (in press). 18 DePasquale EE. Nody AC, DePuey EG. et al. Quantitative rotational thallium-201tomography for identifying and localizing coronary artery disease. Circulation 1988;77:316-27. 19 Kiat H, Berman D, Maddahi J. et al. Late reversibility of tomographic myocardial thallium-201defects: an accurate marker of myocardial viability. J Am Co11Cardiol 1988:12:1456-63. 20 Yang LD. Berman D, Kiat H, et al. Frequency of late reversibility in SPECT TI-201 stress redistribution studies (abstr). J Am Coll Cardiol 1989;13:28A.
1. Berman DS, Garcia EV. Maddahi J, Rozanski A. Thallium-201myocardial perfusion scintigraphy. In: Freeman LM, ed. Freeman and Johnson’s Clinical Radionuclide Imaging. 3rd ed. Orlando, FL: Grune & Stratton, 1984:47%537.
21 Cloninger KG, DePuey EG, Garcia EV, et al. Incomplete redistribution in delayed thallium-201 single photon emission computed tomographic (SPECT) images: an overestimation of myocardial scarring. J Am Co11 Cardiol 1988;12:955-63.
2. Budinger TF. Physical attributes of single photon tomography. J Nucl Med 1980;21:579-512.
22 Garcia EV. Van Train K, Maddahi J, et al. Quantification of rotational thallium-201myocardial tomography. J Nucl Med 1985:26:17-26.
3. Fintel DJ, Links JM. Brinker JA, Frank TL, Parker M, Becker L. Improved diagnostic performance of exercise thallium-201single photon emission computed tomography over planar imaging in the diagnosis of coronary artery disease: a receiver operating characteristic analysis. J Am Co11Cardiol 1989;13:600-12.
23 Tamaki N. Yonekura Y, Mukai T, et al. Stress thallium-201transaxial emission computed tomography: quantitative versus qualitative analysis for evaluation of coronary artery disease. J Am Coll Cardiol 1984;4:121321.
4. Nohara R, Kambara H, Suzuki Y, et al. Stress scintigraphy using single-photon emission computed tomography in the evaluation of coronary artery disease. Am J Cardiol 1984;53:125w. 5. Prigent F. Friedman J. Maddahi J, et al. Comparison of rotational tomography with planar imaging for thallium-201stress myocardial scintigraphy (abstr). J Nucl Med 1983:24:18. 6. Tamaki N, Yonekura Y, Mukal T, et al. Segmental analysis of stress thallium myocardial emission tomography for localization of coronary artery disease. Eur J Nucl Med 1984:9:99-105. 7. Maddahi J. Van Train KF, Wong C, et al. Comparison of Tl-201 single photon emission computerized tomography (SPECT) and planar imaging for evaluation of coronary artery disease (abstr). J Nucl Med 1986;27:999. 8. Metz CE. Basic principles of ROC analysis. Semin Nucl Med 1978:8:28398. 9. Breisblatt WM. Barnes JV. Weiland F, Spaccavento LJ. Incomplete revascularization m multivessel percutaneous transluminal coronary angioplasty: the role for stress thallium-201imaging. J Am Coil Cardiol 1988;11:1183-90.
24 Maddahi J. Van Train K, Wong C, et al. Quantitative analysis of TI-201 myocardial single-photon emission computerized rotational tomography: development. validation, and prospective application of an optimized computer method (abstr). J Am Co11Cardiol 1986:7:22A. 25 Kiat H, Maddahi J. Daly N, Ostrzega E, Van Train K. Accurate detection of symtomatic and asymtomatic single vessel disease by quantitative Thallium-201planar or tomographic imaging vs. exercise electrocardiography (ECG) (abstr). Circulation 1987;76(suppIIV):IV-484. 26 Port SC, Oshima M, Ray G, McNamee P. Schmidt DH. Assessment of single vessel coronary artery disease: results of exercise electrocardiography, thallium-201myocardial perfusion imaging and radionuclide angiography. J Am Co11Cardiol 1985:6:75-83. 27 Prigent F. Maddahi J, Garcia EV. Satoh Y. Van Train K, Berman DS. Quantification of myocardial infarct size by thallium-201 single photon emission computerized tomography: experimental validation in the dog. Circulation 1986:74:852-61. 28. Prigent F. Maddahi J, Garcia EV, Resser K, Lew AS, Berman DS. Comparative methods for quantifying myocardial infarct size by thallium201 SPECT. J Nucl Med 1987:28:325-33.
IO. Gutman J. Berman DS, Freeman M, et al. Time to completed redistribution of thallium-201 in exercise myocardial scintigraphy: relationship to the degree of coronary artery stenosis. Am Heart J 1983;106:989-95.
29. Maublant JC. Peycelon P. Cardot JC. Verdenet JV, Fagret D, Comet M. Value of myocardial defect size measured by thallium-201SPECT: results of a multicenter trial comparing heparin and a new fibrinolytic agent. J Nucl Med 1988;29:1486-91.
II. Schwartz JS, Pontn R, Carlyle P, Forstrom L, Cohn JN. Early redistribution of thallium-201after temporary ischemia. Circulation 1978;57:3325.
30. Friedman J. Berman DS, Van Train K. et al. Patient motion in thallium201 myocardial SPECT imaging: an easily identified frequent source of artifactual defect. Clin Nucl Med 1988;13:3214.
12. Rothendler JA, Okada RD, Wilson RA, et al. Effect of a delay in commencing imaging on the ability to detect transient thallium defects. J Nucl Med 1985:26:X80-3.
31. Friedman J, Van Train K, Maddahi J, et al. “Upward creep” of the heart: a frequent source of false positive perfusion defects on thallium-201 stress-redistribution SPECT (abstr). J Nucl Med 1986:27:899-900.
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KIAT ET AL. EDITORIAL COMMENT
32. Kushner FG, Okada RD, Kirshenbaum HD, Boucher CA, Strauss HW, Pohost GM. Lung thallium-201uptake after stress testing in patients with coronary artery disease. Circulation 1981;63:341-7.
JACC Vol. 13, No. 3 March 1, 1989:613-6
Incremental prognostic power of clinical history, exercise electrocardiography and myocardial perfusion scintigraphy in suspected coronary artery disease. Am J Cardiol 1987;59:27C-7.
33. Levy R, Rozanski A, Berman DS, et al. Analysis of the degree of pulmonary thallium washout after exercise in patients with coronary artery disease. J Am Coil Cardiol 1983;2:719-28.
45. Kotler TS, Maddahi J, Berman DS. Is thallium scintigraphy better than coronary angiography for prognosis? (abstr). Circulation 1986;74(suppl II):II-512.
34. Gill JB, Ruddy TD, Newell JB, Finkelstein DM, Strauss HW, Boucher CA. Prognostic importance of thallium uptake by the lungs during exercise in coronary artery disease. N Engl J Med 1987;317:1485-9.
46. Kaul S, Chesler DA, Okada RD, Boucher CA. Computer versus visual analysis of exercise thallium-201 images: a critical appraisal in 325 patients with chest pain. Am Heart J 1987;114:1129-37.
35. Kaul S, Finkelstein D, Homma S, Leavitt M, Okada RD, Boucher CA. Superiority of quantitative exercise thallium-201variables in determining long-term prognosis in ambulatory patients with chest pain: a comparison with cardiac catheterization. J Am Co11Cardiol 1988;12:25-34.
47. Wilson WW, Gibson RS, Nygaard TW, et al. Acute myocardial infarction associated with single vessel coronary artery disease: an analysis of clinical outcome and the prognostic importance of vessel patency and residual ischemic myocardium. J Am Coil Cardiol 1988;11:223-34.
36. Weiss AT, Berman DS, Lew AS, et al. Transient ischemic dilation of the left ventricle on stress thallium-201scintigraphy: a marker of severe and extensive coronary artery disease. J Am Coll Cardiol 1987;9:752-9.
48. Brown KA, Weiss RM, Clements JP, Wackers F. Usefulness of residual ischemic myocardium within prior infarct zone for identifying patients at high risk late after acute myocardial infarction, Am J Cardiol 1987;60:159.
37. Van Train KF, Berman DS, Garcia EV, et al. Quantitative analysis of stress thallium-201 myocardial scintigrams: a multicenter trial. J Nucl Med 1986;27:17-25. 38. Van Train K, Maddahi J, Berman D. A multicenter trial of the CedarsSinai method for quantitative analysis of TI-201myocardial tomography (abstr). J Nucl Med 1988;29:755. 39. Staniloff HM, Forrester JS, Berman DS, Swan HJC. Prediction of death myocardial infarction, and worsening chest pain using thallium-201scintigraphy and exercise electrocardiography. J Nucl Med 1986;27:1842-8. 40. Brown KA, Boucher CA, Okada RD, Gurney TE, Newell JB, Strauss HW, Pohost GM. Prognostic value of exercise thallium-201imaging in patients presenting for evaluation of chest pain. J Am Co11Cardiol 1984;l: 994-1001. 41. Boucher CA, Brewster DC, Darling RC, Okada RD, Strauss HW, Pohost GM. Determination of cardiac risk by dipyridamole-thallium imaging before peripheral vascular surgery. N Engl J Med 1985;312:389-94. 42. Ladenheim M, Pollock BH, Rozinski A, et al. Extent and severity of myocardial hypoperfusion as orthogonal parameters of prognosis in patients with suspected coronary artery disease. J Am Coll Cardiol 1986; 7:46&71. 43. Leppo J, Plaja J, Gionet M, Tumolo J, Paraskos JA, Cutler BS. Noninvasive evaluation of cardiac risk before elective vascular surgery. J Am Co11Cardiol 1987;9:269-76. 44. Ladenheim ML, Kotler TS, Pollock BH, Berman DS, Diamond GA.
49. Maddahi J, Kiat H, Resser K, et al. Prediction of coronary events by Thallium-201quantitation of jeopardized myocardium and clinical and exercise test variables (abstr). Circulation 1987;76(supplIV):IV-156. 50. Maddahi J, Kiat H, Resser K, et al. A new approach to quantitative interpretation of exercise thallium-201 scintigraphy for assessment of prognosis in patients with prior myocardial infarction (abstr). J Nucl Med 1988;29:951. 51. Kaul S, Lilly DR, Gascho JA, et al. Prognostic utility of the exercise thallium-201test in ambulatory patients with chest pain: comparison with cardiac catheterization. Circulation 1988;77:745-58. 52. Maddahi J, Abdulla A, Garcia EV, Swan HJC, Berman DS. Noninvasive identification of left main and triple vessel coronary artery disease: improved accuracy using quantitative analysis of regional myocardial stress distribution and washout of thallium-201.J Am Co11Cardiol 1986;7: 53-60. 53. Maddahi J, Merz R, Van Train K, Roy L, Wong C, Berman DS. Tc-99m MIBI(RP-30) and Tl-201 myocardial perfusion scintigraphy in patients with coronary artery disease: quantitative comparison of planar and tomographic techniques for perfusion defect intensity and defect reversibility (abstr). J Nucl Med 1987;28:654. 54. Kahn JK, Henderson EB, Akers MS, et al. Gated and ungated tomographic perfusion imagingwith technetium99m RP-30A: comparison with Tl-201tomography in coronary artery disease (abstr). J Am Coil Cardiol 1988;11:32A.
613
Editorial Comment
Comparison of Planar and Tomographic Exercise Thallium-201 Imaging Methods for the Evaluation of Coronary Artery Disease* HOSEN KIAT, MBBS, FRACP, DANIEL
S. BERMAN,
JAMSHID
MADDAHI,
Los Angeles,
Calijkornia
MD, FACC, MD, FACC
Thallium-201 myocardial imaging is the technique of choice for assessing regional myocardial perfusion and is the most widely applied radionuclide technique for detecting and evaluating coronary artery disease (1). Conventional planar thallium-201 imaging, however, is limited by its twodimensional nature, which results in substantial myocardial tissue overlap. The introduction of single photon emission computer tomography (SPECT) (2) offers promise to circumvent the limitations of planar imaging by providing a threedimensional view of the myocardium with reduction in myocardial overlap and enhancement of lesion contrast. With multiple commercial SPECT camera computer systems now available, SPECT is rapidly becoming widely used in clinical practice. A growing body of information (3-7) suggests that it is superior to planar imaging for thallium-201, with the largest comprehensive comparison now being reported by Fintel et al. (3) in this issue of the Journal. Planar versus tomographic imaging by visual analysis. Fintel and associates (3) studied 136 patients by randomly sequenced planar and tomographic imaging and compared the visual diagnostic performance of the two imaging methods. By receiver operating characteristic (ROC) analysis (8) the study reported improved diagnostic performance for
*Editorials published in Journal of the American Colleae of Cardioloav reflect the views of the authors and do not necessarily represent-the viewsof JACC or the American College of Cardiology. From the Departments of Medicine (Division of Cardiology) and Nuclear Medicine, Cedars-Sinai Medical Center and the Department of Medicine, University of California School of Medicine, Los Angeles, California. This work was supported in part by Specialized Center of Research Grant 7651 from the National Institutes of Health, Bethesda, Maryland and a grant from the American Heart Association, Greater Los Angeles Affiliate, Los Angeles, California. Address for reerints: Daniel S. Berman, MD, Nuclear Cardiology, Cedars-Sinai Medical Center, 8700 Beverly Boulevard, Los Angeles, California 90048. 01989 by
the American
College
of Cardiology
detecting and localizing coronary artery disease by tomography over the entire range of clinically relevant specificity values (ie., specificities of ~50%). In subgroup analyses, they found that thallium SPECT was superior in men and in patients with “milder” disease (those with no prior myocardial infarction, single vessel disease and 50 to 69% coronary stenoses). Single photon emission computed tomography was also superior for identifying left anterior descending and left circumflex coronary artery disease. The improved performance by SPECT in localizing disease is of particular clinical importance in the era of coronary angioplasty. Tomographic imaging allows perfusion defects to be more accurately attributed to a branch of a coronary vessel and has been shown to be useful in identifying the “culprit” lesion in myocardial ischemia (9). With respect to correct identijication of the number of d&used vessels, Fintel et al. (3) showed that SPECT was
significantly better than the planar method for identifying patients with single or double vessel diseases. They also demonstrated the superiority of tomographic imaging in detecting multivessel disease in patients with prior myocardial infarction (3) an ability that is of major potential importance in management decisions. Although not shown in this study, others (4,6) have also found tomography to be superior in correct identification of triple vessel disease. In general, therefore, current studies (3-7) are in agreement that SPECT is better than the planar method for assessing disease extent. Order of imaging. Fintel et al. (3) showed that the order of imaging influenced the diagnostic performance of both imaging methods. Each method performed better as the first than as the second test although, even as a second test, imaging by tomography proved to be more accurate than the planar method. The better performance of the first versus the second test is most likely due to early redistribution of thallium-201, which could potentially occur during the 235 min that pass between tracer injection and completion of the initial poststress imaging sequences (10-13). In this respect, radiopharmaceuticals that do not redistribute, such as the technetium-99m isonitriles (14-17), have a potential advantage over thallium-201 for combined planar and tomographic imaging. Optimalthallium-201dose and acquisition protocols. Fintel et al. (3) report that the use of 1.5 to 2 mCi of thallium-201 generally produced “satisfactory” initial poststress and 4 h redistribution images for visual analysis. However, total imaging time was >30 min for tomographic acquisition (30 min of imaging time plus time for camera motion). Because of the relatively uncomfortable position required for tomography (supine with arms extended over the head), long acquisition times increase the possibility of patient motion 0735-1097/89/$3.50
614
KIAT ET AL. EDITORIAL COMMENT
during imaging. We and others (18) have found it preferable to increase the injected dose to 3 to 4 mCi and to reduce the imaging time to about 20 min. This combination results in improved count statistics, enhancing image quality for the initial and 4 h redistribution images. In addition, it allows the acquisition of late (18 to 72 h) redistribution images, which may be necessary to detect late reversibility in patients with nonreversible defects at 4 h (19-21). Visual versus quantitative analysis of thallium-201tomog raphy. Although the results of visual analysis by trained observers as reported by Fintel et al. (3) are likely to remain pivotal in the overall evaluation of tomographic thallium-201 images, objective quantitative analysis methods for SPECT have been developed and validated (18,22-24). These quantitative approaches circumvent the problem of intra- and interobserver variability, help in training of inexperienced readers and assist even experienced observers by providing an objective “second opinion.” Those approaches that utilize a polar display (1822) also provide a method for immediate appreciation of the presence, size and location of perfusion abnormality. Beyond objectifying interpretation, quantitative tomography offers major improvement in diagnostic performance over quantitative planar imaging, with (7,25) and without (26) addition of washout analysis, in the detection of left circumflex artery disease, which is concordant with the visual results of Fintel et al. (3). Additionally, quantitative SPECT thallium-201 imaging is emerging as an important technique for assessing the size of myocardial perfusion defect (27,28) and has been applied clinically to compare the efficacy of various treatment regimens after myocardial infarction (29). Widespread clinical utilization of these objective quantitative approaches is likely in the near future. Quality control with SPECTimaging. Although the available published reports (3-7) demonstrate the improved accuracy of SPECT over planar imaging, it is essential to recognize that tomographic imaging provides many more sources of potential false positive studies. Before image acquisition, SPECT requires greater attention than does planar imaging to collimator integrity, detector uniformity and proper center of rotation. During image acquisition, SPECT is subject to problems related to patient motion (30) and upward motion of the heart (“upward creep”) (31). Even with the shortened (20 min) tomographic imaging time, we (30,31) have found that a substantial proportion of patients demonstrate motion or “upward creep,” which results in artifactual perfusion defects. Thus, a limitation of the study by Fintel et al. (3) is the absence of mention of quality control, specifically as to what extent, if any, patients were excluded because of motion or other artifacts on tomographic images. The problems are not insurmountable. Patient motion can be minimized by keeping the patient awake during acquisition, comfortably restraining the patient’s arms out of the field of view and instructing patients
JACC Vol. 13, No. 3 March 1, 1989:6134
to refrain from talking. Upward creep can be reduced by interposing a 5 min anterior view planar acquisition between the end of exercise and the beginning of tomographic acquisition. As added benefits, addition of this anterior planar view at stress and 4 h allows assessment of the lung uptake of thallium (32), an important marker of disease extent (32,33), as well as evaluation of prognosis (34,35) and transient dilation of the left ventricle, a specific marker of severe and extensive coronary artery disease (36). Quality control for motion and upward creep is achieved by direct evaluation of the projection images in a tine display and by evaluation of added projection images. After acquisition, tomography is more subject than planar imaging to errors in selection of myocardial slices for visual display and quantitation and in alignment of slices for comparison of stress and redistribution images. With its inherently greater complexity and larger number of sources for false positive studies, thallium-201 tomogra-
phy is likely to have a lower specificity than planar imaging whether this is measured by findings in patients with normal coronary arteriograms or patients with a low likelihood of coronary artery disease (37,38). With proper attention to quality control, however, we believe that SPECT is the thallium-201 imaging method of choice. Future directions of tomographic myocardial perfusion imaging. There are many areas in which SPECT requires further development. In the clinical realm, it must be recognized that a large body of data has been collected by planar imaging with respect to the prognostic content of thallium201 studies (35,39-51). By visual analysis (39-45), normal studies have been associated with an excellent prognosis and abnormal studies with a high risk for subsequent cardiac events. This risk has been demonstrated to be a complex function of both the severity and the extent of the perfusion defect (40). Furthermore, the risk stratification by planar thallium-201 scintigraphy was shown to be superior to that of coronary angiography (45). The prognostic importance of pulmonary thallium uptake (34,35) was mentioned earlier. Quantitative analysis of thallium-201 planar studies has also been demonstrated to improve identification of patients with high risk coronary angiographic findings (52), to provide important prognostic information (35,46-51) and to add prognostically to information provided by coronary angiography (51). Studies assessing whether thallium-201 SPECT has similar prognostic power are needed. With respect to technical development, correction for tissue attenuation and scatter could improve the diagnostic accuracy of thallium201 SPECT by minimizing frequently seen basal inferior and basal septal a&factual defects due to variable attenuation as well as problems associated with breast artifact. Image quality may be further enhanced through the employment of an elliptical detector orbiting for acquisition. With the development of the technetium-99m isonitriles and their imminent clinical availability, SPECT may become
JACC Vol. 13, No. 3 March 1. 1989:613-6
KIAT ET AL. EDITORIAL COMMENT
even more important for myocardial perfusion imaging. We have preliminarily observed (53) that image defect contrast is lower by planar imaging with isonitriles than by planar thallium-201 imaging, whereas with SPECT, the isonitrile defect contrast was equal to that observed with thallium-201. Furthermore, isonitrile imaging results in much higher count rates due to the ability to use far higher doses than thallium201, thereby making it possible to perform gated tomographic studies (54), which are likely to improve perfusion defect detection and allow simultaneous assessment of perfusion and function. The work by Fintel et al. (3) has shown the superiority of tomographic over planar thallium-201 imaging at the early stage of visual image assessment. The promise of widespread use of objective quantitative analysis methods, and the forthcoming developments noted, indicate a bright future for single photon tomographic myocardial perfusion imaging.
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