New approach to interpretation of technetium-99m pyrophosphate scintigraphy in detection of acute myocardial infarction

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The American

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1977

of CARDIOLOGY

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V O L U M E 39 NUMBER

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CLINICAL STUDIES

New Approach to Interpretation of Technetium-99m Pyrophosphate Scintigraphy in Detection of Acute Myocardial Infarction Clinical Assessment of Diagnostic Accuracy DANIEL S. BERMAN, MD EZRA A. AMSTERDAM, MD, FACC HORACE H. HINES, MA ANTONE F. SALEL, MD, FACC GERALD J. BAILEY, MD GERALD L. DeNARDO, MD DEAN T. MASON, MD, FACC Davis and Sacramento, California

From the Sectionsof CardiovascularMedicineand Nuclear Medicine, Departmentsof Medicine and Radiology, Universityof Californiaat Davis School of Medicine and Sacramento Medical Center, Davis and Sacramento, California. This studywas supported in part by Research Program Project Grant HL 14780 from The National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland and research grants from California chapters of The American Heart Association, Dallas, Texas. Manuscript received August 17, 1976; revised manuscript received September 27, 1976, accepted September 29, 1976. Address for reprints: Daniel S. Berman, MD, Section of CardiovascularMedicine, Department of Medicine, School of Medicine, University of California, Davis, California 96616.

A modified classification for interpreting technetium-99m pyrophosphate scintigrams defines the 2-t- diffuse pattern of tracer uptake as equivocal rather than positive for acute myocardial infarction. Results of scintigraphy using this classification were compared with results of standard diagnostic tests for myocardial infarction in 235 patients admitted to a coronary care unit with acute chest pain. Of 81 patients with acute transmural infarction by standard clinical, electrocardiographic and serum enzyme criteria, 76 had a positive, 5 an equivocal and none a negative scintigram. Of 18 with acute nontransmural infarction by standard criteria, 7 had a positive, 9 an equivocal and 2 a negative scintigram. Thus it was uncommon for a patient with acute myocardial infarction, transmural or nontransmural, to have a definitely negative technetium-99m pyrophosphate study. Ten patients had equivocal evidence of infarction by standard criteria. Of the remaining 126 patients with no evidence of acute myocardial infarction by standard criteria, 87 had a negative, 35 an equivocal and 4 a definitely positive scintigram. Thus the definitely positive scinUgraphic pattern was relatively highly specific for acute myocardial infarction. If the 2-1- pattern had been considered positive, the specificity of the technique would have been greatly decreased. Computer processing strengthened observer certainty of the visual impression but changed the scinUgraphic evaluation in only eight cases. Thus, use of an equivocal pattern renders techneUum-99m pyrophosphate imaging both an extremely sensitive and specific method for detecting acute myocardial infarction.

After the discovery by Bonte and associates 1,2 that technetium-99m pyrophosphate accumulates in acutely infarcted myocardium, a scintigraphic method was developed that has shown promise in detecting acute myocardial infarction. In our initial studies3 with technetium-99m pyrophosphate, we found relatively good correlation between abnormal images and the presence of acute transmural infarction; however, borderline scintigrams showing moderate diffuse pyrophosphate activity throughout the left ventricular region were difficult to interpret. Most investigators4-7 reporting lack of specificity of the technetium-99m

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pyrophosphate technique for detecting acute myocardial infarction have focused on the problem of the diffuse pattern. We have developed a new classification for interpretation of the technetium-99m pyrophosphate scintigram in which this moderate diffuse pattern of uptake is considered equivocal. In this investigation we evaluate the clinical accuracy of this approach by comparing our results with the results of standard diagnostic tests for infarction in a large group of patients admitted to a coronary care unit with acute chest pain. Materials and Methods Patients: Radionuclide imaging was performed in 235 patients admitted to the coronary care unit of the Sacramento Medical Center with acute chest pain suspected to be due to acute myocardial infarction. Scintigraphy was performed 12 hours to 5 days (mean 40 hours) after the onset of chest pain. The patient population consisted of 169 men and 66 women; mean age (4- standard deviation) of 64 4- 12 years (range 27 to 92 years). Radionuclide studies: After the mixing of stannous pyrophosphate (Mallinckrodt Chemical Works) with technetium-99m pertechnetate (Mallinckrodt Chemical Works or

Squibb & Sons), 15 mCi of the technetium-99m pyrophosphate containing 5 mg of stannous pyrophosphate was injected intravenously into each patient. To minimize blood background radioactivity, imaging was performed 2 hours after injection rather than the conventional 45 minutes to 1 hour after injection. Imaging was performed in the Nuclear Medicine Department, which is adjacent to the coronary care unit. At all times, the patient's electrocardiogram was monitored, and emergency medications as well as defibrillatory equipment were present. Imaging was performed with a PhoGamma IV scintillation camera (Searle Radiographics, Inc.) equipped with a 16,000 hole high resolution collimator. Images containing 500,000 counts were obtained in the anterior, 45 ° leftanterior oblique, leftlateral and 30 ° right anterior oblique positions, using both Polaroid ® and microdot films. In selected patients whose costochondral junctions were superimposed over the lateral left ventricular wall in the anterior view, a modified anterior view was also obtained. In this view the head of the detector was rotated to a 10 ° cephalad and 10 ° right anterior oblique position. The scintigraphic data were also collected and stored on computer disc (Medical Data Systems). The time required for the procedure was approximately 30 minutes. Scintigraphic classification: The scintigrams were assessed by two independent observers (D.B. and G.B.) who

FIGURE 1, Technetium-99m pyrophosphate myocardial scintigrams in a patient without acute infarction (top, A to C) and in a patient with acute anterior myocardial infarction (bottom, D to F). In the former study, the myocardium is not visualized. In the latter, intense accumulation of radioactivity is detected in the anterior wall of the left ventricle. ANT = anterior view; LAO = left anterior oblique view; L LAT = left lateral view; MI = myocardial infarction.

FIGURE 2. Technetium-99m pyrophosphate myocardial scintigrams in a patient with acute inferior myocardial infarction (top, A to C) and in a patient with acute posterolateral infarction (bottom, D to F). In the former there is focal uptake of pyrophosphate in the inferior left ventricular wall; in the latter, the abnormal uptake pattern is demonstrated in the posterior and lateral walls of the ventricle. Abbreviations as in Figure 1.

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were unaware of the patient's clinical state. The degree of radionuclide uptake in the cardiac region was assessed as follows (Table I): 4+ indicated activity greater than that of bone; 3+, activity equal to that of bone; 2+, activity less than that of bone but moderate; 1+, slight activity; and 0, no detectable activity. In addition, the uptake pattern was judged to be focal or diffuse. The scintigraphic study showing 0 to 1+ myocardial activity was considered negative (Fig. 1, A to C). A study showing 3+ to 4+ myocardial activity, in either focal or diffuse patterns, or 2+ focal activity was considered positive (Fig. 1, D to F and Fig. 2, A to F). The interpretation of the 2+ diffuse pattern represents the principal difference in our criteria from those of previous workers1; we classified this type of scintigraphic pattern as equivocal rather than positive (Fig. 3, AtoC). Computer methods: All scintigraphic data were collected on computer disc, and computer evaluation was carried out in 130 of the 235 cases. The computer processing consisted of two basic techniques: (1) background subtraction, and (2) rib subtraction using a program developed by one of us (H.H.). For background subtraction, the background count level was determined and subtracted from the entire image. The resuiting image was then displayed using 16 gray tones. For rib subtraction, regions of interest corresponding to rib areas were defined with use of a light pen, and a variable number of counts determined by the operator were subtracted from these regions. No special hardware was required for either computer processing method. Clinical and laboratory diagnosis: All patients were evaluated with a careful history and physical examination, serial standard 12 lead electrocardiograms and serial serum enzyme determinations (creatine kinase [CK], serum glutamic oxaloacetic transaminase [SGOT] and lactic dehydrogenase [LDH]). All electrocardiographic and myocardial enzyme data were obtained at least daily for 3 consecutive days after admission. When the LDH level was elevated, LDH isoenzymes were also obtained. The development of new Q waves of 0.04 second's duration with typical ST-T wave changes and characteristic clinical and enzyme abnormalities was required for the diagnosis of acute transmural infarction. Typical evolutionary ST-T wave changes without new pathologic Q waves, when accompanied by characteristic clinical and enzymatic changes of acute infarction, were considered diagnostic of nontransmural myocardial infarction. In patients whose clinical presentation, enzyme patterns and serial electrocardiographic changes were not clearly diagnostic of the presence or absence of infarction, the diagnosis was considered equivocal for acute infarction. By reviewing the patient's hospital records after discharge, the director of the coronary care unit (E.A.) determined the absence or presence and type of infarction without knowledge of the scintigraphic results. Certain patients underwent left heart catheterization with left

TABLE I Criteria for Scintigraphic Assessment G rade

Assessment

A. Degree of Tracer Uptake in Region of Myocardium 4+ 3+ 2+ 1+ 0

Greatest intensity Equal to bone intensity Moderate Slight Nonvisualized

B. Interpretation of Myocardial Tracer Uptake 0 to 1+ 3 to 4+ 2+ Focal 2+ Diffuse*

Negative Positive Positive Equivocal*

* This category represents the principal difference between the criteria used in our study and those of previous workers.

ventricular cineangiography and selective coronary arteriography in addition to the standard diagnostic studies. Results

Scintigraphic assessment of presence of infarction (Table II): Clinical, electrocardiographic and enzyme evidence of acute myocardial infarction was present in 99 p a t i e n t s - - 8 1 with acute t r a n s m u r a l infarction and 18 with acute n o n t r a n s m u r a l infarction. Among the 81 patients with acute transmural infarction, the scintigrams were positive in 76 (94 percent), equivocal in 5 (6 percent) and negative in none. T h e sensitivity for the positive detection of acute nont r a n s m u r a l infarction was less: Among the 18 patients, radionuclide studies were positive in only 7 (39 percent) and were equivocal in 9 (50 percent). However, only two patients (11 percent) had a false negative study. Thus, among the 99 patients with myocardial infarction, t e c h n e t i u m - 9 9 m p y r o p h o s p h a t e scintigraphy was negative in only 2 (2 percent). In 10 patients, the chnical, electrocardiographic and enzyme evidence of acute infarction was considered equivocal; 4 of these 10 patients had a positive, 1 an equivocal and 5 a negative scintigraphic study. T h e remaining 126 patients had no evidence of acute myocardial infarction by clinical, electrocardiographic and enzymatic criteria. Eighty-seven (69 percent) of these patients had a negative scintigram and 35 (28 percent) an equivocal scintigram. T h e remaining four

FIGURE 3. Equivocal pattern in technetiurn-99m pyrophosphate myocardial scintigrams. The patient, a 61 year old woman with acute chest pain but no evidence of acute myocardial infarction, was subsequently found to have normal coronary arteriograms. 2 + diffuse radioactivity is evident throughout the region of the left ventricle. Abbreviations as in Figure 1.

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TABLE II Scintigraphic Patterns in 235 Patients with Suspected Acute Myocardial Infarction

NORMAL RIB S U B T R A C T I O N

A.

Technetium-99m Scintigram Clinical Diagnosis*

Patients (no.)

Transmural infarction Nontransmural infarction Equivocal No infarction Totals

81 18 10 126 235

UNPROCESSED

Positive Negative Equivocal 76 7 4 4 91

0 2 5 87 94

_-~~

PROCEDURE

5 9 1 35 50

B.

BACKGROUND SUBTRACTED

~'

* Based on clinical, electrocardiographic and enzymatic data.

c. / , , " patients (3 percent) had a definitely positive scintigram. Two of these four underwent cardiac catheterization; both patients had evidence of three vessel coronary artery disease and a large localized region of akinesia in the left ventricular cineangiogram. In both patients the scintigraphic pattern was 3+ focal and the area of increased activity corresponded to a region of old myocardial infarction in the electrocardiogram and to the large area of akinesia evident in the angiogram. The remaining two of these four patients each had 4+ diffuse uptake of pyrophosphate throughout the left ventricular myocardium in two scintigraphic studies performed 1 year apart. One of these patients had diabetes mellitus, left bundle branch block and a history of two cardiac arrests during abdominal surgery performed 1 year before the initial radionuclide study. The second patient was 87 years old and had no objective noninvasive evidence of heart disease. Neither patient had calcification in the region of the heart on chest X-ray examination or cardiac fluoroscopy with image intensification. Equivocal scintigrams: Fifty of the 235 patients had the equivocal (2+ diffuse) scintigraphic pattern (Table II). By standard clinical, electrocardiographic and enzyme criteria, 5 of these 50 patients had definite acute transmural myocardial infarction, 9 had acute nontransmural infarction and 1 had equivocal evidence of infarction; the remaining 35 had no evidence of acute myocardial infarction. None of the 50 patients had evidence of renal dysfunction. The sex distribution of the 35 patients without infarction and an equivocal scintigram (12 women and 23 men) did not differ significantly (P >0.05) from that of the 87 patients with no infarction and a negative scintigram (27 women and 60 men), but the former were older (mean age 64 4- 14 [standard deviation] years compared with 53 4- 13 years, P <0.05). Twenty-two of the 35 patients with an equivocal scintigram without infarction underwent complete diagnostic cardiac catheterization. Eleven of these 22 were found to have significant coronary arterial stenosis (more than 75 percent luminal narrowing of at least one major coronary artery): Two had one vessel stenosis, three had two vessel obstruction and six had stenosis of all three major coronary arteries. Seven of these 11 patients with coronary disease had localized areas of akinesia or dyskinesia on left ventricular cineangiography, 2 had regions of hypokinesia and 2 had a normal left ventricular contractile pattern. The remaining 11 patients who underwent cardiac catheterization had 344

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N O N - C A R D I A C RIB A R E A S DEFINED

f

RIB S U B T R A C T I O N D. A R E A S DEFINED

~ _

E. RIBS S U B T R A C T E D

LAO FIGURE 4. Method of computer processing for background and rib subtraction in a patient with acute infarction. In the unprocessed image (A) diffuse radioactivity is seen in the region of the heart. After background subtraction (B), the remaining activity appears to be only in the bony structures. Panels C and D demonstrate the steps involved in rib subtraction; the bright linear areas represent the rib regions of interest. After rib subtraction (E), the absence of myocardial radioactivity is confirmed. LAO = left anterior oblique.

normal coronary arteriograms: 6 were found to have idiopathic cardiomyopathy (congestive in 3 and nonobstructive hypertrophic in 3); 3 had valve disease (1 with aortic and mitral prostheses, 1 with a mitral prosthesis and severe aortic regurgitation and 1 with aortic stenosis and regurgitation); I had an atrial septal defect with a 1.5:1 pulmonary to systemic flow ratio; and I had normal findings on cardiac catheterization (Fig. 3). Interobserver variation: The evaluations of the two independent scintigraphic observers were in agreement in 225 of the 235 cases. In the remaining 10 cases the evaluations disagreed by only one scan classification. No scintigraphic study was interpreted as definitely negative by one observer and definitely positive by the other. Computer processing: Background or rib subtraction techniques, or both, were performed in 130 of the 235 patients. A clinical example of the computer processing techniques is demonstrated in Figure 4. In the anterior view, rib subtraction was seldom utilized because in most patients the region of the heart is medial to the costochondral junction and is therefore not obscured by overlying rib activity. In patients with superimposition of the rib ends and the lateral myocardial wall, the modified anterior view was utilized in order to separate the cardiac margin from the overlying ribs, thereby obviating the need for computerized rib sub-

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traction. Thus, rib subtraction was utilized chiefly in the left anterior oblique and left lateral views. The principal effect of computer processing was to strengthen observer certainty about the visual impression of the scintigrams. In only eight studies did the procedure lead to a changed scintigraphic interpretation, and all eight of these studies were considered equivocal by visual inspection without subtraction. Six of the eight were interpreted as definitely negative after minor background subtraction, when the 2+ diffuse activity was no longer observed, and two were interpreted as positive after computer processing in which the 2+ diffuse pattern was shown to be focal on the background-subtracted image. In the 50 patients whose scintigrams were classified equivocal (Table II) this classification was based on visual impression of the unprocessed scintigrams; computer processing reduced the number of equivocal studies to 42. In all eight cases in which computer processing altered the scintigraphic interpretation, the final classification correlated correctly with the clinical, electrocardiographic and enzymatic data. Although the rib subtraction method clarified questionable abnormalities in the left anterior oblique and left lateral views, it did not change the interpretation of the scintigrams in any patient because the anterior view or modified anterior view allowed determination of whether the radioactivity was in the myocardium or ribs. Discussion

After the development of technetium-99m pyrophosphate scintigraphy for studies of acute myocardial infarction, 1,2 we investigated this technique in patients with suspected acute infarction. 3 Despite careful attention to the purity of the radiopharmaceutical agent and the technical quality of our imaging procedure, we quickly found difficulties in interpreting scintigrams showing 2+ diffuse uptake in the region of the myocardium. In this study, we therefore examined a modified set of criteria for scintigraphic interpretation (Table I) in which the 2+ diffuse pattern is considered equivocal rather than definitely positive or definitely negative. Sensitivity of p y r o p h o s p h a t e s c i n t i g r a p h y in d e t e c t i n g p r e s e n c e of a c u t e infarction: Our results demonstrated that with this new modified classification the technetium-99m pyrophosphate imaging technique was a sensitive method for detecting acute myocardial infarction (Table II). Of 81 patients with acute transmural myocardial infarction none had a negative scintigram, and of 18 patients with acute transmural infarction only 2 had a false negative study. Thus only 2 percent of the patients with acute transmural or nontransmural infarction had a negative pyrophosphate scintigram. S p e c i f i c i t y of p y r o p h o s p h a t e s c i n t i g r a p h y for a c u t e m y o c a r d i a l infarction: When our strict criteria for a positive study were used, the specificity of the technique for acute infarction was also good (Fig. 1 and 2). Thus of 126 patients with no evidence of acute myocardial infarction by clinical or laboratory criteria, only 4 demonstrated a definitely positive technetium-

99m pyrophosphate scintigram (Table II). Two of these four patients with "false positive" results had focal uptake in regions of left ventricular dyskinesia secondary to remote myocardial infarction. The findings in these two patients are consistent with those of Ahmad et al., s who previously reported focal uptake of technetium-99m pyrophosphate in patients with coronary artery disease and ventricular aneurysm. The high levels of sensitivity and specificity in our study are a direct result of the use of a system that classifies as equivocal the scintigram with a 2+ diffuse pattern of tracer uptake. Equivocal scintigrams: The clinical importance of this classification is readily demonstrated by further examination of the 50 patients whose scintigrams were judged equivocal (Table II). If this 2+ diffuse pattern had been considered positive, studies in 35 additional patients with no clinical evidence of infarction would have been called positive. Therefore, the separation of this equivocal result from the definitely positive result substantially increased the specificity of the positive scintigram for infarction. Conversely, if the equivocal pattern had been considered negative, studies in 14 additional patients with acute myocardial infarction would have been called negative, thereby greatly decreasing the sensitivity of the examination in detecting infarction. Thus with use of our classification, a deftnitely negative technetium-99m pyrophosphate scintigram provided strong evidence against the presence of acute myocardial infarction, transmural or nontransmural, and a definitely positive radionuclide study indicated that acute infarction was highly probable. Difficulty in interpreting the 2+ diffuse pattern as positive for acute infarction is not unique to our institution. Although Willerson et al.9 showed that this pattern is common in patients with acute subendocardial infarction, the same investigative group 1° also reported that 35 percent of patients with unstable angina pectoris may have positive scintigrams, usually with the 2+ diffuse pattern. Thus classification of the 2+ diffuse pattern as positive decreases the specificity of the positive study for acute myocardial infarction and thereby diminishes the clinical utility of the examination. In addition, this scintigraphic pattern has also been reported in aortic valve disease, 5 in patients with enlargement of the left ventricle secondary to cardiomyopathy 4,7 and in some patients with stable angina pectoris. 6,11 Our results are consistent with these findings: Our patients with the 2+ diffuse pattern included patients with transmural or nontransmural myocardial infarction, unstable angina pectoris or hypertrophic or congestive cardiomyopathy as well as two patients with well documented studies revealing no ischemic or myocardial heart disease (Fig. 3). Thus, interpretation of the 2+ diffuse pattern as positive for acute myocardial infarction has decreased the specificity of the positive pyrophosphate study for acute infarction in many laboratories. A striking example of this effect is seen in the work of Karunaratne et al., 4 whose scintigraphic findings were basically similar to ours when they studied a large group of patients with chest pain. Using the conventional method

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of classifying the 2+ diffuse pattern as positive, they concluded that positive studies occur in a high percentage of patients with no evidence of acute myocardial infarction. In many patients with or without heart disease the 2+ diffuse pattern may represent a small degree of blood pool labeling. When blood pool labeling is more extensive, the scintigraphic pattern is easily differentiated from that of myocardial uptake because the entire blood pool is seen, including that Qf the right heart chambers and the great vessels. However, when blood pool labeling is less extensive, the pattern may be identical to the 2+ diffuse pattern seen in patients with the cardiac conditions listed previously. This phenomenon occurs because the left ventricle is the cardiac chamber with the largest blood pool and may therefore be the only structure visible in the region of the heart in a pyrophosphate scintigram in which there has been slight blood pool labeling. A related occurrence may also explain the appearance of the 2+ diffuse pattern in many patients with an enlarged left ventricle. 4,7 In these patients, although the absolute blood levels of technetium-99m pyrophosphate may not be abnormally high at the time of imaging, the enlarged left ventricular blood pool may cause appearance of the 2+ diffuse left ventricular pattern. Further, when blood pool labeling is present, the pattern persists for many hours after injection, as in myocardial labeling. Thus although the problem of diffuse blood pool labeling is diminished by delaying the time of imaging (2 hours after injection was routinely used in our investigation), this maneuver does not eliminate those patients whose blood pool pattern is secondary to red blood cell labeling 12 rather than to slow clearance of the unbound technetium-99m pyrophosphate moiety. Preliminary data from our laboratories suggest that the slight blood pool labeling may explain the 2+ diffuse pattern even in many patients with a primary cardiac diagnosis other than acute infarction. 13 C o m p u t e r processing and i n t e r o b s e r v e r variation: Our modified scintigraphic classification led to

excellent agreement between the two independent observers. Our findings regarding the utility of computer processing are in accord with those of other investigators. 2 The standard technique for background subtraction and the new method for rib subtraction were not essential for accurate scintigraphic interpretation in more than 96 percent of cases (Fig. 4). The principal effect of the computer analysis was to strengthen observer certainty concerning the scintigraphic classification. Use of the modified anterior view eliminated the need for the rib subtraction method in patients whose cardiac margin was obscured by overlying rib structures in the standard anterior projection. Clinical applications: Although the diagnosis of acute myocardial infarction is frequently apparent from clinical, electrocardiographic and serum enzyme determinations, in a large proportion of patients with acute chest pain admitted to a coronary care unit the presence or absence of acute infarction is initially unclear. In this group are patients without acute infarction as well as patients with nontransmural infarction, isolated posterior myocardial infarction, new infarction in a region adjacent to old myocardial infarction, infarction in the presence of left bundle branch block, intraoperative infarction or infarction superimposed on chronic lung disease. A scintigraphic study that can accurately differentiate patients with and without acute infarction would be a valuable clinical tool. We have found that the scintigraphic criteria described in our study, which provide an equivocal image category, permit the technetium-99m pyrophosphate scintigraphic study to define these two groups. In our institution, these results have established the technetium-99m pyrophosphate scinitigram as a routine and highly useful diagnostic technique for evaluating chest pain of uncertain origin.

Acknowledgment We gratefully acknowledge the technical assistance of Anne-Line Jansholt (MS-Radiopharmacy), Judith Semple, Kay Riggs, Barbara Anderson, Jasie Loving and Leslie Silvernail.

References 1. Bonte FJ, Parkey RW, Graham KD, et ah A new method for radionuclide imaging of myocardial infarcts. Radiology 110:473-474, 1974 2. Parkey RW, Bonte FJ, Meyer SL, et al: A new method for radionuclide imaging of acute myocardial infarction in humans. Circulation 50:540-546, 1974 3. Berman DS, Amsterdam EA, Salel AF, el ah Improved diagnostic assessment of acute myocardial infarction: sensitivity and specificity of Tc-99m pyrophosphate scintigraphy. J Nucl Med 17:523, 1976 4. Karunaratne HB, Walsh WF, Fill HR, et ah Technetium-99m pyrophosphate myocardial scintigraphy in patients with chest pain. Lack of diagnostic specificity. J Nucl Med 17:523-524, 1976 5. Joe SH, Mean I, Jengo JA, et ah False positive myocardial infarction scanning: calcified aortic and mitral valves (abstr): Northern and Southern California Chapters Society of Nuclear Medicine, Los Angeles, 1975, F2 6. Taradash M, Prasquier R, Botvinick E, et ah The specificity of the diffuse pattern of cardiac uptake in myocardial infarction imaging with Tc-99m-pyrophosphate (abstr). Clin Res 24:89A, 1976 7. Gould LA, Perez LA, Hayt DB, et al: Clinical experience: Tc-99m stannous pyrophosphate for myocardial imaging (abstr). Circulation 346

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50:Supp1111:4, 1974 8. Ahmad M, Dubiel J, Verdon T, el ah Technetium 99m stannous pyrophosphate myocardial imaging in patients with and without left ventricular aneurysm. Circulation 53:833-838, 1976 9. Willerson JT, Parkey RW, Bonte FJ, et ah Acute subendocardial infarction in patients: its detection by technetium-99m stannous pyrophosphate myocardial scintigrams. Circulation 51:436-441, 1975 10. Donsky MS, Curry GC, Parkey RW, et ah Unstable angina pectoris: clinical, angiographic, and myocardial scintigraphic observations. Br Heart J 38:257-263, 1976 11. Berman DS, Salel AF, Amsterdam EA, et ah Evaluation of Tc99m-pyrophosphate myocardial uptake in clinical exercise-induced ventricular ischemia. Chest, in press 12. Khentigan A, Garrett M, Lure D, et ah Effects of prior administration of Sn(ll) complexes on in vivo distribution of 99mTc-pertechnetate. J Nucl Med 17:381-384, 1976 13. Berman DS, Amsterdam EA, Salel AF, et ah Enhanced accuracy of Tc-99m-pyrophosphate imaging for acute myocardial infarction by computerized selective blood pool subtraction (abstr). Society of Nuclear Medicine Western Regional Section, San Francisco, 1976, p 42

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