Myocardial uptake and clearance of thallium-201 in normal subjects: comparison of dipyridamole-induced hyperemia with exercise stress

Lessons from the Tc-99m shortage: implications of substituting Tl-201 for Tc-99m single-photon emission computed tomography

BACKGROUND

In 2009, the Chalk River nuclear reactor closed for repairs that led to a critical shortage of technetium-99m (Tc-99m). Several centers used thallium-201 (Tl-201) as an alternative radiotracer for myocardial perfusion imaging. Because Tl-201 is considered by many as a suboptimal radiotracer, we sought to understand the impact of using Tl-201 (during the Tc-99m shortage) on downstream resource utilization.

METHODS AND RESULTS

We performed a retrospective study at the Ottawa Heart Institute of 7402 patients (60% men; mean age, 62.6 ± 11.8 years), patients were referred for myocardial perfusion imaging between May 2008 and January 2011 (PRE_Tc-99m [2938 patients]), during (DURING_Tl-201 [2959 patients]), and after (POST_Tc-99m [1505 patients]) the Tc-99m shortage. Patients were followed for 6 months after their index myocardial perfusion imaging to determine subsequent rates of cardiac catheterization or noninvasive imaging. More downstream testing was seen in the Tl-201 cohort (639 [21.4%] patients) than the Tc-99m cohort (537 [12.1%] patients; P<0.001). After adjustment using propensity scores, differences in downstream referral rates were maintained. The downstream investigations resulted in an estimated increase in per-patient costs ($165.22; 95% confidence interval, 153.00-177.42) in the DURING_Tl-201 cohort compared with the Tc-99m cohort ($90.97; 95% confidence interval, 83.42-98.90; P<0.001). As well, the mean effective radiation dose per-patient was higher in DURING_Tl-201 (23.57 mSv; 95% confidence interval, 23.16-23.96) than in Tc-99m (12.92 mSv; 95% confidence interval, 12.55-13.40; P<0.001).

CONCLUSIONS

In this single-center study, the use of Tl-201 during the Tc-99m shortage was associated with an increase in downstream testing, cost, and patient radiation exposure, but these findings may not be generalizable to other centers. Although Tl-201 provided a short-term solution to the unexpected Tc-99m shortage, long-term cost-effective solutions should be areas of future study.

Comparison of treadmill exercise versus dipyridamole stress with myocardial perfusion imaging using rubidium-82 positron emission tomography

OBJECTIVES

This study assessed the feasibility of treadmill exercise rubidium-82 ((82)Rb) positron emission tomography (PET) and compared image quality and diagnostic content with dipyridamole (82)Rb PET in patients referred for evaluation of coronary artery disease (CAD).

BACKGROUND

Dipyridamole stress (82)Rb PET myocardial perfusion imaging (MPI) is an accurate imaging modality used to diagnose CAD and determine prognosis. Although pharmacologic stress is used routinely, exercise treadmill stress may be an alternative and provide clinical information helpful to decision making, particularly for patients unwilling or unable to tolerate pharmacologic stress.

METHODS

Fifty patients (mean age, 60 +/- 10 years; 47 men) underwent treadmill exercise and dipyridamole (82)Rb PET. Images were assessed: 1) qualitatively using a 17-segment model and a semiquantitative visual score on a five-point scale and with calculation of summed stress score (SSS), summed rest score (SRS), and summed difference score (SDS); and 2) quantitatively with a 70% threshold for abnormal perfusion and expressed as extent of abnormal perfusion (% left ventricular).

RESULTS

Treadmill exercise was preferred by 74% of patients (37 of 50, p < 0.001). The exercise and dipyridamole (82)Rb PET summed scores and quantitative extent of abnormal perfusion were very similar and highly correlated. Results of Bland-Altman analysis showed no significant bias. Image quality was superior with exercise stress with greater myocardial uptake and higher target to background ratios.

CONCLUSIONS

Treadmill exercise (82)Rb PET is feasible and provides imaging results of similar diagnostic content and superior image quality compared with dipyridamole stress. Treadmill exercise is a reasonable alternative to pharmacologic stress with (82)Rb PET MPI.

Comparison between reference values for 201thallium uptake and washout from the myocardium after exercise and after dipyridamole

A slow washout of 201Thallium has a high diagnostic and prognostic value for coronary artery disease. The aim of the present work was to contribute with reference data on 201Tl uptake and washout from the myocardium. With a quantitative method using circumferential profiles and interpolative background subtraction we performed 201Tl myocardial imaging in two groups of subjects with a low probability of coronary artery disease. Washout in per cent is defined as (1-D/I).100, where D is the delayed uptake rate and I the initial uptake rate. In group A (n = 16) myocardial hyperaemia was induced by maximal exercise, in group B (n = 15) by intravenous infusion of dipyridamole. In group A, the mean washout was 52%, and in group B it was 34% (P < 0.001). The delayed uptake rate D was significantly higher after dipyridamole, 58.4 cps, than after exercise, 44.5 cps (P < 0.05). There was no significant differences in I between the two groups. The findings regarding D and I explain that the washout is slower after dipyridamole than after exercise, as the mean D/I ratio was 0.48 in group A and 0.66 in group B (P < 0.001).

Variations in the size of the ischemic myocardium due to differences in the normal file

The myocardial thallium concentration is different during pharmacologic than exercise stress testing due to differences in coronary blood flow and cardiac output. These differences may affect the quantitative measurement of the size of ischemic myocardium if a stress-specific normal file is not used. This study examined this concept in 34 patients with isolated left anterior descending coronary artery disease. All patients underwent tomographic thallium imaging during adenosine infusion (140 micrograms/kg/min for 6 minutes). The size of the ischemic myocardium was measured from the polar map using two different normal files; one derived from normal subjects undergoing treadmill exercise testing, and the second from normal subjects undergoing adenosine stress testing. The extent perfusion abnormality was 19 +/- 13% using the exercise file, and 11 +/- 10% using adenosine file (difference; 8.1 +/- 1.6%, P = 0.0001), the severity score was 580 +/- 480 using exercise file, and 310 +/- 310 using the adenosine file (P < 0.001). The differences were present in the 20 patients with moderate stenosis (50 to 70% diameter stenosis); 17 +/- 10% versus 7 +/- 7% (P = 0.001), and in the 14 patients with severe stenosis (> 70% diameter stenosis); 24 +/- 16% versus 18 +/- 10% (P = 0.03). Thus, stress-specific normal file should be used for sizing the perfusion abnormality. The use of exercise file overestimates defect size in patients undergoing pharmacologic stress testing.

Abnormal nondefect zone myocardial thallium washout ratio in patients with reversible thallium defects and normal coronary arteriograms

Thallium myocardial defects in patients with no angiographic coronary artery stenosis have been attributed to attenuation effects and other artifacts. Of 323 patients having thallium myocardial imaging following dipyridamole infusion, 10 of 159 patients with a segmental perfusion abnormality were found to have no luminal diameter stenosis greater than 20% on coronary angiography and no electrocardiographic (ECG) evidence of myocardial infarction or left bundle branch block. A time-standardized regional thallium washout ratio was calculated as thallium myocardial counts with dipyridamole to counts without dipyridamole. In comparison to 10 age-matched control patients with no evidence of cardiac disease and no myocardial thallium defect, study patients had a lower regional myocardial thallium washout ratio from both the defect and nondefect (1.57 +/- 0.40 versus 2.27 +/- 0.92, p = 0.041) zones. Nine of the 10 study patients had independent evidence of noncoronary heart disease, including echocardiographic or ECG evidence of left ventricular hypertrophy in six patients. In patients with thallium defects and normal coronary arteriograms, a time-standardized regional thallium myocardial washout ratio for the nondefect zone may aid in the differentiation of patients with noncoronary heart disease from those with soft tissue artifacts.

Prognostic implications of a negative dipyridamole-thallium scan: results in 360 patients

PATIENTS AND METHODS

A total of 360 patients with either normal perfusion (314) or fixed defects (46) on dipyridamole-thallium scans were followed over an average period of 16 months. Of the 360 patients, 194 subsequently underwent major noncardiac surgery.

RESULTS

There were a total of eight cardiac events including two postoperative complications (one fatal and one nonfatal myocardial infarction) and six cardiac events during long-term follow-up (one sudden death and five nonfatal infarctions). During the follow-up period, three patients underwent coronary artery bypass surgery. The low cardiac event rate could not be explained by a low pretest likelihood of coronary artery disease: 77% of the 360 patients had either typical angina pectoris, a previous myocardial infarction, or peripheral vascular disease, which is associated with a high prevalence of coronary artery disease.

CONCLUSIONS

In patients with a high pretest likelihood of coronary artery disease, the absence of thallium redistribution on a dipyridamole-thallium scan denotes a very low (1%) cardiac risk for major noncardiac surgery as well as low long-term cardiac mortality (0.3%) and morbidity (1.4%) rates. The coronary death rate is comparable to that of patients with minimal (less than 50%) coronary stenoses.

Prevalence and correlates of increased lung/heart ratio of thallium-201 during dipyridamole stress imaging for suspected coronary artery disease

There is little information concerning the prevalence and clinical correlates of increased pulmonary thallium-201 uptake during dipyridamole thallium-201 stress imaging. Accordingly, the clinical characteristics and quantitative thallium-201 findings were correlated with quantitative lung/heart thallium-201 ratio in 87 patients undergoing dipyridamole thallium-201 stress testing. Nineteen patients (22%) had an elevated ratio (greater than 0.51). These patients were more likely to have had an infarction, to be taking beta blockers, and have a lower rate-pressure product after dipyridamole administration than those with a normal ratio (p less than 0.03). An elevated ratio was associated with a greater likelihood of initial, redistribution and persistent defects, as well as left ventricular cavity dilatation on thallium-201 imaging (p less than 0.05). In addition, the number of myocardial segments demonstrating initial, redistribution and persistent defects was also greater in patients with increased ratios (p less than 0.03). Multivariate analysis demonstrated that the presence of redistribution and left ventricular cavity dilatation were the most significant correlates of lung/heart thallium-201 ratio. It is concluded that the prevalence of increased lung/heart thallium-201 ratio with dipyridamole thallium-201 stress imaging is similar to that seen with exercise stress imaging. As with exercise thallium-201 imaging, increased pulmonary thallium-201 uptake may be a marker of functionally more significant coronary artery disease.

Reverse redistribution of thallium-201 detected by SPECT imaging after dipyridamole in angina pectoris

Reverse redistribution refers to a thallium-201 perfusion defect that develops or becomes more evident on delayed imaging compared with the initial image immediately after stress. To determine the diagnostic importance of reverse redistribution after intravenous dipyridamole, thallium-201 single photon emission computed tomography and quantitative coronary arteriography were performed in 90 men with angina pectoris. Of the 250 myocardial segments analyzed, reverse redistribution was present in 17 (7%). Minimal coronary cross-sectional area in proximal vessel segments was less than or equal to 2.0 mm2 more often in regions with transient perfusion abnormalities than in regions with reverse redistribution (66 vs 29%, p less than 0.05). Compared with regions exhibiting transient perfusion abnormalities, regions with reverse redistribution had larger proximal arterial diameters (1.9 +/- 1.1 vs 1.3 +/- 1.1 mm, p less than 0.001) and cross-sectional areas (3.9 +/- 3.1 vs 2.2 +/- 2.6 mm2, p less than 0.001). Coronary artery dimensions and relative stenosis severity did not differ between those regions with normal perfusion and those with reverse redistribution. Reverse redistribution detected by thallium-201 single photon emission computed tomographic imaging after dipyridamole is uncommon, appears to occur as frequently in normal subjects as in patients undergoing coronary arteriography and does not indicate the presence of severe coronary artery disease.

Detection of human collateral circulation by vasodilation-thallium-201 tomography

Coronary arteriolar vasodilation may provoke redistribution of flow to collateral-dependent jeopardized myocardium. To assess the physiologic significance of collaterals, 80 consecutive post-infarction patients (age 58 +/- 8 years) underwent vasodilation-redistribution thallium-201 tomographic imaging after administration of 0.56 mg of intravenous dipyridamole/kg body weight. Circumferential profile analysis of thallium-201 uptake and redistribution in representative left ventricular tomograms provided quantitative assessment of transient and fixed defects and separation between periinfarctional and distant inducible hypoperfusion. Tomographic perfusion data were correlated to wall motion and collateral circulation between distinct anatomic perfusion territories. Patients were grouped according to presence (59%) or absence (41%) of angiographically visible collateral channels to jeopardized myocardium. In the presence of collaterals, distant reversible defects were larger than in absence of collaterals (p less than 0.05); the extent of combined periinfarctional and distant redistribution was also larger in collateralized patients (p less than 0.025), whereas the size of the persistent perfusion defect was similar in both groups. By prospective analysis the tomographic perfusion pattern of combined periinfarctional and distant redistribution revealed a sensitivity of 85% and a specificity of 78% for the detection of significant collateral circulation in this group of patients. Thus, using the exhausted flow reserve as a diagnostic tool, vasodilation-thallium-201 tomography has the potential to identify and quantitate collateralized myocardium in post-infarction patients and may guide diagnostic and therapeutic decision-making.

Comparison of myocardial washout rate of thallium-201 between rest, dipyridamole with and without aminophylline, and exercise states in normal subjects

The myocardial washout rate of thallium-201 was studied in 85 subjects with a less than 5% likelihood of coronary artery disease undergoing rest (group I, n = 12), dipyridamole (group II, n = 24) and exercise (group III, n = 49) stress thallium-201 scintigraphy. Subjects receiving dipyridamole were subdivided into group IIA (n = 11), who received an aminophylline injection 10 minutes after dipyridamole infusion, and group IIB (n = 13), who did not. The mean and highest washout rate values in each of 3 segments in the anterior, 45 degrees and 85 degrees left anterior oblique views were calculated. In group II the mean washout rate of thallium-201 was similar in all segments of each view and the overall mean washout rate did not differ between the 3 views studied. There was a good correlation between the mean and highest washout rate values in individual subjects (r = 0.98, p less than 0.001). The mean +/- standard deviation myocardial 4-hour washout rate of thallium-201 (anterior view) was 10 +/- 6% in group I compared with 40 +/- 14% in group IIA (p less than 0.05 vs group I), 31 +/- 13% in group IIB (p less than 0.05 vs group I) and 54 +/- 11% in group III (p less than 0.05 each vs group IIA and group IIB, respectively). There was a wide variation in mean washout rate values in group II (range 12 to 58%), and this variation was not altered by aminophylline administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative comparison of thallium-201 scintigraphy after exercise and dipyridamole in coronary artery disease

Data comparing myocardial thallium-201 imaging after exercise and intravenous dipyridamole infusion in the same patients are scarce. Accordingly, this study is a segment-by-segment quantitative analysis of regional uptake and washout of thallium-201 after dipyridamole (0.56 mg/kg) and symptom-limited exercise testing in 21 patients (ages 58 +/- 9.2 years) with chest pain studied 2.5 +/- 1.0 weeks apart. Thallium-201 activity in 9 myocardial segments was measured in initial and delayed anterior and 45 degree left anterior oblique views, producing 184 pairs of segments in the distribution of 63 coronary supply regions for direct comparison. The number of segments with normal thallium-201 uptake and the number of numerically significant defects were similar with exercise and dipyridamole (76 vs 73%, 24 vs 27%, respectively, difference not significant). A slightly higher proportion of redistribution defects was found after dipyridamole infusion compared to exercise (17 vs 10%, p less than 0.05). Agreement between 87% (165 of 189) of segment pairs was found when each was classified as either normal or abnormal. Although 24 of 189 segments were discordant, agreement was observed in 92% (61 of 63) of coronary supply regions determined to be normal (41 of 41) or abnormal (20 of 22). In 15 patients who underwent cardiac catheterization, exercise and dipyridamole-thallium-201 scintigraphy detected 61% (16 of 26) vs 61% (16 of 26) of stenoses greater than 50% (difference not significant) and 100% (19 of 19) vs 100% (19 of 19) (difference not significant) normal vessels, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)