Violet J. Stark

Clinical myocardial imaging with nitrogen-13 ammonia

Myocardial infarcts may be clearly imaged using intravenous nitrogen-13 as carrier-free ammonia in doses of 10–30 mCi. This positron emitter is well imaged with the Nuclear Chicago HP Anger Camera with heavy collimation. The rapid blood disappearance of the agent gives good image contrast, and the short half-life and high isotope dosage give high-count density images with little radiation absorbed dose (5 mrad∕mCi total body).

Quantitative planar imaging of glucose metabolic activity in myocardial segments with exercise thallium-201 perfusion defects in patients with myocardial infarction: comparison with late (24-hour) redistribution thallium imaging for detection of reversible ischemia.

Exercise thallium-201 24-hour redistribution imaging and myocardial glucose metabolism with F-18-deoxyglucose were used to identify reversible ischemia in 30 patients with previous myocardial infarction. Metabolic images were obtained using a planar gamma camera fitted with a rotating tungsten collimator. Of 184 exercise thallium perfusion defects, late redistribution occurred in 88. Metabolic evidence for reversibility (metabolism-perfusion mismatch) was identified in 91% of these 24-hour reversible segments. However, 72% of the segments with fixed perfusion defects also had residual ischemia by F-18-deoxyglucose. Out of 26 fixed severe thallium defects, 69% had F-18-deoxyglucose evidence for residual ischemia. A subset of 14 patients underwent serial exercise thallium scintigraphy or gated equilibrium radionuclide angiography after revascularization or medical therapy. Out of 46 fixed thallium defects in these patients, 30 demonstrated serial scintigraphic improvement. F-18-deoxyglucose-thallium mismatch was present in 81% of these segments, but was absent in the majority of the unimproved segments. Thus quantitative planar imaging of myocardial glucose metabolism with F-18-deoxyglucose using a well-collimated gamma camera can detect clinically important reversible ischemia in segments with fixed thallium defects at late redistribution imaging.

Planar positron imaging of rubidium-82 for myocardial infarction: A comparison with thallium-201 and regional wall motion☆

Rubidium-82 (Rb-82) is a generator-produced, short half-life (76 seconds) positron emitting potassium analog. Using a mobile gamma camera equipped with a rotating tungsten collimator and high-energy shielding, we examined the use of Rb-82 in the coronary care unit and clinical laboratory for detection of perfusion defects due to myocardial infarction. We studied 31 subjects, 10 patients with acute myocardial infarction, 12 with remote myocardial infarction, and nine controls. Rb-82 images were compared with Tl-201 and regional wall motion for detection of infarct-related arteries. Of the 22 patients with myocardial infarction, 16 were identified with Rb-82 and Tl-201. In nine control subjects, eight were normal with each method. Correlation between Rb-82 and Tl-201 defect scores was excellent. Sensitivity and specificity for infarct-related arteries were similar for Rb-82, Tl-201, and wall motion imaging. Thus planar Rb-82 imaging can detect MI reliably in the coronary care unit and in the clinical laboratory.

Cerebral Arteriovenous Shunts Re-examined

Cerebral arteriovenous (A-V) shunts have been described in some histological studies, but their existence has been denied in others. Previous attempts to detect the presence and amount of A-V shunting using radioactive microspheres have suggested that large amounts of blood could bypass cerebral capillaries in this fashion. A reappraisal seemed indicated. The present study, using the radioactive microsphere technique, confined the injected spheres specifically to the cerebral circulation of a nonhuman primate. The conclusion of this study is that structural, noncapillary A-V shunting definitely occurs, but is very small and variable in amount. The implications of this finding in relation to previous studies and to the clinical phenomena of red cerebral veins and early venous filling are discussed.

Accumulation of indium-111-labeled human low density lipoprotein in the rabbit aorta: Implications for nuclear imaging of vascular lesions.

Nuclear imaging of atheromata must distinguish lesions from both blood pool and normal arterial tissue. We have examined spatial and temporal variations of indium-111-labeled human low density lipoprotein (LDL) accumulation in rabbit aortas. LDL-derived In-111 activity was time-independent in lesion-resistant regions of aortas from normal and hypercholesterolemic animals (mean 2.9 × 10(-6) percent injected activity per milligram tissue [%IA/mg]) and in lesion-prone regions of normal aortas (mean 7.1 × 10(-6) %IA/mg). In contrast, activity in sudanophilic lesions of hypercholesterolemic rabbit aortas reached a peak of 31 × 10(-6) %IA/mg at 92 hours postinjection. The mean ratio between activity in lesions versus lesion-resistant regions described a broad convex curve with minima of 4:1 at 14 hours and 136 hours and a peak of 14:1 measured at 72 hours postinjection. The mean ratio between In-111 in lesions and blood followed a sigmoid curve, rising exponentially from 1:25 at 14 hours to 1:3 by 72 hours postinjection. We conclude that optimal signal-to-noise ratios for monitoring atheroma-associated LDL-derived radioactivity occur late, not before about 3 days postinjection. Therefore, LDL labeled with In-111 or even longer-lived radionuclides holds the greatest promise for effective clinical nuclear imaging of atherosclerosis.