Abstract 13133: Heterogeneous Uptake of [ 99m Tc]microspheres and 201 Tl in Skeletal Muscles in Porcine Model of Hindlimb Ischemia

Introduction: There is a need for quantitative methods to evaluate skeletal muscle blood flow in PAD patients. We assessed skeletal muscle blood flow heterogeneity at rest or vasodilator stress using 99m Tc-microspheres (HSA), that do not cross the capillaries, and 201 Tl, a freely diffusible radiotracer, in a pig hindlimb ischemia model. Methods: Following acute right femoral artery ligation, pigs (n=8) were injected with either 201 Tl or HSA at rest (n=2 per tracer) or under adenosine-stress (n=2 per tracer). Ex vivo SPECT/CT imaging was performed on excised ischemic (I) and non-ischemic (NI) gastrocnemius (GM) and soleus (SM) muscles. SPECT images were analyzed for the ischemic/nonischemic (I/NI) activity ratio, tracer heterogeneity as indicated by the coefficient of variance (CV) of signal intensity and image intensity histograms. Results: The rest 201 Tl I/N ratio for SM was higher (0.86±0.08) compared to stress (0.64±0.16), with no difference in GM. In contrast, the I/N ratio for HSA was near normal in both muscle groups at rest (GM: 0.80±0.21, SM: 1.06±0.45) although decreased during stress (GM: 0.70±0.04, SM: 0.71±0.12). Rest 201 Tl displayed uniformly low CV in both I (GM: 0.43, SM: 0.43) and NI muscles (GM: 0.37, SM: 0.43). Corresponding CV for stress 201 Tl were similar (I - GM: 0.41, SM: 0.42; NI - GM: 0.41, SM: 0.46). Rest HSA CV were higher in I (GM: 0.92, SM: 1.03) and NI (GM: 0.67, SM: 0.69) muscle, although there was greater heterogeneity in the I muscles assessed with HSA. Stress HSA CV decreased in both I (GM: 0.60, SM: 0.73) and NI (GM: 0.52, SM: 0.51). Figure displays representative count histograms. Conclusions: 201 Tl displayed less heterogeneity than HSA. 201 Tl appears to be more sensitive at detecting reduced blood flow at rest, due to changes in microcirculation. There was no change in the I/NI ratio during 201 Tl stress due to capillary recruitment, although HSA did demonstrate a decrease in the I/NI ratio with vasodilation, reflecting proximal vascular occlusion.