Absolute activity quantitation in simultaneous I-123/Tc-99m brain SPECT

Dual-isotope imaging can allow simultaneous assessment of brain perfusion u sing a Tc-99m-labeled tracer and neurotransmission using an I-123-labeled t racer. However, the images are affected by scatter, cross talk, attenuation , distance-dependent collimator response (DCR), and partial-volume effect. We determined the accuracy and precision of activity quantitation in simula ted normal and pathologic studies of simultaneous I-123/Tc-99m brain SPECT when compensating for all degrading phenomena. Methods: Monte Carlo simulat ions were performed using the Zubal brain phantom. Contamination caused by high-energy I-123 decay photons was incorporated. Twenty-four Tc-99m and I- 123 activity distributions were simulated on the basis of normal and pathol ogic patient activity distributions. Cross talk and scatter were corrected using a new method based on a multilayer perceptron artificial neural netwo rk (ANN), as well as by the asymmetric window (AW) approach; for comparison , unscattered (U) photons of Tc-99m and I-123 were recorded. Nonuniform att enuation and DCR were modeled in an iterative ordered-subset expectation ma ximization (OSEM) algorithm. Mean percentage biases and SDs over the 12 nor mal and 12 pathologic simulated studies were computed for each structure wi th respect to the known activity distributions. Results: For I-123, AW + OS EM yielded a bias of 7% in the cerebellum, 21% in the frontal cortex, and 3 6% in the corpus callosum in the simulated normal population. The bias was increased significantly in the striata of simulated pathologic studies (P < 0.05). The bias associated with ANN was significantly lower (<9% in these brain structures, P < 0.05). For 99mTc with AW + OSEM, the bias was 60% in the corpus callosum, 36% in the striata, and 18%-22% in the cortical lobes in the simulated normal population. This bias was <11% in all brain structu res with ANN. In the simulated pathologic population, the bias associated w ith AW increased significantly in the cortical lobes to 55% (P < 0.05), alt hough it did not change significantly with ANN. Conclusion: The accuracy an d variability over simulated normal and pathologic studies of both Tc-99m a nd I-123 activity estimates were very close with ANN to those obtained with U + OSEM. ANN + OSEM is a promising approach for absolute activity quantit ation in simultaneous Tc-99m/I-123 SPECT.