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Absolute Activity Quantitation in Simultaneous ¹²³I/^99mTc Brain SPECT

Department of Radiology, Harvard Medical School and Brigham and Women’s Hospital, Boston, Massachusetts; and U494 INSERM, CHU Pitié-Salpêtrière, Paris, France

Dual-isotope imaging can allow simultaneous assessment of brain perfusion using a ^99mTc-labeled tracer and neurotransmission using an ¹²³I-labeled tracer. 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 simulated normal and pathologic studies of simultaneous ¹²³I/^99mTc brain SPECT when compensating for all degrading phenomena. Methods: Monte Carlo simulations were performed using the Zubal brain phantom. Contamination caused by high-energy ¹²³I decay photons was incorporated. Twenty-four ^99mTc and ¹²³I activity distributions were simulated on the basis of normal and pathologic patient activity distributions. Cross talk and scatter were corrected using a new method based on a multilayer perceptron artificial neural network (ANN), as well as by the asymmetric window (AW) approach; for comparison, unscattered (U) photons of ^99mTc and ¹²³I were recorded. Nonuniform attenuation and DCR were modeled in an iterative ordered-subset expectation maximization (OSEM) algorithm. Mean percentage biases and SDs over the 12 normal and 12 pathologic simulated studies were computed for each structure with respect to the known activity distributions. Results: For ¹²³I, AW + OSEM yielded a bias of 7% in the cerebellum, 21% in the frontal cortex, and 36% 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 structures with ANN. In the simulated pathologic population, the bias associated with AW increased significantly in the cortical lobes to 55% (P < 0.05), although it did not change significantly with ANN. Conclusion: The accuracy and variability over simulated normal and pathologic studies of both ^99mTc and ¹²³I activity estimates were very close with ANN to those obtained with U + OSEM. ANN + OSEM is a promising approach for absolute activity quantitation in simultaneous ^99mTc/¹²³I SPECT.

Key Words: dual-isotope brain SPECT • absolute quantitation • Monte Carlo simulation • artificial neural network • cross-talk compensation