DETECTABILITY OF SIMULATED BRAIN ACTIVATION USING DUAL RADIOISOTOPE SPECT BASED ON SIZE AND INTENSITY OF THE FOCAL HYPERACTIVITY

Rationale and Objectives. Simultaneous single-photon emission computed tomography (SPECT) neuroimaging with both technetium-99m (Tc-99m) hex amethylpropyleneamine oxime (HMPAO) and iodine-123 (I-123) N-isopropyl -iodoamphetamine is a recently introduced method with potential for as sessing activation phenomena in the brain. However, there is limited i nformation on the accuracy of the technique for detecting focal cortic al sites of neuroactivation. We determined, in vitro, what levels of a ctivation could be detected as a function of the size of the activated region. Methods. A Lucite brain phantom was filled with both I-123 an d Tc-99m so as to simulate both a nonactivated state (I-123) along wit h focal sites of activation (Tc-99m). Simulated activations ranged fro m 0 to 18% in volumes of 7, 14, 20, and 27 cm(3). Imaging was performe d with a triple-detector gamma camera using a 10% symmetric window at 140 keV and 10% asymmetric window around 159 keV. No correction was ma de for gamma cross-talk. To determine whether a simulated activation w as ''detected,'' the Tc-99m:I-123 count ratios in the activated region s were compared by t test with ratios in nonactivated regions of simil ar volume. Detection sensitivities also were calculated as the fractio n of the activated Tc-99m:I-123 ratios that were greater than the mean + 2 standard deviations of the corresponding nonactivated ratios. Res ults. All sites of simulated activations of 10% or greater were detect ed. The detection sensitivity was 100% (95% confidence interval, 90-10 0%) for the two largest chambers with simulated activations of 13-18%. Activations in the 3-6% range, in the same-sized chambers, were detec ted with a limited sensitivity (67% with a confidence interval of 45-8 4%). In the 14-cm(3) chamber, simulated activations in the 13-18% rang e were detected with 90% sensitivity (confidence interval, 74-98%). In general, the detection sensitivity was greater for larger chambers an d higher levels of simulated activation. Conclusion. We conclude that the dual-radioisotope technique using triple-detector SPECT systems an d low-energy all-purpose (LEAP) collimators should be highly reliable for identifying focal brain activations above 13% that cover at least 14 cm(3) of brain cortex. Smaller, less intense sites of activation wi ll be detected with reduced frequency. These conclusions are based on our assessment of only the physical parameters involved in this method ology and other factors (e.g., the possibility that the relation betwe en cerebral radiotracer concentration and regional cerebral blood flow ) may affect the results obtained with patients.