[Usefulness of change ratio map in 99mTc-HMPAO SPECT with acetazolamide enhancement]

Although a sequential 99mTc-HMPAO SPECT technique with Diamox test (seq-SPECT) is a simple and time-saving procedure to assess brain perfusion reserve, the influence of the first dose of the tracer on the second one is not negligible. Therefore, a subtraction of the rest-SPECT from the 2nd SPECT is widely-used. However, subtracted SPECT images not only need to be corrected for the injected dose and the radiochemical purity due to inherent instability of HMPAO but also are usually degraded in quality. This study was undertaken to resolve these problems utilizing a change ratio (CR) map. The CR map was obtained by dividing 2nd SPECT by rest-SPECT. Prior to subtraction, the 2nd SPECT was normalized with the ratio of the mean whole brain counts between both SPECTs. To validate CR map, 7 patients were studied with both seq-SPECT and 133Xe inhalation CBF measurement (Xe-CBF). The right to left count ratio obtained from the ROIs placed on MCA territory of CR map correlated well with that from Xe-CBF (r = 0.89, p < 0.01). Fifty-three patients with stroke underwent the seq-SPECT which was compared with the cerebral angiography (CAG) and classified into 4 groups according to the CR map. In 25 patients, all of the rest-, the subtracted-SPECT and the CR map did not show any difference between the affected side and the contralateral normal side. Seven patients with normal rest-SPECT showed decreased subtracted-SPECT counts and CR on the affected side. Three of them showed more than 75% stenosis on CAG. Four patients with the decreased counts both at the rest- and the subtracted-SPECT revealed no difference on the CR map suggesting the matched decrease of both blood flow and metabolism in the affected side. In conclusion, the CR map was a simple and useful method to evaluate the brain perfusion reserve with the seq-SPECT.