Validation of the cerebellum as a reference region for SPECT quantification in patients suffering from dementia of the Alzheimer type

In longitudinal brain studies of dementia of the Alzheimer type (DAT), the cerebellum is often used as a reference region for single photon emission c omputed tomography (SPECT) quantification, which assumes no significant reg ional influence of physiological fluctuations or pathology. With the use of absolute quantification in DAT patients, reproducibility of cerebellar upt ake of technetium-99m-d,l-hexamethylpropylene amine oxime (HMPAO) was teste d and compared with the mean absolute cerebellar tracer uptake value in DAT patients and healthy control subjects. In 13 DAT patients SPECT studies we re repeated within 2 weeks to assess reproducibility of cerebellar regional brain uptake (rBU). With calibrated point sources as scaling factors, cere bellar activity was expressed as rBU of HMPAO per cm(3) brain tissue in per cent of the injected lipophilic dose of 740 MBq (20 mCi). Also, mean cerebe llar rBU in patients suffering from DAT was calculated and compared with a previously established database obtained in healthy volunteers. Repeated SP ECT studies within a 2-week interval in clinically stable patients resulted in a mean rBU increase of 6.8 +/- 10.3% in the second SPECT study as compa red with the first. A similar shift was previously reported in healthy volu nteers. Mean cortical cerebellar rBU values in DAT patients and in the heal thy reference population concurred, after cumulative corrections for body s urface and for a mean brain volume of 1350 ml (obtained in healthy control subjects), showing respective mean values of 53.9 +/- 7.4 and of 52.0 +/- 7 .3 x 10(-6) of the injected lipophilic dose 740 MBq (20 mCi) of HMPAO per c m(3) of brain tissue. A unidirectional shift in mean absolute cerebellar up take values occurs between repeat SPECT examinations in DAT patients simila r to previous findings in a group of healthy volunteers. The origin of this phenomenon remains elusive but deserves further study with regard to SPECT (semi)quantification in DAT patients. Most interestingly, the presented fi ndings suggest that with the use of HMPAO SPECT in DAT patients the cerebel lum remains scintigraphically uninvolved. (C) 1999 Elsevier Science ireland Ltd. All rights reserved.