CASE REPORT Annals of Nuclear Medicine Vol. 7, No. 3, 183-186, 1993 Demonstration of abnormal perfusion in the pons with high resolution SPECT and Technetium-99m HMPAO in a patient with acoustic neuroma Nobuharu YUI, Takashi TOGAWA, Fujimi KINOSHITA, Hiroki NAMBA,* and Yoshihisa AKIYAMA** Division of Nuclear Medicine, *Division of Neurosurgery, ***Division of Radiation Physics, Chiba Cancer Center Hospital We detected a regional defect in the pons with ipsilateral cerebellar hypoperfusion in a patient with acoustic neuroma by brain SPECT with 99*Tc HMPAO. A high spatial resolution SPECT system with three detectors equipped with fan-beam collimators was employed. This is the first report of a defect in the brainstem being visualized by perfusion SPECT and this method could make it possible to clarify the cause of a remote effect on the cerebellar perfusion by injury to the brainstem. Key words : Acoustic neuroma, SPECT, Tc-99m HMPAO, Pons INTRODUCTION THERE HAVE BEEN a few reports on perfusion in the brainstem by means of SPECT. In a 123I-IMP study, Minoshimal et al. reported that following injury of the cerebrum there was a decrease in pontine density, but the images were not clear enough to make it pos-sible to estimate regional hypoperfusion in the pons. Perani et al. reported crossed cerebellar diaschisis in a case of pontine ischemia2. However, the pontine image itself was not obtained. We can now use a new SPECT system with a three-head rotating camera which has excellent spatial resolution and provides a normal pontine image with 99*Tc HMPA03,4. This paper reports a case in which brain SPECT clearly visualized a regional defect in the pons of a patient with acoustic neuroma and recovery after surgery. CASE REPORT A 56-year-old male who had been complaining of vertigo and hypesthesia of the right face correspond- ing with the right trigeminal nerve, underwent a brain perfusion SPECT study with 99mTc HMPAO and a three-head rotating gamma-camera (Toshiba GCA 9300A). 740 MBq (20 mCi) of 99*Tc HMPAO was given intravenously and data acquisition was started 1 5 minutes after the injection and continued for 20 minutes. Twenty projection data from each detector (6' steps, 1 20' rotation, 60 seconds per angle), 60 projections altogether, were accumulated in a 256 x 256 matrix. Pixel size was 1.8 mm. The accumu-lated data were converted to parallel-beam projec-tion data in a 128 x 128 format (pixel size=1.7 mm). The data were then smoothed with a Butterworth filter (order : 8, cut off: O. 1 3 cycle/pixel) and recon-structed by means of filtered back projections. Transaxial tomographic images showed asymmetric perfusion in the cerebellum with a relative reduction in the right side and a defect in the right half of the pons suggesting hypoperfusion (Fig. l). MRI per-formed subsequently revealed a mass with 3 cm in diameter at the right cerebello-pontine angle (Fig. 2). This round mass enhanced by Gd-DTPA was thought to be an acoustic neuroma. MRI angiogram re-vealed a shifted right vertebral artery. Vascular stenosis was not seen (Fig. 3). Compression of the pons and right cerebellar hemishere was confirmed. The patient received surgery and the tumor was par-tially removed. The surgical specimen was histologi-cally diagnosed as acoustic neuroma. The right trigeminal hypesthesia recovered after the surgery and the vertigo improved. MRI taken 6 months later showed a reduced mass with a slight compression of the right cerebellar hemisphere and no compression of the pons (Fig. 4). Brain SPECT at that time demonstrated normal perfusion in the pons and a slight reduction in pefusion in the right cerebellum (~Fig. 5). Elimination of the pressure due to the tumor brought recovery of perfusion in the brainstem and cerebellurn. DISCUSSION Acoustic neuroma is a benign tumor which originates in the internal auditory meatus and extends to the cerebello-pontine angle. Compression of cranial nerves, the cerebellum and brain-stem causes vertigo, facial hypesthesia, hearing loss and facial-nerve paresis. Hypoperfusion in cerebellum and brain-stem also would be caused by these extensions, but there is no report about an image showing hypoperfusion visualized by SPECT as a direct effect of the compres-sion. Tamamoto et al. reported a case of acoustic neuroma which showed hypoperfusion in the cere-bellum by means of a single head rotating gamma-camera as a remote effect of compression of the pons5. The decreased blood flow in the cerebe]lum was ipsilateral. No image of the pons was obtained with the camera they used, but MRI demonstrated compression of the pons. No abnormality of the cerebellum was found by X-ray CT, MRI or ver-tebral angiography. They suggested that the pathway from the pontine nuclei to the cerebellum was af-fected at the post-crossed level and the remote effect caused a relative reduction in the blood flow in the ipsilateral cerebellar hemisphere of the compressed pons. On the other hand, Perani et al. reported a case of cerebellar diaschisis in pontine ischemia2. In this report, decreased blood flow was seen in the con-tralateral cerebellum of the pontine ischemic lesion. The explanation was that the pathway from the pontine nuclei to the cerebellum was affected before crossing and a remote effect was contralateral cere-bellar hypoperfusion. And no pons image was shown. It has been reported that thalamic tumors which invaded the brain-stem produced crossed cerebellar diaschisis which was demonstrated by PET6. In any case, these papers failed to report a pons image which would show pons perfusion affected by direct com-pression by the tumor or ischemia. There were only indirect results in the cerebellum due to the remote effects of the affected pathway from the pons. We detected a defect in the pons which occupied one half of it. This remarkable defect extended over the shift in the brainstem caused by the tumor. The finding was more likely to have been caused by hypoperfu-sion of the pons than by the shifted brainstem. The ipsilateral decrease in blood flow in the cerebellum indicated a regional compressions effect by the tumor, but the hypoperfusion covering the whole hemisphere of the right cerebellum even more strongly suggested a remote effect due to the lesion in the brainstem. These findings suggested that the direct compression caused not only the perfusion defect in the pons but also hypoperfusion of the cerebellum and that the remote effect from the post-crossed pathway brought ipsilateral cerebellar hypo-perfus ion . The removal of the tumor excluded the compres-sion of the pons, restored the pons blood fiow and caused the remote effect on the cerebellar perfusion to disappear. The remaining slight hypoperfusion of the cerebellum was thought to be caused by direct pressure exerted by the residual tumor which was seen on the MRI taken after the surgery. Only PET has been thought to be a means to evaluate metabo-lism in the brainstem because of its superior spatial resolution compared to SPECT7,8. However, a re-cently developed high resolution SPECT system enabled the perfusion images of the normal brainstem to be clearly visualized3,4. 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