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Hypofixation and Hyperfixation of ^99mTc-Hexamethyl Propyleneamine Oxime in Subacute Cerebral Infarction

Department of Neurosurgery and Cyclotron Research Center, Iwate Medical University, Morioka
Department of Neurosurgery, Kohnan Hospital, Sendai
Department of Neurosurgery, Tohoku University School of Medicine, Sendai, Japan

Correspondence: For correspondence or reprints contact: Kuniaki Ogasawara, MD, Department of Neurosurgery, Iwate Medical University, 19-1 Uchimanu, Morioka, 020-8505 Japan.

ABSTRACT

The relationship between hypofixation and hyperfixation of ^99mTc-hexamethyl propyleneamine oxime (^99mTc-HMPAO) remains unclear. The purpose of this study was to compare ^99mTc-HMPAO SPECT with regional cerebral blood flow (CBF) imaging using ¹³³Xe inhalation in patients with subacute cerebral infarction and to investigate the behavior of ^99mTc-HMPAO in the infarct area using dynamic SPECT. Methods: ¹³³Xe and consecutive ^99mTc-HMPAO SPECT studies, the latter of which consisted of dynamic and static scanning, were performed on 51 patients (22 women, 29 men; age range, 40–83 y; mean age, 61 y) with cortical infarction in the middle cerebral artery territory 13–15 d after stroke onset. One region of interest (ROI) was drawn in the infarct area. The control ROI was mirrored to the contralateral side, and the same set of ROIs was applied to all SPECT studies. Fractional fixation of ^99mTc-HMPAO in the infarct area was evaluated relatively as the ratio of the infarct-to-control region in ^99mTc-HMPAO static tomograms/the ratio of the infarct-to-control region in CBF images using ¹³³Xe inhalation and was classified as hyperfixation when this value was >1.1 and hypofixation when this value was <0.9. To investigate the behavior of ^99mTc-HMPAO in the infarct area, the second (36–72 s after tracer injection) and eighth (252–288 s after tracer injection) of 8 dynamic scans were selected, and the washout rate was calculated using the formula: 1 - (mean count in the eighth scan/mean count in the second scan). Results: The infarct area showed hyperfixation of ^99mTc-HMPAO when CBF in the area was 35 mL/100 g/min or less and showed hypofixation when CBF was >45 mL/100 g/min. The washout rate was usually negative when CBF imaging using ¹³³Xe inhalation was <20 mL/100 g/min but was positive when it was >45 mL/100 g/min. The washout rate was negative when the infarct area showed hyperfixation of ^99mTc-HMPAO but was positive when it showed hypofixation. Conclusion: ^99mTc-HMPAO SPECT underestimates CBF in high-flow regions and overestimates CBF in low-flow regions of subacute cerebral infarction. ^99mTc-HMPAO hypofixation and hyperfixation are associated with backdiffusion from the brain to blood and gradual accumulation of hydrophilic metabolites, respectively. Dynamic images should be useful for discriminating between ^99mTc-HMPAO hypofixation and hyperfixation.

Key Words: ^99mTc-HMPAO • subacute stroke • brain SPECT

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