Initial cerebral HM-PAO distribution compared to LCBF: use of a model which considers cerebral HM-PAO trapping kinetics.

The cerebral uptake of [99mTc]-d,l-hexamethylpropyleneamine oxime complex (HM-PAO) was compared to LCBF determined simultaneously with [14C]iodoantipyrine (IAP) using double radionuclide quantitative digital autoradiography. Awake male rats were given intravenous injections of a mixture of 50 microCi IAP and 15 mCi of HM-PAO and killed 20 s after tracer activity had first reached the brain. Two separate autoradiograms were produced from each 20 microns brain section. The autoradiograms were digitized, corrected for cross-contamination, and then converted into images of individual tracer concentration. The diffusible tracer model was used to convert the IAP concentration images into LCBF images. Regional HM-PAO concentration was found not to be linearly related to LCBF as determined with the IAP, and therefore a simple microsphere type model was inadequate in relating HM-PAO uptake to LCBF. A better HM-PAO uptake--LCBF correlation was obtained when the HM-PAO arterial input function was corrected for very rapidly produced, non-cerebrally extracted, metabolites and a kinetic model was used that considered the rate of intracerebral metabolism of HM-PAO to a retained metabolite. Even using this model, however, some differences between HM-PAO uptake and LCBF occurred in certain brain regions. Because these differences were small and the HM-PAO uptake pattern has been shown to be constant for many minutes, HM-PAO can probably be used to estimate LCBF in patients with single positron emission computed tomography (SPECT) imaging.