Single-subject statistical mapping of acute brain hypoxia in the rat following middle cerebral artery occlusion: A microPET study Masashi Takasawa a , John S. Beech b , Tim D. Fryer c , P. Simon Jones a , Tahir Ahmed a , Rob Smith c , Franklin I. Aigbirhio c , Jean-Claude Baron a,d, ⁎ a Department of Clinical Neurosciences, Stroke Research Group, University of Cambridge, Cambridge, UK b Department of Medicine, Division of Anaesthesia, University of Cambridge, Cambridge, UK c Department of Clinical Neurosciences, Wolfson Brain Imaging Centre, University of Cambridge, Cambridge, UK d INSERM U894, Université Paris 5, Paris, France abstract article info Article history: Received 21 October 2010 Revised 7 February 2011 Accepted 9 February 2011 Available online 16 February 2011 Keywords: Hypoxia Ischemia Penumbra Positron emission tomography SHR Stroke No study so far has attempted to map the 3D topography of brain hypoxia in the individual rat in vivo following middle cerebral artery occlusion (MCAo). In a previous microPET study, we reported that 18 F- fluoromisonidazole ( 18 F-MISO) trapping in the brain after MCAo was specific for the hypoxic viable tissue. Here, we used 18 F-MISO microPET to map the 3D topography of brain hypoxia in the acute stage of permanent distal MCAo in individual spontaneously hypertensive rats. Normal rats were also studied. 18 F-MISO was intravenously injected approximately 1 h after clip placement and PET data were acquired for 2 hours. Animals were sacrificed and the brains harvested 48 h later for infarct mapping using standard histopathology. As expected, continuous 18 F-MISO trapping was found over the affected relative to unaffected and control MCA cortex. Using single-subject voxel-based statistical mapping, tracer accumulation 90–120 min after injection was consistently significantly higher in the anterior MCA cortex (proximal relative to clip site) and gradually decreased towards posterior areas, a pattern consistent with the classic penumbra concept. The data also suggested that (i) a portion of the significant 18 F-MISO trapping area may sit outside the contours of the final infarct despite the permanent MCAo, suggesting that 18 F-MISO may be a marker not only of severe (penumbral) but also of milder (oligemic) hypoxia, and (ii) small portions of the final infarct may not exhibit early tracer trapping, suggesting that by the time the tracer was administered this tissue had already progressed to irreversible damage. This study shows the feasibility of single-subject mapping of brain hypoxia following MCAo in the rat, which has potential applications in pathophysiological investigations. © 2011 Elsevier Inc. All rights reserved. Introduction In acute ischemic stroke, the penumbra, i.e. the severely hypoxic but potentially salvageable region surrounding the core of irreversible damage, is the main target for therapy (Baron, 2001; Donnan and Davis, 2002). Assessing the temporal and spatial components of the penumbra is key to a full understanding of the ischemic process (Baron, 1999). Positron emission tomography (PET) is a powerful modality to explore these features. Experimental studies measuring brain tissue pO 2 by means of intracerebral probes and electron paramagnetic resonance following acute middle cerebral artery occlusion (MCAo) have disclosed the presence of a gradient of pO 2 values, with hypoxia being most marked in the core and less severe in peripheral areas (Crockard et al., 1976; Liu et al., 2004). However, in addition to their invasiveness and static and limited sampling, intracerebral pO 2 probes are not suited to provide a three-dimensional picture of brain hypoxia after stroke. PET using misonidazole derivatives, mainly 18 F-fluoromisonida- zole ( 18 F-MISO), has been successfully used to map brain hypoxia after stroke in humans (Markus, 2007; Takasawa et al., 2008). However, the analysis of 18 F-MISO trapping pattern is limited in humans because the scanning time point and the topography of penumbra vary widely among patients. Animal experiments involving in vivo imaging following experimental stroke are therefore better suited than clinical studies because they permit full control of strain, age and gender as well as location and duration of the occlusion, in turn achieving optimal homogeneity in penumbral topography and time-course among subjects. Di Rocco et al. (1993) were the first to carry out such investigations using the SPECT hypoxia tracer 99m Tc- BMS-181321 in cats with proximal MCAo, reporting late trapping of the tracer in the affected area, which was validated by terminal autoradiography (ARG). However, they used 2D planar imaging rather than axial tomography, and hence did not assess the 3D topography of tracer trapping. In our recently reported pilot experiments using PET Experimental Neurology 229 (2011) 251–258 ⁎ Corresponding author at: Department of Neurology, Cambridge University, Addenbrooke's Hospital, Box 83, Cambridge CB2 2QQ, UK. Fax: + 44 1223 217 909. E-mail address: jcb54@cam.ac.uk (J.-C. Baron). 0014-4886/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.expneurol.2011.02.005 Contents lists available at ScienceDirect Experimental Neurology journal homepage: www.elsevier.com/locate/yexnr