Window narrowing: a new method for standardized assessment of the tissue at risk-maximum of infarction in CT based brain perfusion maps

Abstract

Background: Mapping of brain perfusion using bolus tracking methods is increasingly used to assess the amount and severity of cerebral ischemia in acute stroke. Using relative perfusionmaps, however, it is difficult to identify the tissue at risk-maximum (TARM) of infarction with sufficient reliability and reproducibility.Methods: We analysed 76 perfusion computed tomography (PCT) derived maps of cerebral blood flow (CBF), cerebral blood volume (CBV) and time-to-peak (TTP) in 40 acute stroke patients using multidetector row technology and standard software (Somatom VolumeZoom, Siemens, Germany). 'Window narrowing' of the color maps was performed until color homogenisation of the contralateral unaffected hemisphere was reached. Tissue still depictable on the affected hemisphere after sufficient window narrowing was defined as the TARM. We analysed presence and size of the TARM on PCT maps, its relative perfusion values by comparison with contralateral, mirrored tissue, and its correlation with occurrence and final size of cerebral infarction on follow-up imaging.Results: An ischemic area was visible in 64, 58.9 and 72.6% on the conventional CBF, CBV and TTP maps, respectively. After window narrowing, a TARM was present in 56.8, 54.1 and 63.0% of slices comprising 11.9, 11.6 and 21.1% of the ipsilateral hemisphere (CBF, CBV and TTP), respectively. The relative perfusion values were 38.7 (CBF) and 43.0% (CBV) for the entire ischemic area and 11.3 (CBF) and 13.3% (CBV) for the TARM. Definite cerebral infarction was visible on 68.1% of the target slices comprising 23.7 ± 22.9% of the ipsilateral hemisphere. The size of the TARM correlated slightly better with the final infarction size (r=0.74–0.82) than the entire ischemic area (r=0.61–0.79). With respect to the occurrence of cerebral infarction, the presence of a TARM on CBF maps showed the best positive (97.9%) and negative (72.7%) predictability.Discussion: On PCT maps, window narrowing provides a standardized display of the TARM in peracute stroke. The severely reduced values of relative CBF and CBV suggest the TARM to indicate tissue most prone to infarction.