Data analysis was performed with Interactive Data Language (IDL (Boulder, CO, USA)) software programs developed in-house. ASL images were corrected for motion, pairwise subtracted between label and control images followed by averaging to generate the mean difference image (ΔM). Quantitative CBF (f) maps were calculated based on the following equation 14 (link),
where R1a (=0.72/0.61sec−1 at 1.5/3T) is the longitudinal relaxation rate of blood, M0 is the equilibrium magnetization of brain tissue, α (=0.8) is the tagging efficiency, τ (=1.5sec) is the duration of the labeling pulse, w (=2sec) is the post-labeling delay time and λ (=0.9g/ml) is blood/tissue water partition coefficient.Equation [1] assumes that the labeled blood spins remain primarily in the vasculature rather than exchanging completely with tissue water, which is justified in stroke patients in whom arterial transit times are likely prolonged 13 (link).
Post-processing of DSC images yielded multi-parametric perfusion maps including CBF, cerebral blood volume (CBV), Tmax and mean transit time (MTT), according to previously described analysis procedures 15 (link). Two CBF values were calculated from DSC data, namely CBFr0 and CBFrm, based on the value at time 0 and Tmax of the tissue residual function (R(t)), respectively. The calculation of CBFr0 may not represent the standard processing of DSC perfusion MRI, but was used to inform the comparison with ASL CBF. In each case, all structural, diffusion and perfusion images were aligned using SPM8 (Wellcome Department of Cognitive Neurology, UCL, UK). Two neuroradiologists and one perfusion MRI expert blinded to treatment and clinical information independently and separately reviewed ASL and DSC perfusion maps, which were scored on a scale of 0–3 to rate image quality and lesion severity/conspicuity, respectively. Both hypo- and hyper-perfusion were noted.
ASL and DSC perfusion images were further normalized into the Montreal Neurological Institute (MNI) template space using SPM8. Subsequently, segmentation of ASL and DSC perfusion images into major vascular territories was performed using an automated region-of-interest (ROI) analysis based on a published template of vascular territories in both hemispheres 16 (link). The vascular territories studied were anterior cerebral artery (ACA), posterior cerebral artery (PCA), and leptomeningeal and lenticulostriate (perforator) distributions of the middle cerebral artery (MCA). In addition, in AIS patients demonstrating hypoperfusion, ROIs defined by Tmax > 6s, 2s < Tmax < 6s and Tmax < 2s were used to extract corresponding ASL and DSC CBF values respectively. Manual restriction of the ROIs was applied when necessary.
where R1a (=0.72/0.61sec−1 at 1.5/3T) is the longitudinal relaxation rate of blood, M0 is the equilibrium magnetization of brain tissue, α (=0.8) is the tagging efficiency, τ (=1.5sec) is the duration of the labeling pulse, w (=2sec) is the post-labeling delay time and λ (=0.9g/ml) is blood/tissue water partition coefficient.
Post-processing of DSC images yielded multi-parametric perfusion maps including CBF, cerebral blood volume (CBV), Tmax and mean transit time (MTT), according to previously described analysis procedures 15 (link). Two CBF values were calculated from DSC data, namely CBFr0 and CBFrm, based on the value at time 0 and Tmax of the tissue residual function (R(t)), respectively. The calculation of CBFr0 may not represent the standard processing of DSC perfusion MRI, but was used to inform the comparison with ASL CBF. In each case, all structural, diffusion and perfusion images were aligned using SPM8 (Wellcome Department of Cognitive Neurology, UCL, UK). Two neuroradiologists and one perfusion MRI expert blinded to treatment and clinical information independently and separately reviewed ASL and DSC perfusion maps, which were scored on a scale of 0–3 to rate image quality and lesion severity/conspicuity, respectively. Both hypo- and hyper-perfusion were noted.
ASL and DSC perfusion images were further normalized into the Montreal Neurological Institute (MNI) template space using SPM8. Subsequently, segmentation of ASL and DSC perfusion images into major vascular territories was performed using an automated region-of-interest (ROI) analysis based on a published template of vascular territories in both hemispheres 16 (link). The vascular territories studied were anterior cerebral artery (ACA), posterior cerebral artery (PCA), and leptomeningeal and lenticulostriate (perforator) distributions of the middle cerebral artery (MCA). In addition, in AIS patients demonstrating hypoperfusion, ROIs defined by Tmax > 6s, 2s < Tmax < 6s and Tmax < 2s were used to extract corresponding ASL and DSC CBF values respectively. Manual restriction of the ROIs was applied when necessary.