Background and Purpose We evaluate associations between the severity of magnetic resonance perfusion-weighted imaging abnormalities, as assessed by the hypoperfusion intensity ratio (HIR), on infarct progression and functional outcome in the Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution Study 2 (DEFUSE 2). Methods Diffusion-weighted magnetic resonance imaging and perfusion-weighted imaging lesion volumes were determined with the RAPID software program. HIR was defined as the proportion of TMax >6 s lesion volume with a Tmax >10 s delay and was dichotomized based on its median value (0.4) into low versus high subgroups as well as quartiles. Final infarct volumes were assessed at day 5. Initial infarct growth velocity was calculated as the baseline diffusion-weighted imaging (DWI) lesion volume divided by the delay from symptom onset to baseline magnetic resonance imaging. Total Infarct growth was determined by the difference between final infarct and baseline DWI volumes. Collateral flow was assessed on conventional angiography and dichotomized into good and poor flow. Good functional outcome was defined as modified Rankin Scale ≤2 at 90 days. Results Ninety-nine patients were included; baseline DWI, perfusion-weighted imaging, and final infarct volumes increased with HIR quartiles (P<0.01). A high HIR predicted poor collaterals with an area under the curve of 0.73. Initial infarct growth velocity and total infarct growth were greater among patients with a high HIR (P<0.001). After adjustment for age, DWI volume, and reperfusion, a low HIR was associated with good functional outcome: odds ratio=4.4 (95% CI, 1.3–14.3); P=0.014. Conclusions HIR can be easily assessed on automatically processed perfusion maps and predicts the rate of collateral flow, infarct growth, and clinical outcome.

Our results suggest that in subacute ischemic stroke patients, Tmax correlates better than absolute mean transit time (MTT) with xenon CT cerebral blood flow (Xe-CT CBF) and that both Tmax >4 seconds and MTT >10 seconds are strongly associated with Xe-CT CBF <20 mL/100 g/minute. CBF = cerebral blood flow; DBP = diastolic blood pressure; DEFUSE = Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution; DWI = diffusion-weighted imaging; EPITHET = Echoplanar Imaging Thrombolytic Evaluation Trial; FOV = field of view; ICA = internal carotid artery; IQR = interquartile range; MCA = middle cerebral artery; MTT = mean transit time; NIHSS = NIH Stroke Scale; PWI = perfusion-weighted imaging; PWI-MRI = magnetic resonance perfusion-weighted imaging; ROC = receiver operating characteristic; ROI = region of interest; SBP = systolic blood pressure; SVD = singular value decomposition; Xe-CT = xenon CT.

We have studied the magnetic interference of the critical currents of synthetic planar thin-film grain-boundary Josephson junctions. We find that the effective area of these junctions scales as the square of the width w in contrast to the usual w(2λ+d) dependence of sandwich-type Josephson junctions. This behavior is a simple consequence of the magnetic response of thin-film superconductors to perpendicular applied fields. A model based on the London theory yields the observed behavior. In addition, we find the correction to the interference pattern due to the effect of the corners.

Arterial spin labeling (ASL) is a completely noninvasive magnetic resonance imaging (MRI) perfusion method for quantitatively measuring cerebral blood flow utilizing magnetically labeled arterial water. Advances in the technique have enabled the major MRI vendors to make the sequence available to the clinical neuroimaging community. Consequently, ASL is being increasingly incorporated into the routine neuroimaging protocol. Although a variety of ASL techniques are available, the ISMRM Perfusion Study Group and the European ASL in Dementia Consortium have released consensus guidelines recommending standardized implementation of 3D pseudocontinuous ASL with background suppression. The purpose of this review, aimed at the large number of neuroimaging clinicians who have either no or limited experience with this 3D pseudocontinuous ASL, is to discuss the common and clinically significant artifacts that may be encountered with this technique. While some of these artifacts hinder accurate interpretation of studies, either by degrading the images or mimicking pathology, there are other artifacts that are of clinical utility, because they increase the conspicuity of pathology. Cognizance of these artifacts will help the physician interpreting ASL to avoid potential diagnostic pitfalls, and increase their level of comfort with the technique.