Cerebral vascular mean transit time (MTT), defined as the ratio of cerebral blood volume to cerebral blood flow (CBV/CBF), is a valuable indicator of the cerebral circulation. Positron emission tomography (PET) and dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSC-MRI) are useful for the quantitative determination of MTT in the clinical setting. The aim of this study was to establish a normal value set of MTT as determined by PET and by DSC-MRI and to identify differences between these methods. Seven healthy volunteers were studied with 15 O-PET (H 2 15 O and C 15 O) and gradient-echo echo-planar DSC-MRI at 1.5 T. In the DSC-MRI study with bolus injection of contrast agent, deconvolution analysis was performed. Comparison of gray-to-white matter ratios showed fairly good agreement between PET and DSC-MRI for all parameters (relative CBV, relative CBF, and relative MTT), confirming the validity of relative measurements with DSC-MRI. However, quantitative MTT measured by DSC-MRI was significantly shorter than that measured by PET in cerebral cortical regions (2.8 to 3.0 secs for DSC-MRI versus 3.9 to 4.3 secs for PET) and the centrum semiovale (3.5 secs for DSC-MRI versus 4.8 secs for PET). These discrepancies may be because of the differences in the intrinsic sensitivity of each imaging modality to vascular components; whereas PET measurement of CBV is equally sensitive to all vascular components, measurement with DSC-MRI originates from the microvasculature in the vicinity of the brain parenchyma. This underlying difference may influence interpretation of MTT determined by PET or by DSC-MRI for patients with cerebrovascular disease.

The duct-penetrating sign on MRCP images was more helpful to distinguish IPM from CPC than were the enhancement patterns on CT and MR images.

Increased ONS diameter measured on coronal orbital thin-slice fat-saturated T2-weighted MR images is a strong indicator of increased ICP, and helps to differentiate between passive subdural fluid collection due to brain atrophy and subdural hygroma with increased ICP.

Motor functional outcome in patients with intracerebral hemorrhage can be predicted by measuring FA values using diffusion tensor tractography.

DWI is useful for differentiating malignant and benign orbital tumors if accompanied by visual assessment of ADC maps and ADC value calculations.

MR images of wall thickening and prominent ring enhancement of the wall of affected large cerebral arteries may be a diagnostic clue in differentiating radiation-induced arteritis from moyamoya disease.

• Diffusion-weighted MRI offers possibilities to assess the aggressiveness of meningiomas. • The q-space imaging-derived mean displacement correlates strongly with apparent diffusion coefficients. • Both diffusion parameters showed a strong negative association with tumour cell counts. • Derived mean displacement may help assess the aggressiveness of meningiomas preoperatively.

Purpose: Deconvolution based on truncated singular value decomposition (SVD deconvolution) is a promising method for measuring cerebral blood ‰ow (CBF) with dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSC-MRI), but it has proved extremely sensitive to tracer delay. The purpose of this study was to investigate the eŠect of regional tracer delay on CBF determined by SVD deconvolution (SVD-CBF). SVD-CBFs with and without correction for the delay were compared with CBF measured by positron emission tomography (PET-CBF), which is regarded as the gold standard for quantiˆcation of CBF.Methods: Perfusion MRI and PET were performed on seven healthy men. In the PET study, the CBF image was obtained with bolus injection of H 2 15 O and continuous arterial sampling. In the DSC-MRI study with bolus injection of Gd-based contrast agent, dynamic perfusion data were obtained with a 1.5T scanner at 1-s intervals by means of gradientecho echo-planar imaging. CBF was determined by the SVD deconvolution method with and without correction for the tracer delay. Region-of-interest measurements were obtained in the gray matter (cerebral cortex in the middle cerebral artery territory) and white matter (centrum semiovale).Results: Tracer delay was signiˆcantly longer in white matter than in gray matter (1.45± 0.61 s vs. 0.59±0.35 s, Pº0.01). Correction for the delay increased SVD-CBF in the white matter and consequently reduced the gray-to-white SVD-CBF ratio. The uncorrected grayto-white SVD-CBF ratio was signiˆcantly larger than that of Pº0.01). However, the gray-to-white delay-corrected SVD-CBF ratio did not diŠer signiˆcantly from that of PET-CBF (2.83±0.31 vs. 2.54±0.49, P＝0.10).Conclusion: The tracer delay in DSC-MRI causes errors in CBF estimates, even in healthy persons, and therefore should be corrected for when delay-sensitive deconvolution, such as SVD deconvolution, is used.