Subjects and MRI Protocol: 27 healthy controls (age 38±13 y) and six patients with biopsy-proven glioblastoma (GB) (age 49±19 y) were included in this study. Scans were performed on a 3.0T GE system and included T₁ weighted imaging (T₁WI) performed before and after contrast agent (Gd), FLAIR images and DCE acquired using multi-phase 3D SPGR with the variable flip angle method used for T₁ maps calculation. Each patient was scanned twice.

Preprocessing: included skull stripping, inhomogeneity correction, T₁ mapping, motion correction and coregistration to DCE space. Plasma volume(v_p) and the volume transfer constant (k^trans) parameters were calculated from the DCE data using DUSTER (DCE-Up-Sampled-TEmporal Resolution), an in-house tool for DCE analysis, based on the Extended-Tofts-Model ^[3] with T₁ maps calculated with correction of inaccuracy in the flip angles and accounting for differences in the bolus arrival time ^{[4, 5]}. The gray matter (GM) and white matter (WM) volumes of interest (VOIs) were extracted for each subject from pre-contrast T₁WI; for the right and the left hemisphere for the healthy controls; and for the contra-lesional hemisphere only, for patients with GB. In patients, the lesion area was extracted at each time point from the T₁WI+Gd images using threshold-based segmentation ^[6]. Lesion VOI was defined as the overlap lesion area between the two time points. Mean and standard deviation (SD) values and histograms of v_p and k^trans in each VOI, were calculated.

Parametric response maps: were calculated separately for the v_p and k^trans parameters. The SD values obtained in the control group from the WM area were used as threshold values for determining voxel-by-voxel differences between scans in patients, where Δ<-2SD=reduction; -2SD<Δ<+2SD=no change; Δ>+2SD=increase.

Healthy controls: Mean and SD values, of v_p and k^trans in the WM and GM of healthy subjects (n=27), are shown in Table 1. Figure 1 shows the mean histograms of (a) v_p and (b) k^trans obtained for the WM (red) and GM (blue) of healthy controls. v_p and k^trans values were significantly (p<0.05) higher in the GM compared with the WM. No age, gender or hemispherical differences were detected for either parameter. The SD values of this group showed the reliability of the values of DCE parameters in the healthy population, and were set as threshold values for patients' assessment.

Patients: The mean values in the contra-lesional hemisphere were similar between the two time points in each patient, with mean differences of v_p: GM=10%, WM=6%; k^trans: GM= 17%, WM=17% across time. Histograms of k^trans and v_p in the WM (red), GM (blues) and lesion (black) areas at time point 1 (solid line) and time point 2 (dashed line), of three patients, are shown in Figure 2, demonstrating the high reproducibility of those parameters between the two time points. Parametric response maps within lesion area obtained for each patient (Fig.1; insets b-c). Patients #1 and #2 (two upper rows) were scanned before and two weeks following bevacizumab therapy, and revealed a substantial reduction in their mean values of: 30% and 81% for the v_p and k^trans, respectively. The radiological assessment of these patients indicated improvement according to RANO criteria. Patient #3 was treated with VBL and bevacizumab therapies and was scanned at two month intervals. This patient demonstrated substantial increase of 52% and 128% for the v^p and k^trans values, supporting the diagnosis of progression.