Comparison of arterial blood volume obtained from model-free arterial spin labelling (ASL) and cerebral blood volume obtained from contrast enhanced dynamic susceptibility weighted imaging (DSC) in brain tumours

Introduction Cerebral blood volume (CBV) in brain tumours reflects vascularity and can be used to differentiate intracranial tumour types. Measurement of CBV with dynamic susceptibility weighted imaging (DSC-MRI) requires injection of a contrast agent. As an alternative, a recently introduced, model-free arterial spin labelling technique (ASL) named QUASAR enables quantification of cerebral blood flow (CBF) and arterial blood volume (aBV) [1]. In this study, aBV and CBF calculated from ASL and CBV and CBF obtained from DSC-MRI were determined in three types of brain tumours. Subjects and Methods Nine subjects with intracranial tumours (3 grade 2-3 gliomas, 3 glioblastomas and 4 meningiomas, two in one patient) were examined on a 3-T MR unit (Philips Achieva, Philips Medical Systems, Best, The Netherlands) with ASL (QUASAR) and DSC-MRI (GRE-EPI). ASL images were obtained with crushed arterial signal using a velocity-encoding gradient (crushed data) as well as with retained arterial signal (non-crushed data); two flip angles were used to obtain equilibrium magnetization in blood. QUASAR parameters were TR/TE/ΔTI/TI1=4000/23/300/40 ms, matrix=64×64, seven slices (6 mm thickness/2 mm slice gap), FOV=240 mm, flip angle=35/11.7°, SENSE factor 2.5, Venc=[∞ ,4 cm/s], and 82 averages (48 using Venc=4 cm/s, 24 using Venc=∞, 10 with smaller flip angle), implemented in a single sequence. For DSC-MRI, GRE-EPI was used with TR/TE=1360/29 ms, flip angle=90°, slice thickness=5 mm, 23 slices, FOV=220 mm, matrix=128×128 and SENSE factor 2.5. In both DSC-MRI and ASL, deconvolution was performed using a block-circulant singular value decomposition method [2]. In the ASL data, arterial signal curves were obtained by subtracting crushed from non-crushed data. aBV was obtained as the time integral of the arterial signal curve divided by the bolus area that corresponds to an initially labelled voxel with 100% blood volume. Under guidance of morphological information, ROIs of 0.08 cm were placed in the lesion and in healthy grey matter (GM) (approximately 30 cm) and the ratio was calculated. Grey matter was used as a reference since the signal-to-noise ratio in white matter is low in ASL. Results Figure 1 shows maps of ASL-aBV, DSC-CBV, ASL-CBF and DSC-CBV in a meningioma. Mean results of the ratios from the four measurements and standard deviations are displayed in table 1. When assuming proportionality between aBV obtained from ASL and CBV obtained from DSC-MRI ratios the correlation was 0.89 (see figure 2). Comparing CBF ratios the correlation was 0.90 (see figure 2).