Section: New Results

Vascular Imaging and Arterial Spin Labelling

Arterial spin labeling for motor activation mapping at 3T

Participants : Jan Petr, Aymeric Stamm, Elise Bannier, Jean-Christophe Ferré, Jean-Yves Gauvrit, Christian Barillot.

Functional arterial spin labeling (fASL) is an innovative biomarker of neuronal activation that allows direct and absolute quantification of activation-related CBF and is less sensitive to venous contamination than BOLD fMRI. This study evaluated fASL for motor activation mapping in comparison with BOLD fMRI in terms of involved anatomical area localization, intra-individual reproducibility of location, quantification of neuronal activation, and spatial accuracy. Imaging was performed at 3T with a 32-channel coil and dedicated post-processing tools were used. Twelve healthy right-handed subjects underwent fASL and BOLD fMRI while performing a right hand motor activation task. Three sessions were performed 7days apart in similar physiological conditions. Our results showed an activation in the left primary hand motor area for all 36 sessions in both fASL and BOLD fMRI. The individual functional maps for fASL demonstrated activation in ipsilateral secondary motor areas more often than the BOLD fMRI maps. This finding was corroborated by the group maps. In terms of activation location, fASL reproducibility was comparable to BOLD fMRI, with a distance between activated volumes of 2.1mm and an overlap ratio for activated volumes of 0.76, over the 3 sessions. In terms of activation quantification, fASL reproducibility was higher, although not significantly, with a CVintra of 11.6% and an ICC value of 0.75. Functional ASL detected smaller activation volumes than BOLD fMRI but the areas had a high degree of co-localization. In terms of spatial accuracy in detecting activation in the hand motor area, fASL had a higher specificity (43.5%) and a higher positive predictive value (69.8%) than BOLD fMRI while maintaining high sensitivity (90.7%). The high intra-individual reproducibility and spatial accuracy of fASL revealed in the present study will subsequently be applied to pathological subjects [25] .

Construction and evaluation of a quantitative ASL brain perfusion template at 3T

Participants : Jan Petr, Elise Bannier, Jean-Christophe Ferré, Jean-Yves Gauvrit, Christian Barillot.

Arterial spin labeling (ASL) allows non-invasive imaging and quantification of brain perfusion by magnetically labeling blood in the brain-feeding arteries. ASL has been used to study cerebrovascular diseases, brain tumors and neurodegenerative disorders as well as for functional imaging. The use of a perfusion template could be of great interest to study inter-subject regional variation of perfusion and to perform automatic detection of individual perfusion abnormalities. However, low spatial resolution and partial volume effects (PVE) issues inherent to ASL acquisitions remain to be solved. The purpose of this study is to enhance the template quality by using DARTEL non-rigid registration and by correcting for PVE. PICORE-Q2TIPS ASL datasets were acquired on 25 healthy volunteers at 3T. Four methods of creating the template were evaluated using leave-one-out cross correlation. Subsequently, these methods were applied to hyper-perfusion detection on functional ASL data of 8 healthy volunteers and compared with the standard generalized linear model (GLM) activation detection [40] .

Evaluation of functional arterial spin labeling data using a perfusion template

Participants : Jan Petr, Elise Bannier, Jean-Christophe Ferré, Jean-Yves Gauvrit, Christian Barillot.

ASL allows non-invasive imaging and quantification of brain perfusion by magnetically labeling blood in the brain-feeding arteries. In this study, a template created from perfusion images of 25 resting healthy subjects was used to automatically detect hyper perfusion patterns of 8 other subjects. DARTEL registration was used to improve the precision of the template and partial volume correction to prevent interpolation artifacts. This study showed that a perfusion template can be used to assess task-related activation zones in functional ASL data while using only activated phase. Two assumptions can be made to explain why standard functional analysis yields slightly larger activation regions. First, the use of FWHM 6mm Gaussian kernel possibly enlarges the detected zones. Second, the data analyzed using SPM contains both resting and activated phases whereas only the activated phase was compared to the template. Future work will focus on detection of hyperperfusion in different neurodegenerative diseases taking into account registration issues of pathological T1 images. [24] .

A contrario detection of focal brain perfusion abnormalities based on an ASL template

Participants : Camille Maumet, Elise Bannier, Jean-Christophe Ferré, Pierre Maurel, Christian Barillot.

Arterial Spin Labeling (ASL) is a recent MRI perfusion technique which enables quantification of cerebral blood flow (CBF). The presence of regions with atypical CBF can characterize a pathology. In brain tumors for instance, perfusion increase can be directly related to the grading of the malignant tissues. It is therefore of great interest to identify these regions in order to provide the patients with the most appropriate therapy. In this work, we proposed to detect abnormal brain perfusion by using an a contrario framework and an ASL template as a model of normal perfusion. Validation was undertaken by qualitative comparison with CBF extracted from dynamic susceptibility weighted contrast enhanced (DSC) sequence. We experimented this framework on four patients presenting brain tumors. Results show that high perfusion regions found in DSC CBF maps are correctly identified as hyperperfusions with a contrario detection. Automatic detection has clear advantages over manual delineation since it is less time-consuming, does not depend on medical expertise and allows quantification of perfusion abnormalites within the detected regions.

Peripheral angiography using non-contrast enhanced imaging

Participants : Elise Bannier, Isabelle Corouge, Nicolas Wiest-Daesslé.

Arteriography, CT and MR angiography are routinely performed in patients presenting peripheral arteriopathy. Yet, contrast agent injection is contraindicated in patients with renal insufficiency and the underlying risk of developing nephrogenic systemic fibrosis further encourages research on non-contrast enhanced MR angiography techniques (NCE MRA). In this context, we evaluated at 3T the ability of a 2 NCE MRA new sequences to reliably detect peripheral vascular abnormalities from the abdominal aorta to the calf in comparison with CE MRA.

A first study including 20 healthy volunteers and 4 patients evaluated the NCE ECG-gated T2 TSE NATIVE SPACE MRI sequence. It demonstrated its potential in noninvasively imaging peripheral vasculature, from the abdominal aorta to the calf, within a clinically acceptable acquisition duration. Although signal inhomogeneity and peristalsis artifacts were sometimes observed in the abdominal aortic station, very good image quality was obtained on all subjects on lower stations, with no venous contamination.

A second study evaluated the NCE ECG-gated Quiescent Interval Single Shot (QISS) MRA sequence. Preliminary results obtained on 11 patients show that several lesions were not detected with QISS MRA especially on the thigh station. Ongoing patient inclusions are required to confirm these results. Finally, a concomitant NCE and CE MRA reading will be performed to compare stenosis grading, stenosis-thrombosis mismatch and lesions not detected with NCE MRA.