Interest of HYPR flow dynamic MRA for characterization of cerebral arteriovenous malformations: comparison with TRICKS MRA and catheter DSA.

OBJECTIVE: HYPR flow is a 3D dynamic contrast-enhanced MRA technique providing isotropic sub-millimetre resolution with half-second temporal resolution. We compared HYPR flow and time-resolved imaging of contrast kinetics (TRICKS) MRA for the characterization of cerebral arteriovenous malformations (cAVMs), using catheter DSA as reference. METHODS: Twenty-two patients underwent HYPR flow and TRICKS MRA within 15 days of DSA. HYPR flow and TRICKS datasets were reviewed separately by two readers for image quality, Spetzler-Martin grade, venous ectasia, and deep venous drainage. RESULTS: Image quality was better for HYPR flow than for TRICKS (narrower full width at half maximum; larger arterial diagnostic window; greater number of arterial frames, P ≤ 0.05). Using HYPR flow, inter-reader agreement was excellent for all cAVM characteristics. The agreement with DSA for the overall Spetzler-Martin grade was excellent for HYPR flow (ICC = 0.96 and 0.98, depending on the reader) and TRICKS (ICC = 0.82 and 0.95). In comparison to TRICKS, HYPR flow showed higher concordance with DSA for the identification of venous ectasia and deep venous drainage. CONCLUSION: Owing to an excellent agreement with DSA with respect to depiction of the vascular architecture of cAVMs, HYPR flow could be useful for the non-invasive characterization of cAVMs. KEY POINTS: • Dynamic MRA is used for cerebral AVM depiction and follow-up • HYPR flow is a new, highly-resolved dynamic MRA sequence • HYPR flow provides whole brain coverage • HYPR flow provides excellent agreement with the Spetzler-Martin grade • Compared to TRICKS MRA, HYPR flow improves cerebral AVM characterization.

Optimization of intra-voxel incoherent motion imaging at 3.0 Tesla for fast liver examination.

BACKGROUND: Optimization of multi b-values MR protocol for fast intra-voxel incoherent motion imaging of the liver at 3.0 Tesla. METHODS: A comparison of four different acquisition protocols were carried out based on estimated IVIM (DSlow , DFast , and f) and ADC-parameters in 25 healthy volunteers. The effects of respiratory gating compared with free breathing acquisition then diffusion gradient scheme (simultaneous or sequential) and finally use of weighted averaging for different b-values were assessed. An optimization study based on Cramer-Rao lower bound theory was then performed to minimize the number of b-values required for a suitable quantification. The duration-optimized protocol was evaluated on 12 patients with chronic liver diseases RESULTS: No significant differences of IVIM parameters were observed between the assessed protocols. Only four b-values (0, 12, 82, and 1310 s.mm(-2) ) were found mandatory to perform a suitable quantification of IVIM parameters. DSlow and DFast significantly decreased between nonadvanced and advanced fibrosis (P < 0.05 and P < 0.01) whereas perfusion fraction and ADC variations were not found to be significant. CONCLUSION: Results showed that IVIM could be performed in free breathing, with a weighted-averaging procedure, a simultaneous diffusion gradient scheme and only four optimized b-values (0, 10, 80, and 800) reducing scan duration by a factor of nine compared with a nonoptimized protocol. Preliminary results have shown that parameters such as DSlow and DFast based on optimized IVIM protocol can be relevant biomarkers to distinguish between nonadvanced and advanced fibrosis.

Comparison of 3D multi-echo gradient-echo and 2D T2* MR sequences for the detection of arterial thrombus in patients with acute stroke.

OBJECTIVES: We compared a multi-echo gradient-echo magnetic resonance sequence (susceptibility-weighted angiography [SWAN]) with the T2* sequence for the detection of an arterial thrombus in acute ischaemic stroke. METHODS: Seventy-four consecutive patients with acute ischaemic stroke were included. Proximal arterial occlusions were diagnosed using time-of-flight (TOF) magnetic resonance angiography (MRA). Two-dimensional (2D) axial reformats from 3D SWAN were generated to match with 2D T2* images. For arterial thrombus detection, each set of MR images (T2*, 2D SWAN reformats and 3D multiplanar SWAN images) was examined independently and separately by three observers who assigned the images to one of three categories: (0) absence of thrombus, (1) uncertain thrombus, (2) certain thrombus. Agreement and diagnostic accuracy were calculated. RESULTS: Twenty-four proximal arterial occlusions involving the anterior (n = 20) or posterior (n = 4) circulation were found. Inter-observer agreement was moderate using T2* images (κ = 0.58), good using 2D SWAN reformats (κ = 0.83) and excellent using multiplanar SWAN images (κ = 0.90). For the diagnosis of thrombus, T2* images were 54% sensitive and 86% specific, 2D SWAN reformats were 83% sensitive and 94% specific and SWAN multiplanar analysis was 96% sensitive and 100% specific. CONCLUSIONS: Three-dimensional SWAN sequence improves the detection of arterial thrombus in patients with acute ischaemic stroke in comparison with the 2D T2* sequence. KEY POINTS: • Multi-echo gradient-echo MR (e.g. susceptibility-weighted angiograph, [SWAN]) is increasingly used in neuroradiology. • Compared with conventional T2* sequences, SWAN improves detection of arterial thrombus. • Multiplanar SWAN analysis had the best diagnostic performance for arterial thrombus detection. • Sensitivity was 96% and specificity 100%. • Findings support combination of time-of-flight and susceptibility effects in suspected acute stroke.

MR selective flow-tracking cartography: a postprocessing procedure applied to four-dimensional flow MR imaging for complete characterization of cranial dural arteriovenous fistulas.

PURPOSE: To assess the feasibility of a selective flow-tracking cartographic procedure applied to four-dimensional (4D) flow imaging and to demonstrate its usefulness in the characterization of dural arteriovenous fistulas (DAVFs). MATERIALS AND METHODS: Institutional review board approval was obtained, and all patients provided written informed consent. Eight patients (nine DAVFs) underwent 3.0-T magnetic resonance (MR) imaging and digital subtraction angiography (DSA). Imaging examinations were performed within 24 hours of each other. 4D flow MR imaging was performed by using a 4D radial phase-contrast vastly undersampled isotropic projection reconstruction pulse sequence with an isotropic spatial resolution of 0.86 mm (5 minutes 35 seconds). Two radiologists independently reviewed images from MR flow-tracking cartography and reported the location of arterial feeder vessels and the venous drainage type and classified DAVFs according to the risk of rupture (Cognard classification). These results were compared with those at DSA. Quadratic weighted κ statistics with their 95% confidence intervals (CIs) were used to test intermodality agreement in the identification of arterial feeder vessels, draining veins, and Cognard classification. RESULTS: Interreader agreement for shunt location on MR images was perfect (κ = 1), with good-to-excellent interreader agreement for arterial feeder vessel identification (κ = 0.97; 95% CI = 0.92, 1.0), and matched in all cases with shunt location defined at DSA. There was good-to-excellent agreement between MR cartography and DSA in the definition of the main feeding arteries (κ = 0.92; 95% CI = 0.83, 1.0), presence of retrograde flow in dural sinuses (κ = 1), presence of retrograde cortical venous drainage (κ = 1), presence of venous ectasia (κ = 1), and final Cognard classification of DAVFs (κ = 1, standard error = 0.35). CONCLUSION: MR selective flow-tracking cartography enabled the noninvasive characterization of cranial DAVFs.

Fluid-attenuated inversion recovery vascular hyperintensities are not visible using 3D CUBE FLAIR sequence.

OBJECTIVE: Fluid-attenuated inversion recovery (FLAIR) vascular hyperintensities (FVH), initially described on 2D FLAIR images, are a useful imaging marker in patients with acute ischaemic stroke. We aimed to compare the sensitivity of the 3D CUBE FLAIR sequence with 2D FLAIR for the detection of FVH. METHODS: Forty-seven consecutive patients admitted for a suspected stroke were explored by 2D and 3D CUBE FLAIR MR sequences at 1.5 and 3 T. Three blinded readers assessed FVH defined as hyperintensities within cerebral arteries. Location of FVH, acute brain infarct and arterial stenosis were also assessed. 2D images were compared with 3D images for the detection of FVH. Agreement between readers was assessed. RESULTS: Of the 47 patients, 21 FVHs were observed on 2D FLAIR images of 15 patients (11 with acute brain infarct and 11 with an arterial stenosis). No FVH was visualised on 3D CUBE FLAIR images for either proximal or distal locations. Agreement between readers was excellent. CONCLUSION: FVHs are not visible using 3D CUBE FLAIR images. This study suggests that, in suspected acute ischaemic stroke, the assessment of FVH should only be performed on conventional 2D FLAIR images. KEY POINTS: • Fluid-attenuated inversion recovery (FLAIR) vascular hyperintensities (FVH) are of neuroradiological importance. • FVHs are useful imaging markers in patients with an acute ischaemic stroke. • FVHs are not visible using 3D CUBE FLAIR images. • Assessment of FVH should be performed on conventional 2D FLAIR images.

Susceptibility-weighted angiography for the detection of high-flow intracranial vascular lesions: preliminary study.

OBJECTIVES: Susceptibility-weighted magnetic resonance imaging (MRI) sequences may demonstrate various signal intensities of draining veins in cases of high-flow vascular malformation (HFVM), including arteriovenous malformation (AVM) and dural arteriovenous fistula (dAVF). Our objective was to evaluate susceptibility-weighted angiography (SWAN) for the detection of HFVM. METHODS: Fifty-eight consecutive patients with a suspected intracranial vascular malformation were explored with SWAN and post-contrast MRI sequences at 3 T. The diagnosis of slow-flow vascular malformation (SFVM), including developmental venous anomaly (DVA) or brain capillary telangiectasia (BCT), was based on MRI. Patients with suspected HFVM underwent digital subtraction angiography (DSA). SWAN images were analysed by three blinded readers according to a three-point scale of the venous signal. RESULTS: Thirty-one patients presented 35 SFVM (26 DVA and 9 BCT) that systematically appeared hypointense on SWAN images. In patients with atypical MRI findings, DSA revealed one patient with an atypical DVA and 26 patients with HFVM (22 AVM and 4 dAVF). SWAN revealed at least one venous hyperintensity in all patients with HFVM. Agreement between readers was excellent. CONCLUSIONS: SWAN appears reliable for characterising blood flow dynamics in brain veins. In clinical practice, SWAN can routinely rule out HFVM in patients with atypical brain veins.

High temporal resolution functional MRI using parallel echo volumar imaging.

PURPOSE: To combine parallel imaging with 3D single-shot acquisition (echo volumar imaging, EVI) in order to acquire high temporal resolution volumar functional MRI (fMRI) data. MATERIALS AND METHODS: An improved EVI sequence was associated with parallel acquisition and field of view reduction in order to acquire a large brain volume in 200 msec. Temporal stability and functional sensitivity were increased through optimization of all imaging parameters and Tikhonov regularization of parallel reconstruction. Two human volunteers were scanned with parallel EVI in a 1.5T whole-body MR system, while submitted to a slow event-related auditory paradigm. RESULTS: Thanks to parallel acquisition, the EVI volumes display a low level of geometric distortions and signal losses. After removal of low-frequency drifts and physiological artifacts, activations were detected in the temporal lobes of both volunteers and voxelwise hemodynamic response functions (HRF) could be computed. On these HRF different habituation behaviors in response to sentence repetition could be identified. CONCLUSION: This work demonstrates the feasibility of high temporal resolution 3D fMRI with parallel EVI. Combined with advanced estimation tools, this acquisition method should prove useful to measure neural activity timing differences or study the nonlinearities and nonstationarities of the BOLD response.