High Resolution 3D Intracranial Imaging at 3.0T

Introduction Vessel wall imaging of intracranial arteries is challenging due to their small sizes and their relatively deep locations inside the brain. 3D Time-of-flight (TOF) and contrast enhanced MRA (ceMRA) are commonly used [1] luminographic techniques. They lack specificity as luminal defects may be caused by various reasons. Dark blood, two dimensional turbo spin echo (2DTSE) has been used to image the vessel walls of intracranials [2] but this 2D approach suffers from low scan efficiency, limited anatomical coverage for the tortuous vessels, partial volume effect and need for careful slice positioning. T1 weighted SPACE (T1w-SPACE) [3] is a TSE variant capable of 3D imaging with high sampling efficiency and good blood suppression, and has previously shown utility for vessel wall imaging of the carotids [4]. We propose here the use of T1w-SPACE for 3D dark blood imaging of the intracranial artery vessel walls at 3.0T with a 32 channel head coil to extend the anatomical coverage and achieve a CT-like spatial resolution of 0.5 mm (isotropic). Method Imaging: The study was approved by the institutional review board. Six healthy volunteers were recruited. Imaging was done on a 3.0T scanner with 32 receiver channels. A 32 channel head coil was used for signal reception. A FLASH localizer was first run, followed by a high resolution TOF acquisition (0.4x0.4x0.5mm) for overview of the intracranial vessels. T1w-SPACE was then run with these parameters: TR/TE=800ms/25ms; echo train length (ETL)=27, NEX=2; fat suppression; bandwidth=475Hz/pixel; true voxel size=0.5mm (isotropic). It was run in two ways: first with iPAT (80 slices for anatomical coverage, 10.6min), and then without iPAT (40 slices for SNR comparison). Lastly, ECG triggered, dark blood multislice 2DTSE [5] of the internal carotid artery (ICA) at the carotid canal was acquired for SNR comparison (imaging parameters: TR/TE = 1RR/9.4ms; ETL=9, NEX=4, bandwidth=302Hz/pixel; fat suppression, dark blood pulse [6] with TI≈450ms). The voxel size was (0.3mm) x 2mm. Interleaved factors of 1 (i.e., no interleaving), 2 or 3 were used, depending on heart rate. 3-4 slices with 2mm gap were acquired. Scan time≈3.3min/interleaved acquisition. Analysis: For each subject, datasets from 2DTSE and T1w-SPACE with no iPAT were co-registered using commercial fusion software. MPR images of T1w-SPACE at identical positions with 2DTSE were generated. In a total of 18 slices, SNR was measured in the vessel wall and white matter. It was defined as [7]: SNR=average signal in ROI/standard deviation of signal in ROI. SNRv (=SNR/voxel volume) was calculated to account for the differences in voxel size between 2D and 3D acquisitions. A paired t-test was used to compare the SNR and SNRv differences between 2D and 3D acquisitions. Finally, the TOF images were co-registered with T1wSPACE with iPAT to check if the 40mm slab covered both the ICA and MCA on both sides in each volunteer.