Arterial spin labeling (ASL) offers non-invasive cerebral blood flow (CBF) measurements, but typically suffers from low signal-to-noise ratio, limiting the achievable spatial resolution. In this work, we employ 3D EPI ASL at 7T, partially-overlapping acquisition of multiple slabs and across-session averaging to achieve a high-quality whole-brain 0.7 mm3 isotropic resolution CBF map from a healthy volunteer. The dataset presents the highest spatial resolution CBF map in humans so far, and a unique opportunity to investigate the cortical distribution of baseline CBF across and within brain areas, including providing a physiological basis for the interpretation of laminar and columnar fMRI.
Measurements were performed on a whole-body 7 Tesla MRI scanner (Siemens Healthineers) with a 32-channel receive head-coil (Nova Medical). A 20-year-old female volunteer was scanned in twelve separate sessions after giving informed consent. Each session lasted 2 hours and contained at least six 10-minute FAIR QUIPSS II ASL runs positioned at different locations over the brain to achieve most efficient coverage of the whole brain (Figure 1). One or more runs were repeated in each session to improve the resulting data quality.
Slab-selective (45 mm) or non-selective inversion was accomplished at 7T using an optimized 10 ms tr-FOCI pulse6. In order to further increase the labeling efficiency at 7T, two rectangular 18x18 cm2 high-permittivity 'dielectric pads' with 5 mm thickness were placed on both sides of the head at the level of the temporal lobes7. The acquisition parameters were: 0.7 mm3 isotropic resolution, 36 slices, 19 degrees nominal flip angle, FLASH-GRAPPA 4, TE/TI1/TI2/TR = 16/700/1890/3000 ms, 61 ms scan-time per slice, 200 TRs. M0 images were also acquired with identical acquisition parameters apart from: inversion and saturation pulses switched off, 192 slices, TR = 12.6 s and 20 TRs. All data within a session were motion-corrected and coregistered with SPM 8 using 6 degrees of freedom (dof) to the M0. Perfusion-weighted data were normalized with its respective M0 before averaging across sessions. Data across sessions were coregistered to the mean M0 (Figure 2) with ANTs using 9 dof8 to account for differences in distortions. Voxel-wise CBF, control-image and perfusion tSNR maps were generated after averaging across sessions.
1. Alsop et al., MRM, 2015, 73:102-16. Recommended Implementation of Arterial Spin Labeled Perfusion MRI for Clinical Applications: A consensus of the ISMRM Perfusion Study Group and the European Consortium for ASL in Dementia.
2. Vidorreta et al., Neuroimage, 2013, 66:662-71. Comparison of 2D and 3D single-shot ASL perfusion fMRI sequences.
3. Ivanov et al., Neuroimage, 2017, 156:363-376. Comparison of 3T and 7T ASL techniques for concurrent functional perfusion and BOLD studies.
4. Huber et al. Neuroimage, 2017, in press, doi: 10.1016/j.neuroimage.2017.07.041. Non-BOLD contrast for laminar fMRI in humans: CBF, CBV, and CMRO2.
5. Gusnard and Raichle, Nat Rev Neurosci. 2001, 2:685-94. Searching for a baseline: functional imaging and the resting human brain.
6. Hurley et al., MRM, 2010, 63:51-8. Tailored RF pulse for magnetization inversion at ultrahigh field.
7. Teeuwisse et al., MRM, 2012, 67:912-8. Simulations of High Permittivity Materials for 7T Neuroimaging and Evaluation of a New Barium Titanate-Based Dielectric.
8. Avants et al., Neuroimage, 2011, 54:2033-44. A reproducible evaluation of ANTs similarity metric performance in brain image registration.
9. Ivanov et al., Proceedings of the High Field Meeting of the ISMRM, 2016, p. 14. Sub-millimeter human brain perfusion imaging using arterial spin labelling at 3 and 7 Tesla.
10. Tse et al., MAGMA, 2016, 29:333-45. Volumetric imaging with homogenised excitation and static field at 9.4 T.