"Fast Imaging Techniques for Brain Imaging" is a wide topic. This presentation will focus on the most recent advances in fast brain imaging towards comprehensive clinical brain exams by acquiring multiple MR contrasts simultaneously in minimal scan time.
For many clinical brain protocols, five MR contrasts are often of particular importance (1):
To shorten a brain MRI exam, parametric and non-parametric multi-contrast pulse sequences have emerged and gained popularity, where the idea is to in a single scan (preferably under about ~5 min) generate many of these MR contrasts at once.
Recent popular parametric approaches are MR Fingerprinting (MRF) (2) and Synthetic MRI (3) (cf. ISMRM18: "Go Faster in Clinical Imaging: Fingerprinting"). In MRF, the flip angle (FA), echo time (TE) and repetition time (TR) are varied for each excitation to produce a unique signal-evolution fingerprint depending on the proton density (PD), T1, and T2 values in each tissue. In the reconstruction, the parametric PD-T1-T2 triplet is determined for each voxel by comparing the signal time course with Bloch simulated time courses in a large dictionary (like a criminal's fingerprint). For clinical use, weighted images (T1-w, T2-w, T1-FLAIR, T2-FLAIR) could be generated via these parametric maps via the signal equations involving FA, TE and TR for each sequence type.
In Synthetic MRI (cf. ISMRM18: "Go Faster in Clinical Imaging: Synthetic MR"), data is acquired to first produce parametric PD, T1 and T2 maps (similar to MRF), from which weighted images can be synthetically generated in the reconstruction.
Important to note is that using a model with simply PD, T1 and T2 values, on cannot produce T2*-w or DW images. Moreover, the T2-FLAIR contrast is difficult to synthesize well in practice due to partial volume effects, which can mimic pathology (4).
Fast brain imaging via multi-contrast MRI
Instead of modeling and fit parametric MR values, one can acquire multiple weighted MR-contrasts in a single fast scan. Breutigam et al. recently acquired T2* and DWI in parallel using an RS-EPI sequence (5). Moreover, a multi-contrast EPI sequence was recently developed (6), able to produce T1-FLAIR, T2-w, DWI (and the parametric ADC), T2*, and T2-FLAIR images with full brain coverage within 1:10 min. This sequence meets the bulk need for MR contrasts in brain MRI without using parametric maps, at the expense of lower matrix size and some degree of geometric distortions. A similar EPI-based multi-contrast technique has independently been developed (7) by another group, able to produce weighted contrasts such as T2*-w, T2-FLAIR, PD, but have also experimented to synthesize T1-w out of ratios of the other contrasts.
Finally, the great new interest in deep learning will likely help teasing out yet more information from both parametric MRI data as well as current and future multi-contrast acquisitions. Another positive effect of these acquisition schemes is that it simplifies the clinical workflow, which may improve data consistency. They also result in less time between each MR contrast due to reduced buttonology and prescans/tuning.