3D QALAS is a promising new technique that simultaneously maps T1, T2, and PD in a single 3D acquisition. Both 3D QALAS and 2D MAGIC were applied for synthetic imaging and quantification in human brains. While T1 and T2 values were comparable between the two techniques, 3D QALAS was simpler to process and achieved smaller voxel sizes over larger matrices with similar acquisition times, resulting in less partial volume effects. 2D MAGIC maintained the high in-plane resolution and efficiency of an interleaved multi-slice technique. 3D QALAS thus presents an attractive quantification method for therapy planning and tissue volume measurement applications.
The 3D QALAS sequence was implemented for untriggered acquisitions in the head and body. This sequence consists of a 5 phases of varying contrasts acquired with 3D gradient echo readouts in a single cycled acquisition. A T2 prep sequence precedes the first phase, followed by an inversion prep pulse and 4 delayed phases (figure 1). One cycle would acquire one segment of Cartesian k-space for each of the 5 phases with spoiled gradient echo readouts, such that repeating the cycle would fill all segments of k-space for 5 image sets. The T2 prep time and the initial delay after the inversion prep pulse were both set at 100 msec. The sequence timing was arranged such that the 5 phases of the acquisition were equally spaced in time at 0.87-1.0 sec, mimicking a triggered cardiac acquisition. Parameter optimization was performed for isotropic voxel 3D axial slab brain imaging, while targeting a total scan time 6:00-6:30.
3D QALAS data was acquired of the brain in 5 healthy volunteers on a 3T scanner (MR750, GE Healthcare, Waukesha, WI). 2D MAGIC was also acquired with FOV=24.0, Matrix = 320x256, slice thickness/skip = 4/1 mm, TR = 5000, BW = +/- 20.83. Quantitative T1, T2, and PD maps were reconstructed from both acquisitions using a research version of SyMRI (SyntheticMR, Linkoping, Sweden) and an estimated B1 correction map was generated from the 2D data. Synthetic T1, T2, and FLAIR images were also reconstructed from the resulting maps. Volume segmentation of grey matter, white matter, and CSF was also performed based on grouped voxels of corresponding T1 and T2 values on the spin parameter maps.
For each subject, parameter maps and synthetic images were compared between the two acquisition types. 3D data was reformatted and viewed in different planes, and 3D and 2D data were compared for visual similarity and effects of partial volume at tissue interfaces.
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3D reformats of synthetic T1 weighted (top), T2 weighted (middle), and FLAIR (bottom) images.