The Compressed Sensing MP2RAGE as a Surrogate to the MPRAGE for Neuroimaging at 3 T

Abstract

Objectives: The magnetization-prepared 2 rapid acquisition gradient echo (MP2RAGE) sequence provides quantitative T1 maps in addition to high-contrast morphological images. Advanced acceleration techniques such as compressed sensing (CS) allow its acquisition time to be compatible with clinical applications. To consider its routine use in future neuroimaging protocols, the repeatability of the segmented brain structures was evaluated and compared with the standard morphological sequence (magnetization-prepared rapid gradient echo [MPRAGE]). The repeatability of the T1 measurements was also assessed.

Materials and methods: Thirteen healthy volunteers were scanned either 3 or 4 times at several days of interval, on a 3 T clinical scanner, with the 2 sequences (CS-MP2RAGE and MPRAGE), set with the same spatial resolution (0.8-mm isotropic) and scan duration (6 minutes 21 seconds). The reconstruction time of the CS-MP2RAGE outputs (including the 2 echo images, the MP2RAGE image, and the T1 map) was 3 minutes 33 seconds, using an open-source in-house algorithm implemented in the Gadgetron framework.Both precision and variability of volume measurements obtained from CAT12 and VolBrain were assessed. The T1 accuracy and repeatability were measured on phantoms and on humans and were compared with literature.Volumes obtained from the CS-MP2RAGE and the MPRAGE images were compared using Student t tests (P < 0.05 was considered significant).

Results: The CS-MP2RAGE acquisition provided morphological images of the same quality and higher contrasts than the standard MPRAGE images. Similar intravolunteer variabilities were obtained with the CS-MP2RAGE and the MPRAGE segmentations. In addition, high-resolution T1 maps were obtained from the CS-MP2RAGE. T1 times of white and gray matters and several deep gray nuclei are consistent with the literature and show very low variability (<1%).

Conclusions: The CS-MP2RAGE can be used in future protocols to rapidly obtain morphological images and quantitative T1 maps in 3-dimensions while maintaining high repeatability in volumetry and relaxation times.

FIGURE 1

Representative MPRAGE and MP2RAGE images of a participant. The T₁ map built from the MP2RAGE images is also shown. The inserts show the striatum (including the putamen and the caudate) to highlight the spatial resolution that is largely preserved on the compressed sensing images.

FIGURE 2

Bland-Altman plot showing the volume differences of WM (green dots), GM (blue dots), and CSF (red dots) between the MPRAGE and the MP2RAGE segmentations. The lines show the mean volume difference, and the dotted lines show the 95% confidence interval.

FIGURE 3

Representative overlays between MP2RAGE and MPRAGE segmentations of WM, GM, and CSF in 1 participant. The white arrow and the blue arrow point at the putamen and the thalamus, respectively, within which some areas are included into the GM segmentation with the MP2RAGE images but not with the MPRAGE images.

FIGURE 4

WM, GM, and CSF volumes segmented from the MPRAGE (circles) or MP2RAGE (dots) images, for each individual (noted Ind_i and represented by different colors).

FIGURE 5

Bland-Altman plot showing the volume differences of the deep gray nuclei studied here (thalamus [green dots], putamen [blue dots], caudate [red dots], hippocampus [pink dots], pallidum [orange dots], amygdala [black dots], accumbens [purple dots]), between the MPRAGE and the MP2RAGE segmentations. The dotted circle shows the large discrepancy between the 2 segmentations of the thalamus, which occurred in 1 patient at 1 scan session.

FIGURE 6

Representative overlays between MP2RAGE and MPRAGE segmentations of several deep gray nuclei: caudate and hippocampus, for which the segmented volumes are smaller from the MP2RAGE images than from the MPRAGE images; thalamus, for which the segmented volumes are larger from the MP2RAGE images than the MPRAGE images. The white arrow points at the larger segmentations obtained from the MP2RAGE images.

FIGURE 7

Volumes of the some deep gray nuclei (caudate, thalamus, putamen, and accumbens) segmented from the MPRAGE (circles) or MP2RAGE (dots) images, for each individual (noted Ind_i and represented by different colors).

FIGURE 8

Mean T₁ values calculated over the multiple scan sessions in each individual of the deep gray nuclei studied here (A) and also of the substructures of the hippocampus (B). The colors represent each individual. The error bars are the standard deviations across the 3 or 4 scans in one given participant.