Three-dimensional motion-corrected T1 relaxometry with MPnRAGE

Abstract

Purpose: To test the performance of the MPnRAGE motion-correction algorithm on quantitative relaxometry estimates.

Methods: Twelve children (9.4 ± 2.6 years, min = 6.5 years, max = 13.8 years) were imaged 3 times in a session without sedation. Stabilization padding was not used for the second and third scans. Quantitative T₁ values were estimated in each voxel on images reconstructed with and without motion correction. Mean T₁ values were assessed in various regions determined from automated segmentation algorithms. Statistical tests were performed on mean values and the coefficient of variation across the measurements. Accuracy of T₁ estimates were determined by scanning the High Precision Devices (Boulder, CO) MRI system phantom with the same protocol.

Results: The T₁ values obtained with MPnRAGE agreed within 4% of the reference values of the High Precision Devices phantom. The best fit line was T₁ (MPnRAGE) = 1.02 T₁ (reference)-0.9 ms, R² = 0.9999. For in vivo studies, motion correction reduced the coefficients of variation of mean T₁ values in whole-brain tissue regions determined by FSL FAST by 74% ± 7%, and subcortical regions determined by FIRST and FreeSurfer by 32% ± 21% and 33% ± 26%, respectively. Across all participants, the mean coefficients of variation ranged from 0.8% to 2.0% for subcortical regions and 0.6% ± 0.5% for cortical regions when motion correction was applied.

Conclusion: The MPnRAGE technique demonstrated highly accurate values in phantom measurements. When combined with retrospective motion correction, MPnRAGE demonstrated highly reproducible T₁ values, even in participants who moved during the acquisition.

Keywords: MPnRAGE; R1; T1; motion correction; relaxometry.

Figure 1:

T1-weighted images and quantitative T1 maps from a 79 month old boy demonstrating ability of MPnRAGE to correct for subtle motion artifacts.

Figure 2:

T1-weighted images and quantitative T1 maps from a 108 month old boy demonstrating ability of MPnRAGE to correct for moderate motion artifacts.

Figure 3:

T1-weighted images and quantitative T1 maps from a 92 month old female demonstrating ability of MPnRAGE to correct for severe motion artifacts arising from nearly continuous motion events throughout the duration of the scan.

Figure 4:

T1 values (top) and coefficients of variation (bottom) from regional measurements in whole brain white cerebrospinal fluid, gray matter, and white matter segmentations from FSL FAST across all subjects. Superscripts ^t,f denote p<0.05 for t-tests and f-tests, used to test for statistical differences of the means (t-tests) and standard deviations (f-tests) between the motion corrected and non-motion corrected T1 values. The T1 values for CSF have been multiplied by 0.5 to preserve dynamic range of the plots.

Figure 5:

A comparison of the quantitative T1 values measured in automatically segmented regions of interest using FAST. Shown are all measurements from all subjects and trials, color coded by region of interest. The y-axis shows the difference of T1 values after motion correction, normalized by the mean value, indicating an average increase of T1 after motion correction was applied. The black dashed line is the mean of the normalized difference, while the thin red lines are at +/− 1.96 standard deviations from the mean.

Figure 6:

T1 values (top) and coefficients of variation (bottom) from regional measurements in subcortical segmentations from FSL FIRST across all subjects. Superscripts ^t,f denote p<0.05 for t-tests and f-tests, used to test for statistical differences of the means (t-tests) and standard deviations (f-tests) between the motion corrected and non-motion corrected T1 values.

Figure 7:

T1 values (top) and coefficients of variation (bottom) from regional measurements in subcortical segmentations from FreeSurfer across all subjects. Superscripts ^t,f denote p<0.05 for t-tests and f-tests, used to test for statistical differences of the means (t-tests) and standard deviations (f-tests) between the motion corrected and non-motion corrected T1 values.

Figure 8:

A comparison of the quantitative T1 values measured in automatically segmented regions of interest using FIRST and FREESURFER. Shown are all measurements from all subjects and trials, color coded by region of interest. The y-axis shows the difference of T1 values after motion correction, normalized by the mean value, indicating an average increase of T1 after motion correction was applied. The black dashed line is the mean of the normalized difference, while the thin red lines are at +/− 1.96 standard deviations from the mean.