4590
High-Resolution Quantitative T2 and T1 Mapping of the Hippocampus
Haley Elizabeth Wiskoski1, Kevin Johnson2, Juan Arias3, Arun Pugazhendhi2, Raza Mushtaq4, Eze Ahanonu5, Ali Bilgin1,2, Craig Weinkauf3, Theodore Trouard1,2,6, and Maria Altbach1,2
1Department of Biomedical Engineering, The University of Arizona, Tucson, AZ, United States, 2Department of Medical Imaging, The University of Arizona, Tucson, AZ, United States, 3Department of Surgery, The University of Arizona, Tucson, AZ, United States, 4Department of Neuroradiology, Barrow Neurological Institute, Phoenix, AZ, United States, 5Department of Electrical and Computer Engineering, The University of Arizona, Tucson, AZ, United States, 6Evelyn F. McKnight Brain Institute, The University of Arizona, Tucson, AZ, United States
1Department of Biomedical Engineering, The University of Arizona, Tucson, AZ, United States, 2Department of Medical Imaging, The University of Arizona, Tucson, AZ, United States, 3Department of Surgery, The University of Arizona, Tucson, AZ, United States, 4Department of Neuroradiology, Barrow Neurological Institute, Phoenix, AZ, United States, 5Department of Electrical and Computer Engineering, The University of Arizona, Tucson, AZ, United States, 6Evelyn F. McKnight Brain Institute, The University of Arizona, Tucson, AZ, United States
Synopsis
Keywords: Quantitative Imaging, Image Reconstruction, Hippocampus
Motivation: The hippocampus is important in memory and is an early target in neurodegenerative diseases. Structural MRI may lack sensitivity to microstructural changes in this region, necessitating more quantitative techniques.
Goal(s): We evaluate reproducibility of new radial MRI methods for T2/T1 mapping in the brain and examine age-related changes.
Approach: Quantitative maps were generated using novel pulse sequences and reconstruction algorithms. We assessed reproducibility and age-related trends.
Results: We observed strong correlations in T2 and T1 measurements between imaging sessions. We observed positive relationships between T2 and T1 measurements and age in white matter and negative relationships in gray matter regions.
Impact: This study introduces novel radial MRI techniques for mapping of the brain and hippocampus, with potential to offer insights into early neurodegenerative changes. Findings on reproducibility and age-related trends contribute to improved understanding of brain health and potential diagnostic applications.
Introduction
The hippocampus, a well-studied brain region crucial for learning and memory, is among the early targets in conditions like Alzheimer's Disease and other neuropsychiatric disorders1. Previous research has mainly centered on using structural T1-weighted MRI to assess hippocampal volume changes as early biomarkers of dementia. However, due to the complex morphology and cytoarchitecture of this region, volumetric MRI techniques may lack the ability to detect subtle changes in microstructure that may precede hippocampal atrophy2. This has led to an increased interest in integrating more quantitative imaging techniques to survey parameters associated with these changes. The challenge in quantitative techniques lies in acquiring multiple data points over extended scan durations for accurate parameter estimation. Here, we investigate reproducibility of two novel radial MRI techniques developed for rapid, high-resolution quantitative T2 and T1 mapping of the hippocampus at 3T and assess relaxation times as a function of age in several regions of the brain.Methods
Radial Turbo Spin-Echo (RADTSE; TR=4000ms, TE=15ms, ETL=12, 648 radial views, 0.48x0.48x2 mm3, 35 slices, imaging time= 7:44 min)3,4 and Inversion-Recovery Radial Gradient Echo (IR-RADGRE; TR=8.75ms, TE=4.34ms, 960 radial views, 0.64x0.64x2 mm3, 33 slices, imaging time= 6:17 min)5 quantitative mapping acquisitions were carried out on a 3T MRI scanner (Skyra, Siemens Healthcare, Malvern, PA) with a 32-channel head coil. 16 healthy subjects (9 female, age range: 20-82, mean age: 46.9) were imaged. Eight of the subjects (3 female, age range: 20-82, mean age: 47.5) were imaged twice to assess reproducibility. An in-house iterative reconstruction algorithm6 was used to generate multiple TE and TI images from RADTSE and IR-RADGRE data, from which final quantitative T2 and T1 maps were produced, respectively (Figure 1).Five regions of interest (ROI) were analyzed: the cornu ammonis 4 (CA4) / dentate gyrus (DG), cornu ammonis 1 (CA1), corpus callosum, cortex, and white matter (WM) of the parahippocampal gyrus. ROIs were manually drawn on a single coronal slice of a reconstructed T2 contrast image as anatomical reference (Figure 2). ROIs were then applied to the quantitative T2 and T1 maps wherein mean T2 and T1 values were recorded. To assess reproducibility, mean T2 and T1 values were compared between imaging sessions for the eight subjects via simple linear regression, Pearson’s correlation, and Bland-Altman analyses. Additionally, mean T2 and T1 values from the first imaging session were assessed as a function of age via simple linear regression for all 16 subjects in these five ROI.
Results
Assessment of reproducibility indicated a statistically significant correlation of T2 measurements between imaging sessions, r(38)=0.86 and P<0.0001, as well as a significant correlation of T1 measurements between sessions, r(38)=0.96 and P<0.0001 (Figure 3). The computed slopes from simple linear regression approximated unity. Bland-Altman analysis found mean and standard deviations of percent differences in T1 measurements to be -1.274 and 6.956, respectively, while those in T2 measurements were found to be 0.5941 and 5.362, respectively (Figure 3). When assessing T2 relaxation as a function of age, a significantly non-zero, positive relationship was observed in white matter, namely the corpus callosum (P=0.0076) (Figure 4). A significantly non-zero, positive relationship was also observed between T1 relaxation and age within the corpus callosum (P=0.0248), contrasting with a significant negative relationship within the cortex (P=0.0392) (Figure 4). In summary, our findings show a negative relationship between T2 and T1 relaxation and age in regions of interest within grey matter, while a positive relationship exists between T2 and T1 relaxation and age in regions of interest within white matter.Conclusion
RADTSE and IR-RADGRE pulse sequences yielded high-quality, high-resolution images at multiple contrasts in ~7 min per sequence. Assessment of reproducibility for these two sequences found concurrence in quantitative T2 and T1 measurements between acquisitions for these five ROI. Results also demonstrated a positive relationship with T2 and T1 relaxation times and age in white matter regions, and a negative relationship in grey matter regions. This is consistent with other reported age-dependent changes7,8. Future research will focus on the potential utility of slice super-resolution neural networks to further reduce scan times for these acquisitions, implementing automated hippocampal subfield segmentation for enhanced precision in region of interest analysis, and examining the relationship between relaxation times and hippocampal subfield volumes and cognitive scores.Acknowledgements
This study was supported by the Arizona Alzheimer’s Consortium, Arizona Biomedical Research Centre (CTR056039), and RO1 AG070987.References
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Figures
Figure 1. A) Representative reconstructed composite image generated from multiple TEs collected with RADTSE. B) Corresponding quantitative T2 map generated from RADTSE. C) Representative quantitative T1 map generated from IR-RADGRE data. Corresponding coronal views of the hippocampus are shown.
Figure 2. Coronal view of a representative composite image generated from RADTSE data which was used as anatomical reference for manual outlining of the five regions of interest. ROI1: cornu ammonis 4 / dentate gyrus, ROI2: cornu ammonis 1, ROI3: corpus callosum, ROI4: cortex, ROI5: white matter of the parahippocampal gyrus.
Figure 3. Results of reproducibility assessment. Reproducibility analysis demonstrated A) a significant positive correlation of T2 relaxation measurements, r(38)=0.86 and P<0.0001, and B) T1 relaxation measurements, r(38)=0.96 and P<0.0001, in five ROI between imaging sessions. Bland-Altman analysis of C) T2 measurements between sessions (Mean=0.5941 and SD=5.362) and D) T1 measurements between sessions (Mean=-1.274 and SD=6.956) are also shown.
Figure 4. Analysis of mean T2 (top row) and T1 (bottom row) relaxation times as a function of age for five ROI. From left to right: cornu ammonis 4 / dentate gyrus, cornu ammonis 1, corpus callosum, cortex, and white matter of the parahippocampal gyrus. Significant relationships were found between T2 and T1 measurements and age in the corpus callosum (P=0.0076 and P=0.0248). A significant negative relationship between T1 measurement and age in the cortex was also observed (P=0.0392).
DOI: https://doi.org/10.58530/2024/4590