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MRI volume of the amygdala: a reliable method allowing separation from the hippocampal formation
1Department of Psychiatry, New York University School of Medicine, Aging and Dementia Research Center, NY 10016, USA. antonio.convit@mcadr.med.nyu.edu
Abstract:
Studies of MRI-derived volume of the amygdala have been mostly performed on coronal sections where its boundaries with the hippocampus and the entorhinal cortex are indistinct. To date, all reports of in vivo amygdala volume have consistently overestimated the size of the structure. We have developed a method for the MRI-based in vivo measurement of the amygdala volume which allows a better separation of the amygdala from the adjoining hippocampal formation. In nine normal volunteers we obtained three-dimensional spoiled gradient recalled acquisition, 1.3-mm thick, T1 weighted sagittal MR images and created electronically linked reformatted images in the coronal and axial planes. On the original sagittal and the reformatted axial planes, where it is more readily apparent, we delineated the boundaries between the amygdala and the hippocampus and the amygdala and the hippocampo-amygdala transition area, respectively. We then projected those markings onto the coronal plane, where the other boundaries of the amygdala are more easily seen. Using these markings as a guide and utilizing extra-amygdalar coronal landmarks for the anterior end, we outlined the whole amygdala on the coronal plane and determined its volume. We observed that 45% of the coronal slices that contained amygdala also contained some hippocampus. The amygdala measurement had high test-retest reliability, with an intra-class correlation coefficient (rICC) of 0.99 for the total volume and an rICC of 0.93 for the measurement at the level of the individual slice. The average amygdala volume was 1.05 +/- 0.17 cm3 on the right and 1.14 +/- 0.15 cm3 on the left. Our amygdala volumes are in agreement with those reported in postmortem studies, which provides the reported method with face validity.
Insights
This study introduces a new MRI method for accurately measuring amygdala volume in vivo, improving boundary delineation with the hippocampus. The developed technique offers high reliability and yields results consistent with postmortem studies.
Area of Science:
- Neuroimaging
- Human Anatomy
- Magnetic Resonance Imaging (MRI)
Background:
- Previous MRI studies of amygdala volume often used coronal sections, leading to indistinct boundaries with adjacent structures like the hippocampus.
- Existing in vivo MRI methods have consistently overestimated amygdala size due to these boundary challenges.
Purpose of the Study:
- To develop and validate a novel MRI-based method for accurate in vivo measurement of amygdala volume.
- To improve the separation of the amygdala from the hippocampal formation and related transition areas.
Main Methods:
- Acquisition of 3D spoiled gradient recalled acquisition, 1.3-mm thick, T1-weighted sagittal MR images in nine healthy volunteers.
- Creation of reformatted coronal and axial images, with boundary delineation performed on sagittal and axial planes and projected onto the coronal plane.
- Utilization of extra-amygdalar coronal landmarks for precise outlining of the amygdala volume.
Main Results:
- The new method demonstrated high test-retest reliability, with an intra-class correlation coefficient (rICC) of 0.99 for total volume.
- Average amygdala volumes were 1.05 ± 0.17 cm³ (right) and 1.14 ± 0.15 cm³ (left).
- Observed that 45% of coronal slices containing amygdala also included hippocampus, highlighting the need for improved delineation.
Conclusions:
- The developed MRI technique provides a more accurate and reliable method for in vivo amygdala volume measurement.
- The results align with postmortem studies, validating the method's face validity for neuroimaging research.