MRI volume of the amygdala: a reliable method allowing separation from thehippocampal formation

Studies of MRI-derived volume of the amygdala have been mostly performed on coronal sections where its boundaries with the hippocampus and the entorhi nal cortex are indistinct. To date, all reports of in vivo amygdala volume have consistently overestimated the size of the structure. We have develope d a method for the MRI-based in vivo measurement of the amygdala volume whi ch allows a better separation of the amygdala from the adjoining hippocampa l formation. In nine normal volunteers we obtained three-dimensional spoile d gradient recalled acquisition, 1.3-mm thick, T1 weighted sagittal MR imag es and created electronically linked reformatted images in the coronal and axial planes. On the original sagittal and the reformatted axial planes, wh ere it is more readily apparent, we delineated the boundaries between the a mygdala and the hippocampus and the amygdala and the hippocampo-amygdala tr ansition area, respectively. We then projected those markings onto the coro nal plane, where the other boundaries of the amygdala are more easily seen. Using these markings as a guide and utilizing extra-amygdalar coronal land marks for the anterior end, we outlined the whole amygdala on the coronal p lane and determined its volume. We observed that 45% of the coronal slices that contained amygdala also contained some hippocampus. The amygdala measu rement had high test-retest reliability, with an intra-class correlation co efficient (r(ICC)) of 0.99 for the total volume and an r(ICC) of 0.93 for t he measurement at the level of the individual slice. The average amygdala v olume was 1.05 +/- 0.17 cm(3) on the right and 1.14 +/- 0.15 cm(3) on the l eft. Our amygdala volumes are in agreement with those reported in postmorte m studies, which provides the reported method with face validity. The abili ty to reliably and validly measure the amygdala in vivo may facilitate the investigation of psychiatric disorders, such as PTSD, anxiety disorders, an d schizophrenia, which are presumed to have amygdala dysfunction and pathol ogy. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.