Using high-resolution MR imaging at 7T to evaluate the anatomy of the midbrain dopaminergic system

Abstract

Background and purpose: Dysfunction of DA neurotransmission from the SN and VTA has been implicated in neuropsychiatric diseases, including Parkinson disease and schizophrenia. Unfortunately, these midbrain DA structures are difficult to define on clinical MR imaging. To more precisely evaluate the anatomic architecture of the DA midbrain, we scanned healthy participants with a 7T MR imaging system. Here we contrast the performance of high-resolution T2- and T2*-weighted GRASE and FFE MR imaging scans at 7T.

Materials and methods: Ten healthy participants were scanned by using GRASE and FFE sequences. CNRs were calculated among the SN, VTA, and RN, and their volumes were estimated by using a segmentation algorithm.

Results: Both GRASE and FFE scans revealed visible contrast between midbrain DA regions. The GRASE scan showed higher CNRs compared with the FFE scan. The T2* contrast of the FFE scan further delineated substructures and microvasculature within the midbrain SN and RN. Segmentation and volume estimation of the midbrain SN, RN, and VTA showed individual differences in the size and volume of these structures across participants.

Conclusions: Both GRASE and FFE provide sufficient CNR to evaluate the anatomy of the midbrain DA system. The FFE in particular reveals vascular details and substructure information within the midbrain regions that could be useful for examining structural changes in midbrain pathologies.

Figures

Fig 1.

GRASE and FFE scans in a single subject next to a histology section of the midbrain at the level of the superior colliculus. A, Histology section of the midbrain (stained by using India ink to highlight vasculature). Reprinted with permission from Duvernoy.B, GRASE image of the midbrain. C, FFE image of the midbrain. The sections are oriented so that the anterior part of the midbrain appears at the bottom of the sections. Legend indicates the location of the midbrain structures marked with respect to a brain atlas.

Fig 2.

FFE image in a single subject in 1 section of the midbrain at the level of the superior colliculus. The image was passed through a lookup table filter to produce a color-contrast scale. Different parts of the RN and the SN are revealed.

Fig 3.

Segmentation process by using a region-growing segmentation algorithm in the FFE scan. A, Sample section in the midbrain at the level of the mammillary bodies where the segmentation algorithm was implemented. B, Segmented region of the SN traced in red within the midbrain. C, Segmented structures (SN is red; VTA, blue; and RN, green) overlaid on the anatomic FFE image.

Fig 4.

Individual-subject absolute-volume measurement for the SN (A), VTA (B), and the RN (C). Volume measurements for 10 subjects are shown here. Blue dots represent the GRASE scan; and red dots, the FFE scan. The dashed line represents mean volume.