Segmented MT maps showed clearly improved delineation of substantia nigra pars compacta (SNc), caudal (posterior) putamen, and rostral (anterior) pallidum, as well as the lateral pulvinar nuclei in the thalamus. These regions exhibited little contrast to white matter on the T1w MDEFT images, whereas they appeared more hypointense on the MT maps, resulting in a higher contrast (for an example, see ). The contrast also improved in the periaqueductal grey, and the dentate nuclei of the cerebellum (not shown). In addition, the MT maps showed a better delineation of WM laminae embedded in GM structures, as seen in the thalamus (b).
Fig. 1 Example of single subject T1w MDEFT (upper row) and MT map (bottom row) data in Montreal Neurological Institute (MNI) standard space. (a) transverse view of whole head; (b) putamen, (c) pallidum, (d) substantia nigra shown as a zoomed view. Consistent (more ...)
CNR values obtained by ROI analysis corroborated the higher visual MT contrast between pallidum and WM with a CNR = 6.2 ± 1.2 (mean ± SD) for MT vs. 3.9 ± 1.5 for MDEFT (p < 0.0015). The same CNR was found for both methods between caudate and WM (9.2 ± 0.8 for MT; 9.3 ± 2.2 for MDEFT, p = 0.93). The SNR of MDEFT, however, was consistently higher (pallidum: 13.8 ± 3.6; caudate: 12.0 ± 2.0; genu: 23.2 ± 4.2) than that of MT maps (8.3 ± 3.6; 10.4 ± 2.7; 12.7 ± 1.0) with all p < 0.013. The difference in SNR can be attributed to the longer acquisition time of the MDEFT image (~ 13 min) compared to the MT map (~ 6 min per data set).
Consistent with the visual inspection of MT maps and CNR comparisons, the population averaged GM maps generated from the MT maps revealed a higher probability of identifying iron-rich midbrain nuclei than the ones based on MDEFT images, i.e., posterior putamen (b), anterior pallidum (c), and SNc, (d). In addition, a larger portion of the pulvinar nuclei of the thalamus could be assigned to GM (b). The statistical comparison in the subcortical ROI corroborated the results, indicating significantly increased GM probabilities for the MT-based maps in the putamen, pallidum, thalamus, and SNc (p
< 0.05 FWE corrected). For details see and a–c. Little or no improvement was seen in posterior pallidum, the subthalamic and red nuclei, and the ventro-anterior and ventro-lateral nuclei of the thalamus (nomenclature according to Hirai and Jones, 1989
). These GM structures contain a larger number of axons, which reduce the contrast on MDEFT and MT maps alike. Between the medio-dorsal, ventro-lateral and pulvinar nuclei in the thalamus, the MT-based segmentation showed a lower GM probability than the MDEFT-based segmentation (p
< 0.05 FWE corrected d). The significant reductions in sub-insular WM and caudate veins were consistent with anatomy.
Fig. 2 Population average maps (n = 49) of gray matter probability in MNI standard space corresponding to the single subject images in : (a) full transverse view, (b) putamen, (c) pallidum, (d) substantia nigra. MT map (bottom row) showing (more ...)
Fig. 3 Statistical comparison between MDEFT- and MT-based population averaged GM probability maps in MNI standard space. t-test was performed on a subcortical ROI and voxels exceeding a threshold of p < 0.05 (FWE corrected) are displayed (more ...)