Axial neuromelanin-sensitive images show distinct band-like high-signal areas in the posteromedial part of the crus cerebri, mainly at the level of the lower midbrain (see Fig. 10.1a) [1, 11]. Horizontal and vertical distributions of the hyperintense areas correspond well with that of neuromelanin-containing neurons in gross specimens, but are substantially different from the distribution of low-signal-intensity areas on T2-weighted images and gray matter signal areas on proton-density-weighted or short inversion–time inversion recovery images [11]. Thus, compared with other structural imaging techniques, neuromelanin-sensitive MRI is more appropriate for direct assessment of the SNc. Sometimes, 3D-GRE images show a punctate hyperintense area at the midline of the midbrain tegmentum, indicating dopaminergic neurons of the ventral tegmental area (see Fig. 10.1c).

The LC is one of the gray matter structures that are “invisible” on conventional MRI but can be visualized as a pair of rod-shaped hyperintense areas along the lateral edge of the fourth ventricle floor in the upper pontine tegmentum by using neuromelanin-sensitive imaging techniques (see Fig. 10.1b, d) [1, 11]. Distribution of the hyperintense areas is identical to that of the LC in gross specimens [10]. In addition, rostrocaudal variations in the signal intensity have been reported to be closely related to the variations in the number of the neuromelanin-containing neurons [12], suggesting that the degree of signal intensity can reflect neuronal number and/or intraneuronal neuromelanin content.

Neuromelanin-sensitive imaging techniques have been mainly utilized for evaluating the pathological changes in the SNc or LC of patients with iPD, because conventional MRI techniques fail to detect significant changes related to the principal pathological changes in iPD, i.e., neuronal depletion of the SNc and LC [11, 13]. Several studies have shown significant signal attenuation and/or volume reduction of hyperintense SNc areas on neuromelanin-sensitive images of iPD patients, indicating degeneration of dopaminergic neurons (Fig. 10.2b) [1, 2, 14]. In addition, such changes were more pronounced in the later stages than in the early stages of the disease, reflecting pathological progression of iPD [2]. Significant signal decrease in the LC areas was also seen in iPD (see Fig. 10.2d) [1]. Thus, neuromelanin-sensitive MRI can be used to visualize degenerative changes in the neuromelanin-containing neurons occurring in iPD. However, the performance of neuromelanin-sensitive MRI in terms of early and differential diagnoses is not known and should be evaluated in further investigations.