In PD patients, much of the work focusing on imaging, delineation, and segmentation of the SN using MRI has concentrated on imaging changes in iron content and NM. While the cause of neuronal death in the SN is in contention, there is consensus that increased iron deposition is related to decreased NM in the SN and an MR sequence sensitive to iron and NM can enhance the study of the progression of PD. It has been hypothesized that increased iron deposition is the primary cause of neuronal death in the SN ( Becker et al., 1995) while other studies have suggested that increased iron deposition in PD is a byproduct of and not a primary cause of neuronal death ( Galazka-Friedman et al., 1996 Uitti et al., 1989). Characteristic effects of PD in the SN include the loss of neuromelanin (NM) generating dopaminergic neurons in the SNpc ( Fearnley and Lees, 1991) and deposition of iron throughout the SN ( Dexter et al., 1991 Zecca et al., 2004). Furthermore, degeneration of the SN is a hallmark of the progression of a number of neurodegenerative diseases including Parkinson's disease (PD) ( Braak et al., 2011). The SN plays a prominent role in many cognitive functions including novelty processing ( Schiffer et al., 2012) and reward based learning ( Guitart-Masip et al., 2012). The substantia nigra (SN) consists of two histologically distinct regions, the SN pars reticulata (SNr) and the SN pars compacta (SNpc). Our results provide a new understanding of the contrast behavior of the SN in the two imaging approaches commonly used to image it and indicate that using both may yield a more comprehensive visualization of the SN. Despite the small overlap, these regions are complementary. Our results indicate that the two contrasts are sensitive to different portions of the SN, with MT seeing the more caudal portion of the SN than SWI, likely due to variations of NM and iron content in the SN. The MT contrasts for the two volumes are 0.16☐.02 and 0.10☐.03 ( p<0.001), respectively. The frequency offsets (homodyne filtered phase/echo time) for the volumes derived from MT (NM) images and SWI images are 0.09☐.32 radians/s and -1.12☐.57 radians/s ( p<0.0001), respectively. Considering the SN volumes in each hemisphere separately, the average distances between the centers of mass of the volumes from the two types images are 5.1☑.1 mm and 4.1☑.2 mm, respectively. The MT volume was more caudal than the SWI volume and with only a 12% overlap between the two volumes. Both images showed the presence of SN but the volumes of the SN identified in the two are spatially incongruent.
The spatial and signal characteristics of the extracted SWI and MT volumes were compared. SN volumes were segmented from these two types of images using a thresholding method.
The first echo was taken as the MT contrast image and the second was used to generate the SWI image. Images were acquired with a MT prepared dual echo gradient echo sequence. We characterize the contrast behavior of substantia nigra (SN) in both magnetization transfer (MT) imaging, which is believed to be sensitive to neuromelanin (NM), and susceptibility weighted imaging (SWI).
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