MRI correlates of pathology in parkinsonism: Segmented inversion recovery ratio imaging (SIRRIM)

The two commonest, clinically well-defined, forms of parkinsonism are idiopathic Parkinson's disease (PD) and progressive supranuclear palsy (PSP). Both involve, inter alia, pathological changes in the substantia nigra pars compacta (SN_C). In PD there is neuronal loss with associated Lewy body pathology and microglial activation. Three morphological features have been identified [Brain 114 (1991), 2283; Greenfield's Neuropathology 2 (1997), 289]. First, there is a gradient of pathological change such that the lateral segments are more affected than the medial. Second, there is thinning of the pigmented tissue, and third, there is a broadening of the overall structure in a dorsal-ventral direction, possibly caused by the migration of melanin-laden macrophages [Greenfield's Neuropathology 2 (1997), 289]. In contrast, PSP is characterized pathologically by intraneuronal neurofibrillary tangles. There are two morphological features in the SN_C [Brain 114 (1991), 2283]. First, the gradient of pathological change is in the opposite direction to that of PD (i.e., the medial segments are more affected than the lateral). Second, there is atrophy. Any technique sensitive to neuropathology should be capable of detecting these features. We have previously reported on a new approach to detecting signal change in the substantia nigra in PD. This makes use of a ratio of images acquired by two distinct inversion recovery sequences [J. Neurol, Neurosurg. Psychiatry 67 (1999), 815; Am. J. Neuroradiol. 21 (2000) 697]. The prior work suggests that the technique is sensitive to, but not necessarily specific for, PD. We present here a preliminary report on an extension of the work. This is a semiautomated segmentation analysis that enables the substantia nigra to be displayed as an isolated structure. The technique is now given the acronym SIRRIM (segmented inversion recovery ratio imaging). In contrast to our earlier work, it allows for a more accurate assessment of the gross abnormalities. We report typical SIRRIM images of the SN_C. Images are shown for three subjects: a normal control, a patient with PD, and a "disease control" (a patient with PSP). In these examples all three morphological features of PD, as well as both morphological features of PSP, have radiological correlates. These preliminary findings suggest that SIRRIM may be specific for both diseases.