The diagnosis of Parkinson disease (PD) continues to be based on clinical findings ¹. Although diagnostic criteria are well established, a large percentage of patients who are initially diagnosed with PD don’t have the disease ². There are significant difficulties differentiating PD from other parkinsonian syndromes, such as multiple system atrophy, progressive supranuclear palsy or corticobasal degeneration ³.

The dorsolateral substantia nigra on axial Susceptibility Weighted Imaging (SWI) from Gradient Echo Imaging (GRE) magnitude/phase datasets in healthy subjects appears as a hypointense stripe that is split posteriorly by a hyperintense cleft, the nigrosome-1. This appearance resembles the split tail of a swallow. Schwarz et al. have described that the loss of this appearance in PD patients yielded high diagnostic accuracy ⁴.

The hyperintense signal in the nigrosome-1 is thought to be due to the presence of neuromelanin-containing dopaminergic cells. In patients with PD this appearance is lost possibly due to the increased iron content, neuromelanin loss or both ^5,6. Iron (deposited as Ferritin) causes stronger susceptibility when compared to other adjacent tissue on SWI ⁷. Quantitative susceptibility mapping (QSM) is an emerging new method which is able to quantify susceptibility from GRE phase images ⁸.

In this retrospective study we compared the susceptibility of the dorsolateral substantia nigra (in the ‘swallow tail’ region) in patients with early Parkinson’s disease and control subjects.

Seven patients with early Parkinson’s disease using clinical criteria and an MRI with SWI/GRE were identified. A group of five control subjects also having SWI/GRE datasets for other reasons like cavernoma, prior trauma were included in the study. Data acquisition was performed at 3 Tesla (Siemens Medical Systems, Erlangen, Germany) using a 32-channel head-coil and a 3D multi-echo GRE sequence with the following parameters: TR = 44 ms, TEs = 4.6, 11.2, 18.0, 24.8, 31.6, 38.4 ms, α = 20°, FOV = 200 x 200 mm², matrix = 256 x 256, resolution = 0.75 x 0.75 mm², slice thickness = 1.5 mm, number of slices = 80. QSM post processing was performed in MATLAB using the STISuite 2.2 toolbox ⁹. After phase unwrapping the back-ground field was removed using the V-SHARP algorithm ¹⁰ and quantitative susceptibility maps were calculated using the iLSQR method (an algorithm for sparse linear equations and sparse least squares) ¹¹. A neuroradiologists identified the swallow tail area on SWI and regions of interest (ROIs) were placed in the left and right dorsolateral substantia nigra (swallow tail area) on the QSM datasets to measure the susceptibility (Figure 2).In Figure 1 the axial SWI images are shown for (Fig. 1a) a PD patient and (Fig. 1b) for a healthy control subject. The corresponding quantitative susceptibility maps are shown for the PD patient in (Fig. 1c) and for the control subject in (Fig. 1d). Figure 2 shows the zoomed details of the purple indicated rectangle area in Figure 1, the substantia nigra is visible in detail. For PD patient the loss of the swallow tail sign (black arrow) is noticeable on the SWI image (Fig. 2a), while this sign can be seen in the control subject (Fig. 2b). The corresponding quantitative susceptibility maps are shown in Fig. 2c, d. The ROIs, where the susceptibility was measured are shown as a purple overlay in Figure 2c, d. The susceptibility in the swallow tail area was higher in the PD group (0.072 ± 0.018 ppm) compared to the control subjects (0.058 ± 0.027 ppm). This likely suggests increased iron deposition in the ‘swallow tail’ area along with dopaminergic neurons loss. There was no significant difference in the susceptibility due to laterality in either group.Different studies found for PD patients increased susceptibility values in the substantia nigra compared to healthy controls. Especially the substantia nigra compacta shows significant differences in susceptibility. In this study we investigated the dorsolateral region of the substantia nigra quantitatively, the region where healthy subjects show a typical swallow tail sign. This visual sign is also present on the corresponding QSM images. The quantification revealed increased susceptibility for the PD group, but the difference to the control group was not significant. The higher standard deviation of the susceptibility in the control group may reflect the variability due to the swallow tail sign.Our study shows increased susceptibility values in the ‘swallow tail’ area in patients with early Parkinson’s disease. This likely suggests increased iron deposition causing a masking effect that contributes to the disappearance of the ‘swallow tail’ in Parkinson’s disease patients.