Synopsis

There is an urgent need for developing diagnostic imaging biomarkers for Parkinson’s Disease (PD). In this work, we applied a standardized substantia nigra pars compacta (SNpc) mask based on neuromelanin-sensitive MR images from healthy subjects to investigate the diagnostic performance of the SNpc overlap percentage and R2* in the SNpc overlap in PD. R2* in the SNpc overlap volume was increased in PD patients as compared to controls. Furthermore, it was significantly positively correlated with the disease duration in PD. We found an excellent diagnostic accuracy for the SNpc overlap percentage (AUC, 0.927) in PD.

Introduction

Parkinson’s disease (PD) affects about seven million people worldwide. Although the exact cause of the disease remains unclear, one possible cause is increased iron accumulation in the substantia nigra (SN) which leads to neurotoxic consequences such as oxidative damage and cell death¹. SN pars compacta (SNpc) is composed of closely packed dopaminergic neurons. Histopathological studies have shown that iron accumulation is severe and mainly occurs in the neuromelanin granules within SNpc in PD^{2, 3}. Further, approximately 50% of melanized dopamine neurons in SNpc have been lost by the time of PD diagnosis⁴. However, the current diagnostic criteria for PD mainly rely on the presence of motor symptoms and a set of clinical assessments ^{5, 6}. Therefore, there is an urgent need for prodromal imaging biomarkers.

Difficulty in identifying SN or SNpc on conventional MR images has been a confounding source of variability in previous studies^7-9. Even with the improved resolution available at 3T, defining the border between the SNpc and the SNr on T2-weighted images has been difficult and remains controversial¹⁰. Previous studies have shown that neuromelanin-sensitive MRI provides contrast sensitive of neuromelanin granules in SNpc¹¹. Here, we apply, a standardized SNpc mask, based on neuromelanin-sensitive MR images from healthy subjects¹², to elucidate region-specific iron deposition in SNpc after PD onset.

Methods

Thirty-nine PD patients were recruited from the movement disorder outpatient clinic diagnosed according to the UK Brain Bank criteria¹³. 33 healthy controls were recruited. The current study was approved by the local ethics committee.

MRI scanning

All subjects underwent scanning with at 3T (Signa HDxt,GE Healthcare) equipped with an eight-channel head coil. A 3D multi-echo GRE sequence was utilized to obtain T2*-weighted images. Imaging parameters were: TR=59.3ms; echoes=16; TE1=2.7ms; TE spacing=2.9ms; FA=12°; FOV=22×22cm2; resolution=0.86×0.86×1.0mm³; acceleration factor=2; and total acquisition time=10minutes 42seconds. Whole brain anatomical images were acquired with a T1-weighted fast spoiled gradient echo sequence for common space registration. Imaging parameters for this sequence were: TR=5.5ms; TE=1.7ms; IT=450ms, resolution = 1×1×1 mm³; FA =12.

Image processing

R2* values were calculated using a mono-exponential fit to the signal decay. The overlap possibility between the whole SNpc (SNpc volume defined by neuromelanin sensitive MR) and R2* hyperintense SN (using a threshold of the mean plus three times the standard deviations) is defined as below, and offers a means to evaluate iron deposition in SNpc. SNpc overlap percentage =Volume (SN_NM intersection with SN_hyperintense)/Volume(SN_NM) where SN_NM and SN_hyperintense denote the NM MRI based SNpc and R2* hyperintense SN volumes, respectively.

Statistical analysis

A chi-square test or an independent two-sample t-test was performed to compare demographic and clinical features between groups. R2* in the SNpc overlap volume and the SNpc overlap percentage between groups were compared using a two-sample t-test. Correlation analysis was performed between R2* in the SNpc overlap volume and the SNpc overlap percentage with disease duration and UPDRS-III score. Diagnostic performance of R2* values in the SNpc overlap volume and the SNpc overlap percentage was performed using a receiver operating characteristics (ROC) analysis. The threshold of statistical significance was set to p<0.05.

Results

All the demographic and clinical data for PD and control groups are shown in Table1. The SNpc overlap volume corresponds to similar regions in SNpc showing reduced NM-sensitive contrast, and presumably NM depletion14 (Figure1) specifically in the lateral ventral tier, which is corroborated by histology4. Significant R2* increases were seen in PD in the SNpc overlap volume (PD: 35.9±5.9, controls: 33.2±5.7, p=0.0286). The SNpc overlap percentage was significantly increased in PD patients as compared to controls (PD: 0.33±0.15;controls: 0.07±0.07, p=1.2e-13) (Figure2).

The SNpc overlap volume R2* (r=0.420; p=0.008) was positively correlated with disease duration in PD patients. ROC analysis of R2* in the SNpc overlap volume and the SNpc overlap percentage in PD and controls found an area under the curve (AUC) of 0.661 (95% CI: 0.531-0.791; P=0.021), and 0.927 (95% CI: 0.867-0.988; P=0.000), respectively (Figure3).

Discussion and conclusion

In this work, we used this standardized SNpc ROI to avoid the problem of variability in SN ROIs used in previous studies^{14, 15}. We found an excellent diagnostic accuracy for the SNpc overlap percentage in PD. This SNpc overlap percentage applied here is based on the overlap between neuromelanin-sensitive and iron-sensitive MRI contrasts, and may represent a promising tool to investigate the nigral PD biology in vivo.

R2* values in the SNpc overlap volume is significantly increased in PD patients, indicating increased iron deposition. We found significant correlation of R2* values in the SNpc overlap volume with disease severity in PD patients which suggests that an increase in SNpc iron content may reflect a marker for disease progression.

Acknowledgements

References