Longitudinal Changes in Neuromelanin MRI Signal in Parkinson's Disease: A Progression Marker

1. Materials and Methods

2. Results

3. Discussion

This study suggests that NM‐sensitive imaging provides reliable progression markers of SN neurodegeneration. First, we confirmed that NM‐based volume and signal intensity were reduced in PD and that this reduction was greater in progressing than in early PD. Second, we characterized the pattern of progression in NM‐based SN volume and signal intensity in PD patients compared with HVs. The results also suggested that using our segmentation method, volume measurements were more effective than signal changes in detecting longitudinal changes in SN. Third, these changes were clinically relevant as they correlated with the clinical severity of disease assessed using MDS‐UPDRS for SN volumes and disease duration for SN signal changes. Fourth, LEDD in patients did not correlate with any SN measurements, suggesting that NM signal changes were not modified by dopaminergic medication. Last, power analysis suggested that NM‐based SN measurements could be effective in detecting significant changes in a neuroprotective clinical trial. Altogether, the results demonstrated the potential of NM as a useful imaging biomarker of PD progression.

NM is a pigment produced in the cytosol of catecholaminergic neurons stored in NM autophagic lysosome organelles in which it is bound to metals, particularly iron.⁹ The NM–iron complex can be found in an extracellular compartment after neuronal death.⁹ The NM–iron complex is paramagnetic and hence can be detected using MRI and appears bright on turbo or fast spin echo T1‐weighted images. Studies have shown that the area of high signal intensity in these images is associated with the NM containing area of SNc at 3 T¹⁰, ¹⁵ and 7 T.¹¹, ¹⁴, ²⁵

The relationships between iron and NM in the SN in PD are also of interest. Iron is involved in the metabolic pathway leading to NM synthesis, and the accumulation of iron when not stored in NM may contribute to neurodegeneration in PD.²⁶, ²⁷ In contrast, the NM–iron complex may have a neuroprotective effect.⁹, ²⁶, ²⁷ Iron‐sensitive MRI techniques have shown raised iron levels in the SN in PD patients.²⁸ A recent study suggested that MRI may help to distinguish NM‐iron from the other iron pool, but this requires further study.¹⁷, ²⁹

SN volume appears highly variable when measured using MRI.¹¹, ¹², ³⁰, ³¹ Many factors contribute to this variability such as the type of sequence that was used including T2*‐weighted imaging at 7 T,³² driven equilibrium single‐pulse observation of T1,³³ Fast Gray Matter Acquisition T1 Inversion Recovery,³⁴ DANTE T1‐SPACE,³⁵ and short‐echo‐time magnitude image derived from quantitative susceptibility mapping,¹¹, ¹², ²³, ³¹, ³⁶, ³⁷ Previous studies using NM‐sensitive images have also measured variable SN volumes ranging from about 110 to 500 mm³.¹¹, ¹², ²³, ³⁰, ³¹ These measurements depended on the method used for volume calculation, the number of image slices used for SN segmentation, and the imaging parameters such as slice thickness, which determined the amount of partial voluming in images. In studies that used semiautomated thresholding, volumes depended on the choice of threshold, which resulted in smaller (<200 mm³),¹¹ intermediate (250–350 mm³),¹², ³⁰ or larger volumes, that is around 500 mm³.³¹ Our values were in the intermediate range, in line with previous studies,¹², ²³, ³⁰, ³², ³⁸ Values reported using MRI were similar³⁹ or greater than those reported using stereological histological methods.⁴⁰

Baseline reductions in volume and signal intensity were observed in early and progressing PD. In early PD, NM‐based SN volume decreased by 11.2% and SNR by 17.4% compared with HV. Some previous studies reported greater reductions in size, of 22% to 29% in de novo patients⁴¹, ⁴², ⁴³, ⁴⁴ and of 29% to 47% in early PD, either using measurements of SN width or area.¹¹, ³⁰, ⁴² Volume reductions were more pronounced at later stages of disease, with a 38.4% decrease here compared with 30%–39% reported in moderate PD¹², ²³, ⁴⁵ and 78% in late PD.¹¹ This decrease was also consistent with the 45% loss reported during the first decade of PD in histology studies.³ Signal reductions also greatly varied across studies ranging from −21% in de novo PD⁴³ to −48% in late PD.⁴⁶ Studies have also reported greater changes in the lateral part of this structure.³⁹, ⁴³, ⁴⁴, ⁴⁷, ⁴⁸, ⁴⁹, ⁵⁰, ⁵¹

Longitudinal reductions in volume and signal intensity were observed in early and progressing PD. Volume decreased by 5.8% per year in early PD patients and by 10.2% per year in late PD patients. One study reported a 17.5% yearly reduction in SN area¹⁸ and a 16.5% reduction in CNR in PD patients with a 3.1‐year disease duration.¹⁸ Longitudinal imaging studies of DA function using radiotracers in PD patients have shown an annual decline rate of DAT binding in the striatum of 4.6% to 11.9% compared with baseline values.⁵² Some studies have reported that DAT binding decreased linearly⁵³, ⁵⁴, ⁵⁵, ⁵⁶, ⁵⁷ and others exponentially.⁵⁶, ⁵⁸, ⁵⁹, ⁶⁰, ⁶¹, ⁶², ⁶³ Exponential decay of vesicular monoamine transporter was also reported.⁶⁴ Although our results suggest a greater decrease in NM content in progressing stages compared with early stages of disease, the dynamics of longitudinal changes in NM in PD, whether linear or exponential, require further investigation.², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁵⁰ Overall, the rate of decline in NM‐based SN volume is in line with the values of DAT binding reported in the literature. Although NM is also found in the extracellular space following DA neuronal death before being degraded,⁹ measurement by NM‐sensitive MRI may provide markers more closely related to SN neurodegeneration than striatal DA function⁹ or free water.⁶⁵ This remains to be confirmed by a direct comparison of the different markers and with postmortem studies.

There were significant differences between men and women in both groups. In the HV group, women had greater SN‐normalized volumes (C_vol) and signal intensities (CNR) and showed greater increase in NM content at V2 compared with men. Larger SN C_vol in female HVs was in line with a previous study.⁶⁶ This was also in line with DAT studies in HVs that have reported higher striatal binding in women compared with men.⁶⁷, ⁶⁸, ⁶⁹ The greater increase in NM content with age in women may explain this discrepancy. In PD, women presented larger baseline volumes and lower decrease in volume and increase in signal intensity compared with men. This was also in line with the higher striatal dopamine transporter binding in women compared with men reported in PD.⁶⁸

In early PD at baseline, SN volume changes correlated with disease severity and signal intensity changes with disease duration, in line with correlations reported between motor severity and SN volume or signal intensity³⁰, ³⁹, ⁴⁵, ⁴⁸, ⁷⁰, ⁷¹ as well as fractional anisotropy in NM‐based SN⁷² and free water in the posterior SN.⁶⁵ No correlations were observed in progressing PD patients, possibly because of the smaller number of subjects.

There were no correlations between NM SN measures in HVs and age, in line with previous studies that showed that SN NM content reached a plateau in the fifth and sixth decades.⁶⁶ This may also apply to progressing PD patients in the same age range. However, there was a significant effect of age on SNR in early PD, as it is possible that increased production of NM may be associated with greater neuronal loss.⁷³

There was a significant scanner effect for SN volume and signal intensity in HVs. This may be because of the differences in sequence parameters, coils, or scanner hardware, resulting in different SNRs. However, head size normalization (C_vol) cancelled the between‐scanner differences in SN volume and hence should be considered for multisite trials investigating NM signal changes in PD patients. Alternatively, a scanner may be entered in the statistical comparison as a covariate.

Sample size estimates were calculated for detecting a slowing of NM decrease in a clinical trial testing a disease‐modifying therapy. These results are comparable with [¹²³I] FP‐CIT SPECT or MRI‐based free water measurements.⁶⁵

This study has several limitations. First, we used manual segmentation to delineate the SN. Automated methods may improve the reproducibility of segmentation techniques.⁷⁴, ⁷⁵, ⁷⁶, ⁷⁷ Manual segmentation has the advantage of allowing careful quality control of images and removal of areas containing artifacts from measurements. Experienced raters can achieve good reproducibility of measurements, as it was in our case, in line with those reported in previous studies.¹⁸, ²³, ³⁰, ³⁹, ⁴⁴, ⁴⁵, ⁴⁶, ⁴⁸, ⁶⁶ Second, 2‐dimensional acquisitions using a relatively thick slice are prone to partial voluming, and the use of 3‐D acquisitions may improve the accuracy of the results.⁷⁸ Third, improved CNR in SN compared with background signal may be obtained by using different acquisition schemes like magnetization transfer¹¹, ³⁷, ⁷² or double inversion T1‐weighted imaging, although improved CNR offered by these MRI protocols comes at the expense of acquisition times. Fourth, direct quantitative methods, including T1 mapping⁷⁹ or magnetization transfer, remains to be evaluated.¹¹, ⁷² Fifth, the sex ratio in HVs was significantly different from that in the PD patients at baseline, thus further validating that the use of a larger number of subjects with balanced sex ratio is warranted to confirm the results. Last, we converted UPDRS‐III scores to MDS‐UPDRS‐III scores, which may be less accurate than the directly obtained scores as in cohort I.

In conclusion, this study showed a progressive and measurable decrease in NM‐based SN volume and signal intensity in PD patients over time that related to the severity of motor symptoms. NM‐sensitive imaging might allow direct, noninvasive assessment of the progression of SN cell loss in PD. In clinical trials, NM‐based SN volume measurements could provide useful biomarkers for clinical trials of disease‐modifying therapies.

Author Contributions

Design, conceptualization and execution of the study, data analysis, statistical analyses and drafting the first and a significant portion of the manuscript, tables and figures: Rahul Gaurav.

Execution of the statistical analyses, review and critique of the manuscript: Lydia Yahia‐Cherif.

Acquisition of clinical data, review and critique of the manuscript: Graziella Mangone.

Execution of the study, data analysis, review and critique of the manuscript: Romain Valabrègue and Nadya Pyatigorskaya.

Review and critique of the statistical analyses and manuscript: Emma Biondetti, Claire Ewenczyk, R. Matthew Hutchison, and Jess M. Cedarbaum.

Design, conceptualization and execution of the study, review and critique of the statistical analyses and manuscript: Jean‐Christophe Corvol and Marie Vidailhet.

Design, organization, conceptualization, and execution of the study, writing of the first draft, overall review and critique of the manuscript: Stéphane Lehéricy.

Supporting information