European Journal of Cardiovascular Medicine

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by motor dysfunction due to dopaminergic neuron loss and Lewy body formation in the nigrostriatal pathway [1–3]. Elevated brain iron, particularly in the substantia nigra (SN), contributes to oxidative stress via the Fenton reaction, producing reactive oxygen species that damage neurons and proteins [4–6].

Quantitative susceptibility mapping (QSM), derived from gradient-echo MRI, quantifies tissue magnetic susceptibility with high sensitivity to iron (e.g., ferritin, hemosiderin), outperforming R2* mapping and susceptibility-weighted imaging (SWI) [7,19,20]. QSM correlates strongly with biochemical iron levels (r = 0.84) [24–27].

The fascicula nigrale (FN), a mineralized tract of striatonigral and nigrostriatal fibers, extends from the globus pallidus (GP) to the SN, potentially mediating iron transport [8–10] (Figure 1). On SWI, it appears as a hyperintense linear structure from the medial GP to the anterior SN (Figure 2).

Prior studies confirm SN iron overload in PD, especially in the SN pars compacta (SNc), correlating with disease severity (e.g., Hoehn and Yahr scores, UPDRS) [11,12,19,20,24,28–35]. Upregulated iron transporters (transferrin, lactoferrin, DMT1) in dopaminergic neurons may disrupt homeostasis, exacerbating age-related accumulation [13–18]. Noninvasive imaging (QSM, SWI, R2*) validates postmortem findings, with QSM offering superior sensitivity [36–41].

This study hypothesized atypical iron patterns in the FN and SN of PD patients, using QSM to quantify deposition and explore age associations in a South Indian cohort.

Study Design and Participants

This prospective observational study, approved by the Institutional Ethics Committee of Government Erode Medical College, Perundurai (Approval No. GEMC/IEC/2025/03), adhered to the Declaration of Helsinki. All participants provided written informed consent.

From March 2025 to September 2025, 25 patients with newly diagnosed idiopathic PD (UK Parkinson’s Disease Society Brain Bank criteria) were enrolled (15 males, 10 females; age 50–72 years; mean 58.6 ± 10.7 years). MRI was performed within 3 months of diagnosis, pre-treatment. Exclusions included atypical, vascular, or drug-induced parkinsonism; cognitive impairment; stroke; head trauma; brain lesions; or other neurodegenerative/psychiatric disorders.

Twenty-five age- and sex-matched healthy volunteers (HVs; 9 males, 16 females; age 54–85 years; mean 61.4 ± 7.3 years) were recruited with identical exclusions.

Imaging Acquisition

Imaging was performed using a 3T Siemens Magnetom Vida MRI scanner equipped with a 12-channel head coil. The imaging protocol included several sequences: sagittal T1-weighted 3D MPRAGE (1 × 1 × 1 mm voxel; TR/TE/TI = 1950/2.26/900 ms), sagittal 3D T2-weighted SPACE (1 × 1 × 1 mm voxel; TR/TE = 3200/458 ms), axial fat-saturated FLAIR (3 mm slice; TR/TE/TI = 9000/77/2500 ms; 0.9 mm gap), axial diffusion-weighted EPI (1.5 × 1.5 × 3 mm voxel; TR/TE = 5700/103 ms; 0.9 mm gap), and axial 3D SWI (0.5 × 0.5 × 2 mm voxel; TR/TE = 29/20 ms). The T1- and T2-weighted sequences were used to aid in anatomical localization..

Image Processing and ROI Analysis

QSM was generated using SPIN software from SWI magnitude and phase images (inverse filter threshold 0.1; vein threshold 200; k-space threshold 0.1; 3 iterations). Minimum intensity projections (mIPs) covered 8 mm (4 × 2 mm slices).

Two blinded radiologists delineated bilateral ROIs for FN, SN, putamen, thalamus, RN, dentate nucleus, and precentral gyrus on mIPs. The FN was traced from GP to SN (3–10 slices; mean length 40–41 mm bilaterally), bisected into rostral/caudal halves (caudal prioritized for odd slices). Susceptibility (ppm) was thresholded at 1000 ppm (subcortical white matter mean: 1000.06 ± 0.5 ppm); maximum values above threshold were recorded. A third reader resolved discrepancies (4/68 cases).Reproducibility was assessed via correlation coefficients.

Statistical Analysis

Sample size was calculated using Statistica v9.0 (StatSoft) for 80% power (α = 0.05). Analyses used SPSS v28.0 (IBM). Data are reported as mean ± SD. Comparisons used independent t-tests and ANOVA with Tukey’s post-hoc. Correlations used Pearson’s r. Significance was set at P < 0.05 (95% CI).

Reproducibility was excellent for FN intra-rater (r = 0.972) and acceptable/good for inter-rater FN (r = 0.761) and SN (r = 0.818) (Table 2). SN susceptibility was significantly higher in PD (P = 0.011).

Mean FN susceptibility trended lower in PD (1123.78 ± 21.00 ppm) than HVs (1179.55 ± 15.16 ppm; P = 0.055), with similar trends in caudal FN (1125.13 ± 55.8 ppm vs. 1163.00 ± 18.8 ppm; P = 0.083) (Table 3). The rostral-to-caudal FN gradient increased in PD, correlating with age (r = 0.62, P < 0.05), unlike HVs.

PD patients showed elevated susceptibility in SNc, GPi, RN, putamen, and caudate (P < 0.05).

Table 1 summarizes the demographic and clinical characteristics of the study participants. The Parkinson’s disease (PD) group consisted of 25 individuals (15 males and 10 females), while the healthy volunteer (HV) group included 25 individuals (9 males and 16 females), with no significant difference in gender distribution (p = 0.078). The mean age of participants in the PD group was 58.6 ± 10.7 years, compared to 61.4 ± 7.3 years in the HV group, which was not statistically significant (p = 0.188). The average duration from diagnosis to MRI in the PD group was 1.4 ± 0.5 months. Regarding handedness, the majority of participants were right-handed in both groups, with a distribution of 20 right-handed and 5 left-handed individuals in the PD group, and 22 right-handed and 3 left-handed individuals in the HV group.

Table 1: Demographic and Clinical Characteristics

Table 2 presents the results of reproducibility analysis and substantia nigra (SN) susceptibility measurements. The intra-rater reliability for fractional anisotropy (FN) was excellent, with a correlation coefficient of r = 0.972, indicating high consistency within the same rater. The inter-rater reliability for FN was acceptable (r = 0.761), while the inter-rater reliability for SN measurements was good (r = 0.818), demonstrating satisfactory agreement between raters. Furthermore, SN susceptibility in patients with Parkinson’s disease (PD) was found to be significantly different compared to healthy controls (p = 0.011), suggesting a meaningful alteration in SN characteristics associated with PD.

Table 2: Reproducibility and SN Susceptibility

Table 3 presents the comparison of fractional nigral (FN) iron levels between patients with Parkinson’s disease (PD) and healthy volunteers (HVs). The mean iron deposition in the PD group was 1123.78 ± 21.00 ppm, while in the HV group it was slightly higher at 1179.55 ± 15.16 ppm; however, this difference did not reach statistical significance (p = 0.055). Similarly, in the caudal aspect of the FN, iron deposition was 1125.13 ± 55.8 ppm in the PD group and 1163.00 ± 18.8 ppm in the HV group, with no significant difference observed (p = 0.083). These findings indicate a trend toward reduced iron levels in the FN among PD patients, though the differences were not statistically significant.

Table 3: FN Iron Levels (ppm)

Figure 1:  Comparison of Mean Iron Deposition (Susceptibility in ppm) in the Fascicula Nigrale Between Parkinson’s Patients and Healthy Volunteers.

This study provides compelling evidence of elevated iron deposition within the substantia nigra (SN) in patients with Parkinson’s disease (PD), aligning with previous findings from quantitative susceptibility mapping (QSM) studies [11,38–40]. A particularly novel observation of this study is the presence of an accentuated rostral-to-caudal gradient in fractional nigral (FN) iron distribution among PD patients. This gradient, which also correlated with advancing age, may indicate an underlying dysfunction in iron transport mechanisms within the nigrostriatal pathway. Such dysregulation could be attributed to the abnormal expression or activity of iron transport proteins such as divalent metal transporter 1 (DMT1) and transferrin [13–17], which may lead to excessive iron accumulation in specific nigral subregions. The resultant increase in iron concentration could promote the generation of reactive oxygen species, contributing to oxidative stress and neuronal degeneration—a well-established pathological feature of PD [4–6]. Interestingly, the observation of lower FN iron levels in PD patients compared to healthy volunteers may suggest a redistribution of iron from the FN to the SN or reflect an early compensatory mechanism due to impaired iron homeostasis. The observed age correlation supports the hypothesis that such iron accumulation and redistribution may begin in the presymptomatic stages of PD, gradually contributing to neurodegeneration over time. The use of QSM in this study, with its superior spatial resolution and quantitative accuracy [40,41], enabled precise detection of subtle regional variations in iron content, thereby strengthening the validity of these findings.

Among the notable strengths of the present study are its prospective design, the blinded analysis of imaging data to reduce bias, and the demonstration of high intra- and inter-rater reproducibility, ensuring methodological reliability. However, certain limitations must be acknowledged. The study was conducted at a single center with a relatively small sample size, which may limit the generalizability of the results. Additionally, potential contamination of small regions of interest (ROIs) by adjacent white matter could not be entirely excluded, although this was mitigated through maximum value analysis. Furthermore, the absence of a priori power calculations for subgroup analyses may have reduced the statistical robustness for detecting smaller effect sizes. To build upon these findings, future research should aim to include larger, multicenter cohorts with longitudinal follow-up to better elucidate the temporal progression of iron accumulation in PD. Correlating imaging findings with detailed clinical, biochemical, and neurobehavioral measures could further clarify the pathophysiological significance of iron dysregulation. Moreover, mechanistic studies focusing on molecular pathways of iron transport and metabolism may help identify potential therapeutic targets for modulating iron homeostasis in Parkinson’s disease

PD is characterized by neuronal loss and iron dysregulation in the nigrostriatal pathway. The exaggerated FN iron gradient, linked to age, suggests tract dysfunction and cumulative pathology. QSM is a valuable noninvasive biomarker for PD, supporting its diagnostic and research utility.

Abbreviations

• PD: Parkinson’s disease
• SN: Substantia nigra
• FN: Fascicula nigrale
• QSM: Quantitative susceptibility mapping
• SWI: Susceptibility-weighted imaging
• SNc: Substantia nigra pars compacta
• GPi: Globus pallidus interna
• RN: Red nucleus
• HVs: Healthy volunteers
• ROI: Region of interest

Limitations

This was a single-center study with a limited sample size, which may restrict the generalizability of the findings. There is also a possibility of white matter contamination in small regions of interest (ROIs); however, this was mitigated by employing maximum value analysis. Additionally, no pretreatment power calculation was performed for all outcome measures, which may affect the statistical robustness of the results

Acknowledgments:

I thank all the faculty , teaching and non-teaching staff of Department of Radiodiagnosis, Government Erode Medical College, Perundurai, Erode and participants of  study for their valuable contribution The authors would like to thank all of the study participants and the administration Department of Radiodiagnosis, Government Erode Medical College, Perundurai, Erode, Tamilnadu, India  for granting permission to carry out the research work.

Conflicts of interest: There are no conflicts of interest.

Ethical statement:

Institutional ethical committee accepted this study. The study was approved by the institutional human ethics committee, Government Erode Medical College, Perundurai, Erode. Informed written consent was obtained from all the study participants and only those participants willing to sign the informed consent were included in the study. The risks and benefits involved in the study and the voluntary nature of participation were explained to the participants before obtaining consent. The confidentiality of the study participants was maintained.

Funding: Nil.

Authors’ contributions: *, ,

Dr.M.Resnik Banu: Conceptualization, Investigation, Methodology, Supervision, Writing – original draft, Writing – review & editing.  Dr.I.Kittu,  Data curation, Formal analysis, Software, Writing – review & editing. Dr.D.Chandra Mohan   Investigation, Project administration, Supervision, Writing – review & editing. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work. All authors have read and agreed to the published version of the manuscript.

DATA AVAILABILITY:

All datasets generated or analyzed during this study are included in the manuscript.

INFORMED CONSENT:

Written informed consent was obtained from the participants before enrolling in the study

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