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In-vivo characterization of the biochemical properties of the locus coeruleus and substantia nigra in healthy controls and Parkinson’s disease

Catarina Rua¹, Claire O'Callaghan², Ron Ye^3,4, Luca Passamonti³, P Simon Jones³, Guy B Williams¹, James B Rowe^3,4, and Christopher T Rodgers¹
¹Wolfson Brain Imaging Centre, Department of Clinical Neurosciences, University of Cambridge, Cambridge, United Kingdom, ²Behavioural and Clinical Neuroscience Institute and Department of Psychology, University of Cambridge, Cambridge, United Kingdom, ³Department of Clinical Neurosciences, University of Cambridge, Cambridge, United Kingdom, ⁴Medical Research Council Cognition and Brain Sciences Unit, University of Cambridge, Cambridge, United Kingdom

Synopsis

In Parkinson’s disease, there is severe loss of dopaminergic projection neurons of the substantia nigra (SN) and locus coeruleus (LC). Histology shows that damage is non-uniform and occurs in stages; i.e. there is preferential degeneration of neurons first in the rostral portion of the SN and only later on in the LC. In this study, we measured the biochemical properties of the SN and LC with T2* and Magnetization Transfer imaging in patients with Parkinson’s disease and two groups of healthy controls.

Purpose

The locus coeruleus (LC) and the substantia nigra (SN) have some common biochemical and cellular features including the accumulation of neuromelanin (NM) in their catecholaminergic neurons (i.e. noradrenergic or dopaminergic). Both LC and SN neurons are vulnerable to the effects of ageing and Parkinson’s disease (PD)^1-3. Neuromelanin-sensitive MRI sequences based on Magnetization Transfer (MT) have been used to image the LC and SN in vivo at 3T and 7T^4-8. In addition, T2*-w MRI correlates with non-heme iron concentrations in the SN.^9-10
In this study, ultra-high resolution imaging of the LC and SN at 7T was performed with MT-w and T2*-w imaging in a group of healthy controls and patients with PD. Data from an age-matched and independent group of healthy controls were also used to derive a template of the LC and SN. The MT-w, magnetization transfer ratio (MTR)¹¹, quantitative susceptibility imaging (QSM) and R2* signal were compared between groups, in LC and SN.

Acquisition

A 3D MT-weighted sequence (MT-w) was developed^5,8 on a 7T Terra MRI scanner (Siemens) applying a train of 20 RF-pulses at 6.72ppm off-resonance before a turbo-flash readout. Imaging parameters were: TE/TR=4.08/1251ms, flip-angle=8º, voxel-size=0.4x0.4x0.5mm³, BW=140Hz/px, no acceleration, slices=112, 14.3%-oversampling, 7min TA. The MT-preparation pulse was calibrated to 420º FA at the middle of the pons. Two MT sequence repeats were acquired (‘MTrun1’ and ‘MTrun2’); a further repeat without off-resonance pulses was also acquired (‘noMT’). 3D multi-echo, 0.7mm isotropic T2*-weighted images were acquired transversely for QSM and R2* mapping: TE1/TR=4.68/27ms; 6 echoes, echo-spacing=3.24ms, FA=15º; BW=430Hz/px; acceleration-factor=2x2; FOV=224x196x157mm³; 7min TA; and Roemer coil combination¹². For anatomical co-registration, 0.7mm isotropic MP2RAGE images were acquired. For the development of a study template data from 29 healthy controls (HCi) (13 female, age 67±8 years) was collected which included the protocol as described above, with three repeats of the MT-w scan (Table 1). For the main study data was independently acquired for a further 23 healthy controls (HC) (11 female, age 66±5 years) and 23 PD patients (6 female, age 67±7 years) (Table 1).

Analysis

QSM data was estimated from the T2* scan using the multi-scale dipole inversion algorithm in QSMbox¹³, and R2* was estimated by fitting a monoexponential curve to the magnitude of the signal decay¹⁴. The MT-w data from each run was first bias-field corrected and then concatenated with ANTs (Figure 1.A)¹⁵.
Using data from the HCi dataset, a T1-w template was first built from the MP2RAGE data (Figure 1.B) and then mapped to MNI152 2009a space (Figure 1.C). MT-w and QSM data were mapped to MNI space via the MP2RAGE study template to guarantee high registration accuracy, and averaged to produce study-specific MT-w and QSM templates (Figure 1.D). On the MT-w group template left and right LC ROIs were defined using a semi-automatic threshold-based method⁴within a defined search area (Figure 2.A). One reference region, labelled ‘REF’, was also defined in the middle of the pons. Similarly, SN and ‘REF’ ROIs were defined based on the MT-w and QSM templates (Figure 2.B).

Results

From all the metrics, MT CNR showed a significant decrease in PD compared to HC group in the LC (p=0.0030, Figure 3). We confirmed that for the SN, the MT-w ROI was segmented caudal to the QSM ROI , consistent with the location of SN pars-compacta (SNc) and reticulata (SNr), respectively (Figure 2.B)¹⁶. In the SN, there was a significant decrease in MT CNR in PD compared to controls within the MT ROI (p=0.00031) but not in the QSM or ‘QSM only’ ROIs (Figure 4.A). In addition, QSM and R2* values obtained from the MT ROI were significantly higher in PD patients than in heathy subjects (QSM: p=0.00032; R2*: 0.00024). Compared to the MT ROI, in all subjects there is, on average, a significantly increase of approx. 22% in QSM and 20% R2* in the ‘QSM only ROI’ (p<0.001). However, no group difference was found in any iron-related metrics studied (QSM, R2* values or QSM and R2* CNR).

Discussion

The MT and R2* MRI properties using ultra-high-field MRI of the LC and SN differ in patients with PD versus controls. Accurate and unbiased ROI definition was obtained via an independent and age-matched dataset at 7T. Group differences in the LC and SN MT contrast were found between patients and controls, together with increased iron-related signal in the SN in patients relative to controls. These results support the hypothesis that neurodegeneration in the SN and LC is associated with reduced neuromelanin and increased iron accumulation^17-20. Longitudinal work is necessary to assess whether the in vivo MRI indices of LC and SN degeneration track disease progression and post mortem variability.

Acknowledgements

This study was funded by the NIHR Biomedical Research Centre and the Isaac Newton Trust. CTR is funded by a Sir Henry Dale Fellowship from the Welcome Trust and the Royal Society [098436/Z/12/B].

References

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15. http://stnava.github.io/ANTs/

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Figures

Table 1: Subject demographics for each dataset.

Figure 1: Analysis Pipeline. On the HCi dataset, MT-w data was concatenated and QSM was estimated (A) and T1-w data was registered across subjects to create a group-specific template (B) which was then mapped to MNI152 2009a space (C). Applying the same registration matrices, MT and QSM data from each subject were transformed to MNI space, and ROIs of the LC and SN created in the subject-average QSM and MT (D). Atlas ROIs were mapped HC and PD groups subject space (E).

Figure 2: Detailed view of MT-w and QSM templates in MNI152 2009a space. (A) Coronal, axial and sagittal views of the substantia nigra (SN) without (left) and with ROIs (right): MT segmented left and right SN (green and red) overlaid on QSM segmented left and right SN (yellow and blue); segmented left and right REF regions (magenta and cyan). (B) Coronal, axial and sagittal views of the locus coeruleus (LC) without (left) and with ROIs (right): MT segmented left and right LC (green and red); segmented REF region (blue).

Figure 3: Boxplots of the average MT CNR (A), MTR (B), QSM CNR (C), QSM (D), R2* CNR (E) R2* (F) obtained from the left and right Locus Coeruleus ROI in healthy controls (HC) and Parkinson’s patients (PD). MTR measured as (noMT-MTrun1)/noMT ¹¹; CNR was measured as (μ_LC-μ_REF)/σ_REF. QSM values were referenced to whole brain QSM. Statistical differences between groups made with a Wilcoxon signed-rank test.

Figure 4: Boxplots of the average MTR (A) MT CNR (B) QSM (C) and QSM CNR (D) obtained from the Substantia Nigra ROIs (left and right) in healthy controls (HC) and Parkinson’s patients (PD): red – segmentation on QSM, green – segmentation on MT, blue – ‘QSM only’ segmentation, i.e. excluding any voxels in the QSM ROI that overlap with the MT segmentation. MTR measured as (noMT-MTrun1)/ noMT ¹¹; CNR measured as (μ_LC-μ_REF)/σ_REF. QSM values were referenced to whole brain QSM. Statistical differences between groups made with a Wilcoxon signed-rank test.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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