Synopsis

The Locus Coeruleus (LC) is the main source of cerebral noradrenaline, which modulates many cognitive domains from attention and motivation to mood and memory. However, the LC is a small structure located in the mid-brain, proven difficult to detect in-vivo. We studied healthy individuals using high-resolution MT-w, R2*, QSM, and fMRI at 7T to characterize the LC’s shape and size, magneto-chemical and functional properties. While no R2* or QSM contrast was found in the LC, it was clearly detected in MT-images and showed widespread functional connectivity towards cortex and cerebellum, These results are the benchmark for future studies in dementia.

Introduction

The human Locus Coeruleus (LC) is the main source of cerebral noradrenaline, which modulates many cognitive domains from attention and motivation to mood and memory¹. Quantitative in vivo imaging of the LC therefore has broad interest in cognitive, ageing and dementia neurosciences. However, the LC is a small tubular structure (approx. 2x2x15 mm³ in size) located close to the fourth ventricle in the pons². Clearly delineating the LC poses technical challenges in achieving sufficient contrast-to-noise, resolution and in avoiding artifacts.

Noradrenergic neurons of the LC produce neuromelanin (NM), which produces intrinsic contrast in magnetization-transfer-weighted (MT-w) images. This has been exploited at 3T and 7T^3,4. The LC also accumulates iron, which might give contrast in R2*-weighted images or Quantitative Susceptibility Mapping (QSM). The concentration of neuromelanin in the LC increases but iron remains constant with age^5,6.

We studied healthy individuals using high-resolution MT-w, R2* and QSM imaging at 7T to characterize the LC’s shape and size, and magneto-chemical properties. We also acquired high-resolution resting state fMRI which allowed the visualization of functional connectivity patterns based on a seed placed in the LC, as defined by the MT-w imaging.

Data Acquisition

Fifteen healthy volunteers (6 female, age 66.6±4.7 years) were scanned at 7T (Magnetom Terra, Siemens). A 3D MT-weighted sequence was developed^3,4 applying a train of 20 RF-pulses at 6.72ppm off-resonance before a turbo-flash readout. Imaging parameters were: TE/TR=4.08/966ms, flip-angle=8º, voxel-size=0.4x0.4x0.5mm³, BW=140Hz/px, no acceleration, slices=112, 14.3%-oversampling, 7min TA. The inversion pulse was calibrated to 420º FA at the middle of the pons. Three MT sequence repeats were acquired per subject; a further repeat without the off-resonance pulses was also acquired. 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 combination⁷.

A 10 minute whole-brain 1.1mm isotropic resolution resting-state fMRI (rsfMRI) was acquired for functional connectivity analysis: 110 slices, TE/TR=24.8/1589ms, multi-band factor=5, FA=64º, partial-Fourier=6/8, iPat=2, BW=1838Hz/px, echo-spacing=0.78ms. Pulse-ox and respiratory traces were logged. For anatomical co-registration, 0.7mm isotropic MP2RAGE images were acquired⁸.

Data Analysis

The three MT-w runs were first concatenated with ANTs⁹, followed by manual segmentation of the LC performed using itksnap. A cylindrical reference region of 37mm³ was drawn starting at the apex of the 4^th-ventricle in the middle of the pons, to calculate the LC Contrast-to-Noise Ratio ($$${CNR}=(\overline{LC}-\overline{Ref})/sd(Ref)$$$). Using ANTs, the LC ROIs were combined into a template prior to mapping to MNI152 2009b space^11,12.

QSM data was obtained using the multi-scale dipole inversion algorithm in QSMbox¹⁰, with $$$\lambda=10^{2.7}$$$ as the optimal regularization term. Voxel-wise R2* maps were obtained by fitting an exponential curve to the magnitude signal decay.

rsfMRI pre-processing included motion, distortion and slice timing correction, registration to MNI152 2009a space, smoothing with a 3.3-mm isotropic Gaussian kernel, and band-pass filtering (0.009-0.1)Hz. A seed-to-voxel analysis for each subject was performed using the first principal component (PC#1) obtained from the LC-left and LC-right as seeds. Nuisance regressors included PCA#1s of WM, CSF and 4^th-ventricle signals, in addition to RETROICOR¹³, and 24 head motion regressors.

Results

The MT-w data showed clear LC contrast in all subjects. Segmented right and left LC regions created a high-resolution LC template which matched previous literature (Figure 1). QSM and R2* showed no contrast in the LC (Figure 2). Quantitative results for LC length, width and volume, CNR, MTR, $$$\Delta\chi$$$ and R2* are reported in Table 1. No statistically significant differences were found between left and right LC in all metrics.

Connectivity analysis showed that the left and right LC were bilaterally positively correlated with middle temporal and occipital regions, parietal, cingulate and cerebellum.

Discussion and Conclusions

We present in-vivo imaging of the LC in healthy older individuals based on its neurochemical and functional properties. MT-weighted imaging revealed clear LC contrast from which subject-wise LC segmentation was performed. Left and right high-resolution probabilistic LC masks are in good agreement with previous reported work¹⁴, and show symmetry in topology as in Betts et al.¹⁵ who used a T1-weighted FLASH sequence.

On average the estimated LC size was smaller than previous estimates¹. However, neuromelanin in the LC is heterogeneous⁵, and may not represent the total LC volume. Susceptibility and R2* values have been reported for the LC. Although no significant intensity contrast was observed in this older adult group, potential changes may be found in patient groups.

We observed widespread connectivity of the LC with cortical regions^16,17, and the cerebellum, which accord with preclinical studies¹⁸.

Acknowledgements

References

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