Magnetic resonance imaging of the human locus coeruleus: A systematic review

Highlights

• Magnetic resonance imaging studies of the human locus coeruleus (LC) are reviewed.

• The methodology and outcomes for 69 structural and functional studies are reported.

• Recommendations to optimize the reliability and validity of future studies are made.

Abstract

The locus coeruleus (LC), the major origin of noradrenergic modulation of the central nervous system, innervates extensive areas throughout the brain and is implicated in a variety of autonomic and cognitive functions. Alterations in the LC-noradrenergic system have been associated with healthy ageing and neuropsychiatric disorders including Parkinson’s disease, Alzheimer’s disease and depression. The last decade has seen advances in imaging the structure and function of the LC, and this paper systematically reviews the methodology and outcomes of sixty-nine structural and functional MRI studies of the LC in humans. Structural MRI studies consistently showed lower LC signal intensity and volume in clinical groups compared to healthy controls. Within functional studies, the LC was activated by a variety of tasks/stimuli and had functional connectivity to a range of brain regions. However, reported functional LC location coordinates were widely distributed compared to previously published neuroanatomical locations. Methodological and demographic factors potentially contributing to these differences are discussed, together with recommendations to optimize the reliability and validity of future LC imaging studies.

Introduction

The locus coeruleus (LC) is a cylindrical, hyperpigmented nucleus located in the rostral pontine brainstem. It is the major origin of noradrenergic neurones in the central nervous system. In histological specimens, the average dimensions of the human LC have been measured as 14.5mm in length and 2.5 mm in thickness (2 mm in the central portion) (Fernandes et al., 2012). Its cylindrical shape is thinner in its central part and diverges laterally in its caudal three-fourths (see Fig. 1).

The LC plays an integral role in the regulation of arousal and autonomic function through connections with widespread areas of the brain and spinal cord (Samuels and Szabadi, 2008a), modulating wakefulness, pupil control, blood pressure and temperature. It also plays a role in anxiogenesis and responses to fear and pain via connections with the amygdala (Samuels and Szabadi, 2008b), and has been implicated in cognitive processes including attention, decision-making and memory through connections with frontal cortex and the hippocampus (Sara, 2009). In humans, LC neuron numbers are believed to gradually decline with increasing age (Mann, 1983) although some reports challenge this view and suggest that LC neuronal death in cognitively normal older adults might indicate a presymptomatic stage of dementia-related physiological decline (Mather and Harley, 2016). Neuromelanin, a dark polymer pigment accumulates inside the noradrenergic neurons of the LC until the age of about 60 years, after which it declines, possibly in association with preferential loss of neurons containing the highest neuromelanin content (Mann and Yates, 1974).

The LC has been the subject of increasing interest within neurology and psychiatry following demonstration of early pathological change in neurodegenerative conditions such as Parkinson’s disease (PD) and Alzheimer’s disease (AD), conditions that have traditionally been more associated with dysfunction in the dopaminergic and cholinergic systems respectively (Zarow et al., 2003). Early tau pathology in the LC may even precede the hallmark appearance of tau and amyloid pathology in the medial temporal lobe and in cortical regions in AD (Braak et al., 2011) and has been shown to be associated with LC volume loss as the disease progresses (Theofilas et al., 2017). LC neurofibrillary tangles and abnormal tau show an age-MCI-AD continuum, with levels of cytopathology being associated with cognitive impairment (Grudzien et al., 2007). Studies have also implicated LC and noradrenergic system alterations in the pathophysiology of other disorders including depression (Arango et al., 1996, Bernard et al., 2011, Ressler and Nemeroff, 1999), schizophrenia (Lohr and Jeste, 1988, Yamamoto and Hornykiewicz, 2004), addiction (Berridge and Waterhouse, 2003) and autism (Mehler and Purpura, 2009).

The human LC can be visualized using T1-weighted MRI (Sasaki et al., 2008b) (see also Fig. 2). Neuromelanin scavenges metals such as iron and copper (Sasaki et al., 2008a), which confers T1-shortening effects resulting in signal hyperintensity on T1-weighted images (Enochs et al., 1997, Trujillo et al., 2017). As neuromelanin is found inside the noradrenergic neurones of the LC and in the dopaminergic neurons of the substantia nigra in humans, these structures can be readily visualized using T1-weighted as well as magnetization transfer (MT) weighted MRI (Nakane et al., 2008, Sasaki et al., 2006).

In addition to neuromelanin-content enhancing T1- or MT-weighted approaches, often referred to as ‘neuromelanin-sensitive’ imaging, functional MRI techniques have also been used to investigate LC activity and connectivity in healthy and clinical human populations. However, no previous review has summarized the characteristics and findings of structural and functional MRI studies of the LC, and no consensus on recommendations to achieve optimal methodological validity and reliability has been published. Optimal in-vivo imaging of the LC is needed to evaluate its potential as a biomarker in neuropsychiatric conditions such as dementia and to assess the efficacy of future treatments targeting the LC-noradrenergic system.

This paper systematically reviews the studies that have published data on the LC using functional or structural MRI techniques. For neuromelanin-optimized structural MRI studies, the review includes the range of scanning parameters used and methods employed to localize and measure the LC, and compares measures of signal intensity obtained in healthy and clinical populations. For functional imaging studies, the review describes the methods used to localize the LC and compares the distribution of location coordinates attributed to the LC with published neuropathological location data. The tasks and conditions associated with LC activation are also reported. Finally, a critical summary of the methodological factors that may limit validity are discussed, together with recommendations to optimize the reliability and validity of future LC imaging studies.