#Early Damage

Importance of Assessing Grey Matter Pathology, in Addition to White Matter

Multiple sclerosis (MS) has long been characterised by the well-known focal inflammatory, demyelinating lesions that are typically seen in the white matter. While white matter lesions and inflammation are certain to contribute to clinical deficits in MS, recent evidence suggests that neurodegeneration and consequent volume loss of the grey matter also play an important role in driving disease progression.1,2

Using MRI Markers to Assess Ongoing Disease Activity in MS

While the assessment of whole brain volume loss is an invaluable marker of disease activity, grey matter volume loss in particular has recently been identified as contributing much of that loss and, increasingly, shown to be associated with clinical outcomes in MS.3,4 Because grey matter loss is shown to be present in the earliest stages of the disease and progresses throughout the course of the disease, accurate measurement of grey matter volume loss is important to better understand MS pathology and quantitatively characterise its effects in patients.2,5

Select Correlations Between Brain Volume Loss and Patient Outcomes6-12

Limitations of Conventional MRI

Conventional magnetic resonance imaging (MRI), such as T2-weighted imaging and GdE T1-weighted imaging, is limited in its ability to assess grey matter damage, due to both technical challenges and patient factors.4,5,7,13 The grey matter/white matter border, as seen on conventional MRI, is generally not as distinctive as the brain/cerebrospinal fluid (CSF) border, and cortical grey matter lesions may be small and difficult to detect on conventional MRI.4,7

Cortical lesions, which make up a significant portion of grey matter damage, are often undetectable on conventional MRI using standard field strength, mainly due to their small size, the anatomical lack of myelin in the cortex generating little MRI contrast upon demyelination, and the partial volume effects from adjacent CSF and white matter. In addition, in contrast to white matter lesions, cortical grey matter lesions lack focal infiltration of immune cells, complement deposition, and blood brain barrier damage that can be seen on conventional MRI.2

Brain volume measurements are also affected by differences in MRI hardware (eg, scanner), scan quality, and analytical software.14 Due to time constraints during routine visits, the spatial resolution and coverage of MRI scans may also be limited compared with scans taken as part of clinical studies.15 Most MRI technology used in general clinical practice uses 1.5 Tesla (T) field strength further limiting the sensitivity of damage that can be observed.16

In addition to technical factors, patient-specific factors such as age, genetics, lifestyle (eg, alcohol, smoking, dehydration), and comorbidities (eg, diabetes, hypertension) can affect the accuracy of brain volume measurements. Pseudoatrophy, in which the resolution of inflammation is thought to decrease water content in the brain with no associated loss of cell structure, can complicate the interpretation of brain volume changes over time.7,8 To account for these physiological confounding factors, normative brain volume changes need to be established, both for healthy individuals and for patients with MS.7 In addition, MRI scans should be obtained at baseline (pretreatment) and within 6 months of treatment initiation to ensure that the treatment has taken effect. Follow-up brain MRI scans should be performed and then compared with the re-baseline reference MRI scan.17

Addressing Challenges of Measuring Grey Matter Volume Loss and Pathology With Advanced MRI Techniques

In recent years, advancements in MRI technology have significantly contributed to identifying the true extent and clinical impact of grey matter involvement in MS. Advanced MRI techniques allow quantification of several pathological processes in vivo and offer insights into MS pathophysiology beyond white matter lesions.1,2 By revealing what is happening beneath the visible surface of MS pathology, these techniques allow early measurement of functional and structural abnormalities and have the potential to delay permanent damage.18

Investigating Subclinical Disease Activity in MS19-21

Grey Matter Volume Loss Measurement Techniques

There are currently commercially available, sophisticated methods to measure regional grey matter volume loss in the brain. Grey matter volume loss, typically characterised by volume decrease of subcortical grey matter structures and volume or thickness decrease of cortical regions, can be measured using standard 3D T1-weighted images acquired by MRI or automated methods, such as FreeSurfer. For measurement of cortical volume, methods such as Jacobian integration and SIENAX (cross-sectional pipeline of SIENA; Structural Image Evaluation using Normalization of Atrophy) can be used.1

  • Jacobian integration is a method that quantifies longitudinal grey matter or white matter volume changes by measuring the total net amount of contraction or expansion of a selected region in an image during an accurate nonlinear registration between the images of the first and last time points. Jacobian integration applied to longitudinal grey matter volume loss analysis has been shown to reduce variability related to measurement error compared to other commonly used methods1
  • FreeSurfer contains a fully automated structural imaging system that allows visualisation of structural MRI data to calculate cortical thickness. FreeSurfer also includes volumetric segmentation of deep grey matter structures. FreeSurfer has a longitudinal pipeline with improved reliability of volume change measurements for analysis of changes over time1
  • SIENAX estimates partial volume fractions of grey matter, white matter, and CSF. The longitudinal SIENA only quantifies overall brain volume change (based on the shift of the parenchyma-CSF border over time), and therefore does not measure grey matter or white matter volume change separately. Extensions of SIENAX to perform specific analysis of grey matter volume loss have recently been developed1

Grey Matter Lesion Measurement Techniques

While it remains unclear whether it could be possible to translate these methods into routine clinical practice, recent improvements in the detection of cortical lesions have been achieved through the introduction of several advanced MRI techniques.2

  • Cortical lesions can be visualised, at 1.5T and 3T, with specific sequences that enhance the contrast between normal-appearing grey matter and focal grey matter lesions. Double inversion recovery (DIR) and phase-sensitive inversion recovery (PSIR) imaging suppresses the signal from CSF and white matter, thus achieving a superior delineation between grey and white matter. These techniques can be used to detect MS cortical pathology, with PSIR showing a higher sensitivity3,22
  • An ultra-high MR field (7T) has further demonstrated that cortical lesions are strongly correlated with disability status and cognitive impairment3,22

Axial DIR Sequences From 4 Different Patients With RRMS2

Several cortical lesions are highlighted; intracortical lesions, leucocortical lesions, and subpial cortical lesions. DIR are red colored to better highlight some cortical lesions.
  • Magnetisation transfer (MT) MRI provides an index, called the MT ratio (MTR), whose values reflect the efficiency of the magnetisation exchange between protons in tissue water and those bound to the macromolecules. MTR maps are sensitive to changes in myelin content in all brain tissues, and although the MTR signal can be influenced by factors such as axonal loss, inflammation, and oedema, their impact is less pronounced in the cortex, supporting its use in investigating myelin loss and repair in cortical areas22
  • Delayed high-resolution postcontrast T2 fluid-attenuated inversion recovery (FLAIR) MRI can be used to visualise leptomeningeal immune cell accumulation, which is often associated with cortical lesions. These infiltrates preferentially involve the subpial cortical layers, are closely associated with subpial demyelination and cortical volume loss, and are present in ~20% of patients with relapsing-remitting MS. They are not exclusively observed in MS, but their relationship with cortical volume loss and lesions appears to be highly specific22
  • Advanced 7T MRI allows detection of several underlying mechanisms that drive pathogenesis in grey matter, including a gradient of subpial cortical abnormalities, strictly associated with increased severity of demyelination, neuronal loss, and microglia activation in the outermost cortical layers. Furthermore, the use of MRI techniques such as 3D DIR has recently shown that cortical lesion susceptibility maps can highly reproduce the heterogeneous activity features of grey matter damage, even at an individual level2

Conclusions

Grey matter pathology has been shown to occur early and extensively throughout all stages of MS disease course, thus influencing the long-term prognosis of the disease.4 Therefore, improving the ability to detect these changes may provide insight on disease management at the earliest phases of disease.4 With MRI measures that are more specific and sensitive to disease pathological substrates, advances in MRI technology promise to improve monitoring of MS, even in the earliest phases of the disease.18

MRI has traditionally been used as a robust tool to monitor lesion load and new disease activity in clinical practice. However, the implementation of a standardized postprocessing protocol for the evaluation of brain volume is not yet feasible.23 The sensitivity associated with advanced MRI techniques is also not currently ideal and therefore, caution is required before presence of grey matter lesions can be reliably assessed.23 The MAGNIMS consortium developed MRI guidelines in 2015 recommending that new T2 lesion count requires high-quality, comparable MRI scans, and must be interpreted by highly qualified, trained readers to minimize observer variability. MRI subtraction facilitates recognition of changes in focal lesions over time, thus increasing the power of serial imaging, and automated subtraction improves accuracy and sensitivity for identifying new and/or enlarged T2 lesions.17 Routine monitoring should be conducted every 3–12 months using at least 1.5 T, depending on patient characteristics such as disease duration, comorbidities and current treatment.17

While advanced MRI techniques have become more commonly used for secondary outcomes in clinical trials in recent years, there are ongoing efforts to make these techniques more widely available and feasible for everyday clinical practice and to reach comparable results between different centres. This will help to achieve personalised patient care and manage various pathological mechanisms that may drive MS disease progression.18,23