Mcdespot-Derived Demyelination Volume in Multiple Sclerosis Patients Correlates with Clinical

Titel;

mcDESPOT-Derived Demyelination Volume in Multiple Sclerosis Patients Correlates with Clinical Disability and Senses Early Myelin Loss

H. H. Kitzler1,3*, J. Su2*, M. Zeineh2, C. Harper-Little3, A. Leung4, M. Kremenchutzky5, S. C. Deoni6, and B. K. Rutt2

1 Department of Neuroadiology, TechnischeUniversitaet Dresden, Dresden, Germany

2 Department of Radiology, Stanford University, Stanford, California, USA

3 Robarts Research Institute, University of Western Ontario, London, Ontario, Canada,

4 Department of Diagnostic Radiology and Nuclear Medicine, University of Western Ontario, London, Ontario, Canada

5 Department of Clinical Neurological Sciences, University of Western Ontario, London, Ontario, Canada

6 Department of Engineering, Brown University, Providence, Rhode Island, USA

(*both authors contributed equally to this work)

Running title: Whole-Brain Demyelination Quantification in MS

Total word count (text body):xxxx

Summary (400 words max; current state: 400) LEFT UNTOUCHED, LEAST TO BE REWRITTEN AS SUGGESTED!

Conventional magnetic resonance (MRMRI) imaging is established as one of the most important surrogate markers of Multiple Sclerosis (MS) development and treatment outcome. Based on the assumption that the clinical course of MS is adequately reflected by focal white matter changes, many clinical trials have used lesion volume as the principal MRMRI-derived measure; however, such measures have been recently criticized as adding little or no independent information over and above non-imaging disability outcome measurements when evaluated retrospectively. [reference required here?] This highlights the need to develop and validate new quantitative WM imaging strategies that aim to characterize the invisible burden of demyelination in the brain and establish much more sensitive and specific markers of MS that correlate strongly with clinical disability. One of the most promising of such arising measures is myelin-selective MR imaging MRI that allows the acquisition of Myelin-Water fraction (MWF) maps, a parameter that is correlated to the brain white matter (WM) myelination. The aim of our study was to apply the newest myelin-selective MRMRI method, multi-component Driven Equilibrium Single Pulse Observation of T1 and T2 (mcDESPOT) in a controlled clinical MS pilot trial. This study was designed to assess the capabilities of this new method to explain differences in disease course and degree of disability in subjects spanning a broad spectrum of MS disease severity. The whole-brain isotropically-resolved 3D acquisition capability of mcDESPOT allowed for the first time the registration of 3D MWF maps to standard space, and consequently a formalized voxel based analysis (VBA) of the data. This VBA approach combined with image segmentation further allowed the derivation of new volumetric measures of disease severity: total demyelinated volume (DV) in WM, DV within WM lesions, DVwithin dirty appearing white matter (DAWM) and DV within normal appearing white matter (NAWM). The analysis confirmed that neither lesion burden nor lesion demyelination correlate well with clinical disease activity measured with the extended disability status scale (EDSS) in MS patients. In contrast, our measurements of demyelination volume in NAWM correlated significantly with the EDSS score (R2 0.405; p<0.01). [will need to update numbers, plus decide what other data is going into this paper and to summarize all the key results in this section here][Our most remarkable result is differentiating CIS from normals. There is some correlation of EDSS with DV but not enough to say “wow.” The other question is whether to include the multi-variate regression analysis.] The same measurement discriminated Clinically Isolated Syndrome (CIS) patients from a normal control population (p<0.001), hence the technique senses very early disease-related myelin loss. Furthermore, the same parameter discriminated patients with the secondary-progressive (SPMS) course from relapsing-remitting MS (RRMS; p<0.01). Overall, our results demonstrate that mcDESPOT-defined demyelination measurements show great promise to act as imaging markers of clinical disease activity in MS. Further investigation will determine if this measure can serve as a risk factor for the conversion into definite MS and for the secondary transition into irreversible disease progression.

Keywords: multiple sclerosis; demyelination; normal appearing white matter; quantitative MRIMRI; myelin-selective imaging

Introduction

Multiple Sclerosis and Imaging Conventional MRI

Multiple Sclerosis (MS) is an immunologically mediated demyelinating and axonal disease of the human central nervous system (CNS) and. It isone of the most common disabling neurological diseases in young people adults with the typical age-at-onset being 20 to 40 years and it is approximately twice as common in women as in men [Platten 2006]. Depending on the location of CNS lesions, [Hagen or others: I simplified your wording, but need to know if what I wrote is still strictly correct] MS patients experience diverse neurological symptoms and impairment of e.g. motor, visual, or, sensory function. Over 80% of MS patients initially present with a relapsing disease course that eventually transitions into permanent disability. More than 50% of patients require a walking aid within 15 years from initial diagnosis [Weinshanker 1989].

The MS etiology[can we just say “the cause of MS”? Or is etiology a standard term in medicine?] is believed to be the result of a complex combination of environmental, genetic, and autoimmune factors resulting in an immune-mediated attack on CNS myelin,[Steinman 2004]. Myelin is the basic structure of the myelin axonal sheath of , surrounding neuronalaxons cells which isand vital for their appropriate function [Steinman 2004]. The pathology of MS , however, lesions is heterogeneous, including inflammatory cell infiltration, astroglial hypertrophy, axonal loss, but demyelination is the recognized hallmark of the disease [Lucchinetti 2001].

The application of conventional magnetic resonance imaging (MRI) techniques has revolutionized the clinical practice in MS [Villenga 2009]. Conventional magnetic resonance (MRMRI)imaging studies reveal focal signal deviations lesions, traditionally called lesions or “plaques” within throughout the white matter (WM) and less frequently also in grey matter (GM) of the brain and spinal cord in both T2 and T1 weighted scans. This appearance of lesions on top of the background of an inflammatory reaction throughout the central nervous system (CNS) defineeds MS early as a multifocal inflammatory demyelinating disease.

Conventional MRI derived lesion numeric and volumetric measures are currently used as paraclinical markers in standardized diagnostic schemes [Polman, 2005]. However, thee lesion-centered view has been challenged by studies investigating the relationship between conventional MRI measures with clinical MS disease severity and neuro-functional scores revealing only dissatisfying and non-significant correlations [Fulton, 1999, more ref]. Despite the acknowledged potential application of conventional MRI, presently available MRI technologies have failed in meeting the critical goal of reflecting MS patient disability status and predicting disease progression. [ref]

Such conventional MRI measures also have been widely employed as presumptive surrogates across a broad spectrum of MS studies, ranging from pilot trials through multi-center pivotal phase III studies. However, the presently employed metrics fail to meet Prentice’s surrogate endpoint validation criteria for reliable surrogate markers in predicting downstream disease activity [Prentice, 1989]; and statistically appear to offer no more valid endpoint than that already offered by clinical outcomes of MS clinical disability as measured by the extended disability status scale (EDSS) and the relapse rate [Daumer 2009].novel quantitative MR technologies, which have provided insights into partial pathological aspects [not sure what you mean by this phrase "partial pathological aspects"] of the disease. It is now thought [known?] that primary demyelination, i.e. selective myelin destruction, is not restricted to focal MS lesions but occurs throughout the entire CNS parenchyma. Moreover such demyelination may be accompanied to a variable degree by remyelination and repair. [should add strategic or key references to some of the above statements]

The current state-of-the-art treatments are disease-modifying agents that at present are able to decrease relapse rates by 30% [Weiner 2009]. However, despite these advances, the field of MS still lacks specific markers to predict clinical relapses and disease progression. Novel immunotherapies are on the rise, but to date non-invasive technologies have failed to provide accurate, reliable tools to assess the state of myelination. Such technologies are particularly needed for testing drug efficacy or for monitoring treatment.

Imaging technologies provide potential instruments to investigate in vivo, real time changes that occur within the CNS over the broad spectrum of natural MS courses as well as during treatment. The application of conventional MR imaging techniques has already revolutionized the clinical practice in MS [Villenga 2009]. Conventional MR imaging derived numeric and volumetric measures are currently used as paraclinical markers in standardized diagnostic schemes [Polman, 2005]. However, despite the widely acknowledged potential application of MRI, presently available MR technologies have failed in meeting the critical goal of predicting disease progression and MS patient disability status. [ref?]

Conventional MR imaging measures also have been widely employed as presumptive surrogates across a broad spectrum of MS studies, ranging from pilot trials through multi-center pivotal phase III studies. However, the presently employed metrics fail to meet Prentice’s surrogate endpoint validation criteria for reliable surrogate markers in predicting downstream disease activity [Prentice, 1989]; and statistically appear to offer no more valid endpoint than that already offered by clinical outcomes of MS relapse rate and clinical disability as measured by EDSS [Daumer 2009].

[Maybe a new sub-heading here?]Quantitative MRI and Imaging Myelin In Vivo

In conventional MRI Imaging studies of MS patients, the WM tissue compartment that does not clearly show lesions or abnormalities is referred to as the Normal Appearing White Matter (NAWM) compartment. Moreover, lesions are not always well-defined areas of MR signal change with sharp boundaries, but often present ill-defined surrounding regions of signal deviation, [I don't like this term circumjacent signal deviation: can you try something simpler / clearer? I've made one suggestion.] the so called Dirty Appearing White Matter (DAWM). Outside of the lesions or DAWM, [do you mean in NAWM here? If so, just say so more clearly] a number of modern Newer, unconventional quantitative MRMRIare often aimed at the derivation of more specific and quantifiable information about MS pathology and its distribution. Quantitative in nature, Tthese quantitative MRI technologies have observed alteration in parameters that may be related tointrinsic tissue integrity myelination and axonal integrity myelination, indicative of a process of diffuse myelin damage and axonalneuronal lossnot restricted to lesion tissue but throughout the entire CNS parenchyma [Seewann, 2009; Vrenken, 2010].

Newer, unconventional MR imaging strategies are often aimed at the derivation of more specific information about MS pathology and its distribution. Progressive changes of intrinsic NAWM microstructure related to the tissue water diffusion characteristics were detected in primary-progressive MS (PPMS) patients in serial diffusion MR imaging study that quantified the apparent diffusion coefficient (ADC) [Schmierer 2006]. Werring et al. investigated the dynamic evolution of water diffusion measurements in pre-lesion NAWM in another serial diffusion MR imaging study and found a steady and moderate increase in ADC, followed by a rapid and marked increase at the time of lesion formation, and even a significant but milder increase in matched NAWM regions [what does this mean, "matched NAWM regions"?] [Werring, 2000].

Widespread tissue changes are found in NAWM of MS patients by measuring the magnetization transfer ratio (MTR). Those changes are mainly explained in terms of axonal damage and loss of one of the major pathological features of multiple sclerosis [Filippi, 1998]. [I don't understand the last half of this sentence: what is the major pathological feature you are referring to? Rewrite to make it clear] A histological analysis of the substrate of those imaging findings revealed that not only MTR but also T1 contrast ratio correlated strongly with axonal density, even in NAWM. However, defining T2 lesions revealed no correlation but a range of pathology, illustrating the low specificity of T2-weighted imaging [van Waesberghe, 1999[HK1]]. Early axonal pathology, can also be quantified with Proton (H+) spectroscopy (S) that provides chemical composition information at the level of metabolites. Early S studies have noted specific changes in metabolite signatures, not only within focal T2 lesions but even a deviation from normal in NAWM areas [Helms 2000]. A measure of ‘whole-brain’ N-acetylaspartate (WBNAA), a marker of axonal integrity, in particular confirmed widespread axonal pathology, largely independent of -visible inflammation in MS patients even in Clinically Isolated Syndrome. No correlation however, was found between the T2 lesion volumes and WBNAA concentrations [is this really a concentration value or a total integrated NAA value?] [Filippi, 2003].

Axons and their myelin sheath form an individually customized unit. [This English doesn't make sense. I'm not even sure what you mean here. Do you mean that the phrase "axonal loss" implies both dymelination and axonal degradation? This definitely needs to be re-worded.] However, axonal loss is not necessarily accompanied by demyelination, moreover both histopathologic changes seem to contribute independently to the appearance in conventional MR imaging scans. An imaging-histopathology case study confirmed that axonal degeneration could occur in the absence of myelin loss as a histopathologic correlate to abnormal MR findings in MS patients [Bjartmar, 2001]. [This last paragraph needs to be improved. Hard to understand.]

Single component T1 relaxation time was found to be abnormal in NAWM in established MS. When compared to MTR, quantitative T1 measurement was more sensitive in detecting subtle pathological change. No correlation was found between NAWM T1 changes and lesion abnormalities [what do you mean here: lesion volume? lesion signal characteristics?] suggesting independent underlying pathologic mechanisms [Griffin, 2002].

Fulton et al. determined the relationship between T2 lesion volume and both neurocognitive and physical disability in untreated relapsing-remitting multiple sclerosis. Despite some correlation to information-processing speed and verbal long-term memory, none of ten other neurocognitive examinations or the physical disability scales as rated according to the EDSS showed significant correlation with total lesion volume. This challenges the view that lesion volume measurement is a robust surrogate marker of impairment in patients with MS [Fulton, 1999]. [Last sentence is awkward and hard to understand]

MTR vs disability correlation?

These studies point to a new direction for MS MR imaging research: to move away from the lesion-centered view and to develop highly sensitive MR methods that accurately and quantitatively reflect the global disease burden even in areas that are apparently normal. Once such methods are developed, important hypotheses can be tested; for example, that such measures reflect the subtle underlying disease-determining pathology and will predict clinical changes in MS disease development, as well as transition towards chronic progression.

Imaging Myelin In Vivo

It seems obvious that novel MSMR imaging MRI strategies should focus on CNS tissue properties that are directly involved in specific disease-related pathological processes, especially the process of demyelination. Direct quantitative myelin assessment would haveimportant application in a variety of inflammatory, degenerative, and developmentaldisorders of the CNS as well as regeneration from injury and trauma.

Current conventional MR imaging MRI methods do not specificallyreflect myelin content. Within the battery ofunconventional MRMRI technologies, magnetization transfer imaging (MTI), diffusion tensor imaging (DTI), and single-component T1 and T2 relaxometry, i.e. the precise measurement of intrinsic magnetic tissue properties,are all thought to provide information related to myelin content. However, these measures are non-specific towards myelination. While quantitative MTI provides an estimate of the macromolecule-bound water fraction, this measure may also reflect inflammation processes[fix] with signal originating froman aspect of pathology in MS other than demyelination [Vavasour 1998, Gareau 2000]. Moreover a histological analysis of the substrate MTI findings revealed a strong correlation with axonal density [van Waesberghe, 1999[HK2]]. With regards to DTI, significant fractional anisotropy (FA) is observed even in non-myelinated nerve tissueindicating that axonal structures may at least in part be responsible for the signal generation [Beaulieu 2002]. [should also mention the crossing fiber problem of DTI which is obviously unrelated to myelination Moreover, the quantification of the integrity of myelinated WM fiber tracts reflected by FA measures may be directly affecteddistorted in regions where there are un-resolvable crossing fiber structures [Oochy 2007].

Finally, both T1 and T2 are influenced by a number of tissue structure and biochemical characteristics, including free water content and the presence of paramagnetic atoms such as iron.

Currently, multi-component relaxometric imaging (MCRI) provides the most direct means of quantifying myelin volume in vivo. In conventional T2 MCRI, the measured MRIMRI signal is decomposed into contributions from two or more water pools, which in brain tissue are attributed to an intra and extra-cellular water pool and water trapped between the hydrophobic bilayers of the myelin sheath [Whittall 1997[HK3]89, Menon 1991 -> ref to be fixed]. [is it really true that there are MCRI methods that use more than a two-pool model?] Through appropriate data acquisition, typically comprising multiple spin-echo images acquired over a range of echo times, and multi-exponential data analysis, maps of the T2 characteristics and volume fractions of each water pool may be estimated. As these volume fraction estimates show strong correlation with ‘gold-standard’ histologic assessments [Webb 2003 -> ref to be fixed, Laule 2006], MCRI has become the de facto standard for non-invasive myelin quantification. Unfortunately, established MCRI methods require lengthy imaging times while providing limited volume coverage. For example, the method of Whittall, MacKay and colleagues [Whittall 1997, Mädler 2006 -> ref to be fixed, ISMRM Proceedings?] requires approximately 16 minutes to acquire 16 contiguous slices with a voxel volume of 10mm3. These volume coverage, / spatial resolution, and / imaging time characteristics are comparable to more recent alternative techniques [Oh 2007 -> ref to be fixed] and make high resolution, whole-brain investigations challenging.

An alternative to MCRI is image combination, in which imaging data acquired with different acquisition parameters are combined so as to emphasize tissues with specific T2 relaxation characteristics [Whittall 1991 -> ref to be fixed, Jones 20042004 - ref to be fixed, Vidarsson 2005]. While multi-slice myelin fraction maps may be estimated in as little as 5 minutes [Vidarsson 2005], these methods are sensitive to T1 effects, depend on the short and long T2 selection criteria, and can suffer from low signal-to-noise ratio (SNR) efficiency.