Chapter 10 Magnetic Resonance Imaging and Spectroscopy: Insights into the Pathology and Pathophysiology of Multiple Sclerosis

10 Magnetic Resonance Imaging and Spectroscopy: Insights into the Pathology and Pathophysiology of Multiple Sclerosis Zografos Caramanos, A. Carlos Santos, and Douglas L. Arnold

Since the time that Ormerod, du Boulay, and McDonald wrote their chapter on the neuroimaging of multiple sclerosis (MS) for the first edition of this volume,1 continuing advances in the field of magnetic resonance (MR) and MR imaging (MRI) have made tremendous impacts in our understanding of this disease.2 Over the last few years, findings from (1) “conventional” MRI techniques (e.g., T2-weighted imaging, proton-density–weighted imaging, and T1-weighted imaging), as well as those from (2) more recently developed “nonconventional” MRI techniques (e.g., magnetization transfer imaging [MTI], diffusionweighted imaging [DWI], diffusion-tensor imaging [DTI], proton magnetic resonance spectroscopy (1H-MRS), and functional MRI [fMRI]) and (3) MRIbased estimates of brain and spinal cord atrophy, have converged with findings from other areas of MS research (e.g., histopathological and clinical research) to give us a more comprehensive picture of MS pathology and pathophysiology. Given that a number of excellent reviews have been written on this topic in recent years,3–9 the purpose of the present chapter is to describe some of the MRI techniques that are most commonly used in the study of MS and to summarize some of the main aspects of our MRI-based understanding of MS. First, however, we briefly review some of the relevant aspects of our current understanding of the pathology and pathophysiology of MS.

THE PATHOLOGY AND PATHOPHYSIOLOGY OF MULTIPLE SCLEROSIS The pathological hallmark of MS is the presence of demyelinating lesions (also referred to as MS plaques) within the central nervous system (CNS) that are 139

140 Multiple Sclerosis 2 disseminated in both space and time.10 Acute and subacute plaques are associated with acute inflammation and myelin breakdown. Chronic plaques are welldemarcated areas within the white matter that are hypocellular and characterized by myelin loss and astrocytic scar formation.11 Although usually described as being “relatively spared,” axons are injured and their density is decreased in both these types of demyelinating lesions. The overt, symptomatic “attacks” of MS that signal the usual initial relapsing-remitting (RR) stage of the disease are generally attributed to focal inflammation, which is associated with axonal injury and demyelination that result in slowing or blockade of axonal conduction. Conversely, the remission of symptoms during this stage is generally attributed to a combination of (1) the resolution of inflammation, (2) the insertion of new sodium channels across demyelinated segments of axons, and (3) the remyelination of axons. The majority of patients will eventually enter a secondary progressive (SP) stage of the disease in which there is progressive neurological disability that is speculated to result from (1) the eventual failure of remyelination, (2) gliosis, and (3) irreversible axonal injury and degeneration. Indeed, as reviewed by Rieckmann and Smith12 and by Bjartmar and Trapp,13 MS is no longer viewed as simply being a disease of inflammation and demyelination of the white matter; rather, axonal degeneration and neuronal damage throughout the brain are now accepted as being prominent features of MS, even early in the disease.

SOME RECENT MAGNETIC RESONANCE IMAGING–BASED INSIGHTS INTO MULTIPLE SCLEROSIS As we will soon see, insights into the pathology and pathophysiology of MS have been greatly advanced by information obtained using MRI. For example, it is now evident that the so-called normal-appearing white matter (NAWM; i.e., white matter that appears normal on gross pathological examination or on conventional MRI) in patients with MS is, in fact, far from normal; this is true both on appropriate histological analysis14,15 and on nonconventional MRI measures, including MTI,16 DWI,17 DTI,18 and 1H-MRS.19,20 Indeed, further changes in patients’ NAWM may become visible on these measures months or years before the lesions associated with MS become detectable on conventional MRI; this is true for MTI,21 DWI,22 and 1H-MRS.23,24 In addition to this pathology of NAWM, there is now growing evidence for a significant involvement of the normal-appearing gray matter (NAGM) of the cerebral cortex in MS. Again, this is true both on histological analysis25,26 and on nonconventional MRI measures, including MTI,16,27,28 DWI,29 and 1HMRS.30,31 Furthermore, there is now evidence from fMRI for adaptive cortical reorganization in patients with MS in the absence of neurological impairment, a finding that suggests that the extent of corticofunctional pathology is greater than that which is manifest clinically.20,32–34 Finally, there is also now MRI-based evidence that brain and spinal cord atrophy (which reflect destructive, irreversible pathology) are common, even early in the course of the disease.35 Of course, all these MR measures that have contributed to our increased understanding of MS are only surrogate markers for different aspects of the

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pathological changes that accompany the disease. To better appreciate what changes in these MR surrogates mean, we now review some of the MR techniques that are currently being used to study MS, first the conventional and then the nonconventional. We then go on to review some of the findings regarding MRI-based analyses of cerebral atrophy in patients with MS.

CONVENTIONAL MAGNETIC RESONANCE IMAGING TECHNIQUES The conventional MRI techniques used to study MS patients produce images that reflect the physicochemical state of protons that are present mainly in the water in the tissue that is being imaged. Contrast in such images is derived primarily from tissue-specific differences in the relaxation times, T1 (i.e., the time constant for the recovery of magnetization in the direction of the magnetic field) and T2 (i.e., the time constant for the decay of magnetization in the plane perpendicular to the magnetic field). (For a review of MRI theory and applications, see Gadian.36) Conventional MRI techniques include (1) T2-weighted imaging, (2) protondensity–weighted imaging, (3) fluid-attenuated inversion-recovery (FLAIR) imaging, (4) standard T1-weighted imaging, and (5) gadolinium-enhanced T1-weighted imaging, each of which is described later. Figure 10.1 presents examples of images obtained using these techniques in patients with MS.

T2-Weighted Imaging MR images are T2-weighted by allowing more time for signal decay to occur because of T2 relaxation during a relatively long echo time (TE). Signals from water protons located in tissues associated with longer T2 values decay less during a long TE; because of this, such tissues appear hyperintense on T2weighted images relative to tissues with shorter T2 values. T2 is prolonged in most pathologies that are associated with (1) inflammatory edema or tissue destruction (i.e., pathologies that increase bulk water, which has less interaction with macromolecules) or (2) gliosis in the white matter of the brain. For these reasons, MS lesions are hyperintense on T2weighted scans both in the early stages of the disease (i.e., when inflammation is most prominent) and in the later stages of the disease (i.e., when tissue injury and gliosis are more prominent).

T2-Weighted Imaging of Multiple Sclerosis Consistent with well-known pathological observations, the T2-weighted MR appearance of MS (see Figure 10.1) is primarily one of multiple, hyperintense white-matter lesions with periventricular predominance.37 (See Narayanan et al38 for an example of the probabilistic mapping of MS lesions.) Because of their exquisite sensitivity to subtle changes in water, T2-weighted hyperintensities can even identify regions of brain tissue that appear normal on gross

142 Multiple Sclerosis 2 Centrum Semiovale T2-Weighted Image

PD-Weighted Image

T1-Weighted Image

Gd-Enhanced Image

PD-Weighted Image

T1-Weighted Image

Gd-Enhanced Image

Lateral Ventricles T2-Weighted Image

Figure 10.1 Cross-sectional slices through the centrum semiovale (top) and the lateral ventricles of a patient with multiple sclerosis as seen on T2-weighted, proton-density– weighted (PD), T1-weighted, and gadolinium (Gd)-enhanced T1-weighted images. Of note at the level of the centrum semiovale: (1) Many of the hyperintense lesions that can be seen on the T2- and PD-weighted images are also seen as hypointensities on the T1-weighted images and (2) two of these lesions are still active and inflammatory (as evidenced by the ringlike gadolinium enhancement). Of note at the level of the lateral ventricles: (1) It is difficult to discriminate between the periventricular lesions and cerebrospinal fluid (CSF) on the T2-weighted image; (2) it is easy to discriminate between the periventricular lesions and the CSF on the PD-weighted image; (3) the extent of T2- and PD-weighted hyperintensity is much more extensive than the hypointensities seen on the T1-weighted images; and (4) there are no active, inflammatory lesions at this level (as evidenced by the lack of gadolinium enhancement). (Images courtesy the Canadian MS/BMT Study Group.)

pathological examination and that are only associated with a very subtle infiltration of inflammatory cells.39 Evolution of T2-Weighted Hyperintense Lesions New lesions that are seen on T2-weighted imaging (or on proton-density–weighted imaging [see below]) have a characteristic evolution.40 Typically they reach a maximum size in approximately 4 weeks, decrease in size over the next 6 to 8 weeks, and leave a residual T2-weighted abnormality37 that is a permanent record of tissue injury.5 For this reason, the total lesion volume that can be measured on such scans is often used as a surrogate measure of disease burden in MS. Furthermore, changes across time in the number and volume of lesions that are visible on T2-weighted imaging can be used as indicators of disease activity and of

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response to treatment. It should be noted, however, that such changes in volume consist partly of inflammatory edema that eventually resolves with an associated decrease in the volume of T2-weighted abnormality.41 Clinical Significance of T2-Weighted Hyperintense Lesions Although the relationship between T2-weighted imaging abnormalities and abnormal findings on histological examination is strong,37,39 the correlation between total cerebral T2-weighted lesion load and clinical disability at any given time is only modest.42,43 Nevertheless, the predictive value of T2-weighted lesions for the future development of clinically definite MS is strong, particularly over the long term. For example, Brex et al.44 recently published the latest results of an ongoing longitudinal study that had, at that point, followed a group of 71 patients for 14 years from the time of their initial episode of presumed CNS demyelination. They found that clinically definite MS eventually developed in 44 of the 50 patients with T2-weighted lesions at presentation (but in only 4 of 21 patients that had presented with normal MRI). Furthermore, the number and the volume of T2-weighted lesions at baseline, as well as the change in lesion volume over the first 5 years, correlated significantly with the patients’ degree of long-term disability as measured by Kurtzke’s Expanded Disability Status Scale45 (EDSS). The latter correlations were, however, of only moderate strength, suggesting that, on its own, T2-weighted lesion data cannot be used to make strong predictions about the prognosis in a patient who is known to have MS. The modest correlation seen between T2-weighted lesion load and concurrent clinical disability may be explained by several factors: (1) the lack of pathological specificity of T2-weighted abnormality; (2) the fact that neurological disability is not easy to quantify and that the instruments used to do so (primarily the EDSS) are limited in their scope (e.g., the EDSS is based primarily on ambulatory ability); (3) the fact that lesions in different CNS locations would be expected to correlate differently with disabilities in different spheres of CNS function (e.g., cerebral lesion load is only weakly related to the sensorimotor dysfunction that results from spinal cord lesions, a dissociation that increases as the MS disease process progresses46); and (4) the fact that lesions may not be the only pathological condition responsible for disability, particularly late in the disease when the effects of a neurodegenerative process may be more evident.13 The correlation between T2-weighted lesion volume and disability is also weakened by (5) the potential of the brain to functionally adapt to injury20,32–34 and (6) the fact that focal lesions have diffuse consequences.19,38 Thus it is not surprising that it has been difficult to demonstrate a strong, direct effect of the localization of cerebral T2-weighted lesions on specific EDSSmeasured functional impairments.47 Nevertheless, there are examples of specific functional deficits that have been shown to correlate with the T2-weighted lesion load that is present in the region of the CNS associated with those particular functions (e.g., olfactory function,48 sustained complex attention,49 verbal working memory,49 and visual function50). T2-Weighted Hypointensities In addition to the T2-weighted white matter hyperintensities that we have dealt with thus far, gray matter hypointensity on T2-weighted imaging of patients with MS (which is thought to reflect pathological iron deposition and brain degeneration) has also been described51 and shown to be related to clinical status52,53 and prognosis54 in MS.

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Proton-Density–Weighted Imaging Because both lesions and cerebrospinal fluid (CSF) are hyperintense on T2weighted images, the discrimination of periventricular lesions can be difficult. One way of increasing this discrimination is to acquire images with so-called proton density weighting.55 Such images do not actually reflect the density of protons; they do, however, have intermediate T1 (see below) and T2 weightings that, as shown in Figure 10.1, result in the CSF appearing dark (because its long T1 value predominates over its long T2) and lesions appearing bright (because their long T2 values predominate over their relatively shorter T1).

Fluid-Attenuated Inversion-Recovery Imaging Another means of increasing contrast between CSF and lesions is through the use of FLAIR images.55 This approach involves the use of an inversion pulse to suppress the signal arising from bulk water in the CSF.56 FLAIR images provide both (1) better discrimination between ventricular CSF and the periventricular T2-weighted hyperintensities that are associated with MS lesions and (2) increased contrast for lesions, particularly for those that are cortical or juxtacortical.57

T1-Weighted Imaging T1-weighted images are produced by shortening the amount of time between successive repetitions of water-proton excitation (i.e., the TR) and thereby allowing less time for water to regain its equilibrium magnetization. Water protons in tissues with a relatively short T1 recover more quickly and produce more signal at relatively shorter TR than do those in tissues with a longer T1. Protons in bulk water (e.g., in CSF or in tissue associated with either extracellular edema or with a loss of structural integrity) have a long T1 and, as shown in Figure 10.1, appear hypointense on T1-weighted sequences. T1-weighted images are less sensitive to changes in either water content or gliosis than are T2-weighted images. Nevertheless, acute, inflammatory lesions can sometimes be associated with so much edema that they can show substantial T1-weighted hypointensity. Chronic T1-weighted hypointensities are, however, much more specific indicators of tissue destruction than are T2weighted hyperintensities.58–60 The term black hole has been used to describe hypointense lesions on T1-weighted images.61 Given the fact, however, that this term is typically used to imply an association with irreversible tissue destruction, use of the term is probably best reserved for lesions that are chronically hypointense on T1-weighted imaging. Interestingly, only about 30 percent of new T2-weighted lesions evolve into chronic black holes.62

T1-Weighted Imaging of Multiple Sclerosis Given the increased pathological specificity associated with T1-weighted hypointensity, it is not surprising that lesions that show this feature on T1-

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weighted imaging are more strongly correlated to disability in MS than are lesions that are T1 isointense.43,59,61 In a recent study, Cid et al63 examined the relationship between RRMS patients’ (1) degree of lesion hypointensity on T1weighted imaging obtained at the time of an MS relapse, (2) change in EDSS score between the time of the relapse and 1 month later, and (3) amount of neuronal apoptosis induced on neuronal cultures by CSF obtained at the time of the relapse. They found a strong relationship between T1-weighted lesion hypointensity and both (1) poor recovery from relapse and (2) the amount of neuronal apoptosis induced by the CSF.

Gadolinium-Enhanced T1-Weighted Imaging As reviewed by Rovaris and Filippi,64 signal intensity on T1-weighted imaging can be increased with the injection of a chelated form of gadolinium (Gd), which interacts with water so as to shorten its T1 relaxation time. Normally, gadolinium does not cross the blood-brain barrier (BBB). However, focal inflammation in the CNS, as occurs during an MS attack, is often associated with an “opening” of the BBB.65 This opening allows gadolinium to pass through in a manner that is graded depending on (1) the extent of the associated increase in BBB permeability, (2) the dose of gadolinium administered, and (3) the interval between gadolinium injection and T1-weighted MR acquisition (i.e., the time available for the gadolinium to leak across the BBB).66,67

Gadolinium-Enhanced T1-Weighted Imaging of Multiple Sclerosis Importantly, the acute inflammatory process just described is transient. As a result, gadolinium only causes MS lesions to enhance for 2 to 6 weeks after they become detectable by conventional MRI.66,68 Thus a useful role for gadolinium-enhanced T1-weighted imaging is to help distinguish recently appearing inflammatory lesions from ones that are more chronic (and no longer associated with sufficient inflammation to result in gadolinium enhancement). Indeed, MRI assessment of disease activity in MS is often based on the number of gadolinium-enhancing lesions that are seen within a T1-weighted scan. The majority of enhancing lesions are “nodular,” but about 25 percent of lesions show ringlike enhancement.69 Such ring-enhancing lesions are associated with a more severe clinical outcome, and it has been suggested that they reflect a more destructive pathology.70 Most lesions that are gadolinium enhancing on T1-weighted images continue to be detectable on T2-weighted images after the acute inflammation and thus the gadolinium enhancement have both resolved. Conversely, it is generally believed that (1) most T2-weighted lesions in the central white matter of MS patients are associated with an initial, variable period of gadolinium enhancement on T1-weighted imaging and (2) gadolinium-enhancing lesions and T2weighted lesions represent two different stages of a single pathological process. There is evidence, however, to suggest that some of the lesions on T2-weighted images can develop independently of gadolinium enhancement,71 perhaps because of (1) ongoing low-grade inflammation that is not detected with

146 Multiple Sclerosis 2 gadolinium enhancement or (2) mechanisms other than inflammation that are responsible for progression in some existing lesions. Clinical Significance of Gadolinium Enhancement About 50 percent of patients with MS will have at least one gadolinium-enhancing lesion at any given time. Surprisingly, in what is referred as the clinicoradiological paradox, a large proportion of these lesions are not associated with clinical manifestations; indeed, on average, gadolinium-enhancing lesions occur about 10 times more often than clinical relapses.3,72,73 Despite the striking difference between the frequency of new gadolinium-enhancing lesions and the frequency of clinical exacerbations, there is still a strong relationship between them.74 Furthermore, the number of enhancing lesions on a single scan is (1) predictive of subsequent relapse rate and (2) correlated with both subsequent enhancing lesion activity and change in T2-weighted lesion load.75 Although the presence of one or more gadolinium-enhancing brain lesions is predictive of conversion to clinically definite MS,76 gadolinium enhancement is not a strong predictor of the development of cumulative impairment or EDSSmeasured disability.74 These findings are consistent with the hypothesis that different pathogenic mechanisms may be responsible for (1) the occurrence of relapses and (2) the development of long-term disability.

NONCONVENTIONAL MAGNETIC RESONANCE IMAGING TECHNIQUES Although the aforementioned conventional MRI techniques have allowed us to image MS lesions with much greater sensitivity, these techniques are not capable of fully characterizing and quantifying the extent of tissue damage in patients with MS. A number of recently developed MR techniques are better suited for such a role. These techniques include MTI, DWI, DTI, 1H-MRS, and fMRI, each of which are described in this section.

Magnetization Transfer Imaging Protons associated with molecules that are large and less mobile than water (e.g., the macromolecules that make up cell membranes) have a very short T2 and are not visible on conventional MRI; this is because their signals decay completely before conventional MRI data is acquired. The effect of these protons can, however, be observed indirectly by the phenomenon of magnetization transfer (MT).77 In MTI, appropriate radiofrequency pulses are applied to selectively saturate the magnetization of the bound protons. This saturated magnetization is then naturally exchanged with those protons that are found in the relatively “mobile” protons of CSF, extracellular water, and intracellular water (i.e., the protons that are normally observed by conventional MRI). This transfer of saturated magnetization to the MRI-observable free-water pool results in a reduction of the signal intensity from the observable protons to a degree that depends on (1) the nature and density of the macromolecules at a given location

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and (2) their interaction with bulk water. In the white matter of the CNS, the MT effect is dominated by the large surface area of myelin and changes in MT are considered more specific for demyelination78,79 than are changes in either T2 or T1 relaxation times. It should be noted that MT is also sensitive to changes in bulk water that are associated with edema,80–82 as well as to macromolecular changes associated with injury to other cell types (e.g., axons60,83). Thus changes in MT should not be considered pathologically specific. An important advantage of MTI is that it can be easily quantified by calculating the magnetization transfer ratio (MTR), which is the relative MRI signal intensity measured in the absence of a saturating pulse compared with the intensity that is measured in the presence of a saturating pulse (Figure 10.2). A low MTR indicates less exchange of magnetization between tissue macromolecules and the surrounding water molecules.

Magnetization Transfer Imaging of Multiple Sclerosis MTR values are typically reduced in MS lesions in a manner that is consistent with the degree of T1-weighted hypointensity,60,84,85 which, as we have seen, is related to tissue destruction; thus lesions that are larger and more destructive have lower MTR values than lesions that are smaller and less destructive. Furthermore, greater reductions in MTR values are seen in lesions that are more inflammatory.86 In addition to the MTR changes observed in MS lesions, numerous studies have also reported decreased MTR in the NAWM of MS patients compared with the white matter of healthy normal controls.21,85,87–89 Greater decreases in MTR are found in patients with SPMS than in patients with RRMS,90–92 and these decreases are related to both increasing EDSS-measured disability43,93 and increasing cognitive impairment.94 Temporal Evolution of Lesions on Magnetization Transfer Imaging Because MTI produces very reproducible MTR values for a given pulse sequence, it can be used to quantify the pathological evolution of MS lesions over time. Such longitudinal MTI studies have shown that, even before the detection of lesions is possible on proton-density—or T2-weighted images, MTR values have begun to decline in the NAWM that is found at these locations.21,95,96 Then, as lesions begin to demonstrate gadolinium enhancement on T1-weighted imaging, there is an explosive decrease in the lesional MTR values because of a combination of inflammatory edema and demyelination; this may be followed by either (1) a stabilization of lesional MTR values at these lower levels, (2) a recovery of lesional MTR values several weeks after the initial decrease, or (3) in a small number of lesions, a continuing decrease in local MTR values.97–99 The recovery of MTR or its continued decline may be related, respectively, to remyelination or to chronic activity of lesions. Thus MTR provides a potentially powerful tool for exploring lesion evolution in MS. Clinical Significance of Magnetization Transfer Imaging The fact that MTR focal abnormalities in NAWM develop before the appearance of lesions implies that MTI might provide information that could predict the future evolution of MS. This has led to the assessment of the predictive value of MTI in MS. For example, the prognostic value of MTI was recently examined by Santos et al.,100 who found that mean NAWM-MTR values were able to successfully predict

148 Multiple Sclerosis 2 MTon with saturation pulse

MToff without saturation pulse

% MTR Image [1⫺(MTon / MToff)] ⫻ 100

Figure 10.2 Examples of T1-weighted images obtained during a magnetization transfer (MT) study of a patient with multiple sclerosis. Images are obtained both with (MTon) and without (MToff) the presence of a saturation pulse, and MT ratios are calculated as shown. Note that the MToff image is just a standard T1-weighted image and MTR values are more reduced in lesions that are more hypointense on this image (e.g., compare A and B). (Images courtesy Dr. GB Pike.)

whether levels of disability would increase at 5-year follow-up in patients with relatively long-standing MS; importantly, MTR values within these individuals’ T2-weighted lesions could not predict such changes. In a related study of patients with a clinically isolated syndrome suggestive of MS, Iannucci et al.101 found that MTR values at the time of presentation were significant predictors of the development of clinically definite MS within the next 25 to 42 months (although not as strong predictors as these patients’ presenting T2weighted lesion volumes). On the other hand, Brex et al102 found that mean NAWM-MTR values in a similar group of patients could not predict whether these individuals would go on to have MS in the following 12 months (at which point these newly diagnosed MS patients still had normal NAWM-MTR). Thus although it is clear that MTI has prognostic value in MS, further studies are necessary to better characterize its strengths and limitations. Magnetization Transfer Imaging of Normal-Appearing Gray Matter As mentioned earlier, there is now an increasing appreciation that, at the microscopic level, there is substantial lesional pathology of the NAGM in patients with MS.25,26 Importantly, the lesions in NAGM are associated with much less inflammation and demyelination than are the lesions in the white matter.26 Perhaps related to this, as well as to their size and to their location (i.e., adjacent to CSF), these lesions in the NAGM are largely undetected by current conventional MRI techniques. Thus it is important that, in addition to the MTI changes in the lesional and normal-appearing white matter of patients with MS, reductions in MTR values have also been found in the NAGM of patients with MS relative to normal controls.16,27,28 Together these findings suggest that the pathological process that is at work in the brains of patients with MS is very diffuse and is not tissue specific.

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Diffusion-Weighted Imaging As reviewed by Cercignani and Horsfield,103 DWI allows for the in vivo measurement of the diffusion of water in the CNS resulting from Brownian motion. Because both the axolemma and the myelin sheath restrict water diffusion in nerve fibers,104 pathological processes (such as those at work in MS) that modify the integrity of such tissues can result in a loss of restricting barriers, thereby increasing the so-called apparent diffusion coefficient (ADC) of water. The ADC is a measure of the random displacement of water molecules in a particular direction. Because of the restricting entities found in biological tissues, ADC values in the CNS are lower than the diffusion coefficient of pure water (hence the term apparent diffusion coefficient of water). A measure of diffusion that is independent of the orientation of structures is provided by the mean diffusivity index, ¯ D. Also referred to as the trace or the directionally averD is the average of the ADCs measured in three orthogaged ADC (or ADCavg), ¯ onal directions. (For a review of DWI theory and applications see, for example, Schaefer et al.105) As reviewed by Filippi and Inglese,106 the pathology associated with MS modifies the water self-diffusion characteristics in the CNS by altering the geometry and the permeability of structural barriers that are found therein. The application of DWI techniques to the study of MS is appealing in that they can provide a quantitative estimate of the degree of fiber disruption and thus potentially provide information on the mechanisms that lead to irreversible disability in this disease.

Diffusion-Weighted Imaging of Multiple Sclerosis DWI studies have consistently shown that the ADC107,108 and ¯ D109 of water is (1) higher in MS lesions than in NAWM (Figure 10.3) and (2) higher in acute lesions than in chronic lesions. Such studies have also consistently demonstrated that mean ¯ D values are increased in the NAWM of MS patients compared with those observed in the white matter of healthy normal controls, a finding that holds true in the brain,17,110,111 the spinal cord,112 and the NAGM.16 ¯ D values have been shown to correlate with individual MS patients’ EDSS D values are higher in scores113–116 and disease durations.114–116 Furthermore, ¯ D patients with SPMS than in those with RRMS.116,117 Increases in lesional ¯ values correlate with hypointensity on T1-weighted images,116 and ringenhancing lesions on gadolinium-enhanced T1-weighted images have higher mean ¯ D values than do nodular-enhancing lesions,118 a pattern that corresponds to reported histopathological differences between these types of lesions.70 Interestingly, individual patients’ ¯ D values are not significantly related to their MTR values119 and are only moderately related to decreases in their 1H-MRSestimated brain tissue concentrations of N-acetyl aspartate120 (the relevance of which is explained in the section on 1H-MRS later in this chapter). These findings suggest that DWI provides information about different aspects of brain pathology in MS than do these other two imaging techniques (i.e., MTI and 1H-MRS).

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Figure 10.3 Cross-sectional slices through the lateral ventricles of a patient with multiple sclerosis as seen on proton-density (PD)–weighted, diffusion-weighted, and diffusiontensor imaging. The asterisks point out the lesions that are seen as (1) hyperintensities on ¯ ) values on diffusion-weighted PD-weighted imaging, (2) increased mean diffusivity (D imaging, and (3) decreased fractional anisotropy values on diffusion-tensor imaging. (Images courtesy JSW Campbell.)

Temporal Evolution of Lesions on Diffusion-Weighted Imaging Serial DWI studies have also been used to investigate the changes in NAWM that precede the development of acute MS lesions. For example, Rocca et al121 found that regions of NAWM that would subsequently become gadolinium-enhancing lesions had a significant increase in their mean ¯ D values starting 6 weeks before the appearance of enhancement. Furthermore, Werring et al,22 who acquired a years’ worth of monthly DWI scans in MS patients, observed (1) a steady and moderate increase in mean NAWM ¯ D values that was followed by (2) a rapid and marked increase at the time of gadolinium enhancement and (3) a slow decay after the end of enhancement. These two studies suggest that new focal lesions that are associated with an eventual breakdown of the BBB are preceded by subtle, progressive alterations in tissue integrity that are below the resolution of conventional MRI. Interestingly, Werring et al22 also found that there was a mild increase in the mean ¯ D values of NAWM regions that were contralateral and homologous to the NAWM regions that evolved into gadoliniumenhancing lesions (but that themselves did not become lesional), a finding that supports the concept that structural damage in lesions can cause disturbances in connected areas of NAWM.122

Diffusion-Tensor Imaging The ADC of water in biological tissue that has a regular and ordered microstructure depends on the direction along which it is measured (i.e., it is D (which has a magnitude but no direction) does not anisotropic).103 Thus ¯ provide a complete description of the diffusion phenomenon. A full characterization can, however, be obtained in terms of a tensor (i.e., a matrix of numbers) that describes the diffusion of water in three dimensions. From such a tensor it

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is possible to derive an index of diffusion anisotropy, the most commonly used index being that of fractional anisotropy (FA123,124). FA values obtained using DTI reflect the degree of cellular-structure alignment within the tissue that is being imaged. In the normal brain, FA images show a marked difference between (1) gray matter and CSF (which are both virtually isotropic) and (2) white matter (which has a variable degree of anisotropy).125 Maximum FA values are found in white matter regions characterized by a strongly ordered parallel arrangement of fibers, whereas much lower values are found in regions where white matter fibers have incoherent orientations or where fiber bundles cross. In DTI voxels that are normally full of highly ordered fibers, a relative decrease in diffusion anisotropy could signal structural disintegration within the CNS and could be used to detect both focal damage to major neuronal pathways and remote damage resulting from wallerian degeneration.126 It should be noted that DTI would be expected to be less sensitive to such damage in voxels that contained less-ordered tissue or that contained crossing fibers.

Diffusion-Tensor Imaging of Multiple Sclerosis Results similar to those found using DWI have also been found using DTI in patients with MS. For example, as shown in Figure 10.3, DTI measures of FA have been shown to be decreased in the NAWM of patients with MS18,29,126,127 and to be even more decreased in lesions,18,126,127 the greatest FA decreases typically being found in the most destructive (i.e., T1-weighted hypointense) D findings described earlier,119 there is also a lack lesions.18,126,127 Similar to the ¯ of relationship between individual MS patients’ FA values and their MTR values,128,129 further suggesting that diffusion imaging and MTI provide somewhat independent measures of brain pathology in MS. Interestingly, whereas significant negative relationships have been found between individuals’ EDSS scores and their FA values,18,29,127 abnormalities in FA have not yet been found in RRMS at the very early stages of their disease130 and mean FA differences have not yet been found among different MS subgroups.16,18,29 If this is not simply the result of a low sensitivity of DTI for detecting structural damage, this would imply that the disability in RRMS has a basis more in dysfunction than in loss of structural integrity.

Proton Magnetic Resonance Spectroscopy None of the water-based imaging methods that we have reviewed so far can provide pathological specificity for injury to a particular cell type. Pathological specificity for injury to neurons and neuronal processes (i.e., axons and dendrites) can, however, be provided by quantification of the neuronal marker compound, N-acetyl aspartate (NAA), using 1H-MRS.131 1 H-MRS is fundamentally different from the water-proton–based MRI techniques that we have discussed thus far in that it records signals that arise from protons in metabolites that are present in brain tissue at concentrations approximately 1000 times lower than that of tissue water.132 Whereas the signal-to-

152 Multiple Sclerosis 2 noise ratio and image resolution possible with these metabolite-based images are much lower than those for water-based images, the resulting images can provide chemicopathological specificity that is not possible with conventional MR images. The various approaches to in vivo 1H-MRS include (1) single-voxel 1 H-MRS studies (in which proton spectra are acquired from a single volume) and (2) 1H-MRS imaging (1H-MRSI) studies (in which proton spectra are obtained from multiple volume elements [i.e., voxels] at the same time). Proton Magnetic Resonance Spectroscopy Metabolites of Interest As shown in Figure 10.4, the water-suppressed, localized 1H-MRS spectrum of the normal human brain that is recorded at relatively long echo times (usually 136 to 272 ms) reveals three major resonance peaks (the locations of which are expressed as the difference in parts per million [ppm] between the resonance frequency of the compound of interest and that of a standard compound [i.e., tetramethyl silane]). These peaks are commonly ascribed to the following metabolites: (1) tetramethyl amines (Cho), which resonate at 3.2 ppm and are mostly cholinecontaining phospholipids that participate in membrane synthesis and degradation; (2) creatine and phosphocreatine (Cr), which resonate at 3.0 ppm and play an important role in energy metabolism; and (3) N-acetyl groups (NA), which resonate at 2.0 ppm and are comprised primarily of the neuronally localized NAA. A fourth peak, usually arising from either the methyl resonance of lactate

Figure 10.4 Proton-density–weighted magnetic resonance images through the centrum semiovale and the results of proton magnetic resonance spectroscopic imaging (1H-MRSI) in a normal control subject and in a patient with multiple sclerosis. The superimposed grid in each image represents individual 1H-MRSI voxels, and the large, thick, white box represents the entire 1H-MRSI volume of interest for that individual. The smaller, numbered boxes represent voxels of normal-appearing white matter (NAWM) and lesional brain tissue in the patient and normal white matter (NWM) in the normal control subject. The 1H-MRSI spectra from within each of these voxels is shown to the right of each image. The areas under the N-acetyl groups (NA) and tetramethyl amines (Cho) peaks (normalized to the area under the peak ascribed to creatine and phosphocreatine [Cr]) are shown above each spectrum. The spectra have been scaled so that the Cr peak in each of them has the same height. Note (1) the decrease in NA/Cr values from the patient’s NAWM voxel relative to the NWM voxels in the control subject, (2) the even greater decrease in lesional NA/Cr, and (3) the increased Cho/Cr in the patient’s NAWM voxel, which may be predictive of a soon-to-appear lesion in that location.

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(LA) or lipids (which both resonate at 1.3 ppm), is normally only barely visible above the baseline noise but can be detected in certain pathological conditions. Spectra acquired at shorter echo times (e.g., 30 ms) are better for detecting resonances that have a short T2 (e.g., lipids and inositol). Unfortunately, such short-T2 1H-MRS records broad, overlapping signals that complicate the quantification of such spectra. The simplest approach to the quantitation of 1H-MR spectra is to normalize the NA and Cho signal intensities to the signal intensity from Cr in the same voxel.131 Of course, this latter method does not provide absolute quantification, and, importantly, the resulting measures of relative concentration are only valid if the underlying pathology does not substantially affect the local concentration of Cr. Thus it is important that Cr concentrations are relatively constant throughout normal brain tissue and that they have also been shown to be relatively constant in both the lesions133,134 and the NAWM134–136 of patients with MS. It should be noted, however, that Cr values have been shown to decrease in acute133 and severely hypointense lesions136; thus it is inappropriate to normalize withinlesion NA and Cho values to within-lesion Cr values in either acute lesions or T1-weighted black holes. The limitations of ratio-based quantitation can be overcome by the various methods of semi-absolute quantification that have been developed.137,138 Unfortunately, such methods have their own limitations; for example, (1) they are dependent on many assumptions, (2) they can be difficult to carry out, and (3) they tend to have more variance than those based on ratios.

Proton Magnetic Resonance Spectroscopy of Multiple Sclerosis The resonance intensity that is ascribed to NAA is arguably the most important 1 H-MRS signal in the characterization of MS pathology because NAA is localized exclusively within neurons and neuronal processes such as axons and dendrites.139,140 Although NAA has been found in cell cultures of oligodendroglial cell lineage,141,142 this appears to be a phenomenon that is limited to cell cultures. Indeed, the specificity of NAA as an axon-specific marker in vivo, even in the presence of injury and significant density of oligodendroglial cell precursors, has been confirmed in a recent biochemical and immunohistochemical study of rat optic nerve transection.143 Furthermore, the validity of NAA as a surrogate for axonal density in MS has been confirmed in studies that correlated (1) findings from in vivo 1H-MRSI and histopathological analysis of cerebral biopsy specimens144 and (2) findings from HPLC and histopathological analysis of spinal cord biopsy specimens.145 N-Acetyl/Creatine For more than a decade 1H-MRSI–measured NA/Cr values have been used to quantify neuronal and axonal integrity in vivo in the brains of patients with MS.146,147 1H-MRS studies have shown that periventricular NA/Cr values are low in both the lesions and, to a lesser extent, the NAWM of patients with MS.19,122,147,148 Furthermore, studies have shown that patients with SPMS are more affected than those with RRMS.19,149 Interestingly, however, this latter finding seems to be related more to NA/Cr differences in NAWM than in lesions.19 Importantly, just as with MTI and DWI, 1H-MRSI–measured values of NA/Cr within the cortical NAGM of patients with MS have also been

154 Multiple Sclerosis 2 shown to be decreased relative to those in the cortical gray matter of healthy normal control subjects.30,31 Decreases in MS patients’ periventricular NA/Cr values are strongly related to both their disease duration and their EDSS scores.149 Importantly, the correlation between patients’ EDSS scores and their periventricular NA/Cr values is as strong or stronger than that of any other MRI measure,150 a relationship that becomes even stronger when EDSS scores are correlated with estimates of NA/Cr in pure periventricular NAWM.19,20 In addition to correlating with a patient’s EDSS scores (which are greatly influenced by a patients’ ambulatory status), periventricular NA/Cr values in MS patients have also been shown to be strongly related to their cognitive abilities.151 Other Metabolites In addition to NA, several other 1H-MRS resonance intensities are also important in understanding the MS disease process. For example, 1 H-MRS–observed Cho and lipid peaks are thought to provide important information regarding myelin breakdown in the MS disease process.23,148 Furthermore, the presence of myo-inositol has been proposed as a marker of glial cells and gliosis.30 Temporal Evolution of Lesions on Proton Magnetic Resonance Spectroscopy As with MTI and DWI, the earliest abnormalities that are visible on 1H-MRS occur months before the appearance of gadolinium-enhanced or T2-weighted lesions. For example, regions of NAWM that will go on to become lesions have been shown to be associated with locally increased levels of Cho24 and lipids,23 both of which are markers of abnormality in cell membranes. As newly developing lesions become detectable on conventional MRI, they are associated with focal inflammation, demyelination, and axonal injury—pathological processes that result in decreases to NA/Cr values,148,152–154 further increases to Cho/Cr values,148,152–154 and acutely increased LA/Cr values.148 Importantly, these NA-related decreases may persist chronically, particularly in the core of chronic lesions.148 On the other hand, the presence of LA is more common in lesions that are gadolinium enhancing154 and seems to resolve within weeks.148 Increases in Cho/Cr are pronounced in gadolinium-enhancing lesions153,154 and may remain elevated for years148 but eventually return to normal.136,148 Spatial Distribution of Proton Magnetic Resonance Spectroscopic Imaging Pathology Changes in 1H-MRSI metabolites are greatest in the core of lesions and decrease with increasing distance from their center.148 Importantly, they do not end at the edge of the T2-weighted abnormality but extend into the surrounding NAWM.148 For example, in the hyperacute phase of the lesion (i.e., when it is still expanding), both the decrease in NA/Cr and the increase in Cho/Cr can be found around the lesion in the NAWM that is well beyond the expanding T2-weighted abnormality. It is still not clear if the NAWM abnormalities in patients with MS result from (1) the sum of the remote effects of focal, lesional pathology or (2) an independent process that is more diffuse.

Functional Magnetic Resonance Imaging fMRI is another MR technique that differs fundamentally from the others discussed so far. For example, the blood oxygen level–dependent method of fMRI

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(a widely used approach to such an analysis) exploits the fact that hemoglobin and deoxyhemoglobin are magnetically different such that hemoglobin shows up better on MRI images than does deoxyhemoglobin.155 Brain activation is associated with increased blood flow and greater blood oxygenation that, in turn, produce an increased MR signal. fMRI involves (1) the acquisition of a series of such MR images of the brain in quick succession and (2) the statistical analysis of these images to quantify subtle changes in the functional state of the brain across time.

Functional Magnetic Resonance Imaging of Multiple Sclerosis Thus far, fMRI has been used in patients with MS to study abnormal patterns of brain activation that occur during the performance of simple motor tasks.20,32–34,156,157 These studies have shown that, as with other forms of brain injury, there is adaptive cortical reorganization in patients with MS as evidenced by extended, bilateral activation in motor-related regions (as opposed to the more constrained, mostly unilateral activation that is seen in normal controls during the tasks that have been used in these studies). Reddy et al156 combined findings from fMRI of a simple finger-flexion task with those from 1H-MRSI. As shown in Figure 10.5, they demonstrated that the extent of this functional reorganization (as expressed in the form of a lateralization index [LI] that reflected the degree of bilateral versus unilateral functional activation) was strongly related to the presence of axonal injury (as measured by decreased periventricular NA/Cr on 1H-MRSI). LI values have since been shown to be even more related to FA and NA/Cr values that are measured specifically within the periventricular NAWM,20 implying that

Normal Control

MS Patient 1 with low disability, normal NA/Cr

MS Patient 2 with low disability, low NA/Cr

Figure 10.5 Examples of fMRI activation maps (in white) obtained during a simple fingerflexion motor task and registered on to anatomical MR images for a normal control subject, an MS patient with normal periventricular NA/Cr, and an MS patient with abnormally low periventricular NA/Cr. Both patients were able to perform the task without difficulty and had low disability ratings. Note the larger, bilateral extent of functional activation in the patient with low NA/Cr. (Images courtesy Dr. PM Matthews.)

156 Multiple Sclerosis 2 NAWM changes are more specifically related to functional change than those in nonsegmented periventricular brain tissue (which contains NAWM, NAGM, and lesions). This reorganized corticomotor activation has been found in regions of the brain that are usually only activated in the execution of more complex motor tasks, suggesting that such activation reflects, at least in part, disinhibition of latent motor pathways that are “recruited” to limit any functional impairment related to the tissue damage associated with MS.157 Individual patients’ levels of activation within these recruited areas have been shown to be related to their (1) EDSS scores, (2) disease duration, (3) extent and number of spinal cord lesions, (4) brain and spinal cord MTR, (5) whole-brain ¯ D, and (6) and whole-brain FA.157 Together these findings suggest that compensation resulting from adaptive cortical changes can contribute to sustaining motor functions during the early stages of MS; as a result, the actual extent of corticofunctional pathology in patients with MS may be greater than that which is clinically evident.

MAGNETIC RESONANCE–BASED ASSESSMENT OF BRAIN ATROPHY Atrophy of the brain or spinal cord at postmortem examination is one of the pathological hallmarks of irreversible CNS damage. As reviewed by Simon,35 with the advent of MRI it is now possible to assess CNS atrophy in vivo using a variety of measures that include, for example, (1) ventricular enlargement, (2) gray and white matter volumes, and (3) the use of more global measures such as (a) the brain parenchymal fraction (BPF; i.e., the ratio of brain parenchymal volume to the total volume within the surface contour of the brain)158 or (b) the brain to intracranial capacity ratio (BICCR; i.e., the ratio of brain parenchymal volume within the surface contour of the inner table of the skull).159

Atrophy in Multiple Sclerosis CNS atrophy in patients with MS has been documented since the original autopsy examinations of such individuals, with atrophy having been shown to reflect (1) injury and loss of both neurons and their processes and oligodendrocytes and the myelin that they produce, as well as (2) changes in the supporting matrix that result from the contraction of glial tissue.35 Until recently, such atrophy has generally been thought to occur late in the disease; 35 this view has changed, however, with the development of neuroimaging techniques that have demonstrated atrophy in the majority of patients with MS, even at very early stages of the disease.158,160,161

Clinical Significance of Central Nervous Systom Atrophy The average amount of accumulated spinal cord atrophy has been shown to be greater in patients with SPMS than in patients with RRMS.162–165 Similarly, as

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Patient with SPMS

Figure 10.6 Cross-sectional slices through the lateral ventricles of a normal control subject, a patient with relapsing-remitting multiple sclerosis (RRMS), and a patient with secondary progressive multiple sclerosis (SPMS) as seen on T2-weighted imaging. Note (1) the high degree of atrophy seen even in the early stage of the disease (as evidenced visually by the ventricular enlargement in the RRMS patient as compared with the control subject) and (2) the even greater degree of atrophy that is seen in the secondary progressive stage of the disease (as evidenced visually by the sulcal enlargement, the decreased volume of the white and gray matter, and the further increased ventricular enlargement in the SPMS patient).

a group, SPMS patients have also been shown to have smaller brain volumes and larger lateral ventricles than RRMS patients159,166 (Figure 10.6). Measures of brain stem and upper spinal cord atrophy have been shown to be strongly correlated with EDSS scores in patients with MS.164,167 This may be due in part to the fact that atrophy in these regions can be related to wallerian degeneration following damage to the cerebrum, to the spinal cord, or to both. Moderate correlations have also been found between individuals’ EDSS scores and their degrees of (1) callosal atrophy,42,168 (2) cerebral white matter atrophy,162 and (3) ventricular enlargement.42,169–172 On the other hand, correlations between EDSS scores and brain parenchyma–based estimates of atrophy have been variable, ranging from strong158,159,173 to statistically insignificant.115,174,175 It should be noted that MRI measures of brain volume and atrophy have been also shown to be significantly related to the presence of depression,176 impaired quality of life,177 and cognitive decline173,178 in patients with MS.

Rate of Atrophy Brain atrophy develops at a remarkably high rate in patients with MS.35 For instance, Fox et al179 showed that (1) the yearly rate of cerebral atrophy in their MS group (0.8% per year) was more than twice that of normal controls (0.3%) and (2) the yearly rate of ventricular enlargement in patients was almost five times greater than in the controls (1.6 versus 0.3 cubic centimeters per year). Furthermore, Simon et al180 studied 85 RRMS patients with mild-to-

158 Multiple Sclerosis 2 moderate disability over the course of 2 years and found that (1) the volume of the lateral ventricles increased at a rate of 5.5 percent per year and (2) the area of the corpus callosum decreased at a rate of 4.9 percent per year. Analysis of a subset of these patients (n = 72) found a yearly decrease of 0.61 percent in their BPF, which translated to a yearly loss of approximately 8 cubic centimeters per year.158 There is some preliminary evidence to suggest that the rate of atrophy in patients with RRMS differs from that in patients with SPMS in a regionspecific manner.181,182 For example, SPMS patients seem to have a significantly faster rate of atrophy around the ventricles than patients with RRMS, suggesting a greater relative volume change along the long projection tracts.182 On the other hand, the rate of spinal cord atrophy has been reported to be faster in patients with RRMS.164 These findings suggest that CNS atrophy may not be a uniform process and that different regions may have distinct responses to disease progression.

Relationship of Atrophy to Other Magnetic Resonance Imaging Measures As might be expected, CNS atrophy in patients with MS has been shown to be related to many of the other MRI measures that we have reviewed in this chapter. For example, patients’ supratentorial brain volumes have been shown to be significantly related to their load of hypointense lesions on T1weighted imaging.183,184 Similarly, patients’ numbers of gadolinium-enhancing T1-weighted lesions have been shown to be well correlated with an increase in their ventricular size,166,185 especially in patients with a high proportion of ringenhancing lesions.185 Furthermore, in two related longitudinal studies, Luks et al172 found that patients’ total numbers of new gadolinium-enhancing lesions were related to their monthly changes in ventricular volume and Simon et al180 found that the degree of cerebral atrophy observed over a 2-year period (as indicated by ventricular enlargement and callosal atrophy) was greater for patients who had entered their trial with gadolinium-enhancing lesions. It should be noted, however, that not all studies have found a relationship between gadolinium enhancement and atrophy.186 The relationship of atrophy to T2-weighted lesion load has also been somewhat inconsistent. For example, whereas some groups have found a significant relationship between their patients’ T2-hyperintense lesion load and the (1) degree of ventricular enlargement,162 (2) callosal atrophy,162 and (3) overall brain volume,90 others have been unable to find a significant relationship with their patients’ supratentorial brain volumes.183,184 The relationship between cerebral atrophy and the newer, nonconventional MRI measures seems to be more consistent. For example, brain volume has been shown to correlate with both (1) MTR values within normal-appearing D values within the brain parenchyma.115 Furthermore, brain tissue90 and (2) ¯ 159 Collins et al found that cerebral atrophy (as measured by BICCR values) was correlated with periventricular axonal injury (as measured by decreases in 1HMRSI NA/Cr values) in their group of patients with SPMS. BICCR values in their group of mildly disabled patients with RRMS were not reduced relative to their normal control group, even though this group of patients did have

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significantly reduced NA/Cr values. Together these findings suggest that microscopic and biochemical changes in the brains of patients with MS are related to the decreases in brain volume found in such individuals; importantly, however, there seems to be a decoupling between axonal damage and atrophy in the very early stages of the disease.

Caveats It should be noted that current MRI analysis techniques allow for the measurement of small changes in volume on the order of 0.2 percent of total brain volume, changes of a magnitude much smaller than those that can be identified on gross pathological examination. Thus far it has been tempting to assume that these small changes in brain volume have the same pathological significance as gross atrophy postmortem and suggest that they provide a measure of a specific pathological feature such as axonal loss. Unfortunately, it is not clear that this is always the case; for example, myelin loss, glial- and matrix-related changes, and shifts in water distribution all occur in MS and may be associated with volume changes of this magnitude. Although it is clear that volume measurements must contribute in some way to estimating the full extent of irreversible axonal damage in MS, further investigations are required to understand the precise pathological significance of atrophy and the mechanisms that contribute to its progression.

SUMMARY AND CONCLUSIONS As we have seen, our understanding of the pathology and pathophysiology of MS has been greatly advanced by information obtained using MRI. For example, the conventional MRI techniques that were described earlier have greatly increased the sensitivity with which lesions can be detected. In addition, they have also provided us with a great deal of in vivo information regarding the spatial distribution, temporal dynamics, and clinical significance of these lesions. Furthermore, the newer, nonconventional methods of MR acquisition and analysis that were also described in this chapter have allowed us to quantify in vivo the microscopic, molecular pathology; the biochemical changes; the corticofunctional adaptations; and the progressive atrophy that may occur in the brain and spinal cord of patients with MS. Based primarily on the findings from the nonconventional MRI methods that were reviewed in this chapter, a number of important insights regarding MS pathology and pathophysiology have become evident. First, there are significant pathological changes in the otherwise normal-appearing gray and white matter of patients with MS, consistent with the emerging view that the pathology in this disease is relatively diffuse and not specifically tied to the white matter. Importantly, the amount of such microscopic, pathological change in any individual patient is highly related to their degree of concurrent disability and is also related to future changes in their disability. Second, certain pathological changes in NAWM can foreshadow the appearance of the focal lesions

160 Multiple Sclerosis 2 that are classically associated with MS. Third, there is significant corticofunctional reorganization that takes place in the brains of patients with MS, a reorganization that, at least at the early stages of the disease, seems to have the potential to be functionally adaptive and to compensate for some of the effects of the ongoing neuropathological processes that are associated with the disease. Fourth, all these changes are occurring in parallel with the progressive CNS atrophy, even in the early course of the disease. Although findings from MRI have taught us much about the MS disease process over the 16 years since the publication of the first edition of this volume, we still have much to learn about the spatial and temporal dynamics of the lesional and nonlesional pathological tissue that characterizes the MS brain. It is the hope and goal of the present authors that a multimodal, multiparametric approach to the analysis of longitudinal data obtained from a combination of conventional and nonconventional imaging techniques will become commonplace. Such an approach still has much to teach us about the spatial and temporal characteristics of the MS disease process, and the knowledge that it will provide us with will undoubtedly lead to better methods of treating and monitoring MS.

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