Abstract
Objective: To identify differences in pathology between the principal clinical phenotypes of MS using conventional and magnetization transfer (MT) MRI.
Methods: T1-weighted and T2-weighted images as well as MT scans were obtained from 20 controls, 21 patients presenting with clinically isolated syndromes suggestive of MS, and 93 MS patients with relapsing-remitting, secondary progressive, benign, or primary progressive course. Metrics considered: hypointense T1 and T2 lesion volumes, average lesion MT ratio, average brain MT ratio, peak height and position from MT histograms.
Results: MS patients had lower MT metrics than controls. Patients with clinically isolated syndromes had MT measures similar to controls, whereas primary progressive MS patients had lower histogram peak height with normal peak position. Relapsing-remitting MS patients had lower MT measures, higher T2 lesion load and ratio of hypointense T1 to T2 lesion volumes than patients with clinically isolated syndromes, and lower MT ratio and peak height than benign MS patients. Benign MS patients were similar to controls and patients with clinically isolated syndromes. Secondary progressive MS patients had the lowest MT measures and highest lesion loads.
Conclusions: Pathology in patients with clinically isolated syndromes is confined to modest tissue damage in the lesions seen on T2-weighted scans. Severe damage is important for the later development of disability. However, microscopic damage in normal-appearing white matter may be a major contributor to disability in primary progressive MS.
Conventional T2-weighted MRI is widely used to assess lesion burden in MS.1-3 However, T2 lesion burden correlates poorly with clinical disability in established MS,1,2 and, as a consequence, it cannot differentiate the major clinical phenotypes of the disease.4-10 Patients with relapsing-remitting (RR), secondary progressive (SP), or benign (B) MS, despite significantly different disease dynamics and disability,4,5 have similar T2 lesion burden,4,6 whereas the lowest burden is seen in primary progressive (PP) MS patients,7-10 whose clinical evolution is usually unfavorable.
There are at least two possible explanations for this clinical–MRI discrepancy. First, conventional T2-weighted imaging gives little information about the pathologic substrate of MS lesions, which ranges from edema and inflammation to severe demyelination and axonal loss.11-14 Although all these pathologies are characterized by an increased water content, giving hyperintense lesions on T2-weighted MR images, they can result in very different neurologic outcomes. Secondly, lesion load estimates from conventional imaging are known not to give a complete picture of the burden of disease.15 Several studies, using different MR techniques, showed that microscopic damage in the normal-appearing white matter (NAWM), which is not detected by conventional imaging,16-20 may account for some of the disability in MS.16,20
Recently, several MR techniques, with the potential for higher pathologic specificity, have been used to monitor MS.2,15 Hypointense MS lesions on T1-weighted scans are thought to represent areas with severe tissue disruption.13,21-23 Low magnetization transfer (MT) ratio (MTR) indicates a reduced capacity of the macromolecules in brain tissue to exchange magnetization with the surrounding water molecules, thus reflecting damage to myelin or to the axonal membrane.24 In MS patients, T1-weighted hypointense MRI lesion load22,23 and average lesion MTR25 correlate better with physical disability than does the volume of abnormalities on conventional T2-weighted MRI. MTR is also reduced in the NAWM of MS patients.17,18 Estimates of the amount and severity of microscopic and macroscopic disease burden can be obtained using MTR histograms,26 which may provide a more global picture of disease burden in MS. The aim of the current cross-sectional study was to use MR metrics derived from T2-weighted, T1-weighted, and MT scans to identify possible differences in pathology between the principal clinical phenotypes of MS.
Patients.
Patients included had clinically definite MS27 for at least 2 years. Their clinical disease phenotypes were classified as RRMS (clearly defined disease relapses with either full recovery or sequelae, but without disease progression during the periods between the relapses), SPMS (initial RR course followed by progression with or without occasional relapses, minor remissions, and plateaux), PPMS (disease progression from onset with occasional plateaux and temporary minor improvements), or BMS (patients fully functional in all neurologic systems 15 years after disease onset) according to the criteria of Lublin and Reingold.28 Patients with a clinically isolated syndrome (CIS) suggestive of MS, with the first clinical attack in the preceding 3 months and at least four focal abnormalities on T2-weighted scans, were also included. None of the patients had had immunosuppressive or immunomodulating treatments for at least 12 months before entry into the study. In addition, they had neither relapses nor steroid treatment during the preceding 3 months. At the time MRI was performed, patients were assessed neurologically by a single physician who was unaware of the MRI results, and disability was measured using the Expanded Disability Status Scale (EDSS).29 Sex- and age-matched controls with no previous history of neurologic diseases and with a normal neurologic examination were also studied. Local Ethical Committee approval and written informed consent from all the patients and controls were obtained before study initiation.
Methods.
MRI.
Brain MRIs were obtained using a scanner operating at 1.5 tesla (Magnetom SP63, Siemens, Erlangen, Germany). During a single session, the following scans were performed without moving the patient from the scanner: dual-echo conventional spin echo (CSE) (repeat time [TR] = 2,400, first echo time [TE] = 30, second TE = 80, number of acquisitions = 1); T1-weighted CSE (TR = 768, TE = 15, number of acquisitions = 2); and 2D gradient echo (GE) (TR = 600, TE = 12, α = 20°) with and without a saturation pulse. The radiofrequency saturation pulse was 1.5 kHz below the water frequency, with a gaussian envelope of duration of 16.4 msec, a bandwidth of 250 Hz, and an amplitude of 3.4 × 10−6 tesla. For the double-echo and T1-weighted scans, 24 contiguous interleaved axial slices were acquired with 5-mm slice thickness, 256 × 256 matrix, and 250 mm field of view, giving an in-plane spatial resolution of approximately 1 × 1 mm. MT scans were obtained with the same acquisition parameters, except for the number of slices (20). The set of slices for the MT images was positioned to obtain the same central 20 slices as for the dual-echo and T1-weighted scans. The slices were positioned to run parallel to a line that joins the most infero-anterior and infero-posterior parts of the corpus callosum according to published guidelines,30 with double obliquing where necessary.
From the two GE images, with and without the saturation pulse, MTR images were derived pixel-by-pixel according to the following equation: MTR = (M0 − MS)/M0 × 100%, in which M0 is the signal intensity for a given pixel without the saturation pulse and MS is the signal intensity for the same pixel when the saturation pulse is applied.
Image review and quantification.
Lesions were first identified by agreement of two experienced observers, without knowing to whom the scans belonged, on the hard copies of the first echo of the dual-echo scans and on the T1-weighted scans; the second echo of the dual-echo scans was always used to increase confidence in lesion identification. For T1-weighted scans, only areas with a signal intensity between that of the gray matter and that of the CSF and with corresponding lesions on both echoes of the dual-echo images were considered as hypointense lesions. Lesion volume measurements and average lesion MTR were then performed by a single observer, again without knowing to whom the scans belonged. A semi-automated segmentation technique based on local thresholding was used for lesion segmentation,31 using the marked hard copies as a reference. The intra-observer coefficient of variation of this technique for measuring MRI abnormalities in MS is less than 5%.32
Using the MTR images, average lesion MTR was calculated for each patient according to the following formula: 
where N is the number of lesions in that patient Ai is the area of lesion i; and MTRi is the average MTR in lesion i.
Brain histograms were obtained from all MS patients and controls following the image postprocessing method described in detail by van Buchem et al.26 The entire brain was first manually segmented from the MTR images by a single observer, without knowing to whom the scans belonged, and MTR histograms (with bins 1% in width) were then created. We excluded from the analysis all the pixels with MTR values lower than 10% to eliminate CSF and points corresponding to noise alone. To correct for the between-patient differences in brain volume, each histogram was normalized by dividing it by the total number of pixels included. The following measures were derived for each histogram: the relative peak height (i.e., proportion of pixels at the most common MTR value), the peak position (i.e., the most common MTR), and the mean brain MTR, as well as MTR25, MTR50, and MTR75, which indicate the MTR at which the integral of the histogram is 25%, 50%, and 75% of the total area under the curve, respectively. All histogram-derived measures were from the whole of the brain tissue, thus including both MS lesions and normal-appearing white and gray matter. MTR is reduced in both lesions and the NAWM in MS patients,17,18,24,26 and this is reflected in the characteristics of the histogram. Focal changes in the normal tissue are expected to decrease the peak height and increase the number of pixels with low MTR values without greatly affecting the peak position. The more severe the pathologic process in these focal abnormalities, the greater will be the relative increase in the histogram at low MTR. Mild but more widespread changes to the white matter would cause a larger reduction in the peak height, accompanied by a broadening of the peak at its left-hand side, because more of the tissue is affected, but with little or no increase at very low MTR. Thus the reduction in the mean MTR might be more subtle. In an extreme case, where most of the white matter is diffusely affected, it would also be possible for the peak position to move to the left because little tissue would remain at truly normal MTR.
Statistical analysis.
Differences in MT metrics between MS patients and controls were evaluated using the two-tailed Student’s t-test for nonpaired data. Differences in hypointense T1 and hyperintense T2 lesion loads as well as the ratio of hypointense T1 to hyperintense T2 lesion loads (denoted as the T1/T2 ratio throughout this article) between the different MS clinical phenotypes were assessed with a one-way analysis of variance. The following five pairwise comparisons were decided a priori (a priori contrasts): controls versus CIS, controls versus PPMS, CIS versus RRMS, RRMS versus BMS, and RRMS versus SPMS. The numbers of a priori contrasts were determined by the available degrees of freedom, and their nature was decided based on the clinical evolution of the disease, i.e., MS onset is either as a CIS or as PPMS; a CIS evolves to RRMS, which can then evolve to SPMS or stabilize to BMS.
Results.
Clinical data.
A total of 114 patients (69 women and 45 men) were included in the study. Their mean age was 34.9 years (SD 9.1), median disease duration 6.0 years (range 0 to 30), and median EDSS score 3.0 (range 0.0 to 8.0). Forty-two patients were classified as having RRMS, 30 with SPMS, 10 with PPMS, 11 with BMS, and 21 with a CIS. Twenty healthy volunteers (12 women and 8 men; mean age 33.8 years [SD 5.0]) served as controls. The demographic and clinical characteristics of the patients in five MS phenotypes studied are reported in table 1.
Demographic and clinical characteristics of the patients with the different MS clinical phenotypes studied
Magnetic resonance imaging results.
No abnormalities were found on the scans from controls. The hyperintense T2 and hypointense T1 lesion loads together with the T1/T2 ratios from the five clinical phenotypes are reported in table 2. MT metrics from the controls and the different clinical MS phenotypes are reported in table 3. Compared with controls, the entire cohort of patients with MS had lower average brain MTR (43.6 ± 3.2% versus 45.8 ± 0.9%, p = 0.002), peak height (59.8 ± 5.8 versus 63.2 ± 3.4, p = 0.05), and peak position (42.6 ± 3.1% versus 43.9 ± 1.1%, p = 0.05). The p values of the five a priori contrasts for the different MRI measures are reported in table 4. Patients with a CIS had MT metrics similar to those of controls (see tables 3 and 4⇓), whereas patients with PPMS had significantly lower histogram peak and MTR25 (see tables 3 and 4⇓). Interestingly, the histograms of patients with PPMS had the lowest peak height and the highest peak position of all five MS clinical phenotypes (see table 3). Patients with PPMS also had the highest average lesion MTR (see table 3). Patients with RRMS had significantly lower MT metrics as well as higher T2 lesion load and T1/T2 ratio (see tables 3 and 4⇓) than did patients with a CIS, as well as lower MTR, MTR25, and peak height than did patients with BMS (tables 3 and 4⇓,), whose MT histogram metrics were similar to those of controls and patients with a CIS (see table 3). Patients with SPMS had the lowest MT metrics and the highest lesion loads of all five clinical MS phenotypes studied (see tables 2 to 4⇓⇓). Compared with patients with RRMS, they had significantly lower MTR25 and higher hypointense T1 and hyperintense T2 lesion loads and T1/T2 ratio (see tables 2 to 4⇓⇓). Average brain MTR (p = 0.09) and MTR50 (p = 0.07) also showed a trend, but the difference was not significant.
Lesion loads on T2- and T1-weighted scans in the different clinical phenotypes studied
Magnetization transfer metrics in controls and in the different MS phenotypes studied
Results of the a priori pairwise comparisons between controls and different MS clinical phenotypes
The figure shows histograms derived from the entire patient population in each clinical group, where they are compared with the control group. The MTR values for the image pixels of all the patients within each group were pooled before the histograms were formed. Note that these average histograms do not necessarily show the same trends as the statistics presented in tables 3 and 4⇑ because the average histograms also reflect heterogeneity within the populations of the separate groups. FIGURE
Figure. Magnetization transfer histograms for all subjects within the individual control and patient groups (black lines). In each graph the histogram derived from controls is shown as a gray line for comparison. (a) Patients at presentation with clinically isolated syndromes suggestive of MS. (b) Patients with benign MS. (c) Patients with primary progressive MS. (d) Patients with relapsing-remitting MS. (e) Patients with secondary progressive MS. MTR = magnetization transfer ratio.
Discussion.
In this cross-sectional study, we compared the ability of several MRI measures to distinguish the major clinical phenotypes of MS. We found that MT-derived metrics and, to a lesser extent, hypointense lesion load on T1-weighted scans match the overall clinical picture of the disease better than do T2 lesion load and, more importantly, give some clues toward understanding the mechanisms underlying the clinical manifestations of the disease.
Several previous studies found poor correlations between clinical and MRI findings in MS.1-3 Of the several possible factors that could lead to such poor correlations, we believe that there are two major ones. First, MS abnormalities as measured on T2-weighted scans are pathologically aspecific.13 This is not the case for T1 hypointense abnormalities and reduced MTR, which have been shown to correlate more strictly with severe axonal loss and demyelination in postmortem studies.21,33 Because demyelination and axonal loss are probably the most disabling aspects of MS, it is not surprising that such techniques contribute to the understanding of MS evolution. Second, T2 lesion load gives inaccurate estimates of the total pathologic burden, which has been demonstrated to extend into the white matter that is considered normal on conventional scans, and NAWM damage may contribute toward disability.17,20 This limitation is overcome by the use of MT histograms, which give information about both the macro- and microscopic aspects of MS pathology in the brain.
The study inclusion criteria used for patients with a CIS at presentation meant that their T2 lesion load was comparable with some of the other clinical phenotypes of established MS, characterized by longer disease durations and more severe disabilities. However, patients with a CIS at presentation had MT histogram metrics completely overlapping those of controls, a relatively high average lesion MTR, and the lowest T1/T2 ratio. This indicates that the pathology in these patients was confined to the discrete lesions visible on the T2-weighted scans and that the degree of tissue destruction occurring within these lesions was modest. These observations have two major implications. First, they suggest that new lesion formation in clinically eloquent areas is the major mechanism underlying the disability in the early phases of the disease. Second, the relatively good preservation of the brain tissue both inside and outside visible T2 lesions suggests that an early treatment preventing new tissue damage may favorably alter the subsequent clinical evolution. This is of particular interest for patients with a CIS and multiple T2-weighted abnormalities at presentation, who have been shown to have the highest risk of developing clinically definite MS and moderate to severe disability in the subsequent few years.34-37
When patients with a CIS at presentation have subsequent relapses, they are said to enter the RR phase of the disease. The demonstration that patients with RRMS have greater T2 lesion load and T1/T2 ratios together with lower MT metrics compared with patients with a CIS at presentation indicates that the subsequent disease evolution involves not only lesion accumulation but also further damage within T2-visible lesions and in the NAWM. This is confirmed by the comparison of patients with the two disease phenotypes that are considered to be the two possible evolution paths of the RR phase, one unfavorable (i.e., SPMS) and the other favorable (i.e., BMS). The demonstration in patients with BMS that the height of the MT histogram peak was significantly greater and that the average brain MTR and MTR25 were higher than they were in patients RRMS indicates that the pathologic changes occurring in their lesions and the NAWM were mild. This is also confirmed by the fact that patients with BMS had an MT histogram similar to controls and patients with a CIS, although they had higher T2 lesion load than did the latter group. Clearly, it remains to be seen whether the more favorable evolution to BMS is the result of only a mildly damaging pathologic process or of more efficient reparative mechanisms. In SPMS the further deterioration of the MT histogram metrics and the increase of hypointense T1 and hyperintense T2 lesion loads and T1/T2 ratio suggests that the intrinsic lesion damage, their accumulation over time, and microscopic damage within the NAWM are all important in determining the evolution from RRMS to SPMS. However, caution must be exercised before drawing firm conclusions about the evolution of MS because this is a cross-sectional study. Many of these issues would be better addressed in a longitudinal study of disease progression.
The nature of the damage seems to be different in PPMS, the other progressive form of the disease. In these patients we confirmed that there are few lesions seen on T2-weighted scans.7,8 We also showed that a minority of them appear as hypointense on T1-weighted scans, in concordance with the observation that the average lesion MTR is highest in these patients. Clearly, these pieces of evidence do not fit in with the typical severely disabling course of this clinical phenotype. However, the MT histograms may shed some light on this discrepancy. We found that patients with PPMS have a histogram peak height lower than all the other clinical phenotypes, with an almost normal average brain MTR and normal peak position. This suggests that such patients have a markedly reduced amount of residual, truly normal brain tissue. Because patients with PPMS characteristically have few lesions visible on conventional T2-weighted imaging, these MT histogram changes may be the consequence of relatively mild pathologic changes within the NAWM. Although clarification is needed about whether such changes are due to a pervasive damage of the NAWM or are confined to certain areas of the brain, our data agree with the result of a preliminary study showing decreased N-acetyl-aspartate in the NAWM of patients with PPMS.38 This would suggest that an important contributor to the MTR changes in the NAWM is diffuse axonal damage, with inflammation playing a lesser role. This should be taken into account when planning clinical trials in PPMS because it may influence both the choice of the treatment and the MRI end points used to monitor treatment efficacy.
Acknowledgments
Supported by grants from the Neurology School, University of Chieti (G.I.); Farmades/Schering, Italy (C.T.); and TEVA Italy (M.P.S.).
Acknowledgment
The authors thank Mr. Clodoaldo Pereira for his skillful technical assistance in collecting the MRIs.
- Received July 20, 1998.
- Accepted October 17, 1998.
References
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