Materials

PATIENTS

Twenty eight patients with clinically definite multiple sclerosis9 were studied. The patients had all been recruited into a double blind, placebo controlled trial of anti-CD4 antibody; full details of the criteria for entry to this study are published elsewhere.10 Fifteen of them had secondary progressive multiple sclerosis (SPMS) and 13 relapsing remitting multiple sclerosis (RRMS). Table 1 shows their clinical data.

MRI

Conventional dual spin echo axial proton density and T2 weighted MRI images were acquired for all patients on a GE Signa 1.5T scanner, using a birdcage head coil. The imaging parameters were: SE 2000/34/90, 5 mm thick contiguous interleaved slices, 256×256 matrix, 24 cm field of view, 1 excitation. The total number of slices was 28 for each echo. The scans were corrected for non-uniformity of the coil before being analysed.11

ANALYSIS

The lesions were identified and then segmented on the proton density weighted scans—under the supervision of a radiologist (MG-C)—by a neurologist who had two years of experience in lesion load measurement (H-ML). The technique used for lesion segmentation is a contouring method provided by D Plummer (Dispimage@DPlummer, University College London). This semiautomated, local thresholding method has been fully described elsewhere,4 and was shown to provide a good within and between rater reproducibility. Lesion areas were obtained by calculating the area of the regions of interests identified. The rater used both the proton density weighted and T2 weighted images to identify multiple sclerosis lesions but segmentation was performed only on proton density images. The rater was able to view the neighbouring slices simultaneously on the computer workstation to assist lesion identification when measuring but was blinded to the patients’ clinical subgroup. (Small hyperintense areas of uncertain cause—for example, possible small blood vessels—were not included in the analysis.)

The within rater coefficient of variation of measurement of lesion volume was 3.0%. This was obtained from 10 measurements on five patients, two measurements on each patient with a two week interval in between.

For this study, we have defined lesions as regions or areas of signal abnormality seen on single slices. These regions or lesion areas are the nearest approximation we have to actual pathological lesions on two dimensional MRI. If a single large pathological lesion actually extended over several slices we have counted it as a separate MRI lesion on every slice that it appeared.

The distribution in size for 1568 lesions in 15 patients with SPMS and 1198 lesions in 13 patients with RRMS was investigated. Lesion area was measured after it had been delineated using the contouring technique on a slice by slice basis. The nominal diameter of a lesion was then calculated by assuming that it had a circular shape. The purpose of calculating a nominal diameter was to help understand the comparisons of lesion sizes. The nominal diameter was not used in calculating the lesion area. Figure 1 shows an example of lesions delineated using the contouring technique. Lesion volumes were calculated by multiplying the slice thickness by the lesion areas of delineated lesion regions only because it is the conventional unit of measurment. All comparisons in size were carried out on lesion areas.

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Figure 1

Example of lesions delineated using the contouring technique. The lesion sizes are 16.2, 129.7, and 17.2 mm²respectively from left, middle, and right. The corresponding nominal diameters are 4.5, 12.9, and 4.7 mm.

Results

Figure 2 depicts the distribution of the percentage of lesions of different sizes. It is evident that the distribution is skewed towards smaller size.

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Figure 2

Distribution of the number of lesions in size.

About 80% of all the lesions were smaller than 8 mm in nominal diameter. The average lesion size was about 36 mm² (7 mm in nominal diameter). About 20% of lesions were small (nominal diameter <3.5 mm). Their contribution to the total lesion loads varied considerably from one subject to another (range 0–5.7%; mean 1.1%; fig 3); it was largest when total lesion loads were small. Thus the percentage contribution of small lesions to the total lesion volume correlated inversely with the total lesion volume (Spearman coefficient of correlation r=−0.65, P<0.001).

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Figure 3

Contribution of the load of small lesions (<10 mm²) to the total lesion load.

Table 2 shows the results for the two clinical subgroups. Lesion sizes were on average smaller in the SPMS group (P<0.001, Mann-Whitney rank sum test) although there were wide overlapping ranges of individual lesion sizes. The total lesion volumes in each subgroup were not significantly different.

Discussion and conclusions

Our evaluation of lesion size was based on the areas of the regions delineated on a slice by slice basis. This practice tends to bias towards the smaller lesion areas, as a confluent biological lesion going through several slices is treated as several smaller lesion regions. This may partly explain why the distribution of the number of lesions was skewed towards smaller sizes. However, as most of the lesion quantification methods used currently are based on region identification on individual slices on two dimensional images, we think that this approach is relevant.

Our findings suggest that small lesions should not be ignored in measurement of lesion volume. They can be quantitatively important, especially in patients with small total lesion loads. A subgroup of patients with multiple sclerosis in whom small lesions are especially prominent are those with primary progressive disease;12 13 it follows that quantification of small lesions in this subgroup should always be attempted. There may also be a case for considering patients with small, moderate, and large lesion loads separately.

The current study applies to 5 mm thick slices. Recent work on 3 mm thick slices14 found an 8% increase in total lesion load which is likely to reflect a contribution from small lesions. The increasing use of thinner slices for clinical trials is likely to result in identification of additional small lesions with important effects on measured lesion load; in the near future sequences such as three dimensional fast spin echo should provide isotropic resolution.

In the SPMS group a slightly higher proportion of the total lesion load was made up by small lesions and the median lesion size was significantly smaller compared with the RRMS group (table 2). Patients with SPMS also exhibited a different pattern of small lesion load contribution in relation to total lesion load from that of RRMS, as shown graphically in fig 3. Small lesions should be investigated more fully to ascertain whether their number and proportion are related to the clinical evolution of the disease. It is not inconceivable, for example, that the accumulation of mainly small lesions, none of which alone is sufficient to cause an acute relapse, might result in a slowly progressive increase in clinical disability. Future prospective studies are needed to consider this question.