THE EFFECT OF IMPRECISE REPOSITIONING ON LESION VOLUME MEASUREMENTS IN PATIENTS WITH MULTIPLE-SCLEROSIS

In this study, we evaluated the effect of imprecision in patient repos itioning encountered in real life on multiple sclerosis (MS) lesion vo lumes measured from MRIs. We also evaluated two putative methods for r educing the variability in these lesion volume measurements: first, a reduction of slice thickness (from the conventional 5 mm to 3 mm) and second, the application of a new repositioning technique based on the use of head immobilization shells. We evaluated the errors in lesion v olume by scanning 10 patients a total of four times using the two slic e thicknesses and two repositioning methods (conventional and using a head immobilization shell). The mean absolute percentage difference be tween two corresponding scans was 6.8% (range, 1.24 to 11%) using conv entional slice thickness and repositioning, 4.1% (range, 0.7 to 5.56%) using conventional slice thickness and head immobilization shells, 2. 6% (range, 0.8 to 6.66%) using the conventional repositioning techniqu e and 3-mm slice thickness, and 1.4% (range, 0.2 to 6.14%) using slice thickness of 3 mm and head immobilization shells. These mean absolute differences were significantly different (p = 0.0008). Our results in dicate that the effect of repositioning errors of the order of those t hat can be encountered in the daily life situation of clinical trials affects significantly lesion load measurements in MS and that the comb ined use of thinner slices and more accurate repositioning techniques can markedly improve the reproducibility of such measurements.