The NIAMS has an ongoing protocol to study polymyocitis. As part of this protocol, subjects are unaged with MRI using a special pulse sequence which suppresses signal from fat, but enhances signal from diseased tissue. In the past, the images were usually evaluated visually to determine extent and progress of disease. A more quantitative method for analysis has been developed and is being tested. This method produces a three dimensional histogram of signal intensity values. In normal subjects this histogram is nearly gaussian in shape, while in abnormal subjects this histogram is skewed to higher intensity values. A method was developed which is hoped will give quantitative indices of both degree of disease as well as volume of diseased tissue. This method is based on assuming that the low signal intensity values within the tissue being studied represent normal tissue. The left half of the histogram is then reflected about its peak to give an estimate of what the subject's histogram would have looked like if it were completely normal. Comparing the reflected histogram to the actual histogram allows estimates of degree and volume of disease. The method is currently under evaluation.
