Quantitative assessment of myositis in thigh muscles using magnetic resonance imaging

Magnetic resonance (MR) imaging has been suggested as a technique for diagn osing and monitoring myositis, an inflammatory muscle disease. To date, the assessment of disease from MR images has been by subjective visual analysi s. We describe here an objective, semi-automatic, computer-based method for quantifying the degree of disease from MR images, without the need for a r adiologist or physician trained in the visual assessment of the MR images. The method is based on analysis of the histogram of intensity values produc ed from the MR images. The analysis yielded measures of the intensity and e xtent of disease. These two measures were combined to produce a calculated myositis index (CMI) which described the degree of disease evident from the MR images, This index was compared with a clinical assessment of the patie nt's condition, based on currently accepted, invasive and non-invasive, non -imaging criteria. Receiver operating characteristic (ROC) curve analysis s howed that calculated myositis index agreed at least as well with clinical assessment as did visual analysis (receiver operating characteristic area = 0.93 and 0.94, p = not significant (NS), respectively, for separating remi ssion from disease). Even using only two central MR slices for each patient , the receiver operating characteristic area for calculated myositis index was 0.92, implying that very short acquisition times are possible. We concl ude that quantitative histogram analysis of MR images can be successfully p erformed with minimal operator input and using few MR slices. Agreement wit h more invasive clinical assessment is good and the method has the advantag es of repeatability, objectivity, and decreased scan and analysis time. (C) 1999 Elsevier Science Inc.