Improved description of MR signal intensity specificity

The variety of MR tissue contrasts based on different types of pulse sequences has suggested the possibility of some improvement in tissue specificity, such as in distinguishing active tumor from benign posttreatment tissue. The lack of an absolute signal intensity scale and the photographic variability are confusing. Previous investigation has shown that intensity relative to that of an internal standard such as fat or muscle is not reliable. A simple formula using regions of interest on both T1- (or 1st echo) and T2-weighted images was designed to compensate for inconsistencies of the internal standards and allow a uniform scale, day to day and patient to patient. This was tested in 70 studies of patients with confirmed active tumor or posttreatment fibrosis. Sensitivity/ specificity improved from 0.75/0.67 to 0.90/0.83 for muscle standards and from 0.80/0.50 to 0.86/0.83 for fat standards. The same simple formula is applicable to other sequences as flip angle or inversion time varies