Evaluation of two cortical fraction estimation algorithms for the calculation of dynamic magnetic resonance renograms

With the high resolution of dynamic magnetic resonance imaging (MRI) scans it is possible to measure cortical renograms directly, but due to partial volume effects this is impossible for medullary renograms. With weighted subtraction of the cortical renogram from a mixed renogram it becomes possible to extract the medullary renogram. For this subtraction the fraction of cortical tissue, present in the region of interest in which the mixed renogram is determined, has to be calculated. We have evaluated two algorithms for calculation of the cortical fraction. Both algorithms use the fact that during an interval after the start of the cortical enhancement no medullary enhancement occurs. One algorithm calculates the ratio between the slopes of both enhancement curves. The other is based on minimising the medullary signal values using a least squares error (LSE) method. Using a computer model of the renograms and measurements on real patients we analysed the accuracy of both methods and determined the best parameters for each.