CT versus MR of the Kidney
by Dr. D Wyner of the University of Florida
CT and MR are useful to evaluate renal cysts that have a Bosniak score (a href=”http://radiology.rsna.org/content/226/1/47.full”>link of 3 or 4. The CT scan uses Hounsefield Units (HUs) to quantitate the relative liquid content of a mass. Cysts, which are fluid-filled, have low HUs (0–5). Masses with more solid components and/or greater vascularity (both of which are features of malignancy) will have greater HUs. While MR also differentiates the relative liquid content, it does so in a qualitative manner.
Once you’ve determined that the mass has more solid features, you need to determine if there is enhancement. Enhancement is the most important factor in identifying malignancies. For CT, iodinated contrast is needed to determine enhancement, while MR can measure the amount of water that can diffuse through tissue. With CT, enhancing lesions light up (appear bright) and have high HUs. With MR, lesions that allow water to diffuse easily (less solid in nature) appear bright.