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Magnetic resonance imaging (MRI) produces high-resolution multiplanar images of the kidney and can provide both morphological and functional information about renal disease. Recent technical developments and the use of gadolinium contrast have considerably improved the performance of MRI.

The detection of renal and pararenal masses on MRI of the kidneys is often difficult because without contrast medium, most pathological lesions of the kidney have a signal intensity similar to that of the surrounding renal parenchyma, which results in low soft-tissue contrast. In addition, motion artifacts caused by respiratory movements and adjacent abdominal vessels may compromise image quality and, therefore, detection of renal masses. With the exception of angiomyolipoma, routine MR sequences have not been shown to be sensitive or specific in the characterization of renal masses. Other than lesion size, the presence of septa, wall and /or septum thickness, the presence of calcification, the most important aspect in characterizing a renal mass is to demonstrate the presence or absence of enhancement on post-contrast images. In most cases, solid or nodular soft-tissue enhancement in a renal mass implies a vascular mass, consistent with a neoplasm.

At computed tomography (CT), Hounsfield unit measurements are obtained in regions of interest (ROI) and are generally reliable for the detection of renal mass enhancement. In MR imaging, there is no universally accepted method of determining the presence or absence of enhancement in a renal mass. There are currently three methods, to determine renal mass enhancement on MR imaging:

Image subtraction is a technique that has been widely used for image postprocessing in MR, specifically in breast MR imaging for the detection of enhancing masses and in contrast-enhanced MR angiography, and is and is currently being studied for its possible application for revealing enhancement in renal masses. This method involves taking each phase from the dynamic contrast-enhanced MR imaging study and using it as a template from which the unenhanced data set is subtracted. Like the quantitative ROI approach, image subtraction also involves the assumption that system gain between acquisitions will be fixed. It is also susceptible to motion and other misregistration artifacts, typically owing to the variability of patient breath holding. In addition, image subtraction does require some subjection determination of enhancement by the radiologist. Although this is not a problem for clearly enhancing masses, there is interobserver variability regarding the presence or absence of subtle enhancement within a renal mass. However, there are several advantages to the subtraction approach for assessing renal mass enhancement. It enables a global assessment of renal enhancement that is not dependent on user-positioned ROIs. Also, the subtraction approach may be particularly well suited for the assessment of lesions that are hyperintense on unenhanced images. The study by Hecht et al, which compared the quantitative analysis of enhancement with signal intensity measurement to the qualitative analysis of enhancement with image subtraction, demonstrated that both quantitative and qualitative methods are sensitive in the detection of enhancement within a renal mass. However, in lesions that are hyperintense on unenhanced T1-weighted MR images, qualitative assessment based on image subtraction should be performed to avoid false-negative quantitative results.

Image subtraction is a technique that has been widely used for image post-processing in MR, specifically in breast MR imaging for the detection of enhancing masses and in contrast-enhanced MR angiography, and is and is currently being studied for its possible application for revealing enhancement in renal masses. This method involves taking each phase from the dynamic contrast-enhanced MR imaging study and using it as a template from which the unenhanced data set is subtracted. Like the quantitative ROI approach, image subtraction also involves the assumption that system gain between acquisitions will be fixed. It is also susceptible to motion and other misregistration artifacts, typically owing to the variability of patient breath-holding. In addition, image subtraction does require some subjection determination of enhancement by the radiologist. Although this is not a problem for clearly enhancing masses, there is interobserver variability regarding the presence or absence of subtle enhancement within a renal mass. However, there are several advantages to the subtraction approach for assessing renal mass enhancement. It enables a global assessment of renal enhancement that is not dependent on user-positioned ROIs. Also, the subtraction approach may be particularly well suited for the assessment of lesions that are hyperintense on unenhanced images. The study by Hecht et al, which compared the quantitative analysis of enhancement with signal intensity measurement to the qualitative analysis of enhancement with image subtraction, demonstrated that both quantitative and qualitative methods are sensitive in the detection of enhancement within a renal mass. However, in lesions that are hyperintense on unenhanced T1-weighted MR images, qualitative assessment based on image subtraction should be performed to avoid false-negative quantitative results.