CT is currently the imaging ''gold standard'' for characterizing renal masses because of long experience with its use and its wide availability. The newest generation of multidetector, multirow CT machines allows for very rapid multiphasic, very thin section image acquisition. Shortened gantry rotation times and increased number of rows of simultaneously irradiated detectors allow for faster coverage and thinner collimation. Lengthy respiratory suspension is no longer necessary, and image misregistration due to breathing and other motion artifacts is considerably reduced. Multiphasic scans through the kidneys can be acquired very rapidly, each taking only a few seconds to accomplish. The resulting improvement in image quality helps minimize some of the technique-related pitfalls in the evaluation of complex renal masses.
A dedicated renal investigation conducted on a multislice CT produces numerous helical data sets. Although manufacturers use different types of detector arrays, a typical scan from a 16-slice machine might utilize a 16 x 1.25 mm detector configuration, 27.5 mm/rotation table speed, 0.5 second gantry rotation speed, and a pitch of 1.37. Because multiphasic, multislice scanning can generate many hundreds of images, the thin section acquisitions are reconstructed into thicker sections (5 mm) for electronic display and archival. If necessary, the original thin sections can be exported to a workstation for multiplanar reformatting, volume rendering, and region of interest analysis.
Serial fast scans capture the progressive flow of intravenous contrast material from the renal arteries through the cortex and medulla and ultimately into the collecting structures after concentration and excretion has occurred. The first scan, obtained before any contrast is administered, is essential because it serves as a baseline for recording attenuation changes in a mass during post-contrast enhancement phases. Even without intravenous enhancement, however, the contrast resolution capability of CT allows the distinction of a water density cyst from the adjacent soft tissue of the renal parenchyma. The unenhanced image also discloses any associated calcifications.
After a bolus intravenous injection of iodinated contrast material (300-350 mgI/mL, 3-4cc/sec), scans can be obtained in any of four visibly distinct temporal phases: the vascular/early corticomedullary (CM) phase, the late CM phase, the tubular-nephrographic or nephrographic phase, and the excretory phase (4). Multiple repeat scans increase radiation dose significantly, so it is important to select only those phases that are likely to be of highest diagnostic yield. Unless a renal cell carcinoma is highly likely, such as in a patient who has had an ultrasound examination suggestive of solid neoplasm, the vascular phase or early CM phase may be omitted. Images obtained in this phase show a brightly enhancing cortex and poorly enhancing medulla. The homogeneously dense nephrographic phase is achieved later, when the cortex and medulla are evenly enhanced. Transiently, in the nephrographic phase, the medulla may eventually even be of higher attenuation than the cortex. Shortly thereafter, the excretory phase begins when the collecting structures begin to fill as the nephrogram decreases in intensity.
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