Computed tomography (CT) scanning provides both anatomic and functional information about the kidney. The renal parenchyma, collecting system, extrarenal space, ureters, and bladder can all be visualized. Renal perfusion can be assessed with the administration of contrast material. CT can identify extrarenal pathology or injury not clinically suspected. As the method of choice in assessing stable patients with suspected renal trauma, it determines the extent of the parenchymal injury, demonstrates urine extravasation or perirenal hemorrhage, and evaluates the integrity of renal vascular pedicle and status of adjacent organs. The disadvantage of CT is the time required to perform the study and the need to administer contrast if optimal results are to be obtained.
With CT, images of the kidney and urinary system are reconstructed from information collected by focused x-ray beams arranged on a plane that cuts through the patient's body in a transverse or horizontal direction. The patient lies on a movable table that passes through the CT gantry, which contains the tightly focused x-ray beams. As these beams pass through the patient, they are absorbed by a ring of detectors on the opposite side of the gantry. The intensity of the x-ray beam that reaches the detector is dependent on the absorption characteristics of the intervening tissue. The radiodensity of a small area of tissue can then be calculated from the absorption pattern of multiple beams crossing the area from different directions.
During spiral CT, the x-ray tube is in continuous rotation while the patient is moved smoothly, at a constant speed, through the scanning field. This technique improves the detection of small lesions, as scanning is performed during a single breath-hold, thus reducing the movement of intraabdominal organs and motion artifact, which occurs when imaging is performed during respiration. Data from multiple scanning planes can be used to produce three-dimensional reconstruction of lesions. In trauma, information about the extent of renal disruption, trauma to the renal pedicle, and viability of disrupted fragments can be provided. In addition, shaded surface displays allow volume surface analysis of parenchymal organs and maximal-intensity projections provide analysis of the vasculature. The selected slice thickness of a scan depends on the clinical presentation. Trauma scans are usually 8 to 10 mm thick and are obtained from the diaphragm down to the bottom of the pelvis. CT scans for renal colic utilize 5-mm slices from the top of the kidneys down to the bladder. If there is an area of subtle or equivocal findings, the radiologist may elect to make smaller cuts for clarification.
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