Recommended Ctu Technique

At the present time, we recommend performing CTU by utilizing the MDCTU approach according to one of the two following techniques: the three-phase protocol or the split-bolus protocol. The two specific protocols that follow are those currently being employed at our institution. For both approaches, scanning begins with a renal stone CT, performed at contiguously reconstructed 5 mm thick axial images obtained from the upper poles of the kidneys to the symphysis pubis.

For the three-phase protocol, the patient is then injected with 150 mL of non-ionic contrast material (at a concentration of 300 mg I/mL and a rate of 3-4 mL/ sec). Axial images are then acquired from the diaphragms through the lower poles of the kidneys (renal mass CT) using a delay of 100 seconds. Finally, at 720 seconds, thin-section (usually 1.25 mm on 16-row and 0.625 or 1.25 mm on 64-row multidetec-tor scanners) excretory-phase images are obtained at 0.625 mm intervals from the tops of the kidneys to the symphysis pubis.

For the split-bolus protocol, after the renal stone CT images are obtained, the patient is injected with 75 mL of nonionic contrast material (at the same concentration used in the three-phase protocol), again at a rate of 3 to 4mL/sec. After 600 seconds, an additional 100 mL of contrast material is administered intravenously at the same rate. Then at 720 seconds, thin-section (0.625 or 1.25 mm thick) images are obtained at 0.625 mm intervals from the diaphragms to the symphysis pubis.

For both the three-phase and the split-bolus protocols, the last thin-section excretory-phase series of images is then used as source images for 3-D reconstructions; however, axial images are also reconstructed at a thickness of 2.5 mm and at intervals of 1.25 mm. It is only these axial images that will be reviewed at the time that the study is interpreted. The initial source axial images are not utilized for image interpretation due to their much larger number.

Postprocessing, which is usually performed by a technologist, involves creating anteroposterior and bilateral 30° coronal oblique maximum-intensity projection and volume-rendered images. Coronal thin-section average-intensity projection images are also obtained (at 3 mm thickness and 3 mm reconstruction intervals). This technique creates, in all, several hundred images that must be reviewed. Such a review is best performed on a workstation.

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