Info

240-480 Adult dose

Gradually over 1 h before scan Gradually over 30 min before scan

Full dose 1 h before scan and a further half-dose immediately prior to scan

Intravenous (IV) contrast should be given in virtually all uro-

logical patients, except when looking for a renal tract calculi.

Intravenous contrast permits—

• Improved delineation of renal masses

• Evaluation of surrounding vasculature

• Characterization of masses by their pattern of contrast enhancement. The contraindications and side effects of intravenous contrast agents have been discussed earlier (see Chapter 3b: IVU)

The standard renal mass protocol is based on renal enhancement post IV contrast injection.

1. Non-contrast scan: best for identification of renal tract calculi, fat, and baseline enhancement

2. Arterial (cortical) phase scan: 15-25 seconds after contrast injection allows evaluation of renal arteries

3. Corticomedullary (nephrographic) phase scan: 70-120 seconds after contrast injection allows visualization of renal parenchy-mal anatomy. Also provides good hepatic and portal vein enhancement

4. Excretory phase scan: 3-5 min after contrast injection allows evaluation of the collecting system and renal pelvis

Delaying the scan beyond this period will demonstrate opacification of the ureters and bladder.

Enhancement (Hounsfield units)

Tissue enhancement (density/attenuation value) is standardized on the Hounsfield scale. The scale extends from -1,000 to +1,000 Hounsfield units (HU) (see Table 3.4). Enhancement of >10 HU post-contrast compared to the non-contrast phase is indicative of a solid, enhancing, malignant mass.

Data processing and images

3D reconstruction—With the increased use of multislice CT scanners, it is now possible to obtain a large number of very thin (<2.5 mm) cross-sectional axial images of the entire urinary tract in a matter of minutes. In addition, these images can be displayed in a variety of multiplanar and 3D reformatted images. Currently, one of three different 3D reconstructions algorithms may be used:

• Average-intensity projection (most closely resembles IVU images)

• Maximum-intensity projection

• Volume rendering

Evidence of the superiority of one over the other is not available yet, but volume rendering seems to be the most preferred owing to the fact that image creation is quick, is least dependent on

Table 3.4. Hounsfield scale

Tissue Hounsfield Units (HU)

Bone +1,000

Renal tract calculus >+400

Non-specific calcification >+150

Acute hemorrhage +50 to 90

Clotted blood +70

Soft tissue +10 to +50

Water 0

Air -1,000

technical factors, and provides excellent data-rich images of the urinary tract in its entirety without any loss of information. The improved resolution and accuracy of reconstructed images, combined with CT angiography, is increasing its use in a range of conditions where demonstration of renal and perirenal vascu-lature is essential, including—

• Assessment and staging of renal tumors

• Planning for nephron-sparing surgery

• Prior to surgery for uretero-pelvic junction obstruction surgery (e.g., endopyelotomy)

• Prior to complex stone surgery

• Assessment of renal transplant donors

• Arterio-venous malformation

• Assessment of complex urinary fistulas

It is important to study the actual axial 2D images as certain small lesions may not be easily identified on reconstructed images. In one recent study, standard 2D CT images correctly demonstrated 89% of small upper tract TCC, compared to a 25% pick-up rate with 3D reconstruction images.

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