Multislice CT urography garners support but needs refining

November 5, 2004



Multislice CT urography has the potential to completely replace conventional intravenous urography (IVU). The technique is more accurate than IVU in assessing the cause of asymptomatic microscopic hematuria, and it can obviate additional imaging. But experience with MSCTU is still preliminary, more studies are required, and refinements in CT technique are needed to maximize study accuracy while minimizing patient radiation exposure.

These were the findings of a joint Anglo-French poster presented at the RSNA meeting last December.

Because patients with suspected urinary tract disease are often referred for multiple studies, a single imaging test that evaluates the entire upper urinary tract has advantages of convenience and cost, according to Dr. Francois Cornud, a radiologist at Cochin Hospital in Paris, and Dr. Nigel C. Cowan, a consultant interventional radiologist at the Churchill Hospital in Oxford, U.K.

"The use of MSCTU requires careful consideration of patient history, the level of risk of urinary tract pathology, and the amount of radiation," they wrote. "Evaluation of the urinary tract using CT, IVU, or a combination of both requires preliminary precontrast images to visualize calculi, followed by post-contrast injection imaging in the nephrographic phase to evaluate the renal parenchyma."

Assessment of the urinary tract lumen is obtained following opacification by contrast excreted in the urine. Direct parallels exist between MSCTU series and IVU films.

MSCTU most commonly involves acquiring several independent series of axial images. Various centers acquire two, three, or four series of axial images. All methods begin with an initial "renal stone" CT, consisting of 2.5 to 5-mm-thick images from the top of the kidneys to just below the symphysis. The purpose of these unenhanced scans is to detect urinary tract stones and assist in the characterization of any detected renal masses.

Cowan uses a two-series split-bolus technique followed by a single combined nephrographic- and excretory-phase acquisition. Cornud employs a single injection of contrast material followed by two contrast-enhanced series, one performed during the nephrographic phase and the other during the excretory phase (three-series acquisition). In theory, the two-series method involves less patient radiation dose while providing nephrographic- and excretory-phase information.

MSCTU may be nondiagnostic in patients who are unable to remain still or hold their breath throughout the protocol. Increased radiation scatter in very obese patients also compromises image quality, according to the authors. Beam-hardening artifact in patients with surgical clips or metallic hardware, such as total hip replacements and ingested lead shot, may limit evaluation of the soft tissues in the region of the artifact. Inadvertently administered oral contrast and overlying calcifications can obscure intrinsic urothelial abnormalities on the 3D images.

For a two-series examination, the mean effective dose has been calculated at 11.6 mSv. This value reaches 14.7 mSv for the three-series study.

"Production of 3D reformatted images is time-consuming, labor-intensive, and costly. For this reason, simultaneous axial pre- and postcontrast images are reviewed in cine mode," the authors said. "For clarification, multiplanar reconstructions are sometimes helpful, and occasionally coronal maximum intensity projections are used to display features to clinicians who prefer to review images in the coronal plane. Using this review method, the time taken for MSCTU image analysis is similar to that of IVU."