Limited CT urography establishes usefulness

October 1, 2006

Comprehensive multiphase protocol proves unnecessary in majority of patients

Comprehensive multiphase protocol proves unnecessary in majority of patients

Advances in CT and MR imaging technology have had an exceptional impact on urinary tract imaging. Many centers are now replacing conventional intravenous urography with an examination based on CT or MRI.

MR urography includes excretory and T2-weighted imaging. Excretory MR urography requires intravenous injection of 0.1 mmol/kg body weight extracellular gadolinium-based contrast and 10 mg frusemide. The technique is similar to that of conventional urography, providing both a morphologic depiction and a functional assessment of the urinary system. Excretory MR urography should be considered whenever iodinated contrast is contraindicated and the use of ionizing radiation must be avoided, providing the renal function of the patient is not markedly reduced.

T2-weighted MR urography uses heavily T2-weighted turbo spin-echo (TSE) sequences to provide water images of the urinary tract. Because the urine acts as an intrinsic contrast medium, no additional contrast is required. The diagnostic technique is independent of renal excretory function and provides excellent urographic views of patients with severe renal impairment, particularly when associated with a dilated collecting system. It is also valuable when evaluating congenital anomalies (Figure 1).

Although radiologists have reached some agreement on how to perform MR urography, consensus is still lacking on the optimum CT urography technique, especially in relation to the number of phases acquired.

- Precontrast scanning. The first phase of multiphase CT urography is precontrast scanning. An unenhanced scan of the upper abdomen is essential in all patients for detecting renal stones, especially those that are small and will not be detected on plain-film abdominal radiography. Precontrast scanning resolves possible confusion associated with the layering effect of contrast in the calyces, which can otherwise be misinterpreted as the presence of renal stones.

This first phase is also useful for assessing space-occupying kidney lesions. It provides a baseline for later measurements of postcontrast lesion enhancement and may be used to detect lesions with a high fat content, suggestive of angiomyolipoma.

The standard practice of routine precontrast scanning of the rest of the abdomen and the pelvis to detect ureteric or bladder stones, however, is not important in most cases referred for CT urography. Ureteric stones are often symptomatic, and patients with renal colic should be investigated with plain spiral abdominal CT. Unexpected clinically silent ureteric stones, which are uncommon, are likely to produce some dilatation of the calyces and ureters proximal to the site of impaction. One study has reported upper tract dilation in 96% of patients with ureteric stones and ureteric wall thickening at the impaction site in 65% of cases.1 Bladder stones are likely to be detected on plain-film abdominal x-ray. Cystoscopy is generally used to investigate abnormalities within the bladder lumen or wall in symptomatic patients.

Precontrast CT scanning of the whole abdomen and pelvis should be performed under the following circumstances: one or both kidneys not visible in the upper abdomen (pelvic position of the kidneys); presence of features suggesting acute renal obstruction (perinephric stranding, perinephric fluid collection, and fullness of the calyces); and suspected urinary tract schistosomiasis (possible calcification of the urinary bladder wall and lower third of the ureter).

- Corticomedullary phase. Next in the comprehensive multiphase approach is the corticomedullary phase. A large dose of contrast (120 to 150 mL) should be injected at a high flow rate (3 to 4 mL per second). Upper abdominal CT scanning should begin 30 to 40 seconds after the start of contrast injection.

The corticomedullary phase is considered important for renal cancer staging. Its depiction of renal arterial and venous anatomy makes it possible to detect any venous extension of tumor tissue. Hypervascular metastases in the liver may also be found. This phase is also useful for assessing possible vascular renal masses, including arteriovenous malformations. But most of this information is not required in the majority of cases referred for urography.

- Nephrographic phase. The upper abdomen is imaged 80 to 100 seconds after the initiation of contrast injection. The nephrographic phase is important for detecting and characterizing renal masses, and it is often considered crucial for the diagnosis or exclusion of renal cancer.

The prevalence of malignant renal tumors is very low, however. The American Cancer Society puts the incidence of this cancer in the general population at 0.002%, rising to about 0.2% in patients over the age of 50.2 Survey data indicate that 4% of patients with macroscopic hematuria and 0.5% of patients with microscopic hematuria will also have renal cancer.3 About 10% of renal cell carcinomas may present as a cystic lesion that can be confused with benign complex renal cysts.4

Because the prevalence of renal cancer is not high, few (about 0.4%)

patients will require good-quality nephrographic phase imaging to differentiate cystic renal cancer from a benign lesion. Renal masses with diameters larger than 10 mm can also be assessed adequately during the excretory phase.5

- Excretory phase. The first excretory phase comes about five minutes after contrast injection with compression. Another after release of the compression is recommended to ensure good distention of the calyces and depiction of ureters. Diuresis induced with intravenous administration of either 10 mg of frusemide or 250 mL of saline has eliminated the need for compression. Research has shown that while diuresis and turning the patient several times before the excretory phase enhance depiction of the pelvicalyceal system and ureter, the prone position does not improve visualization of the distal ureter.6,7

LIMITED APPROACH

Since neither the corticomedullary nor nephrographic phases may be necessary in most patients referred for CT urography, the use of such a high dose of contrast for the excretory phase becomes unwarranted. A lower dose should be administered to avoid the streak artifacts and to mask the filling defects that are associated with high-density pyelograms.8 One study has suggested that only 30 mL of contrast is required for a diagnostic pyelogram.9

We have developed a limited approach to CT urography (Tables 1 and 2). We use axial and coronal images to interpret examination results. The two sets of images are complementary, and this combined approach is better than either method alone.7 We also produce thick coronal slabs. These are useful when examining patients with urinary diversion, where it is helpful to display the entire urinary tract in a single image.

Indications for limited CT urography are the same as those for conventional intravenous urography. We have found CT urography to be particularly helpful for evaluating patients with complicated urinary tract infection, hematuria, upper urinary tract obstruction, urinary diversion, and renal masses (Figures 2 and 3).

The limited protocol can be implemented easily by radiographic staff and exposes patients to a lower dose of ionizing radiation than a full multiphasic CT urography examination. Published figures for the mean effective radiation dose from CT urography are 15 to 25 mSv for three phases and around 10 mSv for two phases. A limited approach employing 80 mAs has a mean effective radiation dose of approximately 4.7 mSv, while the radiation dose associated with conventional intravenous urography (6 to 11 films/examination) ranges from 5 to 10 mSv.10 The risk of a radiation-induced fatal cancer is 1:4200 for limited phase CT urography and from 1:1333 to 1:800 for three-phase CT urography.

Our experience with limited CT urography has been encouraging so far. It has replaced conventional intravenous urography completely in our institution.

PROF. MORCOS is a consultant radiologist at Northern General Hospital, Sheffield Teaching Hospitals NHS Foundation Trust, in Sheffield, U.K.

References

1. Katz DS, Lane MJ, Sommer FG. Unenhanced helical CT of ureteral stones: incidence of associated urinary tract findings. AJR 1996;166(6):1319-1322.

2. Higgins JC, Fitzgerald JM. Am Fam Physician 2001;63(2):288-94, 299.

3. Sutton JM. Evaluation of hematuria in adults. JAMA 1990;263:2475-2480.

4. Hartman DS, Choyke PL, Hartman MS. From the RSNA refresher courses: a practical approach to the cystic renal mass. Radiographics 2004;24 Suppl 1:S101-S115.

5. Yuh BI, Cohan RH. Different phases of renal enhancement: role in detecting and characterizing renal masses during helical CT. AJR 1999;173(3):747-755.

6. Caoili EM, Inampudi P, Cohan RH, Ellis JH. Optimization of multi-detector row CT urography: effect of compression, saline administration, and prolongation of acquisition delay. Radiology 2005;235(1):116-123.

7. McTavish JD, Jinzaki M, Zou KH, et al. Multi-detector row CT urography: comparison of strategies for depicting the normal urinary collecting system. Radiology 2002;225(3):783-790.

8. Raptopoulos V, McNamara A. Improved pelvicalyceal visualization with multidetector computed tomography urography; comparison with helical computed tomography. Europ Radiol 2005;15(9):1834-1840.

9. Sussman SK, Illescas FF, Opalacz JP, et al. Renal streak artifact during contrast-enhanced CT: comparison of low versus high osmolarity contrast media. Abdominal Imaging 1993;18(2):180-185.

10. Nawfel RD, Judy PF, Schleipman AR, Silverman SG. Patient radiation dose at CT urography and conventional urography. Radiology 2004;232(1):126-132.