CT urography with multislice technology has a very high spatial resolution for imaging the urinary tract. It can detect tiny urothelial tumors inside the intrarenal collecting system and ureter, potentially avoiding the need for more invasive endourologic procedures. MR urography is preferred for some patients, however.
"CTU may be better than conventional x-ray urography and ultrasound because it is the one-stop shop to image the collecting system, ureter, and organs of the abdomen all in one scan," said Prof. Dr. Claus Nolte-Ernsting, head of diagnostic and interventional radiology at the Evangelic Hospital of Mulheim in Germany. "It is fast, sensitive, and cost-effective. For at-risk patients, such as those who suffer from painless hematuria and those who are older than 40, CTU is a highly accurate diagnostic test for finding small urothelial tumors."
|T2-weighted static fluid MR urography shows bilateral megaureter with bilateral ureterocele. (Provided by M. Cova)|
To optimize CTU while limiting radiation exposure, radiologists should seek to individualize the examination procedure for patients, according to Nolte-Ernsting, who will be speaking at this morning's European Excellence in Education session about urography examination techniques. The session marks the start of a course on imaging of the urinary tract that concludes on Saturday.
Adequate timing of contrast injection is needed to perform the scan in the urographic phase. To find this phase, Nolte-Ernsting will outline several strategies that can ultimately reduce the number of scans needed. Combining the injection with a low dose of diuretics optimizes specificity to obtain a complete enhancement of the urinary tract in one scan.
"These techniques are still relatively new. Radiologists know how to apply CT to the liver and in angiography, but not the urinary tract," he said. "In some countries, conventional urography is still popular. Radiologists may not know the benefits of combining it with CT in a single procedure."
CTU has been in clinical practice since 2000 in some uroradiology departments. Now, however, Nolte-Ernsting sees the need to distribute technique and expertise to general radiology departments. In Germany, most conventional urography examinations are performed by urologists, but their role in urology may increase through undertaking more urographic procedures.
|T1-weighted excretory MR urography shows normal urinary tract. (Provided by M. Cova)|
The advantage of MR urography lies chiefly in its lack of ionizing radiation and its capacity to image the urinary tract in nonexcreting kidneys, though CTU gives more image information and boasts greater morphological accuracy. MR can also be combined with functional imaging for studying excretory kidney function.
MRU's lower spatial resolution means that visualization of soft-tissue calcifications such as urinary stones is not as good as CT. It is preferred, however, in children, patients with contraindications to contrast media, and pregnant women, said cospeaker Prof. Maria Cova, chair of radiology at the Cattinara University Hospital in Trieste, Italy.
MRU is used mainly to detect congenital anomalies of the urinary tract in children and obstructive uropathy in pregnant women, while in adult patients with hematuria, it can be used to detect urothelial tumors. It is not widely used, however, because longer acquisition times make it sensitive to motion artifacts due to respiration or ureteral peristalsis that can affect image quality. Faster sequences with breath-holding are critical for the detection of small lesions that would otherwise be obscured by even slight respiratory motion.
These sequences can be obtained with new hardware and developments aimed at increasing spatial and temporal resolution, such as the use of multichannel phased-array coils that facilitate faster examination times and better image quality when used with parallel imaging. The use of 3T equipment, providing higher signal-to-noise ratio, may increase image resolution, despite the greater likelihood of susceptibility artifacts and image heterogeneity due to high field strength.
"MR is still an evolving technique that is reliant on developments in hardware and sequences," Cova said. "But good equipment allows doctors the all-in-one approach when imaging the urinary tract with MR. Diffusion imaging of the kidney may help in the evaluation of disease processes through anatomical and functional data acquisition during a single examination."
This all-in-one approach is based on two MR urography techniques that are often combined. The older consists of the heavily T2-weighted turbo spin-echo sequences (T2-weighted static fluid MR urography). The second, more recently developed method comprises T1-weighted sequences with gadolinium (T1-weighted excretory MR urography).
"T2-weighted static fluid urography is good for evaluating the distended and obstructed collecting system, but when the collecting system is not dilated, this technique is not optimal. Excretory MR urography with contrast medium and diuretics is preferred to obtain functional information," she said. "The better the image quality, the more accurate the lesion detection. We still don't know the sensitivity of excretory MR urography in the detection of small lesions, such as urothelial tumors in the collecting systems, as there are still not many studies about its use for patients with hematuria."
Because MR urography takes up to 50 minutes per examination and contrast media is used, it is a relatively expensive procedure that is unlikely to surpass CTU for lesion detection. In selected categories of patients, however, earlier diagnosis of lesions using this technique can save costs globally, Cova said.