New CT technique targets renal artery stenosis

Korean researchers say that gauging the concentration of contrast enhancement material in urine during CT angiography can help pinpoint reliably renal artery stenosis.

Catheter angiography and intra-arterial digital subtraction angiography are the current standard tests for the diagnosis of renal artery stenosis. But neither is suitable as a screening method because each requires invasive procedures. X-ray-based intravenous pyelography (IVP), on the other hand, has been deemed unreliable for this task.

Using instead the urine attenuation ratio out of contrast-enhanced CTA of the kidneys, however, radiologists could perform a reliable noninvasive assessment of renal function, according to the team of investigators led by Dr. Chang Kyu Sung, a radiologist at Seoul National University.

Sung and colleagues prospectively reviewed 33 patients with suspected renovascular hypertension and 43 patients with other conditions but otherwise normal blood pressure. All patients underwent contrast-enhanced CTA followed by intra-arterial DSA during a three-year period.

The researchers observed urine's contrast attenuation on three separate regions of the renal pelvis or of each kidney's proximal ureter and measured them in Hounsfield units. They obtained the urine CT attenuation ratio by dividing the higher attenuation value of one kidney by the lower attenuation value of the contralateral kidney. Two observers reviewed all measurements blinded to angiographic findings.

The investigators found that the CT urography technique helped detect hemodynamically significant renal artery stenosis, accurately indicated the affected kidney, and backed up other CTA findings (AJR 2006;187[2]:532-540).

Of 33 suspected hypertense patients, 26 showed significant renal artery stenosis (20 unilateral and six bilateral), while two had only moderate signs of the condition. The urine CT attenuation ratio in 22 patients with significant renal artery stenosis and two patients with moderate stenoses averaged 2.07 and 1.53 points, respectively. All other patients had an average urine CT attenuation ratio of 1.07 points. The ratio difference between the normal and renal artery stenosis groups was statistically significant (p

The mean urine CT attenuation ratio in patients with unilateral and bilateral renal artery stenosis was 2.18 and 1.75, respectively. The urine CT attenuation ratio's sensitivity, specificity, positive predictive value, and negative predictive value for the diagnosis of significant renal artery stenosis were 95%, 96%, 91%, and 98%, respectively.

There are caveats, though, researchers said. Study limitations include wide variability of attenuation values in some patients due to technically inadequate exams, and anatomic and physiological differences between patients. Further studies using multislice CTA and standardized PACS-based measurements, however, should determine the urine CT attenuation ratio's usefulness.

The urine CT attenuation ratio relies on the functional differences between normally perfused and ischemic kidneys, according to investigators. The technique may thus be used to determine the prognosis of asymptomatic patients with renal artery stenosis after intervention or to help guide therapy to the symptomatic kidney.

"By virtue of the comprehensive information that it offers, the urine CT attenuation ratio can be used for the noninvasive assessment of renal function and as an adjunct to helical CTA," the researchers said.

For more information from the Diagnostic Imaging archives:

Renal CTA moves past arterial stenosis to vessel anomalies

Ultralow-dose CT urography shows promising results

Low-dose MSCT tops x-ray for renal calculi diagnosis

CT tops IVP for imaging asymptomatic microhematuria