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Original Research |
1 Department of Radiology and Institute of Radiation Medicine, Seoul National
University College of Medicine, Seoul 110-744, Korea.
2 Department of Radiology, Seoul National University Boramae Hospital, 425
Shindaebang-2-dong, Dongjak-gu, Seoul 156-707, Korea.
Received January 10, 2005;
accepted after revision June 13, 2005.
Address correspondence to C. K. Sung
(sckmd{at}yahoo.co.kr)
Abstract
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SUBJECTS AND METHODS. In 33 patients with suspected renovascular hypertension and 43 normotensive patients, 5-mm-thick transverse CT scans of the kidney area were obtained 4 min after helical CT angiography (CTA). The attenuation of urine in each kidney was measured, and its ratio was calculated. All 76 patients underwent intraarterial digital subtraction angiography within 2 days after the CT examination. The results of angiography were correlated with the urine attenuation ratio of both kidneys.
RESULTS. Twenty-six patients showed significant RAS: unilaterally in 20 and bilaterally in six patients. Two patients showed moderate stenosis of renal arteries. The other patients with essential hypertension (n = 5) or normal blood pressure (n = 43) showed normal renal arteries. The CT attenuation of urine in each kidney was measured and its ratio calculated in all patients except four patients with unilateral RAS. The urine CT attenuation ratio in 22 patients with significant RAS ranged from 1.11 to 4.76 (mean, 2.07). The two patients with moderate RAS showed ratios of 1.83 and 1.23. The others (n = 48) had a urine CT attenuation ratio that ranged from 1.00 to 1.54 (mean, 1.07). The difference of the ratio between the RAS group and the normal group was statistically significant (p < 0.01). The mean urine CT attenuation ratio was 2.18 in patients with unilateral RAS (n = 16) and 1.75 in patients with bilateral RAS (n = 6). The difference of the ratio between the two groups was not statistically significant (p = 0.16). At a cutoff value of 1.22, the sensitivity, specificity, positive predictive value, and negative predictive value of the urine CT attenuation ratio in the diagnosis of significant RAS were 95%, 96%, 91%, and 98%, respectively.
CONCLUSION. The urine CT attenuation ratio is a simple and reliable indicator with which to detect and lateralize significant RAS and is a useful adjunct to helical CTA.
Keywords: arteriography • CT angiography • hypertension • renal arteries • renal disease • stenosis
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Conventional angiography or intraarterial (IA) digital subtraction angiography (DSA) is considered the reference standard for confirmation of RAS. However, neither is suited to be a screening method because each requires invasive procedures. To identify all curable patients, screening methods with a high sensitivity and less invasiveness are needed. A variety of diagnostic techniques such as captopril scanning, duplex Doppler sonography, CT angiography (CTA), and MR angiography (MRA) are available for this purpose [5-8]. CTA and MRA offer direct identification of RAS, whereas the others identify functional changes in the kidney. Favorable results of helical CTA have been reported in the diagnosis of vascular diseases, including RAS, with minimal invasiveness [7]. In addition, the attenuation of urine can be objectively measured on transverse CT scans. The purpose of this study was to evaluate the value of measuring the attenuation of urine in each kidney and calculating the ratio of those measurements (hereafter referred to as the urine CT attenuation ratio) on delayed CT scans immediately after the acquisition of helical CTA data in the detection and lateralization of significant RAS.
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Thirty-three patients were suspected to have renovascular hypertension on the basis of clinical symptoms, plasma renin assay, renal duplex Doppler sonography, or captopril scintigraphy and were referred to our department for evaluation of the renal arteries (17 men, 16 women; age range, 19-77 years [mean, 39 years]). They met two or more clinical criteria suggestive of renovascular hypertension: onset of hypertension before the age of 25 years or after the age of 45 years, abrupt onset or worsening of hypertension, accelerated or malignant hypertension, uncontrolled hypertension, unexplained hypokalemia, excellent response to angiotensin-converting enzyme inhibitors, or auscultation of an abdominal or flank bruit. The selection of patients was also based on the absence of the following exclusion criteria: known intrinsic renal disease or documented decreased glomerular filtration rate, presence of obstructive uropathy, presence of a single kidney, history of stroke, history of allergy to contrast material, or inability or unwillingness to undergo CTA or DSA. Pediatric and prepubertal patients were excluded.
In 43 normotensive patients, 28 patients were potential renal donors, four patients were suspected of having Takayasu's arteritis with the symptom of claudication, two patients were suspected of having vasculitis, and nine patients were suspected of having nutcracker syndrome (20 men, 23 women; age range, 20-62 years [mean, 37 years]).
Helical CTA
The helical CTA studies were obtained with a single-detector scanner
(Somatom Plus-S, Siemens Medical Solutions), which has a maximum continuous
scanning time of 40 seconds overall, using a 3-mm collimation, 3-6 mm/s table
speed, 3- to 4-mm effective slice thickness, and reconstruction intervals of
1-2 mm. The table speed was determined by the scan range (z-axis
length), which was obtained from the unenhanced localizer image. When the scan
range was below 9 cm, the table speed was set at 3 mm/s; for a 9- to 12-cm
scan range, the table speed was set at 4 mm/s; for a 12- to 15-cm scan range,
the table speed was set at 5 mm/s; and for a scan range of more than 15 cm,
the table speed was set at 6 mm/s. Most of the patients were scanned with a
table speed of 4 or 5 mm/s.
Contrast enhancement was performed with 100-150 mL of nonionic contrast medium (iopromide, 37% iodine concentration [Ultravist 370, Schering]) that was injected into an antecubital vein at a flow rate of 2.8-3.5 mL/s from an automatic injector. The amount of contrast medium was determined with a ratio of 2 mL of contrast material per kilogram of body weight, and the maximum amount was set at 150 mL even if the patient weighed more than 75 kg. The fundamental injection rate of contrast medium was set at 3 mL/s by the protocol. However, the rate was increased to 3.5 mL/s in young (< 35 years) healthy patients, and it was decreased to 2.8 mL/s in underweight patients (< 45 kg).
Helical scanning was started after a delay of 18-22 seconds after the initiation of contrast medium injection, which was determined empirically. A delay of 18 seconds was used for 10- to 30-year-old patients, a delay of 20 seconds was used for 30- to 50-year-old patients, and a delay of 22 seconds was used for patients older than 50 years. No test bolus was performed. The scanning time was 30-40 seconds, with a single breath-hold span at the end of expiration. The average examination time for a patient was approximately 20 minutes.
After scanning, the axial sections and the maximum-intensity-projection images (both in axial and coronal planes) were obtained by radiologists or experienced technologists using an editing program. The images were reconstructed with 2-mm intervals using 180° linear interpolation. Overall, the major scanning parameters were a 3-mm collimation, 4-5 mm/s table speed, and 2-mm reconstruction interval.
Urine CT Attenuation Measurement
In all patients, we also obtained conventional transverse CT scans with a
5-mm collimation and 5-10 sections after CTA. The scans were obtained through
the renal pelvis level 4-5 minutes after the initiation of the IV bolus of
contrast material. From this axial image, the attenuation measurements of
urine were obtained in Hounsfield units. Three separate regions-of-interest
measurements (cursor size, 0.1-0.25 cm2) were obtained from the
renal pelvis or proximal ureter of each individual kidney, and these
measurements were averaged to obtained a mean measurement for each kidney.
Same-sized regions of interests were used in the same patients. The urine CT
attenuation ratio was calculated by dividing the higher urine attenuation
value of one kidney by the lower urine attenuation value of the contralateral
kidney. The observers were not aware of the angiographic findings. All
measurements were performed independently by two observers.
IA DSA
Angiography was performed within 2 days after CT examination using DSA
(Angiostar, Siemens). All studies were performed using the Seldinger technique
with a transfemoral approach. Abdominal aortography was performed with a
5-French pigtail catheter (30 mL of iopromide, 37% iodine concentration
[Ultravist 370, Schering]). When the existence of RAS or accessory renal
arteries was equivocal on the flush aortogram, the examinations were completed
with selective renal arteriography using a 5-French cobra-shaped catheter
(additional 20-50 mL of iopromide).
Image Analysis
All CTA and IA DSA images were interpreted independently by two
radiologists who had no information about the clinical findings. To determine
the percentage of diameter reduction, most cases were measured visually.
However, some borderline stenoses were measured with a precision caliper and a
magnifying lens. When there was a discrepancy in the grading of a stenosis
between the two radiologists, the problem was settled in consensus after
careful reexamination of the renal arteries. When there was a mismatch in the
diagnosis or grading of a stenosis between the two studies, the results of IA
DSA served as the standard of reference.
Renal arteries were classified into five groups: grade 0, normal; grade 1,
0-50% diameter reduction; grade 2, 50-75% diameter reduction (moderate
stenosis); grade 3, 75-99% diameter reduction (severe stenosis); and grade 4,
total occlusion. A significant RAS is defined as a stenosis that causes a
diameter reduction of the renal artery of more than 75% (
grade 3) on CTA
or IA DSA. We considered the proximal part of the stenotic artery as normal
for their standard because several cases showed poststenotic dilatation just
distal to the stenosis.
A comparative analysis of both kidneys with respect to cortical thickness at corresponding cross-sectional levels or kidney size on maximum-intensity-projection images was also performed. A difference in cortical thickness of more than 25% at the level of the renal hilum was considered significant. As for kidney size, a difference of more than 25% compared with the contralateral kidney was considered significant.
Statistical Analysis
The results of the urine CT attenuation ratio of both kidneys were
correlated with the degree of RAS on angiography. The sensitivity,
specificity, and positive and negative predictive values of the urine CT
attenuation ratio in the detection of RAS were calculated with 2 x 2
contingency tables. The receiver operating characteristic (ROC) curve was
generated, and the area under the ROC curve was calculated. The statistical
comparisons were performed using the Mann-Whitney U test. All data
were analyzed with SPSS/PC+ software (SPSS). A statistically significant
difference was considered to exist at a p value of 
0.05 for the
test.
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Concerning the cause of RAS, eight patients had Takayasu's arteritis; three, fibromuscular dysplasia; two, chronic aortic dissection; and the other 15 patients, atherosclerotic, unclassifiable, or miscellaneous causes. None of the 43 normotensive patients showed significant RAS. Only one patient having Takayasu's arteritis showed a mild narrowing—less than 50% in diameter—of the main renal artery.
All stenoses and occlusions were detected and correctly localized with helical CTA. The grades of stenoses based on helical CTA were identical to those based on angiography except in four patients. The four mismatches between CTA and IA DSA were two underestimations of a grade 3 stenosis on IA DSA as a grade 2 stenosis on CTA (urine CT attenuation ratios: 2.04 and 1.52), one underestimation of a grade 4 stenosis on IA DSA as a grade 3 stenosis on CTA (urine CT attenuation ratio, 3.68), and one overestimation of a grade 2 stenosis on IA DSA as a grade 3 stenosis on CTA (urine CT attenuation ratio, 1.23).
A significant thinning of the renal cortex and a concomitant reduction of kidney size (difference in size compared with contralateral kidney > 25%) were seen in five patients. Three of them were the patients in whom the urine CT attenuation ratio measurement could not be obtained because of a small collecting system or a nonfunctioning kidney. The other two patients showed a urine CT attenuation ratio of 1.55 and 2.33, respectively.
In the group of the patients with essential hypertension or normal blood pressure, there were three false-positive diagnoses on CTA compared with IA DSA. Two of the false-positive CTA diagnoses were grade 2 stenoses at a main renal artery and the third was a grade 3 stenosis at one accessory renal artery; in those cases, IA DSA revealed normal renal arteries. The urine CT attenuation ratios for the first two cases were 1.01 and 1.12, respectively. The third case was an overestimation of a grade 1 stenosis on IA DSA as a grade 2 stenosis on CTA, which can be considered as a false-positive result in the diagnosis of RAS greater than 50%. In that case, the urine CT attenuation ratio was 1.08.
The urine CT attenuation ratio in 24 patients with RAS ( 50% stenosis)
and hypertension ranged from 1.11 to 4.76 (mean, 2.02; SD, 0.95) (Figs.
1A,
1B,
2A, and
2B). The two patients with
moderate-degree RAS (grade 2) showed a ratio of 1.83 and 1.23. When the two
patients with a grade 2 RAS were excluded, the urine CT attenuation ratio in
22 patients with significant RAS ( 75% stenosis) ranged from 1.11 to 4.76
(mean, 2.07; SD, 0.97). Among them, the urine CT attenuation ratio was higher
than 1.22 in 21 patients (95%), and the 95% confidence interval (CI) for the
mean ranged from 1.37 to 2.49. The other patients with essential hypertension
(n = 5) or normal blood pressure (n = 43) had a urine CT
attenuation ratio that ranged from 1.00 to 1.54 (mean, 1.07; SD, 0.09) (Figs.
3A and
3B). Among them, the urine CT
attenuation ratio was lower than 1.22 in 46 patients (96%), and the 95% CI for
the mean ranged from 1.03 to 1.07. The mean ratio between the two groups was
statistically different (p < 0.01).
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The mean urine CT attenuation ratio was 2.18 in patients with unilateral RAS (n = 16) and 1.75 in patients with bilateral RAS (n = 6). The difference of the ratio between the two groups was not statistically significant (p = 0.16). When the lateralization of RAS in the unilateral RAS group (n = 16) was considered with urine CT attenuation ratio, 15 patients (94%) showed concordant results, whereas only one patient, who had a ratio of 1.11, had discordant results. In one patient who had RAS of similar degree in bilateral main renal arteries, the urine CT attenuation ratio was 1.26. The other patients who had bilateral RAS showed different severity or location of stenosis and their urine CT attenuation ratios were concordant with the result of morphologic assessment.
Five patients underwent percutaneous transluminal angioplasty (PTA) of the stenosed artery. The follow-up CT examinations were not performed. Only one patient with a significant RAS in the right main and left intrarenal segmental arteries underwent a follow-up CT examination after aortoaortic bypass graft placement with reimplantation of the right renal artery (Figs. 5A, 5B, 5C, 5D, and 5E). The urine CT attenuation ratio in that patient was reversed from 733/378.2 (1.94) to 670/1,379 (0.49; this ratio is changed to 2.06 when it is recalculated in reference to the stenotic side) after operation. This may reflect the presence of functionally significant RAS in the left side.
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To be able to detect all curable patients with suspected secondary hypertension, screening methods that are less invasive and have a high sensitivity are needed. An adequate specificity is also needed to hold the proportion of unnecessary procedures to reasonable limits. Laboratory testing, including plasma renin activity, has relatively poor sensitivity and specificity. Although some abnormalities may be seen on excretory urography, it also has a low sensitivity and specificity for the detection of RAS; therefore, it is not a good screening test for renovascular hypertension.
For patients with suspected renovascular hypertension, captopril-enhanced renal scintigraphy was used. It has a reported sensitivity of 80% and specificity of near 100% for the detection of RAS [5]. Another test that has been used for screening is renal duplex Doppler sonography. It is an easy and noninvasive technique, and it has the potential to provide physiologic information regarding the hemodynamic status of the kidney [13]. A study using a combined approach of imaging both the main renal arteries and the segmental renal arteries reported a sensitivity of 96% and specificity of 98% compared with angiography [6]. Although captopril scanning or Doppler sonography appears to be a useful test for patients who are suspected to have RAS, angiography remains the reference standard for the diagnosis of this disease. However, it requires invasive procedures.
Favorable results of helical CTA and MRA have been reported in the diagnosis of vascular diseases, including RAS, with minimal invasiveness [7, 8]. Researchers found that CTA had a sensitivity of 94% and specificity of 98% in the evaluation of RAS [7]. In another study, researchers showed that MRA had a sensitivity of 97% and specificity of 92% compared with IA DSA for the detection of RAS [8]. In all techniques for the detection of RAS, the reported results are variable because the populations, protocols, and diagnostic criteria varied.
CTA allows simultaneous assessment of the renal vasculature, collecting system, and parenchyma during a single examination. It can be an alternative to conventional arteriography for assessing the renal arteries after endoluminal therapy for RAS, either with PTA or by deployment of metallic stents. There are certain limitations that can be expected of helical CT: difficulty in viewing peripheral stenoses of the segmental arteries and stenoses of small vessels, especially in kidneys with multiple arterial supplies; poor circulatory status; and poor patient cooperation. Today MDCT scanners have replaced single-detector CT scanners as the community standard. It is reasonable to expect that MDCT technology will further improve the accuracy of renal CTA. It allows faster acquisition of images with thinner collimation and influences substantially the overall image quality of 3D reconstructions [14].
For evaluation of RAS on CTA, the identification of ancillary findings such as nephrographic abnormalities or poststenotic dilatation can improve the specificity of identifying hemodynamically significant RAS [15, 16]. It is also useful in the interpretation of equivocal RAS. In the series of Rubin et al. [15], the identification of poststenotic dilatation or nephrogram asymmetry was highly specific for a coexistent RAS of greater than 70%. Galanski et al. [16] reported abnormal renal cortical enhancement in five cases, all associated with an RAS of greater than 50%. However, hemodynamically significant RAS frequently occurs without coexistent poststenotic dilatation or nephrographic abnormalities. Nevertheless, when these signs are used in combination with direct visualization of the stenotic segment, overall grading accuracy should improve by reducing false-negative results. Another associated CT finding in hemodynamically significant RAS may include reduction in kidney size and thinning of the renal cortex. We add differential urine concentration on delayed CT as a useful adjunctive sign in the diagnosis of significant RAS.
On IV excretory urography, findings suggestive of RAS include a small, smooth kidney; delayed nephrogram; delayed pyelogram; late development of a hyperdense pyelogram; and ureteral notching caused by enlarged collateral ureteric arteries. Unilateral RAS leads to a more unilateral hyperconcentration of contrast material in the ischemic kidney than in the normal kidney during the pyelographic phase of excretory urography. This differential in salt and water excretion by the stenosed and unstenosed kidneys serves as the basis for the Howard-Stamey test, which is no longer used, that compared the osmolality and sodium content of urine samples obtained from each kidney by retrograde ureteral catheterization [17]. The stenosed side frequently had higher osmolality and lower sodium content.
Although late development of a hyperdense pyelogram is an interesting urographic sign, it has not been accepted as a reliable indicator of RAS because it is difficult to objectively distinguish increased concentration from increased density. However, objective measurement of urine concentration of contrast material is possible on transverse CT scans. By comparing the CT attenuation values of urine in both kidneys, we can detect and lateralize significant RAS with high specificity. The attenuation value of urine can enhance the confidence or reduce false-positive CTA diagnoses of RAS. At a cutoff value of 1.22, this method showed considerably high sensitivity and specificity, which can be comparable to the result from CTA. However, it should be remembered that the differential urine CT attenuation does not directly define the renal vascular anatomy. It relies on the functional differences between a normally perfused and an ischemic kidney; therefore, it is not a definitive diagnostic tool but a useful adjunct to helical CTA that offers noninvasive assessment of renal function in patients with functionally significant RAS.
Investigators realized the variability of attenuation value measurements from the early days of their clinical use. Physical factors such as patient size and the density of surrounding structures, the kilovoltage used for scanning, and partial volume averaging are well known for their effects on the attenuation value of structures [18]. Despite this wide variability, however, the authors of many studies and landmark articles continue to recommend stringent use of CT attenuation values to aid in the characterization of disease processes [19]. In our study, we could not obtain reliable data in three patients because of a small renal collecting system, which causes wide variability in urine CT attenuation due to partial volume averaging. Attenuation value measurement at a single pixel has wide variability, so we used a relatively large circular cursor and measured attenuation only when there was a collecting system large enough to measure.
Although we performed conventional transverse CT in addition to helical CTA for the measurement of urine CT attenuation, researchers have reported that there is no significant difference between conventional and helical CT in attenuation value measurements [20]. Increased noise associated with the 180° interpolation in helical CT may lead to an increase in the variability of attenuation value measurements; however, the mean value is spared. Therefore, transverse CT scans of the kidney for measuring urine attenuation can be replaced by helical CT scans.
Some information about renal function is available from observing excretion of contrast material on CT. Urine CT attenuation measurement does not require the complex, time-consuming methods that limit clinical applicability; this is a practical method to attempt to derive functional data. Such data may be useful in the noninvasive assessment of kidney function, especially in the detection or lateralization of significant RAS. Most prior descriptions of CT-derived information regarding kidney function were largely qualitative. Some reports showed the usefulness of CT densitometry measures in assessing renal function [21]; however, the application of urine CT attenuation measurements for the assessment of kidney function has not been described, to our knowledge, before our study.
Renin analysis of selective renal veins was suggested to be a method for identifying patients with renin-dependent hypertension and a predictor of the therapeutic benefit from renal revascularization such as PTA [22]. However, it was associated with many false-positive findings (47.8%), a low sensitivity (65%), and a low specificity (52.2%) [23]. Our results showed that the urine CT attenuation ratio is a reliable method for detecting or lateralizing significant RAS with high sensitivity like selective renal vein renin analysis. It does not require additional cost or inconvenience, and it has potential to be an indicator of renal revascularization after surgical or endovascular therapies.
In one patient who had significant RAS in the right main and left intrarenal segmental arteries, a follow-up CT examination was performed after aortoaortic bypass graft placement with right renal artery reimplantation. The urine CT attenuation ratios in that patient were reversed from 1.94 before surgery to 0.49 after the operation. This finding suggests the development of a relatively hyperdense pyelogram in the residual stenotic side (left kidney in this patient) after revascularization of the contralateral side. For the evaluation of the value of the urine CT attenuation ratio in providing functional information after renal revascularization, more follow-up studies are needed.
Renovascular hypertension can be managed by medical therapy with antihypertensive drugs, but surgical and percutaneous therapies offer a potential cure by eliminating the angiotensinogen component of hypertension. Depending on the skill and experience of the interventionist and the clinical circumstances, virtually any RAS can be treated by percutaneous endovascular therapy. Even renal artery occlusions have been successfully treated with PTA or stenting. Although 26 patients showed significant RAS in our study, only five patients underwent PTA or stenting for one or more of the following reasons: Surgical management was performed in many patients by the referring physician's choice; some patients were transferred to a different hospital; and some patients with unilateral RAS and mild symptoms were treated by medical therapy with close monitoring for progression of disease.
The test for detecting RAS with functional information can be limited in patients with unilateral RAS and severely damaged kidneys, in patients with bilateral RAS with equivalently significant stenosis, and in patients with stenosis of an intrarenal or accessory renal artery. In these cases, the sensitivity of the urine CT attenuation ratio in the detection of RAS would be decreased. As in bilateral RAS, blood pressure initially rises in response to increased renin and angiotensin II, and increased aldosterone secretion leads to sodium retention. However, because there is no normal contralateral kidney to excrete the retained sodium and fluid, volume expansion persists and renin levels fall, presumably because of restored perfusion of nephrons by the hypertension and volume expansion. Although bilateral RAS shows different pathophysiology from unilateral RAS, the urine CT attenuation ratio still can be used to detect greater disease on one side in bilateral stenoses.
Our study showed the ratio was elevated in most patients with bilateral RAS or unilateral stenosis of segmental or accessory renal arteries. Although there was no statistical difference, the ratio in the patients with bilateral RAS was relatively lower than that of the patients with unilateral RAS. However, one should keep in mind that the urine CT attenuation ratio is an adjunct to CTA. Also, although this ratio may be useful in determining the prognosis of future interventions or helping guide therapy to the symptomatic kidney, it requires further assessment in the future.
Because renal vascular disease accounts for a low incidence of cases of hypertension, typically the number of patients referred for evaluation of possible RAS exceeds the number who are ultimately diagnosed with RAS. The patient population in this study included a high percentage of patients with morphologic findings on both conventional CT and CTA (26 of 33 patients) who were eventually diagnosed as having renal artery hypertension. The high incidence of RAS among the 33 patients with suspected renovascular hypertension in this study is attributable to the strict clinical criteria and other screening studies, including renal Doppler sonography or captopril scintigraphy, used in the selection of the patient population.
Certain limitations and potential pitfalls of the urine CT attenuation ratio can be seen in this study. First, there is a significant problem of the wide variability of attenuation values in some patients. Partial volume averaging is probably not a limitation given the high density of the urographic contrast material and the fact that most renal pelves are greater than 1 cm in diameter. However, in a kidney that has a small collecting system, it is difficult to reliably measure the urine CT attenuation, and the measurements have wide variability, mainly due to partial volume averaging. Technically inadequate examinations should be treated as false-negatives in detecting RAS and may be minimized by acquisition of thin-slab scans. A poor understanding about the sequential changes of the urine CT attenuation in patients with RAS is another limiting problem. The optimal time for the measurement of urine attenuation is uncertain, and there may be interindividual variation in the excretion pattern of urine. A large series study in the general population with variable imaging protocols is needed.
CT attenuation of urine can be influenced by variation in the size of the renal collecting system. For example, hydronephrosis or a congenitally more capacious renal collecting system on the side of concern would dilute the excreted contrast material, possibly resulting in a false-negative finding. Similarly, a more capacious collecting system on one side in a patient without RAS may result in a false-positive result for the contralateral side. Renal functional status can influence the urine CT attenuation ratio. Although the ratio was still a good indicator of RAS in two patients who had renal cortical thinning and a concomitant reduction of kidney size, such ancillary endorgan morphologic changes make the ratio unreliable when the kidney is aggravated. The measurement of relative CT attenuation of renal collecting systems needs additional time and procedures. However, with the advent of PACS, measurement of attenuation became easier and more convenient. Further studies using MDCT, which is the community standard for renal CTA, and including many patients with moderate stenoses (50-75%) are required to determine the usefulness of the urine CT attenuation ratio.
In conclusion, this study shows that secondary signs on CT urography can corroborate CTA findings. The urine CT attenuation ratio is a simple and reliable indicator to detect and lateralize hemodynamically significant RAS. It relies on the functional differences between a normally perfused kidney and an ischemic kidney. The urine CT attenuation ratio may be used to determine the prognosis of asymptomatic patients with RAS if they undergo interventions 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 can be used as an adjunct to helical CTA.
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