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Ultrasound vs CT for the detection of ureteric stones in patients with renal colic

Departments of ¹Radiology and ²Urology, Shaare Zedek Medical Center, Jerusalem 91031, Israel

Correspondence: Irith Hadas-Halpern, MD

	Abstract

Top Abstract Introduction Patients and methods Results Discussion References

 
The aim of our study was to compare the accuracy of non-contrast spiral CT with ultrasound (US) for the diagnosis of ureteral calculi in the evaluation of patients with acute flank pain. 62 consecutive patients with flank pain were examined with both CT and US over a period of 9 months. All patients were prospectively defined as either positive or negative for ureterolithiasis, based on follow-up evaluation. 43 of the 62 patients were confirmed as having ureteral calculi based on stone recovery or urological interventions. US showed 93% sensitivity and 95% specificity in the diagnosis of ureterolithiasis; CT showed 91% and 95%, respectively. Pathology unrelated to urinary stone disease was demonstrated in six patients. Although both modalities were excellent for detecting ureteral stones, consideration of cost and radiation lead us to suggest that US be employed first and CT be reserved for when US is unavailable or non-diagnostic.

	Introduction

Top Abstract Introduction Patients and methods Results Discussion References

 
Imaging evaluation of patients with acute flank pain is traditionally based on intravenous urography (IVU) as the standard screening tool for detecting urinary calculi. IVU requires iv contrast medium, with its associated potential risks [1]. In addition, the length of this examination may preclude rapid evaluation of patients in an emergency setting. These considerations have led to the use of other techniques, such as the combination of plain abdominal radiography and ultrasound (US) [2], and more recently unenhanced helical CT [3]. Plain radiographs are not sensitive to non-radio-opaque calculi or to non-calculous obstruction. Plain radiography also lacks specificity, as phleboliths, which are common pelvic calcifications, are not always readily differentiated from urinary tract calculi [4]. The advantages of CT over IVU are well documented and include shorter examination time, avoidance of iv contrast medium, greater sensitivity for stone detection and increased detection of abnormalities unrelated to ureteral stones. However, radiation dose is high [5]. Transabdominal US has the advantage of being universally available, does not expose the patient to radiation, requires no iv contrast medium and is independent of kidney function; US is therefore attractive as the modality of choice for the initial evaluation of urinary symptoms. This prospective study compared the accuracy of spiral CT with US in the evaluation of patients with acute flank pain. Plain radiograph studies were excluded since plain radiography and US have already been compared with CT [6].

	Patients and methods

Top Abstract Introduction Patients and methods Results Discussion References

 
62 consecutive patients, seen for suspected renal colic in our emergency department over a 9 month period, were enrolled in a standardized double-blinded protocol that consisted of US examination followed by CT. There were 42 men and 20 women. The age range was 26–89 years. All imaging studies were conducted within 4 h of admission to the emergency department.

US examination was performed transabdominally, after ingestion of 400 ml of water, with an ATL Ultramark 9 system (Advanced Technology Laboratories, Bothell, WA) using 3.5 MHz, 5 MHz and 7.5 MHz probes. Examinations were conducted by one of three experienced senior radiologists (DF, IZ, IHH). US diagnosis of ureteral calculi required the demonstration of an intraluminal hyperechoic structure causing acoustic shadowing. The presence of hydronephrosis and perinephric fluid were also noted.

Non-enhanced helical CT examinations were performed with an Elscint Helicat II scanner (Marconi Medical Systems, Cleveland, OH). CT images were obtained from the upper renal poles to the bladder base. Helical data acquisition consisted of 6.5 mm thick sections and a pitch of 1.5:1. No oral or iv contrast medium was administered. The CT examinations were reviewed by an experienced radiologist (MP) and were evaluated for the presence of ureteral calculi, perinephric or periureteric stranding, and hydronephrosis. CT diagnosis of ureteral calculi was established by visualization of a high attenuation structure (greater than 100 Hounsfield units) within the ureteral lumen.

The two sets of studies were reviewed by independent radiologists who were blinded to the patient's identity and who noted all findings including stone demonstration, stone size, and location and signs of obstruction. Findings not related to calculi were also noted.

	Results

Top Abstract Introduction Patients and methods Results Discussion References

 
43 of the 62 patients were confirmed to have ureteral calculi based on stone recovery or urological interventions. The US and CT findings are summarized in Table 1.

View larger version (160K): [in this window] [in a new window]   Figure 1. Ultrasound shows an intraluminal echogenic focus with an acoustic shadow in the right midureter.

View larger version (94K): [in this window] [in a new window]   Figure 2. Ultrasound demonstration of distal ureteral calculi. (a) Minimal hydronephrosis. (b) Mildly dilated distal ureter with an echogenic focus inside (arrow).

Of the 43 patients with calculi, CT detected 39 (sensitivity 91%, specificity 95%, positive predictive value 98%, negative predictive value 82%). 5 calculi were demonstrated in the proximal ureter, 4 in the midureter (Figure 3) and 30 in the distal ureter (Figure 4).

View larger version (107K): [in this window] [in a new window]   Figure 3. CT shows a calcified stone in the right midureter.

View larger version (128K): [in this window] [in a new window]   Figure 4. CT shows a calcified stone in the right distal ureter.

Pathology unrelated to urinary stones was demonstrated in six patients and included appendicitis, cholelithiasis, cholecystitis and adnexal mass in one patient each, and torted ovarian cyst in two patients. All of these conditions were detected by US and CT except the appendicitis, which was diagnosed by CT alone.

	Discussion

Top Abstract Introduction Patients and methods Results Discussion References

 
Recent studies have shown that non-contrast spiral CT is an excellent method for demonstrating ureteral calculi in patients with suspected renal colic [2]. Smith et al [3] showed non-contrast CT to be more effective than IVU in identifying ureteral stones. In another comparative study, Sommer et al [6] noted that reformatted, non-contrast spiral CT images were superior to a combination of US and plain abdominal radiography for imaging ureteral calculi. In the current study, a comparison was made between spiral CT and US in 62 patients, with comparable results for the two modalities in the demonstration of ureteral calculi. In some cases it was difficult to ascertain on CT whether calcification was within the urinary tract or elsewhere, e.g. calcified phleboliths or a calcified seminal vesicle (Figure 5).

View larger version (131K): [in this window] [in a new window]   Figure 5. CT shows multiple calcifications of the seminal vesicles.

US, which is universally available, non-invasive, inexpensive and radiation free, is preferred by some radiologists as the initial method for evaluation of the kidneys and bladder. However, US is considered to be of limited value in demonstrating pathological conditions of the ureter [8].

All patients with ureterolithiasis described herein had some degree of ureterohydronephrosis, hence US was able to follow the ureter to the level of the stone and demonstrate the exact nature of the obstructing lesion. An intraluminal echogenic focus with acoustic shadowing was clearly depicted in all cases. Technical problems might occur in assessing the ureter when the stone is in the middle third, an area often obscured by bowel gas; we overcame this problem by compressing the area to be examined and changing the patient's position.

Dalla Palma [9] evaluated 120 patients with renal colic using US and plain radiographs, and achieved 95% sensitivity but only 67% specificity. US was classified as positive for ureteric colic in the study when calculi or hydronephrosis were present. In the current study, only cases with a definite demonstration of ureteral calculi were classified as positive and our results show a high specificity of 95%.

We did not evaluate resistive index (RI) values or ureteric jets in our study. Others have recently examined the role of RI with disappointing results. Cronan [10] showed that the addition of RI to renal US did not improve the 77% sensitivity of US in that series. The use of colour Doppler for ureteric jets was studied by Burge et al [11]. Most cases of high-grade urinary tract obstruction had abnormal jets, whereas jets were often normal in low-grade obstruction or non-obstucting stones. In our study, CT and US were equally sensitive in detecting ureteral calculi; 91% and 93%, respectively. In the study by Sommer et al, there were false negative US examinations owing to a lack of significant hydronephrosis detectable on the examination [6]. In our patients, US was also accurate in depicting stones in cases of minimal hydronephrosis.

Extraurinary causes mimicking renal colic were demonstrated by both modalities except in one case of appendicitis that was diagnosed by CT only. However, the small number of cases with extraurinary causes precluded statistical analysis.

In summary, both spiral CT and US were found to be excellent modalities for depicting ureteral stones, but because of high cost, radiation dose and high workload of CT, we suggest that US should be performed first in all cases and CT should be reserved for cases where US is unavailable or fails to provide diagnostic information.

Received for publication January 12, 2001. Revision received May 29, 2001. Accepted for publication June 22, 2001.

	References

Top Abstract Introduction Patients and methods Results Discussion References

 

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