Low-Dose Versus Standard-Dose CT Protocol in Patients with Clinically Suspected Renal Colic

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1 CT of Renal Colic Genitourinary Imaging Original Research Pierre-Alexandre Poletti 1 Alexandra Platon 1 Olivier T. Rutschmann 2 Franz R. Schmidlin 3 Christophe E. Iselin 3 Christoph D. ecker 1 Poletti P-A, Platon A, Rutschmann OT, Schmidlin FR, Iselin CE, ecker CD Keywords: abdominal imaging, CT, low-dose CT, radiation dose, urinary tract DOI: /AJR Received June 16, 2006; accepted after revision September 12, Supported by the grant for research and development of the University Hospital of Geneva. 1 Department of Radiology, University Hospital Geneva, 24 rue Micheli-du-Crest-14, Geneva 1211, Switzerland. Address correspondence to P.-A. Poletti (pierre-alexandre.poletti@hcuge.ch). 2 Department of Internal Medicine, University Hospital Geneva, Geneva, Switzerland. 3 Urology Clinic, University Hospital Geneva, Geneva, Switzerland. AJR 2007; 188: X/07/ American Roentgen Ray Society Low-Dose Versus Standard-Dose CT Protocol in Patients with Clinically Suspected Renal Colic OJECTIVE. The purpose of our study was to compare a low-dose abdominal CT protocol, delivering a dose of radiation close to the dose delivered by abdominal radiography, with standard-dose unenhanced CT in patients with suspected renal colic. MATERIALS AND METHODS. One hundred twenty-five patients (87 men, 38 women; mean age, 45 years) who were admitted with suspected renal colic underwent both abdominal low-dose CT (30 mas) and standard-dose CT (180 mas). Low-dose CT and standard-dose CT were independently reviewed, in a delayed fashion, by two radiologists for the characterization of renal and ureteral calculi (location, size) and for indirect signs of renal colic (renal enlargement, pyeloureteral dilatation, periureteral or renal stranding). Results reported for low-dose CT, with regard to the patients body mass indexes (MIs), were compared with those obtained with standard-dose CT (reference standard). The presence of non urinary tractrelated disorders was also assessed. Informed consent was obtained from all patients. RESULTS. In patients with a MI < 30, low-dose CT achieved 96% sensitivity and 100% specificity for the detection of indirect signs of renal colic and a sensitivity of 95% and a specificity of 97% for detecting ureteral calculi. In patients with a MI < 30, low-dose CT was 86% sensitive for detecting ureteral calculi < 3 mm and 100% sensitive for detecting calculi > 3 mm. Low-dose CT was 100% sensitive and specific for depicting non urinary tract-related disorders (n = 6). CONCLUSION. Low-dose CT achieves sensitivities and specificities close to those of standard-dose CT in assessing the diagnosis of renal colic, depicting ureteral calculi > 3 mm in patients with a MI < 30, and correctly identifying alternative diagnoses. ecause of its high sensitivity and specificity for the detection of ureteral stones [1, 2], CT is now recommended by many authors [3, 4] as the initial diagnostic imaging technique in patients with suspected renal colic. The initial use of CT reveals not only the presence of a calculus but also the stone size and location [5], all of which is useful information for selecting the most appropriate therapeutic approach [6]; this information is not always obtained with abdominal radiography and sonography [7, 8]. However, because renal colic frequently affects young adults, with a rate of recurrence of almost 50% [9], the systematic use of CT at a patient s admission raises an ethical concern about the dose of radiation administered [8, 10]. Several authors have reported that lowdose CT protocols, with substantial reduction of tube charge current or an increase in the pitch, may be used in the screening of patients with suspected renal colic [11 15]. More recently, clinical [16 18] and ex vivo porcine phantom [19] studies have evaluated a lowdose MDCT protocol using low tube charge current ( 30 mas), which delivers a dose of radiation close to that delivered by abdominal radiography. Such low-dose CT protocols resulted in a substantial reduction (50%) of radiation dose when compared with initial management with abdominal radiography and sonography, by reducing the need for further standard-dose CT [18]. Furthermore, lowdose CT might achieve sensitivities and specificities close to those reported for standarddose CT in detecting ureteral stones [16, 17]. To our knowledge, no study has specifically compared low-dose CT protocols ( 30 mas) with those of standard-dose CT as the sole reference standard for the depiction of direct and indirect signs of renal colic and for determination of calculi size in a clinical setting. Some specific concerns have also been raised about using low-dose CT in overweight patients and regarding its capacity to depict AJR:188, April

2 alternate diagnoses [20]. This may explain why low-dose CT protocols using 30 mas (or less) are not yet universally endorsed for the initial investigation of patients admitted with suspected renal colic [20]. The aim of our analysis is to compare lowdose CT using 30 mas with standard-dose unenhanced CT for the detection of renal and ureteral calculi of various sizes and for the visualization of indirect signs of renal colic, according to the patient morphotype. Materials and Methods Patient Population One hundred twenty-five consecutive adult patients who were admitted during the day to our emergency department with suspected renal colic underwent low-dose CT in addition to standarddose CT. Patients admitted after extracorporeal shockwave lithotripsy and pregnant women were excluded. The study protocol was approved by the institutional review board of our hospital. Written informed consent was obtained from each patient. All patients were aware that they would be exposed to an additional dose of radiation, estimated to about one sixth of that delivered by standarddose CT. Each patient s body mass index (MI) was calculated at admission by the attending physician or the emergency radiology fellow, on the basis of weight and height in the patient s history, and was reported on the consent form as MI < 18.5 (underweight), between 18.5 and 24.9 (normal), between 25 and 29.9 (overweight), or 30 (obese) [21]. Technical Imaging Parameters Standard-dose CT and low-dose CT were performed from the lung bases to the pelvis using a 4-MDCT scanner (MX 8000, Philips Medical Systems). Standard-dose CT was obtained with 5- mm contiguous sections, a table speed of 5-mm/s (pitch = 1), 120 kv, and 180 mas. Low-dose CT was performed with the following parameters: reconstruction slice thickness, 5.0 mm; pitch, 1.25; gantry rotation speed, 0.5 second; tube potential, 120 kv; tube charge per gantry rotation, 30 mas (75 ma 0.5 s / 1.25 = 30 mas). The average scanned length, for both standard-dose and lowdose CT, was 40 ± 5 cm for men and 35 ± 5 cm for women. Effective Dose Calculation Low-dose CT According to data provided by the manufacturer, the dose delivered by low-dose CT was estimated using a normalized weighted CT dose index ( n CTDI w ) in air of mgy/mas at 120 kv. Radiation exposure was then calculated as follows: DLP (mgy cm) = CTDI vol L, where DLP is the dose length product, CTDI vol is the volume CT dose index [22], and L is the scan length. For women, the CTDI vol was equal to DLP air = = 192 ± 27 mgy cm. The effective dose (E) was obtained by applying the following relationship: E women = DLP air f [23] (where f is a specific conversion factor) E women = = 2.1 ± 0.3 msv. The same calculation was applied to men using a 40-cm scan length instead of 35 cm: E men = = 1.6 ± 0.2 msv. Standard-dose CT Using 180 mas instead of 30 mas, the same calculation as used for low-dose CT was performed to determine the effective dose delivered by standard-dose CT: E women = 12.6 ± 1.8 msv, and E men = 9.6 ± 1.2 msv. Data Collection and Analysis Standard-dose CT images were immediately interpreted on the PACS by the resident and attending radiologists on duty, and a written report was transmitted to the referring physician (standard procedure in our institution). Low-dose CT images were stored in the PACS but were not interpreted by the radiologists on call. At the end of the study, lowdose CT scans were interpreted independently by two board-certified attending radiologists with 9 and 4 years experience in abdominal CT who were blinded to standard-dose CT findings, patient names, and demographics. Low-dose CT images were analyzed in a random order by both radiologists, using the same workstation and the same visualization software (Cedara I-softview, version 6.1, Cedara software). The following information was recorded for each low-dose CT examination and reported on a standardized form: First, the number, size (largest diameter on the axial plane), and location of calculi in the urinary tract. Renal and ureteral calculi were sorted into three categories according to their size: 0 2.9, 3 4.9, and 5 mm. Location was reported as renal or ureteral. Second, the presence of a rim sign, defined as a halo of soft-tissue attenuation around the circumference of an ureteral stone. This sign has been considered useful for differentiating ureteral calculi from phleboliths [24 27]. Third, any indirect signs of ureteral obstruction (renal enlargement, pyeloureteral dilatation, or periureteral or perirenal stranding). Fourth, the presence of non urinary tract related disorders that could explain the patient s symptoms. Disparities between reviewers were solved by consensus after review of the low-dose CT images and were recorded on a separate form. Once all low-dose CT examinations were interpreted, standard-dose CT images were analyzed a second time, in a random order, in consensus, by the same two reviewers, using standardized forms similar to those used for MDCT. If a patient also underwent contrast-enhanced CT, only the unenhanced CT images were interpreted. An independent nurse was especially employed to help organize this data collection and analysis part. Statistical Analysis Statistical analyses were performed using statistical software SPSS, version 11.5 (SPSS). The twotailed Fisher s exact test was used for group comparison, and the Student s t test was used for means comparisons. Interobserver agreement between the two radiologists for the depiction of renal and ureteral calculi of any size and for the depiction of calculi of at least 5 mm was analyzed for both groups using Cohen kappa statistics. An excellent interobserver agreement was defined as a kappa value of 0.81 or more. A p value of less than 0.05 was indicative of a statistically significant difference between two different sample populations. Data obtained for patients with a MI 30 were analyzed separately. Results Demographics and Diagnosis of Renal Colic The study population consisted of 87 men and 38 women (age range, years; mean age, 45 years; median, 42 years). Eleven (9%) patients had a MI < 18.5; 67 (54%), a MI between 18.5 and 24.9; 34 (27%), a MI between 25 and 29.9; and 13 (10%), a MI 30. When compared with standard-dose CT, low-dose CT was 97% (98/101) sensitive and 96% (23/24) specific for identifying at least one direct or indirect sign suggestive of renal colic. Twenty-six (21%) of 125 patients were considered to have no evidence of renal colic on low-dose CT. A direct or indirect sign of renal colic was detected on CT in three (12%) of these 26 patients. No statistically significant difference was seen between patients with different MIs for the identification of a direct or indirect sign of renal colic. Indirect Signs of Renal Colic An indirect sign of renal colic (pyeloureteral dilatation, renal enlargement, or perirenal or periureteral stranding) (Fig. 1) 928 AJR:188, April 2007

3 CT of Renal Colic A C Fig year-old man with right flank pain. A and, Upper abdomen axial low-dose CT image (120 kv, 30 mas, pitch of 1.25, 5-mm collimation) (A) shows dilatation of pyelocaliceal system (asterisk) in enlarged right kidney and stranding of perirenal fat (arrow). Same findings are shown on standard-dose CT image (120 kv, 180 mas, pitch of 1, 5-mm collimation) (). C, Low-dose CT scan shows calculus of 2 mm in distal aspect of right ureter (arrow) and surrounded by halo of soft-tissue attenuation (rim sign). D, Size of calculus (arrow) is underestimated by < 1 mm when compared with conventional CT. TALE 1: Sensitivity and Specificity of Low-Dose CT for Detecting Direct and Indirect Signs of Renal Colic, by Patient s ody Mass Index (MI) as Compared with Standard CT (Reference Standard) All Patients MI < 30 MI 30 Sign Sensitivity (%) Specificity (%) Sensitivity (%) Specificity (%) Sensitivity (%) Specificity (%) Indirect signs of renal colic a 98 (91/93) 100 (32/32) 96 (84/87) 100 (26/26) 100 (7/7) 100 (6/6) Ureteral calculus 93 (79/85) 95 (38/40) 95 (77/81) 97 (30/31) 50 (2/4) 89 (8/9) Rim sign 77 (17/22) 95 (52/55) 81 (17/21) 96 (51/53) 0 (0/1) 50 (1/2) Note Numbers in parentheses are numbers of patients. a Considered positive (or negative) if at least one (or all) of the following signs are present (or absent): pyeloureteral dilatation, renal enlargement, ureteral dilatation, perirenal stranding, or periureteral stranding. D was detected in 18 (40%) of the 45 patients with no direct sign of calculus on low-dose CT and in 16 (39%) of the 41 patients with no calculus on standard-dose CT. The sensitivity and specificity of low-dose CT for detecting direct or indirect signs of renal colic in patients with a MI < 30 and 30 are reported in Table 1. Direct Signs of Renal Colic and Detection of Ureteral Calculi An interobserver agreement between the two senior radiologists of 98.4% and a AJR:188, April

4 kappa value of 0.97 ± 0.02 were obtained for the identification of ureteral calculi on low-dose CT. An interobserver agreement of 96.1% was achieved for identifying calculi as being < 5 mm, with a kappa value of 0.87 ± Ureteral calculi were identified in 80 (64%) patients on low-dose CT and in 84 (67%) patients on standard-dose CT (Fig. 2). A total of 88 ureteral calculi were detected on standard-dose CT, of which 30 (34%) had a size of mm; 29 (33%), a size of mm; and 29 (33%), a size of 5 mm. A C Fig. 2 Low-dose CT that was false-negative for ureteral stone in 43-year-old overweight patient (body mass index, 30) with intense left flank pain and hematuria. A and, Axial low-dose CT image (120 kv, 30 mas, pitch of 1.25, 5-mm collimation) (A) of upper abdomen shows enlargement of left kidney, dilatation of left pyelocaliceal system (asterisk), stranding of perirenal fat (arrowheads), and 6-mm calculus in pyramid (black arrow). Note also 10-mm renal stone in central aspect of right kidney (white arrow). Same findings are shown on standard-dose CT image (120 kv, 180 mas, pitch of 1, 5-mm collimation) (). C, Axial low-dose CT image at pelvic level shows that multiple streak artifacts reduce quality of interpretation. No ureteral calculus has been reported at low-dose CT analysis. D, Standard-dose CT image at same level as C shows 4.5-mm calculus (arrow) at left ureterovesical junction. No calculus smaller than 3 mm was overestimated at low-dose CT. The size of ureteral calculi was correctly estimated to be between 3 and 4.9 mm on low-dose CT in 69% (20/29) of calculi, underestimated in 24% (7/29), and overestimated in 3% (1/29). Of the 29 calculi 5 mm at standard-dose CT, 76% (22/29) were correctly estimated as being 5 mm at low-dose CT. Twenty-four percent (7/29) of them were underestimated at low-dose CT; in this group, the smallest size reported at low-dose CT was 4.2 mm (5 mm at standard-dose CT). Renal Calculi An interobserver agreement of 98% and a kappa value of 0.89 ± 0.04 were obtained for identification of renal calculi. At least one renal stone was reported in 39 (31%) patients on low-dose CT and in 47 (38%) patients on standard-dose CT. A total of 96 renal calculi were detected at standard-dose CT, of which 58 (60%) were mm; 23 (24%), mm; and 15 (16%), 5 mm. No calculus < 3 mm at standard-dose CT was overestimated at low-dose CT. The size of renal calculi was correctly es- D 930 AJR:188, April 2007

5 CT of Renal Colic TALE 2: Sensitivity of Low-Dose CT for Detecting Ureteral and Renal Calculi, by Calculi Size and Patient ody Mass Index (MI) Ureteral Calculi Renal Calculi MI mm mm 5 mm mm mm 5 mm Any 83 (25/30) 97 (28/29) 100 (29/29) 60 (35/58) 96 (22/23) 100 (15/15) < (25/29) 100 (26/26) 100 (29/29) 63 (33/52) 95 (20/21) 100 (12/12) 30 0 (0/1) 67 (2/3) 33 (2/6) 100 (2/2) 100 (3/3) Note Numbers in parentheses are numbers of calculi. Dash ( ) indicates no data available. timated to be between 3 and 4.9 mm on lowdose CT in 96% (22/23) of calculi, underestimated in 43% (10/23), and overestimated in 4% (1/23). The size of renal calculi was correctly estimated as 5 mm at low-dose CT in 93% (14/15) and underestimated in 7% (1/15). An interobserver agreement of 94.6% A C was achieved for the identification of a renal calculus as being < 5 mm, with a kappa value of 0.89 ± Sensitivities and specificities of low-dose CT for detecting ureteral and renal calculi, according to their size, in patients with MIs < 30 and 30, are reported in Table 2. Fig year-old woman with suspected complicated renal colic (intense left flank pain and fever). A and, Axial low-dose CT image (120 kv, 30 mas, pitch of 1.25, 5-mm collimation) (A) shows wall irregularities of horizontal sigmoid colon (arrowheads) and infiltration of perisigmoid fatty tissue (asterisk), suggesting diverticulitis. Axial standard-dose CT image (120 kv, 180 mas, pitch of 1, 5-mm collimation) () shows same findings as lowdose CT. C, Axial contrast-enhanced CT image with rectal opacification confirms diagnosis of sigmoid diverticulitis. Thickening of inflamed sigmoid part (arrowheads) is well shown. Two-centimeter collection with parietal enhancement, consistent with abscess (arrow), was overlooked on both low-dose CT and unenhanced standarddose CT. Alternate Diagnoses A diagnosis other than renal colic was directly suggested on six low-dose CT and standard-dose CT examinations: appendicitis (n = 3), pyelonephritis (n =1), fecal impaction (n = 1), and diverticulitis (n = 1) (Fig. 3). Standard-dose CT did AJR:188, April

6 not depict more alternative diagnoses than low-dose CT. Discussion To our knowledge, our study is the only one to systematically compare a low-dose CT protocol with standard-dose CT (180 mas) as the sole reference standard in the same population cohort. In this study, low-dose CT achieved excellent sensitivity (97%) and specificity (96%) for diagnosing renal colic on the basis of direct (detection of calculus) and indirect CT signs, using standard-dose unenhanced CT as the reference standard. In addition, the interobserver agreement was excellent when low-dose CT images were evaluated by two independent reviewers. Until our study, the accuracy of low-dose CT protocols for detecting ureteral stones, using a tube charge current < 50 mas, was evaluated in only two series to our knowledge. In a study of 142 patients using a tube charge current of 20 mas, low-dose CT was 97% sensitive and 95% specific in detecting ureteral calculi when compared with clinical and radiologic follow-up [17]. In another study, low-dose CT using a tube charge current of 30 mas achieved a sensitivity between 89.5% and 94.7% and a specificity between 94.1% and 100% for detecting ureteral calculi [16]. oth series were limited by the absence of a reference standard and by the fact that further imaging was not systematically performed after low-dose CT was performed. Nevertheless, those authors had already suggested that low-dose CT protocols delivering a radiation dose close to that of an abdominal radiograph may be comparable to standard-dose CT for detecting ureteral stones. Our study data substantiate these observations. To suspect the diagnosis of renal colic without having further information about the calculus size and location is usually not sufficient to select the most appropriate therapeutic approach, the need for hospitalization, or the need for extracorporeal shockwave lithotripsy or other urologic procedures [3, 28]. The size and location of the calculus are determinants of the likelihood of spontaneous stone passage [6, 29, 30]. Characterization of stone morphology and location are the major advantages of CT over other imaging techniques (abdominal radiography, sonography) commonly used in the initial evaluation of patients with suspected renal colic [1, 3]. A recent study using a higher tube current (50 mas) reported the limitation of low-dose CT for detecting calculi 2 mm [13]. However, to our knowledge no prior study has specifically compared the size of calculi measured on low-dose CT with those measured on standard-dose CT or on direct stone analysis. In the current series, low-dose CT was equivalent to standard-dose CT for detecting stones 3 mm in the ureter of patients with a MI < 30. However, low-dose CT was limited (83% sensitive) in the detection of stones < 3 mm. Nevertheless, these calculi rarely require urologic procedures; 5 mm is generally the critical size for urology referral because the likelihood of spontaneous passage progressively decreases as a calculus exceeds this size [6, 29 31]. The current analysis reveals that low-dose CT does not miss any calculus 3 mm in patients with a MI < 30. Despite this excellent sensitivity, low-dose CT is more limited than standarddose CT in the determination of the exact stone size, which is a major drawback of lowdose CT. Therefore, clinicians should be clearly informed, before determining the optimal treatment, that the size of calculi on low-dose CT may vary by ± 20% compared with standard-dose CT results. This observation does not corroborate the results of a porcine kidney phantom study showing that renal stone detectability and size remained constant on low-dose MDCT, despite a tube current reduction from 170 to 30 ma [19]. It is possible that the detectability and size of the calculi may depend of their chemical content. Indeed, oxalate stones were exclusively used in the porcine kidney phantom, whereas stone composition was unknown in our study, which constitutes a limitation of our data interpretation. Some authors have reported that patients with a MI > 31 should not undergo a lowdose CT examination for the assessment of ureteral calculi [12]. Despite the limited (not statistically significant) number of patients with a MI 30, our data support their observations, although low-dose CT was highly accurate for the detection of indirect signs suggestive of renal colic in obese patients. Our study also analyzed the value of lowdose CT for detecting renal calculi, which is important to evaluate the potential for recurrence. Our data show that the sensitivity of low-dose CT for identifying renal calculi 3 mm is close to that obtained for detecting ureteral stones, but it is more limited in the evaluation of smaller calculi (63% in patients with a MI < 30). This can be explained by the fact that the renal stroma is more heterogeneous than the ureter and periureteral space content; small calculi may therefore be more easily confused with slightly hyperdense pyramids. In our series, no alternative diagnosis was missed by using low-dose CT when compared with standard-dose CT. This observation is mitigated by the small percentage (4.8%, n = 6) of alternative diagnoses depicted on low-dose CT and standard-dose CT, which prevents our performing a statistically significant comparison between the two techniques and which is therefore a limitation of our study. In addition, the percentage of alternative diagnoses is inferior to the range (9.6 31%) reported in prior series [16, 18, 32, 33]. This observation can be explained by the fact that, in our study design, alternative diagnoses did not correspond to the final diagnoses but only to disorders revealed on low-dose CT and on unenhanced standard-dose CT. Some other diagnoses assessed by further examination (i.e., contrastenhanced CT) were not considered in this comparative study. In the absence of a true reference standard, it cannot be inferred from our data that the diagnostic workup of renal colic should be strictly limited to lowdose CT. In unclear clinical presentations, various authors recommend performing higher-dose CT or contrast-enhanced CT after negative results on low-dose CT [16, 18, 20]. However, a recent clinical evaluation of low-dose CT (using 30 mas) at our institution suggested that only a minority of patients (8/27, 30%) with suspected renal colic and normal findings on low-dose CT will require standard-dose CT [18]; most patients are spared the additional dose. In conclusion, low-dose CT achieves sensitivities and specificities close to those of standard-dose CT for diagnosing renal colic, for correctly identifying alternative diagnoses, and for showing ureteral calculi of at least 3 mm in patients with a MI < 30. However, the estimation of the calculi size on lowdose CT may vary by ± 20% with regard to standard-dose CT findings. The results of this comparative study, along with those obtained from prior series [16 18], suggest that a low-dose CT protocol can be used as the first-line imaging tool in the workup of patients with suspected renal colic and a MI < 30, providing that clinicians and patients are aware of the limitations and advantages of this technique with regard to standard-dose CT. 932 AJR:188, April 2007

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