Value of Multislice Helical CT Scans and Maximum-Intensity-Projection Images to Improve Detection of Ureteral Stones at Abdominal Radiography

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1 Bernard E. Van Beers 1 Stéphane Dechambre 1 Pierre Hulcelle 1 Roland Materne 1 Jacques Jamart 2 Received December 11, 2000; accepted after revision May 16, Department of Radiology, Université Catholique de Louvain, Saint-Luc University Hospital, Ave. Hippocrate 10, B-1200 Brussels, Belgium. Address correspondence to B. E. Van Beers. 2 Center of Biostatistics and Medical Documentation, Université Catholique de Louvain, Mont-Godinne University Hospital, Ave. Thérasse 1, B-5530 Yvoir, Belgium. AJR 2001;177: X/01/ American Roentgen Ray Society Value of Multislice Helical CT Scans and Maximum-Intensity-Projection Images to Improve Detection of Ureteral Stones at Abdominal Radiography OBJECTIVE. The purpose of this study was to assess the improvement in the detection of ureteral stones on abdominal radiographs when the stones were viewed on multislice helical CT scans and maximum-intensity-projection (MIP) images. SUBJECTS AND METHODS. The study included 72 patients with renal colic who underwent abdominal radiography and multislice helical CT. For each patient, a frontal MIP image was generated, and the stone, when present, was marked with a cross on the transverse CT scan. The cross appeared automatically on the corresponding MIP image. The CT examination was used as the standard of reference. The presence and location of ureteral stones on the abdominal radiographs were assessed during three interpretation sessions. In the first session, the abdominal radiographs were viewed alone. In the second, they were viewed with the transverse CT scans. In the third, the abdominal radiographs were viewed with the CT scans and the MIP images. RESULTS. Ureteral stones were present in 58 patients. The percentage of stones detected on the abdominal radiographs was 45% when the radiographs were viewed alone, 66% when they were viewed with the CT scans (p = vs radiographs alone), and 78% when viewed with the CT scans and MIP images (p = vs radiographs with CT scans). CONCLUSION. The sensitivity of stone detection on abdominal radiographs was greatest when the interpreters viewed the radiographs in conjunction with the CT scans and MIP images. U nenhanced helical CT has a sensitivity greater than 95% for detecting ureteral stones in patients with acute flank pain [1 7]. In contrast, abdominal radiography has a low sensitivity of 45 58% [6, 8 13]. Several authors believe that abdominal radiographs should not be the first images obtained, but that they are useful after CT in case management in some patients with ureterolithiasis [6, 11, 13]. For example, visualization of a stone on abdominal radiographs is a prerequisite to the performance of extracorporeal shock wave lithotripsy with fluoroscopic guidance without contrast material [11, 14]. In addition, stone migration during conservative management can be assessed with serial radiographs [6, 11 13]. Prior knowledge of the precise location of the ureteral stone should improve detection of stones on abdominal radiographs. Therefore, viewing the abdominal radiographs in combination with CT scans should be useful [6, 11]. To show the precise location of the ureteral stone at CT in a format similar to that of abdominal radiography, we propose a frontal maximum-intensity-projection (MIP) image of the multislice helical CT data. The purpose of this study was to assess the improvement in the detection of ureterolithiasis on abdominal radiographs when the radiographs are viewed in combination with multislice helical CT scans and MIP images. Subjects and Methods Patients Abdominal radiography and unenhanced CT of the urinary tract were routinely performed in patients presenting with acute renal colic to the emergency department of our hospital [6]. From September 1999 to February 2000, our study population consisted of 72 patients (51 men, 21 women; age range, years; mean age, 48 years) with acute renal colic who underwent CT and radiographic examinations with a time interval of less than 4 hr. The patients were referred for an unenhanced CT examination of the urinary tract by the emergency department physicians if the diagnosis of ureteral stone was suspected on the basis of the patients reports of a colicky pain that radiated to the groin with or without hematuria. Patients with non- AJR:177, November

2 Van Beers et al. specific flank pain or patients with flank pain whose clinical examination results and laboratory values were suggestive of another diagnosis were referred for abdominal sonography or unenhanced or contrast-enhanced whole-abdominal CT. These patients were not included in our study. Imaging Techniques The abdominal radiographs were obtained with clinical equipment (Siregraph T.O.P. 33; Siemens, Erlangen, Germany). The exposure factors were kvp, mas, and a 1-mm nominal focal spot. Regular intensifying screens were used with T-MAT L/RA films (Eastman Kodak, Rochester, NY). The abdominal radiographs were obtained before CT was performed. The CT examinations were performed with a multislice helical scanner (MX8000; Marconi Medical Systems, Cleveland, OH). Unenhanced, single breathhold, helical acquisition was obtained from the top of the kidneys to the base of the bladder with 2.5-mm collimation (3.2 mm at full width at half maximum intensity); 120 kvp; 280 mas; table speed, 16.7 mm/ sec; and reconstruction interval, 1.6 mm. One of three attending radiologists experienced with CT reviewed the images from each examination while the patient was still in the CT suite. The radiologist performed an additional contrast-enhanced CT examination in 13 patients in whom the presence of a ureteral stone or another lesion remained doubtful [15]. He also performed a coronal MIP reconstruction of the unenhanced CT data in each patient using a clinical workstation (MX view; Marconi Medical Systems). If the radiologist detected a stone on the transverse CT scans, it was marked with a cross. This cross automatically appeared in the same position on the MIP image if the related slice option of the clinical software was selected. Therefore, in patients with multiple calcifications, the ureteral stone detected on the transverse CT scan could be unequivocally located on the MIP image. A film containing four images the transverse CT scan obtained at the level of the stone with and without the superimposed cross and the MIP image with and without the cross was given to the referring physician. Image Analysis All abdominal radiographs, CT scans, and MIP images were reviewed in consensus by two of the authors. The CT scans and MIP images were reviewed on the same workstation described earlier. For the purpose of the analysis, a patient was considered to have a ureteral stone if it was unequivocally visible on the transverse CT scans. In other words, the CT examination was the standard of reference. The largest transverse diameter and the density of the stones were measured on the transverse CT scans. The location of the stones was considered as proximal (above the sacroiliac junction), middle (at the level of the sacroiliac junction), or distal (beneath the sacroiliac junction). Three interpretations of the abdominal radiographs were performed. In the first interpretation session, only the abdominal radiographs were viewed, and the interpreters had no knowledge of the CT findings. However, they were aware of the side of the body in which the patients felt pain. In the second session, the radiographs were reviewed along with the transverse CT scans. In the third session, the interpreters viewed the radiographs along with the transverse CT scans and the MIP images on which the location of the stone was marked with a cross. The interpretations of the abdominal radiographs were classified into three categories: positive for ureterolithiasis, negative for the condition, and indeterminate. Indeterminate results were considered interpretation errors. In accordance with previous studies [13, 16, 17], the ability of the interpreters to detect ureterolithiasis was estimated by determining the fraction of stones correctly detected and located. The main cause of the interpretation error for the abdominal radiographs was categorized into two classes: poor contrast between ureteral stone and surrounding tissue (because of low stone density or the presence of overlying bone or intestine) and presence of phleboliths or other confounding calcified masses. Applying the binomial test, we compared the detection rates for interpretations that used radiographs alone, radiographs along with transverse CT scans, and radiographs along with CT scans and MIP images. The diameter and densities of stones were compared using the Wilcoxon s rank sum test. Results Ureteral stones were present in 58 patients. Only 26 ureteral stones were correctly detected and located using the abdominal radiographs alone, 38 stones were detected using the radiographs with the CT scans, and 48 stones were detected using the radiographs with the CT scans and MIP images (Figs. 1 3). Therefore, the fraction of stones found on the abdominal radiographs increased significantly from 0.45 during the first interpretation session to 0.66 during the second interpretation (p = vs first interpretation) and 0.78 during the third interpretation using the MIP images (p = vs first interpretation). The main cause of the errors on the initial interpretation was poor contrast between the ureteral stone and surrounding tissue in 24 patients and presence of phleboliths or other confounding calcified masses in 16 patients. Phleboliths were seen in 32 patients (44%) but were not always a cause of interpretation error. The stones were located in the proximal ureter in 15 patients (21% of all patients), the middle ureter in five (7%), and the distal ureter in 38 (53%). The stone detection rate on the abdominal radiographs was 60% (9/15) for the proximal ureter, 40% (2/5) for the middle ureter, and 39% (15/38) for the distal ureter during the initial interpretation. During the second interpretation session, the corresponding figures were 87% (13/15), 40% (2/5), and 61% (23/38). During the last interpretation, they were 93% (14/ 15), 60% (3/5), and 74% (28/38). The diameter of the stones was 4.6 ± 2.4 mm (mean ± SD). The CT density of the stones was 750 ± 331 H. The diameter of the stones detected during the initial interpretation (5.6 ± 2.3 mm) did not differ significantly from the diameter of the additional stones detected during the last interpretation (4.6 ± 1.8 mm, p = 0.25). Similarly, the density of these two categories of stones did not differ significantly (864 ± 333 H vs 779 ± 321 H, p = 0.41). Discussion The rate of detection of ureteral stones on abdominal radiographs is poor [6, 8 12]. CT has become the first examination that patients with renal colic undergo [1 7, 15]. However, abdominal radiographs remain useful in the follow-up of some patients with ureteral calculi for assessing stone migration or for targeting stones for extracorporeal shock wave lithotripsy using fluoroscopic guidance [6, 11 13]. The results of this study show that interpreting the abdominal radiographs in combination with multislice helical CT scans significantly improves the ability of radiologists to detect ureterolithiasis on the radiographs, especially if the CT scans are supplemented with MIP images showing the precise location of the stone in the same format as the abdominal radiographs. In several patients, the precise location of the stones on the abdominal radiograph remained indeterminate when viewed with the transverse CT scans but became obvious when viewed with the MIP image on which the stone had been marked (Figs. 1 and 3). (The contrast of the figures representing the abdominal radiographs has been electronically enhanced to improve the visibility of the stones.) The better detection of the stones on the abdominal radiographs viewed with the MIP images is in accordance with perception studies showing that prior knowledge of the presence and precise location of a lesion improves the interpreter s ability to recognize it [18, 19]. The two main reasons that ureteral stones are not recognized on abdominal radiographs are poor stone-to-tissue contrast and the presence of visual distractors. By knowing the precise stone location on the MIP reconstruction, we can recognize stones that are only faintly hyperdense relative to the surrounding tissue on the radiograph. Of course, not all urinary stones will be detected. Approximately 10 15% of stones appear radiolucent on abdominal radiographs, 1118 AJR:177, November 2001

3 CT and Radiography of Ureteral Stones usually if they are composed of uric acid, cystine, or xanthine [20]. We think it noteworthy that our 78% detection rate of ureterolithiasis during the third interpretation (with MIP images) was similar to the 80% detection rate of an in vitro study of abdominal radiographs performed by Olcott et al. [20]. The presence of phleboliths or other calcified masses is another potential source of error when reviewing abdominal radiographs. Visual distractors are well-known causes of interpretation errors on medical images. This phenomenon has been called satisfaction of search and refers to the fact that the detection of one radiographic finding interferes with that of others [16, 17, 21]. Phleboliths were present in 32 patients in our series. They were not always a cause of diagnostic error because some phleboliths could be recognized by their round shape and radiolucent center [22]. However, phleboliths or other calcified masses were a cause of interpretation error on abdominal radiographs in 16 patients. MIP images were a particularly useful aid in locating a ureteral stone in patients with multiple phleboliths present. We did not observe any major difference in size or density between the stones detected when abdominal radiographs were viewed alone and the stones revealed only when abdominal radiographs were viewed with the CT scans and MIP reconstructions. Regarding stone location, we found that the rate of detection of stones in the middle and distal ureters was lower than that for proximal stones. Similar results have been reported by Assi et al. [12]. A B C D Fig year-old man with right renal colic. A, Transverse CT scan shows right ureteral stone (cross) and calcifications of right internal iliac artery. B, Maximum-intensity-projection image of urinary tract shows right ureteral stone (cross) and several other right pelvic calcifications. C, Magnified maximum-intensity-projection image shows right ureteral stone (long arrow) overlying sacrum, calcifications of right internal iliac artery (short arrows), and phleboliths. D, Abdominal radiograph shows faint density (arrow) corresponding to right ureteral stone. This stone was not detected on radiograph when viewed without MIP image. A B C Fig year-old man with left renal colic. A, Transverse CT scan shows left ureteral stone (arrow) and more external and posterior phleboliths. B, Maximum-intensity-projection image shows left ureteral stone (arrow) and multiple phleboliths. C, Abdominal radiograph shows left ureteral stone (arrow) and phleboliths. Ureteral stone was not recognized on radiograph at initial reading. AJR:177, November

4 Van Beers et al. Fig year-old woman with left renal colic. A, Transverse CT scan shows left ureteral stone (arrow ). B, Maximum-intensity-projection image shows left ureteral stone (arrow ) overlying sacrum. C, Abdominal radiograph in slightly different projection from that of B shows left ureteral stone (arrow). This stone was not recognized on radiograph when viewed without MIP image because it projects onto sacrum. In a previous study comparing the interpretations of abdominal radiographs with and without the use of CT scans, no substantial improvement in the detection of ureterolithiasis was observed when interpreters reviewed the abdominal radiographs along with the CT scans [10]. Several reasons may explain the differences between this previous study and ours. For example, the earlier study did not use MIP images and excluded indeterminate results from the statistical analysis. Reformatting CT images of the ureter has been recommended in patients with ureteral calculi to obtain images in a format that is well known to clinicians [23]. However, creating curved planar reformats may be cumbersome. In contrast, creating global MIP images, as in our study, is rapid because no segmentation is needed. These MIP reconstructions provide images similar to the familiar conventional radiographs of the abdomen. We believe that these MIP images aid in communicating with clinicians. Nevertheless, acceptance by clinicians was not assessed in this study. We also did not calculate the sensitivity of CT scout radiography for the detection of ureteral stones. This sensitivity has been calculated in previous studies, and the researchers have reported that abdominal radiography was more sensitive than CT scout radiography in revealing ureteral calculi [12, 13]. Our study has limitations. First, the frequency of ureterolithiasis in our population was high (80.5%). The reported frequency of ureteral calculi in patients with acute flank pain is variable. In some large series, a frequency of A B C approximately 50% has been reported [3, 24]. In other series, frequencies of up to 85% have been described [2, 5, 6]. The high frequency of individuals with ureterolithiasis in our patient population may have resulted from accurate patient selection by the physicians of the emergency department. Second, the patients underwent both radiographic and CT examinations. Only recently has CT been performed systematically in patients presenting with acute renal colic to our emergency department. Because the availability of CT at night was limited, the previous standard policy for patients with renal colic was to perform abdominal radiography and sonography [9, 25]. During the intermediate period after the start of the new policy, patients routinely underwent abdominal radiography before CT scanning, although we are not recommending performing abdominal radiography in patients who will undergo CT. We, as others [11 13], believe that an abdominal radiograph should be obtained after CT only in patients who will need follow-up imaging or lithotripsy if the CT scout radiography does not reveal the calculus. The results of our study suggest that the detection of ureteral stones in these patients may be improved if abdominal radiographs are viewed with helical CT scans and MIP reconstructions. On the basis of these findings, we recommend that abdominal radiography and frontal MIP reconstruction be performed in patients who will need followup imaging or lithotripsy if the ureteral stones is not visible on the CT scout image. References 1. Smith RC, Rosenfield AT, Choe KA, et al. Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995; 194: Miller OF, Rineer SK, Reichard SR, et al. Prospective comparison of unenhanced spiral computed tomography and intravenous urogram in the evaluation of acute flank pain. Urology 1998;52: Dalrymple NC, Verga M, Anderson KR, et al. The value of unenhanced helical computerized tomography in the management of acute flank pain. J Urol 1998;159: Boulay I, Holtz P, Foley WD, White B, Begun FP. Ureteral calculi: diagnostic efficacy of helical CT and implications for treatment of patients. AJR 1999;172: Sourtzis S, Thibeau JF, Damry N, Raslan A, Vandendris M, Bellemans M. Radiologic investigation of renal colic: unenhanced helical CT compared with excretory urography. AJR 1999; 172: Niall O, Russell J, MacGregor R, Duncan H, Mullins J. A comparison of noncontrast computerized tomography with excretory urography in the assessment of acute flank pain. J Urol 1999; 161: Liu W, Esler SJ, Kenny BJ, Goh RH, Rainbow AJ, Stevenson GW. Low-dose nonenhanced helical CT of renal colic: assessment of ureteric stone detection and measurement of effective dose equivalent. Radiology 2000;215: Mutgi A, Williams JW, Nettleman M. Renal colic: utility of the plain abdominal roentgenogram. Arch Intern Med 1991;151: Haddad MC, Sharif HS, Shahed MS, et al. Renal colic: diagnosis and outcome. Radiology 1992; 184: Levine JA, Neitlich J, Verga M, Dalrymple N, 1120 AJR:177, November 2001

5 CT and Radiography of Ureteral Stones Smith RC. Ureteral calculi in patients with flank pain: correlation of plain radiography with unenhanced helical CT. Radiology 1997; 204: Chu G, Rosenfield AT, Anderson K, Scout L, Smith RC. Sensitivity and value of digital CT scout radiography for detecting ureteral stones in patients with ureterolithiasis diagnosed on unenhanced CT. AJR 1999;173: Assi Z, Platt JF, Francis IR, Cohan RH, Korobkin M. Sensitivity of CT scout radiography and abdominal radiography for revealing ureteral calculi on helical CT. AJR 2000;175: Jackman SV, Potter SR, Regan F, Jarrett TW. Plain abdominal x-ray versus computerized tomography screening: sensitivity for stone localization after nonenhanced spiral computerized tomography. J Urol 2000;164: Logarakis NF, Jewett MA, Luymes J, Honey RJ. Variation in clinical outcome following shock wave lithotripsy. J Urol 2000;163: Cochran ST. Applications of spiral CT in genitourinary imaging. Acad Radiol 1998;5: Berbaum KS, Franken EA, Dorfman DD, et al. Satisfaction of search in diagnostic radiology. Invest Radiol 1990;25: Wester C, Judy PF, Polger M, Swensson RG, Feldman U, Seltzer SE. Influence of visual distractors on detectability of liver nodules on contrast-enhanced spiral computed tomography scans. Acad Radiol 1997;4: Muhm JR, Miller WE, Fontana RS, Sanderson DR, Uhlenhopp MA. Lung cancer detected during a screening program using four-month chest radiographs. Radiology 1983;148: Renfrew DL, Franken EA, Berbaum KS, Weigelt FH, Abu-Yousef MM. Error in radiology: classification and lessons in 182 cases presented at a problem case conference. Radiology 1992;183: Olcott EW, Sommer FG, Napel S. Accuracy of detection and measurement of renal calculi: in vitro comparison of three-dimensional spiral CT, radiography, and nephrotomography. Radiology 1997;204: Ashman CJ, Yu JS, Wolfman D. Satisfaction of search in osteoradiology. AJR 2000;175: Bell TV, Fenlon HM, Davison BD, Ahari HK, Hussain S. Unenhanced helical CT criteria to differentiate distal ureteral calculi from pelvic phleboliths. Radiology 1998;207: Sommer FG, Jeffrey RB, Rubin GD, et al. Detection of ureteral calculi in patients with suspected renal colic: value of reformatted noncontrast helical CT. AJR 1995;165: Katz DS, Scheer M, Lumerman JH, Mellinger BC, Stillman CA, Lane MJ. Alternative or additional diagnoses on unenhanced helical computed tomography for suspected renal colic: experience with 1000 consecutive examinations. Urology 2000;56: Sheafor DH, Hertzberg BS, Freed KS, et al. Nonenhanced helical CT and US in the emergency evaluation of patients with renal colic: prospective comparison. Radiology 2000;217: AJR:177, November