Low-dose unenhanced helical computed tomography (CT) is an effective modality for the assessment of urinary calculus disease ( 1 7 ). It can help diag

Transcription

1 Note: This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at ORIGINAL RESEARCH n GENITOURINARY IMAGING Urinary Calculi Composed of Uric Acid, Cystine, and Mineral Salts: Differentiation with Dual- Energy CT at a Radiation Dose Comparable to That of Intravenous Pyelography1 Christoph Thomas, MD Martin Heuschmid, MD David Schilling, MD Dominik Ketelsen, MD Ilias Tsifl ikas, MD Arnulf Stenzl, MD Claus D. Claussen, MD Heinz-Peter Schlemmer, MD 1 From the Departments of Diagnostic and Interventional Radiology (C.T., M.H., D.K., I.T., C.D.C., H.P.S.) and Urology (D.S., A.S.), University of Tübingen, Hoppe-Seyler-Strasse 3, Tübingen, Germany. From the 2009 RSNA Annual Meeting. Received March 11, 2010; revision requested April 28; revision received June 4; accepted June 9; fi nal version accepted June 16. Address correspondence to C.T. ( christoph.thomas@med.uni-tuebingen.de ). Purpose: Materials and Methods: Results: Conclusion: To retrospectively evaluate radiation dose, image quality, and the ability to differentiate urinary calculi of differing compositions by using low-dose dual-energy computed tomography (CT). The institutional review board approved this retrospective study; informed consent was waived. A low-dose dualenergy CT protocol (tube voltage and reference effective tube current time product, 140 kv and 23 mas and 80 kv and 105 mas; collimation, mm; pitch, 0.7) for the detection of urinary calculi was implemented into routine clinical care. All patients ( n = 112) who were examined with this protocol from July 2008 to August 2009 were included. The composition of urinary calculi was assessed by using commercially available postprocessing software and was compared with results of the reference standard (ex vivo infrared spectroscopy) in 40 patients for whom the reference standard was available. Effective doses were calculated. Image quality was rated subjectively and objectively and was correlated with patient size expressed as body cross-sectional area at the level of acquisition by using Spearman correlation coefficients. One calcified concrement in the distal ureter of an obese patient was mistakenly interpreted as mixed calcified and uric acid. One struvite calculus was falsely interpreted as cystine. All other uric acid, cystine, and calcium-containing calculi were correctly identified by using dual-energy CT. The mean radiation dose was 2.7 msv. The average image quality was rated as acceptable, with a decrease in image quality in larger patients. Low-dose unenhanced dual-source dual-energy CT can help differentiate between calcified, uric acid, and cystine calculi at a radiation dose comparable to that of conventional intravenous pyelography. Because of decreased image quality in obese patients, only nonobese patients should be examined with this protocol. q RSNA, 2010 q RSNA, radiology.rsna.org n Radiology: Volume 257: Number 2 November 2010

2 Low-dose unenhanced helical computed tomography (CT) is an effective modality for the assessment of urinary calculus disease ( 1 7 ). It can help diagnose urinary calculi rapidly, with a high sensitivity and without the need for intravenous administration of nephrotoxic contrast material, and offers additional information with regard to the location of the calculus, its size, and its distance to the skin. Furthermore, CT can help identify complicating features such as hydronephrosis. However, classic single-energy CT does not have the ability to reliably differentiate between different calculus material in vivo ( 8,9 ), despite initial promising results in earlier in vitro studies ( 10,11 ). Determining the composition of a calculus may be clinically useful because the effectiveness of different treatment options is determined by the stone composition: For example, uric acid calculi can be medically dissolved with urine alkalization, and cystine calculi are known to be shock wave resistant and thus should be removed invasively ( 12,13 ). A rapid determination of the composition of a calculus by using CT might help triage patients for treatment and thus avoid unnecessary invasive procedures. Initial efforts using CT at two different energies to identify the composition of urinary calculi were reported decades ago and involved applying consecutive scanning with different photon energies ( 10,14 16 ). This technique was limited by problems of misregistration due to motion in between the two acquisitions ( 17 ) and because of the increased radiation during the two scans. As a result, it was not introduced into wide clinical use. The introduction of dualsource CT with practically dose neutral Advance in Knowledge n Unenhanced low-dose dual- source dual-energy CT can help differentiate uric acid, cystine, and calcium-containing urinary calculi with an acceptable image quality in nonobese patients at a radiation dose comparable to that of intravenous pyelography. simultaneous acquisition of two x-ray spectra has solved these problems, and a number of studies ( 8,9,12,18,19 ) have confirmed the ability of dual-source dual-energy CT to help distinguish uric acid and calcium salts in vitro and in vivo. However, these studies had relatively high radiation doses of around 6.6 msv ( 8 ). According to the as low as reasonably achievable principle, we introduced a custom-designed low-dose dual-energy protocol in our institution, which has radiation exposure similar to that of conventional intravenous pyelography (3 msv [ 20 ]), to reduce the radiation dose to our patients. The purpose of this study was to retrospectively evaluate the radiation dose, image quality, and the ability to differentiate urinary calculi of differing composition by using low-dose dualsource dual-energy CT. Material and Methods Patients and Study Protocol The institutional review board approved this retrospective study; informed consent was waived. The radiology information system at our institution was queried to identify all low-dose dual-energy CT examinations that were performed to confirm or rule out urinary calculus disease from July 2008 to August A total of 112 examinations were found. Patient characteristics (mean age, 50 years; age range, years; 43 female patients) according to subgroup are given in Table 1. In 66 of these examinations, urinary calculi were detected. Whenever a calculus is removed by using ureterorenoscopy or percutaneous nephrolithotripsy, the composition is routinely Implication for Patient Care n The optimal treatment for uri- nary tract calculi may vary depending on the composition of the stone; dual-source dualenergy CT has the potential to help characterize stones and guide therapy for patients with urinary calculus disease. determined by using ex vivo infrared spectroscopy. In cases of spon taneous stone clearance with missing calculi, infrared spectroscopy cannot be performed. Thus, the hospital information system was queried for each patient to assess whether the composition of the calculi had been determined with ex vivo infrared spectroscopy. This resulted in the identification of 40 dual-energy CT examinations with known calculus composition ( Fig 1 ). Low-Dose Dual-Energy CT Protocol All patients were examined with a dualsource CT system (Somatom Definition; Siemens Healthcare, Forchheim, Germany) in dual-energy CT mode, with tube voltages and reference effective tube current time products set to 140 kv and 23 mas and 80 kv and 105 mas. Attenuation-based tube current modulation (CareDose4D; Siemens Healthcare) was used. Collimation on each detector was mm with a z-flying focal spot; the pitch was 0.7. After the examination, axial and coronal nonoverlapping weighted average images containing information from both detectors were reconstructed by using a standard soft-tissue kernel (B30f), with a section thickness of 5 mm (axial) and 3 mm (coronal). Weighted average images were created by fusing the 80- and 140-kV images with a weighting factor of 0.3 (30% of data from the 80-kV source Published online before print /radiol Radiology 2010; 257: Abbreviation: CTDI vol = volume CT dose index Author contributions: Guarantors of integrity of entire study, C.T., M.H., C.D.C., H.P.S.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript fi nal version approval, all authors; literature research, C.T., M.H., D.K., H.P.S.; clinical studies, C.T., M.H., D.S., I.T., H.P.S.; statistical analysis, C.T., M.H., D.K.; and manuscript editing, C.T., M.H., D.S., D.K., I.T., C.D.C., H.P.S. Authors stated no fi nancial relationship to disclose. See also the article by Hidas et al in this issue. Radiology: Volume 257: Number 2 November 2010 n radiology.rsna.org 403

3 Table 1 Patient Characteristics according to Subgroup Parameter No Stone Stone (without Spectroscopy) Stone (with Spectroscopy) No. of patients Mean age (y)* 48 (19 86) 53 (22 82) 50 (11 90) Male-to-female ratio 29:17 18:8 22:18 * Age range is in parentheses. Figure 1 Figure 1: Flowchart summarizes the setup of the study and the numbers of patients in each group. IR = infrared. UA = uric acid. images, 70% of data from the 140-kV series). This data set was primarily used for reading. For kidney stone analysis, two sets of thin overlapping sections containing data from each detector (80 and 140 kv) were reconstructed by using a dedicated nonedge-enhancing D30f kernel and a section thickness and increment of 1.5 and 1.0 mm, respectively. Dual-Energy Calculus Analysis with Standard Postprocessing Software In the dual-energy CT data sets, calculus characterization was performed by using commercially available standard software (Syngo Dual Energy, version VE32B; Siemens Healthcare) ( Fig 2 ). The algorithm has been described in detail elsewhere ( 8,12,21 ). It works by analyzing the ratio of the attenuation of the calculi at 80 and 140 kv (dual-energy ratio). Because uric acid is an organic compound consisting of elements with low atomic numbers (low Z material), the dual-energy ratio is around 1 for uric acid calculi. It is higher (about 1.4 and larger) for calcium salts because of their higher atomic numbers and higher electron density, which leads to an increased attenuation of lower energy photons compared with high energy photons. The dual-energy ratio of cystine is typically between these ratios ( ). Figure 2 Figure 2: Calculus classifi - cation by using commercially available software. According to the defi ned threshold dualenergy ratio, the images are color coded, with blue indicating a larger and red indicating a smaller dual-energy ratio than the threshold. This calculus in the proximal ureter was correctly categorized as uric acid. 404 radiology.rsna.org n Radiology: Volume 257: Number 2 November 2010

4 By using the dual-energy postprocessing software, calculi were categorized as uric acid, cystine, or calcium salts. No further differentiation among types of calcified calculi was attempted because of the known similarity of their dual-energy ratios. The reader (C.T., 3 years of experience with dual-energy CT) performing the dual-energy postprocessing was blinded to the results of infrared spectroscopy. Objective Measurement of Stone Properties To measure the dual-energy ratios and the mean attenuations of the calculi with a completely user-independent approach and with a volumetric approach, dedicated analysis software was written by using the MATLAB environment (version 7.0.1; Mathworks, Natick, Mass). The algorithm automatically segments the calculus in the 140-kV data by applying a threshold (HU t ) calculated from the maximum attenuation (HU t = HU max /2) after manually placing a seed into the calculus without being influenced by the reader or by window settings. The pixels are then mirrored to the 80-kV data, providing a pixel-by-pixel analysis ( 12 ). The dual-energy ratio is calculated by dividing the mean attenuation at 80 kv by the mean attenuation at 140 kv. The dual-energy ratios and the mean attenuations of all groups of calculi as they would appear at 120 kv (calculated as HU 120kV = [0.3 HU 80kV ] + [0.7 HU 140kV ] according to the calculation of the weighted averaged images) were plotted and analyzed for spectral overlap. The sizes of the calculi were determined by deriving the equivalent diameters from the segmented calculi. Calculi with a diameter of less than 2 mm were excluded from the calculation of the dualenergy ratio by the MATLAB software because of expected domination of partial volume effects but were included in the analysis with the dedicated postprocessing software. One mixed uric acid and calcium oxalate monohydrate calculus was excluded from the calculation of the dual-energy ratio because the attenuation of both materials would have been averaged. However, it was included in the evaluation with the commercial analysis software, because this software can detect areas of different attenuation within calculi and is designed to be able to identify mixed calculi. Altogether, 53 single calculi were included in the automated evaluation, and 18 calculi were excluded. Image Quality and Radiation Dose For the evaluation of image quality, the weighted average images from all examinations were reviewed on a standard picture archiving and communication system workstation, and the overall image quality at the level of the kidney and the pelvis was rated subjectively with a four-point scale (4 = unacceptable, 3 = fairly acceptable, 2 = acceptable, 1 = very acceptable) by two radiologists (H.P.S. and C.T., 10 and 4 years of experience, respectively, in abdominal radiology) in consensus. Reading of all cases was performed within 1 week at several reading sessions, during which the images were presented sequentially to the readers. Additional findings such as periureteral rim enhancement, dilatation of the renal pelvis, or pararenal fluid were noted. Furthermore, image noise was measured in circular regions of interest on axial reconstructions at the level of the renal pelvis in the aorta and at the level of the caput femoris in the bladder by using the standard measurement tool of the picture archiving and communication system workstation. The size of the regions of interest was individually adapted to the diameter of the aorta or the size of the bladder. The anteroposterior and lateral diameters ( d ) of the patients were measured at the abovedescribed levels by using an electronic caliper tool, and the equivalent areas of the cross-sections were calculated (Area = p d 1 d 2/4). All objective measurements of image quality were performed by one author (C.T.). To assess the applied radiation dose, the volume CT dose index (CTDI vol ) and the dose length product of each exam ination were obtained from the examination protocol, which is automatically stored by the scanner after each examination. The effective dose in each patient was calculated by multiplying the dose length product by a weighting factor of msv/mgy cm ( 22 ). Statistical Analysis Sensitivity, specificity, positive predictive value, and negative predictive value were calculated for the detection of uric acid, cystine, and calcified calculi with the commercial postprocessing software. The relationships between patient crosssectional area and image noise, image quality, and CTDI vol at the levels of the kidney and the pelvis were assessed by using the Spearman correlation coefficient to rate the ability of the automatic tube current modulation system to compensate for larger patient diameters. All statistical calculations were performed by using software (SigmaStat 3.5; Systat Software, Chicago, Ill). A P value less than.05 was considered to indicate a statistically significant difference. Results Detection of Urinary Calculi In 66 patients, urinary calculi were found. The mean size of the calculi was 4.3 mm (range, mm). Calculi were located in the renal pelvis and calices in 44 patients, in the proximal ureter in 12 patients, in the distal ureter in 13 patients, and in the bladder in four patients. All calculi could be analyzed with the dual-energy CT technique. Among the 40 patients with known calculus composition, 12 patients had multiple calculi. In these patients, dualenergy CT analysis was performed on every single calculus and yielded consistent results in every patient. At infrared spectroscopy, 30 patients had calciumcontaining calculi, five patients had cystine-containing calculi, four patients had uric acid containing calculi, and one patient had a struvite calculus. The composition of the calculi in the 30 patients with calcified calculi was as follows: calcium oxalate monohydrate ( n = 21), carbonate apatite ( n = 2), calcium oxalate dihydrate ( n = 1), calcium phosphate ( n = 1), mixed calcium oxalate dihydrate and carbonate apatite ( n = 3), mixed calcium oxalate monohydrate and carbonate apatite ( n = 1), and mixed uric Radiology: Volume 257: Number 2 November 2010 n radiology.rsna.org 405

5 GENITOURINARY IMAGING: Urinary Calculi Differentiation with Dual-Energy CT Figure 3 Figure 3: A, Coronal reformation image shows decreased image quality in an obese patient. B, Because of the severe ring artifacts, the distal ureteral stone (arrow) was mistakenly interpreted as mixed calcified and uric acid. Figure 4 Figure 4: CT images show struvite calculus (short arrows) next to a double-j catheter (long arrows). The calculus can be detected easily but was falsely interpreted as cystine. 406 acid and calcium oxalate monohydrate (n = 1) with 50% uric acid and 50% calcium oxalate monohydrate. One calcified calculus that was located in the distal ureter in a large patient where high image noise was present (46.8 HU) was falsely characterized as mixed uric acid (Fig 3). The only struvite calculus in the study was falsely interpreted as cystine (Fig 4). The calculi in all other 38 patients with known calculus composition were correctly categorized. Thus, sensitivity, specificity, positive predictive value, and negative predictive value were 100% (four of four), 97% (35 of 36), 80% (four of five), and 100% (35 of 35), respectively, for the detection of uric acid calculi; 100% (five of five), 97% (34 of 35), 83% (five of six), and 100% (34 of 34) for the detection of cystine calculi; and 97% (29 of 30), 100% (10 of 10), 100% (29 of 29), and 91% (10 of 11) for the detection of calcified calculi. Dual-Energy Ratio The following mean dual-energy ratios were calculated for each calculus material by using the pixel-by-pixel analysis: calcium oxalate monohydrate, 1.50 (range, ); calcium oxalate dihydrate, 1.50; carbonate apatite, 1.49 (range, ); calcium phosphate, 1.53 (range, ); mixed calcified calculi, 1.53 (range, ); cystine, 1.36 (range, ); uric acid, 1.06 (range, ); and struvite, radiology.rsna.org n Radiology: Volume 257: Number 2 November 2010

6 Figure 5 Figure 6 Figure 5: Box-and-whisker plot shows ranges (whiskers) and 25th to 75th quartiles (boxes) of dual-energy (DE) ratios and mean attenuations in each group of calculi. CaPhos = calcium phosphate, CarbAp = carbonate apatite, COD = calcium oxalate dihydrate, COM = calcium oxalate monohydrate, Cys = cystine, MixCalc = mixed calcifi ed calculi, Stru = struvite, UA = uric acid. Figure 6: CT images demonstrate image quality in nonobese 46-year-old woman with acute left fl ank pain. Images depict a small distal ureteral stone (arrows), which can be easily identifi ed despite ring artifacts in the bladder. The dual-energy ratios and mean attenuation are plotted in Figure 5. Image Quality and Radiation Dose Figures 6 8 are sample images illustrating the image quality in nonobese patients. Additional findings such as periureteral rim enhancement, dilatation of the renal pelvis, or pararenal fluid were found in 26 patients. The results of the objective and subjective measurements of image quality are given in Table 2. Spearman correlation showed statistically significant positive linear associations between crosssectional area and CTDI vol, image noise, and image quality score at the levels of the kidney and the pelvis ( Table 3 ). Discussion This study examines the use of dualsource dual-energy CT for the differentiation of urinary calculi with a reduced mean effective dose of 2.7 msv, which is an exposure comparable to that of standard intravenous pyelography. By using this technique, kidney stones in 38 of 40 unselected patients were correctly characterized, yielding a high diagnostic accuracy for the detection of uric acid with sensitivity, specificity, positive predictive value, and negative predictive value of 100% (four of four), 97% (35 of 36), 80% (four of five), and 100% (35 of 35), respectively. The positive predictive value for the detection of cystine was comparable (83%). By using weighted average images for reading, the image quality was rated acceptable for most examinations, and a number of additional findings and differential diagnoses were found. Two calculi were not detected correctly: The first calculus was a distal ureteral calcium oxalate monohydrate stone, which was characterized as uric acid, in an obese patient. The combination of obesity and the location of the stone in the pelvis presents two problems: Within the pelvis, the low penetration of the 80-kV photons because of the high attenuation of the osseous pelvis leads to Radiology: Volume 257: Number 2 November 2010 n radiology.rsna.org 407

7 Table 2 Objective and Subjective Evaluation of Image Quality Parameter Kidney Bladder Cross-sectional area (cm 2 ) ( ) ( ) Image noise (HU) 22.3 ( ) 22.3 ( ) Image quality score 1.5 ( 1 4 ) 2.2 (1 4) Note. Data are means, with ranges in parentheses. Figure 7 Figure 8 Table 3 Correlation of Patient Cross-Sectional Area, Radiation Dose, and Image Quality Test and Level r Value * P Value Area versus CTDI vol Kidney 0.873,.001 Pelvis 0.863,.001 Area versus image noise Kidney 0.478,.001 Pelvis 0.333,.001 Area versus image quality score Kidney Pelvis 0.42,.001 * r = Spearman correlation coeffi cient. Figure 7: CT image shows additional fi ndings of ureteral obstruction (proximal ureteral dilatation, periureteral edema, hydronephrosis, and perinephric stranding) caused by a stone in the proximal ureter (arrow) in 52-year-old man with acute left fl ank pain. a decreased detector signal which, when using low-dose protocols, results in decreased signal-to-noise ratios due to an overbalance of electronic detector noise. This effect is further aggravated by obesity. The resulting images are compromised by ring artifacts, which lead to heterogeneous shifts in the Hounsfield scale at certain locations on low energy images. If the calculus is located in these zones, the dual-energy ratio is altered accordingly, and the calculus might be misclassified. The other falsely categorized calculus was a struvite calculus that had the same dual-energy ratio as cystine and was thus incorrectly interpreted as cystine. Contrary to other published studies, we were not able to distinguish calcified calculi from each other by using dual source dual-energy CT. For example, Boll et al ( 12 ) reported that calcium oxalate, calcium phosphate, and brushite can be differentiated by using a pixel-by-pixel approach and a dual-energy ratio based algorithm in a phantom with a mean Figure 8: CT image depicts a differential diagnosis of urinary calculus disease (diverticulitis of the sigmoid colon with extraluminal air and surrounding mesenteric fl uid [arrows]) in 43-year-old man with left-sided abdominal pain. CTDI vol of 22.4 mgy. Despite a similar approach, the spectra of the dual-energy ratios of the calcified calculi overlapped in our study, which is probably because of significantly higher image noise levels (mean CTDI vol = 3.7 mgy) and scatter in the in vivo environment. No other published in vivo study to date has confirmed the successful differentiation of different types of calcified calculi. The correlation of body cross-sectional areas, CTDI vol, and image noise confirms that the radiation dose is automatically increased in larger patients and reduced in smaller patients by the scanner because of the use of tube current modulation. However, despite the increased tube output, the image noise increased in larger patients. Thus, tube current modulation cannot fully compensate for larger patient diameters. This might be because of the exponential increase of attenuation with increasing patient diameter and the limited maximal tube current, especially at lower photon energies. Thus, the use of dual-source dual-energy CT in large patients may be limited when 80 kv is used to provide the low energy spectrum. The recently introduced second-generation dual-source CT system (Somatom Definition FLASH; Siemens Healthcare) is equipped with an additional tin filter that can be used to eliminate low energy partitions of the high energy spectrum (140 kv). This leads to a better separation of the x-ray spectra of the two tubes so that 100 kv instead of 80 kv can be used for the low energy acquisition, which results in better x-ray penetration at a preserved spectral separation ( 23 ). Thus, image quality should be improved in larger patients. Furthermore, the decreased image noise levels due to the use of 100 kv might lead to further dose reduction potential. More in vitro studies are warranted to assess the lowest possible dose levels. If 80 kv instead of 100 kv is used in combination with the tin filter, the difference in dual-energy ratios of the subgroups will increase, leading to potentially better discrimination of urinary calculi. In the clinical setting, the ability to reliably predict stone composition could help to choose the proper diagnostic pathway early and thus to avoid unnecessary invasive procedures. Currently, patients with uric acid calculi can be identified by using dual-source dualenergy CT and can be treated with urine alkalization. Furthermore, cystine calculi can be detected; however, in these 408 radiology.rsna.org n Radiology: Volume 257: Number 2 November 2010

8 patients the presence of struvite calculi should be ruled out because the dualenergy spectra of struvite and cystine are similar. However, further research is warranted to facilitate the prediction of the stone fragility to external shock waves at dual-energy CT. This study had several limitations. Only one technical approach to dualenergy CT was investigated. Besides dualsource CT, sandwich detectors and rapid kilovoltage switching also can be applied to acquire dual-energy CT images. However, because these technologies were not available to the authors, a comparison was not possible. A further limitation was the differing number of patients in each subgroup of calculi, which is because of the relatively rare occurrence of certain calculi such as uric acid and cystine in comparison to calcified calculi. The calculation of sensitivity may be affected by this. Data from several institutions performing dual-energy CT should be pooled to acquire a larger database. In conclusion, we have shown that low-dose unenhanced dual-energy CT can help differentiate between calcified, uric acid, and cystine calculi at a radiation dose comparable to that of conventional intravenous pyelography. Struvite calculi currently cannot be differentiated from cystine calculi by using this protocol. With increasing patient diameter, image noise increases, which can lead to errors in classification of calculi especially in the pelvis. Thus, currently only nonobese patients should be examined with this protocol. Acknowledgments: The authors acknowledge the valuable contribution and the continuous support of our excellent CT technicians, especially Bernadette Kasperek and Eva Erning. References 1. 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 ( 1 ): Meagher T, Sukumar VP, Collingwood J, et al. Low dose computed tomography in suspected acute renal colic. Clin Radiol 2001 ; 56 ( 11 ): Heneghan JP, McGuire KA, Leder RA, DeLong DM, Yoshizumi T, Nelson RC. Helical CT for nephrolithiasis and ureterolithiasis: comparison of conventional and reduced radiation-dose techniques. Radiology 2003 ; 229 ( 2 ): Tack D, Sourtzis S, Delpierre I, de Maertelaer V, Gevenois PA. Low-dose unenhanced multidetector CT of patients with suspected renal colic. AJR Am J Roentgenol 2003 ; 180 ( 2 ): Kalra MK, Maher MM, D Souza RV, et al. Detection of urinary tract stones at lowradiation-dose CT with z-axis automatic tube current modulation: phantom and clinical studies. Radiology 2005 ; 235 ( 2 ): Kluner C, Hein PA, Gralla O, et al. Does ultra-low-dose CT with a radiation dose equivalent to that of KUB suffice to detect renal and ureteral calculi? J Comput Assist Tomogr 2006 ; 30 ( 1 ): Poletti PA, Platon A, Rutschmann OT, Schmidlin FR, Iselin CE, Becker CD. Lowdose versus standard-dose CT protocol in patients with clinically suspected renal colic. AJR Am J Roentgenol 2007 ; 188 ( 4 ): Graser A, Johnson TR, Bader M, et al. Dual energy CT characterization of urinary calculi: initial in vitro and clinical experience. Invest Radiol 2008 ; 43 ( 2 ): Thomas C, Patschan O, Ketelsen D, et al. Dual-energy CT for the characterization of urinary calculi: in vitro and in vivo evaluation of a low-dose scanning protocol. Eur Radiol 2009 ; 19 ( 6 ): Mostafavi MR, Ernst RD, Saltzman B. Accurate determination of chemical composition of urinary calculi by spiral computerized tomography. J Urol 1998 ; 159 ( 3 ): Deveci S, Coșkun M, Tekin MI, Peșkircioglu L, Tarhan NC, Ozkardeș H. Spiral computed tomography: role in determination of chemical compositions of pure and mixed urinary stones an in vitro study. Urology 2004 ; 64 ( 2 ): Boll DT, Patil NA, Paulson EK, et al. Renal stone assessment with dual-energy multidetector CT and advanced postprocessing techniques: improved characterization of renal stone composition pilot study. Radiology 2009 ; 250 ( 3 ): Coe FL, Evan A, Worcester E. Kidney stone disease. J Clin Invest 2005 ; 115 ( 10 ): Hawkes DJ, Jackson DF, Parker RP. Tissue analysis by dual-energy computed tomography. Br J Radiol 1986 ; 59 ( 702 ): Marshall W, Hall E, Doost-Hoseini A, Alvarez R, Macovski A, Cassel D. An implementation of dual energy CT scanning. J Comput Assist Tomogr 1984 ; 8 ( 4 ): Mitcheson HD, Zamenhof RG, Bankoff MS, Prien EL. Determination of the chemical composition of urinary calculi by computerized tomography. J Urol 1983 ; 130 ( 4 ): Grosjean R, Sauer B, Guerra RM, et al. Characterization of human renal stones with MDCT: advantage of dual energy and limitations due to respiratory motion. AJR Am J Roentgenol 2008 ; 190 ( 3 ): Primak AN, Fletcher JG, Vrtiska TJ, et al. Noninvasive differentiation of uric acid versus non-uric acid kidney stones using dual-energy CT. Acad Radiol 2007 ; 14 ( 12 ): Stolzmann P, Kozomara M, Chuck N, et al. In vivo identification of uric acid stones with dual-energy CT: diagnostic performance evaluation in patients. Abdom Imaging doi: /s Published online September 2, Accessed December 15, RadiologyInfo: Intravenous Pyelogram ( IVP ). RSNA and ACR. Reviewed March 15, Accessed January Johnson TR, Krauss B, Sedlmair M, et al. Material differentiation by dual energy CT: initial experience. Eur Radiol 2007 ; 17 ( 6 ): Huda W, Ogden KM, Khorasani MR. Converting dose-length product to effective dose at CT. Radiology 2008 ; 248 ( 3 ): Stolzmann P, Leschka S, Scheffel H, et al. Characterization of urinary stones with dualenergy CT: improved differentiation using a tin filter. Invest Radiol 2010 ; 45 ( 1 ): 1 6. Radiology: Volume 257: Number 2 November 2010 n radiology.rsna.org 409