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Tulane University Medical Center New Orleans, LA 70112
I read with interest the June 1999 AJR article "Ureteral Calculi: Diagnostic Efficacy of Helical CT and Implications for Treatment of Patients" by Boulay et al. [1] and the related article "Radiologic Investigation of Renal Colic: Unenchanced Helical CT Compared with Excretory Urography" by Sourtzis et al. [2]. I find the articles well written, informative, and possibly a turning point in the workup of renal colic. May I commend the authors and the AJR for these articles.
To solidify their places as reference articles, may I ask for additional comment from the authors?
Were oblique radiographs used when the excretory urograms were performed? Oblique views were not mentioned specifically in either article. In my experience, oblique images are at times not done at night and on weekends. Because omitting oblique images would limit an excretory urogram, I would like to be reassured that oblique views were done.
What was the radiation dose to the patient when comparing an average excretory urogram with an average or supplemented helical CT? I am concerned about fertile women. As noted in the American College of Radiology syllabus [3], the uterine-ovarian dose from an excretory urogram is usually 0-1.2 rad (0-0.012 Gy) including supplemental views. If the uterine-ovarian dose from a single-series pelvic CT is 2-5.5 rad (0.02-0.055 Gy) [3], then is it prudent from a risk-benefit point of view to perform unenhanced pelvic CT in young women, then add thin-slice unenhanced CT, and then possibly add enhanced pelvic CT? Could repeated pelvic helical CT examinations elevate young women's uterine-ovarian dose exposure to undesirable levels? On women younger than 45 years old, I personally might opt for sonography with the possible addition of a lowosmolar contrast-enhanced excretory urogram, over a helical CT of the pelvis plus thin-slice CT images or contrast-enhanced images.
Helical CT is valuable, but before performing helical CT routinely for renal colic, could Boulay et al. and Sourtzis et al. clarify what they are comparing from a performance and dose standpoint? Many thanks.
References
Brugmann University Hospital 1020 Brussels, Belgium
In our study [1], all excretory urography and CT were performed during weekdays. On excretory urography, oblique views were obtained whenever the presence of a ureteral stone was suspected, particularly at the ureterovesical junction.
Concerned about the dose delivered by these investigations, we examined four morphologically different patients with a device to measure the skin entry dose. We found an average of 3.5 rad (0.035 Gy) for excretory urographies and 4 rad (0.04 Gy) for CT. These results are in accordance with those described by Fielding et al. [2] and Vieweg et al. [3]. One way to reduce the CT irradiation dose would be to increase the scan pitch to 1:5. Sensitivity to detect stones is preserved, and with the presence of secondary signs, diagnosis can be made with confidence. Trials have been made on a limited series of patients at our institution and results have been satisfactory. This procedure can be enough to establish diagnosis at a less irradiating dose for patients who have repeated episodes of renal colic.
In our institution, whenever renal colic is suspected, we perform abdominal radiography and urinary tract sonography in the emergency department. CT is reserved for patients whose symptoms are clinically suspect and not explained by radiographic and sonographic findings.
References
Medical College of Wisconsin Milwaukee, WI 5322
We thank Dr. Morin for his interest in the recent publications on ureteral and renal calculi as studied by helical CT and excretory urography in the June issue of the AJR [1, 2]. The major issue that Morin raises is the comparative ovarian radiation dose of helical CT and excretory urography, particularly in women during the potential childbearing years. For the techniques used for the patients in our study, Table 1 lists the estimated surface, uterine, and ovarian doses.
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With the standard CT technique of our study (5-mm beam collimation, 7.5 mm per rotation table speed, 120 kVp, 200-240 mAs), the uterine dose is estimated at 0.36-0.43 rad (0.0036-0.0043 Gy). In 7% of the patients in our study, a thin-section small field-of-view technique was used to better define the anatomy of the distal pelvic ureter and to separate phlebolith from calculus. This technique uses 3-mm collimation, 3 mm per rotation table speed, and 140 kVp at 300 mAs, resulting in an estimated uterine dose of 1.1 rad (0.0011 Gy).
In our study, additional pelvic CT with IV contrast material (using standard helical CT techniques) was performed in 20% of the patients. This procedure reflected a learning curve in our practice, particularly with supervising residents. A reasonable standard would be the use of IV contrast material in 10% or fewer of patients to distinguish distal ureteral calculi from phleboliths. Thus, in the patients who had both unenhanced and delayed enhanced (pelvis only) CT scanning, the estimated absorbed uterine radiation dose would be 0.72-0.86 rad (0.0072-0.0086 Gy). Only three patients (3% of total study group) who had thin-section unenhanced CT scanning of the pelvis subsequently had delayed pelvic CT scanning after IV contrast material injection.
Women composed 40% of our study group, slightly greater than the expected male-female ratio of 70-30% in patients with stone disease in the general population. Stone disease is most prevalent in the second to the fourth decade [4, 5]. It is likely that only 20% of patients with a clinical presentation of ureteric colic would be women of reproductive age.
On standard excretory urography at our institution, direct pelvic irradiation occurs on preliminary anteroposterior frontal radiography and on full supine and prone frontal radiography after release of ureteric compression. A standard anteroposterior radiograph of the pelvis is obtained after the patient voids. Oblique radiographs targeting the abdominal or pelvic portion of the ureter are taken at the discretion of the supervising radiologist. Estimated uterine radiation for a standard excretory urogram study is 0.60 rad (0.006 Gy); with additional supplemental oblique views of the pelvis, the dose increases by 0.15 rad (0.0015 Gy) per additional exposure (Table 2) [3].
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The advantages of helical CT over excretory urography in patients with suspected ureteric calculus are speed, improved sensitivity, and avoidance of IV contrast material in most patients. If the results are positive, both studies provide information that is used in patient treatment decisions regarding calculus size and location. Excretory urography may require prolonged imaging with repeated radiographs to determine the site of obstruction, and the diagnostic quality of the study is often impaired by excess bowel content in the unprepared abdomen. Furthermore, excretory urography is a study limited to the urinary tract and cannot aid in the diagnosis of appendicitis, diverticulitis, or leaking abdominal aortic aneurysm, conditions that can mimic ureteric colic. Sonography is not usually efficacious in patients with suspected renal colic. Although dilatation of the upper tract and renal calculi may be shown, only the intramural portion of the ureter can be reliably imaged; therefore, a definitive diagnosis of obstructing ureteral calculus is limited to those patients with an evident calculus in the intramural portion of the ureter.
In summary, uterine-ovarian radiation doses are equivalent for the standard unenhanced CT study and excretory urography. An additional radiation dose will occur when supplemental imaging is used in either technique. For CT, this supplement includes use of a thin-section small-field-of-view high-resolution technique or the use of IV contrast material. For excretory urography, the supplement includes oblique images and delayed full-column radiography. Familiarity with the anatomic course of an unenhanced ureter is important for radiologists to confidently distinguish phleboliths, arterial wall calcified plaque, and the unusual case of a calcified vas deferens from a ureteral calculus on unenhanced helical CT scans. In difficult cases, CT findings of unilateral hydronephrosis on the symptomatic side in conjunction with a pelvic calcification with a rim sign are highly suggestive of ureteral calculus obstruction.
Since the completion of our study, to further modify the CT technique, we have used a 2:1 pitch with a 5-mm beam collimation (Shuman WP, personal communication). This technique modification lowers the estimated uterine radiation dose to 0.30-0.36 rad (0.0030-0.0036 Gy) per examination without any observed deleterious effect on the imaging of ureteral calculi.
We believe that CT provides significant advantages over excretory urography in patients with suspected ureteric calculi. In most patients, central pelvic radiation dose and, in women, ovarian radiation dose, is comparable for the two techniques. The high-resolution thin-section CT technique with increased central pelvic radiation is used infrequently.
References
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This article has been cited by other articles:
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  O. Catalano, A. Nunziata, F. Altei, and A. Siani Suspected Ureteral Colic: Primary Helical CT Versus Selective Helical CT After Unenhanced Radiography and Sonography Am. J. Roentgenol., February 1, 2002; 178(2): 379 - 387. [Abstract] [Full Text] [PDF]
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L. F. Donnelly, K. H. Emery, A. S. Brody, T. Laor, V. M. Gylys-Morin, C. G. Anton, S. R. Thomas, and D. P. Frush Minimizing Radiation Dose for Pediatric Body Applications of Single-Detector Helical CT: Strategies at a Large Children's Hospital Am. J. Roentgenol., February 1, 2001; 176(2): 303 - 306. [Full Text] [PDF]
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