IMAGING

Comparison of Effective Radiation Doses in Patients Undergoing Unenhanced MDCT and Excretory Urography for Acute Flank Pain
Eikefjord EN, Thorsen F, Rorvik J
Department of Radiology, Haukeland University Hospital, Bergen, Norway
AJR Am J Roentgenol. 2007; 188:934-9

• Objective: The purpose of this study was to measure and compare the effective radiation dose in patients undergoing unenhanced MDCT and excretory urography for acute flank pain, and to explore technical and practical factors affecting the effective dose.
• Subjects and Methods: One hundred nineteen patients with acute flank pain were included. All patients were examined using both MDCT and excretory urography. CT involved one acquisition from the upper kidney margin to the symphysis pubis. The only protocol variation was in the tube current (mAs), which was made according to patient body mass. The excretory urography protocol consisted of three images, with more when supplementary images were needed. Effective radiation doses were computer-simulated using dosimetry programs for CT and conventional radiography, based on Norwegian Radiological Protection Board dose data sets. Mean and SDs of measured patient doses were calculated and compared. Further analyses of dose variations in body mass categories (body mass index) were conducted, as were analyses concerning the number of images taken.
• Results: The mean effective doses were 7.7 mSv with MDCT and 3.63 mSv with excretory urography. The effective dose varied both in and between techniques but could be predicted. Radiation risk decreased significantly with increased patient weight.
• Conclusion: The average effective dose with MDCT was more than double that with excretory urography. However, the appropriate dose could be strongly predicted by the patient’s body mass index and by procedure. An optimum low-dose protocol should be considered before initiating unenhanced MDCT for ureteral colic in order to minimize the radiation-induced cancer risk and to secure adequate image quality.
  
  
• Editorial Comment
  In many institutions, nonenhanced computed tomography has largely supplanted intravenous urography as the primary modality for evaluation of patients suspected of having urolithiasis. As we know, nonenhanced multidetector CT (MDCT) examination, on average, doubled the effective radiation dose to the patient when compared with intravenous urography (if a total of 5 films are obtained). This is particularly important to the young female patients due the direct radiation exposure to the gonads. Some young female patients might present with chronic episodes of urolithiasis, and therefore will be submitted to multiple radiologic examinations during their lifetime The aim of this study was to use commercially available software to evaluate effective radiation doses between different radiologic examination procedures and to explore the relationship between technical and practical factors that could affect the effective radiation dose, both during and between the chosen imaging procedures.
  The authors presents an interesting observation; they found that a significantly wide dose range of effective doses with both MDCT and excretory urography, mainly influenced by body size (BMI). Patients of normal weight were exposed to a significantly higher radiation risk with MDCT than with excretory urography when compared with the other weight categories. The mAs with both excretory urography and CT varied considerably according to BMI. With excretory urography, an exponential variation in BMI might be expected from the automatic exposure control system (photo timing). However, with MDCT, the mAs varied as a consequence of subjective considerations. This was possibly caused by operator attempts to avoid an increased noise level for patients with high BMI. Recently, several studies has been show that low-dose MDCT protocols, which delivers radiation dose comparable to those of excretory urography are appropriate for the diagnosis of ureteral stones, and that it provides excellent intraobserver and interobserver agreement and does not obscure alternative diagnosis. Nowadays it is imperative to adapt technical parameters of MDCT on the basis of clinical indication. It is not acceptable to use protocols based on subjective considerations and thus delivering increased radiation risk for patients of normal weight. Based on the authors’ conclusion we should keep in mind that BMI should also be taken into consideration because it is too a risk predictor. As a rule, unenhanced optimized low-dose CT should be used routinely in clinical practice and we must pursue in the development of optimized low-dose MDCT protocols.

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil
E-mail: aprando@mpc.com.br