IMAGING

Patient radiation dose at CT urography and conventional urography
Nawfel RD, Judy PF, Schleipman AR, Silverman SG
Department of Radiology, Brigham and Women’s Hospital, Boston, MA, USA
Radiology 2004; 232: 126-32

• Purpose: To measure and compare patient radiation dose from computed tomographic (CT) urography and conventional urography and to compare these doses with dose estimates determined from phantom measurements.
• Materials and Methods: Patient skin doses were determined by placing a thermoluminescent dosimeter (TLD) strip (six TLD chips) on the abdomen of eight patients examined with CT urography and 11 patients examined with conventional urography. CT urography group consisted of two women and six men (mean age, 55.5 years), and conventional urography group consisted of six women and five men (mean age, 58.9 years). CT urography protocol included three volumetric acquisitions of the abdomen and pelvis. Conventional urography protocol consisted of acquisition of several images involving full nephrotomography and oblique projections. Mean and SD of measured patient doses were compared with corresponding calculated doses and with dose measured on a Lucite pelvic-torso phantom. Correlation coefficient (R(2)) was calculated to compare measured and calculated skin doses for conventional urography examination, and two-tailed P value significance test was used to evaluate variation in effective dose with patient size. Radiation risk was calculated from effective dose estimates.
• Results: Mean patient skin doses for CT urography measured with TLD strips and calculated from phantom data (CT dose index) were 56.3 mGy +/- 11.5 and 54.6 mGy +/- 4.1, respectively. Mean patient skin doses for conventional urography measured with TLD strips and calculated as entrance skin dose were 151 mGy +/- 90 and 145 mGy +/- 76, respectively. Correlation coefficient between measured and calculated skin doses for conventional urography examinations was 0.95. Mean effective dose estimates for CT urography and conventional urography were 14.8 mSv +/- 90.0 and 9.7 mSv +/- 3.0, respectively. Mean effective doses estimated for the pelvic-torso phantom were 15.9 mSv (CT urography) and 7.8 mSv (conventional urography).
• Conclusion: Standard protocol for CT urography led to higher mean effective dose, approximately 1.5 times the radiation risk for conventional urography. Patient dose estimates should be taken into consideration when imaging protocols are established for CT urography.
  
  
• Editorial Comment
  CT urography is an evolving concept and developing technique, which combine the ultimate diagnostic capabilities of intravenous urography and CT. In many institutions, intravenous urography has already been replaced by CT urography to evaluate patients with hematuria and other genitourinary conditions. This paper emphasizes the most important drawback of this technique, which is related to the radiation exposure. In our institution the miliamper seconds (mAs) settings are chosen depending upon clinical indication and patients’ age and body habitus. Recent studies have shown that low-dose (reduced mAs) unenhanced CT is appropriate for the diagnosis of ureteral stones. Similarly efforts have been made in order to perform a low-dose protocol for CT urography. The standard protocol for multislice CT urography usually include 4 phases of imaging [noncontrast, arterial phase (25-30 seconds after intravenous injection of contrast); nephrographic phase (100 seconds) and excretory phase (180 seconds)]. In order to obtain a significant reduction in patient effective radiation dose without deterioration of imaging quality one should optimize the number of phases to be done and also do not include the kidneys and the pelvis in every phase. This can be done by adequate adjustment of the technical parameters to the patient’s weight and clinical situation. To obtain good results with a low-dose CT urography protocol is possible. Since CT urography is still an evolving technique we believe that further improvement of an optimized protocol will be developed very soon.

Dr. Adilson Prando
Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil