IMAGING

Helical CT for nephrolithiasis and ureterolithiasis: comparison of conventional and reduced radiation-dose techniques
Heneghan JP, McGuire KA, Leder RA, DeLong DM, Yoshizumi T, Nelson RC
From the Department of Radiology, Duke University Medical Center, Durham, NC
Radiology. 2003; 229: 575-80

• Purpose: To determine the accuracy of unenhanced helical computed tomography (CT) performed at reduced milliampere-second, and therefore at a reduced patient radiation dose, by using conventional unenhanced helical CT as the standard.
• Materials and Methods: Fifty patients with acute flank pain who weighed less than 200 lb (90 kg) were prospectively recruited for this study. Conventional helical CT scans were obtained with patients in the prone position by using 5-mm-thick sections, 140 kVp, 135-208 mAs (mean, 160 mAs), and a pitch of 1.5 (single-detector row CT) or 0.75 (multi-detector row CT, 4 x 5-mm detector configuration). Conventional CT was immediately followed by low-dose scanning, whereby the tube current was reduced to 100 mA (mean, 76 mAs). All other technical parameters and anatomic coverage remained constant. Three independent readers who were blinded to patient identity interpreted the scans in random order. The observers noted the location, size, and number of calculi; secondary signs of obstruction; and other clinically relevant findings. High- and low-dose scans were compared by using paired t tests and the signed rank test.
• Results: Calculi were found in 33 (66%) patients; 25 (50%) had renal calculi and 19 (38%) had an obstructing ureteral calculus. The accuracy rates (averaged over the three readers) for determining the various findings on the low-dose scan compared with the high-dose scan were as follows: nephrolithiasis, 91%; ureterolithiasis, 94%; obstruction, 91%; and normal findings, 92%. When interpretations between readers were compared, agreement rates were 90%-95% for standard-dose scans and 90%-92% for reduced-dose scans (P > .5). Uncomplicated mild diverticulitis was found in three patients. No other clinically important abnormality was identified. A reduction in the tube current to 100 mA resulted in a dose reduction of 25% for multi-detector row CT and 42% for single-detector row CT.
• Conclusion: In patients who weighed less than 200 lb, unenhanced helical CT performed at a reduced tube current of 100 mA, and therefore at a reduced patient dose, resulted in scans of high accuracy.
  
  
• Editorial Comment
  There is no doubt regarding the crescent acceptance of the unenhanced helical computed tomography (UHCT) for the investigation of patient with acute flank pain and suspected of having urolithiasis. Although UHCT confers diagnostic advantages and avoids the risks of intravenous contrast medium, this should be considered against the increased radiation dose to the patient (particularly to the gonads). Depending on the protocol used, the average dose of an intravenous urography (IVU) vary from 1.5 to 2.0 mSV while for UCHT the effective dose is usually 4.7 mSV. In other words the total dose of radiation of non-optimized UHCT protocol confers a total dose, which is about three times that of an IVU. This study deals with a very important issue in radiology today, which is how to decrease radiation dose to the patients. This issue became more crucial among radiologists after the introduction of the multidetector row CT (MDCT). This diagnostic procedure has become widely used, particularly in the USA, and has been proven to be a valuable tool for various indications. A major issue using this new modality is the inherent risk of applying increased radiation exposure, when compared to single-slice CT or other imaging modalities. Fortunately, radiologists are now able to save radiation exposure from the use of MDCT by choosing optimized exposure parameters or its superior dose efficiency in comparison to single-slice CT. The use of intelligent tools in these modern equipments, such as ECG- or body shape-based real-time dose modulation, can further reduce the radiation dose.
  As we can see all efforts are been doing now by radiologists in order to perform a low-dose CT protocol. While acquiring thin slices with high spatial resolution, we can reduce the dose to similar values as in conventional radiography, especially when examining under high-contrast conditions. Using all these various options available, radiation exposure can sometimes even be lower than using a conventional single-slice helical CT. By using low dose-CT protocol we can reach similar sensitivity, specificity and accuracy. For the detection of urolithiasis, for example, low dose CT protocol is superior to IVU and confers a total dose of 2.8 mSV, which is about double that for IVU and about 75% and 50% of that for non-optimized UHCT protocols. Recently these low dose noncontrast CT protocols has been shown to be useful also for the diagnosis of stones in pregnant women and children.

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