Dr Chotas and colleagues respond: 290 #{149} Radiology October 1993 the fact that many of the risk weighting factors recommended by the ICRP were assigned rather than scientifically estimated based on calculations or actual data, due to lack of adequate data at the time. A more recent version of assigned relative risk estimates was provided by the ICR? in 1990 (4), and even those estimates are not universally accepted. Last, Chotas et al did not discuss the fact that in anteroposte- rior chest exposures, the actual matched exposure risk can vary substantially on the basis of patient sex. Because a very large weighting factor is assigned to the breast and that entrance exposure varies with beam energy (kilovolt peak), Chotas et al should have computed “matched risk” signal-to-noise ratios separately for male and female subjects. In conclusion, because of the large number of uncertainties associated with the risk parameter, we caution against using the matched exposure risk as the basis for comparing image quality parameters (eg, signal-to-noise ratio or others) that typically can be measured with much greater precision than the matched exposure risk itself. Other dose-related param- eters such as midline dose would be substantially more appro- priate for this purpose until the risk can be better quantified and the errors (confidence limits) on the estimated risk mark- edly reduced. This is particularly true for rnultiorgan expo- sures. References 1. Chotas HG, Floyd CE Jr. Dobbins JT, Ravin CE. Digital chest radi- ography with photostimulable storage phosphors: signal-to-noise ratio as a function of kilovoltage with matched exposure risk. Radi- ology 1993; 186:395-398. 2. Recommendations of the International Commission on Radiological Protection (ICRP): ICRP publication 26. Oxford, England: Per- gamon, 1977. 3. Huda W, Bissessur K. Effective dose equivalents, H.,, in diagnostic radiology. Med Phys 1990; 17:998-1003. 4. 1990 Recommendations of the International Commission on Radio- logical Protection (ICRP): ICRP publication 60. Oxford, England: Pergamon, 1990. Drs Gur and Shaw are correct in stating that exposure risk is highly influenced by the organs that are exposed and by pa- tient sex, and that published effective dose equivalence values are estimates only. However, our intention was to evaluate the relationship between signal-to-noise ratio and kilovolt peak with matched exposure risk while holding all other variables constant. Although the absolute exposure risk differs from pa- tient to patient, we estimate that, given a particular patient in a particular geometry, the relative differences in exposure risk that arise due to variations in kilovolt peak alone are ad- equately approximated by the published risk curves. The term estimated relative patient risk might have been used in our article to better draw attention to the uncertainties involved. Harrell C. Chotas, MS, Carey E. Floyd, Jr. PhD, James T. Dobbins III, PhD, and Carl E. Ravin, MD Department of Radiology, Duke University Medical Center Box 3302, Room 139, Bryan Research Building, Durham, NC 27710 Erratum “Preparation of the Colon for Single- and Double-Contrast En- erna Examination: A Simplified Method.” Radiology 1993; 188: 578-580. Page 579, in the Figure, the words “magnesium sulfate (15 rng)” should be replaced with “magnesium sulfate (15 g)” in Steps 1 and 7.