Janice Honeyman-Buck, Ph.D.
Editor
Journal of Digital Imaging
Editorial Office
The Society for Imaging Informatics in Medicine
19440 Golf Vista Plaza, Suite 330
Leesburg, VA 20176
Dear Dr. Honeyman-Buck,
We would like to thank you for the opportunity to respond to the letter from Dr. Willis. We are pleased that Dr. Willis found our paper useful and thank him for his interest and kind comments. Certainly, as Dr. Willis points out, we could have included many more references to the technologies involved, indeed many of his own articles, but our intent was to focus on the measurement of patient dose in the clinical situation.
All computed radiography (CR) and digital radiography (DR) systems provide a proprietary exposure index (EI), which is derived in a different way by each manufacturer. A discussion of the derivation and meaning of EI was therefore felt to be outside the aim of this paper. This problem will not be properly resolved until manufacturers provide an EI based upon air kerma at the cassette (as recommended by the American Association of Physicists in Medicine in their draft report TG116).
We chose to measure patient dose using thermoluminescent dosimeters (TLDs) for film-screen systems and CR, and the dose–area product meter for DR. TLDs measure surface dose including backscatter. For the dose–area product meter, the measured quantity is in-air, so a backscatter factor has to be applied to provide patient surface dose. For posterior anterior chest, we used a factor of 1.3; for anterior posterior abdomen, a factor of 1.41(1). We did not use estimated patient dose based upon output factors as suggested by Dr. Willis.
As far as quality control was concerned, the CR readers were calibrated for sensitivity and uniformity on a semiannual basis, and the DR system was calibrated for sensitivity and uniformity on a monthly basis during the period of the study.
The comments on image quality and automatic exposure control systems are apposite. Certainly, either way of increasing the dose would have been possible. During the transition to digital projection radiography techniques, there is usually a period during which both film and soft copy reporting are utilized. This is partly because of adaptation to soft copy reporting, and partly due to image distribution problems on-site and off-site. At the start of this transition, we had to increase the dose used for chest radiography with CR. This was not necessary for any other organ system and reflects the special requirements for imaging of the chest and, as in any other clinic, the preferences of the radiologists involved.
We give a very full explanation of the concept of reference levels in the “Discussion” section of the paper, and we believe, like many others, that the measurement of patient surface dose and comparison with reference levels are important tools in the optimization process. While reference levels are based on the status quo, they can indicate those clinics that are using high doses and indicate where possible dose reductions could be achieved. In a sense, this approach determines what is unacceptable rather than what is optimum. The determination of the optimum dose—the minimum dose to produce a study with the required diagnostic information—is still elusive, largely because of the problem with measuring image quality objectively.
In summary, our work showed that, in our clinic, there were significant differences in patient dose from DR and CR. For most studies, the patient dose using CR was very slightly different to the dose from previous film-screen systems; for DR the patient dose was about half the dose from film-screen systems.
John Aldrich
Emerancia Duran
Pat Dunlop
John R. Mayo