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Opinion |
1 Department of Pediatric Imaging, Children's Hospital of Michigan, 3901 Beaubien Blvd., Detroit, MI 48201.
Received January 10, 2002;
accepted after revision January 22, 2002.
Address correspondence to T. L. Slovis.
Introduction
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CT is arguably the single most valuable imaging tool. It is therefore imperative to understand how we missed the problem of excessive radiation so we can both remedy it and prevent unnecessary exposure with newer technologies such as computed radiography.
It seems to me there are no "white hats." I consider the radiologist the person ultimately responsible for the care of patients being imaged. Radiologists have been unaware of the "uncoupling effect" in CT—that is, the final image is divorced from the radiation dose because of digital and electronic manipulation [8]. Radiologists wanted the best image and got it without knowing the radiation cost. The radiologist in most cases did not need to devise special protocols for children because "the pictures were great," and we found more and more indications for CT in diagnosing common illnesses. Our clinical colleagues followed eagerly, ordering CT for "everything." (The pictures are great!) Radiologists were unaware that when the radiation doses in our adult protocols are used in neonates or young children, the effective dose is up to 50% greater [9]. This outcome is in part attributable to the fact that the dose in the center of large objects (adults) is about half the surface dose, whereas for small objects (children) the dose is nearly uniform throughout.
Radiologists in general underuse their colleagues in radiation physics. Both groups have little dialogue with radiation biologists or epidemiologists. We don't know each other's literature. The physicists have been telling us that decreasing tube current, kilovoltage, or both lowers the dose [10, 11].
Everything changed in 2000. The radiobiologists unequivocally showed the small individual risk of carcinogenesis from radiation doses comparable to our CT [12]. They restated the radiosensitivity of children. Most of us never got to this literature—but even if some of us did, it seemed to be a price we had to pay to get great pictures.
The manufacturers have some responsibility for explaining the trade-off between the highest quality images and the highest radiation dose. Manufacturers must know children are 10 times more sensitive to radiation; therefore, they have a responsibility to make CT user-friendly for selecting the pediatric, lower dose protocol. It is the manufacturer who must help us find ways to lower the dose and still get great pictures. It is the manufacturers who need to properly train the people on the line when a new instrument is released—the technologist and the radiologist.
Applications training, and training in general, can be woefully inadequate. Training takes place in the hospital or imaging center with all the concomitant distractions. No verification of acquisition of knowledge takes place, and the application specialist does not necessarily have the best training or teaching skills. The equipment is accepted (with a physicist's blessing most of the time), and we can begin to get great images without fully understanding the trade-off between the different techniques (high speed or high quality) or what "overbeaming" means. Our supervisory councils, such as the National Council on Radiation Protection, and the government agencies, such as the United States Food and Drug Administration, have not really helped. We currently use the CT dose index or the weighted CT dose index, which do not adequately convey the risk to the child. The pediatric patient has been forgotten.
These problems prompted the Society for Pediatric Radiology to sponsor (with an unrestricted grant from General Electric Medical Systems) the ALARA conference in August 2001. ALARA means As Low As Reasonably Achievable radiation dose. The 100 multidisciplinary participants and registrants wrestled with these problems. The conference proceedings appear in the April 2002 issue of Pediatric Radiology [13]. The executive summary (Appendix 1) is important for all radiologists who image children.
The driving force for great pictures is the radiologist. We should not do the test if in fact we think the images will not allow us to make the diagnosis or we will miss a lesion. On the other hand, we have done poorly in determining the reduced level of image quality we can accept and still not miss abnormalities [5, 6, 8, 14]. The demand of the radiologist for the best image is understandable but not acceptable unless the cost in radiation dose is fully understood. The manufacturers, radiologists, and physicists must increase research in these areas, because only in this way can we control the technology.
With the February 2001 and subsequent issues of the American Journal of Roentgenology, radiologists should be cognizant of the CT radiation dose issue [4, 5, 7, 8]. The real questions remaining are: Do I believe the data? If I believe the data, what should I do about it? How can I avoid getting blind-sided again?
Clearly, obtaining more information is helping us to believe that a problem exists—and to find solutions. The Society for Pediatric Radiology offered a multidisciplinary session on the topic at its 2002 annual meeting in Philadelphia in April. The National Council on Radiation Protection proposes to conduct a 2-day symposium on patient dose control and protection in CT in the fall of 2002 in Washington, DC. This meeting was initiated by the National Cancer Institute and the Food and Drug Administration. The Society for Pediatric Radiology and the National Cancer Institute are preparing a brochure on the topic for dissemination to physicians—both radiologists and practitioners. Radiation issues were discussed, and a summary of the ALARA conference was presented in an editorial in Radiology [15]. At the 2001 summer meeting of the Intersociety Commission and the annual meeting of the American College of Radiology (2001), each organization passed a resolution on lowering CT doses to children.
Although we have addressed the issues in regard to CT, little has been done to prevent us from being similarly blindsided by excessive radiation dose in computed radiography. Once again, in computed radiography we have the uncoupling effect—that is, the final image is divorced from the radiation dose. Theoretically, we should be able to have comparable images with computed radiography at lower exposures, but this is dependent on how the dynamic range of the instrument is configured. As with CT, what would have been an overexposed image can be manipulated by the computer to give an acceptable image with excessive radiation dose (contrast and density depend on processing and display). It is possible to achieve 35-50% reduction in radiation dose with equal contrast and density if we are willing to accept increased noise (mottle) [15,16,17,18,19]. However, it seems that neither radiologists nor the manufacturers are prepared to take advantage of this aspect of the technology [16]. Do you know that when the numeric exposure number (LgM on Agfa, exposure index on Kodak) is increased (by 0.3 LgM or 300 exposure index number), the radiation dose is doubled? With Fuji, when the "s" number is halved, the dose is doubled. Interesting and confusing. Willis and Parker (Willis CE, Parker BR, presented at Radiological Society of North America meeting, November 2001) stated, "The scientific literature is devoid of any systematic effort to construct techniques quite specific to CR [computed radiography]." It is our job as radiologists to achieve these great diagnostic images at the lowest cost in radiation exposure, be it in CT or computed radiography. The time has come.
Radiologists, physicists, manufacturers, and national oversight groups must work together to minimize the radiation dose in CT and computed radiography. We must all be aware of the greater sensitivity of children to radiation and go out of our way to ensure that the lowest dose diagnostic images are obtained.
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