Computed Tomography Dose Monitoring: Is Radiation Dose the Wrong Patient Imaging Risk to Manage?

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As an early advocate and one of the coauthors of the American Association of Physicists in Medicine (AAPM) Task Group 204, which developed the size-specific dose estimate, I recognize the importance of managing computed tomography (CT) radiation dose risks. Computed tomography dose risks have certainly captured the attention of medical practitioners, researchers, regulators, and the public media. The recent excellent review article by Frush et al encourages a systematic approach to monitor and control CT dose risks (volume 9, number 4, December 2013, “Radiation Protection and Dose Monitoring in Medical Imaging: A Journey From Awareness, Through Accountability, Ability and Action—But Where Will We Arrive?”).
However, there is another risk that deserves equally rigorous and systematic methods of management. The risk of not obtaining the needed diagnostic information from the image can have catastrophic consequences for the patient. As dose is lowered, the x-ray noise in the image increases, reducing the ability to perceive small low-contrast anatomic and diagnostic features of concern. Unfortunately, the risk for degraded image quality and misdiagnosis from using too low a dose is given only a perfunctory acknowledgement in most CT dose literature. Not all CT scanners and their various modes of operation produce the same image quality for a given dose, and not all patients and clinical indications require the same image quality. Thus, dose is a poor surrogate for image quality. The appropriate clinical goal balances the risks and strives to use the lowest dose that results in an image that allows an accurate patient diagnosis.
Although professional organizations (ACR, AAPM, Image Gently, Image Wisely, etc.) provide considerable protocol guidance, the clinical image quality needed for a given diagnostic task is rarely a topic of rigorous medical research. When such research is conducted, results are limited by the study of the specific make and model of the scanner and the selected operating protocol, the range of patient body habitus in the study, the clinical task, and other frequently unstated psychophysical factors. To remedy this, some of the resources that are currently focused on dose measurement and management should be shifted to developing and validating an image quality metric that can be used to guide clinical practice.
Without such an image quality metric, researchers cannot compare and communicate clinical image quality needs to one another and thus cannot confidently manage the trade-offs between a patient's risk for misdiagnosis from a diagnostically compromised image and the concerns regarding medical radiation risks to the population. None of us advocates recklessly reducing dose in a way that increases the risk for an inappropriate medical diagnosis.

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