Dose and Risk Characterization in CT

Computed tomography is an indispensable modality for obtaining critical diagnostic information regarding healthy and diseased tissues of the patient. It is widely accepted that the medical benefit of computed tomography imaging vastly exceeds any harm incurred to the patient following x-ray exposure. Nevertheless, CT imaging exams should be medical justified, and the CT imaging protocols applied should be optimized to balance image quality and patient dose. In this chapter, we review various forms of patient dose metrics and discuss methods by which individual organ doses to patients of both genders, of all ages, and of varying body morphometries may be assessed using computational human phantoms and Monte Carlo radiation transport simulation. These values of individual organ dose may then be used to report nominal values of risk through either radiation detriment, as determined by the quantity effective dose, or the lifetime attributable risk (LAR) of cancer. The former averages population-based risks across both sexes, all ages, and several populations, while the latter retains these individual features of the risk profile. The radiation epidemiological models that underpin these risk estimates are based on dose-response data obtained from long-term follow-up of exposed populations and thus may only be applied, at best, at the population level – such as cohorts of young female patients or those of older male patients – and never at the level of an individual patient. These population-based nominal risks, however, are needed for CT imaging protocol optimization. The chapter concludes with a numerical example of organ doses and risks – both radiation detriment and cancer incidence/mortality – for an age-dependent series of reference patients undergoing 18FDG PE/CT imaging.