Radiation Exposure in Thoracic CT

Abstract

Helical or spiral CT has revolutionized X-ray-based diagnostic medical imaging in the past 15 years (Berland and Smith 1998; Kalender and Polacin 1991; Kalender et al. 1990; Remy-Jardin et al. 2001). Introduced in the late 1980s, single and, more recently, multidetector-row CT scanners have greatly expanded the clinical indications and demand for CT scans of the thorax. This has led to a considerable increase in the number of examinations and in the average scanned volume obtained per examination. This has inevitably led to an increase in the radiation exposure delivered by these studies. The increased utilization of CT has been documented by studies in the USA and UK that have found approximately a twofold increase in the number of CT examinations between the late 1980s and the late 1990s (Mettler et al. 2000; Shrimpton and Edyvean 1998; Shrimpton et al. 1991). A recent report from an American academic institution showed that CT scans formed 11.1% of diagnostic radiologic procedures in 1999 compared with 6.1% in 1990 (Mettler et al. 2000). Because CT is a relatively high-radiation-dose technique, these authors reported that CT delivered approximately 67% of the total effective dose from diagnostic radiology in 1999. This study also noted that 11% of the CT scans were performed in the pediatric population, which was higher than previously estimated (Mettler et al. 2000). The relatively high dose of CT examinations was also noted in a radiation dose survey performed in the UK in 1989 which showed CT studies accounted for 2% of radiographic examinations and 20% of the effective dose (Shrimpton et al. 1991). A follow-up survey in 1995 showed that CT now accounted for 4% of all radiographic examinations (Shrimpton and Edyvean 1998) and for 40% of the medical effective dose. The increase in population radiation exposure from CT examinations, particularly in children, has been of concern to radiologists, medical physicists, government regulators, and the media (Sternberg 2001). The suggestion that excessive radiation doses are being prescribed for CT examinations has appropriately aroused the attention of the radiology community (Rogers 2001). We must be attentive to our responsibility to maintain an appropriate balance between diagnostic image quality and radiation dose (Nickoloff and Alderson 2001). It has been suggested that the rapid expansion of clinical indications for spiral thoracic CT examinations radiation dose issues may have not received adequate attention (Golding and Shrimpton 2002). This chapter reviews (a) measurement units used to quantify radiation exposure, (b) parameters that affect CT radiation dose and effi ciency, and (c) advances in dose reduction in chest CT. Radiation dosimetry and bioeffects are not addressed in detail and interested readers are referred to more complete works in these areas (Golding and Shrimpton 2002; Huda and Atherton 1995; Jones et al. 1992; Mettler and Upton 1995; Metz et al. 1995).