CT radiation dose and image quality

CT scanning burst on the diagnostic imaging scene in 1973, sprinted for almost a decade, and then settled into comfortable midlife. But the quiet life did not last long. Technologic advances thrust CT scanning back to center stage. High-frequency generators with ever-higher power ratings and specially designed CT x-ray tubes with ever-higher heat storage permitted the advent of faster subsecond scans. These advances along with the development of slip ring electrical energy transfer allowed for continuous gantry rotation and the birth of spiral or helical scanning [1]. If that were not enough, multirow detector arrays were developed to increase the area of coverage during one gantry rotation. This facilitates volume CT scanning of whole organs in a single breath-hold. Improvements in software have led to real-time 3-D displays of volume-rendered data, as used in virtual CT colonoscopy, CT angiography, CT coronary calcium scoring, and other useful applications [2]. These technologic advances have expanded the role of CT in diagnostic imaging greatly. The annual number of CT studies in the United States more than tripled from 3.6 million in 1980 to 13.3 million in 1990 and then more than doubled to 33 million in 1998 [3]. By 2001, CT scanning comprised more than 13% of all radiology procedures. Unfortunately, the collective radiation dose from CT procedures increased even faster than the increase in the number of studies. The National Radiological Protection Board in the United Kingdom indicated that in 1989, CT studies comprised only 2% of all imaging studies but contributed to 20% of the total patient dose. Subsequent study analysis showed that CT contribution to overall patient dose has risen to 40% [4]. In a 2000 report of the United Nations Scientific Committee on the Effects of Atomic Radiation, the frequency of CT examinations for all imaging procedures was approximately 5%, but the CT radiation dose was approximately 34% of the total imaging dose and was the largest single sector of radiation dose [5]. In the United States, CT contribution to overall patient dose in some departments may be as high as 67% [4]. The rising total patient radiation dose from CT primarily is the result of its increased use and increased number of images per examination. In the early days, a CT study consisted of 20 similar to 50 images. Today, it is not unusual to have CT studies with 200 - 1000 images or more. Initially, CT was used almost exclusively to rule out malignant disease or replace procedures of even graver danger (Does anyone remember the air contrast pneumo-encephalogram?) and radiation dose was not an issue in most of these cases. Today, with the capability of performing rapid multiphase contrast enhanced studies, screening studies, and increased use, the collective CT radiation patient dose is adding up. What is lacking is good radiation dose management in CT imaging [6]. Since their inception, CT studies have been performed with standard one-size-fits-all technique protocols [7]. A series of journal articles in 2001 concerning the risk of radiation-induced fatal cancer in pediatric patients, and the fact that pediatric patients were being imaged with adult CT protocols, caught media attention and caused considerable public and professional outcry [8 - 11]. This had the positive effect of focusing attention on better CT radiation dose use and implementation of as-low-as-reasonably-achievable (ALARA) concepts into CT technique protocols. Appropriateness criteria now are recommended in the selection of CT imaging examinations. The ALARA concept i evoked for CT radiation dose management. Questions are being asked: What is an appropriate amount of radiation for CT procedures? How can the radiation needed for CT scanning be optimized? Answering these questions requires a basic understanding of CT radiation dose and the factors that determine the amount of radiation used in CT scanning.