CT radiation dose has recently been in the limelight, not only in the news media but also for patients undergoing CT exams. Estimating CT dose is complex and further challenged by the different types of CT scanners available in the market today. In a recent study, investigators collected CT radiation dose measurement data from all the multi-detector CT (MDCT) scanners used in the National Lung Screening Trial. Radiation dose in CT is defined as CT Dose Index (CTDI). CTDI represents the average absorbed dose, along the length of the patient’s body from a series of contiguous scans. Normalized CTDI can be used to determine the efficiency of the CT scanner, and finally the dose imparted to patients. Normalized CTDI represents the dose per mAs (tube current x rotation time) where the mAs determines the number of x-ray photons utilized per rotation of the CT scanner.
The results of this study imply that one needs to be aware of the differences in normalized CTDI – not only between manufacturers, but also between models of CT scanners from the same manufacturer (Table 4 of this study).
This study showed that the average normalized CTDI varied greatly (by a factor of 2) across all scanners from different manufacturers. The dose efficiency of the CT scanners improves as the scanners get more complex, with the number of detectors along the length of the patient increasing from four or eight to sixteen and beyond (e.g.: 4- 8- or 16- slice CT scanner). This has an impact on image quality and patient dose because the more efficient the CT scanner, the less the dose to the patient to acquire the desired image quality.
The dose data in this study was collected for 96 MDCT scanners across 33 participating institutions. While the study did not focus on image quality and patient dose, it did examine scanner-specific radiation dose data across all institutions.