Individually adapted examination protocols for reduction of radiation exposure in chest CT

RATIONALE AND OBJECTIVES. To develop a simple directive for the reduction o f radiation exposure without loss of diagnostic information in routine ches t CT examinations. METHODS. Two hundred fifty adult patients (164 male, 86 female) were entere d into a prospective trial. All examinations were performed with a multisli ce CT technique (Somatom Volume Zoom, Siemens). Four groups of 50 patients each were scanned with patient-related specific parameters: individual mA-s values were derived from the estimated body weight: kilograms + 10, +/- 0, - 10, and - 20 mAs. The results were compared with those of 50 patients wh o were examined by a standard chest protocol by using the parameters 120 mA s and 140 kV. All other parameters including the tube voltage were kept con stant. Subjective image quality was rated on a three-point scale: 1 = excel lent, 2 = fair, 3 = nondiagnostic. In addition, objective criteria based on signal-to-noise measurements were assessed by using a region-of-interest m ethodology. RESULTS. Image quality was sufficient in all cases. Mean subjective grading s of image quality, based on soft-tissue window settings, were 1.1 for the 120-mAs protocol, 1.1 for the (body weight [kg] + 10) mAs protocol, 1.1 for the (body weight [kg] +/- 0) mAs protocol, 1.3 for the (body weight [kg] - 10) mAs protocol, and 1.2 for the (body weight [kg] - 20) mAs protocol. Ob jective criteria based on noise measurements showed mean +/- standard devia tion values of 5.7 +/- 0.8 Hounsfield units (HU) for the 120-mAs protocol. For the reduced-dose protocols, values were calculated as 7.6 +/- 1.2 HU (g roup + 10), 7.9 +/- 1.3 HU (group +/- 0), 8.7 +/- 1.2 HU (group - 10), and finally 9.1 +/- 1.3 HU (group - 20). The best correlation for an entire sub group was achieved with the - 10 protocol (body weight [kg] - 10) mAs, with nearly constant noise related to body weight in all patients. CONCLUSIONS. By deriving mAs values from body weight estimation, an individ ually adapted protocol for chest CT can be recommended and easily employed in a clinical setting. With an adaptation of the tube current-time product based on the estimated body weight of the patient - 10 (body weight [kg] - 10 mAs), a well-balanced examination without significant loss of informatio n, even in soft-tissue window settings, can be performed with this particul ar scanner. For this adapted mAs protocol, a mean reduction of radiation ex posure of 45% was achievable, compared with the standard protocol. A maximu m decrease per case down to 31 mAs was obtained, without relevant loss of i mage quality. Therefore, for other types of CT scanners, analogous protocol s may be adapted.