How high is the radiation dose for a person during a CT examination?

Over the past decades, CT imaging has experienced tremendous development. Today, it is an important instrument in modern diagnostics and numerous procedures exist to calculate the effective dose incurred by a patient. However, all of these procedures are based on phantom-related measurands from which the effective dose is estimated by means of calculated conversion factors for standard patients and for a reference scanner. Moreover, these procedures do not take any patient-specific particularities (such as their size or their weight) into account.

Within the scope of a European cooperation project, PTB has now developed the dosimetric basis for a procedure in which a patient᾽s individual data are to be used in the calculation for the first time. Together with the characteristics of the CT scanner used, the CT sectional views of the patient are entered into a Monte Carlo simulation. Using commercially available software, it is thus possible to generate a simulation of the dose distribution within just a few minutes. For this purpose, however, some important characteristics of the CT scanner used must first be determined experimentally. Among these characteristics are the characteristic fluence spectrum of the photons of the X-ray tube used and the attenuation properties of the integrated bowtie filters. The latter serve to adapt the radiation intensity to the patient᾽s section in order to reduce the dose and to achieve a more homogeneous noise distribution at the image detector.

The novel mobile measuring device determines these parameters, and within the scope of the project it has been possible to constantly reduce the necessary installation effort and the measurement duration. The procedure has been combined with known techniques used to determine the equivalent bowtie filter, so that only one single measuring arrangement and just a few minutes are needed to obtain all the data that are required for the full characterization of the computed tomography system.

To validate this procedure, CT images of anthropomorphic phantoms were taken. These phantoms were fitted inside with real-time dose detectors. The calculated and the measured doses were in agreement within the measurement uncertainties of less than 10 %. Thus, this new procedure has been validated, and an important step toward personalized dosimetry in computed tomography has been achieved.