Reference Organ Absorbed and Effective Dose Coefficients for Computed Tomographic Examinations

Abstract

For many years, ICRP has produced reference dose coefficients for common diagnostic nuclear medicine procedures. Until recently ICRP has not provided reference dose coefficients for x-ray imaging procedures. However, x-ray imaging procedures make the largest contribution to human exposure to ionising radiation from artificial sources. One of the reasons for this is the well documented increase in the use of computed tomography as an imaging modality, resulting in a year-on-year increase in imaging dose per capita associated with such examinations. ICRP has recently published reference dose coefficients for common radiographic examinations, and this publication extends that work to computed tomography. The availability of ICRP reference organ and effective dose coefficients in computed tomography is expected to standardise CT dose assessments internationally and facilitate the work of national and international authorities responsible for the estimation of patient doses. It will also benefit practitioners working in the field. This publication describes the development and implementation of a virtual, representative scanner, the ICRP reference scanner, that incorporates a theoretical, non-manufacturer-specific model of Automatic Tube Current Modulation (ATCM). The reference scanner was developed using detailed data from a series of CT scanners. The ATCM implementation is based on a simple mathematical model and is provided as an educational tool / aid to optimisation only. The reference scanner was used to derive a library of dose coefficients for the adult and paediatric male (M) and female (F) ICRP reference voxel phantoms. The normalisation quantity is CTDI_free-in-air and the body and head CTDI_w to CTDI_free-in-air coefficients are also provided. For each phantom slice, coefficients were determined for organ absorbed doses and for the coefficients of either Σ w_T × H_T^M or Σ w_T × H_T^F, where H_T^M and H_T^F are the equivalent doses to the tissues or organs T, and w_T is the tissue weighting factor for target tissue. The coefficients were derived using the Monte Carlo codes MCNP 6.1 and MCNPX 2.7 and verified using the Monte Carlo code Geant4. Users can use the data themselves with a set of equations provided in the text, or they can make use of a calculator accessible via a graphical user interface that accompanies the publication. No attempt has been made to define specific clinical protocols. The scope of this publication is confined to ‘conventional’ CT; it does not, per se, consider technologies such as photon counting CT or spectral CT, whether using dual detectors or kV switching. The dose coefficients are not intended to be used for individual patient dosimetry.