Comparison of the Dixon and CTDI dose quantification techniques

Quantification of radiation dose in computed tomography(CT) is an ongoing area of research due to the continual evolution of CT technology both physical and software based. CT contributes to nearly 70% of the total dose given to patients during examinations [1] . As the number CT examinations are grown in the world, the radiation dose delivered from CT scanners to patient must be quantified precisely. Current dose measurement from CT is based on Computed Tomography Dose Index(CTDI) which was developed in 1981 [2] . The CTDI method utilizes a 10-cm long pencil ionization chamber placed in a 14-cm long PMMA phantom at 5 independent sites. The CT dose measurement is of a single rotation for each reference site. Modern CT scanners with helical scanning modes, auto exposure control, array detectors and multiple slice planes go beyond the efficacy of the CTDI technique. CTDI excludes the contribution of radiation scattered beyond the 100-mm range along the z axis. Dixon et al. [3] developed a new method to measure the dose from CT scan which is more accurate than the conventional CTDI. By scanning the length of a long phantom containing a small volume farmer chamber we were able to directly measure the cumulative dose D(x) at any point allowing the measurement of a dose equilibrium at the centre of the dose profile. This presentation will compare both techniques (i.e. the dose profiles and output obtained from Dixon and CTDI method in a Clinical CT.