CT output dose performance – conventional approach vs the “ Dixon ” method

Current output dose measurement in CT is based on the Computed Tomography Dose Index (CTDI). The CTDI method utilizes a 10-cm long pencil ionization chamber placed in a 16-cm long Perspex phantom at 5 different sites. The CTDI dose estimation is performed using a single rotation for each reference site and excluding the contribution of radiation scattered beyond the 100-mm range of integration along z. The conventional methodology of CT dosimetric performance characterization is not appropriate to modern CT scanners with helical scanning modes, dose modulation, array detectors and multiple slice planes or cone-beam irradiation geometries. The “Dixon” method (1,2) is a new technique which more accurately characterizes the dose profile from a modern CT scanner. It uses a small volume farmer chamber to directly measure the cumulative dose D(x) at any point by scanning a sufficient length of phantom to produce dose equilibrium at the centre. The purpose of this study is to characterize an in house “Dixon” phantom design using a number of clinical scan sequences, and determine the accumulated dose in a water phantom, determine if the attenuation of the beam is the equal to that of CTDI Perspex phantom and compare CTDI dose estimations using a standard pencil chamber to the Dixon phantom measurements. All dose measurements were independently verified with TLD measurement. Introduction Duo to the modern CT scanner, advanced technologies with helical scanning mode or cone-beam irradiation geometries appeared, the CTDI measurement methods were not suitable. This is because the CTDI are based on two primary arguments, athe pencil ionization chamber (100-mm-long) which used to collect the dose may not be sufficiently long to measure all of the tails of the scattered dose distribution, bthe phantoms used for CTDI measurements are shorter than an adult torso and so do not produce as much scattered radiation as would occur in a typical adult. This means that the average dose (eg, MSAD) that would occur in the much longer “typical-sized” adult torso is underestimated with CTDI measurements in the 14-cm-long body CTDI phantom. New measurement methods are described in the AAPM task group report No.111 (AAPM TG111) (3). small volume ion chamber to directly measure the cumulative dose D(x) at any point by scanning a length of phantom long enough to produce dose equilibrium in the center, and directly measuring Deq(0). The main aim of this work is to follow the AAPM TG 111 steps to measure the accumulative dose and equilibrium dose by farmer chamber with water phantom and compare it with CTDI volume. moreover, using TLD chips to confirm the dose values. Materials & Methods Materials  A “Dixon” water filled phantom was designed and constructed in NUI Galway. It is 32-cm in diameter and 50 cm in length. The water phantom is designed to be transported empty, and once placed on the table, it can be quickly filled or emptied with a small pump operating from a room sink as a reservoir. This phantom size was chosen in order to represent the attenuation and absorption characteristics of the average size adult body.  A Perspex phantom was used to measure the CTDI volume. The phantom consisted of two PMMA cylinders: one cylinder represented the head (160mm diameter) and the other represented the body (320mm diameter). The length of each cylinder was 140mm. Each cylinder contained holes large enough to accept a pencil chamber.  All measurements were taken using a Farmer chamber (PTW type 30013) and pencil chamber (Unfors xitm). Both were ionisation based chambers. The Farmer chamber collection volume is 0.6 cm3. It was calibrated by the National Standards of the German National Laboratory. In contrast, the pencil chamber is 10 cm long and was calibrated by Unfors RaySafe. In addition, dose for verification was perform independently using TLD 100H.  A PC electrometer from Sun Nuclear was used for the Farmer chamber, configured with a cable long enough to allow the electrometer to operate out of the phantom scatter-radiation field, in order to avoid inducing extraneous currents in the electrometer.