Comparison of Planer Dose Equilibrium and Computed Tomography Dose Index and Implications for Reported Patient Dose Information

Technical developments are ongoing in CT, and there has been a continually increasing trend in patient prescription, resulting in increased exposure. Currently, doses delivered during CT are generally evaluated using computed tomography dose index (CTDI), which is measured with a 10 cm pencil ionization chamber placed in a 14 cm PMMA phantom. However, shortfalls in CTDI have been identified by the American Association of Physicists in Medicine (AAPM) who have proposed a new method, dose equilibrium (DEq). In this paper, the dose equilibrium was used to estimate the dose in two protocols (thoracic and abdominopelvic) and compared to CTDI values. In addition, a retrospective correction was applied to 20 patient CTDI’s by characterizing the specific DEq profile of the system scans. The results indicated the dose equilibrium estimations of two protocols, thoracic and abdominopelvic, were 29% and 30% respectively, higher than those informed by the CT scanner. In addition, a retrospective dose correction estimation of a random sample of twenty patients demonstrated an annual underestimation in absorbed dose by between 26% and 28%. Continued use of the CTDI method in quality assurance of modern CT could result in greater patient risk. AAPM Task Group 111 presents a more accurate, safer method to estimate dose and its adoption is paramount.

CT Output Dose Performance-Conventional Approach versus the Dose Equilibrium Method

Current output dose measurement in CT is based on (CTDI). 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. AAPM TG 111 report recognizes the shortfall of the CTDI methods and recommends a new technique that more accurately characterizes the dose profile from modern CT scanners, which utilizes a short conventional ion chamber rather than a pencil chamber. We developed and characterize a in-house phantom design using a three separate anatomical regions of clinical scan sequences (Head, chest and abdomen), and determined the equilibrium dose in our dose equilibrium phantom, measured 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 dose equilibrium phantom measurements. This methodology allows measurements of the accumulated dose for any clinical scan length and allowing measurement of the equilibrium dose. Using the new methodology, we determined that the CTDI approach can underestimate the dose by 25% to 35% and all of our dose values from the water phantom and farmer chamber were independently verified with TLD measurements.

Observatory science with eXTP

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry(eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.