Giovanni Bibbo

An EGS4-ready tomographic computational model of a 14-year-old female torso for calculating organ doses from CT examinations

Fifty-four consecutive CT scans have been used to construct a tomographic computational model of a 14-year-old female torso suitable for the determination of organ doses from CT. The model, known as ADELAIDE, is in the form of an input file compatible with user codes based on XYZDOS.MOR from the readily available EGS4 Monte Carlo radiation transport code. ADELAIDEs dimensions are close to the Australian averages for her age so the model is representative of a 14-year-old girl. The realistic anatomy in the model differs considerably from that in Cristys 15-year-old mathematical computational model by having realistically shaped organs that are appropriately located within a real external contour. Average absorbed dose to organs from simulated CT examinations of the chest and abdomen have been calculated for ADELAIDE using EGS4 within a geometry specific to the General Electric Hi-Speed Advantage CT scanner and using an x-ray spectrum calculated using data from the scanners x-ray tube. The simulations include the scanners beam shaping filter and patient table. It is suggested that the resulting values have fewer possible sources of uncertainty than organ doses derived from dose coefficients calculated for a MIRD style model with mathematical anatomy and a spectrum that may not match that of the scanner. The organ doses were normalized using the scanners CTDI measured free-in-air and an EGS4 simulation of the CTDI measurement. Effective dose to the torso from 26-slice chest and 24-slice abdomen examinations (at 120 kV, 200 mAs, 7 mm slices) is 4.6 +/- 0.1 mSv and 4.3 +/- 0.1 mSv respectively.

A comparison of radiation dose measured in CT dosimetry phantoms with calculations using EGS4 and voxel-based computational models

CT is a high-dose examination and possibly the dominant contributor to dose from diagnostic radiology. Estimates of organ doses are obtained from Monte Carlo calculations and used to quantify radiation risk. To ensure the validity of using Monte Carlo calculations to estimate actual dose, measurements must be compared with calculations. We have measured doses to CT head and chest dosimetry phantoms and compared them with Monte Carlo (EGS4) calculated doses in voxel-based computational models of the phantoms. The simulation used an x-ray spectrum calculated from the specified values of the scanners x-ray tube parameters. The scanners beam-shaping filter was included in the modelling. Measured and calculated doses to both the head and chest phantoms agreed to within 7%. The inclusion of Rayleigh scattering in the calculations has a significant effect if only one slice is scanned but not if multiple slices are scanned.

Off‐axis x‐ray spectra: A comparison of Monte Carlo simulated and computed x‐ray spectra with measured spectra

The off-axis x-ray spectra from a constant potential x-ray generator were measured with a high purity germanium spectrometer cooled to liquid nitrogen temperature. The measured spectra were compared with off-axis x-ray spectra calculated using a code based on the semiempirical model developed by Tucker et al. and Monte Carlo simulated x-ray spectra using the EGS4 code system. In this study, both the Tucker model, and the EGS4 code system, were found to produce off-axis bremsstrahlung x-ray spectra which agreed well with the spectra measured at three emerging angles. In the measured and the EGS4 generated spectra the total K-characteristic peaks were in increasing order, as observed in the anode to cathode direction, whereas the Tucker model produced maximum total K-characteristic peaks at the 6 degrees anode side, and lesser amounts at the central axis and the 6 degrees cathode side. Large differences in the total K-characteristic lines is seen among the three different methods. The EGS4 code system was able to produce x-ray spectra for a combination of target materials.

The effect on dose to computed tomography phantoms of varying the theoretical x-ray spectrum: A comparison of four diagnostic x-ray spectrum calculating codes

Theoretical x-ray spectra calculated by four different codes for the same tube parameters are compared by calculating and measuring doses to computed tomography (CT) body and head phantoms. The effect on the 120 kV spectrum, and hence on the calculated dose, of varying the anode angle, tube voltage, and total filtration of the x-ray tube is investigated. Codes used were those of Nowotny and Höfer (XCOMP), Boone, Iles, and Tucker et al. The code based on the work of Tucker et al. produced calculated doses noticeably lower than the other codes and compared best to the measured value. The variation in calculated dose between the Tucker code and the others is of the same order as the variation introduced by uncertainties in total filtration of about 20%, in peak tube voltage of +/- 4 kV, and in change of anode angle from 7 degrees to 13 degrees.

Diagnostic reference levels for common paediatric fluoroscopic examinations performed at a dedicated paediatric Australian hospital

Diagnostic reference levels (DRL) of procedures involving ionising radiation are important tools for optimising radiation doses delivered to patients and to identify cases where the levels of dose are unusually high. This is particularly important for paediatric patients undergoing fluoroscopic examinations as these examinations can be associated with a high radiation dose. In this study, a large amount of paediatric fluoroscopic data has been analysed to: establish local DRL, identify the most significant factors determining radiation dose to patients, and modify fluoroscopic techniques to optimise the examination protocols.

Diagnostic reference levels of paediatric computed tomography examinations performed at a dedicated Australian paediatric hospital

Diagnostic Reference Levels (DRL) of procedures involving ionizing radiation are important tools to optimizing radiation doses delivered to patients and in identifying cases where the levels of doses are unusually high. This is particularly important for paediatric patients undergoing computed tomography (CT) examinations as these examinations are associated with relatively high‐dose.

Effective doses and standardised risk factors from paediatric diagnostic medical radiation exposures: Information for radiation risk communication

In the paediatric medical radiation setting, there is no consistency on the radiation risk information conveyed to the consumer (patient/carer). Each communicator may convey different information about the level of risk for the same radiation procedure, leaving the consumer confused and frustrated. There is a need to standardise risks resulting from medical radiation exposures. In this study, paediatric radiographic, fluoroscopic, CT and nuclear medicine examination data have been analysed to provide (i) effective doses and radiation induced cancer risk factors from common radiological and nuclear medicine diagnostic procedures in standardised formats, (ii) awareness of the difficulties that may be encountered in communicating risks to the layperson, and (iii) an overview of the deleterious effects of ionising radiation so that the risk communicator can convey with confidence the risks resulting from medical radiation exposures.

Rejoinder to Letters to the Editor: Luke Wilkinson and Donald McLean, in response to Giovanni Bibbo’s letter to the editor: Standardisation of shielding of medical X-ray installations

In replying to my letter to the editor regarding standardisation of shielding of medical X-ray installations [1], both McLean [2] and Wilkinson [3] indicated that there is a need for the standardisation of shielding in medical practices but the stumbling block is the non-uniformity of radiation safety legislations across local regulatory authorities. With reference to shielding, the lack of uniformity is related to the lack of a shielding standard. The local regulatory authorities do not set standards. Their roles are to enforce standards. In Australia, standards in radiation safety can only be established by ARPANSA with the publication of codes of practice or Standards Australia through AS/NZS documents. As a standard, shielding requirements could be as simple as specifying for each medical imaging modality the thicknesses of the shielding barriers in terms of lead equivalent and the height. Wilkinson [3] in his reply expressed his concern regarding over-shielding. A simple cost analysis of lead sheets available in hardware stores indicated that there is only a small difference in the costs per square metre of lead sheets of thickness 10, 15 and 20 kg m−2. This reflects the result of the cost analysis of shielding design performed in 1973 [4] which showed that a tenfold increase in radiation protection could be achieved with only a 25% increase in cost. This statement was also reported on page 6 of the NCRP 49 [5]. Thus, over-shielding is not an issue as it does not represent an additional huge cost in the shielding of X-ray facilities. Even with the currently used shielding methodologies, in specifying the shielding requirements for an X-ray room, the shielding designer should take into account that the lifetime of clinical use of the X-ray room could be many times that of the initially installed X-ray equipment and the uses of the room and adjacent areas with time are subject to changes. Upgrading the shielding of an existing X-ray room will be much more expensive than if the room was properly shielded from its design stage. Standardisation of medical X-ray facilities should be the next step in the evolution of radiation protection in Australia and it can only be achieved through organisations such as ARPANSA and Standards Australia with input from the professional societies ACPSEM and ARPS. As mentioned, it is not the responsibility of local regulatory authorities to set standards; their functions are to enforce established standards.