Beam flatness perturbation effects of a perforated thermoplastic immobilization device on 6 and 12 MeV electron beams.

PURPOSE: Perforated thermoplastic masks are widely used in radiotherapy of head and neck malignancies. They provide for patient immobilization and increase setup reproducibility. Some oncology treatment centers cut mask portals (windows) for the beam to pass through; for those centers that do not, the mask affects beam fluence. The extent to which beam flatness is altered by such a mask is investigated. MATERIALS AND METHODS: The effects of perforated thermoplastic on 6 MeV and 12 MeV electron beams was described in terms of optical density differences in a comparative film study. RESULTS: Variations of beam flatness were documented of up to 11.8% at 5 mm depth for 6 MeV, and 8.1% for 12 MeV electrons. The depth at which this effect may be considered insignificant (mean optical density differences < 2%) is approximately 10 mm for both beam energies. CONCLUSIONS: For clinical situations where the target volume is superficial, some consideration should be given to beam inhomogeneity caused by the mask.

A prototype amorphous selenium imaging plate system for digital radiography.

This paper summarizes the results of research and development of a prototype amorphous selenium digital imaging system. The first phase of this project consists of the preliminary design and fabrication of the system. In this system the conventional film-screen photon receptor is replaced by a charged amorphous selenium imaging plate. After exposure, the latent electrostatic image on the selenium surface is scanned with multiple microelectrometer probes forming a 1024 X 1024 X 12 bit digital image. The second phase investigates the system's physical imaging characteristics and clinical feasibility. X-ray exposure latitude comparable to 200-speed calcium-tungstate film-screen system are shown for three typical diagnostic kVp settings with total filtration of 3 mm aluminum and 9 cm Lucite. Using the modulation transfer function (MTF), resolving power of approximately 1.0 line pair per millimeter and detective quantum efficiency values of approximately 5% have been measured. The clinical evaluation consists of preliminary images of a 16-kg female dog and a 4.5-kg rabbit and comparisons to film-screen images are offered.

Monte Carlo simulation of diagnostic x-ray scatter.

A Monte Carlo method was developed and implemented to simulate x-ray photon transport. Simulations consisted of a pencil beam of monoenergetic photons with energies from 50 to 110 keV incident on water and aluminum slabs. The dependence of scatter fraction and multiple scattering on x-ray energy, scatterer thickness, and material is reported in both number and energy fluence. The average energy of scattered photons reaching the detector plane is also reported. Comparisons are made to previous x-ray scatter computations.

Biomedical image processing for age measurements of intact teeth.

With increasing age the roots of teeth undergo sclerosis. The degree of dental root sclerosis can be demonstrated visually if light is transmitted through the specimen. However, this resultant image is only a two-dimensional (2-D) visualization which misrepresents what in truth is a three-dimensional (3-D) characteristic. We have described an image acquisition and computer processing system for imaging intact teeth, with special reference to the root transparency, which tends to progress from the root apex towards the neck of the teeth as a function of age. The components of our system involve: an energy source with light transmission through table-mounted dental specimens; an image processor with a digitizer; a step motor with a holder to rotate the tooth specimen; and a software package to computerize and reconstruct the sectional digital images. After rotating the position of the specimen, while at each rotation obtaining a 2-D image of the sample, we then can reconstruct the true 3-D cross-sectional or longitudinal morphology or both from these 2-D images. With this new approach, the reconstructed dental images represent segments from different angles of the tooth specimen. Picture element values in each image, quantitatively indicate the optical density, expressing the age dependent pattern of the 3-D anatomy in toto.

An augmented audit program for assuring radiation safety during radiographic examination operations.

Auditing a gamma radiography program is required as part of the authorizing license. Checklists and cursory reviews are the typical approach to addressing program requirements. A more proactive approach is recommended. The audit program described was prepared for a specific set of operating conditions but can be applied to any given program. Improvements in the effectiveness of the radiography safety program can be made with additional examination and emphasis on direct observation of licensed activities.

Computer-assisted management of liquid radioactive waste at the University of California, San Diego.

Commercially available software has been obtained and internal software applications have been developed to implement a tracking system for liquid radioactive wastes. This system utilizes a number of data bases that maintain sampling, waste pickup and disposition information based on various parameters. Computerization has allowed access to summary information and inventory totals that are necessary for radioactive materials license compliance. Comparative reports, which are used to show trends and track historical information, can also be generated.

Effect of bedside shielding on air-kerma rates around gynecologic intracavitary brachytherapy patients containing 226Ra or 137Cs.

An anthropomorphic phantom was implanted with 226Ra or 137Cs gynecologic intracavitary brachytherapy sources. Air-kerma rate measurements were taken at 10-cm increments along a horizontal plane from the side of the bed at 50 cm, 87 cm, and 136 cm heights above the floor. Five portable lead shields were placed at the head, at the foot and along one side of the bed and readings were taken again at the corresponding heights above, below and behind the shields. The readings were normalized to 100-mg Ra equivalence, and air-kerma rate curves were drawn allowing for the comparison of 226Ra and 137Cs with and without lead shields. The data demonstrated that the air-kerma rates for 137Cs were reduced more than those for 226Ra with the use of the portable lead shields. There was four times the transmission with 226Ra than with 137Cs. The optimal placement was with the lateral bedside shields proximal to the head and foot closest to the bed, with the middle shield overlapping in back. The shields at the head and foot should extend out and overlap the bedside shields. The level of the sources should be positioned near the bottom of the shields. This information will provide the medical health physicist with an estimate of air-kerma rates for both 226Ra and 137Cs with and without shielding for evaluating personnel exposures as well as the effectiveness of current shielding in relation to radiation protection requirements in adjacent rooms or hallways.

Digital subtraction angiography: a comparison of 512(2) and 1024(2) imaging.

A commercial DSA unit was modified by the manufacturer to permit 1024 X 1024 8-bit imaging. System upgrade includes a high-resolution 1049-line TV camera that operates with variable aperture to minimize x-ray exposure during 1024(2) imaging. To compare the change in resolution and radiation exposure between 512(2) and 1024(2) imaging with this system, a two-phase phantom study was performed using a high-contrast converging lead line phantom and a specially designed high-resolution low-contrast Lucite phantom. The two-phase phantom study tested general system resolution performance and resolution under simulated and actual clinical conditions for each field size (15, 25, and 36 cm). The 512(2) imaging was performed with the aperture reduced to the 512 setting; 1024(2) imaging was performed with the aperture at the 512 and 1024 values. The 1024(2) imaging resulted in only modest improvement in resolution compared to 512(2). While Nyquist limits were approached with 512(2) imaging, this was not the case with 1024(2) imaging. This suggests other factors such as system noise are playing a significant role in 1024(2) image degradation.