Does 5S RNA function by a switch between two secondary structures?

A significant amount of radiation escapes the rear of an image receptor in conventional dental radiographic procedures despite the placement of a lead-containing backing at the rear of the film pack. The purpose of this investigation was to place additional layers of lead on the film pack in an attempt to reduce transmitted radiation to tissues behind the film. Reduction of dose to tissues behind the film causes a reduction in scatter radiation to critical organs such as the thyroid gland and salivary glands. A thermoluminescent lithium fluoride dosimetry system (TLD) was used and calibrated in both the beam of interest and tissue of interest. A tissue-equivalent anthropomorphic phantom was used for dose determination. Successive layers of lead foils were added to the image receptor, and dose was determined for each additional lead foil in the tissues behind the image receptor. Two sets of exposure conditions were used, namely: 70 kVp, 10 mA, and 0.6 s, and 90 kVp, 10 mA, and 0.6 s. The beam-film-patient orientation used was for a first molar bitewing radiographic projection. Four lead layers (three additional foils equalling 3.92 x 10(-3) mm of lead) on the conventional film package resulted in a significant dose reduction. Four layers of lead resulted in a dose less than one-third that of the conventionally shielded package.

Models, Structural↗

PubMed1992

The use of parallel plate ion chambers to determine surface dose of a 6 MV photon beam.

The shape of the measured photon build-up curve and the surface dose are strongly dependent on the chamber geometry and side wall density. This study examines the response at the phantom surface to a 6 MV beam for the NACP and MARKUS chambers and also for the recently available commercialized ATTIX chamber; and checks the consistency of a published correction term applied to these chambers by comparison of the corrected data sets. The MARKUS and ATTIX chamber results of surface dose versus field size agree to within less than 0.5% over the field size range, whereas the NACP results show an average discrepancy of 22.1% (absolute percentage points), although the slope of the dose versus field size plot is very similar for all three chambers. It is confirmed then that the NACP electron chamber is not optimal for photon dosimetry at an air-tissue interface, and that the chamber of choice from fabrication material and geometrical considerations is the ATTIX solid water chamber.

Models, Structural↗

PubMed1992

Measurements of computed tomography radiation scatter.

This paper describes the measurement of scattered radiation from a computed tomography (CT) scanner in a clinical situation and compares the results with those obtained from a CT performance phantom and with data obtained from CT manufacturers. The results are presented as iso-dose contours. There are significant differences between the data obtained and that supplied by manufacturers, both in the shape of the iso-dose contours and in the nominal values. The observed scatter in a clinical situation (for an abdominal scan) varied between 3% and 430% of the manufacturers' stated values, with a marked reduction in scatter noted at the head and feet of the patient. These differences appear to be due to the fact that manufacturers use CT phantoms to obtain scatter data and these phantoms do not provide the same scatter absorption geometry as patients. CT scatter was observed to increase as scan field size and slice thickness increased, whilst there was little change in scatter with changes in gantry tilt and table slew. Using the iso-dose contours, the orientation of the CT scanner can be optimised with regard to the location and shielding requirements of doors and windows. Additionally, the positioning of staff who must remain in the room during scanning can be optimised to minimise their exposure. The data presented here allows for realistic radiation protection assessments to be made.

Models, Structural↗