The imaging characteristics of an arc-shaped xenon gas ionization chamber for the purpose of megavoltage CT imaging were investigated. The detector consists of several hundred 320 m thick gas cavities separated by thin tungsten plates of the same thickness. Dose response, efficiency and resolution parameters were calculated using Monte Carlo simulations. The calculations were compared to measurements taken in a 4 MV photon beam, assuming that the measured signal in the chambers corresponds to the therein absorbed dose. The measured response profiles for narrow and broad incident photon beams could be well reproduced with the Monte Carlo calculations. They show, that the quantum efficiency is 29.2% and the detective quantum efficiency at zero frequency DQE͑0͒ is 20.4% for the detector arc placed in focus with the photon source. For a detector placed out of focus, these numbers even increase. The efficiency of this kind of radiation detector for megavoltage radiation therefore surpasses the reported efficiency of existing detector technologies. The resolution of the detector is quantified with calculated and measured line spread functions. The corresponding modulation transfer functions were determined for different thicknesses of the tungsten plates. They show that the resolution is only slightly dependent on the plate thickness but is predominantly determined by the cell size of the detector. The optimal plate thickness is determined by a tradeoff between quantum efficiency, total signal generation and resolution. Thicker plates are more efficient but the total signal and the resolution decrease with plate thickness. In conclusion, a gas ionization chamber of the described type is a highly efficient megavoltage radiation detector, allowing to obtain CT images with very little dose for a sufficient image quality for anatomy verification. This kind of detector might serve as a model for a future generation of highly efficient radiation detectors.

This study examined the effects of a systematic, phonics-based reading intervention on the oral reading fluency and accuracy of adolescents with emotional and behavioral disorders (EBD) receiving educational services in a juvenile correctional facility. A multiple-baseline design across four participants was used to calculate the effect of daily, one-on-one, 30-minute reading instructional sessions provided over nine weeks. Oral reading fluency increased markedly and error rates decreased for each participant in the intervention phase. Participants also experienced improvements in reading as measured pre- and postintervention on a standardized reading assessment. Findings underscore the promise of systematic reading intervention for improving academic outcomes for adolescents with EBD confined to juvenile correctional facilities who are also struggling readers.

The UW tomotherapy workbench utilizes a convolution/superposition based dose calculation and optimization program. It specifies the energy fluence that must be delivered from each leaf for each phantom projection angle. This requires that the spectrum of the radiation emitted from the one-dimensional MLC (multileaf collimator) attached to the linear accelerator be determined. The steps involved in that process are described. The spectrum along the central axis of the slit beam was determined, as well as the softening with off-axis position. Moreover, the magnitude of the energy-fluence output had to be quantified on a per MU (monitor unit) basis. This was done for a single leaf along the central axis of the beam. Factors, which modify that energy-fluence output, were investigated. The output increases with off-axis position due to the horns of the beam. The output for a leaf of interest will also increase if additional leaves are open due to the absence of the tongue-and-groove effect and penumbra blurring. The energy-fluence increase per leaf increase by 4.9% if an adjacent leaf is open. No other factors related to the state of additional leaves were found to significantly increase the energy-fluence output for an individual leaf.

Multileaf collimators (MLCs) have advanced past their original design purpose as a replacement for field shaping cerrobend blocks. Typically, MLCs incorporate an interlocking tongue-and-groove design between adjacent leaves to minimize leakage between leaves. They are beginning to be used to provide intensity modulation for conformal three-dimensional radiation therapy. It is possible that a critical target volume may receive an underdose due to the region of overlap if adjacent leaves are allowed to alternate between the open and closed positions, as they might if intensity modulation is employed. This work demonstrates the magnitude of that effect for a commercially available one-dimensional temporally modulated MLC. The magnitude of the transmission between leaves as a function of leaf separation was also studied, as well as the transmission as a function of leaf rotation away from the source. The results of this work were used for the design of a tomotherapy MLC. The radiation leakage considerations for a tomotherapy MLC are discussed.

Abstract-In this work, Monte Carlo methods are used for the design of highly efficient detector structures for megavoltage X-ray imaging. The detector structures consist of a converter material and an active medium ("binary" detector systems). The novel approach is to impose a spatial structure on the converter material that intersperses the active medium and defines the size of a detector cell. The dimensions of these structures have to be optimized with respect to efficiency and spatial resolution. The results show that the quantum efficiency and detective quantum efficiency at zero frequency of such structures surpasse the efficiency of conventional detector structures using the converter material as a buildup layer and depend on the dimensions of the converter structure and the active medium as well as the materials itself. In general, larger converter structures result in a higher efficiency. The detector signal is proportional to the size of the active medium in the cell. However, the size of the structures are limited by the specifications for the desired spatial resolution.