Biomedical subjects

R Rajapakshe

Publications and source records attributed to R Rajapakshe.

4 recordsLinked to original sources

PubMed1994

Optimization of metal/phosphor screens for on-line portal imaging.

Studies were conducted to determine the optimal metal/phosphor screen for on-line video verification of radiation treatment portals. Screens were evaluated for luminance and spatial resolution as a function of composition and thickness at 6- and 23-MV x-ray energies. A new video technique was used to determine modulation transfer functions. Gd2O2S was found to be the most efficient (brightest) phosphor for this application. Luminance was found to vary linearly with phosphor thickness up to a thickness of 500 mg/cm2. Metal plates made of iron, brass, copper, lead, and sintered tungsten of various thicknesses were also tested for luminance and resolution with Gd2O2S phosphor. Brightness peaked at about 2-mm thickness for most metals. Significant contributions to the brightness were found to come from x rays interacting with the phosphor itself.

Evaluation Studies as Topic↗

PubMed1994

Pseudocorrelation: a fast, robust, absolute, grey-level image alignment algorithm.

A new image alignment algorithm--pseudocorrelation--has been developed based on the application of Monte Carlo techniques to the calculation of a cross-correlation integral for grey-scale images. It has many advantages over cross-correlation: it is at least a factor of 10 faster than fast-Fourier-transform-based cross-correlation, and requires 8 times less memory. Its high speed allows for the search space of geometric transformations between images to include magnification and rotation as well as translations without the search time becoming too long. It allows noise to be taken into account, making calculation of a robust, absolute probability of good alignment possible. It is relatively insensitive to differences in quality between images. This article describes the pseudocorrelation algorithm in detail and presents the results of tests of the effects of contrast enhancement, resolution differences, and noise on the algorithm's performance. These tests show that the algorithm is well suited to the task of automated alignment of very low contrast images from video electronic portal imaging devices.

Algorithms↗

PubMed1994

Noise analysis in real-time portal imaging. I. Quantization noise.

The limit at which quantization noise becomes dominant in video-based real-time portal imaging has been studied. Quantization noise due to truncation in integer frame averaging is shown to be dominant over the input analog-to-digital converter (A/D) quantization noise, unless image addition is used in video-based real-time portal imaging systems. Portal images acquired with the Newvicon camera by averaging more than 64 frames are found to be dominated by the quantization noise due to truncation. It has shown that the signal-to-noise ratio (SNR) is limited to 886.8 when using an 8-bit A/D with digital frame averaging, but higher values can be achieved with digital frame addition. It is also shown that digital frame addition together with 16-bit processing can achieve higher contrast resolution than digital frame averaging and 8-bit processing.

Gadolinium↗

PubMedDate not supplied

Monte Carlo optimization of metal/phosphor screens at megavoltage energies.

The physics of imaging with metal/phosphor (Gd2O2S:Tb on brass) screens at megavoltage energies has been investigated using Monte Carlo simulation. It has been found that pair production is a significant contributor to energy deposition for Bremsstrahlung beams with energies greater than 6 MV. The effects of different thicknesses of phosphor and metal have been studied, and it is shown that the metal plays a significant role in establishing electronic equilibrium in the phosphor. The transport of optical photons through the phosphor has been modeled, and was found that only 10% to 20% of the light created in the phosphor escapes from the surface, with much of the loss being due to total internal reflection at the surface. Calculated results have been compared with experimental measurements of screen brightness for different phosphor and metal thicknesses. The SNR of a video electronic portal imaging device (VEPID) has been calculated as a function of x-ray and optical photon detection efficiency. The non-Poisson distribution of energy deposition in the phosphor is an important contributor to the SNR. The results of this paper should serve as a useful guide to the engineering design of future electronic portal imaging systems.

Biophysical Phenomena↗