Biomedical subjects
S Shalev
Publications and source records attributed to S Shalev.
Video techniques for on-line portal imaging.
The application of on-line portal imaging techniques to the verification of treatment precision is reviewed. The design parameters for a video portal imaging system are described, and the optimization of image quality is discussed with particular emphasis on photon noise. On-line images are presented for a head phantom imaged on a 4 MV linac, and compared with a conventional portal film. The relative advantages of an on-line system are compared with conventional portal film analysis.
Development of a tissue-equivalent phantom for diaphanography.
A phantom is proposed for quality control and calibration of instruments used for transillumination of the breast for early detection of cancer. A container is filled with a material having optical properties very similar to those of breast tissue, and internal objects are viewed by transmitted light. The phantom can be used to optimize operating conditions for the visualization of small or deep-seated lesions.
A study on the efficacy of digital enhancement of on-line portal images.
A novel method for evaluation of observer performance with portal images has been developed, in which the observer is required to identify and localize predefined anatomical landmarks in digital portal images. The method was employed to compare the spatial accuracy and decision time for landmark localization in unenhanced on-line portal images and images enhanced digitally by selective adaptive histogram modification. The results indicate that anatomical landmarks were more readily identified in the enhanced images, leading to significantly higher accuracy in landmark localization.
The enhancement of radiotherapy verification images by an automated edge detection technique.
Adaptive histogram equalization techniques are known to be effective for the enhancement of contrast in portal images acquired during radiotherapy treatments. A significant drawback is the loss of definition on the edges of the treatment field. Analysis of this problem shows that it can be remedied by separating the treatment field from the background prior to the enhancement, and using only the pixels within the field boundary in the enhancement procedure. An edge extraction algorithm has been developed for delineating the treatment field in portal images, and consists of four modules that are applied to the original portal image in sequence. In the first step, edges are enhanced with a derivative of Gaussian operator that assures high response to the field edges relative to anatomical or other edges in the image. Pixels for which the response of the edge operator was the strongest are subsequently connected by an edge following algorithm to produce a raw contour of the field. In the last two steps the contour is refined by converting it into straight line segments and appending to the contour any parts of the field edge that might have been missed out during the initial edge following. The final contour encloses exclusively those pixels that belong to the treatment field, and the adaptive histogram equalization is applied selectively to this region. The combination of edge detection and selective enhancement was shown to produce images of superior contrast on the patient's anatomical features as well as accurate definition of treatment field edges.
A rho-theta technique for treatment verification in radiotherapy and its clinical applications.
A new technique based on a rho-theta coordinate system for determining differences in patient position between portal and simulator images is presented. Unlike the conventional point matching method, which requires the fiducial points to be labeled in pairs before the registration, the rho-theta technique avoids this manual procedure. It accomplishes the treatment verification in two major steps; image alignment and field displacement analysis. For the same number of fiducial points in the simulator and portal images, it first finds the corresponding paired points if the points are not distributed symmetrically about their centroid. This is followed by alignment of these paired points using the least squares matching method to find the optimal two-dimensional rigid body transformation parameters (shift, rotation, and scaling factor). The transformation parameters are then used to transform the portal field edge into the simulator image, so that the portal field can be compared with the prescribed field on the simulator image. A number of parameters were explored to describe the field displacement errors, including treatment field size, under/over irradiated size, the shift in center of gravity of the field, the field edge shift, and rotation of the field. The rho-theta technique as implemented is both fast and accurate. Experiments on the registration of radiological phantom portal images acquired with an on-line portal imaging system mounted on a linear accelerator indicate an accuracy on the order of 1 mm in detecting the shift of the field's center of gravity and approximately 1 degree in detecting the field rotation. The results of a clinical trial are also presented.(ABSTRACT TRUNCATED AT 250 WORDS)
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.
Verification of radiotherapy treatments: computerized analysis of the size and shape of radiation fields.
An automated technique has been developed for the verification of treatment field size and shape in external beam radiation therapy. Portal images from film or digital on-line imaging system are analyzed, and basic parameters are derived to describe the field size and shape from the contour points on the field boundary. The initial set of parameters included length of the perimeter, area, aspect ratio, and orientation angle. The parameters found for the actual field in the portal image are compared against those calculated for the prescribed field and any discrepancies indicated to the operator. The accuracy of the field parameterization scheme has been tested on a number of on-line portal images with varying fields. The relative error did not exceed a few percent in perimeter and area or 2 degrees in the angle, which should be sufficiently low for the detection of major errors in field shaping.
Grooved phosphor screens for on-line portal imaging.
Video-based systems for on-line portal imaging utilize a metal plate coated with Gd2O2S phosphor at a typical thickness of 500 mg/cm2. A new screen design is proposed wherein the conventional flat phosphor coating is replaced by a much thicker phosphor layer (1000-2000 mg/cm2) penetrated by either lineal grooves or pyramidal holes comparable to the system pixel size. By increasing the surface area of the phosphor, the grooves or holes allow light from deep layers of the phosphor to escape by a process of internal reflection. In addition, the escaping light is strongly forward peaked, improving optical coupling to the video camera. The processes by which grooved screens intensify light output have been modeled in a simple computer program that gives approximate agreement with experiment. Prototype screens have been constructed that provide several times the forward light output of flat screens, and that improve DQE(f) in light photon limited systems for spatial frequencies below 0.4 mm-1.