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Biomedical subjects

B G Fallone

Publications and source records attributed to B G Fallone.

At least 55 records · Page 3Linked to original sources
PubMed1998

Characteristics of metal-plate/film detectors at therapy energies. I. Modulation transfer function.

Measurements of modulation transfer function (MTF) for front and back metal-plate/film portal detectors are reported for the Cobalt-60 and 10 MV spectra. The detectors consist of a double-emulsion portal film secured between plates of Al, Cu, brass, or Pb with thicknesses varying from 0 to 4.81 mm. Secondary electrons produced within the front plate generate the main signal, but the MTF decreases with an increase in front plate thickness greater than the maximum range of electrons Rmax because of photon scatter in the front plate. Because the decrease of MTF with backplate thickness ceases for backplate thickness greater than Rmax, the MTF is influenced more by the backscatter electrons than the backscatter photons.

Biophysical Phenomena↗
PubMed1998

Characteristics of metal-plate/film detectors at therapy energies. II. Detective quantum efficiency.

Noise power spectrum (NPS) and detective quantum efficiency (DQE) for metal-plate/film portal detectors are reported for the Co-60 and Linac 10 MV spectra. The detectors consist of a double-emulsion portal film secured between plates of aluminum, copper, brass, or lead. The NPS was found to be independent of the detector sensitivity demonstrating that the film grain noise dominates over the quantum noise. Although the detector signal increases with density of the backplates, the resultant electron-backscatter increases detector blur, thus decreasing the DQE. The lowest DQEs are therefore produced with the lead backplates, irrespective of the front plates. For a given backplate and for front-plate thicknesses less than the maximum electron range, the DQE increases with density of the front plate.

Biophysical Phenomena↗
PubMed1997

An active set algorithm for treatment planning optimization.

An active set algorithm for optimization of radiation therapy dose planning by intensity modulated beams has been developed. The algorithm employs a conjugate-gradient routine for subspace minimization in order to achieve a higher rate of convergence than the widely used constrained steepest-descent method at the expense of a negligible amount of overhead calculations. The performance of the new algorithm has been compared to that of the constrained steepest-descent method for various treatment geometries and two different objectives. The active set algorithm is found to be superior to the constrained steepest descent, both in terms of its convergence properties and the residual value of the cost functions at termination. Its use can significantly accelerate the design of conformal plans with intensity modulated beams by decreasing the number of time-consuming dose calculations.

Algorithms↗
PubMed1996

A grey-level image alignment algorithm for registration of portal images and digitally reconstructed radiographs.

An algorithm for automatic registration of pairs of portal images based on image correlation is presented. It uses a fast-Fourier-transform-based cross-correlation operator to find the optimal registration, accounting for both in-plane translations and rotations. Different cross-correlation operators have been tested: the Pearson linear correlation coefficient has been implemented by fast Fourier transform and its performance has been compared to that of the more conventional normalized cross-correlation. A sequential approach has been applied to speed up the registration considerably without degrading the performance of the algorithm. The algorithm has also been applied to the automatic registration of portal images to digitally reconstructed radiographs (DRRs), which have been modified to resemble megavoltage images. The results are indicative of the feasibility of this approach to the inspection of patient setup in radiation therapy.

Algorithms↗
PubMed1995

On the perception of the left thoracic paraspinal line.

RATIONALE AND OBJECTIVES: We investigated whether the left thoracic paraspinal line represents an actual change in optical density corresponding to the posterior pleural reflections or whether it represents a Mach band effect. METHODS: Using photodensitometry, we obtained averaged light intensity tracings across the left thoracic paraspinal line in 14 normal digitized anteroposterior (AP) thoracic spine radiographs. We also performed a mathematical simulation of relative brightness perception by using the contrast sensitivity function of the human visual system (assumed to represent the modulation transfer function) to filter the light intensity profiles by fast Fourier transformation techniques. RESULTS: Results from 14 normal AP thoracic spine radiographs showed no significant increase in light transmitted over the perceived location of the paraspinal line. A simple linear systems analysis of the behavior of the visual system predicted a Mach band effect corresponding in location to the observed paraspinal line. CONCLUSION: The spatial pattern of optical densities in the region of the left thoracic paraspinal line rather than the tissue composition is the crucial factor responsible for the perception of the paraspinal line.

Absorptiometry, Photon↗
PubMed1995

Effect of radiation rate on electret ionization chambers.

The dependence of radiation-induced conductivity (RIC) on radiation rate in 254 microns thick Teflon polyfluoroethylene propylene (FEP) film used in a electret ionization chamber (EIC) is discussed and measured with regard to radiation dosimetry. Ideally, air-kerma, a quantity linearly related to dose, can be determined through measurement of the surface-charge density on the electret surface before and after irradiation and observing the difference. However, RIC permits trapped surface charge to migrate through the polymer and recombine at the collecting electrode which results in a reduced EIC charging efficiency and an overestimation of air-kerma in radiation dosimetry. The response of the EIC to a given air-kerma is recorded as a function of air-kerma rate. It is predicted and observed that surface-charge loss does depend on air-kerma rate. With proper adjustment of physical parameters however, a suitably wide range of air-kerma rates can be found over which response is flat.

Cobalt Radioisotopes↗
PubMed1995

A mathematical model of radiation field edge localization.

A mathematical model of the average edge slope across the radiation field border in a portal image was investigated in order to improve the precision of edge localization in a previously reported algorithm for automatic extraction of the radiation field from double- and single-exposure portals. The model involves a global rather than a local approach to edge localization, and employs a hyperbolic function with four parameters to characterize the behavior of the radiation field penumbra. The location of the radiation field edge is determined from one of these parameters. This model was tested on a group of portal images acquired with different conditions in our clinic. Evaluation results of this model and improvements in the performance of our portal image segmentation algorithm will be presented.

Algorithms↗
PubMed1994

A robust morphological algorithm for automatic radiation field extraction and correlation of portal images.

Accurate extraction of the radiation field boundary from portal images is required in selective histogram equalization techniques for contrast enhancement of portal images, and and is required in portal-simulator image correlation. Local edge detectors are so noise sensitive that delineation of the field from anatomy edges and noise is very difficult. Optimization in edge localization and noise suppression can be achieved with optimal edge detectors, but the required calculation is very time consuming. A portal-image segmentation algorithm based on morphological edge detection and morphological segmentation techniques is presented. Relying on two predefined criteria that are nonsensitive to variation of portal-image types, the algorithm can automatically search the optimal threshold value, which is sensitive to the variation of the type and quality of portal images. With two stages of the searching procedure, the algorithm can accommodate a large variation of single and double exposure portal images obtained from different therapy machines. Results of our morphological edge detector are also compared to that of an optimal edge detector. Portal-simulator image correlation using the inertia moments of the radiation field mask is investigated, and compared to correlation obtained using inertia moments of the radiation field contour. It is shown that the mask method is less sensitive to small variations and distortions in the field shape, resulting in a more accurate correlation. This would substantially simplify the task of treatment verifications.

Algorithms↗
PubMed1993

The design of an adjustable half block for nonasymmetric linear accelerators.

Half-block techniques in radiotherapy may be used to junction two abutting radiation fields. Removable half blocks for linear accelerators not equipped with asymmetrical jaws can accomplish this technique providing the block edge is properly secured with respect to the beam central axis to achieve a smooth beam junction. Successive block edge cuts with a milling machine are, however, required to achieve an acceptable junction of the fields. The design of a half-block device that allows secure adjustment of the block edge on the linac thus alleviating the need for successive milling procedures is presented.

Equipment Design↗
PubMed1992

Feasibility study of an electret dosimetry technique.

Characteristics of a radiation charged electret dosimeter are described. The dosimeter is based on a parallel-plate ionization chamber with the exception that the collecting electrode is covered by a thin polymer, Teflon or Mylar. During the charging of the dosimeter, ions produced in the sensitive volume by an external radiation source drift in the externally applied electric field and become trapped on the polymer surface forming an electret. Once the external supply is removed, a field across the sensitive volume is produced by the electret charge, such that during any subsequent irradiation, ions opposite in sign to those on the electret surface are attracted to the electret and deplete the charge layer in an amount proportional to the air kerma. The remaining charge on the electret is read by an electrometer, through further irradiation. This technique allows the dosimeter to be simultaneously charged and calibrated, used in the field, simultaneously discharged and read, and reused again in situ without dismantling the dosimeter. Various parameters, however, were considered by direct discharge, rather than by initial exposure and subsequent measurement. Calibration, energy dependence, air kerma range, and reproducibility are discussed, and guard-ring effects on the linearity of the chamber are presented. Measurements of natural charge decay on the electrets are introduced.

Feasibility Studies↗
PubMed1988

Elastic characteristics of the self-expanding metallic stents.

We measured elastic properties of the self-expanding metallic (Gianturco) stent using Hooke's law to characterize the stent with respect to its caliber, length, and radius by a coefficient of stiffness. Although this coefficient was independent of the radius of the stent, we found that it was directly dependent on caliber. For a stent of a particular caliber and length, the fractional change in radius determined the force exerted by the stent. For small radial displacements of the stent, the force required to compress it was directly proportional to the radial displacements; for large displacements, the force depended on a power series of the fractional radial displacement. A hyperbolic function was empirically introduced to approximate this type of relationship between force and radial displacement. We calculated tension and pressure exerted by the stent and suggest the use of our findings with normal vessels.

Angioplasty, Balloon↗
PubMed1984

Image subtraction by solid state electroradiography.

An analog radiographic image subtraction technique is presented. A selenium photoconductor is used as the radiation sensitive medium and image subtraction is achieved by electrostatically adding latent images of opposite polarities to get a latent image of the region of interest. In principle, the technique is simple and a comparison with commercial digital techniques shows a similar image quality, suggesting that the electrostatic subtraction technique could be developed into a clinical tool as an inexpensive alternative to digital radiography.

Subtraction Technique↗
PubMedDate not supplied

Noninvasive thermometry with a clinical x-ray CT scanner.

A CT scanner yields CT numbers which are proportional to the fractional difference in effective local electron density of the subject material with respect to that of calibration material. A homogeneous water-equivalent material is used as a calibration phantom under isothermal conditions. Any temperature variation (spatial or temporal) in the subject material subsequently scanned, will generate a CT-number shift in the CT image because of density changes due to thermal expansion. The potential use of the thermally generated CT-number shift in noninvasive thermometry during cancer hyperthermia was studied in vitro in samples of water and muscle tissue. The reproducibility of the area-averaged CT-number measurement on our EMI-7070 scanner was found to depend strongly on the time interval between successive counterclockwise scans and on the size of the region of interest used for area averaging. A linear relationship was found between the CT number and water density in the water temperature range from 10 to 55 degrees C. In the hyperthermia temperature range (36-50 degrees C), the relationship between the CT number and temperature itself is linear, with a CT-number thermal shift of about 0.4 and 0.45 HU/degrees C for water and muscle tissue, respectively. The achievable temperature discrimination is a fraction of degree C at spatial resolutions of the order of a centimeter.

Hot Temperature↗
PubMedDate not supplied

Saturation curves of parallel-plate ionization chambers.

A new analytical expression is presented to describe the full saturation curve of parallel-plate ionization chambers. In contrast to the presently known expressions, which hold only for the near saturation region, this empirically determined expression is in excellent agreement with measurements in the whole collection efficiency range from 0 to 1 for x-ray sources with effective energies from 20 to 150 keV and cobalt-60 gamma rays. The dependence of the ion collection efficiency and the extrapolated electric field, which is a parameter in the new saturation curve expression, on electric field, dose, dose rate, beam quality, and chamber volume, is discussed. The effect of photoemission from the chamber polarizing electrode for low-energy x-ray beams on the saturation current is demonstrated. A universal ionization chamber constant is derived experimentally. It is shown that all parameters of the saturation curve equation and thus the saturation curve itself, can be calculated from one single measurement of ionization current at a given electric field and air gap thickness.

Calibration↗
PubMedDate not supplied

Radiographic image subtraction in gas ionography.

The feasibility of radiographic image subtraction based on electrostatic imaging techniques is demonstrated. Ionographic latent images are essentially foil electrets with surface charge densities modulated by x-ray transmission through the object. Latent image charging at one polarity corresponding to the production of the primary image, and latent image discharging with the opposite polarity, are used to create the final latent image representing the region of interest. The technique is simple and could be in its present form applied to industrial radiography. Using high-pressure xenon ionographic chambers, the technique could be developed into a clinical tool as an inexpensive alternative to conventional digital radiography.

Radiography↗
PubMedDate not supplied

Charging characteristics of ionographic latent images.

The charging characteristics of ionographic latent images are discussed in terms of the saturation characteristics of ionographic chambers. It is shown that latent images are essentially foil electrets with surface charge densities modulated by the x-ray transmission through the object. Empirical methods for the calculation of saturation current densities and extrapolated electric fields are given. Both the saturation current density and the extrapolated field consist of two components: one, which is linear with the air gap thickness, is produced by primary photon interactions in the chamber-sensitive volume; the other, exhibiting an exponential saturation, is attributed to photoelectrons backscattered from the polarizing electrode into the chamber-sensitive volume. The minimum applied electric field needed for an optimized charge collection in the ionographic chamber is presented in terms of both the characteristic polarization time and the electret relaxation time. The maximum possible surface charge density in the latent image is calculated, first exhibiting direct proportionality with the polarizing voltage V0, and then following a V0 (1/2) dependence for voltages larger than the breakpoint voltage. The optimum exposure for latent image production is calculated. Exposures below this optimum give insufficient charge densities for subsequent development, while exposures above it degrade the image and eventually result in a uniform foil electret charged to its maximum theoretical value.

Electric Conductivity↗