摘要:

A formalism is derived that relates the absorbed dose to a medium from photon and electron beams to the photon calibration factor of an ionization chamber. The formalism is applicable to the photon and electron beam energies that are currently of interest in radiation therapy. It is developed in terms of a cavity‐gas calibration factor, a quantity characteristic of the chamber and independent of the energy of the calibration beam assuming the energy expended per ion pair is energy independent. The cavity−gas calibration factor can be obtained from a chamber calibration performed in terms of exposure,absorbed dose to water,or air kerma. The perturbation corrections due to replacement of the surrounding medium by the chamber wall and cavity are identified as ratios of the photon energy fluence,or the electron fluence, at the position of the chamber center. The unmanageable complexities of a theory that covers an ionization chamber made of several materials are avoided by limiting the development to a chamber made of a single material with the expectation that the inhomogeneities of real chambers can be treated as perturbations. Attention is called to certain theoretical aspects of this dosimetry development that do not appear to have been previously recognized.

ISSN:0269-3127    

DOI:10.1118/1.594901

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study measured observers’ ability to detect and locate focal lesions on simulated CT images. The difficulty of the detection task was manipulated by changing the difference in attenuation between the lesion and its background (contrast), the random variation in the CT values (noise) or the lesion's size. The human observers’ performance was compared to that of matched filter detector, modified to include the effects introduced by the display window and the uncertainty about the lesion's location on the image. Changes in lesion contrast, lesion size and noise produced large variations in both the lesion signal–to–noise ratio (a measure of the matched filter detector's performance) and estimated measures of the observer's detection and localization ability. Changes in observers’ performance were closely related to changes in lesion signal–to–noise ratio. Generally, changes in lesion contrast, lesion size or noise that produced similar values of lesion signal–to–noise ratios had equivalent effects upon the observers’ performance.

ISSN:0269-3127    

DOI:10.1118/1.594903

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The detail signal–to–noise ratio model of radiographic imaging is quantitatively analyzed in terms of its accuracy in describing observer threshold perceptibility of radiographic detail. The model is found to adequately describe the effects of magnification, scatter radiation, and system resolution on observer threshold perceptibility. However, it is shown that the model does not apply in screen/film radiography for very low contrasts and high scatter conditions due to insufficient optical density contrast. The dose‐to‐information conversion efficiency of a radiographic imaging system is defined and the effects of magnification, scatter, resolution, image processing, detector efficiency, grids, patient table support, field size, and geometry on the dose efficiency of the imaging system are investigated.

ISSN:0269-3127    

DOI:10.1118/1.594902

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurement results of the relative contribution of primary and secondary radiation are presented. Measurements were done with both monoenergetic and broad spectrum x‐ray beams. A variety of phantoms were used: graphite, water, lucite, a skull phantom, and aluminum. We found that the primary radiation fraction is nearly independent of photon energy and tube potential. Calculations using a simple single Compton scattering model agree with the experiment results.

ISSN:0269-3127    

DOI:10.1118/1.594904

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the U.S. neutron radiotherapy trial centers, absorbed dose is routinely measured using commercially available A‐150 tissue equivalent (TE) plastic ionization chambers. The collecting volumes of these chambers are filled with either methane‐based tissue equivalent gas or air. Absorbed dose in A‐150 plastic, determined with these ionization chambers, was compared to that measured by an A‐150 plastic calorimeter in an A‐150 plastic phantom. These comparisons have yielded the following information: (1) Agreement of the total absorbed dose measured using the ionization chambers was within 2.5% of the calorimeter at all the centers visited to date. (2) For all the neutron fields measured, the product of the stopping power ratio(sw,g)N′between the A‐150 plastic chamber wall and TE gas, and the average energy expended in the gas per ion pair formed,W̄N/e, was computed assuming Bragg–Gray theory and found to be 31.0±0.7 J/C. (3) The displacement correction factor employed to normalize measurements at a depth in a phantom using the type IC‐17 ionization chamber was verified to be approximately 0.97±0.01.

ISSN:0269-3127    

DOI:10.1118/1.594905

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A series of dosimetric measurements was carried out at the Antoni van Leeuwenhoek Hospital, Amsterdam, using a Philips d+T generator, which produces neutrons with energies of approximately 14 MeV. A‐150 plastic ionization chambers, an A‐150 plastic calorimeter, and a GM dosimeter were used to determine total absorbed dose and the photon absorbed dose fraction. The ion chambers were calibrated in a60Co gamma‐ray field in air and compared in a phantom placed in the60Co gamma‐ray beam. Comparison of the total absorbed dose derived from the ionization chamber measurements to that determined using the calorimeter indicated agreement for the neutron measurements between the techniques to within ±2%.

ISSN:0269-3127    

DOI:10.1118/1.594906

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The United States government and the governments of sixteen other countries have established radiation standards laboratories which house the primary standards dosimeters of the respective countries. In the United States, three regional calibration laboratories were established to disseminate the national radiation standards through the use of secondary standards dosimeters. These, in turn, are used for calibration of radiation field instruments used to measure radiation applied in therapy. These laboratories are periodically accredited by the American Assocation of Physicists in Medicine (AAPM). In 1976, AAPM established a Committee on Radiation Calibration Needs in Therapy to conduct a study of the adequacy of the therapy dosimetry network in the United States. According to this study, 1322 centers were performing radiotherapy in 1978, and 1057 radiation instruments were used to calibrate the therapy machines in these facilities. In more than 99% of the institutions, medical physicists are responsible for carrying out the necessary calibrations. The study showed that there is a need, though not an urgent one, for an additional regional calibration laboratory in the Far Western United States. Establishment of such a laboratory would not be excessively disruptive to the existing laboratories.

ISSN:0269-3127    

DOI:10.1118/1.594907

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In dual‐beam selective iodine imaging, images of an object are made with each of two spectrally different x‐ray beams. The mean beam energies may either straddle the 33 keV iodine K‐edge or both lie above the K‐edge. Both patient exposure considerations and the availability of sufficient x‐ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone–to–iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x‐ray spectra. Observed contrast ratios match the predicted value very well. In dual‐beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum energies for dual beam images are discussed.

ISSN:0269-3127    

DOI:10.1118/1.594908

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A survey of six production CT scanners in clinical use has been carried out. The scanner types are the ACTA 0200, EMI 1007, EMI 5005, GE 7800, DELTA 2010, and the AS&E 500. Measurements were made of the noise, spatial resolution, edge enhancement, axial position sensitivity, artefact, and dose. Dose is expressed in terms of the total absorbed energy or the volume integral of the dose, as this is considered the factor most directly related to patient risk. A relationship between dose and picture quality parameters is developed and applied to the scanners for a wide range of settings. Particular emphasis has been placed on evaluating the different operating modes of each scanner. The results show a great variation between machines, especially in the relationship between resolution and noise, and the performance achieved for the dose delivered.

ISSN:0269-3127    

DOI:10.1118/1.594909

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Transthoracic electrical impedanceZconsists of a constant baseline componentZ0and the time variable component ΔZsuch thatZ=Z0+ΔZ. The first time derivative ofZhas a negative peak, the magnitude of which is (dZ/dt)min. Measurements of 170 volunteers show that (dZ/dt)minis a function of the constant baseline componentZ0. This implies that at least a part of the difference of the magnitudes of (dZ/dt)minonZ0must be taken into account in order to avoid erroneous conclusions or inaccurate calculations.

ISSN:0269-3127    

DOI:10.1118/1.594910

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY