摘要:

A general methodology is provided for the absolute assay of radioisotopes decaying with coincident photons in an extended source. In the determination of the source activity, the method requires neither the detailed consideration of the geometric and self‐attenuation processes taking place between the source component points nor a knowledge of the distribution of activity across the source. It derives from the concept of the “equivalent point source,” that is a fictitious point source whose activity would equal that measured for the actual extended source. It has been developed for an arbitrary number of coincident photon types displaying an arbitrary degree of mutual correlation, and for arbitrary detection geometry. A unifying formalism is developed for both point and extended sources and for single and dual detecting systems. It is found that in all cases the various instrumental and spectroscopic uncertainties appear within a composite parameter (herein calledFfactor) that can be determined by standard calibration procedures; this factor is in turn only weakly dependent on its own component parameters. New expressions and relationships are obtained that provide a greater physical insight into coincidence‐counting methods.

ISSN:0269-3127    

DOI:10.1118/1.595769

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Experimental studies of a solid‐state linear detector array developed for a prototype scanning slit digital chest radiographic unit have been completed. The detector consists of a strip of scintillating material, optically coupled to a linear silicon photodiode array. Measured performance characteristics of the detector, such as sensitivity, modulation transfer function, and detective quantum efficiency, are presented for several different scintillators. Results indicate that direct x‐ray absorption events in the silicon photodiode can degrade detective quantum efficiency. Results also indicate that the inexpensive preamplifier circuits used in the digital chest prototype contribute negligible noise at diagnostic x‐ray photon fluence rates.

ISSN:0269-3127    

DOI:10.1118/1.595770

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Artifacts due to periodic motion during the acquisition of magnetic resonance (MR) images have been studied. A mechanical device was constructed to oscillate a small sample along any line within a 0.15‐T Technicare imager. Two‐ and three‐dimensional images were obtained using various frequencies and amplitudes of oscillation. Computer simulations of these experiments yielded images which agreed with the experiments. We demonstrated that movement influences MR images locally through blurring, and also generates ghost artifacts along the phase‐encoding directions of the Fourier transform imaging technique.

ISSN:0269-3127    

DOI:10.1118/1.595782

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We applied Monte Carlo methods for the simulation of x‐ray scattering in water phantoms. The phantom thickness was varied from 5 to 20 cm, and the monoenergetic incident x rays were varied from 15 to 100 keV. Eight screen pairs and a total absorption system were used as x‐ray receptors. We determined the angular, spectral, and spatial distributions of the scattered radiation and the scatter fractions recorded in the image plane. The dependence of these properties on the incident x‐ray energy, the phantom thickness, and the energy response of the recording system was examined. The results of this study provide useful information for the development of antiscatter techniques and for the evaluation of radiographic procedures.

ISSN:0269-3127    

DOI:10.1118/1.595771

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the recently published AAPM protocol for the dosimetry of high‐energy photons and electrons, the response of an ionization chamber is defined as the dose to the gas in the chamber per unit electrometer reading,Ngas. Using a graphite calorimeter,Ngashas been determined for a Farmer‐type ionization chamber using 4‐ and 25‐MV x rays. The procedure was to measure the dose to graphite using the calorimeter, and then obtain the response of the chamber at the same depth in a graphite phantom. Equation (9) of the AAPM protocol was then used to calculateNgas. The values ofNgasdetermined with the calorimeter are within 1% ofNgascalculated according to the AAPM protocol, using the60Co exposure‐calibration factor.

ISSN:0269-3127    

DOI:10.1118/1.595758

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A three‐dimensional photon beam calculation is described which models the primary, first‐scatter, and multiple‐scatter dose components from first principles. Three key features of the model are (1) a multiple‐scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions. The results of calculations for cobalt‐60 in a homogeneous phantom are used to verify the accuracy of the model.

ISSN:0269-3127    

DOI:10.1118/1.595772

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Fermi‐Eyges theory of multiple scattering, applicable to charged‐particle teletherapy beams, has been generalized to second order in small quantities representing deviations from the particles’ initial lines of travel and direction. This second‐order multiple‐scattering theory provides increased accuracy, and in particular it takes into account the skewness of the particles’ paths, in calculating dose. Only the assumptions and certain results of the second‐order theory are presented in this communication, for the immediate use of other investigators. An application to rectangular fields shows that the second‐order theory predicts one component of the buildup of electron central axis depth dose, which the (first‐order) Fermi–Eyges theory cannot do. (The other major component of buildup, which is not yet incorporated into this theoretical work, is due to high‐energy secondary electrons.) Various approximate calculations of electron central axis depth dose are compared with the second‐order calculation.

ISSN:0269-3127    

DOI:10.1118/1.595773

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is presented by which dose distributions in radiotherapy may be judged. This method, based on statistical decision theory, combines the calculated probabilities of radiation induced complications (including failure to control disease) with the therapist's judgment of the morbidity of each complication to yield a single value representing the clinical utility of the dose distribution. Using this figure of merit, alternative dose distributions can be compared on a clinical basis. If the morbidities associated with each injury are satisfactorily estimated, and the dose response parameters are adjusted to match clinical experience, this method can be used to evaluate novel treatment techniques prior to their implementation. Automatic optimization algorithms can be used to maximize the figure of merit as a function of the physical treatment parameters so as to provide a statistically optimal treatment.

ISSN:0269-3127    

DOI:10.1118/1.595707

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting “dose spread arrays” were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three‐dimensional dose distributions. The method gives good agreement for the 15‐MV x‐ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low‐density heterogeneities.

ISSN:0269-3127    

DOI:10.1118/1.595774

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of the collection of free electrons—those which have not formed negative ions in the gas of a parallel‐plate ionization chamber—is considered for the case of continuous radiation. A theory and its solution, valid for the entire voltage range, is given. The solution is based upon the saturation theory of Townsend, and free electrons are included in a perturbative fashion. The collection of free electrons becomes important at exposure rates above 430 R/min. Since the electron contribution involves an extra dependence on exposure rate, some of the wide variation in the cavity gas parameter (mvalue) reported in the literature may be due to the presence of free electrons.

ISSN:0269-3127    

DOI:10.1118/1.595783

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY