摘要:

This work undertakes a detailed systems‐based analysis of the xeromammographic process starting from basic considerations. Both the edge enhancement and wide‐recording latitude, the two principal characteristics of xeroradiography, are shown to bear an intimate relationship to the electric‐field distribution. Criteria and methods are formulated for optimizing xeromammographic image quality and a procedure is developed for calculating the white gap. Densitometric curves are derived for both positive‐ and negative‐mode xeroradiography and found to be in excellent agreement with experiment. The question of image linearity is examined carefully and a threshold value for the electrostatic contrast is established, which sets a natural criterion for the application of Fourier analysis. Furthermore, it is shown that, in xeromammography with its inherently low‐contrast structures, an optimal exposure exists which optimizes simultaneously all low‐contrast edges. This last finding, coupled with experimental results, suggests immediately the possibility of an automatic exposure termination in xeromammography. Beam hardening is also investigated and it is shown that increased filtration combined with a lower bias potential leads to substantial dose reduction without significant loss of image quality. The paper concludes with a discussion of scattered radiation and how it affects xeromammographic image quality.

ISSN:0094-2405    

DOI:10.1118/1.594634

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A system has been developed for the computer generation of dose distributions for the MANTA‐NRL fast‐neutron radiotherapy beam. This program is based on scatter–air ratio (SAR) techniques. A method has been developed to unfold the effect of the neutron‐beam profile in the derivation of SARs so that the SARs obtained are those which would be derived if the beam profile were flat. Tables of zero‐area tissue–air ratios and SARs are presented. Comparisons of calculated and measured dose distributions are shown. An empirical correction to the usual SAR methods was required to obtain agreement between calculated and measured dose distributions at source‐to‐skin distances (SSD) which are different from the SSD at which the SAR are derived.

ISSN:0094-2405    

DOI:10.1118/1.594646

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique is presented for calculating correction factors for neutron dose changes caused by inhomogeneities. The technique is particularly applicable to geometries in which the inhomogeneity lies adjacent to, but not on, a direct path from the radiation source to the point of dosimetric interest; it yields results accurate to a few percent. Best results are obtained for inhomogeneities located at shallow depths.

ISSN:0094-2405    

DOI:10.1118/1.594547

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements of the effects of Teflon and air inhomogeneities on the ionization distributions of clinical negative‐pion beams have been made at the Los Alamos Meson Physics Facility. Inhomogeneity location and pion‐beam energy vary the effect of multiple coulomb scattering on the dose distribution lying in the penumbra of the inhomogeneity. CH2bolus adequately corrects for the effects of these inhomogeneities. Bolus misalignment less than 0.5 cm does not seem critical because of large multiple coulomb scattering of the pion beam. However, this and secondary particles emitted from pion stars prevent the pion beam from being precisely shaped with sharp edges, as demonstrated by measurements under a patient bolus.

ISSN:0094-2405    

DOI:10.1118/1.594548

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Cellulose‐nitrate plastic track detectors have been tested as high spatial‐resolution dosimeters at the Biomedical Channel of the Clinton P. Anderson Meson Physics Facility (LAMPF) at the Los Alamos Scientific Laboratory during pilot clinical trials with negative pi mesons (pions). Six plots of dose contours are presented. Reliable macrodosimetry with track detectors would in principle require detector calibration for the particular conditions of geometry and beam parameters in every case but it is interesting that absolute dose values, estimated from data taken with a quite different calibration geometry, were within 20% of agreement with dose values obtained by electronic dosimetry methods. The track detectors measure only the high linear energy transfer (LET) components produced by the pion beam; it is these components which are of special interest in the negative‐pion therapy program. Plastic track detectors appear to have many favorable properties for pion dosimetry.

ISSN:0094-2405    

DOI:10.1118/1.594635

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The sensitometric properties of radiographic screen–film systems are investigated with regard to their dependence on x‐ray energy. Sensitometric measurements and a heuristic model for density formation are used to show that variation in the relative amount of light emitted by front and back screens may be the most important factor influencing the change in system H and D curves with x‐ray energy.

ISSN:0094-2405    

DOI:10.1118/1.594549

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Clinac‐18 electron‐beam energies and spectral energy distributions were measured by a magnetic analysis method to verify results obtained by the standard range method. A comparison of these measurements with nominal energies quoted by the manufacturer indicates that electron‐beam energies measured by both methods are in agreement with the manufacturer's specifications to ±0.6 MeV. Spectral energy distribution of the electron beam was measured to be approximately 1 MeV (FWHM) at all energies.

ISSN:0094-2405    

DOI:10.1118/1.594550

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Calculated values of the192Ir specific γ‐ray constant Γ, range from the low value of 3.948 R cm2h−1mCi−1recommended in NCRP No. 41 to a high of 4.89 R cm2h−1mCi−1. Measured values of Γ range only from 4.85 to 5.0 R cm2h−1mCi−1. Discrepancies in reported calculated values exist because the isotope decay scheme and other nuclear spectroscopy data pertinent to these calculations were not well known. Using the 28 γ rays and relative intensities from the most recent Evaluated Nuclear Structure Data File (ENSDF), Γ is calculated to be 4.62±0.05 R cm2h−1mCi−1and the exposure rate constant Γδis 4.69±0.05 R cm2h−1mCi−1. These new calculations are presented and previously reported values of Γ and Γδare reviewed.

ISSN:0094-2405    

DOI:10.1118/1.594551

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A system has been implemented for imaging subtle amounts of iodine. Using a grating, information is encoded representing the transmission on either side of the iodineKedge. The information is decoded by filtering and processing the signal on a drum scanner. Experimental results are shown using a thyroid phantom and a rat. This system can be used in a number of studies including hepatography, the radiographic visualization of the liver.

ISSN:0094-2405    

DOI:10.1118/1.594636

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of aluminum attenuation‐equalizing filters in diagnostic radiology may lead to the introduction of intensity blotches inside and outside the image area on the x‐ray film. These blotches are shown to be Laué diffraction spots; the effect can be minimized by selecting filter material with grain size less than about 10−3mm3.

ISSN:0094-2405    

DOI:10.1118/1.594552

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY