摘要:

The accuracy of high‐energy x‐ray dosimetry can be improved by taking account of differences between the compositions of the chamber wall and the buildup cap or dosimetry phantom. The fraction of the ionization due to secondary electrons arising in the chamber wall has been determined as a function of wall thickness for60Co gamma rays and x rays in the range of 2–25 MV for Farmer‐type chambers. Secondary electrons arising in the accelerator head were removed from the x‐ray beams by a magnetic field placed just in front of the ionization chamber. For60Co gamma rays, the fraction increases from 40% to 100% as the wall thickness increases from 0.05 to 0.55 g cm−2. For a 0.05 g cm−2wall, fraction decreases from 60% to 10% as the x‐ray energy is increased from 2 to 25 MV. Limited data obtained with different chambers suggest that the fraction is independent of chamber wall composition when the thickness is expressed in g cm−2.

ISSN:0269-3127    

DOI:10.1118/1.595288

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Slit radiography is a highly efficient means of suppressing scatter. However, when employed with a pulsating x‐ray source, a grid line type of artifact results if the movement of the slit is not properly coupled to the periodicity of the source. Taking the exposure of the film to be the convolution of the slit transmittance and the radiation intensity, the conditions for a uniform exposure have been derived for the realistic configuration of a multiple‐slit array and a slit transmittance that includes the focal spot and slit geometry. Expressions for the amplitude and spatial frequency of the artifact when the conditions are not met are derived. Methods of suppressing the artifact when its vanishing conditions are not met are also discussed.

ISSN:0269-3127    

DOI:10.1118/1.595372

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This report discusses the optical and thermal distribution during photoradiation therapy of malignant tumors. Emphasis is put on the therapeutic procedure with the light dose delivered through an inserted optical fiber. Theoretical predictions and experimental results indicate that the temperature rise during the procedure may give rise to hyperthermal cell kill. The report discusses the extent of the regions with hyperthermal bioeffects in terms of tissue parameters as optical absorption and scattering, thermal conductivity, specific heat, blood flow, and optical dose parameters as optical power and exposure time.

ISSN:0269-3127    

DOI:10.1118/1.595373

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements. A Monte Carlo technique was employed. The calculation was divided into two parts. First, the accelerator treatment head was simulated in detail using the EGS–PEGS electromagnetic shower code. Then, information obtained from these calculations was used to compute dose curves in a polystyrene phantom. Two cases were considered, one in which both electrons and photons were incident upon the phantom, and another in which electrons were eliminated from the incident beam. Results of these calculations agree with recent experimental findings. A decrease in buildup dose as well as a shift indmaxwas observed when electrons were eliminated from the beam.

ISSN:0269-3127    

DOI:10.1118/1.595287

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The propagation of noise in digitized images arrived at by algebraic manipulation of two images has been investigated. The algebraic manipulations investigated were logarithmic subtraction, linear subtraction, and division of the two images. The two images were identical except for the presence of a contrast‐adding component in one of them. The two extreme cases of video‐fluoroscope system behavior, gray level proportional to incident photon flux and gray level proportional to the product of thickness and linear attenuation coefficient, were investigated. The ratio of signal to noise was calculated for each of these cases and a figure of merit established for the ratio of signal to noise of the division process versus each of the subtraction processes for each of the two fluoroscopic system behaviors. In all cases, this figure of merit is greater than unity. Values of these figures of merit for iodine concentrations of 5, 20, and 300 mg/cm3are given in the paper. The process of division appears to be advantageous as a signal‐to‐noise enhancer for all applications not requiring a quantitative relationship between image contrast and photon flux.

ISSN:0269-3127    

DOI:10.1118/1.595289

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An equation was developed to calculate osmotic potential by formulating the several molecular pressures in a simple osmotic system. Its predictability was verified for many organic and inorganic solutes and their mixtures over concentration ranges well in excess of those found in body fluids. As the contribution to osmotic potential of the separate solute components in a solution can be calculated using the equation, it would be useful in the diagnoses and therapy of osmotic pressure related trauma and disease conditions.

ISSN:0269-3127    

DOI:10.1118/1.595371

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A fluorescence bronchoscope system has been developed for imaging lung tumors by fluorescence of a previously injected, tumor‐specific agent hematoporphyrin derivative. Carcinomain situhas been localized, but there are too many false positives and negatives. A new system has been implemented which allows rapid switching between viewing of fluorescence, and viewing of the same area under white light illumination as in conventional bronchoscopy. The excitation source is a violet krypton ion laser coupled to a fused quartz fiber light conductor, with a diverging microlens to spread the light uniformly. A third‐generation, microchannel plate image intensifier amplifies the weak fluorescence for viewing and video display, recording, and analysis. A movable mirror and periscope bypasses the intensifier for normal color viewing and video display and recording, with the laser shutter closed and the white light shutter open. This facilitates accurate localization, comparison of the color and fluorescence images, and precise sampling during biopsy. The improved system should reduce the false positive rate due to biopsy sampling error, and together with the video analyzer should reduce indeterminate results.

ISSN:0269-3127    

DOI:10.1118/1.595374

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation. Single‐scatter calculations, however, show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast, and that they account for a significant fraction of the total scattered energy fluence at the image receptor. Grids and large air gaps are less effective in reducing coherent single scatter than incoherent and multiple scatter. For radiography of the abdomen, coherent first scatter comprises 10% of total scatter and 26% of the primary fluence before a grid, and on the order of 22% and 7.5%, respectively, behind a grid. Coherent first scatter comprises a higher fraction of the total amount of scatter for lower energy examinations such as mammography.

ISSN:0269-3127    

DOI:10.1118/1.595443

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the practice of radiotherapy, various concepts like NSD, CRE, TDF, and BIR are being used to evaluate the biological effectiveness of the treatment schedules on the normal tissues. This has been accepted as the tolerance of the normal tissue is the limiting factor in the treatment of cancers. At present when various schedules are tried, attention is therefore paid to the biological damage of the normal tissues only and it is expected that the damage to the cancerous tissues would be extensive enough to control the cancer. Attempt is made in the present work to evaluate the concent of tumor significant dose (TSD) which will represent the damage to the cancerous tissue. Strandquist in the analysis of a large number of cases of squamous cell carcinoma found that for the 5 fraction/week treatment, the total dose required to bring about the same damage for the cancerous tissue is proportional toT−0.22, whereTis the overall time over which the dose is delivered. Using this finding the TSD was defined asD⋅N−p⋅T−q, whereDis the total dose,Nthe number of fractions,Tthe overall timepandqare the exponents to be suitably chosen. The values ofpand q are adjusted such thatp+q≤0.24, andpvaries from 0.0 to 0.24 andqvaries from 0.0 to 0.22. Cases of cancer of cervix uteri treated between 1978 and 1980 in the V. N. Cancer Centre, Kuppuswamy Naidu Memorial Hospital, Coimbatore, India were analyzed on the basis of these formulations. These data, coupled with the clinical experience, were used for choice of a formula for the TSD. Further, the dose schedules used in the British Institute of Radiology fractionation studies were also used to propose that the tumor significant dose is represented byD⋅N−0.18⋅T−0.06.

ISSN:0269-3127    

DOI:10.1118/1.595375

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Steady‐state temperature solutions to the bioheat equation are presented for magnetic induction heating of a thoracic model consisting of a spherical tumor embedded in lung tissue which is layered by muscle and fatty tissue. Analytical solutions are presented for each of the tissue regions along with their numerical evaluations over a range of physical characteristics, including surface cooling effects. A strong dependence of tumor temperature on size and blood perfusion rate is shown to exist and can be used to optimize treatment parameters. Tendencies of the chest muscles and overlaying fatty tissue to overheat, particularly in the case of an obese patient, are discussed along with the alleviating influence of surface cooling. Healthy lung tissue, on the other hand, is shown to be safe from any significant damage in such a heating situation. Transient times required for tumors to achieve thermal equilibrium are computed and shown to depend strongly on tumor size and, to a lesser extent, on blood perfusion rate. Finally, the overall results obtained from the model are compared with available clinical data and are found to be in line with those observations.

ISSN:0269-3127    

DOI:10.1118/1.595376

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY