摘要:

Storage of radiographs by holographic means is analyzed with emphasis on its gray‐tone reproduction characteristics. The density range and number of gray‐tone levels is found to be limited by random scattering in the holographic medium. The behavior is described quantitatively and is in satisfactory agreement with experimental data. Gray‐tone resolution is shown to be reduced by speckle noise. It is shown that there exists a trade‐off between speckle reduction and reproduced density range.

ISSN:0269-3127    

DOI:10.1118/1.594425

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A proton beam is extracted from the 200‐MeV linear accelerator at the Fermi National Accelerator Laboratory to investigate the efficacy of proton radiography in medical diagnosis. Fluence rates from 2×103to 2×105protons/cm2 s over a 28‐cm diameter field are obtained with a full width at half‐maximum beam‐energy spread of less than 3.61 MeV. The system is designed to radiograph most parts of the human body, including the head, with high‐speed screen–film as the imaging medium. Beam extraction and test results along with the medical implications of the beam quality are reported.

ISSN:0269-3127    

DOI:10.1118/1.594426

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A14CO2‐measuring system for breath‐analysis tests is described which utilizes plastic‐scintillator filaments as radiation‐detector elements. The14C radioactivity in expired breath is measured directly, thus eliminating the need for trapping and counting of liquid scintillation‐counting solutions. Total CO2concentration in expired breath is measured by an infrared detector, making no assumption of endogenous CO2output and enabling results to be expressed as either a concentration (percentage of administered dose per unit of CO2) or total expired14CO2. Advantages of this system over an ionization chamber are: significantly lower background variation and shorter breathing time to fill completely the detecting chamber with expired air. The system is easy to operate, transportable on a small cart to the patient's bed if necessary, and applicable for continuous monitoring of14CO2in experimental animal studies.

ISSN:0269-3127    

DOI:10.1118/1.594427

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An empirical equation is described which accurately fits intensifying‐screen MTF data within the accuracy of the MTF measurements. The equation isS(u) =0.5 erfc[α ln(u/u0)]. The equation was fitted to data for 27 intensifying screens. Graphical and numerical results are presented. The maximum standard error was 0.02 and typical standard error was 0.01. Comparisons between results for three experimenters are also presented.

ISSN:0269-3127    

DOI:10.1118/1.594473

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two important physical characteristics of x‐ray intensifying screens, speed and resolution, are inversely related. However, the exact mathematical relationship, if any, between them is not known. To investigate this matter, a simple model is considered, which predicts that the product of the equivalent passbandNeand the square root of the speed is constant for a given phosphor material and given film. This relationship is found to be in excellent agreement with experimental data. The paper concludes with a discussion of the generalized validity of this relationship.

ISSN:0269-3127    

DOI:10.1118/1.594428

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Corrections for beam‐hardening artifacts in computed tomography can be made by using a model which assumes that water and bone mineral are the only constituents of tissue. With this model, a correction factor for the measured transmission values can be calculated such that the reconstructed attenuation coefficients have values corresponding to a monoenergetic source of known energy. Systematic errors in the uncorrected attenuation coefficients, which may be 5%, can be reduced to less than 1% if corrected transmission values are used.

ISSN:0269-3127    

DOI:10.1118/1.594429

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A flattening filter is an important component in a medical accelerator to modify the photon‐beam properties. To simplify the calculations of the flattening‐filter profile, we have developed a computer program which sums primary and scatter and then makes iterations in the primary component to produce a desired total‐dose profile. The program can account, to first order, for radial spectral changes by using an effective primary attenuation coefficient which varies with radius. Calculations made to model the Clinac‐4 dose profiles using the measured variation of half‐value layer with radius show good agreement with the measured data. It is shown that the variation of quality within the beam impairs the flatness that can be achieved over a range of depths. Since perfect flatness cannot be achieved for small and large fields with one flattening filter, one may choose a primary profile which is a compromise over a range of field sizes and depths. A compromise profile for a 4‐MV beam is discussed.

ISSN:0269-3127    

DOI:10.1118/1.594430

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fast‐ and slow‐neutron contamination in an 18‐MV photon beam from a Philips SL/75‐20 linear accelerator has been measured. Aluminum and indium foils were activated to determine fast‐ and slow‐neutron fluence, which were largely independent of field sizes. Measured fast‐neutron fluences were typically 13.9×104and 4.4×104neutrons/cm2/rad of x ray inside and 5 cm outside the field, respectively. Slow‐neutron fluences, 1.3×104neutrons/cm2/rad of x ray, remained relatively constant inside and outside the field. The reported results are about three times higher than neutron fluences recently reported with a betatron operated at the same energy.

ISSN:0269-3127    

DOI:10.1118/1.594431

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Considerable attention has been paid to the protection against x and γ radiation but comparatively less stress has been given to the possible hazard due to external radiation from high‐energy beta rays. In order to evaluate the magnitude of this hazard, central‐axis depth doses at different source‐to‐skin distances for90Sr‐90Y and32P sources were measured. Isodose curves in a testicular phantom for a90Sr‐90Y source were measured. The data of Haybittle for144Ce for 10 cm SSD has been included. From the measured data, the eye‐lens epithelium dose may be as high as 51%, 21.5%, and 82%, respectively, for the three sources instead of 15% as has been conventionally assumed. The isodose curves obtained in the testicular phantom indicate that an appreciable amount of testicular tissue can be subjected to radiation exposures. The radiation hazards due to high‐energy beta rays are not negligible and considerable care should be exercised while using these sources.

ISSN:0269-3127    

DOI:10.1118/1.594478

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

By applying a transverse magnetic field to a dosimetry phantom, an incident high‐energy electron beam is made to follow a spiral path in the course of slowing down. Certain levels, determined by the electron energy and the magnetic field strength, will be traversed several times by the same electrons. The net result of this process is an enhancement of the depth dose in relation to the entrance dose, and a more sharply defined depth of penetration. Experiments with 50‐ and 55‐MeV electrons traversing a 20.5‐kG field are shown to support the predictions of a detailed Monte Carlo calculation.

ISSN:0269-3127    

DOI:10.1118/1.594420

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY