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Closed‐system ionography for diagnostic radiology

A. Fenster and H. E. Johns

Med. Phys. 3, 379 (1976); http://dx.doi.org/10.1118/1.594250 (5 pages) | Cited 1 time

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Three methods are described whereby radiographic electrostatic images are transferred from inside of a liquid ionography chamber to the outside. One of these methods is implemented showing that multiple‐charge images can be transferred from a single original without significant degradation of image quality and each copy may be developed with a different amount of edge contrast. This new method of imaging was applied to a radiograph of a test pattern and a hand phanton. An exposure of about 6 mR to the imaging chamber is needed to produce a useful image with a resolution of about 8 1p/mm.

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87.57.-s	Medical imaging
87.63.-d	Non-ionizing radiation equipment and techniques
87.85.Pq	Biomedical imaging

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Saturation curve in gases of high atomic number at pressures up to 8 atm. Part II: Freon 13‐B1, and mixtures of Freon with xenon and krypton

W. W. Seelentag, J. W. Boag, and R. J. Barish

Med. Phys. 3, 384 (1976); http://dx.doi.org/10.1118/1.594255 (7 pages) | Cited 3 times

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The saturation curve has been studied in Freon 13‐B1 (CF₃Br) and in mixtures of Freon with xenon and krypton up to a pressure of 8 atm. The enhanced initial recombination due to the electron affinity of Freon has been evaluated and an empirical formulas constructed that describes the fraction of current which escapes initial recombination over a wide range of voltages, pressures, and electrode spacings. After correction for this initial recombination, the general saturation curve for Freon and for mixtures of this gas with krypton and xenon has been derived and, again, convenient empirical formulae established which allow the current collection efficiency to be calculated for any given parameters within the range investigated. These formulae are of practical value in the design of image‐forming ionization chambers.

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87.57.-s	Medical imaging
87.63.-d	Non-ionizing radiation equipment and techniques
87.85.Pq	Biomedical imaging

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In vivo measurements of lung corrections for fast‐neutron therapy

A. R. Smith, J. H. Jardine, G. L. Raulson, P. R. Almond, and D. D. Boyd

Med. Phys. 3, 391 (1976); http://dx.doi.org/10.1118/1.594251 (6 pages)

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Silicon diodes were inserted into the esophagus and bronchus of anesthetized rhesus monkeys in order to measure the corrections to tumor dose resulting from intervening lung tissue during fast‐neutron therapy. The derived corrections were applied to tumor doses for patients being treated for cancer of the esophagus on the fast‐neutron beam at TAMVEC. In vivo dosimetry performed on these patients using silicon diodes in the esophagus confirmed the accuracy of the lung corrections. The measured dose and calculated dose agreed to within 4% for four different patients. The magnitude of the correction is on the order of 16% for the typical esophageal cancer patient. These studies were also done with Cobalt‐60 in order to test, against other data, the results obtained with this animal model.

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87.53.Bn	Dosimetry/exposure assessment
87.53.-j	Effects of ionizing radiation on biological systems

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Dose levels due to neutrons in the vicinity of high‐energy medical accelerators

P. H. McGinley, M. Wood, M. Mills, and R. Rodriguez

Med. Phys. 3, 397 (1976); http://dx.doi.org/10.1118/1.594256 (6 pages) | Cited 3 times

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High‐energy photons are generated for use in radiation therapy by the deceleration of electrons in metal targets. Fast neutrons are also generated as a result of (γ, n) and (e, e′n) interactions in the target, beam compensator filter, and collimator material. In this work the absorbed dose due to neutrons was measured at the center of a 10×10‐cm photon beam and 5 cm outside of the beam edge for a number of treatment units (Varian Clinac‐18, Allis–Chalmers 25‐MeV betatron, and Brown Boveri 45‐MeV betatron). Dose levels due to slow and fast neutrons were also established outside of the treatment rooms and a Bonner sphere neutron spectrometer system was employed to determine the neutron energy spectrum due to stray neutron radiation at each accelerator. For the linac it was found that the neutron dose at the beam center was 0.0039% of the photon dose and values of 0.04% and 0.053% were observed for the Allis–Chalmers betatron and the Brown Boveri betatron. Dose‐equivalent rates in the range 0.3–22.5 mrem were measured for points outside the treatment rooms when the accelerators were operated at a photon dose rate of 100 rad/min at the treatment position.

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87.53.Bn	Dosimetry/exposure assessment
87.53.-j	Effects of ionizing radiation on biological systems

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On the application of a liquid‐dielectric ionization chamber to ²⁵²Cf dosimetry

Daniel R. Bednarek, Lawrence H. Lanzl, and Martin Rozenfeld

Med. Phys. 3, 403 (1976); http://dx.doi.org/10.1118/1.594252 (7 pages) | Cited 2 times

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Our investigations of an iso‐octane‐filled liquid‐dielectric ionization chamber indicate that this dosimeter is well suited for the measurement of dose distributions at distances close to ²⁵²Cf interstitial implant sources. Initial research has shown that such an instrument provides good discrimination between the radiation components of a mixed field and possesses excellent sensitivity. Calculations indicate that iso‐octane will yield a neutron and γ‐ray dose response independent of energy over a major part of the ²⁵²Cf spectral range, while transmission measurements have demonstrated that the liquid causes little perturbation of the radiation field.

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87.53.Bn	Dosimetry/exposure assessment
87.53.-j	Effects of ionizing radiation on biological systems

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NMR relaxation behavior in living and ischemically damaged tissue

L. E. Barroilhet and P. R. Moran

Med. Phys. 3, 410 (1976); http://dx.doi.org/10.1118/1.594258 (5 pages) | Cited 5 times

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A study by pulsed NMR techniques in living liver tissue has led to the discovery that the observed longitudinal relaxation decay behavior is strongly multicomponent. After death of the experimental animal, the relaxation decay curves evolve toward a single‐component behavior. These changes can also be observed within a few minutes after the liver is excised and placed in a test tube, and they involve a high degree of quantitative and qualitative regularity and reproducibility. An excellent description of all observed NMR behavior is obtained from a dynamic two‐compartment model. Rapidly relaxing volumes exchange water molecules with slowly relaxing volumes; associating only an increasing water molecule exchange rate with increasing ischemia accounts in quantitative detail for all observed changes. The exchange‐rate values and their variation with tissue deterioration are in good agreement with that estimated for intra‐ to extracellular water exchange as limited by cell‐membrane osmotic permeabilities. Possible applications of these results in different biomedical areas are discussed.

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87.57.-s	Medical imaging
87.63.-d	Non-ionizing radiation equipment and techniques
87.85.Pq	Biomedical imaging
87.50.C-	Static and low-frequency electric and magnetic fields effects

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Direct‐response ultraviolet thermoluminescent dosimeter

A. Dhar, L. A. DeWerd, and T. G. Stoebe

Med. Phys. 3, 415 (1976); http://dx.doi.org/10.1118/1.594259 (3 pages) | Cited 4 times

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An integrating solid‐state dosimeter has been developed to detect ultraviolet radiation using the process of ‘‘direct’’ ultraviolet stimulation. This process yields a thermoluminescent signal after an exposure to ultraviolet radiation, without prior exposure to ionizing radiation. Magnesium oxide crystals were found to show a directly stimulated thermoluminescent glow peak at 145 °C which can be utilized as a linear, energy‐independent ultraviolet dosimeter.

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87.53.Bn

Dosimetry/exposure assessment

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Frequency of occurrence of various nuclear reactions when fast neutrons (≲50 MeV) pass through tissue‐equivalent material

R. G. Alsmiller, Jr. and J. Barish

Med. Phys. 3, 418 (1976); http://dx.doi.org/10.1118/1.594253 (4 pages)

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Calculated results are presented for the frequency with which various partial nuclear reaction cross sections are utilized when fast neutrons (≲50 MeV) are transported through a tissue‐equivalent phantom to obtain an indication of which cross sections are of most importance for radiotherapy applications and are therefore in need of experimental verification.

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87.53.Bn	Dosimetry/exposure assessment
87.53.-j	Effects of ionizing radiation on biological systems

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Nuclide parameters and S factors for ²⁰¹Tl dosimetry estimates

Paul A. Feller and Kenneth L. Scholz

Med. Phys. 3, 422 (1976); http://dx.doi.org/10.1118/1.594254 (4 pages)

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The mean numbers per disintegration, n, and equilibrium‐dose constants Δ for the various emissions of ²⁰¹Tl were computed. The sums of the penetrating and nonpenetrating equilibrium‐dose constants are 0.1931 and 0.0874 g rad/μCi h, respectively. The S factor, the value of that portion of the MIRD dose equation which is independent of biological distribution, was calculated for various combinations of target and source organs.

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