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Nov 1977

Volume 4, Issue 6, pp. 474-538

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Selective iodine imaging using K‐edge energies in computerized x‐ray tomography

S. J. Riederer and C. A. Mistretta

Med. Phys. 4, 474 (1977); http://dx.doi.org/10.1118/1.594357 (8 pages) | Cited 8 times

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Iodine is commonly used as a contrast material in computerized x‐ray tomography. In some cases the determination of the iodine distribution in the image may be prevented by the presence of bone or tissue variations within the tomographic slice. This paper describes a method for quantitative selective imaging of the iodine concentration in the slice. The method employs scans using three heavily filtered x‐ray beams, two having mean energies which straddle the iodine K edge (33 keV) and another at a slightly higher energy. The results are independent of tissue and bone over a broad range of projection path lengths. It is shown that, for separation of iodine from one other material, a two‐beam K‐edge approach requires less integral dose than a two‐beam technique at conventional CT energies for slice diameters up to 30 cm. For selective iodine imaging in the presence of more than one other material, the three‐spectrum K‐edge technique is a necessity. Exposure requirements and beam‐hardening corrections are discussed in detail and a computer‐simulated CT image generated by the proposed scheme is presented.
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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

A simple source of fluorescent x rays for the study of radiographic imaging systems

Carl J. Vyborny, Kunio Doi, Charles E. Metz, and Arthur G. Haus

Med. Phys. 4, 482 (1977); http://dx.doi.org/10.1118/1.594346 (4 pages) | Cited 1 time

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The properties of a simple source of fluorescent x rays were investigated with regard to its suitability for the study of radiographic imaging systems. The source, consisting of a diagnostic x‐ray tube, fluorescent targets, and filters, was found to yield highly monoenergetic x‐ray fluxes with intensity sufficient to allow the speed of medium‐ and high‐speed screen–film systems to be studied as a function of x‐ray energy.
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07.85.-m X- and γ-ray instruments
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

Physical characterization of neutron beams produced by protons and deuterons of various energies bombarding beryllium and lithium targets of several thicknesses

H. I. Amols, J. F. Dicello, M. Awschalom, L. Coulson, S. W. Johnsen, and R. B. Theus

Med. Phys. 4, 486 (1977); http://dx.doi.org/10.1118/1.594347 (8 pages) | Cited 5 times

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Protons of 35 and 65 MeV and deuterons of 35 MeV were used to bombard beryllium and lithium targets of various thicknesses. Four types of experiments were conducted in order to characterize the neutron fields. They were (1) central axis depth‐dose measurements in a water phantom, (2) dose buildup at small depths in tissue‐equivalent plastic, (3) microdosimetric measurements and LET distributions, and (4) neutron yields and energy spectra at an angle of 0 deg. The results generally show that (a) the central axis depth doses for the 35 and 65 MeV particles roughly approximate those of 60Co and 4‐MeV bremsstrahlung photons, respectively, (b) the neutron dose buildups are more rapid than those of the above‐mentioned photon sources, (c) the microdosimetric spectra show differences which are consistent with the measured neutron energy spectra, and (d) p–Li compared to p–Be neutron spectra have larger high‐energy particle flux for similar target and beam configurations.
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29.25.Dz Neutron sources
07.77.-n Atomic, molecular, and charged-particle sources and detectors

Silicon diode detectors used in radiological physics measurements. Part I: Development of an energy compensating shield

L. David Gager, Ann E. Wright, and Peter R. Almond

Med. Phys. 4, 494 (1977); http://dx.doi.org/10.1118/1.594348 (5 pages) | Cited 12 times

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Silicon diode detectors have the advantages of high resolution, large signal, and fast response, but lack the flat energy response of the Farmer ion chamber. A study was undertaken to develop a compensating shield for a diode which would make it suitable for use in the spectrum of energies produced by a high‐energy radiation beam at depth in a phantom. The energy response of the unshielded diode was quantitated over a range of energies from 18.5 keV to 8 MeV. Shields of different thicknesses, density, and design were tested experimentally. A partial shield of high‐Z material over a diode with miniaturized contacts produced a probe which duplicated the relative dose measurements of the Farmer chamber with less than 1% variation. Typical central axis depth‐dose curves and a beam profile, measured with the chamber and the shielded and unshielded probe, are illustrated.
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87.80.-y Biophysical techniques (research methods)
28.41.Te Protection systems, safety, radiation monitoring, accidents, and dismantling

Silicon diode detectors used in radiological physics measurements. Part II. Measurement of dosimetry data for high‐energy photons

Ann E. Wright and L. David Gager

Med. Phys. 4, 499 (1977); http://dx.doi.org/10.1118/1.594349 (4 pages) | Cited 2 times

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Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth‐dose and off‐axis data, both in and out of the beam. These data for an 8‐MeV unit were first measured using film, a Farmer 0.6‐cm3 ion chamber, a 0.3‐cm3 ion chamber, and a 0.1‐cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X–Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth‐dose and off‐axis data.
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87.80.-y Biophysical techniques (research methods)
28.41.Te Protection systems, safety, radiation monitoring, accidents, and dismantling

Energy dependence of correction factors for some low‐energy direct‐reading pocket dosimeters

Tommie J. Morgan, Libby Brateman, and John Dirkse

Med. Phys. 4, 503 (1977); http://dx.doi.org/10.1118/1.594350 (2 pages)

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The energy dependence for each of five models of low‐energy direct‐reading pocket dosimeters was characterized through an examination of the exposure correction factors determined over the diagnostic x‐ray range (0.4–4.1‐mm Al HVL) for many samples of each model. The results of an analysis of variance performed by model on the correction factors are reported as mean correction factors for the x‐ray beams evaluated, the relative standard error being less than 0.8% in all cases. Energy‐dependence curves for the dosimeter models are given, and their use is described.
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29.40.-n Radiation detectors

Extrapolation of linear attenuation coefficients of biological materials from diagnostic‐energy x‐ray levels to the megavoltage range

William H. Payne, William D. McDavid, Robert G. Waggener, Michael J. Dennis, and Victor J. Sank

Med. Phys. 4, 505 (1977); http://dx.doi.org/10.1118/1.594351 (3 pages) | Cited 3 times

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A dual‐energy algorithm is used in determining the effective atomic number, atomic density, and electron density of biological substances. These quantities are then used to calculate linear attenuation coefficients at the megavoltage level. The validity of the method is checked several ways, including a comparison of extrapolated values with experimental data reported by Rao and Gregg where linear attenuation coefficients at 60 and 122 keV are used to extrapolate to coefficients at 662 keV. Except for a few instances, the extrapolated values agree quite well with the reported experimental values. This method is also used to calculate coefficients at the 60Co range, and these are compared with experimental values measured in water and various types of tissue‐equivalent materials. An additional algorithm is developed to extrapolate coefficients in water and bone up to 10 MeV. These quantities are compared with accepted values previously reported in the literature.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
87.53.Bn Dosimetry/exposure assessment

Neutron spectral measurements in an intense photon field associated with a high‐energy x‐ray radiotherapy machine

G. R. Holeman, K. W. Price, L. F. Friedman, and Ravinder Nath

Med. Phys. 4, 508 (1977); http://dx.doi.org/10.1118/1.594339 (8 pages) | Cited 3 times

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High‐energy x‐ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x‐ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25‐MV x rays) located at Yale–New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2‐, 3‐, 5‐, 8‐, 10‐ and 12‐in.‐diameter polyethylene spheres with 6Li and 7Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium‐covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum. Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time‐of‐flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.
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87.53.Bn Dosimetry/exposure assessment
87.85.-d Biomedical engineering
87.80.-y Biophysical techniques (research methods)

Effects of red cell shape and orientation on propagation of sound in blood

Avtar S. Ahuja and William R. Hendee

Med. Phys. 4, 516 (1977); http://dx.doi.org/10.1118/1.594340 (5 pages)

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In this paper, the red blood cell (RBC) is assumed to have an oblate spheroidal shape and the same volume (87 μm3) and about the same sphericity index (0.7) as a typical human RBC. The acoustic field is assumed to be either parallel or perpendicular to the axis of symmetry of the spheroid, and a wave equation is formulated for a dilute RBC suspension. Because of the small density difference between the RBC and plasma (about 5%), the assumption of spherical shape of the RBC suffices for computation of the velocity and the scattering of sound in blood. For all practical purposes, scattering of sound in blood follows Rayleigh’s law of scattering. However, the viscous absorption coefficient at 1 MHz for a spheroidally shaped RBC oscillating broadside and edgewise to an acoustic field is about 40% and 136%, respectively, of that for a spherically shaped RBC. These results illustrate the significant effects of RBC shape and orientation on the viscous loss of sound energy in blood.
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87.19.-j Properties of higher organisms
43.80.+p Bioacoustics
43.80.-n Bioacoustics
87.50.Y- Biological effects of acoustic and ultrasonic energy

Radiation dose to the lungs from ventilation studies with 133Xe

Evelyn E. Watson, Roger J. Cloutier, and Barbara Y. Howard

Med. Phys. 4, 521 (1977); http://dx.doi.org/10.1118/1.594341 (3 pages)

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The radiation dose to the lungs from 133Xe in lung air is directly proportional to the time integral of 133Xe concentration; i.e., the cumulated concentration in the lungs. Using kinetic models developed to fit clinical observations, we have studied the effect of retention on cumulated concentration in lung air. The models studied were (1) equal exponential washin and washout rate constants, (2) unequal exponential washin and washout rate constants, and (3) single‐compartment washin and two‐compartment washout. Our results show that the radiation dose varies greatly with the model chosen. A simplified method for calculating the average dose to the lungs from activity in lung air is presented. Although we have applied this method only to studies where xenon is rebreathed at constant volume and then washed out, the technique can be adapted to other protocols.
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87.53.Bn Dosimetry/exposure assessment
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
One Physics Ellipse
College Park, Maryland 20740
301-209-3352

© 2013, American Association of Physicists in Medicine. Individual readers of this journal, and nonprofit libraries acting for them, are freely permited to make fair use of the material in it, such as to copy an article for use in teaching or research. (For other kinds of copying see "Copying Fees.") Permission is granted to quote from this journal in scientific works with the customary acknowledgment of the source. To reprint a figure, table, or other excerpt requires, in addition, AAPM may require that permission also be obtained from one of the authors. Address inquiries and notices to Penny Slattery, Journal Manager, Medical Physics Journal, AAPM, One Physics Ellipse, College Park, MD 20740-3846; email: journal@aapm.org.

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