ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1977.tb36096.x

出版商:Wiley    

年代:1998

数据来源: WILEY

摘要:

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 iodineKedge (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‐beamK‐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‐spectrumK‐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.

ISSN:0269-3127    

DOI:10.1118/1.594357

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0269-3127    

DOI:10.1118/1.594346

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 of60Co 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.

ISSN:0269-3127    

DOI:10.1118/1.594347

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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‐Zmaterial 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.

ISSN:0269-3127    

DOI:10.1118/1.594348

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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‐cm3ion chamber, a 0.3‐cm3ion chamber, and a 0.1‐cm3silicon 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 anX–Yrecorder 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.

ISSN:0269-3127    

DOI:10.1118/1.594349

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0269-3127    

DOI:10.1118/1.594350

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 the60Co 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.

ISSN:0269-3127    

DOI:10.1118/1.594351

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 with6Li and7Li 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.

ISSN:0269-3127    

DOI:10.1118/1.594339

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0269-3127    

DOI:10.1118/1.594340

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY