摘要:

A protocol is given to provide radiological physicists with an accurate means of measuring radiation doses on phantoms. Plastics, as well as water, are considered as phantoms. A new quantity, the cavity‐gas calibration factor, is introduced.(AIP)

ISSN:0094-2405    

DOI:10.1118/1.595446

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accurate high‐resolution x‐ray spectral distributions can be obtained with an intrinsic germanium crystal. This technique is difficult to implement since it requires expensive, complicated, and bulky equipment. If energy resolution requirements are relaxed, then the above drawbacks can be overcome and an x‐ray spectrometer that is small, inexpensive could be built. A technique that meets these requirements is described here. X‐ray spectra with about 7‐keV resolution can be achieved by obtaining transmission measurements through a number of differentK‐edge filters. Using these measurements, a system of equations is set up and solved giving the required spectral distribution. The technique has been tested using 80, 100, and 120‐kVcp x‐ray beams with total filtrations of 3.78, 5.78, 9.06 mm Al, and 3.78 mm Al+0.137 mm Ho. The results show that the calculated spectra closely resemble tabulated spectra. The errors ranged from 3% to 13%. The half‐value layers (HVL) were also calculated and compared to the HVL obtained from tabulated spectra and were found to differ by about 7%.

ISSN:0094-2405    

DOI:10.1118/1.595418

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Transmission data for 4‐MV bremsstrahlung beams have been measured with a combination of lead and aluminum attenuators. From these data, the original energy spectra have been reconstructed using an iterative least‐squares technique, previously evaluated by simulation studies. The spectra on the central axis for three similar 4‐MV linear accelerators indicated no significant differences. When studying the spectra at 5° and 9° off the central axis, that at 9° showed the expected increase of low‐energy photons. All these spectra showed a maximum photon energy of 4.5±0.2 MeV. When the magnetron power was reduced, the spectrum on the central axis shifted to lower energies and the maximum photon energy decreased to 3.5±0.2 MeV. The result of this experimental study confirms the conclusions from the previous simulation, that the numerical technique for analysis of transmission data can accurately represent 4‐MV bremsstrahlung spectra and detect differences in energy distribution with changes in machine tuning and position in the radiation field for a 4‐MV bremsstrahlung beam.

ISSN:0094-2405    

DOI:10.1118/1.595356

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x‐ray image intensifier are compared for nine monoenergetic beams of x rays. The agreement between measurement and theory of better than ±5% indicates that we have a sound understanding of the physical parameters controlling the DQE. It is shown that the fraction ofK‐fluorescent x rays escaping from the input phosphor is independent of incident energy. The number of electrons released within the x‐ray image intensifier (XRII) by an incident x ray has been measured. The mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.

ISSN:0094-2405    

DOI:10.1118/1.595357

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The activity of15O and13N per unit volume of air produced by a 25 and a 45‐MeV medical accelerator was determined by direct measurement. The accelerators were operated in such a fashion as to produce maximum activation of the treatment room air. Levels of the order of 1% or less of the maximum permissible concentration in air for15O and13N were found immediately after accelerator shutdown. Three different techniques for calibrations of the air detector were investigated.

ISSN:0094-2405    

DOI:10.1118/1.595358

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Time‐dependent subtraction techniques have tremendously improved the visualization of iodinated vessels. These techniques suffer from motion artifacts, however, since the mask image is acquired several seconds prior to the contrast image. AK‐edge subtraction technique for selectively imaging iodine using the characteristic x rays from a lanthanum secondary target is presented. TheKαlines of lanthanum closely straddle theKedge of iodine. A prepatient iodine filter will alter the ratio ofKα1toKα2intensities; by subtracting two images made with differentKα1/Kα2ratios, iodinated structures can be isolated. Since the method requires no mask image, motion artifacts are reduced. Preliminary images acquired with an image intensifier/photodiode array detector system are shown. Theoretical performance of this method is compared to other iodine‐selective imaging techniques on the basis of exposure and dose. The feasibility of using lanthanumKfluorescence for rapid clinical iodine‐selective imaging is discussed.

ISSN:0094-2405    

DOI:10.1118/1.595447

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The peripheral dose (PD), defined as the dose outside of therapeutic radiation beams, has been investigated for60Co, 4‐, 6‐, and 10‐MV x‐ray machines. The measurements have been carried out down to dose levels of about 0.1% of the peak dose in the beam, since that dose level may be of clinical importance in some situations. The PD measurements for the various machines are qualitatively similar, which allows the identification of a simple basic data set which can characterize the PD for any particular machine. The PD has been separated into two components: in‐phantom scatter dose and transmission (leakage) dose. Knowledge of the two components is important clinically when shielding is considered.

ISSN:0094-2405    

DOI:10.1118/1.595359

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have studied the angular distribution of scattered radiation at low energies for angles between 2° and 15° from the outward normal to the exit surface of several phantoms, with 1° resolution. A cryogenically cooled germanium detector was used to measure the spectra of the scattered radiation. The differential scattering fluences, or numbers of photons per unit solid angle per unit surface area, exhibit distinct peaks at angles in the vicinity of 5°, with the angular position being only slightly energy dependent but very material dependent. The scattered spectra show large changes as a function of angle, in some cases actually becoming harder than the exit unscattered beam. The significance of this behavior relative to the imaging of targets in mammography examinations is discussed.

ISSN:0094-2405    

DOI:10.1118/1.595360

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Monte Carlo computer model has been developed to study the propagation of light in tissues. Light attenuation is assumed to result from absorption and isotropic scattering. The model has been used to predict the distribution of absorbed dose in homogeneous tissues of different absorption/scattering ratios, for illumination both by external light beams and via implanted optical fibers. The photon flux into optical fibers placed in the tissue as detectors has also been investigated. The results are interpreted in relation to the use of visible light irradiation for photo radiation therapy.

ISSN:0094-2405    

DOI:10.1118/1.595361

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An important aspect of heavy charged particle radiotherapy is its ability to localize dose to the target volume. Current techniques generally employ beam delivery schemes which use range modulated beams in which the Bragg peak is spread out over a range of depths. The range modulation is constant over the entire beam cross section. This paper examines the potential gain from the use of variable range modulation over the beam cross section which can be achieved through beam scanning. An analytic technique has been developed to calculate a gain factor, related to integral dose, for a number of different charged particle beams. The dependence of the gain factor on tumor depth, diameter, and size has been examined. These analytic results have been compared with computer calculations of treatment plans simulating beam scanning and fixed modulation delivery techniques for actual clinical cases. The rationale for beam scanning, and its potential impact on tumor control is discussed.

ISSN:0094-2405    

DOI:10.1118/1.595419

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY