摘要:

The purpose of this study is to examine the diagnostic potential of a scanning digital radiography (SDR) system currently used clinically as a CT localization device. The technique of contrast‐detail‐dose analysis is used to compare the low‐contrast sensitivity and dose efficiency of an SDR unit with standard radiography (SR). The theory of threshold perceptibility in both digital radiography and SR is described, including the effects of scattered radiation and antiscatter devices, object attenuation, geometry, system image processing, and recorder quantum efficiency. The concept of dose efficiency is defined and derived from the contrast‐detail‐dose data. The SDR system is shown to operate with a threshold signal‐to‐noise ratio of 7.3±1.2 for large area detection (d≳2 mm); to be relatively scatter‐free; and to be as much as 100× more dose efficient than SR for visualization of large low contrast objects (C≲0.04,d≳2 mm).

ISSN:0269-3127    

DOI:10.1118/1.594958

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The “quality of radiation” for a high energy x‐ray beam can be specified by its attenuation curve in a selected material. The inverse Laplace transform of the attenuation curve can be used as an approximate indication of the energy spectrum of the beam. Existing mathematical procedures for this purpose have been evaluated and were found to poorly represent measured transmission data for 4 MV x‐rays from a linear accelerator. The transmission data between 1 and 0.002 could be fitted within the experimental uncertainty by expressing the logarithmic transmission as a second order polynomial of attenuator thickness. The inverse Laplace transform then becomes a Gaussian function of the attenuation coefficient. This new version of “attenuation analysis” provides a practical method for specification of the quality of the radiation in this energy range.

ISSN:0269-3127    

DOI:10.1118/1.594959

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

With the increased sophistication of lasers used for patient positioning in radiotherapy comes increased possibilities for mistakes in installing the lasers, which can result in serious patient positioning errors. Two principles must be followed when mounting cross‐projecting lasers: (1) the horizontal line must be on the same level as the treatment unit isocenter, and the vertical line must be coplanar with the gantry rotation, and (2) If one of the lines of the cross rotates, it must be the line which is perpendicular to the line joining the two holes from which the laser beams exit the laser casing.

ISSN:0269-3127    

DOI:10.1118/1.594960

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Buildup and surface‐dose measurements were taken for the 6 MV photon beam from a Therac 6 linear accelerator manufactured by Atomic Energy of Canada Limited (AECL) with and without a lucite blocking tray in place. Further measurements were made with a copper filter designed to reduce secondary electrons emitted by photon interactions with the Lucite tray. The results are discussed in relation to skin‐sparing for radiation therapy patients. The measurements were made with a fixed volume PTW parallel‐plate ionization chamber and corrected to zero‐chamber volume. The results were found to be consistent with similar measurements taken with a variable volume extrapolation chamber.

ISSN:0269-3127    

DOI:10.1118/1.594961

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Radiographic resolution and image sharpness are analyzed as a function of magnification for a few screen/film systems and a microfocal spot x‐ray tube. Resolution and sharpness are described in terms of both MTF and effective aperture. The analysis demonstrates advantages of magnification for any combination of radiographic screen/film system and x‐ray focal spot, and, in addition, allows one to compare different screen/film systems each used at different magnifications. Such an analysis is applicable to any radiographic system and may serve as a guide in selecting screen/film systems for use in magnification procedures.

ISSN:0269-3127    

DOI:10.1118/1.594962

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Northwest Center for Radiological Physics has used a well‐defined protocol in the review of radiotherapy machines. The central axis depth dose and off‐axis factor of fifteen Clinac‐4's are reported. The range in depth dose was on the order of 3.5% and the range in off‐axis factor on the order of 8%. Similar measurements on a Theratron‐80 Cobalt‐60 teletherapy machine are reported to demonstrate the reproducibility of the measurement technique.

ISSN:0269-3127    

DOI:10.1118/1.594963

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous authors have suggested that high pass spatial filtering of axial transverse and circular tomograms would improve the perceived image. A preliminary study here shows that good signal‐to‐noise ratio is critical for the success of image processing for the contrasts obtainable in the clinical situation. The successful image processing of a small object on the Philips Polytome is demonstrated, as well as a clinical example. A similar trial with the Toshiba axial transverse tomography machine was not successful due to lower subject contrast with higher kVp techniques. A derivation of the optical transfer function of circular and axial transverse tomograms is given, with emphasis on those features that image processing can improve. To accomplish image processing, two methods employing incoherent light were investigated. Either method may be regarded as an analogue equivalent to the mathematical operations employed by digital computers in the reconstruction of CT images. First, a simple all‐purpose image processor is described that makes use of the Herschel effect. Second, a simple film copying technique that simulates a high pass spatial filter is introduced and used to improve tomographic images.

ISSN:0269-3127    

DOI:10.1118/1.594964

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The energy spectrum of neutrons produced by 35 MeV protons on 1.6 cm thick beryllium has been measured at 0 ° by means of proton recoil spectroscopy. We find substantial agreement with other measurements.

ISSN:0269-3127    

DOI:10.1118/1.594965

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

One of the most accepted methods of characterizing the dose in computed tomograhy (CT) is by measuring the dose profile. Thermoluminescent dosimeters (TLD's) arranged in a stack are scanned in a plexiglass phantom. Unfortunately with this system there is no assurance that the scan properly intercepts the stack. Mispositioning will not be apparent until the TLD's are read, entailing at least a 24 h delay before rescanning. We have designed a simple alignment probe that insures that the scan will be centered on the stack to within 1 mm.

ISSN:0269-3127    

DOI:10.1118/1.594966

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594967

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY