摘要:

Implicit in the concept of conventional tomography and in any attempt to characterize the tomographic process by a modulation transfer function is the assumption that the tomographic process is linear. A Fourier decomposition approach and an analysis of nonlinear contributions to the integrated tomographic image intensity are used in this paper to establish the validity of this assumption and to determine the mechanisms by which the tomographic process is effectively linearized.

ISSN:0094-2405    

DOI:10.1118/1.594400

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Presently, patient exposure and x‐ray tube kilovoltage are determined by image visibility requirements on x‐ray film. With the employment of image‐processing techniques, image visibility may be manipulated and the exposure may be determined only by the desired information content, i.e., by the required degree of tissue‐density discrimination and spatial resolution. This work gives quantitative relationships between the image information content and the patient exposure, give estimates of the minimum exposures required for the detection of image signals associated with particular radiological exams. Also, for subject thickness larger than approximately 5 cm, the results show that the maximum information content may be obtained at a single kilovoltage and filtration with the simultaneous employment of image‐enhancement and antiscatter techniques. This optimization may be used either to reduce the patient exposure or to increase the retrieved information.

ISSN:0094-2405    

DOI:10.1118/1.594405

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Solid‐state thermocurrent and radioconductivity (electrical conductivity during irradiation) experiments are described with attention to the feasibility of using ionic solids for radiographic imaging. The radioconductivity varies with temperature giving rise to temperature windows of potential usefulness. The electrophotographic image is formed at atmospheric pressure using Al2O3, collecting the charge on mylar film. Development is by the power cloud technique. A transparency, which can be viewed as a conventional radiographic image, is easily produced. A successful transfer image, free of electric discharge artifact, was produced.

ISSN:0094-2405    

DOI:10.1118/1.594505

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The resolution has been evaluated for an experimental x‐ray image intensifier employing three microchannel plates (MCPs) as the photon absorber and electron multipliers. The line spread function (LSF) was measured and used for determination of the modulation transfer function (MTF). The MTF was found to be independent of the incident photon energy from 20 to 150 keV. An additional measurement using a lead‐bar test pattern showed that the resolution exceeded 7 line pairs (lp)/mm. The factors influencing the resolution capabilities of the intensifier are discussed. The resolution is limited primarily by the 53‐μm center‐to‐center separation of the channels of the MCPs.

ISSN:0094-2405    

DOI:10.1118/1.594463

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is presented whereby the modulation transfer function can be calculated directly from the edge response function without having to find the line spread function as an intermediate step.

ISSN:0094-2405    

DOI:10.1118/1.594401

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Microdensitometers have both spatial and temporal finite responses which may lead to degradation in images being analyzed. These responses may be quantitated in terms of spatial and temporal modulation transfer functions (MTFs). The temporal response of microdensitometers is studied here. Specifically, the technique of differentiating temporal step‐function responses to determine the temporal MTF is presented. Experimental results illustrating the theory are described using a Baird Atomic microdensitometer.

ISSN:0094-2405    

DOI:10.1118/1.594402

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The photon response of silicon‐diode neutron detectors is analyzed theoretically and measured in the 15–25‐MeV region. The main mechanism for producing a response in the diode is shown to be the displacement of silicon atoms by scattering of electrons. If the photon source is an electron accelerator target, the response is mostly due to electrons originating in the target with a smaller contribution from electrons produced in the diode by photons generated at small angles to the beam.

ISSN:0094-2405    

DOI:10.1118/1.594403

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The accurate assessment of neutron flux and dose‐rate levels in a medical environment is a topic of much current interest. In this paper, a least‐squares data‐analysis technique has been used for extracting neutron spectra and related information from Bonner sphere data. This technique, incorporated in thefortran ivcodenflsis worthy of consideration as an alternative to the count‐rate ratios and iterative‐unfolding techniques used in the past. The analysis provides calculated total neutron flux density, dose‐equivalent rate, and average and median‐neutron‐energy information as well as a plot of integral neutron‐flux‐density spectra. The method allows the calculation of the statistical uncertainty of each of the above quantities, which has not always been possible with other analytical methods. Results of calibration and experimental data analysis are presented and compared to results of the iterative‐unfolding technique.

ISSN:0094-2405    

DOI:10.1118/1.594464

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Cobalt‐60 transmission measurements were made through an Alderson phantom utilizing a transverse axial tomographic device and a NaI (Tl) detector. Measurements were made on different sections of the phantom for as many as 162 angles and 120 linear increments. The attenuation coefficients were reconstructed using both convolution and algebraic reconstruction techniques. Three‐dimensional isodose distributions were obtained using the reconstructed attenuation coefficients. Comparison with standard treatment plans and measured isodose distribution using TLD techniques suggest that a more accurate isodose distribution may be obtained using the reconstructed attenuation coefficients, particularly in regions involving tissue heterogeneities.

ISSN:0094-2405    

DOI:10.1118/1.594393

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The behavior of siliconp–njunction radiation detectors used in the direct‐current short‐circuit mode without bias was examined under such load resistance which ensures operation in a temperature‐compensated state. The objective was twofold; to check whether the compensation is achieved and to investigate the extent of compensation shown at temperatures other than that initially selected as the operation point. It was found that the detector performance in various thermal conditions can be predicted from a knowledge of the behavior of the individual detector and circuit parameters and that it is possible to stabilize the detector response within ±2% over a relatively wide temperature range: 18°–40°C. However, in the case of devices which show thermal currents of large temperature sensitivity, compensation at small and at large dose rates needs to be considered separately.

ISSN:0094-2405    

DOI:10.1118/1.594394

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY