摘要:

The effect of phase on the quality of diagnostic radiological images is investigated on a theoretical basis and with the use of the computer simulations. Phase effects are divided into two categories: linear phase shift and asymmetric phase shift. Using a model of the x‐ray focal spot distribution composed of two Gaussian functions, it is shown that the phase transfer function (PTF) is generally a continuous nonlinear function of spatial frequency. The linear phase shift serves only to shift the location of the image. The asymmetric phase shift in the model studied tends to accompany an improvement in the modulation transfer function (MTF) in the midfrequency region while introducing “wows” into the image. Contrast reversals are shown to be inherent in line spread functions (LSFs) which do not necessarily include linear or asymmetric phase effects, i.e., they are the result of another phenomenon.

ISSN:0094-2405    

DOI:10.1118/1.594282

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Proton‐induced x‐ray fluorescence has been used to determine the concentration of trace elements in human prostate and liver tissue samples. Ca, Fe, Cu, and Zn were observed in all specimens. Small amounts of Br were detected in most of the prostates, and in nearly half of the livers. The data indicate that the Fe content decreases in malignant liver tissues and the quantities of Cu and Zn are also decreased, but to a lesser extent.

ISSN:0094-2405    

DOI:10.1118/1.594233

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The absorbed dose in lung and beyond lung as a result of increased lung transmission of x and γ irradiation is described. The correction factor used to calculate the absorbed dose is a function of beam energy, field area, lung density, and lung and soft tissue depth. Agreement between measurements and calculations in the Alderson phantom is within 3%. An example of how this technique can be used is described.

ISSN:0094-2405    

DOI:10.1118/1.594234

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The radiant intensity along the anode–cathode dimension of a line‐focus x‐ray focal spot tends to peak at the center and decrease gradually to zero at the edges. In recognition of this varying intensity, the ICRU has recommended that the anode–cathode dimension of the focal spot be reported as 0.7 times the dimension measured with a pinhole camera. In this paper more objective correction factors are developed. These correction factors are based upon comparison of calculated MTFs of focal spots with Gaussian and cosine intensity distributions with the MTF for focal spots with a uniform intensity distribution.

ISSN:0094-2405    

DOI:10.1118/1.594238

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Fourier decomposition approach is used to study the imaging properties of conventional tomography. Spatial frequency response curves (MTFs) are calculated for both linear and axial transverse tomography. These curves depend on the product of the spatial frequency of the sinusoidal density variations in thin layers parallel to the tomographic plane and the distance between such layers and the tomographic plane. Based on the spatial frequency response curves, a quantitative definition of tomographic layer thickness is given. Furthermore, the spatial frequency response curves suggest that unattenuated low‐frequency information from outside the tomographic layer limits the resolution in conventional tomograms and that high‐pass spatial filtering of the image may substantially improve the diagnostic quality of tomographic images, particularly in the identification of boundaries.

ISSN:0094-2405    

DOI:10.1118/1.594239

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple method to measure the line spread function of computed tomographic scanners has been developed. The line spread function of the EMI‐Scanner was measured and the MTF of the scanner was calculated. The reproducibility of the method indicated this technique could be utilized to compare the spatial resolution of various computed tomographic scanners.

ISSN:0094-2405    

DOI:10.1118/1.594283

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Statistical fluctuations, or noise, in reconstructed tomograms are analyzed as a function of x‐ray energy. The result is essentially constant for energies above 50 keV, assuming constant dose and spatial resolution, and is in close agreement with the actual performance of the EMI‐Scanner (water‐bag model). The noise is inversely proportional to the square of cell dimensions. For scanners without a water bag, the theoretical noise is approximately 30% less.

ISSN:0094-2405    

DOI:10.1118/1.594240

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The results of neutron‐transport flux‐density and dose rate calculations for implantable Californium‐252 point and line sources in essentially infinite tissue‐equivalent material are presented. The point‐source flux densities were obtained from a discrete ordinates calculation, and the point dose rates were established by multiplying the flux densities by their appropriate kerma factors. Line‐source dose rates were evaluated by integrating the point dose rates over the length of the line source. Dose‐rate data are given within a 20×20‐cm region from the source center for source lengths of 1.5, 2, and 3 cm. The dose rates established by these calculations showed good agreement with an independent Monte Carlo calculation. Detailed point‐source flux‐density data as a function of energy and position are also given.

ISSN:0094-2405    

DOI:10.1118/1.594284

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new mode of localin vivoactivation analysis following neutron therapy in cancer patients has been explored at the Naval Research Laboratory Cyclotron Facility. The level of the therapeutic neutron dose administered to a patient presents a unique opportunity to use a Ge(Li) detector to obtain rapidly a high‐resolution γ‐ray spectrum of the activity induced in and around the tumor mass. Methods of analysis and local geometrical effects are discussed.

ISSN:0094-2405    

DOI:10.1118/1.594285

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Isodose distributions in a tissue‐equivalent phantom produced by fast neutrons from 50‐MeV deuterons incident on a thick beryllium target exhibit strong forward peaking, particularly for large fields. The design by use of decrement lines and the construction of polyethylene filters used to “flatten” those distributions are discussed and the results are illustrated. Also, the compromises of central‐axis attenuation versus effective filter width and of off‐axis peaking versus depth of “flattening” are discussed.

ISSN:0094-2405    

DOI:10.1118/1.594286

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY