摘要:

The star test pattern image contains a large amount of information about the focus OTF which is not presently used. A mathematical analysis of these images for the special case of separable and symmetrical focal spots is presented. Circular blur loci are identified and their changes over the imaging field are described. Harmonic distortion and its application are described. Sources of measurement error are discussed.

ISSN:0094-2405    

DOI:10.1118/1.594379

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the Siemens star image, exact determination of the first disappearance frequency, which is used to measure the focal spot size, is difficult since the disappearance band has a finite width and the image also has other artifacts. The origin of these artifacts and their appearance was studied by Siemens star image simulation on a digital computer. The simulated images were manipulated by using many different modulation and phase transfer functions. It is shown that the bending of spokes is not related to zero contrast; exact triplet splitting can occur only at the disappearance frequency, and therefore splitting is a valuable indicator of that frequency.

ISSN:0094-2405    

DOI:10.1118/1.594352

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The basic theory of field characteristics is discussed. It is shown that the point spread function and optical transfer function (OTF) of geometric unsharpness at any arbitrary field position can be derived from those at the central beam position. At all locations in the radiation field, the OTF in a direction parallel to the x‐ray tube cathode–target axis does not depend on the field positionX, which is perpendicular to the to the tube axis, whereas the OTF in a perpendicular direction does not depend on the field positionY, which is parallel to the tube axis. The theoretical result is used to demonstrate the field characteristic of the uniform, square focal spot.

ISSN:0094-2405    

DOI:10.1118/1.594378

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of asymmetric focal spots in blood vessel imaging is investigated by computer simulation. Phase distortion effects are studied in isolation from MTF effects. Calculations are done for a number of symmetric and asymmetric foci with identical standard deviations. It is concluded that blood vessel image degradation due to focus asymmetryper seis small and that the LSF standard deviation is a useful size parameter for both symmetric and asymmetric foci.

ISSN:0094-2405    

DOI:10.1118/1.594297

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In our previous reports on absorption‐edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1‐kVp, 2‐filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of ±2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accommodated because only two x‐ray energies are involved in the imaging process. Use of a 3‐kVp, 3‐filter technique with logarithmic signal processing is shown to solve this general problem. Computer simulations show that 1‐mg/cm2iodine may be imaged in the presence of 10 cm or more tissue variations and 2000‐mg/cm2bone variations.

ISSN:0094-2405    

DOI:10.1118/1.594298

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A sensitive and relatively simple method for obtaining an index of the spectral quality of high‐energy x rays is presented. The method is based upon the use of photoactivation threshold detectors. Photonuclear reactions produce measurable amounts of radioactivity for radiation exposures of several thousand roentgens. Since the threshold energies for photonuclear reactions vary for different nuclei, the ratio of radioactivity induced in two appropriately selected foils is very sensitive to the x‐ray spectrum. Photoactivation ratios (PAR) have been measured for 20–35‐MV x rays using Co, Cu, Y, Zr, and Au activation foils. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra, comparison of high‐energy accelerators, and the measurement of variations of spectral quality control at different points in an irradiated volume.

ISSN:0094-2405    

DOI:10.1118/1.594299

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A sensitive and relatively simple method for obtaining an index of the spectral quality of x rays in the 4–15‐MV range is presented. The method is based upon the simultaneous production of115mIn by inelastic scattering of x rays by an indium foil and the production of116mIn by neutron capture, where the neutrons are obtained from the photodisintegration of deuterium. Since the cross sections for the production of115mIn and116mIn have markedly different energy response functions, the ratio of radioactivities is very sensitive to the incident x‐ray spectrum. Using a detector consisting of an indium foil sandwiched between two plastic bottles of D2O, photoactivation ratios (PAR) have been measured for 4‐, 6‐, and 10‐MV x rays. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra and comparison of high‐energy accelerators.

ISSN:0094-2405    

DOI:10.1118/1.594300

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The angular dependence of the ESR line shape from polycrystalline samples due to incomplete randomness in the orientation of the crystallites is calculated for centers with axial symmetry. The effect of a preferential axis of orientation is considered and the results are applied to radiation produce centers in bone.

ISSN:0094-2405    

DOI:10.1118/1.594301

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements of dynamic shear compliance versus dead time at room temperatures are reported for a sample of rabbit bone and several samples of human bone, including an extracted wisdom tooth. In all cases an abrupt, second‐order transition is observed in one or both components of the audio‐frequency complex shear compliance,Jb*=Jb′−iJb″, at about 5 hours after death; this is considered to be an indication of the life‐to‐death transition in bones.

ISSN:0094-2405    

DOI:10.1118/1.594383

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three‐dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed‐length water‐bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross‐sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.

ISSN:0094-2405    

DOI:10.1118/1.594302

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY