ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1983.tb36192.x

出版商:Wiley    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1983.tb36193.x

出版商:Wiley    

年代:1998

数据来源: WILEY

摘要:

An improved reconstructed stereoscopic x‐ray image display system, based on the principle of binocular parallax, is described. Pulse‐synchronized operation of two triode x‐ray tubes and one plumbicon TV camera is employed to separate the left and right images for presentation to the two eyes, respectively. The principle of binocular perception, quantitative characteristics of the stereoscopic depth, the overcoming of the inertia problem of the single‐camera scheme, and a way to achieve low x‐ray dosage and enhanced contrast of image are presented. Experimental results and applications are discussed.

ISSN:0094-2405    

DOI:10.1118/1.595365

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new approach to the measurement of the spatial resolution of a computed tomography (CT) scanner system is presented. The method is based on a direct least‐squares fit of an analytical expression to a set of data obtained from a CT image of the interface between two materials. The implementation of the method in connection with the G. E. RTPLAN computer configuration is described. The method has been applied in determining the resolution of an EMI‐7070 scanner and it is shown that the assumption of uniformity of the system resolution across the CT image is fulfilled within the accuracy of the present method. The reproducibility of the method has been estimated from a series of spatial resolution determinations performed on ten images taken with identical scan parameters. The standard deviation of this series was 3.2%.

ISSN:0094-2405    

DOI:10.1118/1.595328

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Central axis electron beam depth dose distributions can be transformed by replacing dose by fluence and depth by a measure of angular dispersion. This transformation was applied to a set of broad beam central axis depth dose distributions calculated by Monte Carlo code for beams with initial energies ranging from 1 to 60 MeV in homogeneous media of water, aluminum, and copper. The resulting fluence distributions belong to a family of curves that can be parametrized by a single‐valued function of initial beam energy and medium and can be used to calculate fluence distributions for accelerators. Fluence curves can be easily transformed to depth dose curves.

ISSN:0094-2405    

DOI:10.1118/1.595329

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In clinical radiation oncology, it is commonly reported that complications of normal tissue occur more readily at larger field sizes for a given dose and recurrence of disease is observed more frequently from the larger tumors for a given dose. Cognate phenomena have long been observed in the study of the strength of materials. That is, the larger specimens will fracture under less applied stress, breakdown under less applied voltage, corrode in a shorter time, etc. The statistical theory of extreme values has provided both a rational explanation and a technique for exploitation of these “size effects” on the likelihood of specimen failure. This theory describes the relation which exists between the parameters (in particular, the location parameter) of the frequency distributions of the extreme values [smallestx(1)and largestx(n)] in a sample from a population of observationsxiand the sample sizen. It is shown in the present paper that the clinical failure phenomena are not inconsistent with the statistical theory of extreme values. The paper presents heuristic comparisons of the predictions of this theory with the received clinical observations of the effect of the size of the volume of irradiated tissues on the likelihood of occurrence of the misadventures of clinical radiation oncology: recurrence of disease and complication of normal tissue. The concordance of observations and predictions is acceptable. The quality and quantity of the currently available data have precluded the construction of any apodictic representations.

ISSN:0094-2405    

DOI:10.1118/1.595366

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is known that the ratio (R) of the detected coherent and Compton scattered photons from bone can be used in order to determine its mineral density. This technique utilizes the dependence of the coherent scattering on the effective atomic number (Z̄) of the scattering medium. It is generally accepted that a small scatter angle is preferred in order to ensure adequate counting statistics by favoring the detection of more coherent photons. Moreover, it has been assumed that a change in the scatter angle does not affect the sensitivity of the measurement. Our theoretical calculations for 60‐keV photons and for the range ofZ̄ that corresponds to trabecular bone, indicate that increasing the scatter angle results in a stronger power dependence of the measured ratio onZ̄. This implies that by increasing the scatter angle, smaller changes in the mineral density can be detected, thus improving the sensitivity of the measurement. This effect was investigated experimentally by using a collimated beam of 59.54‐keV photons from Am‐241 (44.4 GBq) and a collimated intrinsic germanium detector. Solutions of K2HPO4with different concentrations were used in order to simulate trabecular bone. The scatter spectra were recorded for all solutions at six scatter angles between 37° and 98° and the value ofRwas computed for each spectrum. The sensitivity of the measurement, evaluated from these experiments increased, with the increase of the scatter angle.

ISSN:0094-2405    

DOI:10.1118/1.595330

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The fundamental operations of nuclear magnetic resonance (NMR) imaging can be formulated, for a large number of methods, as sampling the object distribution in the Fourier spatial‐frequency domain, followed by processing the digitized data (often simply by Fourier transformation) to produce a digital image. In these methods, which include reconstruction from projections, Fourier imaging, spin‐warp imaging, and echo‐planar imaging, controllable gradient fields determine the points in the spatial‐frequency domain which are sampled at any given time during the acquisition of data (the free induction decay, or FID). The detailed time dependence of the resulting trajectory of sample points (thektrajectory) determines the relative weight and accuracy with which image information at each spatial frequency is measured, establishing theoretical limitations on image quality achievable with a given imaging method. We demonstrate here that these considerations may be used to compare the theoretical capabilities of NMR imaging methods, and to derive new imaging methods with optimal theoretical imaging properties.

ISSN:0094-2405    

DOI:10.1118/1.595331

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co‐60 γ‐ray beams. Either the Klein–Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source–surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

ISSN:0094-2405    

DOI:10.1118/1.595332

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We present a technique to simplify the calibration procedure of the dual‐energy scanning, using aluminum (AL) and plastic (PL) as basis materials. In essence, this method determines the “basis functions” of the basis materials at the chosen energies. This technique replaces the two‐dimensional calibration procedure, which is tedious, by two simple one‐dimensional calibration steps. In addition, this technique also provides an alternative approach for solving the dual‐energy problem.

ISSN:0094-2405    

DOI:10.1118/1.595367

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY