摘要:

The introduction of negative pions into cancer therapy has required the construction of large new proton accelerators together with special magnetic systems to form and direct the pion beam to a patient. A summary is presented of the fundamental properties of pions and of the methods used to study the therapeutic beams. The dosimetry of these beams requires the use of the older techniques as well as new methods for determining the different LET components. The data for a number of beams is given and the utilization of this data in treatment planning is reviewed. An important problem for therapy is the behavior of inhomogeneities in the pion beam, and experimental methods are described which illuminate this problem. The studies of the effects of inhomogeneities in a beam point the way toward fruitful comparisons with the computerized treatment planning codes known as PION‐1 and PIPLAN. A useful step in treatment is the verification of doses in patients during therapy. For this purpose the new methods for measuring the high LET doses in patients are described as well as a timing measurement for checking the stopping effect of the tissues as obtained from the CT scans.

ISSN:0269-3127    

DOI:10.1118/1.594868

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fission track detectors were applied to a unique problem in neutron dosimetry. Measurements of neutron doses were required at locations within a tumor of 1 cm diameter implanted on the back of a mouse and surrounded by a square array of four252Cf medical sources. Measurements made in a tissue‐equivalent mouse phantom showed that the neutron dose rate to the center of the tumor was 2.18 rads μ g−1h−1±8.4%. The spatial variation of neutron dose to the tumor ranged from 1.88 to 2.55 rads μ g−1h−1. These measurements agree with calculated values of neutron dose to those locations in the phantom. Fission track detectors have been found to be a reliable tool for neutron dosimetry for geometries in which one wishes to know neutron dose values which may vary considerably over distances of 1 cm or less.

ISSN:0269-3127    

DOI:10.1118/1.594874

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A novel method is presented for estimating regional myocardial blood flow (RMBF) using x‐ray computed tomography (CT). Two major setbacks are removed from the existing methods; namely, the requirement for intra‐arterial bolus injection of tracer, and the inability to determine regional volumes of tracer distribution when the corresponding partition coefficients are unknown. The mathematical model developed for RMBF estimation combines both the tracer dispersion process and the CT measurement process. Intravenously administered contrast media (tracer) is assumed to be perfectly mixed by the myocardium. Tracer dynamics, as measured by CT (CT♯s. vs time curves) in the myocardial regions and in the left ventricular chamber, can then be used to compute cardiac output, regional volumes of tracer distribution, and absolute RMBF. These computations use the zeroth and the first moments of the measured CT♯s. vs time curves. Errors due to partial volume effects, and methods for their correction, are analysed. This formulation is readily adapted to existing computed tomographic systems having the capability to produce multiple sequential CT scans.

ISSN:0269-3127    

DOI:10.1118/1.594875

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Mercury exposure in dental offices may represent a health hazard. Previous studies evaluated mercury exposure through blood, urine or hair analysis. The mercury content of body fluids in hair depends on the time since mercury exposure, mode of intake and its excretion, and sequestration by different tissues. Utilizing an x‐ray fluorescence technique, the mercury burden in bone, liver, lung, spleen, and kidney was measuredin vitro. The x‐ray fluorescence, XRF, and chemical assays of the mercury content of these samples correlated. The XRF technique was used to assay the mercury content of tissues irradiatedin vivo. It was found that the mercury signal arising at a depth in tissue was reduced as the thickness of overlying material increased. Thus, the XRF assay for tissuesin vivois confined to organs near the surface. The mercury content of the superficial layers of the head and wrist were determinedin situfor a dental population. Concentrations as low as 20 μg/g could be detected with an exposure of 60 mR. Of the 298 dentists evaluated, 28% had greater than 20 μg/g of mercury in the head.

ISSN:0269-3127    

DOI:10.1118/1.594876

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The 18 MV photon beam characteristics of a Therac 20 Medical Linear Accelerator manufactured by Atomic Energy of Canada Ltd, are presented. Tissue phantom ratios (TPR's) and percent depth dose data are given; for a 10×10 cm field, the percent depth dose at a depth of 10 cm is 78.5 (SSD 100 cm). The relative dose factors (RDF'S) are given and are analyzed to elucidate the relative contributions from phantom scatter, collimator scatter, and backscatter from the top of the collimators into the monitor chambers. The effect of field size and depth on the penumbra is described. Crossplots of the beam at a depth of 5 cm indicate that the flattening filter could be improved; there are hot spots of 108% near the corners of 40×40 fields.

ISSN:0269-3127    

DOI:10.1118/1.594833

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Laplace transform theory is used to investigate the relationship between x‐ray spectra and the corresponding attenuation curves. It is shown that in principle, each is sufficient to determine the other; but in practice, experimental considerations lead one to require conformity with some preconceived mathematical model.

ISSN:0269-3127    

DOI:10.1118/1.594834

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A direct, noninvasive method for measuring absolute values of specific capillary blood flow in living tissue is described. The method is based on the photon activation,in situ, of tissue elements and the measurement of the subsequent decay of the positron activity induced, employing coincidence detection of the photon pairs produced in positron annihilation. Analysis of the time‐dependent coincidence spectrum reveals the contribution to the total signal from the decay of15O, from which the specific capillary blood flow in the imaged, activated volume is ultimately determined. By virtue of its introduction of the radioisotope of interest (15O) directly and uniformly into the tissue volume under investigation, the method described permits both the nonperfused and well perfused fractions of an activated volume to be estimated and hence, the average specific blood flow within imaged tumor volumes to be computed. The model employed to describe and analyze the data is discussed in detail. Results of application of the technique to measurement of specific blood flow in rhabdomyosarcoma tumors grown in WAG/Rij rats are presented and discussed. The method is shown to be reliable and well suited to studies designed to determined the effects of various agents, such as heat, radiation and drugs, on tumor blood flow.

ISSN:0269-3127    

DOI:10.1118/1.594835

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The image information transfer efficiency for five x‐ray fluorescent screens (calcium tungstate, barium halide, and three rare earth screens) has been experimentally determined with monoenergetic x‐ray beams at energies of 18, 22, 32, 49, 51, 58, and 69 keV. The transfer efficiency, which is defined by the ratio of the output to input signal‐to‐noise ratios, was determined from measurements of (a) the fraction of incident x rays absorbed in the screen and (b) the statistical distribution of the number of light photons emitted from the screen per absorbed x ray which was determined by light photon counting techniques. Comparisons of the information transfer efficiency, the average number of light photons emitted per absorbed x ray, and the light output energy per Roentgen are given for the above screens and x‐ray energies.

ISSN:0269-3127    

DOI:10.1118/1.594836

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have devised a transit dose technique for fast neutron therapy treatment planning based on the16O(n,p)16N reaction in recirculating water, and have determined the effect of simulated bone and lung inhomogeneities in phantom. An effective threshold of 10.2 MeV in the16O(n,p) reaction is exploited to detect transmitted neutrons without the need for detector collimation. This system has been demonstrated with 14 MeV (d,T) neutrons and with cyclotron producedp(42)+Be neutrons.16N decays to the 6.13 MeV excited states of16O in 7.14 s, allowing for easy identification by NaI(Tl) and for rapid recirculation. The transmission of fast neutrons can thus be related to an effective thickness of soft tissue, providing a rapid and direct measure of the effects of inhomogeneities under actual treatment conditions, with the 10 MeV threshold providing a useful degree of insensitivity to multiply scattered neutrons. Equivalent thicknesses of compact bone and lung relative to water were found to be 1.4 and 0.34 respectively, closely resembling the effective thicknesses for Cobalt‐60 gamma rays.

ISSN:0269-3127    

DOI:10.1118/1.594837

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Information contained in the x‐ray energy spectrum can be used to produce selective radiographic images of bone or soft tissue. A method has been devised to separate bone and soft tissue based upon differences in photoelectric absorption and Compton scattering using an appropriate combination of images obtained with radiographic exposures at 70 KVP and 140 KVP. Since photoelectric absorption is highly dependent upon atomic number, high atomic number materials such as calcium can be easily separated from water density substances. Using a prototype system for line‐scanned radiography, selective subtraction of bone or soft‐tissue has been implemented. Because this method uses a conventional broad‐spectrum x‐ray source, it was necessary to develop a nonlinear polynomial approximation to estimate tissue and bone thickness. The model was verified with phantom studies using water and aluminum. The application of this dual‐energy bone and soft‐tissue separation to chest radiography is demonstrated. This method allows accurate estimation of tissue and bone thickness and should find application to chest radiography for improved lesion detection and for bone mineral assessment.

ISSN:0269-3127    

DOI:10.1118/1.594957

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY