摘要:

The immediate goal of clinically based x‐ray‐transmission computed tomography (CT) is to provide a measurement of the x‐ray linear attenuation coefficient in cross section with the ultimate goal of impacting on patient managerment and care. To do this with the accuracy needed for clinical goals requires the careful integration of x‐ray physics, detector technology, and mathematical reconstruction theory. Performance evaluation and quality assurance are necessary adjuncts to a CT scanning program. A number of investigative studies are under way.

ISSN:0269-3127    

DOI:10.1118/1.594381

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Compton profiles and total scattered intensities have been measured to determine the total sample mass, analyzed by an x‐ray probe energy‐dispersive analyzer. Under photon excitation, fluorescent x rays are emitted from the trace elements in a biological matrix. From incident radiation, the number of photons which are Compton and elastically scattered by low‐atomic‐number elements is directly proportional to the total specimen mass. Tissue specimen masses have been measured from Compton intensities using Zr Kα and Ma Kα excitation x rays and mass calibration standards based upon carbon scattering. This procedure has been extended to include lower‐energy excitation radiation, such as Cu, where the resolution of an energy‐dispersion system requires that the total scattered intensity be used to determine the sample mass. Trace element weight‐fraction concentrations are determined from this scheme with precisions of 1% in 2%, relying only upon information contained in the energy‐dispersive x‐ray spectrum. By adjusting for the difference between tissue and carbon scattering, the accuracy of the elemental weight‐fraction concentrations is brought to within l0% of elemental concentrations measured by atomic absorption spectroscopy, for samples weighing up to 25 mg. In the case of heavier samples, absorption corrections are necessary to achieve this accuracy.

ISSN:0269-3127    

DOI:10.1118/1.594388

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The suitability of the intenseKαx rays of terbium emitted in the electron‐capture decay of159Dy for use in transmission imaging and bone mineral analysis is investigated. It is found that this radionuclide offers all the advantages of radiations from210Pb and none of the disadvantages inherent in the use of the latter. Yields of theKαandKβx rays of terbium and the 58‐keV γ rays emitted in159Dy decay are determined using a high‐resolution Si(Li) photon spectrometer. Attenuation coefficients for these photons in gadolinium (critical) absorber are measured in a narrow‐beam geometry. For TbKβx rays, whose average energy is only about 0.4 keV above theKedge of Gd, our experimental attenuation coefficient is about 10% less than the theoretical value given by Storm and Israel. Transmission images of regular and irregular bones obtained using159Dy are presented.

ISSN:0269-3127    

DOI:10.1118/1.594307

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The depth of penetration of heavy charged‐particle therapy beams is sensitive to the density of tissues traversed. Maximum depth of dose contours will vary appreciably as the beam passes through bone, muscle, lung, and air or gas. Calculations suggest that beam activation of the short‐lived positron‐emitting isotope15Oin vivowill permit localization of proton therapy beams with reasonable detected‐event density and dose. Preliminary tests of this method indicate that the beam can be located at depth with a typical dose of 15 rad, using a large field‐of‐view positron camera on‐line. This technique is also applicable to other heavy charged‐particle beams, negative pions, and heavy ions.

ISSN:0269-3127    

DOI:10.1118/1.594389

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fast‐neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three‐component model, from measurements made with three ionization chambers (tissue‐equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35‐MeV D+on Be). This method may prove useful for measurements in phantoms as well.

ISSN:0269-3127    

DOI:10.1118/1.594382

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Multiplicative corrections for percent depth‐dose values were measured for situations with nonmaximal backscatter because of reduced thickness of the irradiated phantom. Data were obtained for common clinical field sizes for a60Co beam as well as beams from a 2.5‐MV and a 4.0‐MV generator. Functional forms, which summarize the results and include field size effects, depth, and the thickness of the backscatter medium as variables, were obtained by regression analysis.

ISSN:0269-3127    

DOI:10.1118/1.594308

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have measured the effect of a 10‐kG magnetic field on the dose distribution of electrons in a polystyrene phantom. Isodensity plots and depth‐dose curves are presented for 22‐ and 28‐MeV electron beams with and without the magnetic field applied. The measurements show that magnetic fields as low as 10 kG can produce a substantial modification of the absorbed dose distribution. When compared with the zero‐magnetic‐field distribution of the same energy, the magnetic field significantly improves theDmax‐surface dose ratio and increases the fall off in dose past theDmaxregion.

ISSN:0269-3127    

DOI:10.1118/1.594309

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The relative sensitivity of various materials for the measurement of half‐value‐layer thickness has been calculated for bremsstrahlung x‐ray spectra in the energy range 5–40 MeV. It is concluded that low‐atomic‐number materials such as water are more sensitive to changes in spectral quality of megavoltage x rays than high‐atomic‐number materials such as lead.

ISSN:0269-3127    

DOI:10.1118/1.594390

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Central‐axis percentage depth doses and tissue–maximum ratios (TMR) for 45‐MV photon beams from a betatron have been measured in water. Also the influence of field size and collimator scatter on the dose in the buildup region have been investigated. The maximum dose for TMR has been shown to occur at a point about 2 cm deeper than the maximum dose for percentage depth dose. This difference is significant in the characterization of the photon beam at this high energy. The measured physical data have been computerized for use in routine treatment planning. Computer‐generated beams have been found to be in close agreement with measured isodose curves. Computer‐generated isodose distributions for typical clinical irradiation techniques have been verified using RP/V film in an Alderson phantom.

ISSN:0269-3127    

DOI:10.1118/1.594391

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The calibration (in rad/nC) of an air‐filled tissue‐equivalent ionization chamber for neutrons of l5‐MeV average energy was determined by measuring absolute fluences and calculating the kerma per unit fluence. The neutron calibration determined from the γ‐ray calibration and application of the Bragg–Gray relation was found to be 4% higher than that based on the fluence measurement. Additional data were taken to obtain calibration factors with tissue‐equivalent gas; the same difference betwen the two methods was observed.

ISSN:0269-3127    

DOI:10.1118/1.594310

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY