摘要:

X‐ray source filtration as a means to reduce patient dose while maintaining image quality was investigated for CT scanners. The CT values, their variances for various materials, and the surface dose to a cylindrical phantom were calculated for different filter thicknesses and composition as well as for different tube potentials. Thermoluminescent dosimetry indicated that the maximum dose could be predicted by calculation with an accuracy of 10% (±2 s.d.). The product of the variance of the CT values times surface dose was used to establish the appropriate thickness and composition of the filter, a figure of merit that was independent of dose and noise when the sole source of noise was Poisson statistics. This analysis indicated that source filter materials with an atomic number from 29 to 40 are optimum, and if aluminum is used, the minimum thickness, at 120 kVp, should be 4 mm.

ISSN:0094-2405    

DOI:10.1118/1.594609

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The radiographic contrast of metal screen‐film detectors was investigated in order to determine the contrast capabilities of these detectors applied to megavoltage x‐ray imaging. The film contrast γ was found to be independent of the metal screen composition. Measurement of the scatter‐to‐primary film dose ratio in contact geometry demonstrated that a thick front screen of either 1.5 g/cm2copper or 2.5 g/cm2lead provides optimum contrast for the photon energies studied (60Co and 4‐ and 8‐MV x rays). The same thicknesses were also found to be suitable in an air gap geometry which significantly improved the contrast compared to the contact geometry. Rear lead screens were found to provide no contrast improvement.

ISSN:0094-2405    

DOI:10.1118/1.594610

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A thermoluminescent (TL) glow peak has been shown to exist at 80°C in synthetic hydroxyapatite, in amorphous calcium phosphate, and in deproteinated rat bone mineral. This TL glow peak is excited by a thermal pretreatment at elevated temperature, followed by exposure to ultraviolet (UV) light. When measured on a cylic TL‐readout UV‐exposure basis after an original thermal pretreatment, the intensity of this 85°C glow peak displays a logarithmic decay curve characteristic of a transfer thermoluminescence process: A deep trap is activated during the annealing process, and the subsequent UV exposure causes charge transfer to the 85°C glow peak, as is verified using electron spin resonance. Exposure to the atmosphere appears to decrease the TL intensity levels, presumably due to water adsorption; vacuum drying and additional thermal pretreatment reverse this effect on the TL intensities. TL measurements are applied to the comparison of bone mineral samples of varying age. Increases in overall TL intensity are demonstrated between a four‐week old normal rat group and eight‐ and fourteen‐week‐old normal rat groupings. The TL intensity is also increased by metabolic disorder which causes changes in bone mineral chemistry and crystallography.

ISSN:0094-2405    

DOI:10.1118/1.594611

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The LSF's and MTF's of two high resolution rare‐earth screen‐film combinations were measured at two beam qualities. The two beam qualities were chosen to provide x‐ray spectral distributions either above or below theK‐edge of the screen phosphor. The LSF's were found to be photon energy dependent. This energy dependence is attributed to the generation and reabsorption of phosphorKx‐rays resulting in a broadening of the LSF.

ISSN:0094-2405    

DOI:10.1118/1.594612

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Administration of x‐ray contrast agents during heart catheterization examination increases the absorbed radiation dose in tissue. To estimate the dose absorbed by the blood of children undergoing diagnostic heart catheterization and angiocardiography, a number of measurements and calculations were conducted. First, entrance and exit exposures to the patient were measured with thermoluminescent dosimeters calibrated for the diagnostic x‐ray energy range. Second, a dose enhancement factor was calculated from mass energy absorption coefficients for various concentrations of the contrast media and at selected x‐ray energies. Third, the dose enhancement factor was estimated from survival of peripheral blood lymphocytes suspended in varying concentrations of the contrast agent during exposure to graded doses of x‐rays. Fourth, a mean absorbed dose to the patient's blood was calculated using (a) the dose enhancement factor determined above, (b) an estimate of the mean exposure in the irradiated body volume calculated from the entrance and exit exposure measurements, (c) an effective iodine concentration in the blood during the exposure time, and (d) a ratio correcting for the distribution and circulation of the blood. For eight pediatric patients monitored, absorbed doses to the blood ranged between 3 and 12 rad. These values were two to three times greater than the expected dose without administration of a contrast agent.

ISSN:0094-2405    

DOI:10.1118/1.594613

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This work considers the problems associated in megavoltage x‐ray dosimetry concepts with the current techniques used for normalizing the various dose parameters. The depth of transient charged particle equilibrium (CPE) is introduced as the optimum choice of the normalization point. Properties of this depth are discussed and its uniqueness as a measure of beam quality is illustrated.

ISSN:0094-2405    

DOI:10.1118/1.594614

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The MTF of metal screen film detectors used in radiation treatment verification has been measured at 4 and 8 MV x‐ray energies. The results show that lead screens provide better resolution than copper screens, and a single‐emulsion film offers considerable advantage over the traditional double‐emulsion film. A rear lead screen was found to seriously degrade the resolution properties of a front lead screen single‐emulsion film detector. The detector MTF was found to be energy dependent. In general, both the low and the high spatial frequency response decreased with increasing x‐ray energy. This, in part, accounts for the noticeable image quality difference between 4 and 8 MV radiographs.

ISSN:0094-2405    

DOI:10.1118/1.594615

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The determination of the physical density by Compton scatter densitometry is dependent upon the geometry used for the measurement. The diameter of the sample, the density range, and the scattering volume size and shape influence the relative electron density values obtained by such a system. This work presents the results of an experimental analysis of these parameters in which the importance of each is evaluated for samples of known density. A bias in the computed Compton density, due to multiple scattering and attenuation, is defined; and an expression relating the density bias to these geometrical parameters is presented. The importance of applying corrections to the computed physical density for samples of large diameter and/or high density (cortical bone tissue and large diameter cancellous bone tissue) is discussed.

ISSN:0094-2405    

DOI:10.1118/1.594616

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A system for imaging occult bronchogenic carcinoma by the fluorescence of previously‐injected, tumor‐specific compound hematoporphyrin‐derivative has been assembled and successfully used to locate a tumor l mm thick. The violet excitation source is a krypton ion laser coupled to fused quartz fiber light conductor. An electrostatic image intensifier attached to a standard flexible fiberoptic bronchoscope provides a bright image even at relatively low irradiance. A red secondary filter rejects most reflected background and autofluorescence. Sensitivity and contrast capability of the system should permit detection of a tumor less than 0.1 mm thick.

ISSN:0094-2405    

DOI:10.1118/1.594617

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is developed by which the computed tomography scans of a medium carried out at a number of diagnostic energies can be utilized to obtain theinsitu“Effective Atomic Number” and “Effective Density”. The electron depth dose curves in water when scaled by these factors estimate the actual electron depth dose distribution in that medium. It appears that the use of CT scans for electron beam treatment planning, in the management of cancer, is quite promising.

ISSN:0094-2405    

DOI:10.1118/1.594644

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY