摘要:

From the inauspicious beginning arising in the aftermath of a reactor accident in 1957,in vivoanalysis of body elements by neutron activation has become an important tool in medical research. In particular, it provides a much needed means to make quantitative assessments of body composition of human beingsin vivo. The data are useful both for basic physiological understanding and for diagnosis and management of a variety of diseases and disorders. The paper traces the development of thein vivoneutron activation technique in the past quarter of a century from basic systems to the present state of the art facilities, and discusses the advantages and disadvantages of the various systems. A scan of some of the numerous clinical applications that have been made of this technique reveals the broad potentialities ofin vivoneutron activation. The paper also considers alternative routes of future development and raises some of the questions now faced in making the techniques more widely available to both medical practitioners and medical investigators.

ISSN:0094-2405    

DOI:10.1118/1.594899

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A phantom consisting of a gel closely resembling human liver onB‐mode scanning, and containing a rounded zone of decreased echogenicity, was used to create a series ofB‐mode ultrasonic images containing simulated “lesions” in random locations on the final recorded image. A homogeneous gel phantom was used to create a series of blank images. Images were recorded in three display formats: conventional white on black background, conventional black on white background, and black on a white background with luminance normalized to that of the conventional white on black image. Six subjects were presented with a randomized series of scans in the 3 display formats, and receiver operating characteristic analysis was used to assess the effect of display format on low‐contrast detectability. Detectability of low‐contrast “lesions” was lower in a high contrast image with black on white background compared to the medium contrast image in white on black format. Detectability with the black on white format having normalized luminance was generally intermediate between that observed for the two conventional formats.

ISSN:0094-2405    

DOI:10.1118/1.594900

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A nonconventional and sensitive method of measuring scatter is described. The method was employed to quantitate the scatter imaged in mammography with a conventional unit, a unit equipped with a grid, and a unit equipped with a scanning multiple slit assembly (SMSA). The results indicate that the grid technique significantly reduces the scatter imaged, while the SMSA virtually eliminates it. The resultant increase in large area contrast is readily apparent on radiographs with greater improvement obtained with the SMSA than with the grid. The effect of the increase in contrast on small detail visibility was assessed with a phantom having simulated fibrils and calcifications. Significantly more fibril and calcification detail was visible with the grid and SMSA technique than with the conventional technique. The detail visible with the grid and SMSA technique was comparable, and the lack of better performance by the SMSA unit is attributed to its poorer MTF.

ISSN:0094-2405    

DOI:10.1118/1.594927

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is argued that the technique of choice for measuring neutrons from medical electron accelerators is neutron remmeters with gold activation foils as thermal neutron detectors. This technique is to be preferred because of its simplicity, its accuracy, and its lack of dependence on any calculated responses or neutron spectral shapes. Measurements of the response of this detector are presented along with the results of representative measurements made at seven medical linacs and one betatron.

ISSN:0094-2405    

DOI:10.1118/1.594928

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this paper we have developed expressions for assessing radiographic systems where grids, air gaps, or both are used. The application of these expressions to film‐screen detectors gives predictions in good agreement with data that are found in the literature. Using this analytical tool, image intensified fluoroscopy has been studied. We have shown that under certain circumstances with air gaps there are potential gains in the efficient use of dose in fluoroscopy. The air gaps are of particular value where relatively thin body parts or small lesions are being examined using small focal spot generators. An independent gain control on the image intensifier would be useful for achieving the greatest flexibility when using magnification in fluoroscopy.

ISSN:0094-2405    

DOI:10.1118/1.594929

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Hyperthermia has emerged as a promising alternative or adjunct to other forms of cancer therapy. In order to utilize hyperthermia in very localized volumes immersed in regions of vital normal tissue, an invasive microwave coaxial monopole antenna has been developed. An experimental approach has been taken to characterize and optimize the electromagnetic properties and heating capabilities of bare and insulated antennas imbedded in tissue equivalent phantoms and dog brain. Four methods have been used to visualize the thermal profiles of the microwave probes: the liquid crystal technique, the gelatin technique, and the direct measurement of temperature with thermistor probes in phantom and dog brain. Among the parameters studied are: antenna impedance, insertion depth, antenna insulation (dielectric constant and thickness), shaft insulation, and frequency.

ISSN:0094-2405    

DOI:10.1118/1.594930

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Analytical formulae have been developed to describe the scatter dose on the central axis of rectangular fields as a function of depth and field size for60Co γ radiation and 4 and 8 MV x rays. The basic relationship expresses the scatter–to–primary dose ratio as a function of the area–to–perimeter ratioA/P, with depth and energy dependent parameters for which analytical expressions are given. The accuracy of the total dose on and off the central axis, calculated with these formulae, is compared to that resulting from the use of Day's method with tabulated values. The differences are generally less than 1% of the total dose on the central axis.

ISSN:0094-2405    

DOI:10.1118/1.594931

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dosimetry in phototherapy involves a determination of the energy absorbed per unit mass of tissue, corrected for the quantum yield in a photochemical reaction. The dose rate in photochemotherapy of cancer with hematoporphyrin derivative and visible light is related to the extinction coefficient, quantum yield for singlet oxygen production, concentration of sensitizer and energy flux density at depth. Data or methods of determining these quantities are presented. Calculations have been performed for the energy flux density at depth, as a function of the total attenuation coefficient and ratio of scattering coefficient to total attenuation coefficient, for isotropic scattering in slab geometry. For small absorption, these depth dose curves exhibit a maximum within the tissue followed by an exponential decrease.

ISSN:0094-2405    

DOI:10.1118/1.594932

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A comparison was made of ferrous sulfate (Fricke) and ionometric methods for determining the absorbed dose in a phantom irradiated with 4‐MV x‐rays, 25‐MV x‐rays, or electron beams having various incident energies between 10 and 32 MeV. Both chemical and ionization instruments were calibrated in a60Co beam at a point in water where the absorbed dose had been previously determined. The chemical yield measurements were corrected for spatial variations in dose within the volume of the solution and used to obtain a value of the absorbed dose for each of the x‐ray and electron beams. The ratios ofG‐values required for these determinations were taken from ICRU reports 14(1)and 21.(11)Ionization instrument readings from three types of commercial ionization chambers were used to obtain alternate values of the absorbed dose for each radiation.CλandCEvalues used in determining these ionization values of dose were also taken from the above ICRU reports. For 4‐MV x‐rays the values of absorbed dose obtained from chemical measurements agreed to within 0.5% with values obtained from ionization measurements; for 25‐MV x‐rays the chemical values were about 1% higher than the ionization values; for the electron beams the chemical values were 1%‐4% below the ionization values. These discrepancies suggest an inconsistency among the recommendedG,Cλ, andCEvalues similar to that which has been noted by other workers.

ISSN:0094-2405    

DOI:10.1118/1.594934

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several authors have suggested using heavy metal filters withKedges in the diagnostic energy range to reduce the width of the x‐ray spectrum and hence reduce patient radiation exposure. This spectral narrowing also increases subject contrast and permits an increase in tube potential. Results of contrast measurements are presented for a 250 μ gadolinium filter. It was found that aluminum filter contrast could be matched by using 8 to 10 kVp higher potential with the gadolinium filter. Similar results were found for calcium tungstate and rare‐earth screens. Measurements were also done to determine skin exposure and mAs ratios for both constant contrast and constant kVp technique conversion methods. A simple theory with one adjustable parameter gives a reasonable fit to the experimental results.

ISSN:0094-2405    

DOI:10.1118/1.594933

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY