摘要:

Optical diffraction is used to study the structure of the Fourier transform of a thin suspension of human albumin microspheres. The intensity profile in the transform plane is then used to derive information about the particle size distribution.

ISSN:0094-2405    

DOI:10.1118/1.594214

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A line‐focusing x‐ray monochromator has been designed and developed to be used with an energy dispersive detection system for the quantitative analysis of trace elements in biological specimens. This instrument uses a cylindrically curved Johansson‐type crystal to monochromatize and focus the excitation radiation into a line 0.5 mm wide and 40 mm long. The excitation radiation chosen for these experiments was MoKα and the total intensity of the line‐focused beam was estimated to be 2×107cps. Because of the narrow width and high power density of the excitation beam, small tissue specimens 1×1×12 mm in size along with a single strand of hair were analyzed for trace elements from Al through Sr. For the transition elements, concentrations of a few parts per million were easily detected with this analyzer.

ISSN:0094-2405    

DOI:10.1118/1.594207

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A portable tissue equivalent calorimeter has been constructed and employed to measure absorbed dose in a mixed fast‐neutron and γ‐ray field. Design details and measurement techniques are described along with the limits of precision and absolute accuracy. Experiments have been carried out with a cyclotron produced neutron beam using the9Be(d,n)10B reaction at dose rates ranging from 3 to 25 rad/min. A series of ten measurements at a dose rate of approximately 10 rad/min were performed with a precision of ±2% (standard deviation about the mean). The compact design of this calorimetric dosimeter system facilitates transport to other laboratories for the measurement of absorbed dose in several complex radiation fields.

ISSN:0094-2405    

DOI:10.1118/1.594273

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The displacement correction factor to be used for analysis of fast‐neutron dosimetric measurements using air‐filled EG and G tissue‐equivalent ion chambers in a tissue‐equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35‐MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC‐17 (1.0‐cm3) ion chamber, and is 0.989 for the EG and G IC‐18 (0.1‐cm3) ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

ISSN:0094-2405    

DOI:10.1118/1.594208

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The elemental variations in whole blood following γ‐radiation injury of mice have been studied. The trace‐element analyses were performed using the proton‐induced x‐ray emission technique, and the results were compared with the red blood‐cell count. It is shown that, by using this method, the variation of the six elements, K, Ca, Fe, Cu, Zn, and Rb, can be studied simultaneously.

ISSN:0094-2405    

DOI:10.1118/1.594209

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The distribution of absorbed dose in a medium irradiated by broad electron beams has been analyzed from physical and therapeutic points of view. A number of parameters which describe the beam quality and the shape of the absorbed‐dose distribution along the central axis are defined. Several comparisons are made of the values of these parameters obtained from measurements made on different therapy accelerators and from theoretically calculated dose distributions for monoenergetic and monodirectional beams. It is found that the major part of the observed differences between experimental and theoretical results can be attributed to the energy distribution of the accelerator beam and to processes in the scattering material, such as energy straggling and the production of secondary electrons and photons. A simple expression is derived relating the slower falloff in dose found for most accelerator beams to the large energy spread found in these beams. Further, a semiempirical relationship is found which makes it possible to obtain the mean electron energy at the phantom surface from the depth where the absorbed dose has decreased to 50% of its maximum value along the central axis.

ISSN:0094-2405    

DOI:10.1118/1.594210

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A procedure is developed for characterizing and measuring the distortion and intensity variations which are present in x‐ray image‐intensifier systems. These effects often seriously limit the usefulness of such systems, and the results here make possible an evaluation and comparison of competing intensifier instruments.

ISSN:0094-2405    

DOI:10.1118/1.594275

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Variations in the output with a fixed monitor unit setting as a function of magnetron power of a 4‐MV linear accelerator has been studied. The relationship between radiation output and monitor units was found to vary by as much as 10%. The central‐axis depth dose was found to remain constant within experimental error. Field flatness measurements indicated that for lower power outputs, the radiation field was less flat than for higher power outputs. A mechanism for the change in calibration is suggested based upon field flatness measurements.

ISSN:0094-2405    

DOI:10.1118/1.594211

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A high‐sensitivity biomedical radiation probe which employs a fiber‐optic coupled NaI(Tl) scintillator as a detector is described. It was developed forin vivocounting of low‐energy239Pu photons from material located in the tracheobronchial lymph nodes. This probe is 20 times as sensitive as a solid‐state probe (avalanche diode) previously developed for this application. Tests with99mTc show a sensitivity more than 90 times greater than biomedical probes using DcTe of GaAs; however, the improved sensitivity is largely due to an increased sensitive volume. Probes were evaluated in animals and phantoms for detection of239Pu and for location of lung tumors labeled with111In.

ISSN:0094-2405    

DOI:10.1118/1.594274

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Weighting of given doses from parallel opposed fields is a widespread practice in radiation therapy of cancer in the tonsillar region. In order to determine when weighting is useful, dose distributions on the central axis were calculated for various energies (60Co, 4‐, 8‐, 15‐, and 24‐MVp photons), weighting factors, and field separations. Criteria for judging the usefulness of weighting were (i) that variation in the dose across the tumor volume not exceed 10%, and (ii) that the dose to critical tissues outside the target volume be at least 20% less than target volume dose. The clinical situation that met these criteria was a lateral lesion that extended to less than one‐third of the transverse diameter of the head, treated with either60Co, 8, or 4 MVp with 4:1 weighting. Under these conditions, weighting was considered to be justified.

ISSN:0094-2405    

DOI:10.1118/1.594276

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY