摘要:

This paper traces the development of the early history of radiological physics beginning with the early speculations of Faraday that “...matter may be classed into four states, solid, liquid, gaseous, and radiant,” and concluding just before the discovery of x‐rays. Throughout the main theme of the narrative is the discovery of the properties of cathode rays and changing contemporary explanations of their nature. The interaction between the experimental and theoretical work of such well‐known scientists as Faraday, Dalton, Hertz, and Maxwell is also stressed. (AIP)

ISSN:0094-2405    

DOI:10.1118/1.594578

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Echocardiogram analysis is treated in a pattern recognition framework. Anterior mitral leaflet waveforms are classified for the four‐class problem consisting of the classes “normal,” “mitral stenosis,” “mitral valve prolapse,” and “idiopathic hypertrophic subaortic stenosis.” In addition, aortic root waveforms and left ventricular wall waveforms are classified for the two‐class problem consisting of the classes “normal” and “idiopathic hypertrophic subaortic stenosis.” One common method of analysis (Fourier analysis) underlies each classification scheme. Classification accuracy is sufficiently good to warrant the inference that successful automated decision‐making based on the algorithms investigated is feasible.

ISSN:0094-2405    

DOI:10.1118/1.594579

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique to measure the MTF of metal screen‐film detectors at megavoltage x‐ray energies has been devised. It employs an exposure through a slit 25 μm wide and a method to average numerous line spread functions from a single film image. That the slit is wide compared to those used at diagnostic x‐ray energies serves two purposes: it facilitates the use of a laser alignment procedure designed to ensure correct orientation of the slit‐forming tungsten blocks in the x‐ray beam, and it also increases the contrast in the slit image, thereby reducing the data analysis problems caused by film noise. Further reduction in the effects of film noise is accomplished by the averaging method. A correction is made to account for the effect of the wide slit on the MTF. The validity of this correction, the laser alignment, and the averaging method have been confirmed by simulations.

ISSN:0094-2405    

DOI:10.1118/1.594581

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The calibration of a cobalt‐60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor,Ac, a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue–air ratios were measured with these chambers for cobalt‐60 gamma rays in a polystyrene phantom, and the ratioAc/Aeqwas experimentally determined. In order to calculateAcfrom this ratio,Aeqwas determined also. It was found thatAcdepended on chamber diameter only, and not on field size or depth. A value of 0.990 forAeqis recommended and a table ofAcfor chambers of different outer diameters is included.

ISSN:0094-2405    

DOI:10.1118/1.594582

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When there is an absence of scattering material adjacent to the skin on the exit side of a megavoltage beam the dose to the skin is less than would be calculated using depth dose tables or isodose distributions measured in “semi‐infinite” phantoms. Ionization measurements using a thin‐window parallel‐plate chamber show that the dose at 4 mg/cm2from the exit surface is 14% to 16% less for cobalt‐60 gamma rays and about 8% less for 25‐MV x rays compared to the dose with full backscatter. As the angle of incidence increases the skin dose increases due to radiation scattered toward the surface. A method for the calculation of skin doses from tangential therapy beams is described.

ISSN:0094-2405    

DOI:10.1118/1.594583

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For megavoltage x‐ray beams, it is well known that the percent depth‐dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose,dmax. The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25‐MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 g cm−2, of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.

ISSN:0094-2405    

DOI:10.1118/1.594580

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The attributes of high energy photon beams, i.e., low surface dose, largedmaxand improved %DD, are compromised with increase in field size. This is due to the relative increase with field size of the electron component in the beam, as shown by recent experiments done here using a sweeping magnet. The present study shows that the advantages can be partially regained with the use of foils to remove electrons. Various thicknesses of Al, Cu, Sn, and Pb were placed in a 25‐MV linac x‐ray beam at several sites in the treatment head. Buildup curves were measured with a “pancake” chamber for various SSD and field sizes. The magnitude of improvement achieved is dependent upon field size, SSD, the atomic number of the foil material, and foil thickness. Pb foil (0.55 g cm−2) provided the best overall improvement. Surface dose reduction of 10%–20% can be achieved along with significant increase indmaxand %DD. These findings suggest a new design feature for the next generation of high energy linacs.

ISSN:0094-2405    

DOI:10.1118/1.594585

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dose distribution pattern from a Delta‐25 head scanner was evaluated experimentally in a 17.5‐cm‐diam cylinder water phantom for different scan speed and slice thickness combinations. TLD chips were used to measure the dose at different points across the scan section. The isodose curves thus obtained were found to be asymmetric, with the regions of maximum and minimum dose near the surface on the opposite sides of the phantom. The effect of changing the scan thickness and speed on dose was evaluated and a linear increase in dose with the increase of slice thickness was observed. This was consistent with the design of the line focus and the beam collimators used in this system. The effect of multiple slice scan on the dose was also evaluated.

ISSN:0094-2405    

DOI:10.1118/1.594632

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The generation of long, high quality random number sequences for Monte Carlo simulations using minicomputers is considered. The importance of the thorough testing of Monte Carlo random number generators is emphasized. A recommendation is given to authors of Monte Carlo papers to specify their random number generator and to describe the randomness testing which that generator has undergone.

ISSN:0094-2405    

DOI:10.1118/1.594584

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two methods for calculating the radiation dose from remaining body activity have been suggested. One requires correction of the cumulated activities so that they reflect the activity uniformly distributed in the total body. The other method requires correction of theSvalues so that a value ofSfor the target organ from the remainder of the body is obtained. These two methods give the same answer. We have examined these methods and the number of steps required to calculate the radiation dose in each case. Our results show that the method of correcting the cumulated activities is preferred, especially if the number of source and target organs is large and a computer equipped with the necessary software is not available.

ISSN:0094-2405    

DOI:10.1118/1.594586

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY