摘要:

The effects of small cavities on dose in water and the dose in a homogeneous nonunit density medium illustrate that inhomogeneities do not act independently in photon dose perturbation, and serve as two constraints which should be satisfied by approximate methods of computed tomography (CT) pixel‐based dose calculations. Current methods at best satisfy only one of the two constraints and show inadequacies in some intermediate geometries. We have developed an approximate method that satisfies both these constraints and treats much of the synergistic effect of multiple inhomogeneities correctly. The method calculates primary and first‐scatter doses by first‐order ray tracing with the first‐scatter contribution augmented by a component of second scatter that behaves like first scatter. Multiple‐scatter dose perturbation values extracted from small cavity experiments are used in a function which approximates the small residual multiple‐scatter dose. For a wide range of geometries tested, our method agrees very well with measurements. The average deviation is less than 2% with a maximum of 3%. In comparison, calculations based on existing methods can have errors larger than 10%.

ISSN:0269-3127    

DOI:10.1118/1.595294

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The technical characteristics of a new digital fluorographic image processing method called matched filtering are presented. This technique, a type of extensive temporal integration, takes a weighted sum of images acquired during passage of a contrast bolus through some area of interest. The weight of each image is governed by the magnitude of the contrast bolus in that image. An essential requirement of the matched filter is that its integral be zero. It is shown for equal exposure rates and typical bolus characteristics that matched filtering provides a factor of two higher signal‐to‐noise ratio (SNR) than conventional methods for bolus transit times of 10 s or higher. Equilvalently, matched filtering can yield images with quality comparable to conventional digital subtraction angiography (DSA) at a factor of four less patient exposure. The SNR obtained with matched filtering is shown to be within 30% of an ideal bound. Comparisons of matched filtering to standard recursive methods and simple integration are made. Experimental canine studies are presented which compare matched filtering with conventional DSA.

ISSN:0269-3127    

DOI:10.1118/1.595238

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Grid transmission characteristics have been predicted, with good agreement demonstrated between experiment and theory. Differences between the present work and an earlier theoretical analysis have been noted. It is shown that comparison between experiment and theory can lead to a sensitive method for determining grid parameters. Generally, the derived grid parameters deviate somewhat from manufacturer supplied data. Use of the manufacturer's nominal parameters can lead to large errors in predicting grid transmission.

ISSN:0269-3127    

DOI:10.1118/1.595239

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The desirable features of equipment intended for the clinical application of local microwave hyperthermia are identified and the design of a machine incorporating these features is described. An assessment of the technical aspects of the performance of the equipment is given.

ISSN:0269-3127    

DOI:10.1118/1.595240

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes a system for the transport of compounds labeled with short‐lived positron emitting isotopes from a clinical cyclotron to a remote imaging suite. The system consists of a pneumatic transport line for the sending of batch lots of radioactive material and a pressurized gas line for the continuous transmission of radioactive gases. Experimental performance of both systems has been measured and a simple model for the pressurized gas line is compared to experimental results. The radiation dosimetry and safety aspects of the systems are also discussed.

ISSN:0269-3127    

DOI:10.1118/1.595290

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A number of medical phenomena may be discussed in terms of the probability of something “surviving” as a function of time, dose, distance, etc. Employing elementary results of engineering reliability theory, we examine the different shapes of survival curves which can arise when several modes of failure compete for the members of a homogeneous or heterogeneous population. A variety of examples of medical interest illustrate the approach.

ISSN:0269-3127    

DOI:10.1118/1.595241

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The characteristics of 15‐MV photon beam of Mevatron 77 have been examined with respect to clinical parameters. Some of the characteristics of this 15‐MV photon beam are different from those of similar machines used in radiotherapy. The tissue–maximum ratio values have been determined experimentally for SAD 100 cm and percentage depth dose values have been determined from tissue–maximum ratio data. It is found that the experimentally determined PDD values for SSD 80, 100, and 120 cm are within ±1% of the calculated data. The scatter–maximum ratio data have been calculated from the experimentally determined zero area attenuation coefficient and tissue–maximum ratio data and are presented as function of depth and radii of circular fields. The relative dose factors have been determined both for square and rectangular fields. The relative dose factor for rectangular fields depends on the two pairs of jaws differently and the maximum difference is about 1.5%. The flatness and symmetry are within the specifications set by the manufacturer.

ISSN:0269-3127    

DOI:10.1118/1.595242

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been developed for the quantitative determination of the tolerance doses for preoperative and postoperative radiotherapy of bones in New Zealand white rabbits. The midshaft osteotomy was carried out on both ulnae of each rabbit. Localized radiation of 2000, 3000, and 4000 rad (20, 30, and 40 Gy) was given in daily fractions of 500 rad to different groups of rabbits. The diaphyseal area of one forearm of each rabbit was irradiated and the other forearm was used as a control. After healing, the mechanical strength of each ulna was measured. The end point is defined as “strength reduction dose‐63” (SRD63), i.e., the dose which would be expected to reduce the mechanical strength of the irradiated, fractured, and healed bone compared with the contralateral unirradiated diaphysis by 63%. The SRD63's for preoperative and postoperative radiotherapy were 4350 rad (43.5 Gy) and 6200 rad (62 Gy), respectively, with a 500 rad (5 Gy)/fraction schedule. Through the use of the nominal standard dose (NSD) formula at 300 rad (3 Gy)/fraction and five fraction per week, these SRD63's are equivalent to 5700 rad (57 Gy) and 8100 rad (81 Gy), respectively.

ISSN:0269-3127    

DOI:10.1118/1.595298

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The regional silver content of radiographic film measured by x‐ray fluorescence is compared to light absorption measured by a densitometer. Silver content analysis appears to permit a greater dynamic range of useful exposure levels than does light absorption densitometry. This improvement in latitude, however, is not considered great enough to warrant development of a complex system for scanning silver distribution in radiographic applications.

ISSN:0269-3127    

DOI:10.1118/1.595243

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A postmortem study was conducted to assess the feasibility of measuring bone lead concentrations noninvasivelyin vivo. CharacteristicLx rays were induced with an external source of125I in the superficial tibial cortex of the intact legs of six adults who had no history of occupational exposure to lead. Tibial lead concentrations in the same bones subsequently determined by flameless atomic absorption spectroscopy varied from 15 to 35 μg Pb/g wet weight. The upper limit for the modern normal range of lead in the bone is about 25 mg Pb/g wet tissue. The linear correlation coefficient (r) between the measurements made with x‐ray fluorescence and lead concentration by absorption spectroscopy was 0.90. Radiation doses of 10 mGy (1 rad) to 1 cm2of skin, with associated doses to the marrow of adjacent bone of about 0.6 mGy (60 mrad), yielded net lead fluorescence signals ranging from one to seven times the standard deviation of background.

ISSN:0269-3127    

DOI:10.1118/1.595244

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY