摘要:

We describe a program for planning treatments of the eye with charged particle beams. Many of its features apply to treatment planning programs in general. These include the three‐dimensional definition of the tumor volume and of normal structures, the possibility of delivering the treatment beam from any direction in space, the provision of arbitrary viewpoints including a “beam's eye” point of view, the design and fabrication of the field shaping aperture, the specification of the necessary beam quality (range and modulation), the calculation and display of isodose contours on any plane in space and on relevant surfaces, the identification of the extent to which normal structures are included or excluded from the beam, and the simulation of the desired alignment film with which patient alignment can be verified.

ISSN:0269-3127    

DOI:10.1118/1.595258

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Xenon ionization detectors have been used successfully in computed tomography (CT) scanners; however, the detector design used in CT scanners does not provide sufficient spatial resolution for scanned projection radiography. We have been investigating a new design of xenon detector with individual element widths of 0.5 mm. In this design, there are no metal septa separating individual elements. As a result, detection efficiency is better than the design with septa, and construction of an array with submillimeter element widths is simpler; however, crosstalk will now occur between elements. Theoretical calculations of efficiency and resolution for our septaless design of xenon detector are presented. Results of these calculations indicate that for a spectrum of 100 kVp, element dimensions of 0.5 mm×0.5 mm×10 cm, a front window of 0.5‐mm aluminum, and a xenon pressure of 20 atm, quantum efficiency will be greater than 95%, detective quantum efficiency (DQE) will be approximately 75%, and both energy and conversion efficiencies will be limited to 50% byK‐fluorescent escape. The calculations also predict that for the same design, the reduction in lesion contrast induced by crosstalk will be less than 10% for all typical spectra. These theoretical results have encouraged us to pursue the construction of a prototype septaless xenon detector for scanned projection radiography.

ISSN:0269-3127    

DOI:10.1118/1.595259

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A more complete computational method is developed to account for the effect of the spectral distribution of the incident x‐ray fluence on the minimum exposure required to record a specified information set in a diagnostic radiograph. It is shown that an earlier, less rigorous, but simpler computational technique does not introduce serious errors provided that both a good estimate of the mean energy per photon can be made and the detector does not contain an absorption edge in the spectral range. Also shown is that to a first approximation, it is immaterial whether the detecting surface counts the number of photons incident from each pixel or measures the energy incident on each pixel. A previous result is confirmed that, for mammography, the present methods of processing data from the detector utilize only a few percent of the incident information, suggesting that techniques can be developed for obtaining mammograms at substantially lower doses than those presently used. When used with film–screen combinations, x‐ray tubes with tungsten anodes should require substantially lower exposures than devices using molybdenum anodes, when both are operated at their optimal voltage.

ISSN:0269-3127    

DOI:10.1118/1.595304

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron “contamination.” The present study of a60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb‐loaded acrylic, or Ba‐ or Pb‐loaded nonbrowning glass placed 57 cm from the source, using 5×5, 20×20, and 35×35‐cm2beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He‐gas‐filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a “clear”60Co γ‐ray beam of 1‐cm diameter.

ISSN:0269-3127    

DOI:10.1118/1.595305

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new technique is presented for determination of the effective point of measurement when cavity ionization chambers are used to measure the absorbed dose due to ionizing radiation in a dense medium. An algorithm is derived relating the effective point of measurement to the displacement correction factor. This algorithm relates variations of the displacement factor to the radiation field gradient. The technique is applied to derive the magnitudes of the corrections for several chambers in ap(66)Be(49) neutron therapy beam.

ISSN:0269-3127    

DOI:10.1118/1.595260

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Monte Carlo program has been devised that estimates the total absorbed energy and mean dose in defined regions within phantoms exposed to rotating x‐ray sources. The calculations deal with fanlike beams of rectangular cross section projected onto elliptic cylindrical phantoms, which may be inhomogeneous. The effects of coherent scattering, which may be significant in a narrow‐beam geometry, can be studied. The code can score the exit flux, allowing study of the relationship between dose and image quality. The model is described, the code is verified, and typical results are presented.

ISSN:0269-3127    

DOI:10.1118/1.595306

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper represents a preliminary study of the effects of regional temperature distribution in two‐dimensional nuclear magnetic resonance (NMR)T1 imaging. It is found, as expected, that variations in local temperature appear as variations in the correspondingT1 image. The potential use of NMRT1 imaging in temperature measurements is evaluated in the case of water and blood samples. Using containers where the temperature could be either known or directly controlled with reasonable accuracy, images are obtained with samples having at least two regions at different temperatures. As expected,T1 is found to vary linearly with 1/Tover the range of 0 °C to about 40 °C for blood. The potential use ofT1 imaging in hyperthermia applications is also discussed.

ISSN:0269-3127    

DOI:10.1118/1.595307

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accurate and reliable thermometry is essential in the development of microwave‐induced hyperthermal cancer therapy. While temperature measurements in strong electromagnetic fields usually require special sensors, this does not hold true for interstitial radiator/sensor systems. Miniature thermistors (with metallic leads) bonded to invasive microwave applicators exhibit a sensor error linearly related to the radiator's transmitted power. This relationship permits thermistor sensor error correction and temperature measurements to within ±0.1 °C or better. The instrumental methods and empirical validation are presented.

ISSN:0269-3127    

DOI:10.1118/1.595261

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dosimetry for 10‐MV x rays has been extended to radiation fields smaller than 4×4 cm which may be suitable for radiation therapy of small lesions, e.g., intracranial tumors, benign or malignant. Special consideration in this study was given to (i) the variation of dose with field size (collimator and phantom scatter), (ii) the central axis percentage depth doses, and (iii) the moving‐beam therapy dose distribution. We conclude that simple dosimetric techniques can provide adequate physics background for stereotaxic radiosurgery with small radiation fields and high‐energy x rays.

ISSN:0269-3127    

DOI:10.1118/1.595262

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A quality assurance (QA) test has been developed as a practical means to routinely monitor digital image resolution. The method is based upon a parameter termed the image modulationM. When noise is presentMcannot be directly measured. Instead, a biased estimate ofM, designatedM′, is measured. This report describes the application of the parameterM′ to QA monitoring of digital image resolution. A measurement ofM′ is generally simpler than alternative measurements, and is shown to outperform visual evaluation in the clinical QA environment.

ISSN:0269-3127    

DOI:10.1118/1.595263

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY