ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1988.tb36285.x

出版商:Wiley    

年代:1998

数据来源: WILEY

摘要:

Pencil beam algorithms for the calculation of electron beam dose distributions have come into widespread use. These algorithms, however, have generally exhibited difficulties in reproducing dose distributions for small field dimensions or, more specifically, for those conditions in which lateral scatter equilibrium does not exist. The work described here has determined that this difficulty can arise from the manner in which the width of the pencil beam is calculated. A unique approach for determining the pencil beam widths required to accurately reproduce small field dose distributions in a homogeneous phantom is described and compared with measurements and the results of other calculations. This method has also been extended to calculate electron beam dose distributions in heterogeneous media and the results of this work are presented. Suggestions for further improvements are discussed.

ISSN:0094-2405    

DOI:10.1118/1.596303

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root‐mean‐square spatial Gaussian spread σ(z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source–surface distance (SSD) for a 7‐MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.

ISSN:0094-2405    

DOI:10.1118/1.596201

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measured and calculated dose profiles under partial attenuators which cover only part of the radiation beam are presented. The study was performed for x‐ray beams generated with a 4‐MV linear accelerator at a source–surface distance of 120 cm in a water phantom for lead attenuators of arbitrary shape but constant thickness. Dose correction factors, which are used to multiply the open beam data to predict doses under partial attenuators, depend strongly on attenuator parameters, such as its thickness, lateral dimensions, and distance from phantom or patient surface, in addition to depending on depths in the phantom. The dose correction factors are calculated with Clarkson sector integration techniques, and the results, in spite of the simplifying assumptions used in the algorithm, generally agree with measured data to within 3%. The calculational method therefore may be applied to general clinical situations in which partial attenuators are used.

ISSN:0094-2405    

DOI:10.1118/1.596202

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have developed a novel method, which employs large lead collimators and computed tomography reconstruction techniques, to measure the intensity distributions of x‐ray sources of radiotherapy devices. Using this method, we have measured the intensity distributions of x‐ray sources from60Co, 6‐, 18‐, and 25‐MV radiotherapy devices. The x‐ray sources of the accelerators were all elliptical in shape, but varied in eccentricity, and the sizes of the accelerator sources varied from 0.7 to 3.3 mm full width at half‐maximum. The60Co source was circular in shape and 20 mm in diameter, however, the output from this source was not uniform across its face. The modulation transfer functions (MTF's) (at the image plane) calculated for the accelerator sources, assuming an image magnification of 1.2, had similar magnitudes at low spatial frequencies as the MTF's of the metal plate/film detectors commonly used for therapy imaging. However, the source MTF's declined much more rapidly at high spatial frequencies. Therefore, for geometries commonly found in radiotherapy, the loss in spatial resolution due to the x‐ray source was at least equal to that caused by electron and photon scatter within the metal plate/film detectors.

ISSN:0094-2405    

DOI:10.1118/1.596295

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The first Mitsubishi medical linear accelerator in the United States was commissioned in April 1985. This unit EXL‐8 (marketed by Mitsubishi International Corporation) produces 8‐MeV electron beams in addition to 6‐MV x rays. It is a 100‐cm source–axis distance isocentric machine. Acceptance testing and performance evaluation of this accelerator were completed. Our measurements included beam characteristics and dosimetry parameters for both modalities. Central axis % depth dose (% DD), tissue‐maximum ratio, field size output factors, wedge factors, etc., for this Linac 6‐MV beam are reported. Characteristics of the 8‐MeV electron beam, namely % DD data, isodose curves, and cone ratios for various electron applicators are presented.

ISSN:0094-2405    

DOI:10.1118/1.596203

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In intraoperative radiation therapy, it is critical to reduce the radiation contamination outside the useful field by as much as physically feasible. Additionally, a uniform dose is clinically desirable across the tumor volume. A study of the Medical College of Ohio applicators indicates that the radiation contamination outside the field can be as high as 18% of the central axis dose. The effects of the photon collimator setting on the magnitude and energy of the radiation contamination are discussed and means are presented for reducing this unwanted radiation. The dose nonuniformity across the field is found to be virtually independent of the photon collimator setting and is shown to be mostly due to the transparent applicator wall. The clinical significance of the findings is discussed.

ISSN:0094-2405    

DOI:10.1118/1.596204

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This is a review of the density matrix formalism of quantum statistical mechanics as it relates to nuclear relaxation. The single spin and coupled exchanging two spin systems are treated explicitly. We show the circumstances under which these models reduce to the more familiar expressions used to describe proton relaxation in tissue. The effect of radiofrequency (rf) pulses on the density matrix is presented as well as the density matrix description of the Carr–Purcell pulse technique and conditions under which even and odd echo trains will superpose or are independent of rf pulse rate.

ISSN:0094-2405    

DOI:10.1118/1.596205

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new algorithm to compute spin‐lattice relaxation (T1) maps in magnetic resonance imaging (MRI) by using fast imaging protocols and monitoring the magnetization evolution towards a steady state is proposed. This algorithm uses a general least‐squares fit to multiple point experimental data and is thus much more precise than the standard two‐point fit. Since this imaging protocol is designed to consume time comparable to that of standard clinical protocols, it has an advantage in comparison to traditional multiple point protocols, which require considerably longer scanning times. In addition toT1maps the protocol can generate final images suitable for clinical diagnosis. Thus it is possible to generate relaxation data without considerably lengthening the total required MRI study time. The results of phantom testing of this protocol are reported.

ISSN:0094-2405    

DOI:10.1118/1.596206

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Deep and superficial body temperatures were measured byin vitroandin vivoexperiments, using a fluoroptic procedure and a variety of magnetic and electromagnetic fields, in the course of magnetic resonance imaging (0.35 T; 1.5 T).In vitroexperiments were performed to select and check the appropriate temperature method. Temperature measurements in the human body were carried out centrally (esophageal and rectal measurements).In vivoexperiments in 30 volunteers showed no significant changes (p=0.05) in central or peripheral temperatures resulting from the application of static or dynamic fields or radiofrequency.

ISSN:0094-2405    

DOI:10.1118/1.596207

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY