ISSN:0269-3127    

DOI:10.1118/1.595311

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Kermas for various substances averaged over the energy spectra of fast neutron therapy beams, as well as ratios of average kermas relative to muscle, were calculated in an attempt to estimate the uncertainties introduced in these quantities by the poor knowledge of the elemental kerma functions, actual neutron energy spectra, and composition of tissues and other materials. Average kermas have uncertainties of the order of 7%–25%, while for ratios of average kermas the uncertainties are of the order of 2%–5% for materials of clinical interest. It is concluded that the ratio of average kerma of muscle to A‐150 tissue‐equivalent plastic should be 0.93±0.03 for the newp+Be clinical neutron beams.

ISSN:0269-3127    

DOI:10.1118/1.595390

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model for the dependence of normal tissue radiation dose response functions on volume variations and dose inhomogeneities is derived using probability theory. Power law volume correction factors and the complication probability factor are shown to be special cases arising from approximations applied to this model. Both require the assumption of small probabilities of complication. Power law volume corrections are shown to require a homogeneous dose distribution. The general model is tissue specific and can be used to calculate probabilities of complication for individual organs or isoprobability doses for radiation injury. The model is applicable to both homogeneous and inhomogeneous dose distributions and has been used in computer determination of optimal treatment parameters. Experimental data are presented which are consistent with the general model alone and which demonstrate the limits of applicability of previous models.

ISSN:0269-3127    

DOI:10.1118/1.595312

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An earlier paper [Simpsonet al., Med. Phys.9, 574 (1982)] described a computed tomography CT scanner that was constructed by adding a detector array to a 4‐MV isocentric linear accelerator. Since the previous article, the detector array has been improved and we now demonstrate better than 3‐mm spatial resolution and better than 1% relative electron density discrimination. A series of pictures from volunteer patients is included. Normal anatomy is visualized with bone, muscle, fat, and air being clearly delineated.

ISSN:0269-3127    

DOI:10.1118/1.595391

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The detection and imaging characteristics of a new type ofgamma cameraforsingle photon emission computed tomographyhave been investigated. Unlike conventional gamma cameras which use mechanical collimation, the new gamma camera utilizeselectronic collimationwhich is obtained from a sequential interaction of gamma radiation with a dual position‐and‐energy sensitive detection system. Coincident counting between the two detectors provides localization of activity upon a multitude of conical surfaces throughout the object, wherefrom the three‐dimensional activity distribution can be reconstructed. Not only does electronic collimation provide simultaneous multiple views of the object, but a large gain in sensitivity is also indicated over a conventionally collimated gamma camera under conditions of similar spatial resolution. Detector optimization studies have been performed to design a prototype system comprising a 33×33 array of high‐purity germanium detectors coupled to an uncollimated conventional scintillation camera. The cumulative signal‐to‐noise ratio in projection images obtained with this system is expected to be about a factor of 4 higher (sensitivity about a factor of 15 higher) than that obtained in a corresponding projection image with a conventional gamma camera for imaging a uniformly distributed Tc‐99msource in a 20‐cm‐diam ×20‐cm‐tall cylinder. A similar gain is expected in the tomographic images.

ISSN:0269-3127    

DOI:10.1118/1.595313

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Iterative algorithmshave been investigated forreconstructing imagesfrom data acquired with a new type of gamma camera based upon an electronic method of collimating gamma radiation. The camera is composed of two detection systems which record a sequential interaction of the emitted gamma radiation. Coincident counting in accordance with Compton scattering kinematics leads to a localization of activity upon a multitude of conical surfaces throughout the object. A two‐stage reconstruction procedure in which conical line projection images as seen by each position sensing element of the first detector are reconstructed in the first stage, andtomographic imagesare reconstructed in the second stage, has been developed.Computer simulationstudies of both stages and first‐stage reconstruction studies with preliminary experimental data are reported. Experimental data were obtained with one detection element of a prototypegermanium detector. A microcomputer based circuit was developed to record coincident counts between the germanium detector and an uncollimated conventional scintillation camera. Point sources of Tc‐99mand Cs‐137 were used to perform preliminary measurements ofsensitivityandpoint spread functioncharacteristics ofelectronic collimation.

ISSN:0269-3127    

DOI:10.1118/1.595314

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Central axis depth dose (CADD) and off‐axis absorbed dose ratio (OAR) measurements were made in water, muscle and whole skeletal bone tissue‐equivalent (TE) solutions, mineral oil, and glycerin with a clinical neutron therapy beam. These measurements show that, for a given neutron beam quality and field size, there is a universal CADD distribution at infinity if the depth in the phantom is expressed in terms of appropriate scaling lengths. These are essentially the kerma‐weighted neutron mean free paths in the media. The method used in ICRU Report No. 26 to scale the CADD by the ratio of the densities is shown to give incorrect results. The OARs measured in different media at depths proportional to the respective mean free paths were also found to be independent of the media to a good approximation. Therefore, neutron beam CADDs and OARs may be measured in either TE solution (USA practice) or water (European practice), and having determined the respective scaling lengths, all measurements may be scaled from one medium to any other. It is recommended that for general treatment planning purposes, scaling be made to TE muscle with a density of 1.04 g cm−3, since this value represents muscle and other soft tissues better than TE solution of density 1.07 g cm−3. For such a transformation, relative measurements made in water are found to require very small corrections. Hence, it is further recommended that relative CADD and OAR measurements be performed in water because of its universality and convenience. Finally, a table of calculated scaling lengths is given for various neutron energy spectra and for various tissues and materials of practical importance in neutron dosimetry.

ISSN:0269-3127    

DOI:10.1118/1.595364

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Computed tomography (CT) scanning techniques designed to visualize the beating heart utilize the electrocardiogram (ECG) waveform to gate or select the phase of the cardiac cycle being imaged. Most such methods require that each slice to be imaged be scanned many times to obtain the usual number of projections in a single phase interval of the cardiac cycle. We are studying a new method in which only a single scan of 20 s should be required per imaged slice. The central problem in the proposed technique is to test the diagnostic utility of images reconstructed from a very limited number (about 25) of projections. An essential part of the proposed algorithm is the elimination of all material outside a circle chosen to contain only the heart and the structures immediately surrounding it. Experiments are reported here on scans of calf 's heart in an 11‐cm container of water, with most views deleted so as to simulate the results from this type of gated scan. The results show that if no high‐contrast objects are within the chosen circle, then image quality is satisfactory and could be clinically useful.

ISSN:0269-3127    

DOI:10.1118/1.595315

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Methods of correcting for tissue inhomogeneities which consider only changes in the photon fluence are of limited usefulness for 10‐MV x rays. Although there is normally transient electronic equilibrium on the central axis beyond the depth of maximum buildup in soft tissue, when the beam enters a low‐density material the beam profile is degraded and there is a loss of lateral electron equilibrium which reduces the dose both within and beyond the inhomogeneity. This paper describes experimental measurements of the dose within and near large, low‐density inhomogeneities simulating lungs and also behind air channels (where similar effects occur). Methods of calculating correction factors are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595392

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Computerized dosimetric studies of single‐ and double‐plane iridium‐192 (Ir‐192) planar implants were performed. With respect to dose homogeneity, we found that the optimal source and ribbon separation for single‐plane implants was 1.0 cm. For double‐plane implants, the preferred ribbon and plane separation was 1.5 cm, maintaining a 1‐cm separation for the sources. Using these separations, standard dose rate curves for single‐ and double‐plane Ir‐192 implants were generated by computer calculations. These standard curves are useful for quickly and fairly accurately estimating the dose from any size planar implant, without requiring more time‐consuming individual computer dosimetry. We believe that the curves will prove to be of practical clinical value to physicists and radiotherapists.

ISSN:0269-3127    

DOI:10.1118/1.595308

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY