摘要:

In this fourth article in a series on the calculation of electron dose using multiple‐scattering theory, we deal with localized inhomogeneities by solving the Fermi equation for scattering power which is an arbitrary function of position. In fact, we go further, by solving the second‐order multiple‐scattering equation which supersedes the (first‐order) Fermi equation, again for scattering power which is an arbitrary function of position. Thus, we are no longer restricted to a horizontally layered medium, as is the case with the Fermi–Eyges theory. Our general solution is in the form of a perturbation series which evidently converges rapidly enough that only its first two or three terms need be taken for accurate dose calculation. Regarding the energy directly deposited by the primary electrons, the formulas developed in this article give very good agreement with Monte Carlo calculations for the thick half‐slab configuration, as will be seen in the next article in this series. Moreover, our first‐rank, second‐order formulas, when expressed in Fourier‐transformed space, are simple enough to be implemented in a treatment planning system providing full three‐dimensional electron dose calculation for arbitrary configurations of inhomogeneities.

ISSN:0269-3127    

DOI:10.1118/1.596698

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Presented here is a method of designing compensators for a single beam or one or more pairs of beams, not necessarily parallel opposed. The objective is to produce a flat distribution in a plane that may be perpendicular to the central ray or may be an arbitrarily oriented plane, for example, a plane that bisects the hinge angle between two beams. The method takes into account not only surface irregularities but also tissue inhomogeneities, hinge angles between beams, distance from the source, and even “horns” in the beam. The design process employs convolution of Monte Carlo generated pencil beams with photon fluence distributions, appropriately modified for the presence of beam modifiers (blocks and compensators), to compute dose in a flat homogeneous phantom. Corrections for inhomogeneities and surface curvature are applied by using computerized tomography information to determine the effective path length through tissue. Multiple interations are used to arrive at a compensator that properly incorporates changes in radiation transport, and therefore dose distribution, resulting from the presence of beam‐shaping devices. In each iteration it is assumed that the required reduction in dose at a point can be achieved by reducing the fluence along the ray joining the source to computation point proportionately. The compensator design is represented as a finely spaced matrix of thickness values which is entered into a prorammable milling maching for fabrication. Dose measurements in phantom exposed to 6‐MV x rays with and without compensation are presented.

ISSN:0269-3127    

DOI:10.1118/1.596699

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Linear programming is a versatile mathematical tool for optimizing radiation therapy treatment plans. For planning purposes, dose constraint points, possible treatment beams, and an objective function are defined. Dose constraint points are specified in and about the target volume and normal structures with minimum and maximum dose values assigned to each point. A linear objective function is designed that defines the goal of optimization. A list of potential treatment beams is defined by energy, angle, and wedge selection. Then, linear programming calculates the relative weights of all the potential beams such that the objective function is optimized and doses to all constraint points are within the prescribed limits. Historically, linear programming has been used to improve conventional treatment techniques. It can also be used to create sophisticated, complex treatment plans suitable for delivery by computer‐controlled therapy techniques.

ISSN:0269-3127    

DOI:10.1118/1.596700

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using an approximation of Bjarngard and Petti [Phys. Med. Biol.33, 21–32 (1988)] that the scatter‐to‐primary ratio SPR(r,d) is a linear function of the geometrical parameterz=rd/(r+d) (whereris the radius of photon beam anddis depth in phantom), a method was developed to determine primary dose directly from depth‐dose data, measured within the range of fields with lateral electronic equilibrium. This method was evaluated using tabulated TAR data for Co‐60 γ radiation in water. The results for primary dose agree within ∼1% of those from other studies. It was shown that this method is applicable within the energy range of therapeutic x rays. Primary dose was determined in the case of 6‐ and 18‐MV x rays using proposed method.

ISSN:0269-3127    

DOI:10.1118/1.596701

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Calibration of an imported strontium‐90 ophthalmic applicator at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology) has disclosed a significant discrepancy in dose rate calibration (32%–35%) with that quoted by the manufacturer. The University of Wisconsin has investigated this discrepancy and found that both laboratories use similar techniques and a version of the Bragg–Gray equation to yield dose rate estimates. Experimental results indicate a strong relationship between the size of the collecting electrode used in the extrapolation chamber and the resulting estimate of absorbed dose rate. Calibration of these applicators is reviewed and suggestions for improvement and further research are proposed.

ISSN:0269-3127    

DOI:10.1118/1.596702

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is presented for integrating stereotactic projection and tomographic image data to give composite 3‐D images (stereo pairs) of cerebral anatomy and vasculature. The technique serves to combine complementary information from each modality and allows the imaged volume to be viewed directly. The procedure is largely automated and requires no additional apparatus or information beyond that which is ordinarily employed during stereotactic surgical planning. The two types of data are combined by superimposing the projection angiogram (DSA) onto a translucent volume rendered CT or MR image. Since the rendering algorithm employs an orthographic projection technique, the tomographic volume must first be reshaped and oriented to yield a perspective view that matches the DSA projection. During this process, the data undergo various interpolations which consequently affect the accuracy of target identification based on the resulting images. The integrity of the matching procedure was assessed using simulated data sets. Also, calculations were performed to estimate the resolution of measurements made from digitized stereoscopic images. The resulting sub‐pixel accuracy of the matched images suggests that the technique has potential for stereotactic applications. Preliminary results are presented illustrating combined CT‐DSA and MR‐DSA data sets.

ISSN:0269-3127    

DOI:10.1118/1.596703

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A “shortened” multiantenna hyperthermia applicator has been designed and tested at the Mallinckrodt Institute of Radiology at Washington University School of Medicine. By shortening the distance from antenna to aperture, an applicator is obtained that produces an SAR pattern that is essentially the same as produced by a monopole antenna. By placing several properly spaced probe antennas into the same “shortened” applicator, an applicator is obtained that produces a SAR distribution that is essentially a composite of small overlapping SAR patterns produced by weakly interacting incoherently driven antennas. Such a design significantly improves the applicator's lateral heating efficiency and allows the independent control of temperatures in certain tumor areas by changing the input power to the respective antennas.

ISSN:0269-3127    

DOI:10.1118/1.596704

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A study of activity quantitation with prereconstruction Metz filtering and use of the scatter degradation factor (SDF) to numerically correct for scatter was conducted. The ratio of the count rate per unit activity for source locations within a 30×23‐cm water‐filled tub phantom to the count rate per unit activity for Tc‐99m point sources of known activity imaged in air was used to judge the accuracy of activity determination. The investigation was conducted for certain locations within the tub when it was uniformly filled with Tc‐99m activity, and for the same locations at the center of 5, 4, 3, and 2‐cm diam, hot spheres imaged in a cold background. The source locations were the center, and one‐fourth, one‐half, and three‐fourths the major axis. Various methods of combining the conjugate views for use with prereconstruction attenuation correction (arithmetic and geometric mean), and extent to which the Metz filter followed the inverse filter before rolling off to suppress noise were investigated. Without Metz filtering, attenuation correction was performed using a transmission curve that included buildup. With Metz filtering, the good‐geometry attenuation coefficient was used and the combined views were scaled by the SDF calculated for the average body thickness. Depending on the size of the sphere and the extent to which the inverse filter was followed, Metz filtering combined with use of the SDF improved the accuracy of activity quantitation.

ISSN:0269-3127    

DOI:10.1118/1.596705

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is generally recognized that single‐photon emission computed tomography (SPECT) imposes very stringent requirements on gamma camera uniformity to prevent the occurrence of ring artifacts. The purpose of this study was to examine the relationship between nonuniformities in the planar data and the magnitude of the consequential ring artifacts in the transaxial data, and how the perception of these artifacts is influenced by factors such as reconstruction matrix size, reconstruction filter, and image noise. The study indicates that the relationship between ring artifact magnitude and image noise is essentially independent of the acquisition or reconstruction matrix sizes, but is strongly dependent upon the type of smoothing filter applied during the reconstruction process. Furthermore, the degree to which a ring artifact can be perceived above image noise is dependent on the size and location of the nonuniformity in the planar data, with small nonuniformities (1–2 pixels wide) close to the center of rotation being less perceptible than those further out (8–20 pixels). Small defects or nonuniformities close to the center of rotation are thought to cause the greatest potential corruption to tomographic data. The study indicates that such may not be the case. Hence the uniformity requirements for SPECT may be less demanding than was previously thought.

ISSN:0269-3127    

DOI:10.1118/1.596706

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Video cameras are used in many film digitization and teleradiology systems. However, the density range of medical radiographs often exceeds the dynamic range of the camera, and all diagnostic information in the original image may not be captured. Information in both the high and low density areas of the film can be captured in a single video frame if the transmitted luminance range of the radiograph is reduced. This can be accomplished by spatially modulating the back illumination of the film such that areas of lesser density receive less illumination while areas of greater density receive greater illumination. In this work, the use of a video monitor is shown to be an effective means to provide spatially modulated light for compressing the transmitted luminance range and thereby expanding the apparent dynamic range of the video camera. A simple computer‐interfaced video feedback system that determines the appropriate compression mask and a scheme for linearization of system response are described. This system provides an interactive means for control of the degree of range compression.

ISSN:0269-3127    

DOI:10.1118/1.596707

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY