摘要:

A detector has been developed and used to measure dose distributions versus linear energy transfer to thin gas targets in spherical geometry from fast neutron irradiation of tissue‐equivalent plastic and carbon. The detector is a hemispherical proportional counter with a Cs(T1) scintillator at the center of the hemisphere. The coincidence of the proportional counter signals constrain the measurements to charged particles traversing the radius of the hemisphere. The charged particle energy deposition distributions are directly measured for a known pathlength. The A‐150 kerma factor was measured at a neutron energy of 14.8 MeV and is in agreement with tabulated values. The carbon kerma factor measurements are less than the tabulated value at 14.8 MeV. The alpha‐particle production in carbon was measured for neutron energies from 14.1 to 14.8 MeV and is compared with existing data.

ISSN:0094-2405    

DOI:10.1118/1.596523

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin‐window parallel‐plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co‐60 gamma rays to 24‐MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel‐plate ion chambers. It is observed that for energies>10 MV most parallel‐plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high‐Zinterfaces and lower energies, the dose measurements should be performed with ultrathin‐window parallel‐plate ion chambers or extrapolation chambers.

ISSN:0094-2405    

DOI:10.1118/1.596553

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed‐separation plane‐parallel ionization chambers are most commonly used for this purpose. Recent papers have re‐emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed‐separation plane‐parallel ionization chambers. Data for Co‐60, 6‐, 10‐, 18‐, and 24‐MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane‐parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of>19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.

ISSN:0094-2405    

DOI:10.1118/1.596522

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dosimetry measurements at nominal electron energies of 4, 6, 8, 10, 12, 15, 17, 20, and 22 MeV were made for different sized, open‐sided applicators on two Philips SL25 linear accelerators. Measurements include beam flatness, percentage depth dose, surface dose, isodose curves, field size dependence, output at extended distances, virtual source position, and required low melting point alloy thickness for field shaping. These measurements are presented to document the characteristics of electron beams with a new type of applicator design on this series of Philips accelerators.

ISSN:0094-2405    

DOI:10.1118/1.596524

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An algorithm is described for generating a treatment plan with minimal input from the user for a remote high‐dose‐rate afterloading irradiator. The algorithm generates a plan after locating all catheters involved and an area of interest on each catheter, and two additional numbers are specified: a radial distance and a target dose. The treatment volume becomes the locus of all points that are within the specified radial distance from any point within the area of interest on any catheter (except for the end points). For a single catheter, the volume may be alternately outlined on an x‐ray film of the implant. The routine uses a linear programming formulism to compute which dwell positions are to be used, as well as the dwell time at each position, to irradiate the treatment volume to the target dose while minimizing the total volume integrated dose to the patient.

ISSN:0094-2405    

DOI:10.1118/1.596525

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo calculations were used to simulate the propagation of visible and near‐infrared light through homogenous tissue in order to quantitate the potential spatial resolution performance for transillumination imaging. Specifically, the relative effectiveness of coaxial collimation and time of flight (TOF) detection for improving spatial resolution was investigated. The results demonstrate that significant improvements in spatial resolution can be achieved through these techniques, with TOF methods offering superior performance for a given level of detected signal intensity.

ISSN:0094-2405    

DOI:10.1118/1.596526

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We report on the reproducibility of human observers' vanishing detection thresholds for visual targets in contrast–detail (C/D) analysis of ultrasound B‐mode images. The images used in this study contain visual targets which are circular cross sections of constant‐contrast conical structures in the C/D phantom. The vanishing threshold diameters for these targets vary as a function of the perceived size of the imaged target, target‐to‐background contrast, image noise content, and reproducibility of the decision levels of human observers for repeated observations. Our study indicates that the determination of absolute vanishing threshold diameter values for several targets of different contrast by human observers yields a high degree of error that is not predicted by existing theoretical assumptions based on a static threshold detector. We find that systematic error is introduced by the observers during the course of the experiment and that the levels of sensitivity of the observers differ widely at all times, and increase the amount of total observer error. These results suggest that, due to the large total observer error, C/D analysis may be impractical in a clinical environment, unless there is access to a team of observers specifically and extensively trained in this task. We suggest that a computer‐based observer may be more reliable for the objective performance of contrast–detail analysis as a method for evaluating ultrasound image quality and comparison of imaging systems.

ISSN:0094-2405    

DOI:10.1118/1.596527

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurement of spatial noise power spectra from television based radiographic and fluoroscopic systems is essential to the understanding of their operation and optimization. However, conventional methods require acquisition and processing of large numbers of complete images, thus confining such measurements to special applications where accessible frame buffers already exist or elaborately equipped laboratories. We have developed a method which only requires storage of single TV lines or point scans. A digital oscilloscope captures these point scans and a laboratory microcomputer facilitates manipulation of the data to separate out different components of the noise power spectra. The x‐ray dependent component of the noise power spectrum so produced is not the ordinary Wiener spectrum. However, it is shown that reconstruction of the full Wiener spectrum from this is possible subject only to the requirement that the x‐ray noise spectrum at the output of the imaging system is circularly symmetric.

ISSN:0094-2405    

DOI:10.1118/1.596528

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous studies [J. Opt. Soc. Am.4A895–901 (1987)] have shown the utility of multivariate moment‐generating functions for analyzing the influence of stochastic amplifying and scattering mechanisms on the transfer of signal and noise through multistage imaging systems. Recently, these studies were extended to include cases in which the amplification or scattering parameters are themselves stochastic variables [J. Opt. Soc. Am.6A, 1156–1164 (1989)]. In this paper we consider a special case in which amplification is followed by scattering such that the same random variable which characterizes the parameters of each amplification process also characterizes the parameters of the subsequent scattering of the amplified output events. In radiographic imaging, this can be used to describe the physics of the depth dependence of emission efficiency and light scatter in x‐ray intensifying screens, which was originally treated by Lubberts [J. Opt. Soc. Am.58, 1475–1483 (1968)]. In this work Lubberts' original results are rederived in a more general form. They are then illustrated in terms of a diffusion model [Appl. Opt.12, 1865–1870 (1973)] for light scatter within the intensifying screen.

ISSN:0094-2405    

DOI:10.1118/1.596529

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We are developing a computerized method for measurement of lung texture in digital chest radiographs for detection and characterization of interstitial disease. Physical texture measures are obtained from analysis of the power spectrum of the lung texture. We have investigated the effect of digital parameters such as pixel size, regions of interest size, the number of quantization levels, and the peak frequency of the visual system response, as well as the effect of the unsharp masking technique on the performance of this computerized method. We calculated the texture measures by changing digital parameters for 100 normal lungs and 100 abnormal lungs in our database. Receiver operating characteristic (ROC) curves were employed for evaluation of the performance of this computerized method for distinguishing between normal and abnormal lungs. We used the area under the ROC curve to compare the detection accuracy for interstitial infiltrates. We believe that the results of this study may be useful as a guide in the design of computerized schemes for lung texture analysis in digital chest radiographs.

ISSN:0094-2405    

DOI:10.1118/1.596530

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY