摘要:

This article is the first in a series on the calculation of electron dose using multiple‐scattering theory. In it we develop a unified theory, which we term Gaussian multiple‐scattering theory, starting from a number of contributions already in the literature: the Fermi–Eyges multiple‐scattering theory, the Yang path length distribution, the second‐order multiple‐scattering theory of Jette [Med. Phys.12, 178 (1985)], and the diffusion theory of Betheet al. [Proc. Am. Philos. Soc.78, 573 (1938)]. After examining in detail the ramifications and limitations of Gaussian multiple‐scattering theory, we derive basic formulas generalizing the Fermi–Eyges theory, for use in subsequent articles. We also find explicit, accurate expressions for incorporating the scattering power into the theory.

ISSN:0269-3127    

DOI:10.1118/1.596166

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been developed to extract pencil beam kernels from measured broad beam profiles. In theory, the convolution of a symmetric kernel with a step function will yield a function that is symmetric about the inflection point. Conversely, by deconvolution, the kernel may be extracted from a measured distribution. In practice, however, due to the uncertainties and errors associated with the measurements and due to the singularities produced in the fast Fourier transforms employed in the deconvolution process, the kernels thus obtained and the dose distributions calculated therefrom, often exhibit erratic fluctuations. We propose a method that transforms measured profiles to new, modified distributions so that they satisfy the theoretical symmetry condition. The resultant kernel from the deconvolution is then free of fluctuations. We applied this method to compute photon and electron dose distributions at various depths in water and electron fluence distributions in air. The agreement between measured and computed profiles is within 1% in dose or 1 mm in distance in high dose gradient regions.

ISSN:0269-3127    

DOI:10.1118/1.596267

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measured and calculated dose distributions under attenuators, which are of smaller cross‐sectional dimensions than the radiation field, are presented. The study was performed on a 4‐MV linac at a source–surface distance of 120 cm on the beam central axis in a water phantom for several thicknesses and cross sections of lead attenuators. Dose correction factors, which are used to multiply the open beam data to get dose distributions under partial attenuators, depend strongly on attenuator parameters and on depths in phantom. A method to calculate dose correction factors for any combination of attenuator parameters and any phantom depth is presented. The calculated dose distributions under partial attenuators agree well with measured data, which indicates that the method can be applied in clinical situations.

ISSN:0269-3127    

DOI:10.1118/1.596167

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

All previous methods for measuring image noise spectra require a noise realization, a static image, typified as a photograph which can be scanned to create the Wiener spectrum. We wished to analyze the spatial noise power spectrum at the output phosphor of a continuously irradiated imaging device, an x‐ray image intensifier (XRII), which is incapable of image storage and thus the image is continually changing as a function of both time and space. Our new method utilizes a pair of slits to measure the relative Wiener spectrum of the temporally changing components of the image (i.e., x‐ray quantum and XRII gain noises). By measuring the modulation transfer function and the Wiener spectrum of the same XRII on the same apparatus it was possible to demonstrate the spatial frequency dependence of the detective quantum efficiency. Adaptations of the method should permit the measurement of Wiener spectra of fluoroscopic television systems directly from the TV monitor.

ISSN:0269-3127    

DOI:10.1118/1.596246

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We are investigating the characteristic features of lung nodules and the surrounding normal anatomic background in order to develop an algorithm of computer vision for use as an aid in the detection of nodules in digital chest radiographs. Our technique involves an attempt to eliminate the background anatomic structures in the lung fields by means of a difference image approach. Then, feature‐extraction techniques, such as tests for circularity, size, and their variation with threshold level, are applied so that suspected nodules can be isolated. Preliminary results of this automated detection scheme yielded high true‐positive rates and low false‐positive rates in the peripheral lung regions of the chest. This detection scheme, which can assist the final diagnosis by the clinician, has the potential to improve the early detection of lung carcinomas.

ISSN:0269-3127    

DOI:10.1118/1.596247

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is proposed that digital scanned projection radiography of the chest be performed by using an energy‐sensitive septaless xenon ionization detector (SXID) to obtain dual‐energy images. The proposed detector is composed of a front region, sensitive to low‐energy x rays, and a rear region, sensitive to high‐energy x rays, separated by a suitable filter layer. We have developed a simple, precise theoretical formulation for dual‐energy optimization, and applied it to the split SXID. We describe the variation of optimum detector performance with source kilovoltage and filtration (material and thickness), and hence heat loading, under conditions of constant exposure and constant dose. We estimate dose as the average absorbed dose to an equivalent water layer of suitable thickness, assuming slab geometry, so that the calculation is as simple as that for exposure.

ISSN:0269-3127    

DOI:10.1118/1.596248

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Diaphanography is an imaging technique used in diagnosis of breast disease including cancer. The breast is illuminated with low intensity light and the transmission pattern of red and near‐infrared radiation is detected, amplified, reconstructed and displayed in a monitor. The instrumentation for diaphanography has evolved empirically, mostly through clinical practice, without a very clear understanding of the scientific basis of the technique. This research is concerned with investigating theoretically the dependence of the contrast produced by a lesion in a diaphanography image on the size, depth at which a tumor is located, photon energy, and photon angular flux distribution. Contrast calculations using thedotcomputer code in a two‐dimensional geometry showed that decreasing the size of a tumor by 50% decreases the contrast by a factor of 3 and 4 for 695‐ and 853‐nm photons, respectively. Decreasing the size of the normal tissue where a tumor is imbedded by 25% (from 4 to 3 cm) does not change the contrast very much (less than 20%) for both 695‐ and 853‐nm photons. The contrast for 950‐ and 695‐nm photons is comparable while the values for 853‐nm photons are smaller by a factor of 5 for similar cases. The contrast was also found to be dependent on the angle at which the diffuse light is detected after it transverses the host tissue, maximum contrast was found for 695‐ and 853‐nm photons at about 55°. For a detection angle of 77° the contrast observed is 3× and 12× smaller for 695‐ and 853‐nm photons, respectively. For smaller angles such as 18° the contrast was found to be a factor of 2 and 4 smaller for 695‐ and 853‐nm photons, respectively. It was also found that a tumor perturbs the photon flux density over very short distances in normal tissue (the effect is very localized). The flux recuperates at a distance of about two diffusion lengths. The condition that the lesion must be located next to the skin surface where a detector or camera is placed overrides any other effect.

ISSN:0269-3127    

DOI:10.1118/1.596249

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This present study reports the results of a computer simulation whose aim was to predict the low‐contrast imaging performance of which a conventional x‐ray image intensifier with charge coupled device (CCD) camera would be capable if incorporated into a computed tomography (CT) volume imager. A vascular imaging task was modeled in our simulation. The effects of detector noise, x‐ray exposure levels, analog‐to‐digital conversion (ADC) precision and residual levels of detected x‐ray scatter were considered. The results of this simulation indicate that the low‐contrast imaging performance of an image intensifier‐based CT system was most limited by the CCD detector readout noise. Given this limitation the detection of greater than about 100 000 detected photons/pixel/projection gave marginal improvement in low‐contrast resolution. At these exposures 12 bit ADC precision resulted in little additional image noise. The effects of detecting scattered x rays are twofold; decreasing the signal‐to‐noise ratio associated with our modeled artery and introducing a cupping artifact. Based on the results from the simulation, it appears that an image intensifier‐based CT system is a feasible concept from a noise viewpoint, if the anticipated imaging task is intravenous angiography.

ISSN:0269-3127    

DOI:10.1118/1.596250

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Images obtained by digital fluorography were checked for compressability. These images include images of coronary vessels and images of peripheral vessels. These images have a very low signal‐to‐noise ratio compared to the optical images usually used for developing compression methods. Configurational entropy was used to represent the information content of these images. Reversible prediction algorithms were extensively checked in a search for minimal residual information, enabling more efficient reversible compression. Optimal results were obtained for algorithms based on two or three neighboring pixels and a semiempirical rule, based on the noise level, was found which decides on the best approach. It was found that raw data images are more predictable than subtracted images although the latter are visually preferred.

ISSN:0269-3127    

DOI:10.1118/1.596272

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The authors study the acceptance of a Capintec parallel‐plate ionization chamber. The Capintec chamber is used for dose measurements in a lead and polystyrene slab phantom irradiated with cobalt‐60 γ rays. The authors define an enhancement ratio to quantify the dose measurements. The enhancement ratio equals the ratio of dose measured with the lead slab present to dose measured under equilibrium conditions in polystyrene at equal primary beam attenuation. The measured enhancement ratio at the exit side of the lead/polystyrene interface is 25% lower than the Monte Carlo predicted enhancement ratio. The authors propose that geometric acceptance limitations of the Capintec chamber to large‐angle, low‐energy electrons are the cause for this difference. A Monte Carlo simulation of the Capintec chamber acceptance confirms the hypothesis.

ISSN:0269-3127    

DOI:10.1118/1.596251

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY