摘要:

A study was conducted to evaluate the subjective improvement in portal film image quality resulting from the contact copy contrast enhancement technique which was introduced six years ago. Five observers were asked to identify and orient polyvinyl chloride cylinder images on both original and contrast‐enhanced portal films taken with a 10‐MeV linear accelerator. Fixed reviewing periods (T) were alloted of 20, 40, and 60 s as well as unlimited viewing time in order to increase the clinical relevance of this comparison. A scoring system and a probability representation were used to compare the original and enhanced films as a function ofT. The results show a substantial increase in object detectability for the enhanced films at the short viewing times (T=20, 40, and 60 s). For longer times (Tgreater than or equal to 80 s) the object detectability for enhanced and original films is not statistically different.

ISSN:0094-2405    

DOI:10.1118/1.596125

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A digital beam attenuator system has been developed to automatically generate patient‐specific compensating filters for chest radiography. An initial low‐dose test image is used to generate the attenuator, which is fabricated by overprinting multiple layers of a heavy‐metal material onto a nonattenuating substrate. The attenuator is subsequently inserted into the x‐ray beam for a final compensated radiograph. The effects of focal spot blurring and limited attenuator resolution result in the final compensated image containing only high‐spatial frequency information. The frequency response of the process is not strictly describable by a modulation transfer function, but an approximation of the frequencies remaining in the compensated image is obtained for low‐contrast conditions. It is found that a 4×4 blurring function on the original 64×64 test image is required for the attenuator to give appropriate compensated image appearance. A proposed attenuator printing scheme prints the attenuator in a 16×16 matrix, staggering successively printed layers to achieve the required 64×64 sampling with appropriate blurring. The resulting compensated image has good anatomical definition and contains a frequency response similar to that obtained by compensation techniques being investigated by Plewes and Sorenson.

ISSN:0094-2405    

DOI:10.1118/1.596086

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper summarizes the results of research and development of a prototype amorphous selenium digital imaging system. The first phase of this project consists of the preliminary design and fabrication of the system. In this system the conventional film–screen photon receptor is replaced by a charged amorphous selenium imaging plate. After exposure, the latent electrostatic image on the selenium surface is scanned with multiple microelectrometer probes forming a 1024×1024×12 bit digital image. The second phase investigates the system's physical imaging characteristics and clinical feasibility. X‐ray exposure latitude comparable to 200‐speed calcium–tungstate film–screen system are shown for three typical diagnostic kVp settings with total filtration of 3 mm aluminum and 9 cm Lucite. Using the modulation transfer function (MTF), resolving power of approximately 1.0 line pair per millimeter and detective quantum efficiency values of approximately 5% have been measured. The clinical evaluation consists of preliminary images of a 16‐kg female dog and a 4.5‐kg rabbit and comparisons to film–screen images are offered.

ISSN:0094-2405    

DOI:10.1118/1.596087

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several investigators have approximated the scatter component of digital radiographic images by applying a constant weighting factor to a convolved version of the detected image. The scatter approximation is then subtracted from the detected image. When a constant weighting factor is used, the scatter in areas of low transmission is underestimated. We have extended this technique by allowing for a spatially variable weighting factor based on the local image intensity. This technique improves the scatter estimate and should provide better results for general videodensitometric applications.

ISSN:0094-2405    

DOI:10.1118/1.596088

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A cylindrical aluminum/Plexiglas phantom representing trabecular bone surrounded by various amounts of cortical bone was constructed. Measurements of this phantom using a computed tomography scanner with a125I photon source demonstrated errors of 0% to 28% in the density of trabecular bone. Two contributing factors are identified: scatter and exponential edge‐gradient effect. A simple first‐order correction is developed to correct for the scatter‐induced error. Relative to the exponential edge‐gradient effect, which contributes up to 3.4% error over the range of cortical thicknesses measured, the correction procedure reduces the scatter‐induced error to a level of −0.66% to +0.61%. The consistency of the optimized correction parameters with the physical model as well as the effect of scatter measured by the same phantom on a GE 8800 scanner are shown.

ISSN:0094-2405    

DOI:10.1118/1.596089

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Adaptation of existing fluoroscopic television rooms to permit radiographs to be recorded directly by the television system would be a very practical way to introduce digital radiography into general use if the image quality was adequate. The main limitation of current systems is spatial resolution. Although a digital system can change the spatial frequency response by application of spatial frequency enhancement, the amount of enhancement which can be usefully applied is a function both of the spatial frequency response of the whole system and image noise. Slow scan television systems have been proposed as a way to reduce noise; and hence, as a way to potentially improve limiting resolution. We examine all the consequences of changing readout scan rate, both theoretically and with measurements on a commercial camera with a lead oxide image tube. It is found that recent improvements in camera tube design, specifically the increase in current capability of the electron gun, have reduced the advantages theoretically available with slow scan. However, an order of magnitude increase in useful dynamic range is still possible.

ISSN:0094-2405    

DOI:10.1118/1.596090

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15‐MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low‐density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.

ISSN:0094-2405    

DOI:10.1118/1.596048

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Therac 20 6‐MeV scanned electron beam may be used for partial or total skin therapy. The maximum field size at 1 m is 30×30 cm defined by a set of primary photon collimators in conjunction with secondary trimmers. We have studied electron beam profiles with and without trimmers at the nominal source–skin distance of 1 m versus extended distances of 3–5 m. We find that the trimmers limit the field size and add little to the beam uniformity at extended distances. Beam energy, dose distributions, and output factors at extended distances were measured for single and multiple field arrangements with and without trimmers. Beam parameters were measured after introducing a degrader that lowered the energy to 3.7 MeV.

ISSN:0094-2405    

DOI:10.1118/1.596049

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An approximation to saturation curves of parallel‐plate ionization chambers for various gases under continuous irradiation is given. The proposed fit asymptotically agrees with Mie's set of equations for the low and the high collection efficiency regions. It is shown that this fit which, for gases containing electronegative atoms, is dependent upon a parameter obtained from physical constants agrees with measurement and with values calculated from Mie's set of equations. However, for gases containing nonelectronegative atoms, it is shown that an additional experimental parameter is required to fit the measurements.

ISSN:0094-2405    

DOI:10.1118/1.596050

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The potential performance of certain photonuclear target systems designed to produce short‐lived positron emitters for nuclear medicine applications was evaluated. Included were calculations of photonuclear target activity yields, relative radiolysis product yields and relative specific activities of products as a function of photonuclear target shapes, dimensions, spacing from the bremsstrahlung photon source, and the energy of the electron beam using the reactions12C(γ,n)11C,14N(γ,n)13N, and16O(γ,n)15O. Results indicate that substantial activity yields can be achieved in these photonuclear target systems and that specific activity of the product varies widely as a function of the energy of the electron beam used to produce the bremsstrahlung and the included angle from the central beam axis that the photonuclear target occupies. Results indicate that several commercial radiotherapy accelerators appear to have electron beam energy and current capabilities suitable for producing multiple mCi (1 mCi=3.7×107Bq) amounts of the above radionuclides.

ISSN:0094-2405    

DOI:10.1118/1.596141

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY