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G T Barnes

Publications and source records attributed to G T Barnes.

88 records · Page 5Linked to original sources
PubMedDate not supplied

Experimental measurements of the scatter reduction obtained in mammography with a scanning multiple slit assembly.

A nonconventional and sensitive method of measuring scatter is described. The method was employed to quantitate the scatter imaged in mammography with a conventional unit, a unit equipped with a grid, and a unit equipped with a scanning multiple slit assembly (SMSA). The results indicate that the grid technique significantly reduces the scatter imaged, while the SMSA virtually eliminates it. The resultant increase in large area contrast is readily apparent on radiographs with greater improvement obtained with the SMSA than with the grid. The effect of the increase in contrast on small detail visibility was assessed with a phantom having simulated fibrils and calcifications. Significantly more fibril and calcification detail was visible with the grid and SMSA technique than with the conventional technique. The detail visible with the grid and SMSA technique was comparable, and the lack of better performance by the SMSA unit is attributed to its poorer MTF.

Female↗
PubMedDate not supplied

Radiographic mottle: a comprehensive theory.

A comprehensive theory of radiographic density fluctuations is developed. The theory incorporates film granularity, quantum mottle, the absorbed x-ray photon energy distribution, fluctuations in phosphor light photon yield, and phosphor coating weight variations. The resulting expressions allow one to calculate the density fluctuations or, alternatively, the Wiener spectrum that will be obtained with a radiographic screen/film system from the physical parameters of the system. Reasonable agreement is obtained between experimental results and calculated values for three typical screen/film systems. An expression is also derived for the detective quantum efficiency (DQE) of screen/film systems, and the results of DQE calculations are presented for a commonly used screen/film combination as a function of density and of object size.

Mathematics↗
PubMedDate not supplied

An examination of errors in characteristic curve measurements of radiographic screen/film systems.

The precision and accuracy achieved in the measurement of characteristic curves for radiographic screen/film systems is quantitatively investigated for three techniques: inverse square, kVp bootstrap, and step-wedge bootstrap. Precision of all techniques is generally better than +/- 1.5% while the agreement among all intensity-scale techniques is better than 2% over the useful exposure latitude. However, the accuracy of the sensitometry will depend on several factors, including linearity and energy dependence of the calibration instrument, that may introduce larger errors. Comparisons of time-scale and intensity-scale methods are made and a means of measuring reciprocity law failure is demonstrated.

Radiographic Image Enhancement↗
PubMedDate not supplied

Signal-to-noise ratio considerations in radiographic imaging.

The methodology employed to calculate radiographic signal-to-noise ratio (SNR) for the commonly used disk-shaped test object is reviewed. Although the "matched" aperture yields the maximum SNR, its shape is difficult to achieve experimentally. On the other hand, a circular aperture having the same size as the object is much simpler to realize. We have performed a numerical analysis of the SNRs obtained with matched and circular apertures for two screen/film systems (a mammographic and a general purpose combination) for a range of test-object sizes. We find that for object sizes greater than 0.5 mm in diameter, there is a less than 4% difference between the SNRs obtained with the two apertures. The shapes of some of the matched apertures are also presented. We conclude that the SNR determined with the circular aperture is a simpler and more useful determinant of system performance for the screen/film combinations studied.

Information Theory↗
PubMedDate not supplied

Exposure uniformity considerations in slit radiography.

Slit radiography is a highly efficient means of suppressing scatter. However, when employed with a pulsating x-ray source, a grid line type of artifact results if the movement of the slit is not properly coupled to the periodicity of the source. Taking the exposure of the film to be the convolution of the slit transmittance and the radiation intensity, the conditions for a uniform exposure have been derived for the realistic configuration of a multiple-slit array and a slit transmittance that includes the focal spot and slit geometry. Expressions for the amplitude and spatial frequency of the artifact when the conditions are not met are derived. Methods of suppressing the artifact when its vanishing conditions are not met are also discussed.

Mathematics↗
PubMedDate not supplied

A method to measure the MTF of digital x-ray systems.

A method has been devised to accurately measure the modulation transfer function (MTF) of digital x-ray systems up to and, for undersampled systems, beyond the pixel Nyquist frequency (fN). A phantom consisting of an array of parallel tungsten or similar wires is imaged, and discrete Fourier transforms of rows of pixel values are computed. Under suitable conditions of phantom orientation, wire diameter, wire spacing, and image magnification, the envelope of the modulus of the mean Fourier transform represents the system MTF. Experimental results extending beyond fN are presented for an undersampled prototype digital chest x-ray system and shown to be in reasonable agreement with predicted values. Employment of the method with other digital imaging modalities [i.e., computerized tomography (CT) scanners and nuclear magnetic resonance (NMR) units] is also discussed as well as error considerations and practical problems in implementing the method.

Humans↗
PubMedDate not supplied

Performance characteristics of a dual-energy detector for digital scan projection radiography.

An energy discriminating x-ray detector has been developed for dual-energy, scan projection digital radiography. The detector is comprised of a pair of x-ray intensifying screen/linear photodiode arrays, aligned one behind the other. Energy discrimination is achieved by employing a low atomic number phosphor in the front screen and a high atomic number phosphor in the back screen. The x-ray response, modulation transfer function, and defective quantum efficiency of the detector are reported along with the experimental methodology utilized for the measurements. Also presented is an analysis which indicates that in a typical patient's lung field, the detector can resolve the projected density (g/cm2) of a 3-mm-thick, 1-cm2 area of bone to better than 1.5%.

Humans↗
PubMedDate not supplied

Noise correlations in images acquired simultaneously with a dual-energy sandwich detector.

An energy discriminating detector for dual-energy radiography can be configured as a two-layer sandwich, where the mean energy of photons detected by the two layers differs. To characterize the quantum noise of such a detector, the noise covariance between the two layers must be known in addition to the noise variance in each layer. A theory is presented which permits the calculation of the noise covariance, and it is found to be negligibly small. Experimental results, based on measurements with a Na1 sandwich detector and an isotope gamma ray source, are reported and shown to confirm the theory. The quantum noise in each layer is independent and Poisson.

Radiographic Image Enhancement↗
PubMedDate not supplied

An energy sensitive cassette for dual-energy mammography.

A cassette for simultaneously acquiring dual-energy mammographic images is proposed and studied utilizing a theoretical noise analysis model. The cassette consists of a sandwich of two storage phosphor plates separated by a copper filter. The front, low atomic number, plate was assumed to be comprised of SrFBr and the rear, high atomic number, plate of commercially available BaFBr (98 mg/cm2 coating weight). Assuming a constant x-ray tube voltage of 50 kVp and a typical breast thickness, the theoretical model yielded a front SrFBr phosphor coating weight of approximately 21 mg/cm2. The study indicates that a relatively large separation in the average of x-ray photon energies absorbed in the two plates can be obtained. It also indicates that both a high-quality conventional (single energy) digital image and a tissue canceled digital image (i.e., a calcium image) can be obtained at dose levels comparable to those currently employed. The latter image could potentially improve the early detection of cancerous microcalcifications and also lends itself to computer aided diagnosis.

Female↗
PubMedDate not supplied

Assessing fluoroscopic contrast resolution: a practical and quantitative test tool.

Fluoroscopic contrast resolution is commonly determined at a specified kVp by imaging a test object comprised of targets where contrast decreases gradually and sequentially. Threshold contrast or contrast resolution is the contrast of the lowest contrast target that can be perceived. This approach suffers from two problems. First, test object contrast is specified at a x-ray tube voltage that is not always obtainable in practice. Second, the small change in contrast between adjacent targets contributes to observer variability making consistent and reproducible contrast threshold determinations difficult. Described is a contrast resolution test tool that eliminates or reduces these problems. The novel target arrangement allows one to quickly and easily specify the contrast resolution of a fluoroscopic imaging chain to a precision approximately equal to 0.5%. Tables of target contrast versus x-ray tube potential are developed that permit one to employ the test object for contrast resolution determination over the normal range of tube potentials encountered on clinical units.

Fluoroscopy↗
PubMedDate not supplied

Bone mineral densitometry with x-ray and radionuclide sources: a theoretical comparison.

Two methods of dual-photon absorptiometry (DPA) utilizing an x-ray tube instead of a radionuclide source have recently been introduced. In one method kVp switching is employed and two transmitted intensities at each pixel are determined. In the other method, K-edge filtration combined with a single kVp spectrum is used, but photons in two energy windows are counted. We present a theoretical analysis of the two methods, focusing on a figure of merit which is essentially the exposure efficiency (the precision for a given entrance exposure) and tube loading. We also compare their exposure efficiencies to theoretical limits that no DPA system can exceed. Our study indicates that the K-edge-filtered method is more exposure efficient by about a factor of 2. The switched-kVp method requires less heat units per scan by about a factor of 3. A hybrid K-edge switched-kVp method is suggested which achieves the same exposure efficiency as the K-edge-filtered method at lesser tube loading. Our theoretical model is based on published x-ray spectra and attenuation coefficients and is in good agreement with other simulation work. It is of interest that a point source of Gd-153 would be even more exposure efficient, achieving about 90% of the theoretical limit. However, in practice, the Gd source is of finite size and limited strength, and consequently the radionuclide method cannot achieve as good a precision as either x-ray method in similar scan times.

Bone Density↗
PubMedDate not supplied

Molybdenum target x-ray spectra: a semiempirical model.

A semiempirical model for generating molybdenum target x-ray spectra is presented. The model is an extension of a previous model developed by the authors for tungsten and takes into account the depth of production for both bremsstrahlung and characteristic x-ray photons. As in the previous work, the optimal model parameters were determined using nonlinear least-squares fits to experimental data. Good agreement between the two was obtained. By varying target angle, off-axis angle, and filtration in the model in accordance with the x-ray tube and geometry of interest, results consistent with tabulated spectra for different conditions have been obtained.

Models, Theoretical↗
PubMedDate not supplied

Semiempirical model for generating tungsten target x-ray spectra.

A semiempirical model for generating tungsten target x-ray spectra is presented. This model extends earlier work in two significant areas. First, both bremsstrahlung and characteristic x-ray production are assumed to occur at varying depths within the target. Second, optimal parameters for the model were determined from experimental spectra utilizing nonlinear least-squares techniques. As a result, good agreement is obtained between calculated and measured x-ray tube spectra and output for different target angles and a wide range of x-ray tube potentials. Such is not the case with previously published models.

Mathematics↗
PubMedDate not supplied

Economic analysis of a comprehensive quality assurance program.

The quality assurance program at The University of Alabama in Birmingham is briefly reviewed and its economic efficacy examined. The development of the program was the result of the cooperative effort of the quality control technologists, the radiological service engineering staff, and the radiation physicists. Analysis of our film and chemistry consumption before and after the program was established indicates that its implementation has saved the hospital approximately $27,000 per year after personnel cost for the last four years. A further $32,000 to $55,000 savings per year is realized by having in house rather than contracted service.

Alabama↗