Bowtie filters for dedicated breast CT: theory and computational implementation

PURPOSE

To design bowtie filters with improved properties for dedicated breast CT to improve image quality and reduce dose to the patient.

METHODS

The authors present three different bowtie filters designed for a cylindrical 14-cm diameter phantom with a uniform composition of 40/60 breast tissue, which vary in their design objectives and performance improvements. Bowtie design #1 is based on single material spectral matching and produces nearly uniform spectral shape for radiation incident upon the detector. Bowtie design #2 uses the idea of basis material decomposition to produce the same spectral shape and intensity at the detector, using two different materials. Bowtie design #3 eliminates the beam hardening effect in the reconstructed image by adjusting the bowtie filter thickness so that the effective attenuation coefficient for every ray is the same. All three designs are obtained using analytical computational methods and linear attenuation coefficients. Thus, the designs do not take into account the effects of scatter. The authors considered this to be a reasonable approach to the filter design problem since the use of Monte Carlo methods would have been computationally intensive. The filter profiles for a cone-angle of 0° were used for the entire length of each filter because the differences between those profiles and the correct cone-beam profiles for the cone angles in our system are very small, and the constant profiles allowed construction of the filters with the facilities available to us. For evaluation of the filters, we used Monte Carlo simulation techniques and the full cone-beam geometry. Images were generated with and without each bowtie filter to analyze the effect on dose distribution, noise uniformity, and contrast-to-noise ratio (CNR) homogeneity. Line profiles through the reconstructed images generated from the simulated projection images were also used as validation for the filter designs.

RESULTS

Examples of the three designs are presented. Initial verification of performance of the designs was done using analytical computations of HVL, intensity, and effective attenuation coefficient behind the phantom as a function of fan-angle with a cone-angle of 0°. The performance of the designs depends only weakly on incident spectrum and tissue composition. For all designs, the dynamic range requirement on the detector was reduced compared to the no-bowtie-filter case. Further verification of the filter designs was achieved through analysis of reconstructed images from simulations. Simulation data also showed that the use of our bowtie filters can reduce peripheral dose to the breast by 61% and provide uniform noise and CNR distributions. The bowtie filter design concepts validated in this work were then used to create a computational realization of a 3D anthropomorphic bowtie filter capable of achieving a constant effective attenuation coefficient behind the entire field-of-view of an anthropomorphic breast phantom.

CONCLUSIONS

Three different bowtie filter designs that vary in performance improvements were described and evaluated using computational and simulation techniques. Results indicate that the designs are robust against variations in breast diameter, breast composition, and tube voltage, and that the use of these filters can reduce patient dose and improve image quality compared to the no-bowtie-filter case.

Bowtie filters for dedicated breast CT: Analysis of bowtie filter material selection

PURPOSE

For a given bowtie filter design, both the selection of material and the physical design control the energy fluence, and consequently the dose distribution, in the object. Using three previously described bowtie filter designs, the goal of this work is to demonstrate the effect that different materials have on the bowtie filter performance measures.

METHODS

Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to the differences in path length in a projection have been designed. The nature of the designs allows for their realization using a variety of materials. The designs were based on a phantom, 14 cm in diameter, composed of 40% fibroglandular and 60% adipose tissue. Bowtie design #1 is based on single material spectral matching and produces nearly uniform spectral shape for radiation incident upon the detector. Bowtie design #2 uses the idea of basis-material decomposition to produce the same spectral shape and intensity at the detector, using two different materials. With bowtie design #3, it is possible to eliminate the beam hardening effect in the reconstructed image by adjusting the bowtie filter thickness so that the effective attenuation coefficient for every ray is the same. Seven different materials were chosen to represent a range of chemical compositions and densities. After calculation of construction parameters for each bowtie filter design, a bowtie filter was created using each of these materials (assuming reasonable construction parameters were obtained), resulting in a total of 26 bowtie filters modeled analytically and in the penelope Monte Carlo simulation environment. Using the analytical model of each bowtie filter, design profiles were obtained and energy fluence as a function of fan-angle was calculated. Projection images with and without each bowtie filter design were also generated using penelope and reconstructed using FBP. Parameters such as dose distribution, noise uniformity, and scatter were investigated.

RESULTS

Analytical calculations with and without each bowtie filter show that some materials for a given design produce bowtie filters that are too large for implementation in breast CT scanners or too small to accurately manufacture. Results also demonstrate the ability to manipulate the energy fluence distribution (dynamic range) by using different materials, or different combinations of materials, for a given bowtie filter design. This feature is especially advantageous when using photon counting detector technology. Monte Carlo simulation results from penelope show that all studied material choices for bowtie design #2 achieve nearly uniform dose distribution, noise uniformity index less than 5%, and nearly uniform scatter-to-primary ratio. These same features can also be obtained using certain materials with bowtie designs #1 and #3.

CONCLUSIONS

With the three bowtie filter designs used in this work, the selection of material is an important design consideration. An appropriate material choice can improve image quality, dose uniformity, and dynamic range.

Characterization of scatter magnitude and distribution in dedicated breast computed tomography with bowtie filters

Scatter contamination of projection images in cone-beam computed tomography (CT) degrades the image quality. The use of bowtie filters in dedicated breast CT can decrease this scatter contribution. Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to differences in path length in a projection were studied. These designs have been investigated in terms of their ability to reduce the scatter contamination in projection images acquired in a dedicated breast CT geometry. The scatter magnitude was measured as the scatter-to-primary ratio (SPR) using experimental and Monte Carlo techniques for various breast phantom diameters and tube voltages. The results show that a 55% reduction in the center SPR value could be obtained with the bowtie filter designs. On average, the bowtie filters reduced the center SPR by approximately 18% over all breast diameters. The distribution of the scatter was calculated at a range of different locations to produce scatter distribution maps for all three bowtie filter designs. With the inclusion of the bowtie filters, the scatter distribution was more uniform for all breast diameters. The results of this study will be useful in designing scatter correction methods and understanding the benefits of bowtie filters in dedicated breast CT.

A virtual trial framework for quantifying the detectability of masses in breast tomosynthesis projection data

PURPOSE

Digital breast tomosynthesis (DBT) is a promising breast cancer screening tool that has already begun making inroads into clinical practice. However, there is ongoing debate over how to quantitatively evaluate and optimize these systems, because different definitions of image quality can lead to different optimal design strategies. Powerful and accurate tools are desired to extend our understanding of DBT system optimization and validate published design principles.

METHODS

The authors developed a virtual trial framework for task-specific DBT assessment that uses digital phantoms, open-source x-ray transport codes, and a projection-space, spatial-domain observer model for quantitative system evaluation. The authors considered evaluation of reconstruction algorithms as a separate problem and focused on the information content in the raw, unfiltered projection images. Specifically, the authors investigated the effects of scan angle and number of angular projections on detectability of a small (3 mm diameter) signal embedded in randomly-varying anatomical backgrounds. Detectability was measured by the area under the receiver-operating characteristic curve (AUC). Experiments were repeated for three test cases where the detectability-limiting factor was anatomical variability, quantum noise, or electronic noise. The authors also juxtaposed the virtual trial framework with other published studies to illustrate its advantages and disadvantages.

RESULTS

The large number of variables in a virtual DBT study make it difficult to directly compare different authors' results, so each result must be interpreted within the context of the specific virtual trial framework. The following results apply to 25% density phantoms with 5.15 cm compressed thickness and 500 μm(3) voxels (larger 500 μm(2) detector pixels were used to avoid voxel-edge artifacts): 1. For raw, unfiltered projection images in the anatomical-variability-limited regime, AUC appeared to remain constant or increase slightly with scan angle. 2. In the same regime, when the authors fixed the scan angle, AUC increased asymptotically with the number of projections. The threshold number of projections for asymptotic AUC performance depended on the scan angle. In the quantum- and electronic-noise dominant regimes, AUC behaviors as a function of scan angle and number of projections sometimes differed from the anatomy-limited regime. For example, with a fixed scan angle, AUC generally decreased with the number of projections in the electronic-noise dominant regime. These results are intended to demonstrate the capabilities of the virtual trial framework, not to be used as optimization rules for DBT.

CONCLUSIONS

The authors have demonstrated a novel simulation framework and tools for evaluating DBT systems in an objective, task-specific manner. This framework facilitates further investigation of image quality tradeoffs in DBT.

X-ray properties of an anthropomorphic breast phantom for MRI and x-ray imaging

The purpose of this study is to characterize the x-ray properties of a dual-modality, anthropomorphic breast phantom whose MRI properties have been previously evaluated. The goal of this phantom is to provide a platform for optimization and standardization of two- and three-dimensional x-ray and MRI breast imaging modalities for the purpose of lesion detection and discrimination. The phantom is constructed using a mixture of lard and egg whites, resulting in a variable, tissue-mimicking structure with separate adipose- and glandular-mimicking components. The phantom can be produced with either a compressed or uncompressed shape. Mass attenuation coefficients of the phantom materials were estimated using elemental compositions from the USDA National Nutrient Database for Standard Reference and the atomic interaction models from the Monte Carlo code PENELOPE and compared with human values from the literature. The image structure was examined quantitatively by calculating and comparing spatial covariance matrices of the phantom and patient mammography images. Finally, a computerized version of the phantom was created by segmenting a computed tomography scan and used to simulate x-ray scatter of the phantom in a mammography geometry. Mass attenuation coefficients of the phantom materials were within 20% and 15% of the values for adipose and glandular tissues, respectively, which is within the estimation error of these values. Matching was improved at higher energies (>20 keV). Tissue structures in the phantom have a size similar to those in the patient data, but are slightly larger on average. Correlations in the patient data appear to be longer than those in the phantom data in the anterior-posterior direction; however, they are within the error bars of the measurement. Simulated scatter-to-primary ratio values of the phantom images were as high as 85% in some areas and were strongly affected by the heterogeneous nature of the phantom. Key physical x-ray properties of the phantom have been quantitatively evaluated and shown to be comparable to those of breast tissue. Since the MRI properties of the phantom have been previously evaluated, we believe it is a useful tool for quantitative evaluation of two- and three-dimensional x-ray and MRI breast imaging modalities for the purpose of lesion detection and characterization.

Effect of oblique X-ray incidence in flat-panel computed tomography of the breast

We quantify the variation in resolution due to anisotropy caused by oblique X-ray incidence in indirect flat-panel detectors for computed tomography breast imaging systems. We consider a geometry and detector type utilized in breast computed tomography (CT) systems currently being developed. Our methods rely on mantis, a combined X-ray, electron, and optical Monte Carlo transport open source code. The physics models are the most accurate available in general-purpose Monte Carlo packages in the diagnostic energy range. We consider maximum-obliquity angles of 10 ( degrees ) and 13 ( degrees ) at the centers of the 30 and 40 cm detector edges, respectively, and 16 ( degrees ) at the corner of the detector. Our results indicate that blur is asymmetric and that the resolution properties vary significantly with the angle (or location) of incidence. Our results suggest that the asymmetry can be as high as a factor of 2.6 between orthogonal directions. Anisotropy maps predicted by mantis provide an understanding of the effect that such variations have on the imaging system and allow more accurate modeling and optimization of breast CT systems. These maps of anisotropy across the detector could lead to improved reconstruction and help motivate physics-based strategies for computer detection of breast lesions.

Anisotropic imaging performance in breast tomosynthesis

We describe the anisotropy in imaging performance caused by oblique x-ray incidence in indirect detectors for breast tomosynthesis based on columnar scintillator screens. We use MANTIS, a freely available combined x-ray, electron, and optical Monte Carlo transport package which models the indirect detection processes in columnar screens, interaction by interaction. The code has been previously validated against published optical distributions. In this article, initial validation results are provided concerning the blur for particular designs of phosphor screens for which some details with respect to the columnar geometry are available from scanning electron microscopy. The polyenergetic x-ray spectrum utilized comes from a database of experimental data for three different anode/filter/kVp combinations: Mo/Mo at 28 kVp, Rh/Rh at 28 kVp, and W/Al at 42 kVp. The x-ray spectra were then filtered with breast tissue (3, 4, and 6 cm thickness), compression paddle, and support base, according to the oblique paths determined by the incidence angle. The composition of the breast tissue was 50%/50% adipose/glandular tissue mass ratio. Results are reported on the pulse-height statistics of the light output and on spatial blur, expressed as the response of the detector to a pencil beam with a certain incidence angle. Results suggest that the response is nonsymmetrical and that the resolution properties of a tomosynthesis system vary significantly with the angle of x-ray incidence. In contrast, it is found that the noise due to the variability in the number of light photons detected per primary x-ray interaction changes only a few percent. The anisotropy in the response is not less in screens with absorptive backings while the noise introduced by variations in the depth-dependent light output and optical transport is larger. The results suggest that anisotropic imaging performance across the detector area can be incorporated into reconstruction algorithms for improving the image quality of breast tomosynthesis. This study also demonstrates that the assessment of image quality of breast tomosynthesis systems requires a more complete description of the detector response beyond local, center measurements of resolution and noise that assume some degree of symmetry in the detector performance.

Lubberts effect in columnar phosphors

Noise transfer in granular x-ray imaging phosphor screens is not proportional to the square of the magnitude of the signal transfer when the transfer properties are considered for the entire screen thickness, unless appropriately weighted at each depth of interaction. This property, known as the Lubberts effect, has not yet been studied in columnar structured screens because of a lack of a generalized description of the depth-dependent light transport. In this paper, we investigate the signal and noise transfer characteristics of columnar phosphors used in digital mammography detectors using DETECT-II, an optical Monte Carlo light transport simulation code. We first validate our choice of optical parameters for the description of granular and columnar screens using published normalized modulation transfer (MTF) experimental data. Our calculations of MTF match empirically measured MTFs for a granular film/screen analog system, and for an indirect x-ray digital imaging system with CsI:Tl screen representative of digital mammography systems. Using the depth-dependent spread functions and collection efficiencies, we calculate the signal and noise transfer functions and the Lubberts fraction, which is the ratio of the signal transfer function to the noise transfer function, for different screen thicknesses of granular and columnar phosphors. We find that the Lubberts fraction of a 85 microm granular screen model corresponding to a Gd2O2S:Tb screen is similar to the fraction for a 100 microm columnar CsI:Tl screen.

Noise in flat-panel displays with subpixel structure

Subpixel structures found in medical monochrome active-matrix liquid crystal displays (AMLCDs) affect noise estimates measured with conventional methods. In this work, we discuss methods that identify sources of noise and permit the comparison of luminance noise estimates across technologies independent of pixel design and device technology. We used a three-million pixel AMLCD with a pixel structure consisting of three color stripes, each in a two-domain, in-plane switching mode. Images of uniform fields displayed on the AMLCD were acquired using a low-noise, high-resolution CCD camera. The camera noise and flat-field response were characterized using a uniform light source constructed for this purpose. We show results in terms of spatial luminance noise and noise power spectrum for high-resolution images and for the same images processed with a pixel-aligned aperture. We find that the pixel-aligned aperture eliminates almost all the noise found in the high-resolution images, suggesting that most of the luminance noise in AMLCDs comes from the subpixel structure and less-than-100% aperture ratio, rather than from interpixel variations.

Study design in the evaluation of breast cancer imaging technologies

RATIONALE AND OBJECTIVES

Bringing a new imaging technology to market is a complex process. Beyond conceptualization and proof of concept, obtaining U.S. Food and Drug Administration (FDA) approval for clinical use depends on the documented experimental establishment of safety and efficacy. In turn, safety and efficacy are evaluated in the context of the intended use of the technology. The purpose of this study was to examine a conceptual framework for technology development and evaluation, focusing on new breast imaging technologies as a highly visible and current case in point.

MATERIALS AND METHODS

The FDA views technology development in terms of a preclinical and four clinical phases of assessment. With a concept of research and development as a learning model, this phased-assessment concept of regulatory review against intended use was integrated with a five-level version of a hierarchy-of-efficacy framework for evaluating imaging technologies. Study design and analysis issues are presented in this context, as are approaches to supporting expanded clinical indications and new intended uses after a new technology is marketed.

CONCLUSION

Breast imaging technologies may be intended for use as replacements for standard-of-care technologies, as adjuncts, or as complementary technologies. Study designs must be appropriate to establish claims of superiority or equivalence to the standard for the intended use. Screening technologies are ultimately judged on their demonstrated effectiveness in decreasing cause-specific mortality through early detection, but they may be brought to market for other uses on the basis of lesser standards of efficacy (eg, sensitivity, specificity, positive and negative predictive value, and stage of disease detected).

Molybdenum, rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography

Computer simulation is a convenient and frequently used tool in the study of x-ray mammography, for the design of novel detector systems, the evaluation of dose deposition, x-ray technique optimization, and other applications. An important component in the simulation process is the accurate computer-generation of x-ray spectra. A computer model for the generation of x-ray spectra in the mammographic energy range from 18 kV to 40 kV has been developed. The proposed model requires no assumptions concerning the physics of x-ray production in an x-ray tube, but rather makes use of x-ray spectra recently measured experimentally in the laboratories of the Center for Devices and Radiological Health. Using x-ray spectra measured for molybdenum, rhodium, and tungsten anode x-ray tubes at 13 different kV's (18, 20, 22, ..., 42 kV), a spectral model using interpolating polynomials was developed. At each energy in the spectrum, the x-ray photon fluence was fit using 2, 3, or 4 term (depending on the energy) polynomials as a function of the applied tube voltage (kV). Using the polynomial fit coefficients determined at each 0.5 keV interval in the x-ray spectrum, accurate x-ray spectra can be generated for any arbitrary kV between 18 and 40 kV. Each anode material (Mo, Rh, W) uses a different set of polynomial coefficients. The molybdenum anode spectral model using interpolating polynomials is given the acronym MASMIP, and the rhodium and tungsten spectral models are called RASMIP and TASMIP, respectively. It is shown that the mean differences in photon fluence calculated over the energy channels and over the kV range from 20 to 40 kV were -0.073% (sigma = 1.58%) for MASMIP, -0.145% (sigma = 1.263%) for RASMIP, and 0.611% (sigma = 2.07%) for TASMIP. The polynomial coefficients for all three models are given in an Appendix. A short C subroutine which uses the polynomial coefficients and generates x-ray spectra based on the proposed model is available on the World Wide Web at http:/(/)www.aip.org/epaps/epaps.html.

Absence of enhanced sympathoadrenal activity and behaviorally evoked cardiovascular reactivity among offspring of hypertensives

To determine whether offspring of hypertensives show enhanced sympathetic nervous system activity, we evaluated several indices of sympathoadrenal activation and cardiovascular responsiveness to behavioral stimuli among 90 normotensive, young adult men having either one or two hypertensive parents (PH+(-), PH++) or normotensive parents only (PH--) (n = 30/group). Measurements included heart rate (HR) and blood pressure (BP) reactions to three mental stressors (the Stroop test, mental arithmetic, mirror tracing), a cold pressor test, postural adjustment (60 degrees upright tilt), isometric exercise and bicycle ergometry, as well as the 24-h excretion of catecholamines (epinephrine [E], norepinephrine [NE]) and venous plasma catecholamine concentrations, both at rest (seated and supine) and in response to the Stroop test and upright tilt. The three groups did not differ in age, education, body mass index (BMI), estimated aerobic fitness, resting HR, cardiac preejection period (PEP) and PEP:LVET (left ventricular ejection time) ratio, 24-h Na or K excretion, or fasting lipids, insulin or plasma renin activity. Resting systolic and diastolic BP varied as a function of parental hypertension, and were significantly higher in PH++ than among PH-- subjects (P < .05). No significant group difference was observed on any measure of plasma or urinary catecholamines, nor did offspring of hypertensives (PH++ or PH+-) showed greater HR or BP reactions than PH-- subjects to any of the several laboratory challenges. In sum, we find no evidence of enhanced sympathetic activity or heightened cardiovascular responsiveness among normotensive young adults who are familially predisposed to essential hypertension.

Comparison of beam-hardening and K-edge filters for imaging barium and iodine during fluoroscopy

This study investigated the dose reduction performance of several beam-hardening and K-edge filter materials for the imaging of barium or iodine during fluoroscopy. A computer model was developed to simulate the effect of added filtration on entrance exposure rate (Xp), integral dose rate (Di), contrast (C), signal to noise ratio (SNR), imaging performance per dose (SNR2/Di), and tube load. The model incorporated the response characteristics, in both manual and automatic control modes of operation, of fluoroscopic systems to increasing or decreasing x-ray intensity at the input of the image intensifier. Input parameters to the computer model included choice of filter material and thickness, a barium or iodine test object, tube potential, phantom thickness, a CsI input phosphor, and a set of algorithms for controlling the fluoroscopic system. In all cases, the performance of systems with added filtration was judged with respect to a reference system operating under comparable conditions. In general, either beam-hardening or K-edge filters provided a significant reduction in entrance exposure and integral dose rates, but with an attendant increase in tube load. For a fluoroscopic system constrained to follow a representative automatic brightness control algorithm, added filtration provided a reduction in entrance exposure and integral dose rates for all phantom or uniformly distributed barium thickness. However, the imaging performance per dose, in some cases, decreased rapidly and was less than that of the reference system at large thicknesses. Only as change in the algorithm controlling the kVcp and mA operating points on the fluoroscopic system provided an imaging performance per dose greater than the reference system's at large thicknesses. The practical implementation of adding filtration to fluoroscopic systems is most simply accomplished with beam-hardening filters rather than K-edge filters. However, the systems with K-edge added filtration can provide slightly better performance when used over a limited range of phantom thicknesses such as the range normally associated with pediatric patients.

A patient-equivalent attenuation phantom for estimating patient exposures from automatic exposure controlled x-ray examinations of the abdomen and lumbo-sacral spine

The Joint Commission on Accreditation of Healthcare Organizations requires diagnostic radiology facilities to known the approximate amount of radiation received by an average patient during radiographic examinations at the facility. Automatic exposure controlled (AEC) techniques are used for many of these exams, and a standard patient-equivalent phantom is necessary when estimating patient exposure on such systems. This is of particular importance if exposures are to be compared among AEC systems with different entrance x-ray spectra. We have developed a phantom, LucA1 Abdomen, to facilitate determining the average patient exposure from AEC anteroposterior (AP) abdomen and lumbo-sacral (LS) spine radiography. The phantom is relatively lightweight, transportable, sturdy, and made of readily available inexpensive materials (Lucite and aluminum). It accurately simulates the primary and scatter transmission through the soft tissue and L-4 spinal regions of a patient-equivalent anthropomorphic phantom for x-ray spectra typically used in abdomen/LS spine radiography. A clinical evaluation to verify the patient-equivalence of three commercial anthropomorphic phantoms (Humanoid, Rando, 3-M) and two acrylic/aluminum phantoms (ANSI and LucA1 Abdomen) has been conducted. The design and development of the LucA1 Abdomen phantom and the evaluation of all phantoms is described.

A method for comparing beam-hardening filter materials for diagnostic radiology

The necessity for using adequate beam filtration in diagnostic radiology is well known. Although aluminum is the most widely used filter material for diagnostic x-ray applications, the possibility that other materials might have superior properties has prompted a number of studies that have attempted to determine both the type and the amount of filtration most appropriate for a given situation. This paper describes a method based on precise matching of spectral shape that permits the absolute ranking of beam-hardening materials. Matching of spectral shape ensures equality of such parameters as image contrast and patient dose. Spectrally equivalent filters can then be ranked on the basis of the transmission of one relative to another. Following the development of the theory behind the method and an algorithm for implementing it, the method is applied to the evaluation of a variety of materials for use as filters in diagnostic radiology. Experimental verification of a few of the calculated results is also described. Both calculated and experimental results show that normal aluminum filters are about 10% less efficient than filters of materials such as copper, brass, or iron. Since the approach followed here was the basis for several early investigations of filtration for orthovoltage therapy, a brief comparison of results from these early reports with results calculated using the method developed here is also presented.

Diagnostic imaging: finding the critical features

The clinical interpreter of a diagnostic image has the task of reducing the information displayed-in the order of a megabyte-to a set of classifications that convey answers to specialized diagnostic questions-in the order of just a few bits. A high-dimensional feature space is thereby reduced to a low-dimensional feature space. This paper reviews the features available in x-ray imaging, magnetic resonance imaging, and diagnostic ultrasound that derive from the geometry and the physics of the imaging process. An example is drawn from diagnostic ultrasound to demonstrate an objective assessment of a small feature space and the utility of adding back additional features. The assessment is based on the methodology of the receiver-operating characteristic (ROC) curve. Motivation is provided for use of this fundamental tool from statistical decision theory, together with definitions of the quantities involved in scoring performance.

Noise analysis of scintillation camera images: stochastic and non-stochastic effects

An analysis of noise was performed on scintillation camera images. The analysis was performed on sets of field floods, uniform exposures over the field of view, in such a way that stochastic and non-stochastic effects were separable. Stochastic noise for the digitally acquired images was found to approach the limit of Poisson count statistics. The non-stochastic component of the calculations dominated the results at the lower spatial frequencies. These results hold true even after standard methods of uniformity correction are applied, primarily due to the effects of Compton scatter events. The noise power spectrum calculations carry information about the patterns of the noise in flood images. Examples of digitisation, photomultiplier tube mottle and edge-packing artefacts are presented.

Beam quality independent attenuation phantom for estimating patient exposure from x-ray automatic exposure controlled chest examinations

The periodic assessment of exposures in diagnostic radiology is an important part of a comprehensive quality assurance program. The most frequent radiologic examination conducted in the United States is chest radiography. Automatic exposure controlled (AEC) techniques are often used for this exam, and a standard patient-equivalent chest phantom is useful when estimating patient exposures on such systems. This is of particular importance if exposures are to be compared among AEC systems with different entrance x-ray spectra. Such a phantom has been developed to facilitate surveys of the average patient exposure from AEC posteroanterior chest radiography. The phantom is relatively lightweight and easily transportable, sturdy and made of readily available and relatively inexpensive materials (Lucite and aluminum). It accurately simulates the primary and scatter transmission through the lung-field regions of a patient-equivalent anthropomorphic phantom for x-ray spectra typically used in chest radiography. A clinical evaluation has been conducted to verify the patient equivalence of the phantom. Measurements of patient entrance skin exposure were obtained for a large number of patients on a variety of x-ray systems operated in the AEC mode using one or both lung-field detectors. Comparison of these data with exposure estimates derived from the phantom indicate that the phantom attenuates the x-ray beam in such a way that it can be employed to accurately and consistently estimate the mean exposure of the average patient under a variety of radiographic conditions. The design, development, and evaluation of the patient-equivalent attenuation phantom is described.

Effects of chronic modification of dietary fat and carbohydrate in rats

1. Rats were fed on diets enriched with starch, sucrose, corn oil or beef tallow for 3 weeks and the activities of various enzymes in the liver were measured. 2. The mitochondrial glycerol phosphate acyltransferase activity was lower in rats fed on the starch diet than on the two high-fat diets. 3. The non-microsomal (presumably peroxisomal) dihydroxyacetone phosphate acyltransferase activity was higher in rats fed on the starch diet and corn-oil diets than in those fed on the sucrose and beef-tallow diets. Urate oxidase activity was higher in rats fed on the starch diet than in the three other groups. There were no significant differences in the activity of acyl-CoA oxidase among the groups. 4. The activity of soluble phosphatidate phosphohydrolase was not significantly different among the dietary groups. There were increases of 3.3--4.3-fold in this activity in the dietary groups 6h after injection of corticotropin. The equivalent increases for the mitochondrial glycerol phosphate acyltransferase were 1.4--1.6 fold. 5. The corticosterone responses to the corticotropin injection were not significantly different between dietary groups. However, the corticosterone response of the rats fed on the two high-fat diets was prolonged when the rats were given an acute load of fructose [Brindley, Cooling, Glenny, Burditt & McKechnie (1981) Biochem. J. 200. 275--283]. 6. Rats fed on the high-fat diets had higher concentrations of circulating cholesterol than those fed on the starch and sucrose diets. Serum triacylglycerol concentrations were lower in the rats fed on the starch diet than in the three other groups. 7. The results are discussed in terms of the relationship between diet, hormonal balance and hepatic glycerolipid metabolism.

The activities of lipoprotein lipase and of enzymes involved in triacylglycerol synthesis in rat adipose tissue. Effects of starvation, dietary modification and of corticotropin injection

1. The effects of dietary modification, including starvation, and of corticotropin injection on the activities of acyl-CoA synthetase, glycerol phosphate acyltransferase, dihydroxyacetone phosphate acyltransferase, phosphatidate phosphohydrolase, diacylglycerol acyltransferase and lipoprotein lipase were measured in adipose tissue. 2. Lipoprotein lipase activities in heart were increased and those in adipose tissue were decreased when rats were fed on diets enriched with corn oil or beef tallow rather than with sucrose or starch. The lipoprotein lipase activity was lower in the adipose tissue of rats fed on the sucrose rather than on the starch diet. 3. Rats fed on the beef tallow diet had slightly higher activities of the total glycerol phosphate acyltransferase in adipose tissue than did rats fed on the sucrose or starch diet. The diacylglycerol acyltransferase and the mitochondrial glycerol phosphate acyltransferase activities were higher for the rats fed on the tallow diet than for those fed on the corn-oil diet. 4. Starvation significantly decreased the activities of lipoprotein lipase (after 24 and 48 h), acyl-CoA synthetase (after 24 h) and of the mitochondrial glycerol phosphate acyltransferase and the N-ethylmaleimide-insensitive dihydroxyacetone phosphate acyltransferase (after 48 h) in adipose tissue. The activities of the microsomal glycerol phosphate acyltransferase, diacylglycerol acyltransferase and the soluble phosphatidate phosphohydrolase were not significantly changed after 24 or 48 h of starvation. 5. The activities of lipoprotein lipase and phosphatidate phosphohydrolase in adipose tissue were decreased 15 min after corticotropin was injected into rats during November to December. No statistically significant differences were found when these experiments were performed during March to September. These differences may be related to the seasonal variation in acute lipolytic responses. 6. These results are discussed in relation to the control of triacylglycerol synthesis and lipoprotein metabolism.

Efficiency of human visual signal discrimination

We have measured the overall statistical efficiency of human subjects discriminating the amplitude of visual pattern signals added to noisy backgrounds. By changing the noise amplitude, the amount of intrinsic noise can be estimated and allowed for. For a target containing a few cycles of a spatial sinusoid of about 5 cycles per degree, the overall statistical efficiency is as high as 0.7 +/- 0.07, and after correction for intrinsic noise, efficiency reaches 0.83 +/- 0.15. Such a high figure leaves little room for residual inefficiencies in the neural mechanisms that handle these patterns.

Optimal x-ray spectra for screen-film mammography

Theoretical and experimental techniques have been used to study optimal x-ray for screen-film mammography. A simple model of mammographic imaging predicts optimum x-ray energies which are significantly higher than the K-characteristic energies of Mo. A subjective comparison of x-ray spectra from Mo-anode and W-anode tubes indicates that spectra produced by a W-anode tube filtered with materials of atomic number just above that of Mo are more suitable for screen-film mammography than spectra produced by the Mo-anode/Mo-filter system. The imaging performance of K-edge filtered, W-anode tube spectra was compared to the performance of Mo-anode spectra using phantom measurements and mastectomy specimen radiography. It was shown that optimal W-anode spectra can produce equal contrast with an exposure reduction of a factor of two to three, a dose reduction of a factor of two, and equal or reducing tube loading, compared to Mo-anode spectra. A computer simulation was carried out to extend the initial, monoenergetic theory to the case of real, polychromatic sources. The effects of varying filter material and thickness, tube operating potential, and breast thickness were all studied. Since W-anode x-ray tubes are considered to be better for Xerox mammography than Mo-anode tubes, this study has shown that both Xerox and screen-film techniques can be performed optimally with a single, properly designed, W-anode x-ray tube.