Biomedical subjects

I A Cunningham

Publications and source records attributed to I A Cunningham.

At least 19 recordsLinked to original sources

PubMed2006

Signal and noise transfer properties of photoelectric interactions in diagnostic x-ray imaging detectors.

Image quality in diagnostic x-ray imaging is ultimately limited by the statistical properties governing how, and where, x-ray energy is deposited in a detector. This in turn depends on the physics of the underlying x-ray interactions. In the diagnostic energy range (10-100 keV), most of the energy deposited in a detector is through photoelectric interactions. We present a theoretical model of the photoelectric effect that specifically addresses the statistical nature of energy absorption by photoelectrons, K and L characteristic x rays, and Auger electrons. A cascaded-systems approach is used that employs a complex structure of parallel cascades to describe signal and noise transfer through the photoelectric effect in terms of the modulation transfer function, Wiener noise power spectrum, and detective quantum efficiency (DQE). The model was evaluated by comparing results with Monte Carlo calculations for x-ray converters based on amorphous selenium (a-Se) and lead (Pb), representing both low and high-Z materials. When electron transport considerations can be neglected, excellent agreement (within 3%) is obtained for each metric over the entire diagnostic energy range in both a-Se and Pb detectors up to 30 cycles/mm, the highest frequency tested. The cascaded model overstates the DQE when the electron range cannot be ignored. This occurs at approximately two cycles/mm in a-Se at an incident photon energy of 80 keV, whereas in Pb, excellent agreement is obtained for the DQE over the entire diagnostic energy range. However, within the context of mammography (20 keV) and micro-computed tomography (40 keV), the effects of electron transport on the DQE are negligible compared to fluorescence reabsorption, which can lead to decreases of up to 30% and 20% in a-Se and Pb, respectively, at 20 keV; and 10% and 5%, respectively, at 40 keV. It is shown that when Swank noise is identified in a Fourier model, the Swank factor must be frequency dependent. This factor decreases quickly with frequency, and in the case of a-Se and Pb, decreases by up to a factor of 3 at five cycles/mm immediately above the K edge. The frequency-dependent Swank factor is also equivalent to what we call the "photoelectric DQE," which describes signal and noise transfer through photoelectric interactions.

Electrons↗

PubMed2005

Computational engine for development of complex cascaded models of signal and noise in X-ray imaging systems.

The detective quantum efficiency (DQE) is generally accepted as the primary metric of signal-to-noise performance in medical X-ray imaging systems. Simple theoretical models of the Wiener noise power spectrum (NPS) and DQE can be developed using a cascaded-systems approach to assess particular system designs and establish operational benchmarks. However, the cascaded approach is often impractical for the development of comprehensive models due to the complexity and extremely large number of algebraic terms that must be manipulated to describe signal and noise transfer. We have developed a computational engine that overcomes this limitation. Using a predefined library of elementary physical processes, complex models are assembled and input-output relationships established using a graphical interface. A novel recursive algorithm is described that allows the signal and noise analyses of models with arbitrary complexity including the use of multiple parallel cascades. Symbolic mathematics is used to develop analytic expressions for the NPS and DQE. The algorithm is validated by manual calculation for simple models and by Monte Carlo calculation for complex models. We believe our approach enables the use of complex cascaded models to design better detectors with improved image quality.

Computer Simulation↗

PubMed2003

Monte Carlo simulation of the image formation process in portal imaging.

We have written Monte Carlo programs to simulate the formation of radiological images. Our code is used to propagate a simulated x-ray fluence through each component of an existing video-based portal imaging system. This simulated fluence consists of a 512 x 512 pixel image containing both contrast-detail patterns as well as checker patterns to assess spatial resolution of the simulated portal imager. All of the components of the portal imaging system were modeled as a cascade of eight linear stages. Using this code, one can assess the visual impact of changing components in the imaging chain by changing the appropriate probability density function. Virtual experiments were performed to assess the visual impact of replacing the lens and TV camera by an amorphous silicon array, and the effect of scattered radiation on portal images.

Computer Simulation↗

PubMed2002

A framework for noise-power spectrum analysis of multidimensional images.

A methodological framework for experimental analysis of the noise-power spectrum (NPS) of multidimensional images is presented that employs well-known properties of the n-dimensional (nD) Fourier transform. The approach is generalized to n dimensions, reducing to familiar cases for n = 1 (e.g., time series) and n = 2 (e.g., projection radiography) and demonstrated experimentally for two cases in which n = 3 (viz., using an active matrix flat-panel imager for x-ray fluoroscopy and cone-beam CT to form three-dimensional (3D) images in spatiotemporal and volumetric domains, respectively). The relationship between fully nD NPS analysis and various techniques for analyzing a "central slice" of the NPS is formulated in a manner that is directly applicable to measured nD data, highlights the effects of correlation, and renders issues of NPS normalization transparent. The spatiotemporal NPS of fluoroscopic images is analyzed under varying conditions of temporal correlation (image lag) to investigate the degree to which the NPS is reduced by such correlation. For first-frame image lag of approximately 5-8%, the NPS is reduced by approximately 20% compared to the lag-free case. A simple model is presented that results in an approximate rule of thumb for computing the effect of image lag on NPS under conditions of spatiotemporal separability. The volumetric NPS of cone-beam CT images is analyzed under varying conditions of spatial correlation, controlled by adjustment of the reconstruction filter. The volumetric NPS is found to be highly asymmetric, exhibiting a ramp characteristic in transverse planes (typical of filtered back-rojection) and a band-limited characteristic in the longitudinal direction (resulting from low-pass characteristics of the imager). Such asymmetry could have implications regarding the detectability of structures visualized in transverse versus sagittal or coronal planes. In all cases, appreciation of the full dimensionality of the image data is essential to obtaining meaningful NPS results. The framework may be applied to NPS analysis of image data of arbitrary dimensionality provided the system satisfies conditions of NPS existence.

Algorithms↗

PubMed2001

Parallel cascades: new ways to describe noise transfer in medical imaging systems.

A generalized approach to describing transfer of the noise power spectrum through medical imaging systems has been developed over the past several years in which image-forming processes are represented in terms of a cascade of amplified point processes. Until recently, this approach has been restricted to serial cascades only. Here we develop a generalized expression for the cross covariance of amplified point processes and an expression for the cross spectral density for wide-sense stationary conditions. These results extend the generalized transfer-theory approach to include the description of more complex image-forming processes involving parallel cascades of quantum amplification processes. This parallel-cascade approach is used to develop a theoretical expression for noise-power transfer in a simple radiographic screen that includes the effect of characteristic x-ray reabsorption. The result confirms earlier work by Metz and Vyborny, who showed that reabsorption increases image noise and decreases the detective quantum efficiency at low spatial frequencies. Use of the transfer-theory approach facilitates a straightforward generalization to many new digital imaging systems including conventional angiographic and active-matrix flat-panel systems.

Biophysical Phenomena↗

PubMed1998

Signal, noise power spectrum, and detective quantum efficiency of indirect-detection flat-panel imagers for diagnostic radiology.

The performance of an indirect-detection, active matrix flat-panel imager (FPI) at diagnostic energies is reported in terms of measured and theoretical signal size, noise power spectrum (NPS), and detective quantum efficiency (DQE). Based upon a 1536 x 1920 pixel, 127 microns pitch array of a-Si:H thin-film transistors and photodiodes, the FPI was developed as a prototype for examination of the potential of flat-panel technology in diagnostic x-ray imaging. The signal size per unit exposure (x-ray sensitivity) was measured for the FPI incorporating five commercially available Gd2O2S:Tb converting screens at energies 70-120 kVp. One-dimensional and two-dimensional NPS and DQE were measured for the FPI incorporating three such converters and as a function of the incident exposure. The measurements support the hypothesis that FPIs have significant potential for application in diagnostic radiology. A cascaded systems model that has shown good agreement with measured individual pixel signal and noise properties is employed to describe the performance of various FPI designs and configurations under a variety of diagnostic imaging conditions. Theoretical x-ray sensitivity, NPS, and DQE are compared to empirical results, and good agreement is observed in each case. The model is used to describe the potential performance of FPIs incorporating a recently developed, enhanced array that is commercially available and has been proposed for testing and application in diagnostic radiography and fluoroscopy. Under conditions corresponding to chest radiography, the analysis suggests that such systems can potentially meet or even exceed the DQE performance of existing technology, such as screen-film and storage phosphor systems; however, under conditions corresponding to general fluoroscopy, the typical exposure per frame is such that the DQE is limited by the total system gain and additive electronic noise. The cascaded systems analysis provides a valuable means of identifying the limiting stages of the imaging system, a tool for system optimization, and a guide for developing strategies of FPI design for various imaging applications.

Equipment Design↗

PubMed1997

Tomographic imaging of the angular-dependent coherent-scatter cross section.

A new special-purpose computed tomographic (CT) imaging system is described which produces images based on measurements of the low-angle (0-10 degrees) x-ray diffraction properties of an object. Low-angle scatter in the diagnostic x-ray energy range is dominated by coherent scatter, and the system uses first-generation CT geometry to acquire a diffraction pattern for each pencil beam. The patterns are used to reconstruct a series of images which represent the coherent-scatter intensity at a series of scatter angles. To demonstrate the potential of coherent-scatter CT (CSCT), the scanner has been built and used to image a phantom consisting of a water-filled Lucite cylinder containing rods of polyethylene, Lucite, polycarbonate, and nylon. In this paper, the system is described and a sequence of CSCT images of this phantom is shown. Coherent-scatter cross sections of these materials are generated for each pixel from this sequence of images and compared with cross sections measured separately. The resulting excellent agreement shows that the angular-dependent coherent-scatter cross section can be accurately imaged in a tomographic slice through an object. These cross sections give material-specific information about the object. The long-term goal of this research is to make measurements of bone-mineral content for every pixel in a tomographic slice.

Bone Density↗

PubMed1997

Empirical and theoretical investigation of the noise performance of indirect detection, active matrix flat-panel imagers (AMFPIs) for diagnostic radiology.

Noise properties of active matrix, flat-panel imagers under conditions relevant to diagnostic radiology are investigated. These studies focus on imagers based upon arrays with pixels incorporating a discrete photodiode coupled to a thin-film transistor, both fabricated from hydrogenated amorphous silicon. These optically sensitive arrays are operated with an overlying x-ray converter to allow indirect detection of incident x rays. External electronics, including gate driver circuits and preamplification circuits, are also required to operate the arrays. A theoretical model describing the signal and noise transfer properties of the imagers under conditions relevant to diagnostic radiography, fluoroscopy, and mammography is developed. This frequency-dependent model is based upon a cascaded systems analysis wherein the imager is conceptually divided into a series of stages having intrinsic gain and spreading properties. Predictions from the model are compared with x-ray sensitivity and noise measurements obtained from individual pixels from an imager with a pixel format of 1536 x 1920 pixels at a pixel pitch of 127 microns. The model is shown to be in excellent agreement with measurements obtained with diagnostic x rays using various phosphor screens. The model is used to explore the potential performance of existing and hypothetical imagers for application in radiography, fluoroscopy, and mammography as a function of exposure, additive noise, and fill factor. These theoretical predictions suggest that imagers of this general design incorporating a CsI: Tl intensifying screen can be optimized to provide detective quantum efficiency (DQE) superior to existing screen-film and storage phosphor systems for general radiography and mammography. For fluoroscopy, the model predicts that with further optimization of a-Si:H imagers, DQE performance approaching that of the best x-ray image intensifier systems may be possible. The results of this analysis suggest strategies for future improvements of this imaging technology.

Electronics↗

PubMed1997

Optimal phosphor thickness for portal imaging.

A theoretical approach known as quantum accounting diagram (QAD) analysis has been used to calculate the spatial-frequency-dependent detective quantum efficiency (DQE) of two portal imaging systems: one based on a video camera and another based on an amorphous silicon array. The spatial frequency-dependent DQEs have then been used to determine indices of displayed and perceived image quality. These indices are figures of merit that can be used to optimize the design of linear imaging systems. We have used this approach to determine which of eight phosphor screen thicknesses (ranging between 67 and 947 mg/cm2) is optimal for the two designs of portal imaging systems. The physical characteristics (i.e., detection efficiencies, gains, and MTFs) of each of the eight x-ray detectors have been measured and combined with the physical characteristics of the remaining components to calculate the theoretical DQEs. In turn, the DQEs have been used to calculate theoretical indices of displayed and perceived image quality for two types of objects: a pelvis object and a pointlike object. The maximal indices of displayed and perceived image quality were obtained with screen thickness ranging between 358 and 947 mg/cm2, depending upon the imaging system design and the object being imaged. Importantly, the results showed that there is no single optimal screen thickness. The optimal thickness depended upon imaging task (e.g., detecting large, low-contrast structures, or detecting edges and small structures). Nevertheless, the results showed that there were only modest improvements in the indices of image quality for phosphor screens thicker than 350-400 mg/cm2.

Biophysical Phenomena↗

PubMed1997

A quantum accounting and detective quantum efficiency analysis for video-based portal imaging.

The quality of images generated with radiographic imaging systems can be degraded if an inadequate number of secondary quanta are used at any stage before production of the final image. A theoretical technique known as a "quantum accounting diagram" (QAD) analysis has been developed recently to predict the detective quantum efficiency (DQE) of an imaging system as a function of spatial frequency based on an analysis of the propagation of quanta. It is used to determine the "quantum sink" stage(s) (stages which degrade the DQE of an imaging system due to quantum noise caused by a finite number of quanta), and to suggest design improvements to maximize image quality. We have used this QAD analysis to evaluate a video-based portal imaging system to determine where changes in design will have the most benefit. The system consists of a thick phosphor layer bonded to a 1 mm thick copper plate which is viewed by a T.V. camera. The imaging system has been modeled as ten cascaded stages, including: (i) conversion of x-ray quanta to light quanta; (ii) collection of light by a lens; (iii) detection of light quanta by a T.V. camera; (iv) the various blurring processes involved with each component of the imaging system; and, (v) addition of noise from the T.V. camera. The theoretical DQE obtained with the QAD analysis is in excellent agreement with the experimental DQE determined from previously published data. It is shown that the DQE is degraded at low spatial frequencies (< 0.25 cycles/mm) by quantum sinks both in the number of detected x rays and the number of detected optical quanta. At higher spatial frequencies, the optical quantum sink becomes the limiting factor in image quality. The secondary quantum sinks can be prevented, up to a spatial frequency of 0.5 cycles/mm, by increasing the overall system gain by a factor of 9 or more, or by improving the modulation transfer function (MTF) of components in the optical chain.

Biophysical Phenomena↗

PubMed1996

Angular-dependent coherent scatter measured with a diagnostic x-ray image intensifier-based imaging system.

Low-angle scatter of x rays at diagnostic energies is primarily coherent. This coherence gives rise to interference effects resulting in x-ray diffraction patterns that are characteristic of the scattering material. A method is described of imaging these low-angle (0 degree-10 degrees) x-ray diffraction properties of tissue specimens using a diagnostic x-ray beam and image intensifier-based system. The coherent-scatter cross sections of several materials measured this way are presented. It is shown theoretically that the measurements made with this system can be expressed as the mono-energetic cross section "blurred" by the x-ray spectrum using a linear superposition integral. Experimental results using aluminum powder confirm this. Using a 70 kVp x-ray beam filtered with gadolinium to reduce the spectral width, materials such as water, Lucite, and hydroxyapatite all have significantly different diffraction patterns. The cross sections determined from this analysis from the basis of a unique method of characterizing and identifying tissue samples according to their atomic structure rather than x-ray attenuation properties.

Durapatite↗

PubMed1996

Technical exhibits.

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Exhibitions as Topic↗

PubMed1996

A prospective randomized clinical trial comparing two film-screen systems for chest radiography.

OBJECTIVE: To compare conventional and asymmetric film-screen chest radiography systems in a prospective, randomized trial. PATIENTS AND METHODS: Posteroanterior and lateral films were obtained with each system for one healthy volunteer and 49 consenting patients referred from pulmonary clinics and wards (for a total of 27 male and 23 female subjects ranging in age from 16 to 82 [mean 58] years). The radiographs, obtained and presented in random order, were reviewed and rated independently by two experienced radiologists and one resident in radiology; all observers were blinded to patient identification and film type. The Wilcoxon signed rank sum nonparametric test for paired samples was used to test for significant differences between the two film-screen systems. A second evaluation involving direct (blinded) comparison of the two types of films was then performed for each of the 25 patients in whom abnormality was noted during the first evaluation. RESULTS: For the posteroanterior radiographs, the asymmetric film-screen system was significantly better for assessing the trachea and mainstem bronchi, the descending thoracic-aortic edge, the left paraspinal line, the thoracic vertebral body interspace and the azygo-esophageal line (p < 0.05), whereas the conventional system had superior conspicuity in the lateral subpleural zones (p < 0.05). For the lateral radiographs, the asymmetric system was superior for assessing retrosternal lung markings (p < 0.05) but inferior for assessing fissures (p < 0.05). CONCLUSIONS: In general, the asymmetric system was superior for assessing mediastinal features and inferior for assessing the lateral subpleural zones in the posteroanterior radiographs. The asymmetric system was superior for assessing retrosternal lung markings and inferior for assessing fissures in the lateral radiographs. The results for the posteroanterior radiographs were consistent with the results of nonblinded studies reported elsewhere.

Adolescent↗

PubMed1995

A family of hsp60-related proteins in pancreatic beta cells of non-obese diabetic (NOD) mice.

Eukaryotic hsp60s are plastid-specific molecular chaperones implicated in the pathogenesis of many inflammatory and autoimmune diseases. We have used immunoelectron microscopy, immunoblotting and subcellular fractionation of islet cells to determine whether analogous proteins with related function are expressed in other cellular structures and whether such hsp60-related proteins could serve as antigenic targets in autoimmune diabetes. Using a panel of monoclonal and polyclonal antibodies to human and yeast hsp60s and immunoelectron microscopy, the hsp60 antibody cross-reactive proteins were detected in secretory granules, mitochondria, synaptic-like microvesicles and microtubules of mouse pancreatic beta cells. The expression of microtubule-associated hsp60 was induced by an infiltration of islets by mononuclear cells. This novel inducible-form of hsp60-related protein was recognized as an antigen by sera from diabetic mice. Subcellular fractionation of islets indicated that the molecular size of hsp60-related proteins included 66, 62, 58, 55, 52 and 38 kDa. These results demonstrate that the pancreatic beta cells express a family of hsp60-related proteins, with members differentially expressed in distinct cellular compartments. These proteins bearing hsp60 epitopes were antigenic targets for autoimmune responses in diabetic NOD mice.

Animals↗

PubMed1995

Size estimation and magnification error in radiographic imaging: implications for classification of arteriovenous malformations.

PURPOSE: To assess magnification error in digital subtraction angiography as it pertains to arteriovenous malformation (AVM) size. METHODS: A rectangular grid phantom with equally spaced markers mounted in a stereotactic frame was imaged with digital angiographic equipment. The location and orientation of the grid was altered relative to the central plane of the phantom. Both linear and area measurements were made according to the perceived location of phantom markers using a standard catheter calibration technique and compared with stereotactically derived estimates. Finally, a single case example of an angiographically imaged rolandic AVM was used to compare linear dimensions obtained with both described techniques. RESULTS: The determination of location and size with standard angiographic imaging is subject to error because of the divergent geometry of the incident x-ray beam. The resulting nonconstant geometric magnification causes errors in linear measurements of 10% to 13% at depths of 7 cm from the calibration plane. Errors in area measurements at the same position increase by 20% to 25%. Measurements of maximum diameter or cross-sectional area may have an additional error when nonspherical objects are inclined to the viewing direction (40% at 45 degrees inclination). These errors are reduced to less than 1 mm using the stereotactic technique. Some commercial angiographic systems have internal software to enable a spatial calibration based on known distances in the image or on the diameter of a catheter. The catheter technique was accurate in the calibration direction (perpendicular to the catheter axis) but had a 12% error in the direction parallel to the catheter because of a nonunity aspect ratio in the video system. Measurement of the dimensions of a rolandic AVM using the catheter calibration technique had an error that ranged from -3% to +26% (standard error, 20%) with respect to the stereotactic technique. CONCLUSIONS: Numerous nonstereotactic referential systems for determining linear distances are inherently erroneous by varying degrees compared with the stereotactic technique. Area and volume determinations naturally increase this error further. To the extent that no standardized method for determining linear distances exists, significant variations in estimation of AVM size result. Classification schemes for AVMs have been hampered by this technical error.

Algorithms↗

PubMed1994

A spatial-frequency dependent quantum accounting diagram and detective quantum efficiency model of signal and noise propagation in cascaded imaging systems.

The detective quantum efficiency (DQE) is a system parameter that can be used to accurately describe image noise transfer characteristics through many imaging systems. A simpler approach used by some investigators, particularly when evaluating new ideas and system designs, is to describe the system as a series of cascaded stages. Each stage may correspond to either an increase in the number of quanta (e.g., conversion from x-ray to optical quanta in a radiographic screen), or a loss (a detection or coupling probability). The number of secondary quanta at each stage per incident primary quantum is given by the product of all preceding gains, and can be displayed graphically for convenient interpretation. The stage with the fewest quanta is called the "quantum sink," limiting the pixel signal-to-noise ratio to less than the square root of the number of quanta per pixel. This conventional zero-spatial-frequency "quantum accounting diagram" (QAD), however, neglects the spatial spreading of secondary quanta and can seriously underestimate image noise. It is shown that this problem is avoided with the introduction of a spatial-frequency dependent QAD, expressed as the product of the gains and squared modulation-transfer functions (MTF) of each stage. A generalized expression is developed for the DQE of a cascaded imaging system that is dependent only on the gain, gain Poisson excess (related to the variance), and MTF, of each stage. A direct relationship is then shown to exist between the DQE and values in the QAD. The QAD of a hypothetical system consisting of a charge-coupled device camera and a scintillating screen is evaluated as an illustrative example. The conventional zero-frequency analysis suggests two quantum sinks occur with approximately equal importance: one in the number of x rays, and one in the number of optical quanta. The spatial-frequency dependent analysis, however, shows the optical quantum sink becomes severe and dominates at nonzero frequencies. The necessary increase in gain or optical numerical aperture required to prevent the optical quantum sink for spatial frequencies of interest is determined from the QAD analysis. The visual impact of this nonzero spatial-frequency quantum sink is shown in images generated using a Monte Carlo simulation of the cascading process.

Algorithms↗

PubMed1994

Technical exhibits.

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Diagnostic Imaging↗

PubMed1986

A new system for quantitative arterial imaging and blood flow measurements.

A new x-ray imaging system is being developed for quantitative arterial imaging and blood flow measurements. The system consists of an x-ray image intensifier optically coupled with a 1024-element photo-diode detector array. Low-noise, quantitative images are obtained by irradiating small regions of interest to minimize the detection of scattered radiation and intensifier tube veiling glare, and by making use of the large dynamic range (8000:1) and response linearity of the solid-state photo-detector. In the first of two modes of operation, low-noise scanned projection images are produced. Stenosis size (reduction of lumen area) in phantoms is determined with a maximum uncertainty of 10% over a range of iodine contrast agent concentrations of 4 to 100 mg/ml in a 1.0 cm2 cross-sectional area tube, independent of stenosis orientation and size. In the second mode, flow information is obtained by detecting the movement of a small, locally injected iodine bolus. Peak flow velocity and locations of flow separation and turbulence resulting from simulated stenoses are determined with stenosis sizes ranging from 0 to 84% area reduction.

Angiography↗