Search NASA⌕ Search

SEARCH · Search NASA

Results for “modulation transfer function”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Lithium indium diselenide: A new scintillator for neutron imaging

Lithium indium diselenide, 6 LiInSe 2 or LISe, is a newly developed neutron detection material that shows both semiconducting and scintillating properties. The 24% atomic density of 6 Li yields a thermal neutron mean free path of only 920 μm. This paper reports on the performance of LISe crystals in scintillation mode for its potential use as a converter screen for thermal/cold neutron imaging. The spatial resolution of LISe, determined using a 10% value of the Modulation Transfer Function (MTF), was found to not scale linearly with thickness. Crystals having a thickness of 450 μm or larger resulted in an average spatial resolution of 67 μm, and the thinner crystals exhibited an increase in spatial resolution down to the Nyquist frequency of the CCD. The highest measured spatial resolution of 198 μm thick LISe (27 μm) outperforms a commercial 50 μm thick ZnS(Cu): 6 LiF scintillation screen (100 μm) by more than a factor of three. For the thicknesses considered in this study, it has been found that the light yield of LISe did not scale with its thickness, suggesting the need for optimizing the synthesis to enhance the scintillation mechanism. Absorption measurements indicate that the 6 Li concentration is uniform throughout the samples and its absorption efficiency as a function of thickness follows general nuclear theory, indicating that the variation in apparent brightness is likely due to a combination of particle escape, light transport, and activation of the scintillation mechanisms. As a result, the presence of 115 In and its long-lived 116 In activation product did not result in ghosting (memory of past neutron exposure), demonstrating potential for using LISe for imaging transient systems.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2018

High-energy radiographic imaging performance of LYSO

Here, a comprehensive comparison of the dominant sources of radiation-induced blur for radiographic imaging system performance is made. End-point energies of 6, 10, 15, and 20 MeV bremsstrahlung photon radiation produced at the Los Alamos National Laboratory Microtron facility were used to examine the performance of large-panel cerium-doped lutetium yttrium silicon oxide (LYSO:Ce) scintillators 3, 5 and 10 mm thick. The system resolution was measured and compared between the various end-point energies and scintillator thicknesses. Contrary to expectations, it is found that there was only a minor dependence of system resolution on scintillator thickness or beam end-point energy. This indicates that increased scintillator thickness does not have a dramatic effect on system performance. The data are then compared to Geant4 simulations to assess contributions to the system performance through the examination of modulation transfer functions. It was determined that the low-frequency response of the system is dominated by the radiation-induced signal, while the higher-frequency response of the system is dominated by the optical imaging of the scintillation emission.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

2D perovskite-based high spatial resolution X-ray detectors

Abstract X-ray radiography is the most widely used imaging technique with applications encompassing medical and industrial imaging, homeland security, and materials research. Although a significant amount of research and development has gone into improving the spatial resolution of the current state-of-the-art indirect X-ray detectors, it is still limited by the detector thickness and microcolumnar structure quality. This paper demonstrates high spatial resolution X-ray imaging with solution-processable two-dimensional hybrid perovskite single-crystal scintillators grown inside microcapillary channels as small as 20 µm. These highly scalable non-hygroscopic detectors demonstrate excellent spatial resolution similar to the direct X-ray detectors. X-ray imaging results of a camera constructed using this scintillator show Modulation Transfer Function values significantly better than the current state-of-the-art X-ray detectors. These structured detectors open up a new era of low-cost large-area ultrahigh spatial resolution high frame rate X-ray imaging with numerous applications.

2D perovskite↗
DOE OSTI2010

Design and implementation of a full profile sub-cm ruby laser based Thomson scattering system for MAST

A major upgrade to the ruby Thomson scattering (TS) system has been designed and implemented on the Mega-ampere spherical tokamak (MAST). MAST is equipped with two TS systems, a Nd:YAG laser system and a ruby laser system. Apart from common collection optics each system provides independent measurements of the electron temperature and density profile. This paper focuses on the recent upgrades to the ruby TS system. The upgraded ruby TS system measures 512 points across the major radius of the MAST vessel. The ruby laser can deliver one 10 J 40 ns pulse at 1 Hz or two 5 J pulses separated by 100-800 {mu}s. The Thomson scattered light is collected at F/15 over 1.4 m. This system can resolve small (7 mm) structures at 200 points in both the electron temperature and density channels at high optical contrast; {approx}50% modulated transfer function. The system is fully automated for each MAST discharge and requires little adjustment. The estimated measurement error for a 7 mm radial point is <4% of T{sub e} and <3% of n{sub e} in the range of 40 eV to 2 keV, for a density of n{sub e}=2x10{sup 19} m{sup -3}. The photon statistics at lower density can be increased by binning in the radial direction as desired. A new intensified CCD camera design allows the ruby TS system to take two snapshots separated with a minimum time of 230 {mu}s. This is exploited to measure two density and temperature profiles or to measure the plasma background light.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Investigating the relationship between noise transfer inside the x-ray framing cameras and their imaging ability

We apply a cascaded linear model analysis to a micro-channel plate x-ray framing camera. We establish a theoretical expression of the Noise Power Spectrum (NPS) at the detector’s output and assess its accuracy by comparing it to the NPS of Monte Carlo simulations of the detector’s response to a uniform illumination. We also reflect that fitting the NPS of experimental data against a parametric model based on this expression can yield valuable information on the imaging ability of framing cameras, offering an alternative method to the usual avenue employed to measure their modulation transfer functions.

25 ENERGY STORAGE↗
DOE OSTI2021

High resolution >40 keV x-ray radiography using an edge-on micro-flag backlighter at NIF-ARC

Radiography of low-contrast features in high-density materials evolving on a nanosecond timescale requires a bright photon source in the tens of keV range with high temporal and spatial resolution. One application for sources in this category is the study of dynamic material strength in samples compressed to Mbar pressures at the National Ignition Facility, high-resolution measurements of plastic deformation under conditions relevant to meteor impacts, geophysics, armor development, and inertial confinement fusion. Here, we present radiographic data and the modulation transfer function (MTF) analysis of a multi-component test object probed at ~100 keV effective backlighter energy using a 5 μm-thin dysprosium foil driven by the NIF Advanced Radiographic Capability (ARC) short-pulse laser (~2 kJ, 10 ps). The thin edge of the foil acts as a bright line-projection source of hard x rays, which images the test object at 13.2× magnification into a filtered and shielded image plate detector stack. The system demonstrates a superior contrast of shallow (5 μm amplitude) sinusoidal ripples on gold samples up to 90 μm thick as well as enhanced spatial and temporal resolution using only a small fraction of the laser energy compared to an existing long-pulse-driven backlighter used routinely at the NIF for dynamic strength experiments.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Characterization of a 1D-imaging high-energy x-ray backlighter driven by the National Ignition Facility Advanced Radiographic Capability laser

Plastic deformation of samples compressed to Mbar pressures at high strain rates at the National Ignition Facility (NIF) forms the basis of ongoing material strength experiments in conditions relevant to meteor impacts, geophysics, armor development, and inertial confinement fusion. Hard x-ray radiography is the primary means of measuring the evolution of these samples, typically employing a slit-collimated high-Z microdot driven by the NIF laser to generate >40 keV x rays [E. Gumbrell et al., Rev. Sci. Instrum. 89, 10G118 (2018) and C. M. Huntington et al., Rev. Sci. Instrum. 89, 10G121 (2018)]. Alternatively, a dysprosium “micro-flag” target driven by the Advanced Radiographic Capability laser (~2 kJ, 10 ps) can deliver significantly higher spatiotemporal resolution [M. P. Hill et al., Rev. Sci. Instrum. 92, 033535 (2021)], especially in high-opacity samples. Here, initial experiments revealed problematic brightness and spectral gradients from this source, but by radiographing a set of diamond-turned, 105 µm-thick Pb test objects and supported by simulations using the 3D Monte Carlo code GEANT4, these geometry-dependent gradients across the field of view are quantified and mitigation strategies are assessed. In addition to significantly enhancing the modulation transfer function compared to the existing system, image stacking from multiple layers of image plate is shown to almost double the signal to noise ratio that will reduce uncertainties in future dynamic strength experiments.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Windowed Fourier transform and cross-correlation algorithms for molecular tagging velocimetry

In this study, simulated and experimental molecular tagging velocimetry (MTV) images have been analyzed with a technique commonly used to process grid images on surfaces, the windowed Fourier transform with local spectrum analysis (WFT-LSA). A systematic synthetic image study of the modulation transfer function (MTF) and error tendencies of the WFT-LSA was performed and compared with a PIV-style cross-correlation algorithm to see if advanced strategies such as iterative image deformation can improve analysis of gridded images with high noise levels. Testing of single-pass algorithms showed that in typical MTV images, the WFT-LSA yields significantly lower bias errors than cross-correlation (CC) at displacements greater than 1 pixel but slightly higher random error at all displacements and image conditions. Analysis of the MTF shows that CC provided better resolution of spatial fluctuations than the WFT-LSA in many combinations of grid size and interrogation window. Tests of image deformation algorithms showed that the gap in performance between CC and WFT-LSA is maintained even as both methods improve. Additionally, WFT-LSA and CC methods are applied to real MTV experiments in high-speed gas jet flows. A preliminary analysis of phosphorescence lifetime provided by acetone vapor excited at 266 nm is used for assessing the required gas speeds for making MTV application feasible. The application of WFT-LSA to real MTV images demonstrates the ability of the algorithm to handle further real-world effects that could not be considered in the synthetic image analysis, like reduced signal-to-noise ratio and non-uniform intensity of the tagging grid across the image introduced by the actual laser beam energy distribution. With experimental images, CC is more accurate with shear flows but less robust to high noise levels than WFT-LSA, as predicted by the synthetic image analysis.

42 ENGINEERING↗
DOE OSTI2023

Deep Learning Approach for High-accuracy Electron Counting of Monolithic Active Pixel Sensor-type Direct Electron Detectors at Increased Electron Dose

Abstract Electron counting can be performed algorithmically for monolithic active pixel sensor direct electron detectors to eliminate readout noise and Landau noise arising from the variability in the amount of deposited energy for each electron. Errors in existing counting algorithms include mistakenly counting a multielectron strike as a single electron event, and inaccurately locating the incident position of the electron due to lateral spread of deposited energy and dark noise. Here, we report a supervised deep learning (DL) approach based on Faster region-based convolutional neural network (R-CNN) to recognize single electron events at varying electron doses and voltages. The DL approach shows high accuracy according to the near-ideal modulation transfer function (MTF) and detector quantum efficiency for sparse images. It predicts, on average, 0.47 pixel deviation from the incident positions for 200 kV electrons versus 0.59 pixel using the conventional counting method. The DL approach also shows better robustness against coincidence loss as the electron dose increases, maintaining the MTF at half Nyquist frequency above 0.83 as the electron density increases to 0.06 e−/pixel. Thus, the DL model extends the advantages of counting analysis to higher dose rates than conventional methods.

Materials Science↗
DOE OSTI2021

High-energy micrometre-scale pixel direct conversion X-ray detector

The objective of this work was to fabricate and characterize a new X-ray imaging detector with micrometre-scale pixel dimensions (7.8 µm) and high detection efficiency for hard X-ray energies above 20 keV. A key technology component consists of a monolithic hybrid detector built by direct deposition of an amorphous selenium film on a custom designed CMOS readout integrated circuit. Characterization was carried out at the synchrotron beamline 1-BM-B at the Advanced Photon Source of Argonne National Laboratory. The direct conversion detector demonstrated micrometre-scale spatial resolution with a 63 keV modulation transfer function of 10% at Nyquist frequency. In addition, spatial resolving power down to 8 µm was determined by imaging a transmission bar target at 21 keV. X-ray signal linearity, responsivity and lag were also characterized in the same energy range. Finally, phase contrast edge enhancement was observed in a phase object placed in the beam path. This amorphous selenium/CMOS detector technology can address gaps in commercially available X-ray detectors which limit their usefulness for existing synchrotron applications at energies greater than 50 keV; for example, phase contrast tomography and high-resolution imaging of nanoscale lattice distortions in bulk crystalline materials using Bragg coherent diffraction imaging. Here, the technology will also facilitate the creation of novel synchrotron imaging applications for X-ray energies at or above 20 keV.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Development and Characterization of a High-Energy Neutron Time-of-Flight Imaging System

We present that Los Alamos National Laboratory has developed a prototype of a high-energy neutron time-of-flight imaging system for the non-destructive evaluation of dense, massive, and/or high atomic number objects. High-energy neutrons provide the penetrating power, and thus the high dynamic range necessary to image internal features and defects of such objects. The addition of the time gating capability allows for scatter rejection when paired with a pulsed monoenergetic beam, or neutron energy selection when paired with a pulsed broad-spectrum neutron source. The Time Gating to Reject Scatter and Select Energy (TiGReSSE) system was tested at the Los Alamos Neutron Science Center’s (LANSCE) Weapons Nuclear Research (WNR) facility, a spallation neutron source, to provide proof of concept measurements and to characterize the instrument response. This paper will show results of several objects imaged during this run cycle. In addition, results from system performance metrics such as the Modulation Transfer Function and the Detective Quantum Efficiency measured as a function of neutron energy, characterize the current system performance and inform the next generation of neutron imaging instrument.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Modeling blur in various detector geometries for MeV radiography

Monte Carlo transport codes have been used to model the detector blur and energy deposition in various detector geometries for applications in MeV radiography. Segmented scintillating detectors, where low Z scintillators combined with a high-Z metal matrix, can be designed in which the resolution increases with increasing metal fraction. Here, the combination of various types of metal intensification screens and storage phosphor imaging plates has also been studied. A storage phosphor coated directly onto a metal intensification screen has superior performance over a commercial plate. Stacks of storage phosphor plates and tantalum intensification screens show an increase in energy deposited and detective quantum efficiency with increasing plate number, at the expense of resolution. Select detector geometries were tested by comparing simulation and experimental modulation transfer functions to validate the approach.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

Characterization and operation optimization of large aperture optical interferometers using binary pseudorandom array test standards

Recently, a technique for calibration of the Modulation Transfer Function (MTF) of a broad variety of metrology instrumentation has been established. The technique is based on test samples structured according to binary pseudorandom (BPR) one-dimensional sequences and two-dimensional arrays. The inherent power spectral density of BPR gratings and arrays, has a deterministic white-noise-like character that allows a direct determination of the MTF with a uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: Functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments working with support of the U.S. Department of Energy on industrialization of the technique. The goal is to develop affordable BPR test samples, application procedures, and data processing software, suitable for thorough characterization of optical interferometers and microscopes, as well as x-ray, electron (scanning and transmission), and atomic force microscopes. We report on the development of BPR array test samples optimized for advanced characterization (including the instrumental MTF and aberrations) and operation optimization of large aperture optical interferometers. We describe the sample fabrication process and tests to verify the compliance to desired surface topography. The data acquisition and analysis procedures for application of the technique for precise focusing of Fizeau interferometer are discussed in detail.

Yashchuk, VV↗
DOE OSTI2020

Binary pseudorandom array test standard optimized for characterization of large field-of-view optical interferometers

Recently, a technique for calibrating the modulation transfer function (MTF) of a broad variety of metrology instrumentation has been demonstrated. This technique is based on test samples structured as one-dimensional binary pseudo-random (BPR) sequences and two-dimensional BPR arrays (BPRAs). The inherent power spectral density of BPR gratings (sequences) and arrays has a deterministic white-noise-like character that allows direct determination of the MTF with uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments directed to the optimization of the sample design, fabrication, application, and data processing procedures, suitable for thorough characterization of large aperture optical interferometers. Compared with the previous coded-aperture based design, the improved, 'highly randomized' BPRA pattern of the new test standard provides better accuracy and reliability of instrument MTF and aberration characterization, and enables operation optimization of large aperture optical interferometers. We describe the pattern generation algorithm and tests to verify the compliance to desired BPRA topography. The data acquisition and analysis procedures for different applications of the technique are also discussed.

Yashchuk, Valeriy V.↗
DOE OSTI2010

A spatio-temporal detective quantum efficiency and its application to fluoroscopic systems

Purpose: Fluoroscopic x-ray imaging systems are used extensively in spatio-temporal detection tasks and require a spatio-temporal description of system performance. No accepted metric exists that describes spatio-temporal fluoroscopic performance. The detective quantum efficiency (DQE) is a metric widely used in radiography to quantify system performance and as a surrogate measure of patient ''dose efficiency.'' It has been applied previously to fluoroscopic systems with the introduction of a temporal correction factor. However, the use of a temporally-corrected DQE does not provide system temporal information and it is only valid under specific conditions, many of which are not likely to be satisfied by suboptimal systems. The authors propose a spatio-temporal DQE that describes performance in both space and time and is applicable to all spatio-temporal quantum-based imaging systems. Methods: The authors define a spatio-temporal DQE (two spatial-frequency axes and one temporal-frequency axis) in terms of a small-signal spatio-temporal modulation transfer function (MTF) and spatio-temporal noise power spectrum (NPS). Measurements were made on an x-ray image intensifier-based bench-top system using continuous fluoroscopy with an RQA-5 beam at 3.9 {mu}R/frame and hardened 50 kVp beam (0.8 mm Cu filtration added) at 1.9 {mu}R/frame. Results: A zero-frequency DQE value of 0.64 was measured under both conditions. Nonideal performance was noted at both larger spatial and temporal frequencies; DQE values decreased by {approx}50% at the cutoff temporal frequency of 15 Hz. Conclusions: The spatio-temporal DQE enables measurements of decreased temporal system performance at larger temporal frequencies analogous to previous measurements of decreased (spatial) performance. This marks the first time that system performance and dose efficiency in both space and time have been measured on a fluoroscopic system using DQE and is the first step toward the generalized use of DQE on clinical fluoroscopic systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Objective characterization of GE Discovery CT750 HD scanner: Gemstone spectral imaging mode

Purpose: To objectively characterize the performance of the gemstone spectral imaging (GSI) mode of GE CT750 HD scanner from a user's perspective. Methods: A regular scan protocol that approximates the adult abdomen scan protocol frequently used in the authors' institute was selected as the baseline, and a GSI protocol (preset 11) that is similar to the regular protocol and has a moderate dose level (CTDI{sub vol}=26.27 mGy) was compared to the baseline protocol. The resolving power of both protocols was characterized in terms of modulation transfer functions and high contrast resolution bar readings. Their noise characteristics were studied through noise power spectra, and their low contrast detectability was compared via contrast-to-noise ratio. Material decomposition capability of GSI was evaluated by scanning iodine solutions of 9-24 mg/ml iodine concentration in a Gammex CT phantom and by examining the estimated iodine concentration. In addition, a formula describing the dependency of HU in iodine enhanced area on GSI monochromatic energies and iodine concentrations was provided and the theoretical values were compared with the measured results. Results: The resolutions levels of 50%, 10%, and 5% MTF of GSI monochromatic images at 65 keV agree with those of the regular protocol within 0.1 lp/cm. GSI monochromatic images at 65 keV demonstrated the lowest noise level among GSI images of different monochromatic energies and showed very similar noise magnitude and noise power distribution as compared to the regular protocol images. The CNR of 60 and 65 keV GSI monoimages are approximately 100% of those of the regular protocol images. Estimated iodine concentration levels agreed with the actual values within 2% when the iodine solutions were placed at 3, 9, 12 o'clock positions of the phantom; when iodine solutions were placed at the phantom center and at 6 o'clock position, higher discrepancies of 2%-10% were observed. The observed dependency of HU on keV and iodine concentration levels agreed with the expectation from x-ray attenuations. Conclusions: Equivalent performances were observed in the comparison between GSI 65 keV monochromatic images and images from a regular abdomen scan protocol. This suggests the possibility of GSI to be employed in routine abdominal scans, which would potentially offer more information through its capabilities of material decomposition.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2011

A dedicated cone-beam CT system for musculoskeletal extremities imaging: Design, optimization, and initial performance characterization

Purpose: This paper reports on the design and initial imaging performance of a dedicated cone-beam CT (CBCT) system for musculoskeletal (MSK) extremities. The system complements conventional CT and MR and offers a variety of potential clinical and logistical advantages that are likely to be of benefit to diagnosis, treatment planning, and assessment of therapy response in MSK radiology, orthopaedic surgery, and rheumatology. Methods: The scanner design incorporated a host of clinical requirements (e.g., ability to scan the weight-bearing knee in a natural stance) and was guided by theoretical and experimental analysis of image quality and dose. Such criteria identified the following basic scanner components and system configuration: a flat-panel detector (FPD, Varian 3030+, 0.194 mm pixels); and a low-power, fixed anode x-ray source with 0.5 mm focal spot (SourceRay XRS-125-7K-P, 0.875 kW) mounted on a retractable C-arm allowing for two scanning orientations with the capability for side entry, viz. a standing configuration for imaging of weight-bearing lower extremities and a sitting configuration for imaging of tensioned upper extremity and unloaded lower extremity. Theoretical modeling employed cascaded systems analysis of modulation transfer function (MTF) and detective quantum efficiency (DQE) computed as a function of system geometry, kVp and filtration, dose, source power, etc. Physical experimentation utilized an imaging bench simulating the scanner geometry for verification of theoretical results and investigation of other factors, such as antiscatter grid selection and 3D image quality in phantom and cadaver, including qualitative comparison to conventional CT. Results: Theoretical modeling and benchtop experimentation confirmed the basic suitability of the FPD and x-ray source mentioned above. Clinical requirements combined with analysis of MTF and DQE yielded the following system geometry: a {approx}55 cm source-to-detector distance; 1.3 magnification; a 20 cm diameter bore (20 x 20 x 20 cm{sup 3} field of view); total acquisition arc of {approx}240 deg. The system MTF declines to 50% at {approx}1.3 mm{sup -1} and to 10% at {approx}2.7 mm{sup -1}, consistent with sub-millimeter spatial resolution. Analysis of DQE suggested a nominal technique of 90 kVp (+0.3 mm Cu added filtration) to provide high imaging performance from {approx}500 projections at less than {approx}0.5 kW power, implying {approx}6.4 mGy (0.064 mSv) for low-dose protocols and {approx}15 mGy (0.15 mSv) for high-quality protocols. The experimental studies show improved image uniformity and contrast-to-noise ratio (without increase in dose) through incorporation of a custom 10:1 GR antiscatter grid. Cadaver images demonstrate exquisite bone detail, visualization of articular morphology, and soft-tissue visibility comparable to diagnostic CT (10-20 HU contrast resolution). Conclusions: The results indicate that the proposed system will deliver volumetric images of the extremities with soft-tissue contrast resolution comparable to diagnostic CT and improved spatial resolution at potentially reduced dose. Cascaded systems analysis provided a useful basis for system design and optimization without costly repeated experimentation. A combined process of design specification, image quality analysis, clinical feedback, and revision yielded a prototype that is now awaiting clinical pilot studies. Potential advantages of the proposed system include reduced space and cost, imaging of load-bearing extremities, and combined volumetric imaging with real-time fluoroscopy and digital radiography.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2011

Three-dimensional anisotropic adaptive filtering of projection data for noise reduction in cone beam CT

Purpose: The combination of quickly rotating C-arm gantry with digital flat panel has enabled the acquisition of three-dimensional data (3D) in the interventional suite. However, image quality is still somewhat limited since the hardware has not been optimized for CT imaging. Adaptive anisotropic filtering has the ability to improve image quality by reducing the noise level and therewith the radiation dose without introducing noticeable blurring. By applying the filtering prior to 3D reconstruction, noise-induced streak artifacts are reduced as compared to processing in the image domain. Methods: 3D anisotropic adaptive filtering was used to process an ensemble of 2D x-ray views acquired along a circular trajectory around an object. After arranging the input data into a 3D space (2D projections + angle), the orientation of structures was estimated using a set of differently oriented filters. The resulting tensor representation of local orientation was utilized to control the anisotropic filtering. Low-pass filtering is applied only along structures to maintain high spatial frequency components perpendicular to these. The evaluation of the proposed algorithm includes numerical simulations, phantom experiments, and in-vivo data which were acquired using an AXIOM Artis dTA C-arm system (Siemens AG, Healthcare Sector, Forchheim, Germany). Spatial resolution and noise levels were compared with and without adaptive filtering. A human observer study was carried out to evaluate low-contrast detectability. Results: The adaptive anisotropic filtering algorithm was found to significantly improve low-contrast detectability by reducing the noise level by half (reduction of the standard deviation in certain areas from 74 to 30 HU). Virtually no degradation of high contrast spatial resolution was observed in the modulation transfer function (MTF) analysis. Although the algorithm is computationally intensive, hardware acceleration using Nvidia's CUDA Interface provided an 8.9-fold speed-up of the processing (from 1336 to 150 s). Conclusions: Adaptive anisotropic filtering has the potential to substantially improve image quality and/or reduce the radiation dose required for obtaining 3D image data using cone beam CT.

62 RADIOLOGY AND NUCLEAR MEDICINE↗