Modulation Transfer Function Measurement Research Articles

36. Optimization of the acquisition threshold of Photon Counting Detectors (PCDs) used in X-ray medical imaging

Purpose PCDs with energy discrimination led to new possibilities of improving the quality of X-ray medical images. PCDs are direct detection devices in which the charge delivered by a single interacting photon is directly collected and discriminated at single pixels level. The limit of high densely pixelated PCDs is the charge sharing which can lead to multiple counts from a single interaction degrading the spatial resolution of the system. Increasing the discrimination threshold, multiple counts can be partially or totally removed from images improving the spatial resolution at the cost of a reduction of the detection area of single pixels with a consequent loss of statistics. This work aims to show a simple method to find the optimal threshold which gives the best trade-off between spatial resolution and statistics. Methods The acquisitions have been performed at the SYRMEP beamline (Elettra Trieste) using the tomographic experimental set-up of INFN project SYnchrotron Radiation MAmmography (SYRMA-3D). SYRMA-3D uses ‘Pixirad-8’, a PCD with CdTe sensor and a pixel size of 60 μm. For this study, we used a monochromatic beam of 38 keV acquiring a set of images of an edge and a set of images of the beam with different detection thresholds. The two sets of images have been used respectively for the measurement of the Modulation transfer function (MTF) and the noise power spectrum (NPS). Combining both NPS and MTF the Number of Equivalent Quanta (NEQ) can be determined. The NEQ summarizes both informations about spatial resolution and noise and is an useful metrics for the global image quality assessment. Results The comparison between NEQ curves shows that lower thresholds give better NEQ response. Conclusions The increase of the discrimination threshold improves the spatial resolution but worsens the noise. The global image quality, measured using NEQ, is higher for lower thresholds.

Modulation transfer function measurements of Type-II mid- wavelength and long-wavelength infrared superlattice focal plane arrays

Mid-wavelength Infrared (MWIR) and long-wavelength infrared (LWIR) Barrier Infrared Detector (BIRD) based on Antimonides Type-II superlattice detector array material are hybridized to 640 × 512, 1024 × 1024 and 1344 × 784 pixels format read out integrated circuits. The focal plane arrays (FPAs) Noise Equivalent Temperature Difference (NEΔT), Quantum Efficiency (η), Dark Current Density (JD), and Modulation Transfer Function (MTF) performance were measured at operating temperatures. High performance 640 × 512 array format MWIR High Operating Temperature (HOT) FPA has typical NEΔT ∼ 19 mK, JD ∼ 6.2 × 10−7 A/cm2, and η ∼ 53% using 3–5 μm band pass filter at 115 K operating temperatures. Best performance 640 × 512 array format LWIR FPA has NEΔT ∼ 25 mK, JD ∼ 2.5 × 10−5 A/cm2 and η ∼ 41% using 8–9.4 μm band pass filter at 60 K operating temperatures. For MTF investigation, a novel direct one-to-one image approach of Line Spread Function (LSF) and Edge Spread Function (ESF) are patterned onto substrate-removed FPAs. The direct LSF and ESF image are produced by 1000 A° thick gold layer deposition. The direct image horizontal and vertical MTFs are compared with projected knife-edge ESF-derived MTFs and results indicate that direct image MTFs have higher values compared to projected knife-edge ESF-derived MTFs at a Nyquist frequency. However, direct image MTF requires no lens MTF correction. The only relevant requirement for the direct image MTF is for the deposited pattern and substrate to be extremely thinned in which the HOT-BIRD FPAs fulfilled for this investigation.

Study of the MTF of a MWIR T2SL focal plane array in IDDCA configuration

Type-II InAs/GaSb superlattice (T2SL) has recently matured into a commercially available technology addressing both MWIR and LWIR spectral domains. As the prerequisites such as Quantum Efficiency (QE) and dark current were met, more advanced figures of merits related to the ElectroOptic (EO) system as a whole can now be studied to position this technology. In this paper, we focus on modulation transfer function (MTF) measurements. Knowing the MTF of a detector is indeed of primary importance for the EO system designers, since spatial filtering affects the system range. We realized MTF measurements on a 320 × 256 MWIR T2SL FPA provided by IRnova, using a Continuously Self Imaging Grating (CSIG). The advantage of this experimental configuration is that no high performance projection optics is required thanks to the self-imaging property (known as Talbot effect) to project a pattern with known spatial frequencies on the photodetector. Besides, the pattern being propagation invariant, alignment is easier and the bench does not require exact knowledge of the focal plane distance to operate, opening measurement in Integrated Detector Dewar Cooler Assembly (IDDCA) configuration. Extracted from measurements, the pixel size is 26 µm for a pitch of 30 µm.

Measurement of the modulation transfer function for a backscatter radiography system used in nondestructive examination

The need for subsurface flaw detection in materials has led to the recent development of a backscatter radiography system. In order to achieve the optimal system design, the modulation transfer function (MTF) must be characterized for the backscatter system. Although several established methods exist for medical imaging systems, these techniques cannot be directly applied to the backscatter system. The goal of this study was to alter the transmission MTF measurement methods using the bar pattern and edge technique so that accurate and reliable results can be obtained for the backscatter system. Test tools were developed in order to carry out these measurements, with the ultimate goal of providing a target for quality assurance testing of the equipment. Although the edge method results were higher than the bar pattern method, the difference in magnitude was less than 0.1. The MTF measurements indicated the importance of target design and positioning in order to achieve accurate results for a backscatter radiography system used in nondestructive examination.

Realtime Measurement of Ultrahigh-Definition Camera Modulation Transfer Function

Herein, experimental results are presented for a system capable of measuring the modulation transfer function (MTF) of a broadcast camera based on its knife-edge responses in realtime. This method can be applied to multiple edges in arbitrary directions, unlike the conventional ISO 12233 edge-based method in which only a near-horizontal or near-vertical edge can be analyzed. The new measurement system significantly facilitates the evaluation of the performances of the camera and lens as well as accurate focusing in an objective manner. The analysis of multidirectional edges on a starburst chart yields a contour plot of the multidirectional MTF for direct observation of anisotropy due not only to the misalignment of the optical system but also to the pixel arrangement of the image sensor.

An algorithm for automated modulation transfer function measurement using an edge of a PMMA phantom: Impact of field of view on spatial resolution of CT images.

PurposeThe purpose of this study was to introduce a new algorithm for automated measurement of the modulation transfer function (MTF) using an edge of a readily available phantom and to evaluate the effect of reconstruction filter and field of view (FOV) on the spatial resolution in the CT images.MethodsOur automated MTF measurement consisted of several steps. The center of the image was established and an appropriate region of interest (ROI) designated. The edge spread function (ESF) was determined, and a suitably interpolated ESF curve was differentiated to obtain the line spread function (LSF). The LSF was Fourier transformed to obtain the MTF. All these steps were accomplished automatically without user intervention. The results of the automated MTF from the edge phantom were validated by comparing them with a point image, and the results of the automated calculation were validated by the standard fitting method. The automated MTF calculation was then applied to the images of two polymethyl methacrylate (PMMA) phantoms and a wire phantom which had been scanned by a Toshiba Alexion 4‐slice CT scanner and reconstructed with various filter types and FOVs.ResultsThe difference in the 50% MTF values obtained from the edge and point phantoms were within ±4%. The values from the automated and fitted methods agreed to within ±2%, indicating that the automated MTF calculation was accurate. The automated MTF calculation was able to differentiate MTF curves for various filters. The spatial resolution values were 0.37 ± 0.00, 0.71 ± 0.01, and 0.78 ± 0.01 cycles/mm for FC13, FC30 and FC52 filters, respectively. The spatial resolution of the images decrease linearly (R 2 > 0.98) with increasing FOVs.ConclusionAn automated MTF method was successfully developed using an edge phantom, the PMMA phantom. The method is easy to implement in a clinical environment and is not influenced by user experience.

Regularized slanted-edge method for measuring the modulation transfer function of imaging systems

The slanted-edge method for modulation transfer function (MTF) measurement uses edge target images whose gray values are often affected by noise and other factors, decreasing its accuracy. We first analyze the ill-posedness in the edge spread function (ESF) regression caused by noise. Second, we propose a regularized slanted-edge method to solve this problem by incorporating a Tikhonov regularization term. Combined with varying precision weights, the ESF is solved using the variational principle, and the MTF is estimated using the regularized ESF. The regularized slanted-edge method is verified for Gaussian, gamma, and Rayleigh noise. The results show that our method improves the accuracy by 0.01-9.02% and 4.33% on average. The proposed method is more robust to noise and accurate than the slanted-edge method.

Characterizing the digital radiography system in terms of effective detective quantum efficiency and CDRAD measurement

The aim of this study was to assess the performance of a digital radiography system in terms of effective detective quantum efficiency (eDQE) for different tube voltages, polymethyl methacrylate (PMMA) phantom thicknesses and different grid types. The image performance of the digital radiography system was also evaluated by using CDRAD measurements at the same conditions and the correlation of CDRAD results with eDQE was compared. The eDQE was calculated via measurement of effective modulation transfer function (eMTF), effective normalized noise power spectra (eNNPS), scatter fraction (SF) and transmission factors (TF). SFs and TFs were also calculated for different beam qualities by using MCNP4C Monte Carlo simulation code. The integrated eDQE (IeDQE) over the frequency range was used to find the correlation with the inverse image quality figure (IQFinv) obtained from CDRAD measurements. The highest eDQE was obtained with 60 lp/cm grid frequency and 10:1 grid ratio. No remarkable effect was observed on eDQE with different grid frequency, but eDQE decreased with increasing grid ratio. A significant correlation was found between IeDQE and IQFinv.

MTF measurements of a type-II superlattice infrared focal plane array sealed in a cryocooler

In operational electro-optical systems, infrared focal plane arrays (IR FPA) are integrated in cryocoolers which induce vibrations that may strongly affect their modulation transfer function (MTF). In this paper, we present the MTF measurement of an IR FPA sealed in its cryocooler. The method we use to measure the MTF decorrelates operational constraints and the technological limitations of the IR FPA. The bench is based on the diffraction properties of a continuously self imaging grating (CSIG). The 26 µm pixel size extracted from the MTF measurement is in good agreement with the expected value.

High-Resolution Extremity Cone-Beam CT with a CMOS Detector: Evaluation of a Clinical Prototype in Quantitative Assessment of Bone Microarchitecture.

A prototype high-resolution extremity cone-beam CT (CBCT) system based on a CMOS detector was developed to support quantitative in vivo assessment of bone microarchitecture. We compare the performance of CMOS CBCT to an amorphous silicon (a-Si:H) FPD extremity CBCT in imaging of trabecular bone. The prototype CMOS-based CBCT involves a DALSA Xineos3030 detector (99 μm pixels) with 400 μm-thick CsI scintillator and a compact 0.3 FS rotating anode x-ray source. We compare the performance of CMOS CBCT to an a-Si:H FPD scanner built on a similar gantry, but using a Varian PaxScan2530 detector with 0.137 mm pixels and a 0.5 FS stationary anode x-ray source. Experimental studies include measurements of Modulation Transfer Function (MTF) for the detectors and in 3D image reconstructions. Image quality in clinical scenarios is evaluated in scans of a cadaver ankle. Metrics of trabecular microarchitecture (BV/TV, Bone Volume/Total Volume, TbSp, Trabecular Spacing, and TbTh, trabecular thickness) are obtained in a human ulna using CMOS CBCT and a-Si:H FPD CBCT and compared to gold standard μCT. The CMOS detector achieves ~40% increase in the f20 value (frequency at which MTF reduces to 0.20) compared to the a-Si:H FPD. In the reconstruction domain, the FWHM of a 127 μm tungsten wire is also improved by ~40%. Reconstructions of a cadaveric ankle reveal enhanced modulation of trabecular structures with the CMOS detector and soft-tissue visibility that is similar to that of the a-Si:H FPD system. Correlations of the metrics of bone microarchitecture with gold-standard μCT are improved with CMOS CBCT: from 0.93 to 0.98 for BV/TV, from 0.49 to 0.74 for TbTh, and from 0.9 to 0.96 for TbSp. Adoption of a CMOS detector in extremity CBCT improved spatial resolution and enhanced performance in metrics of bone microarchitecture compared to a conventional a-Si:H FPD. The results support development of clinical applications of CMOS CBCT in quantitative imaging of bone health.

Tilted-wire method for measuring resolution properties of CT images under extremely low-contrast and high-noise conditions.

Edge methods are predominantly used for modulation transfer function (MTF) measurements in computed tomography (CT) images reconstructed using iterative methods. However, edge methods employ a relatively large and distinct test object, which is intended to simulate relatively large and distinct clinical organs. If one wants to assess the image quality of a small low-contrast object that is visually indistinct against a noisy background, a small and indistinct test object is desired. Another concern is that information related to the signal amount is discarded during MTF measurements. Choosing a weak impulse as the ultimately small test object, we have developed a tilted-wire method, which is a type of point spread function (PSF) method compatible with extremely low contrast-to-noise ratio (CNR) conditions. The signal amount is measured as the PSF volume. We used two commercial CT systems to evaluate the measurement accuracy of the tilted-wire method. When ensemble-averaged images are used, one can measure the MTF even when the wire is indiscernible from noise. The measurement error under such conditions is a few percent for both the MTF and signal amount. We also applied the tilted-wire method to two hybrid iterative reconstruction methods, namely AIDR-3D and ASiR. The results show that the MTF of ASiR is completely CNR-dependent, but that of AIDR-3D is noise-dependent. The signal amount obtained with ASiR is unchanged from that obtained through filtered back-projection (FBP). The signal amount obtained with AIDR-3D is less than that obtained through FBP, depending on the noise level.

Contrast computation methods for interferometric measurement of sensor modulation transfer function

Accurate measurement of image-sensor frequency response over a wide range of spatial frequencies is very important for analyzing pixel array characteristics, such as modulation transfer function (MTF), crosstalk, and active pixel shape. Such analysis is especially significant in computational photography for the purposes of deconvolution, multi-image superresolution, and improved light-field capture. We use a lensless interferometric setup that produces high-quality fringes for measuring MTF over a wide range of frequencies (here, 37 to 434 line pairs per mm). We discuss the theoretical framework, involving Michelson and Fourier contrast measurement of the MTF, addressing phase alignment problems using a moiré pattern. We solidify the definition of Fourier contrast mathematically and compare it to Michelson contrast. Our interferometric measurement method shows high detail in the MTF, especially at high frequencies (above Nyquist frequency). We are able to estimate active pixel size and pixel pitch from measurements. We compare both simulation and experimental MTF results to a lens-free slanted-edge implementation using commercial software.

Bridging the Gap Between Imaging Performance and Image Quality Measures

Imaging system performance measures and Image Quality Metrics (IQM) are reviewed from a systems engineering perspective, focusing on spatial quality of still image capture systems. We classify IQMs broadly as: Computational IQMs (CP-IQM), Multivariate Formalism IQMs (MF-IQM), Image Fidelity Metrics (IF-IQM), and Signal Transfer Visual IQMs (STV-IQM). Comparison of each genre finds STV-IQMs well suited for capture system quality evaluation: they incorporate performance measures relevant to optical systems design, such as Modulation Transfer Function (MTF) and Noise-Power Spectrum (NPS); their bottom-up, modular approach enables system components to be optimized separately. We suggest that correlation between STV-IQMs and observer quality scores is limited by three factors: current MTF and NPS measures do not characterize scene-dependent performance introduced by imaging system non-linearities; contrast sensitivity models employed do not account for contextual masking effects; cognitive factors are not considered. We hypothesize that implementation of scene and process-dependent MTF (SPD-MTF) and NPS (SPD-NPS) measures should mitigate errors originating from scene dependent system performance. Further, we propose implementation of contextual contrast detection and discrimination models to better represent low-level visual performance in image quality analysis. Finally, we discuss image quality optimization functions that may potentially close the gap between contrast detection/discrimination and quality.

Error of the slanted edge method for measuring the modulation transfer function of imaging systems.

The slanted edge method is a basic approach for measuring the modulation transfer function (MTF) of imaging systems; however, its measurement accuracy is limited in practice. Theoretical analysis of the slanted edge MTF measurement method performed in this paper reveals that inappropriate edge angles and random noise reduce this accuracy. The error caused by edge angles is analyzed using sampling and reconstruction theory. Furthermore, an error model combining noise and edge angles is proposed. We verify the analyses and model with respect to (i)the edge angle, (ii)a statistical analysis of the measurement error, (iii)the full width at half-maximum of a point spread function, and (iv)the error model. The experimental results verify the theoretical findings. This research can be referential for applications of the slanted edge MTF measurement method.

Modified slanted-edge method for camera modulation transfer function measurement using nonuniform fast Fourier transform technique

ISO 12233 slanted-edge method experiences errors using fast Fourier transform (FFT) in the camera modulation transfer function (MTF) measurement due to tilt angle errors in the knife-edge resulting in nonuniform sampling of the edge spread function (ESF). In order to resolve this problem, a modified slanted-edge method using nonuniform fast Fourier transform (NUFFT) for camera MTF measurement is proposed. Theoretical simulations for images with noise at a different nonuniform sampling rate of ESF are performed using the proposed modified slanted-edge method. It is shown that the proposed method successfully eliminates the error due to the nonuniform sampling of the ESF. An experimental setup for camera MTF measurement is established to verify the accuracy of the proposed method. The experiment results show that under different nonuniform sampling rates of ESF, the proposed modified slanted-edge method has improved accuracy for the camera MTF measurement compared to the ISO 12233 slanted-edge method.

Accuracy and Precision of Edge-Based Modulation Transfer Function Measurement for Sampled Imaging Systems

The real-time, edge-based, multidirectional modulation transfer function (MTF) measurement system has elucidated the MTF characteristics of sampled imaging systems. However, the accuracy and precision of edge-based MTF measurements are not well understood quantitatively because of the complicated shift-variant nature of the sampling process by the image sensor and the binning process in the edge-based method for producing a supersampled edge spread function. The edge-based MTF measurement is valid only when the edge positions imaged are assumed to be distributed uniformly relative to the sampling sites. This study demonstrates that precision, which is dependent on the edge angle, is counterintuitively high at a higher oversampling ratio of the binning regardless of the smaller pixel counts per bin even with camera noise, and that significantly high precision can be achieved by taking an ensemble average of MTFs computed over a single cycle of the binning phase, even at low oversampling ratios, with high accuracy through proper corrections to compensate for the attenuation of the MTF values due to the discrete processing of the edge-based method.

基于月球目标的FY-2G在轨调制传递函数评估

基于月球目标的FY-2G在轨调制传递函数评估

Modeling and evaluation of a high-resolution CMOS detector for cone-beam CT of the extremities.

Quantitative assessment of trabecular bone microarchitecture in extremity cone-beam CT (CBCT) would benefit from the high spatial resolution, low electronic noise, and fast scan time provided by complementary metal-oxide semiconductor (CMOS) x-ray detectors. We investigate the performance of CMOS sensors in extremity CBCT, in particular with respect to potential advantages of thin (<0.7mm) scintillators offering higher spatial resolution. A cascaded systems model of a CMOS x-ray detector incorporating the effects of CsI:Tl scintillator thickness was developed. Simulation studies were performed using nominal extremity CBCT acquisition protocols (90kVp, 0.126mAs/projection). A range of scintillator thickness (0.35-0.75mm), pixel size (0.05-0.4mm), focal spot size (0.05-0.7mm), magnification (1.1-2.1), and dose (15-40mGy) was considered. The detectability index was evaluated for both CMOS and a-Si:H flat-panel detector (FPD) configurations for a range of imaging tasks emphasizing spatial frequencies associated with feature size aobj. Experimental validation was performed on a CBCT test bench in the geometry of a compact orthopedic CBCT system (SAD=43.1cm, SDD=56.0cm, matching that of the Carestream OnSight 3D system). The test-bench studies involved a 0.3mm focal spot x-ray source and two CMOS detectors (Dalsa Xineos-3030HR, 0.099mm pixel pitch) - one with the standard CsI:Tl thickness of 0.7mm (C700) and one with a custom 0.4mm thick scintillator (C400). Measurements of modulation transfer function (MTF), detective quantum efficiency (DQE), and CBCT scans of a cadaveric knee (15mGy) were obtained for each detector. Optimal detectability for high-frequency tasks (feature size of ~0.06mm, consistent with the size of trabeculae) was ~4× for the C700 CMOS detector compared to the a-Si:H FPD at nominal system geometry of extremity CBCT. This is due to ~5× lower electronic noise of a CMOS sensor, which enables input quantum-limited imaging at smaller pixel size. Optimal pixel size for high-frequency tasks was <0.1mm for a CMOS, compared to ~0.14mm for an a-Si:H FPD. For this fine pixel pitch, detectability of fine features could be improved by using a thinner scintillator to reduce light spread blur. A 22% increase in detectability of 0.06mm features was found for the C400 configuration compared to C700. An improvement in the frequency at 50% modulation (f50 ) of MTF was measured, increasing from 1.8lp/mm for C700 to 2.5lp/mm for C400. The C400 configuration also achieved equivalent or better DQE as C700 for frequencies above ~2mm-1 . Images of cadaver specimens confirmed improved visualization of trabeculae with the C400 sensor. The small pixel size of CMOS detectors yields improved performance in high-resolution extremity CBCT compared to a-Si:H FPDs, particularly when coupled with a custom 0.4mm thick scintillator. The results indicate that adoption of a CMOS detector in extremity CBCT can benefit applications in quantitative imaging of trabecular microstructure in humans.

Performance characteristics of the Inveon micro-CT scanner in small animal imaging

InveonTM is a tri-modality (Siemens Medical Solutions) imaging system designed to image rodents such as mice and rats. The objective of this study was to characterize the performance of the CT component of the scanner in terms of image noise, uniformity, spatial resolution as well as image quality. A clinically relevant water phantom was used for the measurement of image noise and uniformity. Spatial resolution was measured by calculating the modulation transfer function (MTF) using the slanted edge method. Image noise was estimated under several conditions of beam amperage (50–500 μA) and x-ray CT voltages (40–80 kVp) at an exposure time of 200 ms. The CT number for different tissues was measured using a manufacturer-supplied phantom of different densities (0, 50, 250, 750 mg/cc). System linearity was assessed using a wide range of iodine concentrations (Ultravist 370) ranging from 5 to 70 mg/cc. No significant change was noticed for image uniformity across the whole range of tube currents at voltage of 80 kVp. However, a significant reduction of the noise was noticed when the current increased from 50 to 500 μA and also with gradual increment of the tube voltage. The best noise measure was obtained at 500 μA and 80 kVp. Noise or ring reduction feature showed a significant reduction of the noise level. The relationship between noise and the current was shown to be dominated by the photon statistics with less contribution from the system noise. The MTF measurements showed a resolution of 198 μm, 43 μm and 9.52 μm respectively at 10% modulation. A good linear correlation was found between the CT number and iodine concentrations with high statistical significance. The system has shown good CT performance measures in terms of uniformity, spatial resolution, image noise and contrast linearity. Clinical results showed the capability of the system in acquiring high spatial resolution images. One of the shortcomings is disability to perform 4D CT and thus additional engineering work is requested to further enhance the sensitivity and temporal resolution of the scanner.

Modified slanted-edge method to measure the modulation transfer function of camera

The slanted-edge method specified in ISO Standard 12223, which experiences large errors in the camera modulation transfer function(MTF) due to tilt angle error in the knife-edge resulting in non-uniform sampling of the edge spread function. In order to resolve this problem, a modified slanted-edge method for camera MTF measurement is proposed. By applying a simple direct calculation of the Fourier transform of the derivative for the non-uniform sampling data, the camera super-sampled MTF results are obtained. Theoretical simulations for images with and without noise under different tilt angle errors are run using the proposed method. It is demonstrated that the MTF results are insensitive to tilt angle errors. To verify the accuracy of the proposed method, an experimental setup for camera MTF measurement is established. Measurement results show that the proposed method is superior to traditional methods, and improves the universality of the slanted knife-edge method for camera MTF measurement.