Modulation Transfer Function Measurement Research Articles

Measurement of the modulation transfer function of a charge-coupled device array by the combination of the self-imaging effect and slanted edge method

In this paper, by a combination of the self-imaging effect for Ronchi gratings and the standard slanted edge modulation transfer function (MTF) measurement method for CCD cameras, the MTF of the CCD array without optics is measured. For this purpose, a Ronchi-type grating is illuminated by an expanded He-Ne laser. A self-image of the grating appears without optics on the CCD array that is located on the Talbot distance. The lines of the self-image of the grating are used as a slanted edge array. This method has all the advantages of the slanted edge method, and also since the array of the edge is ready, the total area of the CCD can be tested. The measured MTF is related to the CCD array without optics.

Echelette optical low-pass filter as a Bangerter filter for diagnosis and treatment of amblyopia and diplopia

This paper presents a new echelette optical low-pass filter that serves as a Bangerter filter (BF) for diagnosis and treatment of amblyopia and diplopia. Theoretical analysis and experimental measurements showed that echelette BFs have several advantages over the modern BFs used in medicine. Four echelette BFs with different grating periods and one BF were manufactured, and image blurring in an optical visual system with an echelette BF was experimentally investigated. The experimentally measured modulation transfer function and line spread function demonstrated definite image blurring (line expansion) that is inversely proportional to the width of the grating period. The results of measurements of the angular resolution of the optical system with an echelette BF coincide well with the results obtained by theory.

Alternative methodology for intraocular lens characterisation

To present an alternative methodology to characterise intraocular lenses (IOLs) using an optical setup with a Hartmann-Shack wavefront sensor. The methodology was inspired by the ANSI Z80.30 Standard, which addresses IOL optical properties and test methods, and aims to simplify the procedure to allow complete IOL characterisation with a reduced number of measurement steps. Seven spherical and three toric IOLs were characterised with the alternative methodology and by commercial equipment. Three spherical IOLs measured with the alternative methodology obtained dioptric spherical errors larger than the tolerance established by the ANSI Z80.30 Standard when compared to the IOLs theoretical designs, probably due to manufacturing deviations. All cylindrical dioptric power measurements were within the Standard tolerance. The longitudinal spherical aberration of the IOLs were measured and 2-dimensional Modulation Transfer Functions (MTF) were calculated in order to complementarily assess manufacturing and design quality. The alternative methodology allows complete IOL characterisation with a reduced number of steps compared to those required by the Standard ANSI Z80.30, achieving satisfactory results in measurements of IOL dioptric power, spherical aberration, cylindrical power and MTF. Although MTF measurements were not performed with a model eye, they may contain additional relevant information regarding the design of ophthalmic lenses.

SU-D-141-03: Evaluation of Cone-Beam CT Imaging Quality Using a Clinical Detective Quantum Efficiency Measure (cDQE)

Purpose: To quantitatively analyze the image quality of cone-beam CT(CBCT) imaging using a robust image evaluation metric, a clinical analogue of detective quantum efficiency (DQE), to allow for image assessment under variable acquisition parameters. Methods: CBCT image quality assessment, especially between different systems, remains semi-quantitative and lacks the fundamental metrics found in 2D x-ray imaging. Clinical DQE(cDQE) is based on measuring modulation transfer function (MTF) and noise power spectrum(NPS) using a cylindrical acrylic phantom containing modules with line-pairs of various spatial frequencies, and uniform section. Imaging was acquired using a linac based CBCT (Elekta XVI) and a commercial cylindrical imaging QA phantom (Phantom Laboratory Catphan 500). CBCT imaging was obtained with various acquisition settings for collimation size (small S20, medium M20 field-of-views), beam filter (with and without bow-tie filter), and adjustable scatter-to-primary ratios(SPRs). Incident beam fluence was determined based on dose measurements, and fluence-dose conversion ratio computed by Monte Carlo simulations. A standard vendor supplied reconstruction algorithm was used with various SPRs (0, 0.1, 0.2, 0.33). Results: Use of bow-tie filter improved CBCT image quality, as measured by cDQE, by 42% compared to imaging with neutral filter. As expected, cDQE confirmed that small collimation produced superior imaging compared to medium collimation due to reduced scattering. Interestingly, CBCT images reconstructed under various SPRs (0, 0.1, 0.2, 0.33) showed no significant difference in cDQE values. The results of cDQE calculations were consistent with evaluations of the phantom imaging based on traditional contrast-to-noise ratio, spatial resolution visibility, and qualitative observer assessment. Conclusion: cDQE was shown to be an effective metric to assess CBCT imaging under variable acquisition parameters. The proposed approach can be extended to other imaging phantoms and CBCTs, and has significant potential as a QA tool in monitoring and optimizing the imaging performance of CBCT systems for clinical use.

Spatial resolution characterization of differential phase contrast CT systems via modulation transfer function (MTF) measurements

By adding a Talbot–Lau interferometer to a conventional x-ray absorption computed tomography (CT) imaging system, both differential phase contrast (DPC) signal and absorption contrast signal can be simultaneously measured from the same set of CT measurements. The imaging performance of such multi-contrast x-ray CT imaging systems can be characterized with standard metrics such as noise variance, noise power spectrum, contrast-to-noise ratio, modulation transfer function (MTF), and task-based detectability index. Among these metrics, the measurement of the MTF can be challenging in DPC-CT systems due to several confounding factors such as phase wrapping and the difficulty of using fine wires as probes. To address these technical challenges, this paper discusses a viable and reliable method to experimentally measure the MTF of DPC-CT. It has been found that the spatial resolution of DPC-CT is degraded, when compared to that of the corresponding absorption CT, due to the presence of a source grating G0 in the Talbot–Lau interferometer. An effective MTF was introduced and experimentally estimated to describe the impact of the Talbot–Lau interferometer on the system MTF.

Investigation of image lag and modulation transfer function in fluoroscopy images obtained with a dynamic flat-panel detector

Digital imaging with a dynamic flat-panel detector (FPD) is commonly used in clinical practice. However, several factors reduce the accuracy of target tracking in fluoroscopic imaging, including image lag and blurring. There have been several reports focusing on the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) in different types of FPD. However, there have been no studies comparing image lag and MTF properties in dynamic images obtained with indirect- and direct-conversion FPDs. We investigated the image lag and MTF under several imaging conditions in fluoroscopic images obtained with an indirect-conversion and a direct-conversion FPD system. The measurements of image lag and MTF were obtained under several conditions according to IEC 62220-1-3 standards. We examined whether the image lag and MTF were influenced by the dose level and target movement speed. Indirect-conversion FPD showed dependence on the dose level, which was not observed for direct-conversion FPD. Furthermore, there were large differences in MTF between images of static and moving plate with indirect-conversion FPD in comparison to the differences observed with direct-conversion FPD. These results will be useful for the determination of imaging conditions for target tracking and other types of dynamic imaging.

Auto‐calibration approach for k–t SENSE

The goal of this work is to increase the spatial resolution of training data, used by reconstruction methods such as k-t SENSE in order to calculate the missing data in a series of dynamic images, without compromising their temporal resolution or acquisition time. The k-t SENSE method allows dynamic imaging at high acceleration factors with high reconstruction quality. However, the low resolution training data required by k-t SENSE may cause undesired temporal filtering effects in the reconstructed images. In this work, a feedback regularization approach is applied to realize auto-calibration of the k-t SENSE algorithm. To that end, a full resolution training data set is calculated from the accelerated data itself using a TSENSE reconstruction. The reconstructed training data are then fed back for the actual k-t SENSE reconstruction. For evaluation of our approach, temporal filtering effects are quantified by calculating the modulation transfer function and noise measurements are done by Monte-Carlo simulations. Computer simulations and cardiac imaging experiments demonstrate an improved temporal fidelity of auto-calibrated k-t SENSE compared to standard k-t SENSE. Auto-calibrated k-t SENSE provides high quality reconstructions for dynamic imaging applications.

Comparison of approaches and artefacts in the measurement of detector modulation transfer functions

In order to investigate the reproducibility of measurements of transmission electron microscope detector modulation transfer functions (MTFs) we measure the MTF of a charge-coupled device (CCD) camera using five different methods. MTFs derived from a sharp edge, a circular aperture and electron holographic interference fringes are found to agree closely with one other. The difficulty of obtaining accurate measurements of MTFs and the potential of using focused electron probes to make direct measurements of MTFs is discussed. We highlight the sensitivity of image contrast after deconvolution to small differences in the measured MTF.

디지털 방사선촬영 환경에서 선량의 최적화 및 영상품질에 대한 연구

Digital Radiography (DR) has improved a quality of resolution based on a wide dynamic range, high detective quantum efficiency (DQE), and modulation transfer function (MTF), compared with film/screen(F/s). Unlike expectation that a low level of radiation can be used in examination, high level of signal to noise ratio(SNR) due to over-exposure caused increase of exposed dose to patients. Also, the auto exposure control (AEC) using Kilovolage(kVp) in F/S can cause over-exposure. Hence, in this study, we proposed a proper method for using DR, in which effect of tubing Kilovolage on device's image, DR MTF measurement with changes of tubing current (mA), and the quantitative evaluation of skull phantom captured images' PSNR were evaluated. Changes of contrast with tubing Kilovolage can be improved by retouching, and MTF changes according to tubing current(1.41~1.39 lp/mm in 50% area, and 3.19~2.8 lp/mm in 10% area) does not influence on resolution of image. As a result, high tubing Kilovoltage, and tubing current will be suitable to use of DR.

Resolution and sensitivity of wafer-level multi-aperture cameras

The scaling limits of multi-aperture systems have been widely discussed from an information-theoretical standpoint. While these arguments are valid as an upper limit, the real-world performance of systems for mobile devices remains restricted by optical aberrations. We argue that aberrations can be more easily controlled with certain architectures of multi-aperture systems, especially those manufactured on wafer scale (wafer-level optics, WLO). We complement our analysis with measurements of one single- and one multi-aperture WLO camera. We examine both sharpness and sensitivity, giving measurements of modulation transfer function and temporal noise, and showing that multi-aperture systems can indeed reduce size without compromising performance.

High-resolution X-ray imaging with samarium-doped fluoroaluminate and fluorophosphate glass

We report the use of samarium-doped (Sm3+) fluoroaluminate and fluorophosphate glasses for X-ray imaging. Incident X-rays convert samarium ions from the trivalent to the divalent state on a semi-permanent but erasable basis. Of the three fluorophosphate glass compositions investigated, that with the smallest fraction of phosphate bonds showed the greatest conversion, but fluoroaluminate glass showed better conversion than any of them. The effects of polyvalent co-dopants Ce, Eu, Pb, Sn, and In, (which may potentially act as electron donors or acceptors) on the conversion efficiency were investigated in fluoroaluminate glass, and whilst In and Sn inhibit the conversion, none of the co-dopants produced significant enhancement. The valence conversion may be used for X-ray imaging through the photoluminescence of the X-ray generated Sm2+ ions. With an optimal choice of the excitation wavelength and appropriate optical filtration in the camera-based recording system, very high-resolution X-ray imaging can be achieved. Recorded images show resolved detail down to the level of 25μm, and the measured modulation transfer function extends out to 11line pairs/mm.

Speckle-based modulation transfer function measurements for comparative evaluation of CCD and CMOS detector arrays

Charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) matrices offer excellent features in imaging systems. For assessing the suitability of each technology according to the application, the complete characterization of the detector arrays becomes necessary. A system is optically characterized by the modulation transfer function (MTF). We have comparatively studied the results provided by the speckle method for detectors of two types: CCD and CMOS. To do so, we first analysed the precision in determining the MTF of the CCD using two apertures at the exit port of an integrating sphere: a single and a double-slit. For the single-slit, we propose a new procedure of fitting the experimental data which overcomes the drawbacks of the conventional procedure. Since it offered lower uncertainty and better reproducibility, the single-slit was used for the study with the CMOS detector. Significant differences were found between the MTF of the CCD and the CMOS detectors.

Modulation transfer function of QWIP and superlattice focal plane arrays

Modulation transfer function (MTF) is the ability of an imaging system to faithfully image a given object. The MTF of an imaging system quantifies the ability of the system to resolve or transfer spatial frequencies. In this paper we will discuss the detail MTF measurements of a 1024×1024pixel multi-band quantum well infrared photodetector and 320×256pixel long-wavelength InAs/GaSb superlattice infrared focal plane arrays.

Modulation Transfer Function Measurement System at Different Contrasts for CCD Camera

In order to achieve MTFs at different contrasts, the MTF measurement system is designed and developed. Two integrating spheres are used to illuminate the face and back of the test target uniformly. The target luminance and background luminance of the test target are regulated by adjusting the attenuators near the entrance of each integrating sphere. The many groups’ experimental results indicate that the luminance differences between the values by the system and those by L88 first level luminance meter, which is checked by National Institute of Metrology P. R. China, are within ±0.3 cd/m2. Thereby the measurement precision can be ensured in MTF test. MTFs of Sony camera and Cannon camera at different contrasts are measured by this system. The measurement values show that MTFs at different contrasts can demonstrate the imaging quality fully and objectively. This study provides an effective method to evaluate the imaging quality of visible imaging systems.

Measurements of system sharpness for two digital breast tomosynthesis systems

The aim of this work was to propose system sharpness parameters for digital breast tomosynthesis (DBT) systems that include the influence of focus size and focus motion for use in quality assurance protocols. X-ray focus size was measured using a multiple pinhole test object, while detector presampling modulation transfer function (MTF) was measured from projection images of a 10 cm × 10 cm, 1 mm thick steel edge, for the Siemens Inspiration and Hologic Selenia Dimensions DBT systems. The height of the edge above the table was then varied from 1 to 78 mm. The MTF expected from theory for the projection images was calculated from the measured detector MTF, focus size MTF and focus motion MTF and was compared against measured curves. Two methods were used to measure the in-plane MTF in the DBT volume: a tungsten wire of diameter 25 µm and an Al edge 0.2 mm thick, both imaged with a 15 mm thick poly(methyl methacrylate) (PMMA) plate. The in-depth point spread function (PSF) was measured using an angled tungsten wire. The full 3D MTF was estimated with a 0.5 mm diameter aluminium bead held in a 45 mm thick PMMA phantom, with the bead 15 and 65 mm above the table. Inspiration DBT projection images are saved at native detector resolution (85 µm), while the Dimensions re-bins projections to 140 µm pixels (2 × 2 binning); both systems used 2 × 2 binning of projection data before reconstruction. The 50% point for the MTF (MTF0.50) measured in the DBT projection images for the tube-travel direction fell as a function of height above the table from 3.60 to 0.90 mm−1 for the Inspiration system and from 2.50 to 1.20 mm−1 for the Dimensions unit. The maximum deviation of measured MTF0.50 from the calculated value was 13%. MTF0.50 measured in-plane (tube-travel direction) fell as a function of height above the table from 1.66 to 0.97 mm−1 for the Inspiration system and from 2.21 to 1.31 mm−1 for the Dimensions system. The full-width half-maximum for the in-depth PSF was 3.0 and 5.9 mm for the Inspiration and Dimensions systems, respectively. There was no difference in the 3D MTF curves, sectioned in the tube-travel direction, for bead heights of 15 and 65 mm above the table. A 25 µm tungsten wire held within a 15 mm thick PMMA plate was found to be a suitable test object for measurement of in-plane MTF. Evaluation of MTF as a function of height above the table, both in the projection images and in the reconstructed planes, provides important information on the impact of focus size and focus motion on the DBT system's imaging performance.

Estimation of the two‐dimensional presampled modulation transfer function of digital radiography devices using one‐dimensional test objects

The modulation transfer function (MTF) of medical imaging devices is commonly reported in the form of orthogonal one-dimensional (1D) measurements made near the vertical and horizontal axes with a slit or edge test device. A more complete description is found by measuring the two-dimensional (2D) MTF. Some 2D test devices have been proposed, but there are some issues associated with their use: (1) they are not generally available; (2) they may require many images; (3) the results may have diminished accuracy; and (4) their implementation may be particularly cumbersome. This current work proposes the application of commonly available 1D test devices for practical and accurate estimation of the 2D presampled MTF of digital imaging systems. Theory was developed and applied to ensure adequate fine sampling of the system line spread function for 1D test devices at orientations other than approximately vertical and horizontal. Methods were also derived and tested for slit nonuniformity correction at arbitrary angle. Techniques were validated with experimental measurements at ten angles using an edge test object and three angles using a slit test device on an indirect-detection flat-panel system [GE Revolution XQ∕i (GE Healthcare, Waukesha, WI)]. The 2D MTF was estimated through a simple surface fit with interpolation based on Delaunay triangulation of the 1D edge-based MTF measurements. Validation by synthesis was also performed with simulated images from a hypothetical direct-detection flat-panel device. The 2D MTF derived from physical measurements yielded an average relative precision error of 0.26% for frequencies below the cutoff (2.5 mm(-1)) and approximate circular symmetry at frequencies below 4 mm(-1). While slit analysis generally agreed with the results of edge analysis, the two showed subtle differences at frequencies above 4 mm(-1). Slit measurement near 45° revealed radial asymmetry in the MTF resulting from the square pixel aperture (0.2 mm × 0.2 mm), a characteristic which was not necessarily appreciated with the orthogonal 1D MTF measurements. In simulation experiments, both slit- and edge-based measurements resolved the radial asymmetries in the 2D MTF. The average absolute relative accuracy error in the 2D MTF between the DC and cutoff (2.5 mm(-1)) frequencies was 0.13% with average relative precision error of 0.11%. Other simulation results were similar to those derived from physical data. Overall, the general availability, acceptance, accuracy, and ease of implementation of 1D test devices for MTF assessment make this a valuable technique for 2D MTF estimation.

A robust geometry estimation method for spiral, sequential and circular cone‐beam micro‐CT

The authors propose a novel method for misalignment estimation of micro-CT scanners using an adaptive genetic algorithm. The proposed algorithm is able to estimate the rotational geometry, the direction vector of table movement and the displacement between different imaging threads of a dual source or even multisource scanner. The calibration procedure does not rely on dedicated calibration phantoms and a sequence scan of a single metal bead is sufficient to geometrically calibrate the whole imaging system for spiral, sequential, and circular scan protocols. Dual source spiral and sequential scan protocols in micro-computed tomography result in projection data that-besides the source and detector positions and orientations-also require a precise knowledge of the table direction vector to be reconstructed properly. If those geometric parameters are not known accurately severe artifacts and a loss in spatial resolution appear in the reconstructed images as long as no geometry calibration is performed. The table direction vector is further required to ensure that consecutive volumes of a sequence scan can be stitched together and to allow the reconstruction of spiral data at all. The algorithm's performance is evaluated using simulations of a micro-CT system with known geometry and misalignment. To assess the quality of the algorithm in a real world scenario the calibration of a micro-CT scanner is performed and several reconstructions with and without geometry estimation are presented. The results indicate that the algorithm successfully estimates all geometry parameters, misalignment artifacts in the reconstructed volumes vanish, and the spatial resolution is increased as can be shown by the evaluation of modulation transfer function measurements.

Random laser speckle based modulation transfer function measurement of midwave infrared focal plane arrays

Direct measurement of the modulation transfer function (MTF) of focal plane arrays (FPAs) using random laser speckle approaches for the visible/near-infrared wavelength band has been well documented over the last 20 years. These methods have not transitioned to the midwave infrared (MWIR) primarily because other techniques have been sufficient and MWIR laser sources with sufficient output power have been unavailable. However, as the detector pitch decreases, MTF measurements become more difficult due to diffraction, while potential MTF degradation due to lateral carrier diffusion crosstalk makes accurate MTF characterization critical for sensor system design. Here, a random laser speckle FPA MTF measurement approach is adapted for use in the MWIR that utilizes a quantum cascade laser coupled with an integrating sphere to generate the appropriate in-band random speckle. Specific challenges associated with the technique are addressed including the validity of the Fresnel diffraction assumptions describing the propagation of the random speckle field from the integrating sphere to the FPA. Improved methods for estimating the power spectral density (PSD) of the measured speckle that reduce data requirements are presented. The statistics and uniformity of the laser speckle are presented along with PSD measurements and estimated MTFs of a MWIR FPA.

Initial On-Orbit Modulation Transfer Function Performance Analysis for Geostationary Ocean Color Imager

The world`s first geostationary ocean color imager (GOCI) is a three-mirror anastigmat optical system 140 mm in diameter. Designed for 500 m ground sampling distance, this paper deals with on-orbit modulation transfer function (MTF)measurement and analysis for GOCI. First, the knife-edge and point source methods were applied to the 8th band (865 nm) image measured April 5th, 2011. The target details used are the coastlines of the Korean peninsula and of Japan, and an island 400 meters in diameter. The resulting MTFs are 0.35 and 0.34 for the Korean East Coastline and Japanese West Coastline edge targets, respectively, and 0.38 for the island target. The daily and seasonal MTF variations at the Nyquist frequency were also checked, and the result is on average. From these results, we confirm that the GOCI on-orbit MTF performance satisfies the design requirements of 0.32 for 865 nm wavelength.

Towards task-based assessment of CT performance: System and object MTF across different reconstruction algorithms

To investigate a measurement method for evaluating the resolution properties of CT imaging systems across reconstruction algorithms, dose, and contrast. An algorithm was developed to extract the task-based modulation transfer function (MTF) from disk images generated from the rod inserts in the ACR phantom (model 464 Gammex, WI). These inserts are conventionally employed for HU accuracy assessment. The edge of the disk objects was analyzed to determine the edge-spread function, which was differentiated to yield the line-spread function and Fourier-transformed to generate the object-specific MTF for task-based assessment, denoted MTF(Task). The proposed MTF measurement method was validated against the conventional wire technique and further applied to measure the MTF of CT images reconstructed with an adaptive statistical iterative algorithm (ASIR) and a model-based iterative (MBIR) algorithm. Results were further compared to the standard filtered back projection (FBP) algorithm. Measurements were performed and compared across different doses and contrast levels to ascertain the MTF(Task) dependencies on those factors. For the FBP reconstructed images, the MTF(Task) measured with the inserts were the same as the MTF measured from the wire-based method. For the ASIR and MBIR data, the MTF(Task) using the high contrast insert was similar to the wire-based MTF and equal or superior to that of FBP. However, results for the MTF(Task) measured using the low-contrast inserts, the MTF(Task) for ASIR and MBIR data was lower than for the FBP, which was constant throughout all measurements. Similarly, as a function of mA, the MTF(Task) for ASIR and MBIR varied as a function of noise--with MTF(Task) being proportional to mA. Overall greater variability of MTF(Task) across dose and contrast was observed for MBIR than for ASIR. This approach provides a method for assessing the task-based MTF of a CT system using conventional and iterative reconstructions. Results demonstrated that the object-specific MTF can vary as a function of dose and contrast. The analysis highlighted the paradigm shift for iterative reconstructions when compared to FBP, where iterative reconstructions generally offer superior noise performance but with varying resolution as a function of dose and contrast. The MTF(Task) generated by this method is expected to provide a more comprehensive assessment of image resolution across different reconstruction algorithms and imaging tasks.