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.

PURPOSE: The 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. METHODS: Our 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. RESULTS: The 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 (R2  > 0.98) with increasing FOVs. CONCLUSION: An 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.

Investigation of the Section Thickness Measurement in Tomosynthesis by Thin Metal Plate Edge Method.

When performing tomosynthesis, the section thickness needs to be set depending on a radiographic part and its diagnostic purpose. However, the section thickness in tomosynthesis has not been clearly defined and its measurement method has not been established yet. In this study, we devised the alternative measurement method to diagnose the section thickness using an edge of thin metal plate, and compared with the simulation results, the wire and bead method reported in the previous papers. The tomographic image of the thin metal plate positioned on the table top inclining 30 degrees, which showed the edge spread function (ESF) of each tomographic height, was taken, and then the line spread function (LSF) was obtained by differentiating the ESF image. For the next, a profile curve was plotted by maximum values of LSF of each tomographic height, and a section thickness was calculated using the full width at half maximum (FWHM) of the profile curve. The edge method derived the section thickness close to the simulation results than the other methods. Further, the section thickness depends on the thickness of the metal plate and not the material. The thickness of the metal plate suitable for the evaluation of section thickness is 0.3 mm that is equivalent to pixel size of the flat panel detector (FPD). We conducted quantitative verification to establish the measurement method of the section thickness. The edge method is a useful technique as well as the wire and bead method for grasping basic characteristics of an imaging system.

Improving the accuracy of MTF measurement at low frequencies based on oversampled edge spread function deconvolution.

The modulation transfer function (MTF) of a radiographic system is often evaluated by measuring the system's edge spread function (ESF) using edge device. However, the numerical differentiation procedure of the traditional slanted edge method amplifies noises in the line spread function (LSF) and limits the accuracy of the MTF measurement at low frequencies. The purpose of this study is to improve the accuracy of low-frequency MTF measurement for digital x-ray imaging systems. An edge spread function (ESF) deconvolution technique was developed for MTF measurement based on the degradation model of slanted edge images. Specifically, symmetric oversampled ESFs were constructed by subtracting a shifted version of the ESF from the original one. For validation, the proposed MTF technique was compared with conventional slanted edge method through computer simulations as well as experiments on two digital radiography systems. The simulation results show that the average errors of the proposed ESF deconvolution technique were 0.11% ± 0.09% and 0.23% ± 0.14%, and they outperformed the conventional edge method (0.64% ± 0.57% and 1.04% ± 0.82% respectively) at low-frequencies. On the experimental edge images, the proposed technique achieved better uncertainty performance than the conventional method. As a result, both computer simulation and experiments have demonstrated that the accuracy of MTF measurement at low frequencies can be improved by using the proposed ESF deconvolution technique.

Multifocal contact lens myopia control: central and peripheral retinal image quality.

CLINICAL RELEVANCE: That myopic defocus, even if restricted to the peripheral retina, inhibits eye growth in young monkey eyes has motivated the therapy of myopia control through multifocal contact lens wear in children. BACKGROUND: To understand how eye-length regulating mechanisms are triggered by light requires knowledge of retinal light spread. That is largely lacking for the multifocal contact lenses used in the therapy because empirical methods identifying just the defocus in dioptres are inadequate. METHODS: "Through-focus" diffraction computations in contact lens/eye models with typical normal eye parameters, including polychromatic light, the chromatic aberrations and an M-cone phototransduction layer, offer estimates of retinal image spread for a range of viewing distances. RESULTS: Point- and edge-spread distributions of activation of phototransduction in the central retina show that the addition of multifocal zones produces some veiling for in-focus viewing and substantial improvement of image quality for near targets in the unaccommodated eye. These effects are much reduced in the retinal periphery. CONCLUSION: Whatever therapeutic value there is in prescribing multifocal contact lenses for myopia control, it is not particularly dependent on the precise configuration of the multifocal zones, nor can it be ascribed to changes in image quality specific to the retinal periphery; its origin is more likely less blur for near targets, reducing the stimulus to accommodation.

Enhancing Applicability of Reversible UV Thermochromic Offset Inks: Edge Quality Parameters and Thermochromic Printing System Modulation Transfer Function.

Modern logo design is characterized by its ability to convey information through the use of various images and text compositions. These designs often use simple elements such as lines to capture the essence of a product. When using thermochromic inks in logo design, it is important to consider their composition and behavior, as they differ significantly from conventional printing inks. This study aimed to determine the resolution capabilities of the dry offset printing technique when using thermochromic ink, with the ultimate goal of optimizing the thermochromic ink printing process. Horizontal and vertical lines were printed using both thermochromic and conventional inks to compare the edge reproduction characteristics of the two ink types. Moreover, the impact of the type of applied ink on the share of mechanical dot gain of the print was investigated. Additionally, modulation transfer function (MTF) reproduction curves were generated for each print. Moreover, scanning electron microscopy (SEM) was conducted to investigate the surface of the substrate and prints. It was found that the quality of the printed edge produced by thermochromic inks can rival that of conventional inks. Thermochromic edges showed lower raggedness and blurriness values for horizontal lines, whereas line orientation proved to be insignificant in the case of vertical lines. MTF reproduction curves confirmed higher spatial resolution for vertical lines in the case of conventional inks, whereas they were identical for horizontal lines. The share of mechanical dot gain is not highly influenced by the ink type. SEM micrographs confirmed that the conventional ink smooths out the micro-roughness of the substrate. However, on the surface, the microcapsules of thermochromic ink (measuring 0.5-2 µm) are observable.

Characterization of x-ray focal spots using a rotating edge.

Purpose: The focal spot size and shape of an x-ray system are critical factors to the spatial resolution. Conventional approaches to characterizing the focal spot use specialized tools that usually require careful calibration. We propose an alternative to characterize the x-ray source's focal spot, simply using a rotating edge and flat-panel detector. Methods: An edge is moved to the beam axis, and an edge spread function (ESF) is obtained at a specific angle. Taking the derivative of the ESF provides the line spread function, which is the Radon transform of the focal spot in the direction parallel to the edge. By rotating the edge about the beam axis for 360 deg, we obtain a complete Radon transform, which is used for reconstructing the focal spot. We conducted a study on a clinical C-arm system with three focal spot sizes (0.3, 0.6, and 1.0 mm nominal size), then compared the focal spot imaged using the proposed method against the conventional pinhole approach. The full width at half maximum (FWHM) of the focal spots along the width and height of the focal spot were used for quantitative comparisons. Results: Using the pinhole method as ground truth, the proposed method accurately characterized the focal spot shapes and sizes. Quantitatively, the FWHM widths were 0.37, 0.65, and 1.14 mm for the pinhole method and 0.33, 0.60, and 1.15 mm for the proposed method for the 0.3, 0.6, and 1.0 mm nominal focal spots, respectively. Similar levels of agreement were found for the FWHM heights. Conclusions: The method uses a rotating edge to characterize the focal spot and could be automated in the future using a system's built-in collimator. The method could be included as part of quality assurance tests of image quality and tube health.

Reconstruction of x-ray focal spot distribution using a rotating edge.

The size and shape of an x-ray source's focal spot is a critical factor in the imaging system's overall spatial resolution. The conventional approach to imaging the focal spot uses a pinhole camera, but this requires careful, manual measurements. Instead, we propose a novel alternative, simply using the collimator available on many x-ray systems. After placing the edge of a collimator blade in the center of the beam, we can obtain an image of its edge spread function (ESF). Each ESF provides information about the focal spot distribution - specifically, the parallel projection of the focal spot in the direction parallel to the edge. If the edge is then rotated about the beam axis, each image provides a different parallel projection of the focal spot until a complete Radon transform of the focal spot distribution is obtained. The focal spot can then be reconstructed by the inverse Radon transform, or parallel-beam filtered backprojection. We conducted a study on a clinical C-arm system with 3 focal spot sizes (0.3, 0.6, 1.0 mm nominal size), comparing the focal spot obtained using the rotating edge method against the conventional pinhole approach. Our results demonstrate accurate characterization of the size and shape of the focal spot.