Phase and dark‐field imaging with mesh‐based structured illumination and polycapillary optics

Abstract

Purpose: X-ray phase and dark-field (DF) imaging have been shown to improve the diagnostic capabilities of X-ray systems. However, these methods have found limited clinical use due to the need for multiple precision gratings with limited field of view or requirements on X-ray coherence that may not be easily translated to clinical practice. This work aims to develop a practicable X-ray phase and DF imaging system that could be translated and practiced in the clinic. Methods: This work employs a conventional source to create structured illumination with a simple wire mesh. A mesh-shifting algorithm is used to allow wider Fourier windowing to enhance resolution. Deconvolution of the source spot width and camera resolution improves accuracy. Polycapillary optics are employed to enhance coherence.The effects of incorporating optics with two different focal lengths are compared.Information apparent in enhanced absorption images, phase images, and DF images of fat embedded phantoms were compared and subjected to a limited receiver operator characteristic (ROC) study. The DF images of the moist and dry porous object (sponges) were compared. Results:The mesh-based phase and DF imaging system constructs images with three different information types:scatter-free absorption images,differential phase images,and scatter magnitude/DF images,simultaneously from the same original image. The polycapillary optic enhances the coherence of the beam. The deblurring technique corrects the phase signal error due to geometrical blur and the limitation of the camera modulation transfer function (MTF) and removes image artifacts to improve the resolution in a single shot. The meshshifting method allows the use of a wider Fourier processing window, which gives even higher resolution, at the expense of an increased dose. The limited ROC study confirms the efficacy of the system over the conventional system. DF images of moist and dry porous object show the significance of the system in the imaging of lung infections. Conclusion:The mesh-based X-ray phase and DF imaging system is an inexpensive and easy setup in terms of alignment and data acquisition and can produce phase and DF images in a single shot with wide field of view. The system shows significant potential for use in diagnostic imaging in a clinical setting. K E Y W O R D Simage processing, mammography, radiography, X-ray dark-field imaging, X-ray phase imaging 6642

Dark field signal maximization in mesh-based x-ray imaging

“…This work employs a conventional source combined with focusing polycapillary optics to create structured illumination with a simple wire mesh. 6,16 The periodicity of the mesh allows the use of a Fourier processing technique to obtain differential phase contrast, dark field, and absorption images. 17 Source and detector deblurring were used to improve accuracy, which is particularly important for the phase contrast images.…”

“…17 Source and detector deblurring were used to improve accuracy, which is particularly important for the phase contrast images. 6 When an object is placed in the path of the beam, its attenuation, phase delay and ultra-small angle scatter will change the structure of the mesh harmonics. The high contrast object transmission image, referred to as the TRANS image, is obtained from inverse fast Fourier transforming the windowed normalized central harmonic, 7,8 0…”

“…where y I is proportional to the visibility of the horizontally oriented mesh wires with the object in place relative to the visibility of the mesh-only image, such that a value of Similarly, the horizontal DPC and DF images are obtained from the windowed Fourier transform of the first order harmonic in the horizontal direction. 6,19 The dark field signal SDF was taken as the difference between the value in the thickest part of the object and the background…”

“…We have previously observed increased phase signal with the optic even though the focal spot size is similar to the x-ray source size. 6 The measured focal spot size of the optic is as shown in Exposure was 96 mAs and 40 kV for the experiment with varying geometry and 28mAs for the experiments with varying tube voltages. Because the transmission of the polycapillary optic was not accounted for in the exposure calculation, the actual dose to the sample was less than the apparent value.…”

“…Instead, we have demonstrated simple and effective phase contrast and dark field imaging by using a simple, coarse wire mesh to pattern the beam from a conventional source. 6,7,8 Mesh-based dark field imaging shares similarities with grating interferometry and speckle-based imaging in that all three methods retrieve phase and dark field information by measuring the distortion of a reference pattern on the beam due to the phase shift imparted by object. Talbot interferometers use fine diffraction gratings to produce periodic Talbot patterns on the beam.…”