X-ray diffraction enhanced imaging based on synchrotron radiation has extremely high sensitivity of weakly absorbing low-Z samples in medical and biological fields. This letter is dedicated to a direct reconstruction algorithm for directional-derivative projections of computed tomography of diffraction enhanced imaging. It is a “one-step” algorithm and does not require any restoration processing compared with the current “two-step” methods. The actual values of the sample’s refractive index decrement can be estimated from its reconstruction images directly. The algorithm is proven by the actual experiment at the Beijing Synchrotron Radiation Facility and the reconstructed images are described finally.
Metal artifacts remain a challenge for computed tomography (CT) reconstruction, especially for medical CT, in which the radiation dose for patients is strictly limited. This study presents a novel method of reducing the metal artifacts for cone-beam CT by renovating the metal areas in the 2D cone-beam projection. We first calculate the 3D coordinates of all the metallic implants, which will then be projected into the projection images in each angle as the metal seed points (MSPs), increasing the precision of the segmentation of the metal areas. Different from the existing methods of calculating the metal positions with two projection images, we proposed a new method to calculate the 3D coordinates of all the metallic implants using three different projection images that are nearly orthogonal. One is the parallel-beam projection image along the Z-axial direction (i.e. along the rotation axis) calculated by the projection synthesizing method, while the other two are the cone-beam projections at two different views chosen from the scanned data. The three nearly orthogonal views will greatly help to accurately locate the metallic implants when there is more than one implant in the object. Then the MSPs can be located in each projection by geometry calculations. The region growing segmentation method and the bilinear interpolation method are used to modify the projection areas of the metallic implants. Experimental studies demonstrate that this method is accurate in locating MSPs and efficient in reducing metal artifacts.
Terahertz phase imaging can reveal the depth information of an optically opaque object and provide much better contrast for weak-absorption materials. We demonstrate a continuous-wave terahertz interferometric imaging method in which a far-infrared laser interferometer is used to measure the phase distribution with diffraction-limited lateral resolution and subwavelength axial resolution. An improved four-step phase-shifting algorithm is introduced to retrieve the phase map with very high accuracy and low distortion. The relative depth profiles of two transparent samples are successfully extracted by using this method. Experimental results verify that terahertz interferometric imaging in combination with the phase-shifting technique enables effective reconstruction of the phase image of the object under test.
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