Biomedical subjects

Daxin Shi

Publications and source records attributed to Daxin Shi.

8 recordsLinked to original sources

PubMed2006

Off-axis holographic tomography for diffracting scalar wavefields.

Inverse scattering theories are available that permit tomographic reconstruction of the complex-valued refractive index distribution of weakly scattering objects from knowledge of intensity measurements. These imaging methods are valuable in applications that involve high-frequency optical wavefields, in which direct wavefield phase measurements can be difficult experimentally. The so-called in-line holographic imaging geometry has been well-studied, in which two measurements of the forward scattered wavefield intensity are acquired on distinct parallel detector planes that are perpendicular to the direction of the illuminating wavefields. In this work, based on the principles of intensity diffraction tomography, a reconstruction theory for off-axis holographic tomography with diffracting scalar wavefields is developed and investigated. A distinct feature of the method is that, at each tomographic view angle, the object is illuminated by use of two plane waves that propagate in different directions. The intensities of the forward scattered wavefields are measured on a single detector behind the object. This permits the direction of the probing wavefield to be varied for acquisition of the necessary measurement data, rather than the detector placement as required by conventional in-line holographic methods. The developed image reconstruction method is validated and investigated by use of computer-simulation studies.

Journal Article↗

PubMed2006

Multispectral intensity diffraction tomography reconstruction theory: quasi-nondispersive objects.

A multispectral intensity diffraction tomography (I-DT) reconstruction theory for quasi-nondispersive scattering objects is developed and investigated. By "quasi-nondispersive" we refer to an object that is characterized by a refractive index distribution that is approximately nondispersive over a predefined finite temporal frequency interval in which the tomographic measurements are acquired. The scanning requirements and measurement data are shown to be different than in conventional I-DT. Unlike conventional I-DT that requires intensity measurements on a pair of detector planes for each probing wave field, this new method uses measurements on a single detector plane at two frequencies. Computer simulation studies are conducted to demonstrate the method.

Journal Article↗

PubMed2005

Weighted expectation maximization reconstruction algorithms with application to gated megavoltage tomography.

We propose and investigate weighted expectation maximization (EM) algorithms for image reconstruction in x-ray tomography. The development of the algorithms is motivated by the respiratory-gated megavoltage tomography problem, in which the acquired asymmetric cone-beam projections are limited in number and unevenly sampled over view angle. In these cases, images reconstructed by use of the conventional EM algorithm can contain ring- and streak-like artefacts that are attributable to a combination of data inconsistencies and truncation of the projection data. By use of computer-simulated and clinical gated fan-beam megavoltage projection data, we demonstrate that the proposed weighted EM algorithms effectively mitigate such image artefacts.

Algorithms↗

PubMed2005

Spherical-wave intensity diffraction tomography.

An extension of the recently developed method of intensity diffraction tomography is derived that assumes that the probing field is a spherical wave produced by a point source sufficiently far from the scatterer. A discussion of the method and numerical reconstructions of a simulated three-dimensional scattering object are presented.

Journal Article↗

PubMed2005

Image reconstruction in spherical-wave intensity diffraction tomography.

A reconstruction theory for intensity diffraction tomography (I-DT) has been proposed that permits reconstruction of a weakly scattering object without explicit knowledge of phase information. We investigate the I-DT reconstruction problem assuming an incident (paraxial) spherical wave and scanning geometries that employ fixed source-to-object distances. Novel reconstruction methods are derived by identifying and exploiting tomographic symmetries and the rotational invariance of the problem. An underlying theme is that symmetries in tomographic imaging systems can facilitate solutions for phase-retrieval problems. A preliminary numerical investigation of the developed reconstruction methods is presented.

Algorithms↗

PubMed2004

Half-scan and single-plane intensity diffraction tomography for phase objects.

A reconstruction theory for intensity diffraction tomography (I-DT) has been proposed that permits reconstruction of a weakly scattering object without explicit knowledge of phase information. In this work, we examine the application of I-DT, using either planar- or spherical-wave incident wavefields, for imaging three-dimensional (3D) phase objects. We develop and investigate two algorithms for reconstructing phase objects that utilize only half of the measurements that would be needed to reconstruct a complex-valued object function. Each reconstruction algorithm reconstructs the phase object by use of different sets of intensity measurements. Although the developed reconstruction algorithms are equivalent mathematically, we demonstrate that their numerical and noise propagation properties differ considerably. We implement numerically the reconstruction algorithms and present reconstructed images to demonstrate their use and to corroborate our theoretical assertions.

Algorithms↗

PubMed2004

Analytic image reconstruction in local phase-contrast tomography.

Phase-contrast tomography is a non-interferometric imaging technique for reconstructing the refractive index distribution of a weakly absorbing object from a set of tomographic projection measurements. In many practical situations, the spatial resolution of the reconstructed image can be increased by minimizing the field of view (FOV) of the imaging system. When the object of interest is larger than the FOV, the measured projections are truncated and one is faced with a local tomography reconstruction problem. In this work, we analytically and numerically investigate the problem of reconstructing tomographic images from truncated phase-contrast projection data. A simple backprojection algorithm for reconstructing object discontinuities from truncated phase-contrast projection data is proposed and investigated that involves no explicit filtering of the projection data. We also investigate the use of the filtered backprojection algorithm and a local tomography reconstruction algorithm developed for absorption CT. These reconstruction algorithms are implemented and numerically investigated to corroborate our theoretical assertions.

Algorithms↗

PubMed2003

A preliminary investigation of local tomography for megavoltage CT imaging.

We investigate the problem of reconstructing a two-dimensional (2-D) cross-sectional image of a tumor volume from a set of truncated MV projections that are produced by radiation therapy treatment beams. Our proposed approach is conceptually distinct from previously investigated approaches in that it utilizes a noniterative local tomography reconstruction algorithm. A local tomography reconstruction algorithm is implemented and systematically investigated using several sets of simulated and experimental MV projection data. We demonstrate that the conventional (nonlocal) filtered backprojection reconstruction algorithm cannot, in general, accurately reconstruct the edges and boundaries of low-contrast features from truncated MV projection data. We demonstrate that the local tomography algorithm is not adversely affected by projection truncation and can reconstruct accurately the boundaries of low-contrast structures within the region of interest from truncated MV projections.

Algorithms↗