Abstract
X-ray tomography can be used to study the structure of whole cells in close to their native state. Ptychographic Fresnel coherent diffractive imaging (FCDI) holds particular promise for high-resolution tomographic imaging with quantitative phase sensitivity. To avoid the common missing wedge problem in tomography, cells can be mounted in thin glass capillaries that allow access to the full 180° angular field. However, soft x-rays, which are preferred for cellular imaging, interact strongly with capillaries, sometimes leading to violation of the usual assumptions for coherent diffractive imaging (CDI) and introducing artifacts (i.e., phase wrapping) in the reconstructed images. Here, we describe a method of applying ptychographic FCDI to obtain quantitative x-ray phase images of whole eukaryotic cells mounted in capillaries. The approach eliminates phase-wrapping artifacts due to thick capillaries without the deterioration in image quality that occurs at shallow angles of incidence when using planar mounting schemes. This technique makes it possible to apply CDI tomography to the study of various specimens that can be supported in capillaries and is compatible with established methods of cryogenic preparation.
Highlights
High-brilliance and highly coherent x-ray light sources have enabled x-ray coherent diffractive imaging (CDI) methods to be applied to three-dimensional (3D) reconstruction of samples with elemental sensitivity [1, 2] and at nanoscale resolution [3,4,5]
We demonstrate a method to improve the quality of reconstructed phase images of a specimen mounted in a capillary using ptychographic Fresnel coherent diffractive imaging (FCDI)
The model described by equations (1.1) and (1.2) assumes the capillary can be described as a symmetric tube with the inner and outer radii to be r1, r2 and homogenous composition characterized by its refractive index of which the decrement from unity of the real part is δC and imaginary part is βC
Summary
High-brilliance and highly coherent x-ray light sources have enabled x-ray coherent diffractive imaging (CDI) methods to be applied to three-dimensional (3D) reconstruction of samples with elemental sensitivity [1, 2] and at nanoscale resolution [3,4,5]. A common method for obtaining 3D structure information is to mount the sample on a planar membrane and measure a series of projections. The increasing absorption of x-rays through the sample at shallower incident angles can be even more of an issue for CDI since the reconstructed phase information may be degraded long before the measured intensity disappears. This problem can be avoided by mounting samples inside suitably prepared, hollow glass capillaries, which allow a full 360° rotation. The capillaries themselves are relatively robust and can be modified to preserve biological samples in a hydrated state, for instance, within the vacuum environment of a soft x-ray microscope [8, 14]
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