The grating based approach to phase contrast imaging is rather inefficient in the use of the available x-ray flux
due to the presence of two absorption gratings and it requires longer scan times compared to conventional CT
because multiple images are needed at each projection angle. To avoid these drawbacks, a proof-of-principle
experiment was developed to obtain absorption, phase contrast (DPC) and dark field images (DCI) in a single
exposure using only a non-absorbing phase grating, a micro-focus source in cone-beam geometry and a highresolution
x-ray detector.
We have developed a compact grating-based in-vivo phase-contrast micro-CT system with a rotating gantry. The 50 W microfocus x-ray source operates with 20 to 50 kV peak energy. The length of the rotating interferometer is around 47 cm. Pixel size in the object is 30 micron; the field of view is approx. 35 mm in diameter, suited to image a mouse. The interferometer consists of three gratings: an absorption grating close to the x-ray source, a phase grating to introduce a π/2 phase shift and an absorption analyzer grating positioned at the first fractional Talbot distance. Numerous drives and actuators are used to provide angular and linear grating alignment, phase stepping and object/gantry precision positioning. Phantom studies were conducted to investigate performance, accuracy and stability of the scanner. In particular, the influences of gantry rotation and of temperature fluctuations on the interferometric image acquisition were characterized. Also dose measurements were performed. The first imaging results obtained with the system show the complementary nature of phase-contrast micro-CT images with respect to absorption-based micro-CT. Future improvements, necessary to optimize the scanner for in-vivo small-animal CT scanning on a regular and easy-to-use basis, are also discussed.
Proper selection of modern key components allows eliminating most artifacts in micro-CT and nano-CT systems already
during data acquisition. X-ray cameras with direct photon detection allow avoiding ring artifacts. Newly developed fully
depleted CCD sensors show an energy response similar to traditional cameras with a thin scintillator, but without any
geometrical distortions and flashes from x-ray photons penetrating through the fiber optics. Air-bearing rotation stages
and piezo-positioning minimizes mechanical inaccuracies in acquiring angular projections. Beam hardening can be
eliminated by energy-selective photon counting imaging.
Most X-ray systems are limited in spatial resolution by the x-ray source performance. In laboratory sources, x-rays are
generated by the interaction of an electron beam with a metal target. Bulk target sources produce a spot size in the
micron range. Thin layer targets allow a spot size improvement down to hundreds of nanometers, but with a significant
flux reduction. Until now a spatial resolution under 100 nm could only be achieved by imaging with Fresnel zone plates
with limited depth of focus, typically - several microns. This is acceptable for imaging of flat objects, but it creates a
problem for tomography, which requires all parts of a bulk object to be in focus.
To overcome the limitations, we invented an x-ray source with a new type of target. Because x-ray cameras can only
collect photons from a small angle, the new emitter is physically shaped in such way that the camera can see it as a small
dot, but it has a big length along the direction perpendicular to the camera creating a significant flux without
compromising the resolution. Evaluation shows that structures down to 50 nm can be distinguished while maintaining a
significant x-ray flux and infinite depth of focus required for nano-tomographical reconstruction.
We have developed an x-ray computer tomography (CT) add-on to perform X-ray micro- and nanotomography in any
scanning electron microscope (SEM). The electron beam inside the SEM is focused on a metal target to generate x-rays.
Part of the X-rays pass through the object that is installed on a rotation stage. Shadow X-ray images are collected by a
CCD camera with direct photon detection mounted on the external wall of the SEM specimen chamber. An extensive
description on the working principles of this micro/nano-CT add-on together with some examples of CT-scans will be
given in this paper. The resolution that can be obtained with this set-up and the influence of the shape of the electron
beam are discussed. Furthermore, possible improvements on this SEM-CT set-up will be discussed: replacing the backilluminated
CCD with a fully depleted CCD with improved quantum efficiency (QE) for higher energies, reduces the
exposure time by 6 when using metal targets with x-ray characteristic lines around 10 keV.
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