X-Ray phase contrast imaging (PCI) is being developed as an alternative to overcome the poor contrast sensitivity of existing attenuation imaging techniques. The “phase sensitivity” can be achieved using a number of phase-enhancing geometries such as free space propagation, grating interferometry and edge illumination (also known as coded aperture) technique. The enhanced contrast in the projected intensities (that combine absorption and phase effect) can vary by object shape, size and its material properties as well as the particular PCI method used. We show a comparison of this signal enhancement for both FSP and coded aperture (CA) PCI. Our data shows that the phase enhancement is significantly higher for CA in comparison to FSP. Our preliminary results indicate that the enhanced phase effect decreases in all PCI techniques with increasing background thickness. Investigations involving signal location and background tissue thickness dependent signal enhancement (and/or loss of this signal) are very important in determining the true benefit of PCI methods in a practical application involving thick organs like breast imaging.
Photon counting spectral detectors are being investigated to allow better discrimination of multiple materials by collecting spectral data for every detector pixel. The process of material decomposition or discrimination starts with an accurate estimation of energy dependent attenuation of the composite object. Photoelectric effect and Compton scattering are two important constituents of the attenuation. Compton scattering while results in a loss of primary photon, also results in an increase in photon counts in the lower ene1rgy bins via multiple orders of scatter. This contribution to each energy bin may change with material properties, thickness and x-ray energies. There has been little investigation into the effect of this increase in counts at lower energies due to presence of these Compton scattered photons using photon counting detectors. Our investigations show that it is important to account for this effect in spectral decomposition problems.
X-ray phase contrast imaging has been investigated during the last two decades for potential benefits in soft tissue
imaging. Long imaging time, high radiation dose and general measurement complexity involving motion of x-ray
optical components have prevented the clinical translation of these methods. In all existing popular phase contrast
imaging methods, multiple measurements per projection angle involving motion of optical components are required to
achieve quantitatively accurate estimation of absorption, phase and differential phase. Recently we proposed an
algorithmic approach to use spectral detection data in a phase contrast imaging setup to obtain absorption, phase and
differential phase in a single-step. Our generic approach has been shown via simulations in all three types of phase
contrast imaging: propagation, coded aperture and grating interferometry. While other groups have used spectral
detector in phase contrast imaging setups, our proposed method is unique in outlining an approach to use this spectral
data to simplify phase contrast imaging. In this abstract we show the first experimental proof of our single-shot phase
retrieval using a Medipix3 photon counting detector in an edge illumination aperture (also referred to as coded aperture)
phase contrast set up as well as for a free space propagation setup. Our preliminary results validate our new transport
equation for edge illumination PCI and our spectral phase retrieval algorithm for both PCI methods being investigated.
Comparison with simulations also point to excellent performance of Medipix3 built-in charge sharing correction
mechanism.
Photon counting spectral detectors (PCSD) with smaller pixels and efficient sensors are desirable in applications like material decomposition and phase contrast x-ray imaging where discrimination of small signals and fine structure may be desired. Charge sharing in PCSD increases with decreasing pixel sizes and increasing sensor thickness such that the energy calibration or utility of spectral information can become a major hurdle. Utility of a combination of high Z sensors and small pixel sizes in PCSD is limited without efficient threshold calibration and charge sharing mitigation. Here we explore the utility of x-ray tube kVp as a reference to achieve efficient and fast calibration of PCSDs. This calibration method itself does not require rearranging the imaging setup and is not impacted by charge sharing. Our preliminary results indicate that this method can be useful even in scenarios where metal fluorescence and radioactive source based calibration techniques may be practically impossible. Our results are validated using x-ray fluorescence based calibration for a Silicon detector with moderate charge sharing. Calibration of a particularly challenging case of a Medipix2 detector (55 μm pixel size) with a 1 mm thick CdTe sensor and a Medipix3 detector with CdTe sensor is also demonstrated. A cross validation with K-edge identification of Gd is also presented here.
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