Contrast and resolution in imaging with a microfocus x-ray source

OA248] Computer-based platform for phase contrast breast imaging

Portal vein imaging is an important method for investigating portal venous disorders. However, the diagnostic requirements are not usually satisfied when using single imaging techniques. Diagnostic accuracy can be improved by combining different imaging techniques. Contrast agents that can be used for combined imaging modalities are needed. In this study, the feasibility of using microbubbles containing gadolinium (MCG) as contrast agents for both phase contrast imaging (PCI) and magnetic resonance imaging (MRI) are investigated. MCG were made by encapsulating sulfur hexafluoride (SF6) gas with gadolinium and lyophilized powder. Absorption contrast imaging (ACI) and PCI of MCG were performed and compared in vitro. MCG were injected into the main portal trunk of living rats. PCI and MRI were performed at 2 min and 10 min after MCG injection, respectively. PCI exploited the differences in the refractive index and visibly showed the MCG, which were not detectable by ACI. PCI could facilitate clear revelation of the MCG-infused portal veins. The diameter of the portal veins could be determined by the largest MCG in the same portal vein. The minimum diameter of clearly detected portal veins was about 300 µm by MRI. These results indicate that MCG could enhance both PCI and MRI for imaging portal veins. The detection sensitivity of PCI and MRI could compensate for each other when using MCG contrast agents for animals.

In available synchrotron radiation facilities all over the world, the size of electron beam bunch has distinct difference between the horizontal direction and the vertical direction at the cross section vertical to electron moving direction. It will cause low symmetry of source size in horizontal and vertical directions, and result in big difference in the spacial resolution and phase contrast between these two directions in synchrotron radiation X-ray in-line phase contrast imaging. The superior resolution and phase contrast of one direction will couple with the inferior ones of the other direction in actual object imaging, this interaction will degrade both spacial resolution and phase contrast of the whole image and lead to poor image quality. In this paper, a four-knife white beam slit is used in Beijing synchrotron radiation facility (BSRF) 4W1A beamline to adjust source size in horizontal and vertical directions and restrict the point spread function (PSF) of source, the source size is limited from 1.23 mm (horizontal)×0.46 mm (vertical) to 0.35 mm×0.35 mm. By this way, a highly symmetric effective X-ray source is obtained. The symmetry of source size is improved from 2.7 to about 1. Although the total X-ray flux loses 99.95%, the X-ray flux density is only reduced to 22%. It because total flux is taken account of the expanded spot size due to divergence, while flux density is mainly related to effective source size, which is limited not severely as expanded spot size by white beam slit. The flux density reduction by this method is not a critical defect in high resolution X-ray phase contrast imaging and can be improved by other ways. The polyamide fiber net is imaged using X-ray in-line phase contrast imaging at 4W1A, BSRF. Diameter of fiber is about 50 microns, X-ray energy is 13 keV, the effective pixel size is 0.325 micron, the experiment images are obtained at object-to-detector distances of 4, 10, 16, 36, 100, 200 and 300 mm with source-to-object distance of about 43 m. The results show that when source size in horizontal direction is limited from 1.23 to 0.35 mm (symmetry of source size is about 1 at this condition), the maximal phase contrast of image is advanced from 0.057 to 0.35. It means the density sensitivity is promoted about 5 times than in ordinary source size conditions. The spacial resolutions in different source size symmetry conditions are also primary analyzed based on Rayleigh criterion. The result shows that spacial resolution can be improved notably in horizontal direction by limiting horizontal source size to a nearly symmetric source size condition. Our work demonstrates that the spacial resolution and phase contrast can be promoted at the same level by source size symmetry improved, which markedly improved the imaging quality in synchrotron radiation X-ray in-line phase-contrast imaging. It will have great benefit to high-resolution X-ray in-line phase contrast imaging. The reasons of these improvements are analyzed and developments in future are also discussed.

We present an improved diffraction model for phase contrast imaging (PCI), relevant to the measurement of electron density fluctuations in magnetically confined fusion plasmas. The model applies to optical systems containing multiple special components, such as phase plates, localization masks, and apertures, connected by segments describable in terms of ray transfer matrices. Fresnel diffraction in the connecting segments may be described using linear canonical transforms, while the special components are characterized by their respective transfer functions. For a PCI system with ideal placement of the phase plate and localization mask, deviations between the Fresnel and Fraunhofer diffraction models are entirely determined by the distance from the diffraction plane to the object plane of the optical system. Application of the developed model to the synthetic PCI diagnostic at the Wendelstein 7-X (W7-X) stellarator shows minimal impact of diffraction on the DC detector power. However, significant amplitude modifications due to phase scintillation from diffraction are expected for high-wavenumber PCI sound wave calibration signals at W7-X, in qualitative agreement with the experimental observations. In contrast to previous expectations, diffraction is found to have only a small impact on the localization mask response characteristics at W7-X. Based on this observation, we show that the synthetic PCI mask response at W7-X can be reproduced by a model which only considers the signal at the beam center in the image plane without diffraction. We further use a similar model to reproduce the experimentally observed mask response from the DIII-D tokamak, indicating the general applicability of such models.

X-ray phase-contrast imaging produces significantly higher contrast than the conventional attenuation-based imaging. However, phase contrast typically requires small low-power sources or precisely machined and aligned gratings to generate the required spatial coherence. To overcome this limitation, we use a simple wire mesh to produce a periodic intensity pattern in the illumination. Distortions in this pattern upon propagation through an object can be used to produce phase contrast images. Unlike Talbot-grating-based techniques, this directly provides a measure of the phase gradient. Due to periodicity, fast Fourier transforms enable rapid retrieval of the phase contrast image. The high contrast of the mesh pattern allows a significant relaxation of the coherence requirement. However, the resolution of the phase contrast image is normally limited by the mesh period. We demonstrate that employing wider windowing functions during the Fourier processing substantially improves the spatial resolution at the cost of introducing artifacts in the reconstructed images. In order to remove these artifacts, a method is presented to combine images acquired while spatially shifting the mesh. Experimental results demonstrate the successful removal of these artifacts. This offers a method for significant resolution improvement in mesh-based phase-contrast imaging, enabling high resolution phase contrast images with a significant relaxation of source coherence requirements.

The objective of this study was to demonstrate the potential benefits of using high energy x-rays for phase sensitive breast imaging through a comparison with conventional mammography imaging. We compared images of a contrast-detail phantom acquired on a prototype phase sensitive x-ray imaging system with images acquired on a commercial flat panel digital mammography unit. The phase contrast images were acquired using a micro-focus x-ray source with a 50 µm focal spot at 120 kVp and 4.5 mAs, with a magnification factor of 2.46 and a 50 µm pixel pitch. A phase attenuation duality-based phase retrieval algorithm that requires only a single phase contrast image was applied. Conventional digital mammography images were acquired at 27 kVp, 131 mAs and 28 kVp, 54 mAs. For the same radiation dose, both the observer study and signal-to-noise ratio (SNR)/figure of merit comparisons indicated a large improvement by the phase retrieved image as compared to the clinical system for the larger disc sizes, but the improvement was not enough to detect the smallest discs. Compared to the double dose image acquired with the clinical system, the observer study also indicated that the phase retrieved image provided improved detection capabilities for all disc sizes except the smallest discs. Thus the SNR improvement provided by phase contrast imaging is not yet enough to offset the noise reduction provided by the clinical system at the doubled dose level. However, the potential demonstrated by this study for high energy phase sensitive x-ray imaging to improve lesion detection and reduce radiation dose in mammography warrants further investigation of this technique.

In tapping mode atomic force microscopy (TM‐AFM), the phase image can reveal more about the physicochemical properties of the sample surface than the topography image, making it widely utilized in fields such as materials science and life sciences. However, obtaining high phase contrast in phase images during experiments is challenging, thus studying the phase imaging theory of TM‐AFM and optimizing phase contrast methods hold significant importance. This paper derives a theoretical expression for phase contrast based on phase theory, discovering that phase contrast is directly related to the frequency ratio and background dissipation quality factor. Through theoretical, experimental, and simulation approaches, optimization methods for phase contrast were proposed from the perspectives of frequency ratio and background dissipation quality factor as follows: (1) By fitting experimental results with the theoretical expression for phase contrast, the optimal frequency ratio for scanning can be determined, and imaging with this optimal ratio can enhance phase contrast and (2) reducing the width ratio between the free and fixed ends of the probe can decrease background dissipation, increase the proportion of core dissipation in the total energy dissipation of the probe, and thereby improve phase imaging contrast. These findings are significant in guiding TM‐AFM phase scanning experiments and optimizing phase imaging contrast.

X-ray phase and dark-field contrast have recently been the source of much attention in the field of X-ray imaging, as they both contribute new imaging signals based on physical principles that differ from conventional X-ray imaging. With a so-called Talbot grating interferometer, both phase-contrast and dark-field images are obtained simultaneously with the conventional attenuation-based X-ray image, providing three complementary image modalities that are intrinsically registered. Whereas the physical contrast mechanisms behind attenuation and phase contrast are well understood, a formalism to describe the dark-field signal is still in progress. In this article, we report on correlative experimental results obtained with a grating interferometer and with small-angle X-ray scattering. Furthermore, we use a proposed model to quantitatively describe the results, which could be of great importance for future clinical and biomedical applications of grating-based X-ray imaging.

To investigate the properties of tomographic grating-based phase contrast imaging with respect to its noise power spectrum and the energy dependence of the achievable contrast to noise ratio. Tomographic simulations of an object with 11 cm diameter constituted of materials of biological interest were conducted at different energies ranging from 25 to 85 keV by using a wave propagation approach. Using a Monte Carlo simulation of the x-ray attenuation within the object, it is verified that the simulated measurement deposits the same dose within the object at each energy. The noise in reconstructed phase contrast computed tomography images shows a maximum at low spatial frequencies. The contrast to noise ratio reaches a maximum around 45 keV for the simulated object. The general dependence of the contrast to noise on the energy appears to be independent of the material. Compared with reconstructed absorption contrast images, the reconstructed phase contrast images show sometimes better, sometimes worse, and sometimes similar contrast to noise, depending on the material and the energy. Phase contrast images provide additional information to the conventional absorption contrast images and might thus be useful for medical applications. However, the observed noise power spectrum in reconstructed phase contrast images implies that the usual trade-off between noise and resolution is less efficient for phase contrast imaging compared with absorption contrast imaging. Therefore, high-resolution imaging is a strength of phase contrast imaging, but low-resolution imaging is not. This might hamper the clinical application of the method, in cases where a low spatial resolution is sufficient for diagnosis.

Conventional radiography is based on absorption contrast and geometrical (ray) optics. After an outline of the relevant theory, this article reports results displaying both phase- and absorption-contrast, collected with a technique which utilizes a micro-focus x-ray source to achieve a high degree of spatial coherence, and relatively large object-to-image distances to enable (wave) interference effects (Fresnel diffraction) to occur and manifest themselves as phase contrast in the image plane. Both soft tissue (chicken knee) and hard tissue (finger bone) samples are investigated for a range of source sizes and object-to-image distances, encompassing conditions somewhat analogous to conventional radiography. Variation in image contrast and resolution as a function of these variables is observed and discussed.

A program devoted to performing the first in vivo synchrotron radiation (SR) breast computed tomography (BCT) is ongoing at the Elettra facility. Using the high spatial coherence of SR, phase-contrast (PhC) imaging techniques can be used. The latest high-resolution BCT acquisitions of breast specimens, obtained with the propagation-based PhC approach, are herein presented as part of the SYRMA-3D collaboration effort toward the clinical exam. Images are acquired with a dead-time-free single-photon-counting CdTe detector. The samples are imaged at 32 and 38keV in a continuous rotating mode, delivering 5 to 20mGy of mean glandular dose. Contrast-to-noise ratio (CNR) and spatial resolution performances are evaluated for both PhC and phase-retrieved images, showing that by applying the phase-retrieval algorithm a five-time CNR increase can be obtained with a minor loss in spatial resolution across soft tissue interfaces. It is shown that, despite having a poorer CNR, PhC images can provide a sharper visualization of microcalcifications, thus being complementary to phase-retrieved images. Furthermore, the first full-volume scan of a mastectomy sample ( ) is reported. This investigation into surgical specimens indicates that SR BCT in terms of CNR, spatial resolution, scan duration, and scan volume is feasible.

The basic principles of x-ray image formation in radiography have remained essentially unchanged since Röntgen first discovered x-rays over a hundred years ago. The conventional approach relies on x-ray absorption as the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image formation. This approach ignores another, potentially more useful source of contrast, namely phase and scattering information. Phase-contrast imaging techniques, which can be understood using wave optics rather than ray optics, offer ways to augment or complement standard absorption contrast by incorporating phase information. The recent development of grating based phase- and darkfield-contrast imaging with x-rays¹ pawed the way for many potential applications to medical imaging and structure determination in material science.<br />Here we present our recent contributions to the field of interferometric phase-contrast and dark-field x-ray imaging. We introduce a new material dependent scattering parameter, the <em>Linear Diffusion Coefficient</em>, and a quantitative mathematical formalism to extend the dark-field x-ray images into three dimensions by tomographic reconstruction. Further, the results of two experiments that illustrate the potential of dark-field imaging for computed tomography are shown.

This study explores the ability of a hard Kα x-ray source (17.48 keV) produced by a 10 TW class laser system operated at high temporal contrast ratio and high repetition rate for phase contrast imaging. For demonstration, a parametric study based on a known object (PET films) shows clear evidence of feasibility of phase contrast imaging over a large range of laser intensity on target (from ~1017 W/cm2 to 7.0 × 1018 W/cm2). To highlight this result, a comparison of raw phase contrast and retrieved phase images of a biological object (a wasp) is done at different laser intensities below the relativistic intensity regime and up to 1.3 × 1019 W/cm2. This brings out attractive imaging strategies by selecting suitable laser intensity for optimizing either high spatial resolution and high quality of image or short acquisition time.

Phase sensitive X-ray imaging methods can provide substantially increased contrast over conventional absorption-based imaging and therefore new and otherwise inaccessible information. The use of gratings as optical elements in hard X-ray phase imaging overcomes some of the problems that have impaired the wider use of phase contrast in X-ray radiography and tomography. So far, to separate the phase information from other contributions detected with a grating interferometer, a phase-stepping approach has been considered, which implies the acquisition of multiple radiographic projections. Here we present an innovative, highly sensitive X-ray tomographic phase-contrast imaging approach based on grating interferometry, which extracts the phase-contrast signal without the need of phase stepping. Compared to the existing phase-stepping approach, the main advantages of this new method dubbed "reverse projection" are not only the significantly reduced delivered dose, without the degradation of the image quality, but also the much higher efficiency. The new technique sets the prerequisites for future fast and low-dose phase-contrast imaging methods, fundamental for imaging biological specimens and in vivo studies.

The objective of this study was to investigate the potential of hydrogen peroxide-generated oxygen gas-based phase contrast imaging (PCI) for visualizing mouse hepatic portal veins. The O2 gas was made from the reaction between H2O2 and catalase. The gas production was imaged by PCI in real time. The H2O2 was injected into the enteric cavity of the lower sigmoid colon to produce O2 in the submucosal venous plexus. The generated O2 gas could be finally drained into hepatic portal veins. Absorption contrast imaging (ACI) and PCI of O2-filled portal veins were performed and compared. PCI offers high resolution and real-time visualization of the O2 gas production. Compared with O2-based ACI, O2-based PCI significantly enhanced the revealing of the portal vein in vivo. It is concluded that O2-based PCI is a novel and promising imaging modality for future studies of portal venous disorders in mice models.