Dual focal-spot imaging for phase extraction in phase-contrast radiography

X-ray phase contrast imaging of spherical capsules

We demonstrate that a laser-based synchrotron X-ray source can be used to image and characterize in a single laser shot spherical capsules similar to ICF targets. Thus, we establish this source potential for real-time ultrafast imaging of the ICF laser driver interaction with the target. To produce the X-ray beam we used a 160 TW high power laser system with 3.2 J and 20 fs incident on a supersonic gas jet target at 2.5 Hz repetition rate. We produced 2.7 × 10⁹ photons/0.1% BW/sr/shot at 10 keV with a critical energy E_c = 15.1 keV. In our experimental conditions the spatial resolution was 4.3 μm in the object plane. We show that it is feasible to image the capsule structure and experimentally retrieve the phase information.

Phase-contrast and magnification radiography at diagnostic X-ray energies using a pseudo-microfocus X-ray source

OBJECTIVE

To investigate the use of conventional diagnostic X-ray tubes for applications in which specialist microfocus sources are normally required.

METHODS

A conventional diagnostic X-ray tube was used in conjunction with a range of apertures to investigate improvements in spatial resolution using a line-pairs test object. Phase-contrast effects were investigated by varying source-to-object and object-to-receptor distances using a 2-French catheter as a clinically realistic test object.

RESULTS

For magnification radiography using a computed radiography receptor and conventional X-ray tube with a 1-mm nominal focus size, the limiting spatial resolution was improved from 3.55 line-pairs per millimetre, for a conventional contact image, to 5.6 line-pairs per millimetre, for a ×2 magnified view with a 250-µm aperture. For inline phase-contrast radiography, phase contrast enhancement of a 2-French catheter was demonstrated, and the expected trends with variations in source-to-object and object-to-receptor distances were found. Images of a neonatal phantom demonstrated a subtle improvement in visibility of a superimposed 1-French catheter simulating a percutaneously inserted central catheter for no increase in patient radiation dose.

CONCLUSION

Spatial resolution improvement and visible phase contrast can be produced in clinically relevant objects using a pseudo-microfocus geometry at X-ray energies in the normal diagnostic range, using conventional diagnostic X-ray tubes and image receptors. The disadvantages of the proposal are the large distances required to produce phase contrast and limitations imposed by the resulting tube loading.

ADVANCES IN KNOWLEDGE

It is possible to use conventional diagnostic X-ray equipment in applications that normally require microfocus X-ray sources. This presents some possibilities for clinical applications.

Analysis of ideal observer signal detectability in phase-contrast imaging employing linear shift-invariant optical systems

Phase-contrast imaging methods exploit variations in an object's refractive index distribution to permit the visualization of subtle features that may have very similar optical absorption properties. Although phase-contrast is often viewed as being desirable in many biomedical applications, its relative influence on signal detectability when both absorption- and phase-contrast are present remains relatively unexplored. In this work, we investigate the ideal Bayesian observer signal-to-noise ratio in phase-contrast imaging for a signal-known-exactly/background-known exactly detection task involving a weak signal. We demonstrate that this signal detectability measure can be decomposed into three contributions that have distinct interpretations associated with the imaging physics.

Relationships between smooth- and small-phase conditions in X-ray phase-contrast imaging

We analyze some relationships between the small- and smooth-phase conditions in propagation-based X-ray phase-contrast imaging. Although these conditions are generally well-understood, our analysis yields the identification of physical conditions under which they are mathematically equivalent. We also demonstrate that the smooth-phase condition depends not only on the imaging system resolution and object-to-detector distance, but also on the topography of the wavefield phase function.

Influence of imaging geometry on noise texture in quantitative in-line X-ray phase-contrast imaging

Quantitative in-line X-ray phase-contrast imaging methods seek to reconstruct separate images that depict an object's projected absorption and refractive properties. An understanding of the statistical properties of the reconstructed images can facilitate the identification of optimal imaging parameters for specific diagnostic tasks. However, the statistical properties of quantitative X-ray phase-contrast imaging remain largely unexplored. In this work, we derive analytic expressions that describe the second-order statistics of the reconstructed absorption and phase images. Concepts from statistical decision theory are applied to demonstrate how the statistical properties of images corresponding to distinct imaging geometries can influence signal detectability.

Feasibility study of the iterative x-ray phase retrieval algorithm

An iterative phase retrieval algorithm was previously investigated for in-line x-ray phase imaging. Through detailed theoretical analysis and computer simulations, we now discuss the limitations, robustness, and efficiency of the algorithm. The iterative algorithm was proved robust against imaging noise but sensitive to the variations of several system parameters. It is also efficient in terms of calculation time. It was shown that the algorithm can be applied to phase retrieval based on one phase-contrast image and one attenuation image, or two phase-contrast images; in both cases, the two images can be obtained either by one detector in two exposures, or by two detectors in only one exposure as in the dual-detector scheme.

Statistical properties of X-ray phase-contrast tomography

Quantitative in-line X-ray phase-contrast tomography methods seek to reconstruct separate images that depict an object's absorption and real-valued refractive index distributions. They hold great promise for biomedical applications due to their ability to distinguish soft tissue structures based on their complex X-ray refractive index values. In this work, we investigate the second-order statistical properties of images in phase-contrast tomography and describe how they are distinct from those associated with conventional absorption-based tomography.

Preliminary feasibility study of an in-line phase contrast X-ray imaging prototype

In this study, a series of imaging experiments on biological specimens, including human breast core biopsies, lumpectomy, and chicken tissues, as well as standard phantoms, were performed in an effort to investigate the feasibility of an in-line phase contrast X-ray imaging prototype. The prototype system employed in the study consists of a microfocus X-ray source with tungsten target and a digital flat panel detector, and it can be operated in both conventional attenuation-based imaging mode and in-line phase contrast imaging mode. Biological specimens were imaged in the conventional mode and phase contrast mode with the same source-to-image-detector distance (SID), and phase contrast images exhibited both improved image quality compared with conventional images, and the overshooting patterns along the boundaries in the specimens, which revealed the occurrence of the edge enhancement effect provided by the phase contrast technique. In addition, the performance of the phase contrast mode and conventional mode was compared based on the American College of Radiology (ACR) phantom imaging and contrast detail mammography (CDMAM) phantom-based contrast detail analysis with two experimental settings: one with the same SID and the other with the same object entrance exposure. In both pairs of comparison under our experimental conditions, the phase contrast imaging mode exhibited improved image quality as compared to the conventional mode, which further supported the feasibility of the prototype.

An attenuation-partition based iterative phase retrieval algorithm for in-line phase-contrast imaging

For medical applications of the in-line phase-contrast x-ray imaging, phase retrieval is a crucial step for quantitative imaging such as reconstructing the 3-D distribution of tissue linear attenuation coefficients and refraction indices. The conventional phase retrieval algorithms, such as the transport of intensity equation (TIE) based algorithms, are not robust against the quantum noise that appears in acquired images due to the radiation dose constraints in medical imaging. In this work a new attenuation-partition based iterative phase retrieval algorithm is proposed. This new algorithm takes advantage of the correlation between the attenuation and phase-shift, and is much robust against noise in acquired images. Phase retrieval results from experimental images show that this new iterative algorithm is fast and robust, and it has good potential for medical x-ray imaging applications.

Clarification of aspects in in-line phase-sensitive x-ray imaging

In this report we clarify two aspects for in-line phase-sensitive x-ray imaging, which includes phase-contrast imaging and phase imaging. First, we point out that there is confusion in the literature about the lateral coherence length, which is widely adopted as the coherence criteria for implementing phase-sensitive imaging. The confusion exaggerates the coherence requirement for clinical implementation of in-line phase-sensitive imaging. Instead we show that the ratio of the phase-space shearing length to lateral coherence length is a good measure for gauging the partial coherence realized in a specific image setting. Second, based on the general intensity equation for in-line phase-sensitive imaging, we discuss the differences between the phase-contrast imaging and phase imaging in terms of the physics mechanism, image acquisition approaches, computation algorithm development, and the potentials for tissue quantitative characterization.

Gene therapy targeting the Tie2 function ameliorates collagen-induced arthritis and protects against bone destruction

OBJECTIVE

In a previous study, we demonstrated that Tie2 regulates angiogenesis in arthritis. The current study was performed to determine whether systemic delivery of a soluble Tie2 receptor (ExTek) using an adenoviral vector (AdExTek) as a Tie2 inhibitor affects arthritis development and progression in an animal model.

METHODS

We used a collagen-induced arthritis (CIA) mouse model to study the outcome of treatment with either AdExTek or a control vector. The onset, incidence, and severity of arthritis were quantified. Immunohistologic analysis of endothelium obtained from the paws was performed. Bone destruction in paws was analyzed using phase-contrast radiography.

RESULTS

The data showed that systemic delivery of ExTek before disease development significantly inhibited the onset, incidence, and severity of arthritis. When AdExTek was given after disease onset, the severity of disease in mice treated with AdExTek was significantly lower than that in the control group at 35 days postimmunization, which correlated with significantly diminished angiogenesis in mouse paws. Strikingly, AdExTek treatment protected bone from erosion in the CIA model and reduced levels of RANKL. No differences in collagen-specific antibodies were detected between these 2 groups.

CONCLUSION

We demonstrated that blocking Tie2 receptor activation inhibits angiogenesis and arthritis development and protects against bone destruction in a CIA mouse model. These findings identify Tie2 as a therapeutic target for arthritis treatment and imply that interventions designed to target the Tie2 pathway could be clinically beneficial.

A new theory of phase-contrast x-ray imaging based on Wigner distributions

There is a pressing need for a comprehensive theory for phase-contrast x-ray imaging to guide its development and clinical applications. This work presents such a theory as the foundation for deriving these guidelines. The new theory is based on the Wigner-distributions for the parabolic wave equations, and it is more general than the present theories based on the Fresnel-Kirchhoff diffraction theory. The new theory shows for the first time how the complex degree of coherence (CDC) of the incident x-ray beam determines the phase-contrast visibility in general, and how the reduced complex degree of coherence (RCDC) for an anode-source is equal to the system's optical transfer function for geometric unsharpness in particular. The role of detector resolution in phase visibility has been clarified as well. Computer simulations based on the new theory were conducted and optimal design parameters were derived for phase-contrast mammography systems.

X-ray phase-attenuation duality and phase retrieval

Phase retrieval is the key to quantitative x-ray phase-contrast imaging. To retrieve the phase image of an x-ray wave field, in general one needs multiple phase-contrast images. We have made a new observation of phase-attenuation duality for soft tissues, and we show how only a single phase-contrast image is needed for successful phase retrieval based on this duality. The phase-retrieval formula based on a single phase-contrast image of inhomogeneous soft tissue is derived and presented. We show the striking enhancement of the tissue contrast in simulated phase images that this new approach produces.

Quantification of the effect of system and object parameters on edge enhancement in phase-contrast radiography

The purpose of this study was to evaluate the effects of system parameters (focal spot size, tube voltage, geometry, detector resolution, and image noise) and object characteristics (edge gradient/ shape, composition, thickness, and overlying attenuating material) upon the edge enhancement effect in phase-contrast radiography. Each variable of interest was adjusted and images of a 3 mm lucite phantom were obtained with the other variables remaining constant. A microfocus x-ray source coupled to a CCD camera with an intensifying screen was used to acquire the digital images. Two parameters of image analysis were used to quantify the effects. The edge enhancement index (EEI) was used to measure the absolute degree of edge enhancement, while the edge enhancement to noise ratio (EE/N) was used to measure the conspicuity of the edge enhancement relative to image noise. Little effect on EEI was seen from tube voltage, object thickness, overlying attenuating material, while focal spot size and system geometry demonstrated measurable effects upon the degree of edge enhancement. It was also shown that while the edge enhancement effect over straight edges is highly dependent upon how the edge aligns with the x-ray beam, rounded edges, which better model biological objects, do not suffer from this dependence and the EEI reaches its maximal level at any alignment. Decreasing detector resolution diminished the EEI slightly, but even with pixel sizes of 0.360 x 0.360 mm edge enhancement effects were readily visible. The effect of image noise on EE/N was evaluated using different exposure times showing an expected improvement with longer exposure time with EE/N approaching a plateau at 5 min. Many of the parameters that will go into the design of a future PC-R imaging system have been quantified in terms of their effect on the degree of edge enhancement in the acquired image. These results, taken together, indicate that either a specimen or even clinical breast imaging system could be created with currently available technology. The major limitation to a clinical system would be the low x-ray flux from the microfocal x-ray source.

An experimental method of determining relative phase-contrast factor for x-ray imaging systems

The relative phase-contrast factor (RPF) represents a quantitative measure of the phase-contrast transfer in x-ray in-line phase-contrast imaging. The larger the modulus of RPF(u, v) is, the more the phase-contrast manifests. In this work we show how the RPF can be determined by measurements of the focal spot size and x-ray spectra for a x-ray imaging system with a micro-focus x-ray tube. The results show the significant effects of x-ray beam hardening on the visibility of phase-contrast, and reveal a new dimension in seeking optimal techniques for x-ray phase-contrast imaging.