Improved phase-contrast flow quantification by three-dimensional vessel localization

In this paper, a method of three-dimensional (3D) vessel localization is presented to allow the identification of a vessel of interest, the selection of a vessel segment, and the determination of a slice orientation to improve the accuracy of phase-contrast magnetic resonance (PCMR) angiography. A marching-cube surface-rendering algorithm was used to reconstruct the 3D vasculature. Surface-rendering was obtained using an iso-surface value determined from a maximum intensity projection (MIP) image. This 3D vasculature was used to find a vessel of interest, select a vessel segment, and to determine the slice orientation perpendicular to the vessel axis. Volumetric flow rate (VFR) was obtained in a phantom model and in vivo using 3D localization with double oblique cine PCMR scanning. PCMR flow measurements in the phantom showed 5. 2% maximum error and a standard deviation of 9 mL/min during steady flow, 7.9% maximum error and a standard deviation of 13 mL/min during pulsatile flow compared with measurements using an ultrasonic transit-time flowmeter. PCMR VFR measurement error increased with misalignment at 10, 20, and 30 degrees oblique to the perpendicular slice in vitro and in vivo. The 3D localization technique allowed precise localization of the vessel of interest and optimal placement of the slice orientation for minimum error in flow measurements.     

Application of perceptual difference model on regularization techniques of parallel MR imaging

Parallel magnetic resonance imaging through sensitivity encoding using multiple receiver coils has emerged as an effective tool to reduce imaging time or to improve image SNR. The quality of reconstructed images is limited by the inaccurate estimation of the sensitivity map, noise in the acquired k-space data and the ill-conditioned nature of the coefficient matrix. Tikhonov regularization is a popular method to reduce or eliminate the ill-conditioned nature of the problem. In this approach, selection of the regularization map and the regularization parameter is very important. Perceptual difference model (PDM) is a quantitative image quality evaluation tool that has been successfully applied to varieties of MR applications. High correlation between the human rating and PDM score shows that PDM should be suitable to evaluate image quality in parallel MR imaging. By applying PDM, we compared four methods of selecting the regularization map and four methods of selecting the regularization parameter. We found that a regularization map obtained using generalized series (GS) together with a spatially adaptive regularization parameter gave the best reconstructions. PDM was also used as an objective function for optimizing two important parameters in the spatially adaptive method. We conclude that PDM enables one to do comprehensive experiments and that it is an effective tool for designing and optimizing reconstruction methods in parallel MR imaging.     

Improved simultaneous multislice magnetic resonance imaging using total variation regularization

Controlled aliasing in parallel imaging results in higher acceleration(CAIPIRINHA) for simultaneous multislice imaging has been proposed recently, which combines multiband excitation and phase cycling techniques to reduce scan time and improve subsequent imaging reconstruction. In this work, the total variation(TV) regularization method is used to further improve CAIPIRINHA. The TV regularization uses an edge-preserving prior, which establishes a relationship between neighboring pixels for image reconstruction. It reduces artifacts and suppresses noise amplification simultaneously.The results are presented with a standard eight-channel head coil with an acceleration factor of 4, where the TV-regularized CAIPIRINHA generates an improved reconstruction as compared with a typical nonregularized CAIPIRINHA.     

基于激光驱动等离子体X光源的X射线相衬成像

为了诊断惯性约束聚变(ICF)内爆靶丸球壳的多层信息,在神光Ⅱ激光器上对激光驱动等离子体X光源的相衬成像进行了研究。利用神光Ⅱ第9路激光驱动平面Ti靶获得X光源,在10μm的针孔约束下作为次级点光源对样品成像,用X光胶片记录。成功地将相衬成像技术应用于ICF实验,综合考虑成像放大倍数、分辨力、成像衬度和抑制烧蚀碎片等因素,选择合适的实验条件,成功获得了清晰的双层内爆靶丸球壳结构,空间分辨力优于10μm。     

基于激光驱动等离子体X光源的X射线相衬成像

为了诊断惯性约束聚变（ICF）内爆靶丸球壳的多层信息,在神光Ⅱ激光器上对激光驱动等离子体X光源的相衬成像进行了研究。利用神光Ⅱ第9路激光驱动平面Ti靶获得X光源,在10 μm的针孔约束下作为次级点光源对样品成像,用X光胶片记录。成功地将相衬成像技术应用于ICF实验,综合考虑成像放大倍数、分辨力、成像衬度和抑制烧蚀碎片等因素,选择合适的实验条件,成功获得了清晰的双层内爆靶丸球壳结构,空间分辨力优于10 μm。     

Visibility in differential phase-contrast imaging with partial coherence source

This paper gives theoretical analysis of visibility of fringes, which is influenced by distances, temporal and spatial coherence of source, in hard x-ray differential phase-contrast imaging with microfocus x-ray source. According to the character of longitudinal periodicity of the interferogram, the setup is insensitive to mechanical drift and vibrations. The effect of temporal coherence of x-ray source is investigated and its related bandwidth is derived. Based on the theory of partially coherent light, it shows that the requirement for the spatial coherence of x-ray source is not strict and can be met by the general microfocus x-ray tube for x-ray differential phase-contrast imaging.     

Quantitative in-line phase-contrast imaging with multienergy X rays.

We present a new method for quantitative nondestructive characterization of objects by x-ray phase-contrast imaging. Spatial distributions of the projected values of the complex refractive index in the sample are reconstructed by processing near-field images collected at a fixed sample-to-detector distance using a polychromatic incident beam and an energy-sensitive area detector, such as a CCD used in the photon-counting spectroscopy mode. The method has the potential advantages of decreased radiation dose and increased accuracy compared to conventional techniques of x-ray imaging.     

Quantitative phase-contrast imaging of cells with phase-sensitive optical coherence microscopy

We describe a method for en face phase-contrast imaging of cells with a fiber-based differential phase-contrast optical coherence microscopy system. Recorded en face images are quantitative phase-contrast maps of cells due to spatial variation of the refractive index and (or) thickness of various cellular components. Quantitative phase-contrast images of human epithelial cheek cells obtained with the fiber-based differential phase-contrast optical coherence microscopy system are presented.     

Three-vessel study of cerebral blood flow using phase-contrast magnetic resonance imaging: Effect of physical characteristics

The development of phase-contrast magnetic resonance imaging (P-C MRI) provides a noninvasive method for measurement of volumetric blood flow (VFR). We performed P-C MRI to study the effects of physical characteristics on cerebral blood flow. VFR of the left and right internal carotid arteries and basilar artery were measured using P-C MRI and total cerebral blood flow (tCBF) was calculated by summing up the VFR values in the three vessels. Moreover, we investigated the changes in these blood flows as influenced by age, head size, height, weight, body surface area, and handedness. The blood flows were 142 ± 58 ml/min (mean ± standard deviation) in the basilar artery; and 229 ± 86 ml/min in the left, and 223 ± 58 ml/min in the right internal carotid artery; and tCBF was 617 ± 128 ml/min. Significant increases were observed in head size-related change of VFR in the basilar artery (p = .028) and height-related change of tCBF (p = .045). The other characteristics did not significantly influence any VFR. The results suggest that head size and height may reflect CBF, and that these effects should be considered when changes of CBF are diagnosed. Phase-contrast MRI is useful for a noninvasive and rapid analysis of cerebral VFR and has potential for clinical use.     

Noise analysis of grating-based x-ray differential phase-contrast imaging with angular signal radiography

X-ray phase-contrast imaging is one of the novel techniques,and has potential to enhance image quality and provide the details of inner structures nondestructively.In this work,we investigate quantitatively signal-to-noise ratio(SNR) of grating-based x-ray phase contrast imaging(GBPCI) system by employing angular signal radiography(ASR).Moreover,photon statistics and mechanical error that is a major source of noise are investigated in detail.Results show the dependence of SNR on the system parameters and the effects on the extracted absorption,refraction and scattering images.Our conclusions can be used to optimize the system design for upcoming practical applications in the areas such as material science and biomedical imaging.     

Theory and method of dual-energy x-ray grating phase-contrast imaging

The principle of dual-energy x-ray grating phase-contrast imaging(DEPCI) is clarified by using the theory of x-ray interference and Fresnel diffraction. A new method of retrieving phase from the two interferograms is proposed for DEPCI,and its feasibility is verified via simulation. Finally, the proposed method applied to DEPCI experiment demonstrates the effectiveness of the method. This paper lays the theoretical foundation for performance optimization of DEPCI and the further integration of DEPCI and computed tomography.     

Combined analyser-based and propagation-based phase-contrast imaging of weak objects

A new theoretical method combining analyser-based and propagation-based hard X-ray phase-contrast imaging is investigated. Unlike the previous theoretical model of the combined imaging method constructed under the assumption of slow variation of the individual transfer functions (large Fresnel numbers), a new model proposed in this paper uses the assumption of a weak scatterer (analogous to the first Born approximation). Consequently, the results are not limited to the case of short propagation distances or low-resolution imaging. An explicit expression for the combined transfer function is derived and analytical and numerical examples solving related inverse imaging problems are presented.     

Improved renormalization group equations with dimensional regularization and the ε expansions

Due to the absence of dimensional cut-off parameters in the dimensional regularization scheme, vanishing of the renormalized mass of the scalar boson implies vanishing of its renormalized mass; thus the masses of both bosons and fermions in renormalizable field theories can be made finite by multiplicative mass renormalizations. The improved renormalization group equations in D dimensions are derived in such a way that both the large (or the small) momentum limits and the Wilson ? expansions can be uniformly treated for the fermion as well as the boson cases. We discuss the improved equations for φ₆³ theory, φ₄⁴ theory, quantumelectrodynamics, massive vector-gluon model, and non-Abelian guage theories incorporating fermions. For the latter three classes of theories, the gauge dependent problem of the coefficient functions in the improved renormalization group equations is discussed.     

双面声阵列波束形成的正则化改进算法

双面声阵列波束形成能够区分识别位于不同扫描平面的相干声源,然而该算法在低信噪比条件下识别精度较低。针对此问题提出一种迭代正则化改进算法,通过迭代方法更新正则化矩阵与波束形成输出,在不断提升正则化稳定性、抑制干扰旁瓣的基础上使声学云图主瓣向实际相干声源点处聚焦。数值仿真与实验算例结果显示,改进算法在中高频代表频率下能够正确区分相干声源前后方位,并具有相对原算法更高的识别精度。从而表明:从反问题正则化角度对原算法进行优化改进是理论可行的;正则化矩阵的具体形式与广义逆波束形成输出的空间分辨率紧密相关,且可通过迭代方法将二者整合以提高声源识别精度。     

Noise properties of multi-combination information in x-ray grating-based phase-contrast imaging

Grating-based x-ray phase contrast imaging has attracted increasing interest in recent decades as multimodal and laboratory source usable method. Specific efforts have been focused on establishing a new extraction method to perform practical applications. In this work, noise properties of multi-combination information of newly established information extraction method, so-called angular signal radiography method, are investigated to provide guidelines for targeted and specific applications. The results show that how multi-combination of images can be used in targeted practical applications to obtain a high-quality image in terms of signal-to-noise ratio. Our conclusions can also hold true for upcoming targeted practical applications such as biomedical imaging, non-destructive imaging, and materials science.     

A robust electrical conductivity imaging method with total variation and wavelet regularization

PurposeThis study aims to develop and evaluate a robust conductivity imaging method that combines total variation and wavelet regularization to enhance the accuracy of conductivity maps.Theory and methodsThe proposed approach is based on a gradient-based method. The central equation is derived from Maxwell's equation and describes the relationship between conductivity and the transceive phase. A linear system equation is obtained via a finite-difference method and solved using a least-squares method. Total variation and wavelet transform regularization terms are added to the minimization problem and solved using the Split Bregman method to improve reconstruction stability. The proposed approach is compared with conventional and gradient-based methods. Numerical simulations are performed to validate the accuracy of the developed method, and the effects of noise are determined. Phantom and in vivo experiments are conducted at 3 T to verify the clinical applicability of the proposed method.ResultsNumerical simulations show that the proposed method is more robust than other methods and can suppress the effects of noise. The quantitative conductivity value of the phantom experiment agrees with the measured value. The in vivo experiment results present a clear structure, and the conductivity value of the tumor region is significantly higher than that around healthy tissues.ConclusionThe proposed electrical conductivity imaging method can improve the quality of conductivity reconstruction, and thus, has future clinical applications.     

Retrospectively gated cardiac cine imaging with temporal and spatial acceleration

Parallel imaging methods are routinely used to accelerate the image acquisition process in cardiac cine imaging. The addition of a temporal acceleration method, whereby k-space is sampled differently for different time frames, has been shown in prior work to improve image quality as compared to parallel imaging by itself. However, such temporal acceleration strategies prove difficult to combine with retrospectively gated cine imaging. The only currently published method to feature such combination, by Hansen et al. [Magn Reson Med 55 (2006) 85-91] tends to be associated with prohibitively long reconstruction times. The goal of the present work was to develop a retrospectively gated cardiac cine method that features both parallel imaging and temporal acceleration, capable of achieving significant acceleration factors on commonly available hardware and associated with reconstruction times short enough for practical use in a clinical context.Seven cardiac patients and a healthy volunteer were recruited and imaged, with acceleration factors of 3.5 or 4.5, using an eight-channel product cardiac array on a 1.5-T system. The prescribed FOV value proved slightly too small in three patients, and one of the patients had a bigemini condition. Despite these additional challenges, good-quality results were obtained for all slices and all patients, with a reconstruction time of 0.98±0.07 s per frame, or about 20 s for a 20-frame slice, using a single processor on a single PC. As compared to using parallel imaging by itself, the addition of a temporal acceleration strategy provided much resistance to artifacts.     

Long-term quantitative phase-contrast imaging of living cells by digital holographic microscopy

The dynamic analysis of biological living samples is one of the particular interests in life sciences. An improved digital holographic microscope for long-term quantitative phase-contrast imaging of living cells is presented in this paper. The optical configuration is optimized in the form of a free-space-fiber hybrid system which promotes the flexibility of imaging in complex or semi-enclosed experimental environment. Aberrations compensation is implemented taking into account the additional phase aberration induced by liquid culture medium in long-term observation. The proposed approach is applied to investigate living samples of MC3T3-E1 and MLO-Y4 cells. The experimental results demonstrate its availability in the analysis of cellular changes.     

On the origins of decoherence and extinction contrast in phase-contrast imaging

A theoretical formalism describing the formation of images in a linear shift invariant X-ray optical system is derived within the wave-optical theory. It is applicable to a non-crystalline object consisting of two types of features, with the characteristic sizes which are respectively not smaller and much smaller than the resolution of the imaging system. This formalism is then applied to two phase-contrast imaging techniques, the propagation-based and analyser-based imaging. The obtained formulae for the intensity distribution in the image well explain the “decoherence effect” which is observed in the former technique and the “extinction contrast” which is a characteristic of the latter technique. This formalism is shown to be in good agreement with the results of the accurate numerical simulations, using rigorous wave-optical theory, of the propagation-based and analyser-based phase-contrast images of the model objects.