Congbo Cai

Iterative thresholding compressed sensing MRI based on contourlet transform

Reducing the acquisition time is important for clinical magnetic resonance imaging (MRI). Compressed sensing has recently emerged as a theoretical foundation for the reconstruction of magnetic resonance images from undersampled k-space measurements, assuming those images are sparse in a certain transform domain. However, most real-world signals are compressible rather than exactly sparse. For example, the commonly used two-dimensional wavelet for compressed sensing MRI (CS-MRI) does not sparsely represent curves and edges. In this article, we introduce a geometric image transform, the contourlet, to overcome this shortage. In addition, the improved redundancy provided by the contourlet can successfully suppress the pseudo-Gibbs phenomenon, a tiresome artefact produced by undersampling of k-space, around the singularities of images. For numerical calculation, a simple but effective iterative thresholding algorithm is employed to solve l 1 norm optimization for CS-MRI. Considering the recovered information and image features, we introduce three objective criteria, which are the peak signal-to-noise ratio (PSNR), mutual information and transferred edge information, to evaluate the performance of different image transforms. Simulation results demonstrate that contourlet-based CS-MRI can better reconstruct the curves and edges than traditional wavelet-based methods, especially at low k-space sampling rate.

Partial Fourier transform reconstruction for single-shot MRI with linear frequency-swept excitation.

A novel image encoding approach based on linear frequency‐swept excitation has been recently proposed to overcome artifacts induced by various field perturbations in single‐shot echo planar imaging. In this article, we develop a new super‐resolved reconstruction method for it using the concepts of local k‐space and partial Fourier transform. This method is superior to the originally developed conjugate gradient algorithm in convenience, image quality, and stability of solution. Reduced field‐of‐view is applied to the phase encoding direction to further enhance the spatial resolution and field perturbation immunity of the image obtained. Effectiveness of this new combined reconstruction method is demonstrated with a series of experiments on biological samples. Two single‐shot sequences with different encoding features are tested. The results show that this reconstruction method maintains excellent field perturbation immunity and improves fidelity of the images. In vivo experiments on rat indicate that this solution is favorable for ultrafast imaging applications in which severe susceptibility heterogeneities around the tissue–air or tissue–bone interfaces, motion and oblique plane effects usually compromise the echo planar imaging image quality. Magn Reson Med, 2013.

An efficient de-convolution reconstruction method for spatiotemporal- encoding single-scan 2D MRI

Spatiotemporal-encoding single-scan MRI method is relatively insensitive to field inhomogeneity compared to EPI method. Conjugate gradient (CG) method has been used to reconstruct super-resolved images from the original blurred ones based on coarse magnitude-calculation. In this article, a new de-convolution reconstruction method is proposed. Through removing the quadratic phase modulation from the signal acquired with spatiotemporal-encoding MRI, the signal can be described as a convolution of desired super-resolved image and a point spread function. The de-convolution method proposed herein not only is simpler than the CG method, but also provides super-resolved images with better quality. This new reconstruction method may make the spatiotemporal-encoding 2D MRI technique more valuable for clinic applications.

Observation of true and pseudo NOE signals using CEST-MRI and CEST-MRS sequences with and without lipid suppression.

To investigate the characteristics of nuclear Overhauser enhancement (NOE) imaging signals in the brain at 7T.

High-resolution two-dimensional correlation spectroscopy in inhomogeneous fields: New application of intermolecular zero-quantum coherences

A new pulse sequence is proposed based on intermolecular zero-quantum coherences (iZQCs) to obtain high-resolution two-dimensional (2D) correlation spectroscopy (COSY) in inhomogeneous fields via three-dimensional (3D) acquisition. This sequence extends the high-resolution iZQC approaches from one dimension to two dimensions. Since the iZQC evolution periods in the new sequence are insensitive to the field inhomogeneities, high-resolution COSY spectra can be recovered from inhomogeneous fields by projecting the 3D data onto the indirectly acquired 2D plane. Theoretical expressions were derived according to the distant dipolar field treatment combined with product operator formalism. Both the experimental observations and computer simulations are consistent with the theoretical predictions. The new sequence thus provides an attractive way to eliminate the influences of field inhomogeneity on the conventional COSY methods, which may be useful for the study of chemical and biological materials.

An aliasing artifacts reducing approach with random undersampling for spatiotemporally encoded single-shot MRI

Compared to the echo planar imaging (EPI), spatiotemporally encoded (SPEN) single-shot MRI holds better immunity to the field inhomogeneity, while retaining comparable spatial and temporal resolutions after the super-resolved reconstruction. Though various reconstruction methods have been proposed, the reconstructed SPEN images usually contain aliasing artifacts because of vast undersampling. A hybrid scheme based on random sampling, singular value decomposition (SVD) and compressed sensing (CS) was introduced to reduce these aliasing artifacts and improve the image quality. The efficiency of this hybrid scheme was demonstrated by numerical simulations and experiments on water phantom and in vivo rat brain. The hybrid scheme provided herein would benefit the SPEN approach in vast undersampling situation.

Brown adipose tissue mapping in rats with combined intermolecular double-quantum coherence and Dixon water-fat MRI

Brown adipose tissue (BAT) is a promising therapeutic target in obesity studies. Recently, MRI has been proposed for the mapping of BAT. However, because of the limitation of spatial resolution, similar to the existing positron emission tomography and computed tomography techniques for BAT detection, it fails to distinguish BAT cells when they are mixed with other cells. In this work, a new MRI method is proposed, combining intermolecular double‐quantum coherence and the chemical shift‐encoded Dixon method. Its contrast depends on the water to fat ratio at the cellular scale, which is smaller than the imaging voxel size. The feasibility of this MRI method was shown with computer simulations and phantoms, and preliminary imaging of BAT of rats at 7 T. Both computer simulations and experimental results are consistent with theoretical predictions. The method provides a novel contrast mechanism and can map BAT distribution exclusively. In particular, a mixture of BAT cells and white adipose tissue cells was detected in an older rat, which was undetectable by other noninvasive methods. This method may be applicable to a wide range of uses in BAT‐related studies, including the formation and variation of BAT. Copyright

Draining Algorithm for the Maximum Flow Problem

A new augmenting path based algorithm called draining algorithm is proposed for the maximum flow problem in this letter. Unlike other augmenting path based algorithms which augment gradually the flow from zero-flow to the maximum flow, the proposed algorithm drains the redundant capacities out of the network to achieve the maximum flow. Experimental results shown the high efficiency of the proposed algorithm in near saturated network, thought it has a same computational complex with the traditional augmenting path approach for regular flow networks.

Compressed sensing MRI based on nonsubsampled contourlet transform

How to reduce acquisition time is very important in magnetic resonance imaging (MRI). Compressed sensing MRI emerges recently to suppress the aliasing when undersampling k-space data is employed. However, typical sparse transform for compressed sensing MRI ever used is wavelet, which only captures limited directional information with decay rate M1. In this paper, we introduce contourlet into compressed sensing to obtain a sparse expansion for smooth contours with decay rate C(logM)3M2 and employ nonsubsampled contourlet to increase the redundancy of basis for magnetic resonance images. We propose compressed sensing MRI based on nonsubsampled contourlet transform (NSCT). Experimental results demonstrate that NSCT outperforms wavelet on suppressing the aliasing and improves the visual appearance of magnetic resonance images.

High-resolution NMR spectroscopy in inhomogeneous fields via Hadamard-encoded intermolecular double-quantum coherences.

A new pulse sequence based on intermolecular double‐quantum coherences was proposed to obtain one‐dimensional high‐resolution liquid NMR spectra in inhomogeneous magnetic fields via Hadamard encoding. In contrast with the conventional intermolecular multiple‐quantum coherences method with a two‐dimensional acquisition to obtain one one‐dimensional high‐resolution spectrum, the new method can provide relatively high‐resolution spectra directly through one‐dimensional acquisition, and can greatly improve the signal‐to‐noise ratio of the spectrum within a relatively short acquisition time. Theoretical derivation was performed and analytical expressions of the resulting signals are given. Solution samples in purposely de‐shimmed magnetic fields and pig brain tissue samples were tested. The experimental results demonstrate that this sequence can retain useful structural information, even when the field inhomogeneity is sufficiently severe to erase almost all spectral information with conventional one‐dimensional single‐quantum coherence techniques, and good solvent suppression can be achieved. This method may provide a promising technique for applications in in vivo and in vitro NMR. Copyright