Hien M. Nguyen

Denoising MR Spectroscopic Imaging Data With Low-Rank Approximations

This paper addresses the denoising problem associated with magnetic resonance spectroscopic imaging (MRSI), where signal-to-noise ratio (SNR) has been a critical problem. A new scheme is proposed, which exploits two low-rank structures that exist in MRSI data, one due to partial separability and the other due to linear predictability. Denoising is performed by arranging the measured data in appropriate matrix forms (i.e., Casorati and Hankel) and applying low-rank approximations by singular value decomposition (SVD). The proposed method has been validated using simulated and experimental data, producing encouraging results. Specifically, the method can effectively denoise MRSI data in a wide range of SNR values while preserving spatial-spectral features. The method could prove useful for denoising MRSI data and other spatial-spectral and spatial-temporal imaging data as well.

A Modified Generalized Series Approach: Application to Sparsely Sampled fMRI

In functional MRI, it is often desirable to reduce the readout duration to make the acquired data less prone to T2* susceptibility artifacts. In addition, a shorter readout length allows for a shorter minimum TE, which is important for optimizing SNR. This can be achieved by undersampling the k-space. However, the conventional Fourier transform-based reconstruction method suffers from under-sampling artifacts such as high-frequency ringing and loss of resolution. To address this problem, we revisit the constrained-model approach using the generalized-series (GS) which has been proposed to address the undersampling problem for dynamic MRI. We propose a modification to the conventional use of the model in order to reflect small hemodynamic signal changes typical in fMRI. Specifically, while realizing that having high model order is necessary to capture missing information, we found that it is not necessary to span all frequencies of GS basis functions uniformly. Instead, having k-space and GS “sampling” trajectories covering low-frequencies uniformly while spanning high-frequencies sparsely, was observed to be an efficient strategy. The ability of the method over the conventional GS approach in improving resolution of functional images and activation maps while reducing undersampling ringing is demonstrated by simulations and experiments at 3T. Reduction in the readout time allowed an increase of statistical signal power as compared to the fully sampled acquisition. Unlike compressed sensing approaches, the proposed method is linear and hence has lower computational complexity. The method could prove useful for other imaging modalities where the signal change is smaller than the baseline component.

Spatiotemporal denoising of MR spectroscopic imaging data by low-rank approximations

This paper addresses the denoising problem associated with magnetic resonance spectroscopic imaging (MRSI), where low signal-to-noise ratio (SNR) has been a critical problem. A new scheme is proposed, which exploits two low-rank structures that exist in MRSI data, one due to partial separability and the other is due to linear predictability. Experimental results from practical data demonstrate that the proposed method provides an effective way to denoise MRSI data while preserving spatial-spectral features in a wide range of SNR values.

Denoising of MR spectroscopic imaging data with spatial-spectral regularization

Low signal-to-noise ratio has been a significant limitation for clinical applications of magnetic resonance spectroscopic imaging (MRSI). This paper investigates a new scheme for denoising MRSI data, incorporating both an anatomically-adapted spatial-smoothness constraint and an autoregressive spectral constraint within the penalized maximum-likelihood framework. Both theoretical analysis and simulation results are provided to characterize the denoising performance of this approach.

Joint Estimation and Correction of Geometric Distortions for EPI Functional MRI Using Harmonic Retrieval

Magnetic resonance imaging (MRI) uses applied spatial variations in the magnetic field to encode spatial position. Therefore, nonuniformities in the main magnetic field can cause image distortions. In order to correct the image distortions, it is desirable to simultaneously acquire data with a field map in registration. We propose a joint estimation (JE) framework with a fast, noniterative approach using harmonic retrieval (HR) methods, combined with a multi-echo echo-planar imaging (EPI) acquisition. The connection with HR establishes an elegant framework to solve the JE problem through a sequence of 1-D HR problems in which efficient solutions are available. We also derive the condition on the smoothness of the field map in order for HR techniques to recover the image with high signal-to-noise ratio. Compared to other dynamic field mapping methods, this method is not constrained by the absolute level of the field inhomogeneity over the slice, but relies on a generous pixel-to-pixel smoothness. Moreover, this method can recover image, field map, and T2* map simultaneously.

The acoustic lens design and in vivo use of a multifunctional catheter combining intracardiac ultrasound imaging and electrophysiology sensing

A multifunctional 9F intracardiac imaging and electrophysiology mapping catheter was developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology (EP) procedures. The catheter tip includes a 7.25-MHz, 64-element, side-looking phased array for high resolution sector scanning. Multiple electrophysiology mapping sensors were mounted as ring electrodes near the array for electrocardiographic synchronization of ultrasound images. The catheter array elevation beam performance in particular was investigated. An acoustic lens for the distal tip array designed with a round cross section can produce an acceptable elevation beam shape; however, the velocity of sound in the lens material should be approximately 155 m/s slower than in tissue for the best beam shape and wide bandwidth performance. To help establish the catheters unique ability for integration with electrophysiology interventional procedures, it was used in vivo in a porcine animal model, and demonstrated both useful intracardiac echocardiographic visualization and simultaneous 3- D positional information using integrated electroanatomical mapping techniques. The catheter also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.

Clinical application and technical challenges for intracardiac ultrasound imaging catheter based ICE imaging with EP mapping

A 9F combination intracardiac imaging and electrophysiology mapping catheter has been developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology procedures. A 7.5 MHz, 64 element, side looking phased array was used for sector scanning from the tip of the catheter. Multiple electrophysiology (EP) mapping sensors were mounted as ring electrodes just proximal to the array for electrocardiographic synchronization of ultrasound images. The catheter has been used in vivo in a porcine animal model and has demonstrated useful intracardiac echocardiographic (ICE) visualization of both cardiac tissue and electrophysiology catheters in the right atrium. The catheter performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.

Cramér-rao bound analysis of echo time selection for 1 H-MR spectroscopy

The choice of echo time (TE) is a complicated and controversial issue in proton MR spectroscopy, and represents a balancing act between signal-to-noise ratio and signal complexity. The TE values used in previous literature were selected either heuristically or based on limited empirical studies. In this work, we reconsider this problem from an estimation theoretic perspective. Specifically, we analyze the Crame´r-Rao lower bound on estimated spectral parameters as a function of TE, which serves as a metric to quantify the reliability of the estimation procedure. This analysis reveals that a good choice of TE often depends on the particular metabolite of interest, and is a function of both the coupling properties of the metabolites and the general complexity of the spectrum.

Joint estimation in MRI using harmonic retrieval methods

One of the key factors affecting functional MRI image reconstruction is field inhomogeneity. It is desirable to estimate both the distortion-free MRI image and field map simultaneously, thus compensating for image distortions caused by the field inhomogeneity. To solve this problem, which is called joint estimation problem, we propose a new non-iterative approach using harmonic retrieval (HR) methods. This connection establishes an elegant framework to solve the joint estimation problem through a sequence of one dimensional HR problems in which efficient solutions are available. We also derive the condition on the smoothness of the field map in order for HR techniques to recover the image with high SNR. Experimental results with the proposed method show significant improvements in MRI image reconstruction compared to methods that do not correct for field inhomogeneity

5G-6 Forward Looking Intracardiac Imaging Catheters for Electrophysiology

Minimally invasive electrophysiology interventions to treat cardiac arrhythmias are increasing worldwide due to advances in technologies that enable more effective clinical procedures. A forward imaging ultrasound catheter design has been developed and tested to advance the methods of integration of intracardiac imaging and electrophysiology sensing. The first catheters built have been constructed with a 9F (3 mm) shaft and a large (15F) tip to support experimental wire ports adjacent to a 24 element phased array operating at 14 MHz. The final tip design construction size will be 9F and possess an integrated metal electrode at the catheter distal end. Two forward looking array designs have been developed in parallel to produce two contrasting fine pitch (65 microns) 24 element phased array construction approaches. The first array has been assembled with a standard 2-2 composite PZT technology with the flex circuit mounted on the front facing side, and the second design is a cMUT version with a flip-chip bonded silicon die bonded to a backside flex circuit. The cMUT design requires a special interface to assure a safe element biasing scheme while enhancing the arrays linearity and sensitivity. The first PZT array prototypes, built without explicit matching layers, have been characterized and agree with FEA and KLM analyses in operation at 14 MHz. Matching layer variations have been used on the front layer flex circuit to optimize the sensitivity and bandwidth of the PZT arrays while minimizing the thermal boundary layer. Specially designed assembly approaches addressed the challenging forward looking array configurations that utilize interconnection flex circuits with bend radii at 250 microns. Animal studies have been performed utilizing beam forming adaptations for the forward looking imaging catheter operation on a Vingmed Vivid-7 system. The first piezoceramic array devices were used successfully to image the myocardium of the right atrium of a pig while simultaneous tissue ablation was performed