Journal of Healthcare Engineering

Volume 2017 (2017), Article ID 9856058, 9 pages

https://doi.org/10.1155/2017/9856058

## Low-Rank and Sparse Decomposition Model for Accelerating Dynamic MRI Reconstruction

^{1}College of Physical Science and Technology, Central China Normal University, Wuhan 430079, Hubei, China^{2}Key Laboratory of Cognitive Science of State Ethnic Affairs Commission, South-Central University for Nationalities, Wuhan 430074, Hubei, China^{3}Hubei Key Laboatory of Medical Information Analysis & Tumor Diagnosis and Treatment, Wuhan 430074, Hubei, China

Correspondence should be addressed to Shouyin Liu; nc.ude.uncc.yhp@uilys

Received 10 March 2017; Accepted 17 May 2017; Published 8 August 2017

Academic Editor: Feng-Huei Lin

Copyright © 2017 Junbo Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The reconstruction of dynamic magnetic resonance imaging (dMRI) from partially sampled *k*-space data has to deal with a trade-off between the spatial resolution and temporal resolution. In this paper, a low-rank and sparse decomposition model is introduced to resolve this issue, which is formulated as an inverse problem regularized by robust principal component analysis (RPCA). The inverse problem can be solved by convex optimization method. We propose a scalable and fast algorithm based on the inexact augmented Lagrange multipliers (IALM) to carry out the convex optimization. The experimental results demonstrate that our proposed algorithm can achieve superior reconstruction quality and faster reconstruction speed in cardiac cine image compared to existing state-of-art reconstruction methods.

#### 1. Introduction

Dynamic MRI (magnetic resonance imaging), an essential medical imaging technique, allows noninvasiveness, nonionization visualization, and analysis of anatomical and functional changes of internal body structure through time. However, MRI sampling speed is relatively slow due to the need of physical and physiological conditions such as nuclear relaxation and peripheral nerve stimulation [1]. One way for accelerating MRI is to reconstruct high-resolution images from undersampled *k*-space data. However, such undersampling violates the Nyquist criterion and often results in aliasing artifacts if the traditional linear reconstruction is directly applied.

To address this issue, there are so much research efforts to accelerate MRI acquisition process using hardware and software [2–4]. Among them, compressed sensing (CS) has been proved to be able to increase imaging speed and efficiency in MRI application [5–7]. The CS theory requires image sparsity and incoherence between the acquisition space and representation space [8]. Fortunately, the MR image sequence often provides redundant information in both spatial and temporal domains, which presents favorable conditions for the application of CS. In addition, the idea is easily extended to the reconstruction of dynamic MRI (dMRI) images due to extensive spatio-temporal correlations that result in sparser representations. The *k-t* FOCUSS is a successful method, which imposes a sparsity constraint in the temporal transform domain by using the FOCUSS algorithm [9], and extends the FOCUSS technique with motion estimation and compensation to compressed sensing framework for cardiac cine MRI. But the limitation of the prediction schemes on sparsifying the residual signal sets back the further improvement when the motion is aperiodic.

Recently, researchers have made great efforts to exploit the low-rank property of matrices instead of simply sparsity of vectors. Lingala et al. proposed a *k-t* SLR algorithm that exploited the low rank prior and global sparsity in Karhunen-Louve Transform (KLT) domain for MRI reconstruction [10]. However, the algorithm failed to take into account the structural sparsity of the MRI image, and the limitation held back the further improvement. Some studies presented patch-based dictionary learning techniques for dMRI reconstruction [11, 12]. However, a major challenge in learning sparse dictionary is that such patch-based learning cannot be effectively employed for dMRI reconstruction. Because the size of dMRI sequence is large, it is inefficient to learn dictionaries for such large datasets [13]. Even though we take no account of computational limitations, it is not practical to acquire such huge dMRI training sequences for learning sparsifying dictionaries. Currently, robust principal component analysis (RPCA) has been used in recovering dynamic images to explore the low-rank structure of data [14, 15]. The RPCA decomposes the data in low rank and sparse components, where the low rank component models the temporally correlated background information and the sparse component represents the dynamic information. *k-t* RPCA [16], a method developed for dMRI, uses the low-rank plus sparse decomposition prior to reconstructing dynamic MRI from part of the *k*-space measurements. In this method [16], the image reconstruction is regularized by a low-rank plus sparse prior, where the Fourier transform is used as the sparsifying transform and the alternating direction methods of multipliers (ADMM) is applied to solve the minimization problem in the temporal direction. The shortcoming of *k-t* RPCA is that the results of reconstructed image are easily affected by the noise, since the noise will generally be represented by highly sparse coefficients during the sparsifying transform.

In this paper, aims to the shortcoming of *k-t* RPCA, we propose an efficient numerical algorithm based on inexact augmented Lagrangian method (IALM) instead of ADMM to solve the optimization problem and accelerate the dMRI reconstruction. The experimental results demonstrate that our proposed algorithm can achieve more satisfactory reconstruction performance and faster reconstruction speed in given cardiac cine sets.

#### 2. Theory Background

The dynamic MRI data acquisition in the *k-t* space can be expressed as follows:
where represents the measured *k-t* space signal, denotes the desired dynamic image series, and is the measured noise, which can be reasonably modeled by an additive white Gaussian distribution [16, 17].

In this paper, the solution of this problem is to find the closest representation of the MR image from undersampled measurement . Since the *k-t* space is partially sampled, (1) is converted to an inverse problem and can be rewritten as a vector [18].
where , , , is the total number of frames, is the Fourier transform operator, and the measurement matrix is the undersampled mask applied on the *k*-space.

##### 2.1. CS-Based MR Image Reconstruction

The CS approach [5, 19] was proposed to reconstruct the MR image from the partially sampled *k*-space data by exploiting the sparsity transform and convex optimization algorithms. The problem will be solved if we can find the sparsest vector satisfying (2),
where is *l*_{0}-norm, counting the number of nonzero entries in the vector, is the sparsifying transform or dictionary, and is a small constant. Unfortunately, (3) is NP-hard problem, which needs to be solved by a brute force search. The CS theory [8] proves that the convex relaxation approach referred to as *l _{1}* minimization can be replaced with the

*l*

_{0}-norm in (3), where is

*l*

_{1}-norm, meaning the sum of absolute values of the vector.

##### 2.2. Low-Rank and Sparse Decomposition Model for MR Image Reconstruction

CS-based techniques that exploit sparsity of the image in the transform domain have been successfully used for MR image reconstruction. However, the performance of CS is primarily dependent on the specific dictionary or sparsifying operator, which limits the maximum achievable acceleration rate. Therefore, some researchers tried to investigate a few new approaches to reconstruct MR image [20–24]. In those methods, low-rank matrix recovery is a popular technique in medical image processing.

The basic assumption is the same as [18], that is, the image is simultaneously sparse (in a transform domain) and low rank. The problem is to recover , given fewer *k*-space samples than the number of elements in the matrix. We assume that the approximate rank of the matrix is and the size of single frame image is . When the matrix is low rank, which has only degrees of freedom instead of , it is possible to recover the matrix from lesser number of samples by solving the rank minimization problem,

However, the rank minimization problem, that is, solving (5), is combinatorial and known to be NP-hard [25]. Therefore, convex relaxation is often used to make the minimization tractable. where denotes any linear operator and is the nuclear norm, which is defined as where are the singular values of and is the rank of .

To recover from the given , can be decomposed into a superposition of a low-rank matrix and a sparse matrix .

is recovered as the solution of the following optimization:
where low-rank matrix has few nonzero singular values and represents the background component, sparse matrix has few nonzero entries and corresponds to the changes, and is a tuning parameter that balances the contribution of the *l*_{1}-norm relative to the nuclear norm.

#### 3. The Proposed Method

In principal component pursuit (PCP) model [26], to solve (9) can be posed as an optimization problem by using regularization rather than strict constraints [15]. Hence, (9) can be converted as where the parameters and trade off data consistency and is a sparse transform basis.

Equation (10) is a RPCA problem that involves minimizing a combination of the nuclear norm and *l _{1}*-norm. Otazo et al. Study [15] adopted the iterative thresholding scheme to solve (10); however, the iterative thresholding technique converges slowly. So, we presented an inexact augmented Lagrange multipliers (IALM) algorithm to solve the RPCA problem [27]. According to the constraint conditions of (6),
where is a dual operator, contains the measurement noise, and and are low-rank element and sparse element, respectively. We applied IALM method to solve the following optimization problem:
where is a Lagrange multiplier to remove the equality constraint and is a small positive scalar. The condition implies that cannot grow too fast. The IALM method for solving the RPCA problem can be described as Algorithm 1.