Biomedical subjects

Qing-San Xiang

Publications and source records attributed to Qing-San Xiang.

6 recordsLinked to original sources

PubMed2006

Two-point water-fat imaging with partially-opposed-phase (POP) acquisition: an asymmetric Dixon method.

A novel two-point water-fat imaging method is introduced. In addition to the in-phase acquisition, water and fat magnetization vectors are sampled at partially-opposed-phase (POP) rather than exactly antiparallel as in the original Dixon method. This asymmetric sampling encodes more valuable phase information for identifying water and fat. From the magnitudes of the two complex images, a big and a small chemical component are first robustly obtained pixel by pixel and then used to form two possible error phasor candidates. The true error phasor is extracted from the two error phasor candidates through a simple procedure of regional iterative phasor extraction (RIPE). Finally, least-squares solutions of water and fat are obtained after the extracted error phasor is smoothed and removed from the complex images. For noise behavior, the effective number of signal averages NSA* is typically in the range of 1.87-1.96, very close to the maximum possible value of 2. Compared to earlier approaches, the proposed method is more efficient in data acquisition and straightforward in processing, and the final results are more robust. At both 1.5T and 0.3T, well separated and identified in vivo water and fat images covering a broad range of anatomical regions have been obtained, supporting the clinical utility of the method.

Adipose Tissue↗

PubMed2005

Accelerating MRI by skipped phase encoding and edge deghosting (SPEED).

A fast imaging method called skipped phase encoding and edge deghosting (SPEED) is introduced. The k-space is sparsely sampled into three interleaved datasets, each with a skip-size N and a relative shift in phase encoding (PE). These datasets are separately reconstructed by 2DFT and edge-enhanced by a differential filter in the PE direction, resulting in edge maps with phase-shifted aliasing ghosts. The sparseness of edges reduces the chance of ghost overlapping. Typical ghosted-edge maps can be adequately modeled with only two dominating ghost layers that are resolved from a set of three equations using least-square error minimization, yielding N ghost maps of different orders that can be registered and averaged into a single deghosted-edge map for noise and artifact reduction. Finally, the deghosted-edge map is transformed into a deghosted image by an inverse filter. A few central k-space lines are collected without PE skip to aid the inverse filtering. SPEED has been demonstrated by in vivo data to reduce scan time considerably without noticeable artifacts. It has various potential applications, such as MR angiography (MRA), where the signal itself is sparse. As an independent method, SPEED can be combined with other fast imaging methods for further acceleration.

Algorithms↗

PubMed2005

Nonlinear phase correction with an extended statistical algorithm.

This paper presents a new magnetic resonance imaging (MRI) phase correction method. The linear phase correction method using autocorrelation proposed by Ahn and Cho (AC method) is extended to handle nonlinear terms, which are often important for polynomial expansion of phase variation in MRI. The polynomial coefficients are statistically determined from a cascade series of n-pixel-shift rotational differential fields (RDFs). The n-pixel-shift RDF represents local vector rotations of a complex field relative to itself after being shifted by n pixels. We have found that increasing the shift enhances the signal significantly and extends the AC method to handle higher order nonlinear phase error terms. The n-pixel-shift RDF can also be applied to improve other methods such as the weighted least squares phase unwrapping method proposed by Liang. The feasibility of the method has been demonstrated with two-dimensional (2-D) in vivo inversion-recovery MRI data.

Algorithms↗

PubMed2004

Quantitative evaluation of metal artifact reduction techniques.

PURPOSE: To develop a technique to quantify artifact, and to use it to compare the effectiveness of several approaches to metal artifact reduction, including view angle tilting and increasing the slice select and image bandwidths (BWs), in terms of metal artifact reduction, noise, and blur. MATERIALS AND METHODS: Nonmetallic replicas of two metal implants (stainless steel and titanium/chromium-cobalt femoral prostheses) were fabricated from wax, and MR images were obtained of each component immersed in water. The differences between the images of each metal prosthesis and its wax counterpart were measured. The contributions from noise and blur were isolated, resulting in a measure of the metal artifact. Several off-resonance artifact reduction techniques were assessed in terms of metal artifact reduction capability, as well as signal to noise ratio and blur. RESULTS: Increasing the image BW from +/-16 kHz to +/-64 kHz was found to reduce the artifact by an average of 60%, while employing view angle tilting (VAT) alone was found to reduce the artifact by an average of 63%. The metal artifact reduction sequence (MARS), which combines several susceptibility artifact reduction techniques, resulted in the least amount of image distortion, reducing the artifact by an average of 79%. CONCLUSION: The results indicate that while VAT alone (with an image BW of +/-16 kHz) resulted in the smallest amount of total energy and no reduction in the signal-to-noise ratio compared to a conventional spin-echo pulse sequence, MARS resulted in significantly less artifact and dramatically less blur.

Artifacts↗

PubMed2004

Linear combination of multiecho data: short T2 component selection.

The myelin sheath, which is wrapped around the axons in the brain, can be affected by many diseases, resulting in cognitive and physical disability. Other work showed water in the myelin sheath has a T2 approximately 15 ms. The current standard technique to estimate the fraction of myelin water in vivo is to collect multiecho data and fit the decay curves using a nonnegative least-squares (NNLS) algorithm. A new algorithm was developed to calculate optimized coefficients which were used to linearly combine multiecho data to estimate the myelin water signal. A set of simulations showed the new technique was accurate over a broad range of myelin water signal. The myelin water fraction from brain regions in scans from five volunteers, estimated by the linear combination method, agreed with the myelin water fraction estimated by the standard technique. The strength of the new technique is that the linear combination does not assume an underlying T2 model and is 20,000 times faster than NNLS.

Algorithms↗

PubMed2002

Understanding phase maps in MRI: a new cutline phase unwrapping method.

This paper describes phase maps. A review of the phase unwrapping problem is given. Different structures, in particular fringelines, cutlines, and poles, contained within a phase map are described and their origin and behavior investigated. The problem of phase unwrapping can then be addressed with a better understanding of the source of poles or inconsistencies. This understanding, along with some assumptions about what is being encoded in the phase of a magnetic resonance image, are used to derive a new method for phase unwrapping which relies only on the phase map. The method detects cutlines and distinguishes between noise-induced poles and signal undersampling poles based on the length of the fringelines. The method was shown to be robust to noise and successful in unwrapping challenging clinical cases.

Algorithms↗