Biomedical subjects

Q S Xiang

Publications and source records attributed to Q S Xiang.

12 recordsLinked to original sources

PubMed1994

A quantitative interpretation of IVIM measurements of vascular perfusion in the rat brain.

Pulsed gradient spin echo (PGSE) sequences have been used to measure the signal loss of 19F in perfluorinated hydrocarbon blood substitutes moving within the vasculature of the rat brain in the experimental conditions of the study. The signal loss is not characterized by a single apparent pseudodiffusion coefficient. A simple vascular network model based on self-similarity has been used to calculate the shape of the signal loss. Excellent agreement with the experiment has been obtained showing that the IVIM measurements are sensitive to flow over a wide range of vessel diameters and flow rates. This model of vascular structure may serve well for other MR measurements that are sensitive to perfusion.

Animals↗

PubMed1993

K-space description for MR imaging of dynamic objects.

The spatial frequency (k) space concept is extended to describe the imaging of time-dependent objects. This work builds on the existing k-space description of MRI and is useful for simplifying the analysis of explanations of motion artifacts, algorithms for the correction of motion, and efficient imaging schemes for dynamic objects. Specific examples of the use of this concept for different imaging techniques are presented.

Artifacts↗

PubMed1993

Quantitative interpretation of magnetization transfer.

Magnetization transfer contrast (MTC) experiments using off-resonance irradiation have been performed with an agar gel model by systematically varying offset frequency, amplitude of the RF irradiation and gel concentration. The experimental results are shown to be quantitatively modelled by a two-pool system consisting of a liquid pool with a Lorentzian line shape and a small semisolid pool with a Gaussian lineshape. The fitted model yields physically realistic fundamental parameters with a T2 of the semisolid pool of 13 microseconds. Further analysis shows that the off-resonance irradiation MTC experiment had significant limitations in its ability to saturate the semisolid pool without directly affecting the liquid component.

Agar↗

PubMed1990

Projection images of the position-velocity joint spin density distribution.

A new concept of phase encoding called position-velocity combined oblique Fourier phase encoding is introduced. It encodes both spatial and velocity information in a single oblique direction in the position-velocity space. Using this method, two-dimensional projection images of the three-dimensional position-velocity joint spin density distribution along different directions can be obtained. These projection images can provide detailed information on the flow system under study. The imaging of the two-dimensional projection is less time consuming compared to three-dimensional Fourier flow imaging and can be easily implemented on a conventional magnetic resonance imaging scanner.

Fourier Analysis↗

PubMed1989

Differential flow imaging by NMR.

A new method for spatially resolved NMR flow measurements, named differential flow imaging (DFI), is introduced and experimentally verified. The DFI technique is based on the fact that flow velocity in any direction may cause a pixel position shift in the phase-encoding direction of a 2DFT NMR image. In this method two flow-influenced magnitude images are obtained by properly encoding and/or compensating the flow velocity. A spatial map of the desired component of the flow velocity can consequently be calculated from these two images. Since the DFI technique uses only the magnitude information of the complex images, it is not sensitive to systematic phase errors in contrast to other methods which are based on the phase measurements. On the other hand, the DFI technique can be combined with the phase measurement methods to perform multidimensional flow measurements in a shorter data acquisition time when the phase errors are small or corrected.

Magnetic Resonance Imaging↗

PubMedDate not supplied

Motion artifact reduction with three-point ghost phase cancellation.

A novel method for "ghost" artifact suppression is introduced. It suppresses ghosts induced by motion in any direction, as well as other types of quasi-periodic signal modulation. Because it requires neither special hardware nor intensive data processing, it can be easily implemented on conventional magnetic resonance (MR) imagers. The method is based on the concept of decomposition of a ghosted complex image into a ghost mask and ideal image. A set of deliberately designed acquisitions are used to generate a set of ghosted complex images in which the ghost components are related in a simple manner. With use of equations describing image decomposition and ghost correlation, the ideal image can be calculated pixel by pixel. The ideal image obtained (representing the time-averaged spin-density distribution) is shown to be a truer representation of physical reality than the ghost-free image obtained with ordered phase encoding. In this technique, both interview and intraview effects are taken into account. The technique is also useful in simultaneously suppressing ghosts from multifrequency signal modulations such as respiratory and cardiac motions. The method was successfully tested with three time-interleaved, phase-encoding-order-shifted acquisitions. Experimental results have shown that it is a simple but effective technique.

Abdomen↗

PubMedDate not supplied

Dynamic image reconstruction: MR movies from motion ghosts.

It has been previously shown that an image with motion ghost artifacts can be decomposed into a ghost mask superimposed over a ghost-free image. The present study demonstrates that the ghost components carry useful dynamic information and should not be discarded. Specifically, ghosts of different orders indicate the intensity and phase of the corresponding harmonics contained in the quasi-periodically varying spin-density distribution. A summation of the ghosts weighted by appropriate temporal phase factors can give a time-dependent dynamic image that is a movie of the object motion. This dynamic image reconstruction technique does not necessarily require monitoring of the motion and thus is easy to implement and operate. It also has a shorter imaging time than point-by-point imaging of temporal variation, because the periodic motion is more efficiently sampled with a limited number of harmonics recorded in the motion ghosts. This technique was tested in both moving phantoms and volunteers. It is believed to be useful for dynamic imaging of time-varying anatomic structures, such as in the cardiovascular system.

Artifacts↗

PubMedDate not supplied

Ghost phase cancellation with phase-encoding gradient modulation.

Motion artifacts are a dominant cause of magnetic resonance image quality degradation. Periodic or nearly periodic motion results in image replicates of the moving structures in spin-warp Fourier imaging. The replicates, or ghosts, propagate in the image in the phase-encoding, or y, direction. These ghosted images can be considered to consist of the time-averaged spin density I0 and a ghost mask g. A set of j ghosted images Ij may be acquired in which the ghost mask is intentionally phase shifted by varying amounts relative to I0 with interleaved acquisitions that have shifted phase-encoding orders or by acquiring multiple images during a single readout period in the presence of an oscillating phase-encoding gradient. The resulting complex images Ij have the same time-averaged spin density I0 but have ghost contributions gj that, on a pixel-by-pixel basis, trace part of a circle around I0. The source images Ij can then be used to estimate I0. Simulations and experiments with the phase-encoding gradient modulation method show good general ghost suppression for a variety of quasi-periodic motion sources including both respiratory-type artifacts and flow artifacts. The primary limitation of the method is the need for rapid gradient switching.

Artifacts↗

PubMedDate not supplied

Two-point interference method for suppression of ghost artifacts due to motion.

A two-point interference method is introduced for suppression of ghosting due to motion in magnetic resonance imaging. The method requires only two time-interleaved data acquisitions, without any monitoring of the motion. A postprocessing technique is used to produce a weighted sum of the two acquired images, in which ghosts are suppressed by interference through an automatic regional tuning procedure. The appropriate complex weighting factors are regionally chosen by minimizing the "gradient energy," which is defined as the sum of squared pixel values in the partial-derivative maps. The method was tested in both phantoms and volunteers with a variety of imaging protocols. The level of ghost suppression with the two-point method was found to be comparable to that of the three-point method described previously by the authors.

Artifacts↗

PubMedDate not supplied

Temporal phase unwrapping for CINE velocity imaging.

A simple algorithm named temporal phase unwrapping (TPU) is introduced to address the phase aliasing problem in time-dependent phase contrast (CINE-PC) velocity imaging. The method exploits the temporal continuity of velocity field and unwraps the phase along time. TPU only involves a one-dimensional (1D) temporal integration; therefore, many complications in 2D or 3D spatial phase unwrapping are avoided. Differential velocity maps (DVM) between adjacent movie frames are first calculated from the complex MR images. The DVMs have no phase aliasing as the differential velocities are much smaller than the absolute velocities. Aliasing-free velocity maps are obtained by integrating the DVMs along the time direction provided an aliasing-free reference velocity map (RVM) is found as a starting point of the integration. Typically, such RVMs are always available within the cardiac cycle, especially in diastole where the blood flow is the lowest. In vivo results from fully automated processing and detailed discussion on noise behavior are presented.

Algorithms↗

PubMedDate not supplied

Inversion recovery image reconstruction with multiseed region-growing spin reversal.

A new algorithm is introduced for inversion recovery (IR) image reconstruction. The original complex image is modeled as a product of three factors: magnitude, polarity, and a smoothly changing phase factor. The simple binary polarity factor is first unified by a region-growing spin reversal (RGSR) operation, allowing the phase factor to be extracted. Multiplying the complex conjugate of the phase factor with the original complex data yields the desired IR contrast. The RGSR process is repeated with multiple seeds distributed in the field of view (FOV), and the results are added together, enabling disconnected tissues in the FOV to be handled. The extracted phase factor is filtered to reduce noise and artifacts, without losing useful information. The method is fully automatic and has been used practically in a large number of clinical examinations. The algorithm may also be useful for phase correction in simple proton spectroscopic imaging.

Algorithms↗

PubMedDate not supplied

Water-fat imaging with direct phase encoding.

A new method is introduced for water-fat imaging. With three acquisitions, a general direct phase encoding (DPE) of the chemical shift information is achieved. Pixels containing both water and fat are solved directly. Pixels with only a single component are resolved with local and global orientation filters, which use phase information from neighboring pixels. The fact that a single component is more likely to be water than fat in living tissues is also useful. A second pass solution yields water and fat images with superior signal-to-noise ratio. Unlike other methods, DPE does not rely on the error-prone phase unwrapping; also, it easily handles disconnected tissues. Because the magnetization vectors of water and fat are sampled not only at parallel or antiparallel, they can be not only separated but also identified respectively, which is desirable for routine clinical work. DPE has been implemented on several imagers at various field strengths and has been demonstrated in a large number of clinical cases to be useful and robust in various parts of the body.

Adipose Tissue↗