The University of British Columbia

Sunnybrook Health Science Center

General Electric Company

Qing-San Xiang

Ross M Henkelman

Li An

Douglas George Connell

Dennis Lyle Janzen

Alexander Lloyd MacKay

A method of correcting for magnetic field inhomogeneity caused by various factors, such as implanted metal and air/tissue interfaces, in magnetic resonance imaging (MRI) is provided. Geometric distortion due to inhomogeneity in a static magnetic field B.sub.0 is corrected for by addition of a compensation gradient. The compensation gradient is applied in the slice selection direction Z, has a timing substantially identical to the standard frequency encoding gradient G.sub.x, and has an amplitude identical to the slice selection gradient G.sub.z that is applied during the initial RF excitation. Inhomogeneity in an RF field B.sub.1 is compensated for by utilizing an RF coil that is large enough in size as compared with a metal implant to make the volumetric percentage of the metal in the coil insignificant. Inhomogeneity in a gradient field G=(G.sub.x, G.sub.y, G.sub.z) is corrected for by a treatment of the most significant error factor G.sub.z that causes slice thickness error. Specifically, the method acquires two images with complementary slice thickness error by using two pulse sequences with flipped slice selection gradients G.sub.z ; combination of the two images successfully cancels the effect of the slice thickness error. Local dephasive MRI signal loss due to magnetic field inhomogeneity is corrected for by acquisition of two images with positive and negative offset G.sub.z gradient lobes, respectively. The pair of images are combined to cancel the effect of local signal loss error.

Method of correcting for magnetic field inhomogeneity in magnetic

A method acquires three complex images, wherein their respective phase differences between water and fat components are .alpha..sub.0, .alpha..sub.0 +.alpha., and .alpha..sub.0 +2.alpha., respectively. The method obtains from these three complex images two possible solution sets for water and fat images. For pixels with both water and fat components, one correct solution set is selected using a binary choice based on the relative Larmor frequencies of water and fat. If a pixel contains only one component, a known statistical bias is applied to identify the component and, thus, the pixel. To correct misidentified pixels, various filters may be applied to all the pixels. The water and fat solutions obtained from the three complex images are used to produce separate images of water and fat. Second pass solutions of water and fat with improved signal-to-noise ratio can be obtained by either averaging or a least square error method.

Water-fat imaging with direct phase encoding (DPE)

Chemical shift imaging with spectrum modeling (CSISM) models the general chemical shift spectrum as a system with N distinct peaks with known resonant frequencies and unknown amplitudes. Based on the N peak spectrum model, a set of nonlinear complex equations is set up that contains N+1 unknowns of two kdnds: the magnitudes of the N peaks, and a phasor map caused by main magnetic field inhomogeneity. Using these equations, the timing parameters for shifting the 180.degree. RF refocusing pulses for acquiring spin-echo images are optimally chosen. Corresponding timing parameters for other pulse sequences can also be optimized similarly. Using the chosen timing parameters, a plurality of images are acquired. Next, acquired image data are automatically processed to solve the complex linear equations. First, the phasor map is found by fitting various phasor map values over a small number of pixels, or 'seeds', that are picked sparsely in a field of view. Second, from the original 'seeds', the region of pixels that are picked to find the best-fit phasor map is grown into the entire field of view, based on a predetermined phase difference between the original seed and a neighboring pixel. The optimal phasor map value is then entered into the complex linear equations to derive the only unknown values at this point--the peak amplitudes. Optionally, second pass solutions of the peak amplitudes may be obtained using a smoothed phasor map value. When the equations are solved, the spectroscopic images are output.

Chemical shift imaging with spectrum modeling

A method for manipulating motion ghosts is disclosed. The method includes acquiring two image data sets, each including a time averaged image component and a ghost component and processing these components to separate the ghosts from the desired image and further using the ghost information to produce a dynamic image. Motion monitoring systems are not required. Improved scan times and noise control are allowed over previous methods.

Motion ghost manipulation in magnetic resonance imaging

A method for producing dynamic images from a ghosted MR image is disclosed. The method includes acquiring an image data set including a time averaged image component and a ghost component and processing these components to produce a dynamic image. A motion monitoring system is not required as in conventional cine magnetic resonance imaging.

Producing dynamic images from motion ghosts

An NMR imaging system performs a scan to acquire three sets of data which are Fourier transformed to produce three image data arrays. A corrected image data array is calculated from corresponding elements in the three acquired image data arrays. Image artifacts which cause ghosts in the reconstructed image are masked from the corrected image data by the calculations.

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Vancouver, Canada

Qing-San Xiang