Picker International, Inc.

Philips Medical Systems (cleveland), Inc.

Heidi A Schlitt

Labros S Petropoulos

Mark J Loncar

Michael R Thompson

Wayne R Dannels

James A Halamek

A magnetic resonance apparatus includes a main magnet ( ) which generates a main magnetic field through and surrounding an examination region ( ). The main magnetic field contains inhomogeneities which affect image quality. A gradient coil assembly ( ) generates gradient magnetic fields across the examination region, which contain higher order harmonics causing inhomogeneities in the gradient magnetic field. A multi-axis shim set ( ) is selectively excited in order to cancel both the main magnetic and gradient magnetic field inhomogeneities. More particularly, DC currents are applied by a shim coil power supply ( ) to cancel the main magnetic field inhomogeneities. AC current pulses are superimposed on the DC currents by the shim coil power supply ( ) in order to correct the gradient magnetic field inhomogeneities. The existence of correctable higher order gradient magnetic field harmonics provides for fast switching, high duty cycles, and high peak gradients, without adding distortions to the resulting magnetic resonance images.

Real-time multi-axis gradient distortion correction using an interactive shim set

A gradient coil assembly (,) generates substantially linear magnetic gradients across the central portion of an examination region (,). The gradient coil assembly (,) includes primary x, y, and z-gradient coils (,) which generate a gradient magnetic field (,) having a non-zero first derivative in and adjacent the examination region. Preferably, the gradient coil assembly (,) includes secondary, shielding x, y, and z coils which generate a magnetic field which substantially cancels, in an area outside a region defined by the shielding coils, a fringe magnetic field generated by the primary gradient coils. The existence of a non-zero first derivative in and adjacent the examination region eliminates aliasing effects attributable to the non-unique gradient field values on either side of a rollover point (,). The non-unique values of the gradient magnetic field adjacent the rollover point caused structure near the rollover point to overlay each other (FIGS.,B,,B). The unique non-linearity of the present gradient (,) adjacent the edges expands (magnifies) the image adjacent the edges (FIGS.,A,,A). Because the expansion is unique, distortions at the edges are readily and accurately mapped (,) back to linear.

Gradient coil set with non-zero first gradient field vector derivative

A method of calibrating pre-emphasis gradient pulse conditioning for long term eddy current compensation in an MRI system is provided. It includes positioning an object in the MRI system for imaging therewith. A long term eddy current generating pre-scan gradient pulse is applied, and after a delay, an EPI sequence is run such that a resulting image of the object is acquired. This process of applying the pre-scan gradient pulse followed by the EPI sequence is repeated while varying the delay therebetween. Size variations in the resulting images of the object are observed, and these size variations are fit to an exponential curve to obtain one or more time constants for the long term eddy currents. Next, a reference EPI sequence is run without applying the long term eddy current generating pre-scan gradient pulse such that a reference image of the object is acquired. The long term eddy current generating pre-scan gradient pulse is reintroduced, and after a delay which is shorter than the time constants, a test EPI sequence is run such that a test image of the object is acquired. The process of applying the pre-scan gradient pulse followed by the test EPI sequence is repeated while adjusting the pre-emphasis gradient pulse conditioning which is based upon the time constants obtained. This continues until the test images of the object have a size substantially equivalent to that of the reference image.

EPI image based long term eddy current pre-emphasis calibration

A method (100) of automatically tuning a radio frequency transmitter (24) and receiver (38) in a magnetic resonance imaging apparatus to an optimum frequency includes generating and acquiring (104) a magnetic resonance signal. The magnetic resonance signal is transformed to the frequency domain and spectral magnitude of the signal is computed (106). A center of gravity interpolation is performed (110) on the spectral magnitude to obtain a desired frequency sampling. A model function is generated based upon a strength of a main magnetic field which has peaks separated by the same separation as that for fat and water signals. The spectral magnitude is correlated (112) with the model function and a peak having the greatest magnitude is located therefrom. The location of a species peak along the spectral magnitude is estimated (114) from empirically derived information, the strength of the main magnetic field and the location of the correlation peak. A region of the spectral magnitude to be examined is defined (116) from empirically derived information, and a peak within this region is located (118) which is nearest the estimated location. Ultimately, the frequency of the radio frequency transmitter and receiver is set (122) to align with the peak located within the region that is nearest the estimated location.

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Chesterland, OH, United States of America

Heidi A Schlitt