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Method for Shimming a Static Magnetic Field in a Local MRI Coil

MCW #1124

 

 Key Inventor

Andrzej Jesmanowicz, PhD

 State-of-the-Art

Magnetic resonance imaging (MRI) systems include active shimming capability built into the body coil assembly to reduce artifacts due to inhomogeneities of the static magnetic field. These inhomogeneous regions are especially present when scanning areas of the patient where there are air-tissue interfaces. Various types of pulse sequences can be employed to attempt to correct for this and materials such as pyrolytic carbon can also be placed on the patient to reduce artifacts (e.g. on the patient’s forehead to reduce magnetic susceptibility differences in the area of the sinuses).

 Problem

While active shimming works fine for standard anatomical imaging, because of the sensitivity of echo planar imaging (EPI) to magnetic susceptibility artifacts, it is not very effective for functional MRI – resulting in magnetic distortion and signal drop-out, especially in area like the frontal lobe. Using pulse sequences to correct for this problem produces increased scan times and applying pyrolytic carbon is not as accurate as direct shimming of the field.

 Solution

The system described in this intellectual property utilizes special pulse sequences to acquire measurement signals while the patient is positioned in the bore of the MRI system. These signals are used to construct a map that identifies changes in the magnetic field throughout the region of interest. This data is used to calculate a ferroshim matrix (grid) to compensate/correct for these changes/inhomogeneities. The data is converted into a postscript file and a standard photocopier is used to produce a passive shim with ferroshims placed in areas indicated by the insert matrix. Each higher order shim that is produced is unique to that patient and corrects for distortion or signal drop-out. Standard copier toner (the ink has magnetic properties) is used to produce the custom insert matrix and, depending on the amount of correction required, multiple passes may be required to deposit the correct amount of toner ink. This passive shim assembly is wrapped into a cylinder and placed in a local gradient/RF coil (or directly into the bore of the system) to improve the homogeneity of the static field.

 Benefit
  • For fMRI acquisitions the signal-to-noise-ratio (SNR) can be improved by as much as 50%.
  • This technology allows for field corrections to achieve realistic SNR in regions critical to many fMRI studies.
  • This technique does not interfere with stimulation devices.
 Stage of Development

Pre-clinical

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MEDICAL COLLEGE OF WISCONSIN
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Summary Information

Patent Status:
US 6,294,972

Patent Coverage Type:
Method of Use

Geographical Coverage:
US Patent

Related Areas of Interest:
Central Nervous System 

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