System and Method for Mode Mixing in Magnetic Resonance Imaging

• US 20100289494A1
• Filed: 02/02/2010
• Published: 11/18/2010
• Est. Priority Date: 02/02/2009
• Status: Active Grant

• Current Assignee: The General Hospital Corporation (Mass General Brigham, Inc.)
• Original Assignee: The General Hospital Corporation (Mass General Brigham, Inc.)
• Inventors: Wald, Lawrence L., Alagappan, Vijayanand, Polimeni, Jonathan R.
• Granted Patent: US 8,581,589 B2
• Time in Patent Office: Days
• US Class Current: 324/318
• CPC Class Codes: G01R 33/3415 comprising arrays of sub-coils , i.e. phased-array coils with flexible receiver channels
• CPC Class Codes: G01R 33/3621 NMR receivers or demodulators, e.g. preamplifiers, means for frequency modulation of the MR signal using a digital down converter, means for analog to digital conversion [ADC] or for filtering or processing of the MR signal such as bandpass filtering, resampling, decimation or interpolation

Abstract

The present invention provides a system and method for using a hardware-based compression of signals acquired with an magnetic resonance imaging (MRI) system. This allows a first multi-channel MR signal to be compressed to a second multi-channel MR signal having fewer channels than the first MR signal. This system and method reduces the number of RF receivers needed to achieve the sensitivity encoding benefits associated with highly parallel detection in MRI. Furthermore, the system and method reduces bottlenecks connection an MRI system'"'"'s RF receiver and reconstruction computer and reduces the computational burden of image reconstruction.

Claims

1. A magnetic resonance imaging (MRI) system comprising:

a computer system configured to operate the MRI system during an imaging process to image a subject;

an RF system including a receive coil array having a first number of channels configured to acquire signals from the subject during the imaging process;

mode-mixing apparatus configured to receive the signals acquired by the receive coil array to form a coil basis set having the first number of channels and the mode-mixing apparatus comprising;

a plurality of splitters configured to receive the coil basis set and produce a plurality of sub-signals;

a plurality of amplifiers configured to adjust the respective sub-signals by a respective adjustable amplification value;

a plurality of phase shifters configured to phase shift the respective sub-signals by a respective adjustable phase-shift value; and

a plurality of combiners configured to receive the plurality of amplified and phase shifted sub-signals and produce an output basis set having a second number of channels.

2. The MRI system of claim 1 wherein the second number of channels is less than the first number of channels.

3. The MRI system of claim 1 wherein the computer system includes a user input configured to receive user selection of the adjustable respective amplification value and the adjustable respective phase-shift value and the computer system is configured to provide the adjustable respective amplification value and the adjustable respective phase-shift value to the plurality of amplifiers and plurality of phase shifters.

4. The MRI system of claim 3 wherein the computer system is configured to select the adjustable respective amplification value and the adjustable respective phase-shift value based on at least one of a plurality of coil sensitivity profiles of a receive coil being used with the MRI system and a matrix decomposition analysis.

5. The MRI system of claim 1 wherein the computer is configured to acquire coil sensitivity data during the imaging process and generate a coil sensitivity matrix using the coil sensitivity data and wherein the computer is further configured to generate a signal correlation matrix by matrix-multiplying the coil sensitivity matrix with the coil sensitivity matrix'"'"'s Hermitian conjugate.

6. The MRI system of claim 5 wherein the computer system is configured to select the adjustable respective amplification value and the adjustable respective phase-shift value using an eigenvector matrix and corresponding eigenvalues determined by performing matrix decomposition on the signal correlation matrix.

7. The MRI system of claim 6 wherein the computer system is configured to store the adjustable respective amplification value as a magnitude of an eigenvector matrix and the adjustable respective phase-shift value as the phase of an eigenvector matrix.

8. The MRI system of claim 7 wherein the eigenvalue matrix and the eigenvector matrix include information regarding noise covariances in the coil basis set.

9. The MRI system of claim 5 wherein the signal correlation matrix is a whitened signal correlation matrix generated by matrix-multiplying a whitened coil sensitivity matrix with the whitened coil sensitivity matrix'"'"'s Hermitian conjugate.

10. The MRI system of claim 1 wherein the first number of channels is 96, the second number of channels is 32, the plurality of splitters includes 96 32-way splitters, the plurality of combiners includes 32 96-way combiners, the plurality of amplifiers includes 3072 amplifiers, and the plurality of phase shifters includes 3072 phase shifters.

11. A method for calibrating a mode-mixing apparatus including a plurality of hardware-based amplifiers and phase shifters for use in a magnetic resonance imaging (MRI) system, the steps comprising:

a) performing a prescan to determine a plurality of sensitivity profiles of a receiver coil array;

b) analyzing the sensitivity profiles of the receiver coils to generate a set of mode-mixing coefficients; and

c) loading the coefficients determined in step b) in the mode-mixing apparatus to select a value of amplification at each of the plurality of hardware-based amplifiers and a value of phase shift at each of the plurality of hardware-based phase shifters.

12. The method of claim 11 further including:

b) i) calculating a signal correlation matrix;

b) ii) calculating an eigenvalue matrix and an eigenvector matrix from the signal correlation matrix;

b) iii) truncating the eigenvalue matrix and the eigenvector matrix; and

wherein the truncated eigenvalue matrix selects the value of amplification at each of the plurality of hardware-based amplifiers and the truncated eigenvector matrix selects the value of phase shift at each of the plurality of hardware-based phase shifters.

13. The method of claim 12 wherein the eigenvector matrix and corresponding eigenvalues are determined by performing matrix decomposition on a signal correlation matrix.

14. The method of claim 12 wherein the signal correlation matrix is generated by matrix-multiplying a coil sensitivity matrix with the coil sensitivity matrix'"'"'s Hermitian conjugate.

15. The method of claim 12 wherein the eigenvalue matrix and the eigenvector matrix include information regarding noise covariances in the coil basis set.

16. The method of claim 12 wherein the signal correlation matrix is a whitened signal correlation matrix generated by matrix-multiplying a whitened coil sensitivity matrix with the whitened coil sensitivity matrix'"'"'s Hermitian conjugate.

a conventional loop coil “

basis set”

of sensitivity patterns is transformed in hardware to a linear combination of array elements with spatially orthogonal properties

17. The method of claim 16 wherein step a) includes collecting noise data and step b) includes calculating a noise covariance for use with the whitened signal correlation matrix.

18. A mode-mixing apparatus for use in a magnetic resonance imaging (MRI) system to receive and process a coil basis set having a first number of channels, the mode-mixing apparatus comprising:

a plurality of splitters configured to receive the coil basis set and produce a plurality of sub-signals;

a plurality of amplifiers configured to adjust the respective sub-signals by a respective adjustable amplification value;

a plurality of phase shifters configured to phase shift the respective sub-signals by a respective adjustable phase-shift value; and

a plurality of combiners configured to receive the plurality of amplified and phase shifted sub-signals and produce an output basis set having a second number of channels that is less than the first number of channels of the coil basis set.

19. The apparatus of claim 18 wherein the plurality of amplifiers and plurality of phase shifters are configured to receive the adjustable respective amplification value and the adjustable respective phase-shift value from a user input.

20. The apparatus of claim 18 wherein the plurality of amplifiers and plurality of phase shifters are configured to receive the adjustable respective amplification value and the adjustable respective phase-shift value from a pulse sequence server of the MRI system.

21. The apparatus of claim 20 wherein the adjustable respective amplification value and the adjustable respective phase-shift value are selected by the pulse sequence server of the MRI system based on at least one of a plurality of coil sensitivity profiles of a receive coil being used with the MRI system and a matrix decomposition analysis.