Magnetic resonance imaging systems and methods

Patent metadata

Field	Value
Publication number	US-10914802-B2
Application number	US-201916449624-A
Country	US
Kind code	B2
Filing date	Jun 24, 2019
Priority date	Jun 24, 2019
Publication date	Feb 9, 2021
Grant date	Feb 9, 2021

Abstract

A method is provided for magnetic resonance (MR) imaging near metal, including acquiring an image at a first magnetic field from a subject that includes a metal object, acquiring an image at a second magnetic field, and combining the images to provide a corrected image with reduced metal distortion. An MR imaging system for measuring near metal is also provided including a superconducting magnet to provide a magnetic field, a power supply for a current to ramp the magnetic field, a cryocooler in contact with the superconducting magnet, a magnetic field controller programmed to ramp the main magnetic field by adjusting the current generated by the power supply, a radio frequency system for transmitting and receiving signals, and a data aquisition and processing system to receive the MR signals, generate image data sets and combine the image data sets to provide a corrected image having a reduced metal distortion.

First claim

What is claimed is:

1. A method for magnetic resonance (MR) imaging near metal comprising:
acquiring a first image data from a subject including a metal object using an MR imaging system having a main magnetic field at a first magnetic field strength and using a first transmit frequency;
adjusting the main magnetic field of the MRI system to a second magnetic field strength;
acquiring a second image data from the subject using the MR imaging system while the main magnetic field of the MR imaging system is at the second magnetic field strength; and

combining the first and second image data to provide a corrected image having a reduced metal distortion,
wherein the second magnetic field strength is lower than the first magnetic field strength; the second transmit frequency comprises a resonant frequency and at least one offset frequency; and the number of offset frequencies of the first transmit frequency is fewer than the number of offset frequencies of the second transmit frequency.

2. The method for MR imaging near metal as in claim 1 , further comprising using a second transmit frequency while the main magnetic field of the MR imaging system is at the second magnetic field strength.

3. The method for MR imaging near metal as in claim 1 , wherein the first transmit frequency comprises a resonant frequency and at least one offset frequency, and the first image data is a sum of the image data from the resonant frequency and the at least one offset frequency.

4. The method for MR imaging near metal as in claim 2 , wherein the second transmit frequency comprises a resonant frequency and at least one offset frequency, and the second image data is a sum of the image data from the resonant frequency and the at least one offset frequency.

5. The method for MR imaging near metal as in claim 1 , wherein the first magnetic field strength and the second magnetic field strength are selected based on a material of a metal implant.

6. The method for MR imaging near metal as in claim 1 , wherein the magnetic field is adjusted to the second magnetic field strength by:
selecting a ramp function defining at least one ramp rate;
setting a current generated by a power supply to an initial current value;
activating a superconducting switch to its closed position, thereby connecting a superconducting magnet of the MR imaging system and the power supply in a connected circuit;
adjusting the current generated by the power supply according to the selected ramp function; and
activating the superconducting switch to its open position when the second magnetic field strength is reached, thereby disconnecting the superconducting magnet and the power supply from the connected circuit and placing the superconducting magnet in a closed circuit.

7. The method for MR imaging near metal as in claim 6 , wherein selecting a ramp function comprises selecting a short ramp time for adjusting the magnetic field to the second magnetic field strength.

8. A magnetic resonance (MR) imaging system for measuring near metal comprising:
a superconducting magnet for generating a main magnetic field;
a power supply for providing a current for ramping the main magnetic field;
a switch selectively connecting the superconducting magnet to the power supply and having an open state and a closed state, wherein when in the closed state the switch connects the superconducting magnet and the power supply in a connected circuit;
a mechanical cryocooler in thermal contact with the superconducting magnet and operable to reduce and maintain a temperature of the superconducting magnet below a transition temperature of the superconducting magnet;
a magnetic field controller programmed to ramp the main magnetic field from a first magnetic field strength to a second magnetic field strength by:
setting a current generated by the power supply to an initial current value;
activating the switch to its closed position, thereby connecting the superconducting magnet and the power supply in the connected circuit;
adjusting the current generated by the power supply; and
activating the switch to its open position when the second magnetic field strength is reached, thereby disconnecting the superconducting magnet and the power supply from the connected circuit and placing the superconducting magnet in a closed circuit;
a gradient system positioned about a bore of the superconducting magnet for producing magnetic field gradients;
a radio frequency (RF) system for transmitting RF excitation signals and receiving MR signals;
a data acquisition system connected to the RF system to receive the MR signals from the RF system;
a data processing system configured to:
receive the MR signals from the data acquisition system;
sort the MR signals into a plurality of MR data sets, each MR data set acquired at a distinct magnetic field strength;
generate a plurality of image data sets corresponding to the plurality of MR data sets; and
combine the plurality of image data sets to provide a corrected image having a reduced metal distortion;
a pulse controller connected to the RF system, gradient system and data acquisition system that generates pulse sequences that include RF pulses from the RF system and gradient pulses from the gradient system; and
a computer storage for storing the corrected image.

9. The MR imaging system as in claim 8 , further comprising: a controller connected to the RF system for setting a plurality of offset frequencies, wherein each offset frequency comprises a central transmit frequency and a central receive frequency set to an offset frequency value that is distinct for each offset frequency.

10. The MR imaging system as in claim 9 , wherein the MR data sets further comprise the MR signals received for the offset frequencies.

11. The MR imaging system as in claim 8 , wherein the switch is a superconducting switch.

12. The MR imaging system as in claim 8 , wherein the magnetic field controller is programmed to receive at least one operating parameter value indicative of a present state of the MR imaging system and to select the ramp function based on the at least one operating parameter value and the second magnetic field strength.

13. The MR imaging system as in claim 12 , further comprising a temperature monitor in thermal contact with the superconducting magnet so as to measure the temperature of the superconducting magnet, and wherein the at least one operating parameter value includes the temperature of the superconducting magnet.

14. The MRI system as in claim 12 , further comprising a magnetic field sensor proximate the superconducting magnet so as to measure the first magnetic field strength of the magnetic field generated by the superconducting magnet, and wherein the at least one operating parameter value includes the first magnetic field strength.

15. The MRI system as in claim 12 , further comprising a current monitor in electrical communication with the power supply so as to measure the current generated by the power supply, and wherein the at least one operating parameter value includes the current generated by the power supply.

Assignees

• Synaptive Medical Barbados Inc
• Synaptive Medical Inc

Inventors

• Stainsby Jeff Alan
• Harris Chad Tyler

Classifications

• G01R33/56536

  due to magnetic susceptibility variations · CPC title

• G01R33/446

  Multifrequency selective RF pulses, e.g. multinuclear acquisition mode (spatially selective RF pulses G01R33/4833) · CPC title

• G01R33/445 Primary

  MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging · CPC title

• H01F6/008

  Electric circuit arrangements for energising superconductive electromagnets · CPC title

• G01R33/072

  Constructional adaptation of the sensor to specific applications · CPC title