System and method to improve performance of asymmetrical gradient coils by allowing a uniform offset field

Abstract

An asymmetric electromagnet system, method, and method of producing an asymmetric electromagnet system, wherein the asymmetric electromagnet system is for generating an imaging magnetic field in an imaging region with an imaging isocentre, the imaging region being asymmetrically positioned within a gradient coil bore inside a magnetic resonance imaging (MRI) system during imaging, the electromagnet assembly comprising: an asymmetric gradient coil configured to generate a gradient field in the asymmetrically positioned imaging region, at least one gradient axis having the gradient field with a constant offset component such that the position at which the gradient field passes through zero is offset with respect to the imaging isocentre of the asymmetrically positioned imaging region.

1 . An asymmetric electromagnet system for generating an imaging magnetic field in an imaging region with an imaging isocentre, the imaging region being asymmetrically positioned within a gradient coil bore inside a magnetic resonance imaging (MRI) system during imaging, the electromagnet system comprising:
 an asymmetric gradient coil configured to generate a gradient field in the asymmetrically positioned imaging region, at least one gradient axis having the gradient field with a constant offset component such that the position at which the gradient field passes through zero is offset with respect to the imaging isocentre of the asymmetrically positioned imaging region; and   a radiofrequency (RF) compensator operatively coupled to the asymmetric gradient coil, the RF compensator configured to counteract for a change in frequency to a main magnetic field caused by the gradient field offset.   
     
     
         2 . The electromagnet system of  claim 1 , wherein the asymmetric gradient coil includes an asymmetric z-gradient coil. 
     
     
         3 . The electromagnet system of  claim 2 , wherein the gradient field is offset from the imaging isocentre between 7 cm to 15 cm. 
     
     
         4 . The electromagnet system of  claim 2 , wherein the asymmetric gradient coil is further configured to be force balanced. 
     
     
         5 . The electromagnet system of  claim 1 , wherein the RF compensator is operatively coupled to an RF transmitter and an RF receiver of the MRI system, the RF compensator being configured to counteract the change in frequency to the main magnetic field by modulating a frequency response of the RF transmitter when transmitting a pulse waveform, and of the RF receiver during signal reception. 
     
     
         6 . The electromagnet system of  claim 5 , wherein the RF compensator is configured to calculate a time-varying demodulation frequency based on timing, a known amount of gradient field offset per unit of control and a gradient control waveform. 
     
     
         7 . The electromagnet system of  claim 6 , wherein the calculated time-varying demodulation frequency is stored in memory and the RF compensator is coupled to the memory to retrieve the time-varying demodulation frequency during imaging for transmission to the RF transmitter and the RF receiver. 
     
     
         8 . The electromagnet system of  claim 6 , wherein the RF compensator is configured to calculate the time-varying demodulation frequency in real-time during imaging. 
     
     
         9 . A method for generating an imaging magnetic field in an imaging region with an imaging isocentre, the imaging region being asymmetrically positioned within a gradient coil bore inside a magnetic resonance imaging (MRI) system during imaging, the method comprising:
 generating a gradient field in the asymmetrically positioned imaging region with an asymmetric gradient coil, at least one gradient axis having the gradient field with a constant offset component whereby the position at which the gradient field passes through zero is offset with respect to the imaging isocentre of the asymmetrically positioned imaging region;   counteracting a change in frequency to a main magnetic field caused by the gradient field offset with a radiofrequency (RF) compensator.   
     
     
         10 . The method of  claim 9 , wherein the gradient field generated is shifted from the imaging isocentre along the z-axis. 
     
     
         11 . The method of  claim 10 , wherein the gradient field is generated with an offset between 7 cm to 15 cm from the imaging isocentre. 
     
     
         12 . The method of  claim 11 , wherein counteracting the change in frequency to the main magnetic field comprises the RF compensator:
 modulating a frequency response of an RF transmitter when transmitting a time-varying gradient waveform, and   demodulating a frequency response of an RF receiver during signal reception.   
     
     
         13 . The method of  claim 12 , wherein the modulating comprises calculating a time-varying demodulation frequency from timing, a known amount of gradient field offset per unit of control and a gradient control waveform. 
     
     
         14 . The method of  claim 12 , wherein the modulating further comprises storing the calculated time-varying demodulation frequency in memory for retrieval by 0 an RF compensator during imaging. 
     
     
         15 . The method of  claim 12 , wherein the modulating comprises calculating the time-varying demodulation frequency in real-time during imaging. 
     
     
         16 . A method of producing an asymmetric gradient coil system for generating an imaging magnetic field in an imaging region with an isocentre, the imaging region being asymmetrically positioned within a bore inside a magnetic resonance imaging (MRI) system, the steps of the method comprising:
 (a) forming a coil representation of a coil surface for an asymmetric gradient coil;   (b) setting a plurality of magnetic field targets for the asymmetric gradient coil, the plurality of field targets including a gradient field having a constant offset component such that the position at which the gradient field passes through zero is offset with respect to the isocentre of the asymmetrically positioned imaging region;   (c) forming a performance functional, based on the coil representation and the plurality of magnetic field targets, for generating a current density pattern over the coil surface;   (d) optimizing the performance functional based on the plurality of magnetic field targets;   (e) generating a current density pattern over the coil surface based on the optimized performance functional;   (f) obtaining coil windings from the current density pattern for the asymmetric gradient coil; and   (g) operatively coupling a radiofrequency (RF) compensator to the asymmetric gradient coil, the RF compensator configured in use to counteract for a change in frequency to a main magnetic field caused by the gradient field offset.   
     
     
         17 . The method of  claim 16 , wherein the position at which the gradient field passes through zero is shifted from the isocentre is along the z-axis. 
     
     
         18 . The method of  claim 17 , wherein the position at which the gradient field passes through zero is shifted from the isocentre by between 7 cm to 15 cm. 
     
     
         19 . The method of  claim 16 , wherein the performance functional further includes a balancing of Lorentz forces. 
     
     
         20 . The method of  claim 16 , wherein the RF compensator is configured to counteract the change in frequency to the main magnetic field by:
 modulating a frequency response of an RF transmitter when transmitting a time-varying gradient waveform, and   demodulating a frequency response of an RF receiver during signal reception.