Method for acquiring a magnetic field inhomogeneity value and distortion correction method for magnetic resonance imaging system

Abstract

A method for acquiring a basic magnetic field inhomogeneity value of a magnetic resonance imaging (MRI) system includes homogenizing an original basic magnetic field of the MRI system into a target magnetic field, providing a magnetic field compensation amount for the MRI system by a dynamic shimming method. The dynamic shimming method includes performing a 3D low-resolution dual-echo gradient echo sequence, and using a general formula to acquire the magnetic field inhomogeneity value, the general formula being: ΔB=ΔB.sub.original+ΔB.sub.compensating, wherein ΔB is the magnetic field inhomogeneity value, ΔB.sub.original is a difference value between the original magnetic field and the target magnetic field, and ΔB.sub.compensating is the magnetic field compensation amount. This method for acquiring a magnetic field inhomogeneity value for an MRI system saves considerable time to map the magnetic field again, thereby shortening the magnetic resonance imaging time, and increasing the efficiency of magnetic resonance imaging.

Claims

1. A method for acquiring a magnetic field inhomogeneity value of a magnetic resonance imaging (MRI) system, comprising: homogenizing an original basic magnetic field of a magnetic resonance scanner into a target magnetic field, by providing a magnetic field compensation amount for the scanner by a dynamic shimming method executed in a processor, wherein the dynamic shimming method comprises performing a three-dimensional (3D) low-resolution dual-echo gradient echo sequence; in said processor, using a general formula to acquire the magnetic field inhomogeneity value, the general formula being
ΔB =ΔB.sub.original+ΔB.sub.compensating, wherein ΔB is the magnetic field inhomogeneity value, ΔB.sub.original is a difference value between the original basic magnetic field and the target magnetic field, and ΔB.sub.compensating is the magnetic field compensation amount; using a further general formula to acquire the difference value, the further general formula being:
ΔB.sub.original=Δφ/(γ.Math.ΔTE), wherein ΔTE is a difference value of echo times of dual echoes of the 3D low-resolution dual-echo gradient echo sequence, Δφ is a phase difference of two gradient echo images generated by the 3D low-resolution dual-echo gradient echo sequence, and γ is a gyromagnetic ratio; and making the magnetic field inhomogeneity value available from the processor as an electronic signal.

2. A distortion correction method for a magnetic resonance imaging (MRI) system, comprising: based on magnetic field inhomogeneity values of pixels of a magnetic resonance image obtained in the MRI system using a basic magnetic field of the MRI system, obtaining, in a processor, pixel offsets of the pixels in a phase encoding direction; using the pixel offsets to subject the pixels to distortion correction in said processor; using a general formula
Δn.sub.PE=α.Math.ΔB +β, wherein Δn.sub.PE is the pixel offsets, ΔB is magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, α is a conversion parameter, and β is an adjustment parameter; and making the distortion-corrected pixels available from the processor in electronic form as a data file.

3. The distortion correction method as claimed in claim 2, comprising operating the scanner of the magnetic resonance system with an echo planar imaging method, and obtaining the conversion parameter according to a further general formula, the further general formula being:
α=γ.Math.T.sub.esp.Math.N.sub.PE, wherein α is the conversion parameter, γ is a gyromagnetic ratio, T.sub.esp is a magnetic resonance echo spacing obtained by the echo planar imaging method, and N.sub.PE is the number of steps in the phase encoding direction in the magnetic resonance scanner.

4. A device for acquiring a magnetic field inhomogeneity value for a magnetic resonance imaging apparatus comprising: a processor configured to homogenize an original basic magnetic field of a magnetic resonance scanner into a target magnetic field, by providing a magnetic field compensation amount for the scanner by a dynamic shimming method executed in a processor, wherein the dynamic shimming method comprises performing a three-dimensional (3D) low-resolution dual-echo gradient echo sequence; said processor being configured to use a general formula to acquire the magnetic field inhomogeneity value, the general formula being
ΔB =ΔB.sub.original+ΔB.sub.compensating, wherein ΔB is the magnetic field inhomogeneity value, ΔB.sub.original is a difference value between the original basic magnetic field and the target magnetic field, and ΔB.sub.compensating is the magnetic field compensation amount; using a further general formula to acquire the difference value, the further general formula being:
ΔB.sub.original=Δφ/(γ.Math.ΔTE), wherein ΔTE is a difference value of echo times of dual echoes of the 3D low-resolution dual-echo gradient echo sequence, Δφ is a phase difference of two gradient echo images generated by the 3D low-resolution dual-echo gradient echo sequence, and γis a gyromagnetic ratio; and said processor being configured to make the magnetic field inhomogeneity value available from the processor as an electronic signal.

5. A distortion correction device for a magnetic resonance imaging (MRI) system comprising: a processor configured to obtain pixel offsets of pixels in a phase encoding direction based on magnetic field inhomogeneity values of pixels of a magnetic resonance image obtained in the MRI system using a basic magnetic field of the MRI system; said processor being configured to use the pixel offsets to subject the pixels to distortion correction in said processor; using a the general formula
αn.sub.PE=α.Math.ΔB +β, wherein Δn.sub.PE is the pixel offsets, ΔB is magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, α is a conversion parameter, and β is an adjustment parameter; and said processor being configured to make the distortion-corrected pixels available from the processor in electronic form as a data file.

6. The distortion correction device as claimed in claim 5, wherein the processor is configured to operate the scanner of the MRI system with an echo planar imaging method, and to obtain the conversion parameter according to a further general formula, the further general formula being:
α=γ.Math.T.sub.esp.Math.N.sub.PE, wherein α is the conversion parameter, γ is a gyromagnetic ratio, T.sub.esp, is a magnetic resonance echo spacing obtained by the echo planar imaging method, and N.sub.PE is the number of steps in the phase encoding direction in the magnetic resonance scanner.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flowchart of a distortion correction method for an MRI system according to a particular embodiment of the present invention.

(2) FIG. 2 is a flowchart of a method for acquiring a magnetic field inhomogeneity value for an MRI system according to a particular embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The fundamental feature of the method for acquiring a magnetic field inhomogeneity value for an MRI system according to a particular embodiment of the present invention is the use of magnetic field inhomogeneity values to calculate pixel offsets of pixels of a magnetic resonance image in a phase encoding direction, and the use of the pixel offsets to subject the pixels to distortion correction (i.e. image registration).

(4) FIG. 1 is a flowchart of a distortion correction method for an MRI method according to a particular embodiment of the present invention. As FIG. 1 shows, the distortion correction method 100 for an MRI method according to a particular embodiment of the present invention comprises: step 101, based on magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by the basic magnetic field of the scanner of the MRI system, obtaining pixel offsets of the pixels in a phase encoding direction; and step 102, using the pixel offsets of the pixels to subject the pixels to distortion correction.

(5) Specifically, a step of using a first general formula to obtain, based on magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, pixel offsets of the pixels in a phase encoding direction, the first general formula being:
Δn.sub.PE=α.Math.ΔB+β,
wherein Δn.sub.PE is the pixel offsets of the pixels in a phase encoding direction, ΔB is magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, α is a conversion parameter, and β is an adjustment parameter. Clearly, there is a linear mapping relationship between magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, and pixel offsets of the pixels in a phase encoding direction.

(6) There is a specific mapping relationship, linear or non-linear depending on the particular MRI method, between magnetic field inhomogeneity values on pixels of a magnetic resonance image obtained in the MRI system by a main magnetic field of the MRI system, and pixel offsets of the pixels in a phase encoding direction.

(7) Taking an MRI method based on an echo planar imaging (EPI) method as an example, the magnetic resonance image is generated by an echo planar imaging method, wherein the conversion parameter is obtained according to a second general formula, the second general formula being:
α=γ.Math.T.sub.esp.Math.N.sub.PE,
wherein α is the conversion parameter, γ is the gyromagnetic ratio, T.sub.esp is a magnetic resonance echo spacing obtained by the echo planar imaging method, N.sub.PE is the number of steps in the phase encoding direction of the magnetic resonance image, and the adjustment parameter is a value set by a user or a value calculated by linear fitting. Specifically, in the distortion correction method for an MRI method according to a particular embodiment of the present invention, the adjustment parameter β is 0.

(8) Using the pixel offsets of pixels in a phase encoding direction, obtained in the above step, the pixels in a magnetic resonance image are subjected to distortion correction, i.e. image registration, in subsequent processing; in other words, the original coordinates of pixels in the phase encoding direction are added to the pixel offsets in the phase encoding direction, in order to obtain corrected coordinates of pixels in the phase encoding direction, and in turn obtain a magnetic resonance image formed by the pixels according to the corrected coordinates.

(9) The fundamental feature of the method for acquiring a magnetic field inhomogeneity value for an MRI system according to a particular embodiment of the present invention is: first performing dynamic shimming, then using data generated during dynamic shimming to acquire a magnetic field inhomogeneity value.

(10) FIG. 2 is a flowchart of a method for acquiring a magnetic field inhomogeneity value for an MRI system according to a particular embodiment of the present invention. As FIG. 2 shows, a method 200 for acquiring a magnetic field inhomogeneity value for an MRI system according to a particular embodiment of the present invention comprises the following steps. In step 201, in order to homogenize an original magnetic field of an MRI system into a target magnetic field, a magnetic field compensation amount is provided for the MRI system by a dynamic shimming method, wherein the dynamic shimming method comprises performing a 3D low-resolution dual-echo gradient echo sequence Step 101 from the embodiment of FIG. 1 is then implemented, followed by step 202, which is comparable to step 102 of FIG. 1, but wherein a third general formula is used to acquire the magnetic field inhomogeneity value, the third general formula being:
ΔB=ΔB.sub.original+ΔB.sub.compensating
wherein ΔB is the magnetic field inhomogeneity value, ΔB.sub.original is the difference value between the original magnetic field and the target magnetic field, and ΔB.sub.compensating is the magnetic field compensation amount.

(11) Specifically, the MRI system makes use of a dynamic shimming method to improve magnetic field homogeneity, in other words to homogenize the original magnetic field of the MRI system into a target magnetic field. In the dynamic shimming method, a magnetic field compensation amount is superposed on the original basic magnetic field, so as to form a shimmed actual magnetic field. The magnetic field inhomogeneity value is just the difference value between the target magnetic field and the actual magnetic field; at the same time, the magnetic inhomogeneity value is also just the sum of the magnetic field compensation amount and the difference between the original magnetic field and the target magnetic field.

(12) Specifically, a fourth general formula is used to acquire the difference value, the fourth general formula being:
ΔB.sub.original=Δφ/(γ.Math.ΔTE),
wherein ΔTE is a difference value of echo times of dual echoes of the 3D low-resolution dual-echo gradient echo sequence, Δφ is a phase difference of two gradient echo images generated by the 3D low-resolution dual-echo gradient echo sequence, and γ is the gyromagnetic ratio. At the same time, ΔB.sub.original is the difference value between the original magnetic field and the target magnetic field and can be obtained in various other ways, such as be measurement, etc.

(13) Specifically, a fifth general formula is used to acquire the difference value, the fifth general formula being:

(14) $B_{\mathrm{compensating}}\left(r,\theta ,\varphi \right)=\underset{n=0}{\overset{\infty }{.Math.}}\underset{m=0}{\overset{n}{.Math.}}{\left(\frac{r}{R_0}\right)}^n\left(A_n^m{I}_n^m\cos \left(m\varphi \right)+B_n^m{I}_n^m\sin \left(m\varphi \right)\right)P_n^m\left(\cos \theta \right)$

(15) The fifth general formula describes the situation in a spherical coordinate system, wherein (r, θ, φ) are coordinates in the spherical coordinate system; due to the presence of multiple shimming coils, (m,n) is used to distinguish each shimming coil; R.sub.0 denotes the radius of a shimming region; A.sub.n.sup.m and B.sub.n.sup.m denote the sensitivity of the (m,n)th shimming coil; I.sub.n.sup.m denotes the size of the current passing into the (m,n)th coil; P.sub.n.sup.m is a Legendre polynomial. R.sub.0, A.sub.n.sup.m, B.sub.n.sup.m and I.sub.n.sup.m can all be acquired from the MRI system. Generally, R.sub.0, A.sub.n.sup.m and B.sub.n.sup.m are related to the system hardware, and will not vary; I.sub.n.sup.m is calculated by dynamic shimming technology according to ΔB.sub.original and will vary with the scanned object.

(16) In a dynamic shimming method, a magnetic field measurement sequence, i.e. the 3D low-resolution dual-echo gradient echo sequence, acquires 3D volume data of a region to be shimmed, for the purpose of assessing a shimming current needed in each shimming coil so as to optimize magnetic field homogeneity.

(17) The method for acquiring a magnetic field inhomogeneity value for an MRI system according to particular embodiments of the present invention can use data generated in a dynamic shimming method directly to calculate magnetic field inhomogeneity, and therefore saves the considerable amount of time taken to map the magnetic field again, thereby shortening the magnetic resonance imaging time, and increasing the efficiency of magnetic resonance imaging. Thus this technology is of great value with regard to the large amount of strenuous scanning work carried out in hospitals.

(18) The distortion correction method for an MRI method according to particular embodiments of the present invention can use magnetic field inhomogeneity values directly to calculate pixel shifts without the need for measurement, and can therefore save substantial time costs in the case of certain MRI methods (such as echo planar imaging methods and Dixon water-fat imaging methods).

(19) Specifically, taking echo planar imaging methods and Dixon water-fat imaging methods as an example: these use a dual-echo sequence based on a gradient echo to map an original magnetic field; shimming is then performed based on the original magnetic field, i.e. in order to turn the main magnetic field of the magnetic resonance system into a homogeneous magnetic field (i.e. target magnetic field); an actual magnetic field is generated by superposing a compensating magnetic field on the original magnetic field (but a disparity remains between the actual magnetic field and the target magnetic field; for example, a shimming operation comprises various methods, e.g. static shimming methods and dynamic shimming methods, wherein a dynamic shimming method uses a shimming current to generate a compensating magnetic field. Thus, the vector sum of the original magnetic field and compensating magnetic field forms the actual magnetic field, and then the difference between the target magnetic field and actual magnetic field forms the magnetic field inhomogeneity value. In the distortion correction method for an MRI method according to particular embodiments of the present invention, the magnetic field inhomogeneity value is used to calculate pixel offsets, and therefore saves the considerable amount of time taken to map the magnetic field again, thereby shortening the magnetic resonance imaging time, and increasing the efficiency of magnetic resonance imaging. Thus this technology is of great value with regard to the large amount of strenuous scanning work carried out in hospitals.

(20) Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.