System and method for image warp correction for magnetic resonance imaging
View Patent ↗A method of correcting warping of an acquired image in an MRI system, caused by non-linearities in gradient field profiles of gradient coils is set forth, comprising a) constructing a computer model representing conducting pathways for each gradient coil in said MRI system; b) calculating a predicted magnetic field at each point in space for each said gradient coil in said model; c) measuring actual magnetic field at each point in space for each said gradient coil in said MRI system; d) verifying accuracy of said model by comparing said predicted magnetic field to said actual magnetic field at each said point in space and in the event said model is not accurate then repeating a)-d), and in the event said model is accurate then; constructing a distortion map for mapping coordinates in real space to coordinates in warped space of said acquired image based on deviations of said predicted magnetic field from linearity; and unwarping said warping of the acquired image using said distortion map.
1. A method of correcting warping of an acquired image in an MRI system, caused by non-linearities in gradient field profiles of gradient coils, comprising:
a) constructing a computer model representing conducting pathways for each gradient coil in said MRI system;
b) calculating a predicted magnetic field at each point in space for each said gradient coil in said model;
c) measuring actual magnetic field at each point in space for each said gradient coil in said MRI system;
d) verifying accuracy of said model by comparing said predicted magnetic field to said actual magnetic field at each said point in space and in the event said model is not accurate then repeating a)-d), and in the event said model is accurate then;
e) constructing a distortion map for mapping coordinates in real space to coordinates in warped space of said acquired image based on deviations of said predicted magnetic field from linearity; and
f) unwarping said warping of the acquired image using said distortion map,
wherein said predicted magnetic field is calculated using one of either Biot-Savart equations or an equivalent thereof,
wherein said distortion map is constructed for mapping each point x,y,z in real space to a point x′,y′,z′ of the image in warped space by obtaining B(x,y,z) values from said Biot-Savart equations or equivalent for each said gradient coil,
wherein values of x′, y′ are computed directly from k-space data using inverse gridding from uniform to non-uniform sample locations, and
wherein said inverse gridding is effected by performing an apodization correction in k-space, followed by an Inverse Fourier Transform and then using convolution interpolation in image space with a Kaiser-Bessel kernel.
2. The method of claim 1 , wherein said measuring is performed using a field measuring device.
3. The method of claim 2 , wherein said field measuring device comprises 3-axis Hall probe.
4. The method of claim 1 , wherein said computer model is an element array model of a gradient coil electromagnet comprising a set of elements joined together to form a spatial representation of a wire pattern for each said gradient coil.
5. The method of claim 1 , wherein said Biot-Savart equations or equivalent comprise:
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and wherein f is a function of x, y, z, B x represents a component coaxial with a main static field of the predicted magnetic field produced along the X-axis of said MRI system, B y represents a component coaxial with the main static field of the predicted magnetic field produced along the Y-axis of said MRI system, and B z represents a component coaxial with the main static field of the predicted magnetic field produced along the Z-axis of said MRI system.
6. The method of claim 1 , wherein said unwarping is performed in image space using one of either nearest neighbor, linear, or cubic interpolation.
7. The method of claim 1 , wherein the size of the Fourier Transform is determined by the k-space matrix size and an oversampling factor.
8. The method of claim 7 , wherein said Kaiser-Bessel kernel is pre-stored and re-sampled during interpolation.
9. The method of claim 1 , further comprising adjusting a field of view of said acquired image by one of either enlarging or reducing the field of view to one of either eliminate aliasing artifacts that occur where portions of the image in warped space are mapped beyond the boundaries of the field of view of the image in real space or to improve image resolution.
10. The method of claim 9 , wherein said enlarging comprises:
calculating a maximum amount of expected distortion for said field of view using said distortion map; and
using the calculated maximum amount of expected distortion to create an enlarged field of view.
11. The method of claim 9 , further comprising generating a pulse sequence for said MRI system based on the adjusted field of view.
12. The method of claim 10 , wherein said image is acquired according to the enlarged field of view by over sampling said k-space.
13. The method of claim 1 , further comprising increasing the acquired image to a higher sampling density to avoid image artifacts resulting from collapsing a plurality of voxels onto a single voxel in said warped space.
14. The method of claim 13 , wherein the acquired image is increased to a higher sampling density by zero padding said k-space.
15. The method of claim 13 , wherein said zero-padding comprises adding zeroes to an outer portion of said k-space prior to said convolution.
16. The method of claim 15 , further comprising applying an apodization function to the zero-padded k-space to aide in removal of Gibb's ringing artifacts.