Claims
- 1. A method of correcting spatial distortion in magnetic resonance images due to magnetic field inhomogeneity including inhomogeneity due to susceptibility variations in an object being imaged comprising the steps of
- a) placing the object in a magnetic field (B.sub.o) oriented along an axis in a three axis coordinate system;
- b) applying first RF pulses to said object and a magnetic gradient along a first axis;
- c) applying first magnetic gradients along at least one axis in a plurality of first read-out of sequences,
- d) detecting a plurality of magnetic resonance signals in response to said plurality of first read-out sequences;
- e) processing said plurality of magnetic resonance signals to obtain a first complex image in an image plane;
- f) repeating steps b) through e) for second RF pulses, second magnetic gradients, and second read-out sequences and obtaining a second complex image in said image plane;
- g) obtaining a position-error profile for pixels from said complex images; and
- h) remapping the locations of pixels in the image plane using said position-error profile of said pixels.
- 2. The method as defined by claim 1 wherein said read-out sequences are spin echo sequences and include 180.degree. phase inversion pulses following 90.degree. phase excitation pulses with the 180.degree. pulses occurring at different times following the 90.degree. pulses.
- 3. The method as defined by claim 1 wherein step c) and f) include applying at least two gradient-recalled echo sequences having echoes at different times.
- 4. The method as defined by claim 3 wherein step (g) includes obtaining registration error .DELTA.z(z) from
- .gamma.G.sub.z .DELTA.z(z)=.OMEGA.(z)
- where .gamma. is the gyromagnetic ratio for protons, G.sub.z is a magnetic gradient along a Z axis and .OMEGA.(z) is frequency shift at the location, z, due to magnetic field inhomogeneity including inhomogeneity due to magnetic susceptibility variations defined as ##EQU5## where p(z,T+.DELTA.T) is the image intensity for echo at time T+.DELTA.T, p(z, T) is the image intensity for echo at time T, .DELTA.T=2.pi./.omega..sub.f, and .omega..sub.f is the chemical shift between water and lipid.
- 5. The method as defined by claim 3 wherein step (h) includes remapping the image using new values of z from
- p'(x,y,z)=p(x,y,z-.DELTA.z)
- where p'=new image intensity and p=old image intensity.
- 6. Apparatus for use in correcting spatial distortion in magnetic resonance images due to magnetic field inhomogeneity including inhomogeneity due to susceptibility variations in a object being imaged comprising
- a) means for establishing a magnetic field (B.sub.0) through said object along an axis in a three axis coordinate system;
- b) means for applying first RF pulses to said object and a first magnetic gradient along a first axis;
- c) means for applying first magnetic gradients along at least one axis in a plurality of first read-out sequences,
- d) means for detecting a plurality of magnetic resonance signals in response to said plurality of first read-out sequences;
- e) means for processing said plurality of first magnetic resonance signals to obtain a first complex image in an image plane;
- f) means for applying second RF pulses to said object and a second magnetic gradient along said first axis;
- g) means for applying second magnetic gradients along said at least one axis in a plurality of second read-out sequences;
- h) means for detecting a plurality of second magnetic resonance signals in response to said plurality of second read-out sequences;
- i) means for processing said plurality of second magnetic resonance signals to obtain a second complex image in said image plane;
- j) means for obtaining a position-error profile for pixels from said complex images; and
- k) means for shifting the location of pixels in the image plane using said position-error profile of said pixels.
- 7. Apparatus as defined by claim 6 wherein said read-out sequences are spin echo sequences and include 180.degree. phase inversion pulses following 90.degree. phase pulses with the 180.degree. pulses occurring at different times following the 90.degree. pulses.
- 8. Apparatus as defined by claim 6 wherein said read-out sequences are gradient-recalled echo sequences having echoes at different times.
- 9. Apparatus as defined by claim 7 wherein said means for obtaining a position error profile includes means for obtaining registration error .DELTA.z(z) from
- .gamma.G.sub.z .DELTA.z(z)=.OMEGA.(z)
- where .gamma. is the gyromagnetic ratio for protons, G.sub.z is a magnetic gradient along a Z axis and .OMEGA.(z) is frequency shift at the location, z, due to magnetic field inhomogeneity including inhomogeneity due to magnetic susceptibility variations and defined as ##EQU6## where p(z,T+.DELTA.T) is the image intensity for echo time T+.DELTA.T, p(z,T) is the image intensity for echo time T, .DELTA.T=2.pi./.omega..sub.f, and .omega..sub.f is the chemical shift between water and lipid.
- 10. Apparatus as defined by claim 8 wherein said means for shifting the location of pixels includes means for remapping the image using new values of z from
- p'(x,y,z)=p(x,y,z-.DELTA.z).
- where p'=new image intensity and p=old image intensity.
Parent Case Info
This is a continuation of application Ser. No. 07/834,178, filed Feb. 7, 1992, now abandoned.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
4823085 |
Fuderer et al. |
Apr 1989 |
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4876509 |
Perlmutter |
Oct 1989 |
|
Non-Patent Literature Citations (3)
Entry |
"Geometrical Image Transformation to Compensate for MRI Distortions" by Chang et al. SPIE vol. 1233 Med. Imaging IV (1990) (month of pub. unknown). |
"Total Inhomogeneity Correction Including Chem. Shifts and Susceptibility by View Angle Tilting" Med. Physics vol. 15 No. 1 Jan. 1988, Cho, Z. H. et al. |
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Continuations (1)
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Number |
Date |
Country |
Parent |
834178 |
Feb 1992 |
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