Spectral component imaging using phased array coils

• US 6,177,795 B1
• Filed: 05/19/1998
• Issued: 01/23/2001
• Est. Priority Date: 05/19/1998
• Status: Expired due to Term

• Current Assignee: Elscint Limited (Elbit Imaging Limited)
• Original Assignee: Elscint Limited (Elbit Imaging Limited)
• Inventors: Huang, Jian, Hariharan, Hari, Freeland, Stephen J., Zhu, Gang Gary
• Primary Examiner(s): Arana, Louis
• Application Number:
  US09/080,920
• Time in Patent Office: 980 Days
• Field of Search: 324/307, 324/309, 324/310, 324/312, 324/314, 324/300
• US Class Current: 324/307
• CPC Class Codes: G01R 33/3415 comprising arrays of sub-coils , i.e. phased-array coils with flexible receiver channels
• CPC Class Codes: G01R 33/4828 Resolving the MR signals of different chemical species, e.g. water-fat imaging

Abstract

A method of using magnetic resonance imaging (MRI) systems for acquiring separate data contributions in images of subjects derived from at least a first and a second spectral components, the method comprising:

applying RF pulses;

receiving RF signals with a phased array coil arrangement having a plurality of segments;

acquiring two initial complex images, the first of the two initial images being a complex image of each of the spectral components in phase; the second of the two initial images being a complex image of each of the spectral components out of phase, the first of said two initial complex images ostensibly being a plus image, and the second of said two complex images ostensibly being a minus image initially; and

matching the ostensible plus and minus images so that the first complex images and the second complex images are grouped together for further processing. Preferably, a two-point Dixon method is used to generate the components.

Claims

1. A method of using magnetic resonance imaging (MRI) systems for acquiring separate data contributions in images of subjects derived from at least a first and a second spectral components, the method comprising:

applying RF pulses;

receiving RF signals with a phased array coil arrangement having a plurality of segments;

acquiring two initial complex images, the first of the two initial images being a complex image of each of the spectral components in phase;

the second of the two initial images being a complex image of each of the spectral components out of phase, the first of said two initial complex images ostensibly being a plus image, and the second of said two complex images ostensibly being a minus image initially; and

matching the ostensible plus and minus images so that the first complex images and the second complex images are grouped together for further processing.

2. The method of claim 1 wherein further processing includes determining whether the ostensible plus and minus images are truly plus or minus images.

3. The method of claim 1 including:

processing the images of the subjects to obtain a wrapped phase map and an unwrapped phase map;

adding a phase value C to the wrapped phase map; and

subtracting the phase value C from the unwrapped phase map, to assure that at least a portion of the image will be free of errors.

4. The method of claim 3, wherein said portion comprises a center of the image.

5. The method of claim 3, wherein said phase value is a same value for the entire phase map.

6. The method of claim 3 including applying a low pass filter to the unwrapped phase map to remove possible error “

spots”

along phase jump lines.

7. The method of claim 1, wherein said phase array coil comprises at least four segments.

8. The method of claim 1, wherein said phase array coil comprises at least six segments.

9. The method of claim 1, wherein said spectral component images are acquired using a Dixon method.

10. A method of using magnetic resonance imaging (MRI) systems for acquiring separate data contributions in images derived from at least first and second spectral components, the method comprising:

using a phased array coil for detecting signals from said MRI systems;

said phased array coil comprising a plurality of coil segments, each of said segments acquiring signals from a patient being imaged;

acquiring at least two initial complex images per segment using said phased array coil, the first of said two initial complex images including said at least first and second spectral components in-phase, the second of said two complex images including said at least two first and second spectral components out of phase; and

using the in-phase and the out-of-phase images to identify an image of the first spectral component and an image of the second spectral component.

11. The method of claim 10 including:

generating a wrapped phase map from said two initial complex images; and

unwrapping the wrapped phase map.

12. The method of claim 11 wherein unwrapping the wrapped phase map comprises:

solving a Poisson equation with a weighting map to derive an unwrapped phase map;

using the unwrapped phase map to correct the phase of the out-of-phase image of spectral components;

using the image with the corrected phase to obtain a plus image and a minus image; and

determining whether the plus image and the minus image are truly plus and minus images.

13. The method of claim 10 wherein the first spectral component is water and the second spectral component is a lipid.

14. The method of claim 12 wherein the unwrapped phase map includes a bias for all of the pixels, comprising:

removing the bias to obtain a true unwrapped phase map; and

using said true unwrapped phase map to correct the phase of the out-of-phase complex image.

15. The method of claim 14 wherein removing the bias comprises:

determining the bias per pixel; and

using the determined bias to remove the bias from the unwrapped phase map.

16. A method of using magnetic resonance imaging (MRI) systems having phased array coils made up of a plurality of segments for acquiring separate data contribution in images of subjects derived from at least a first and second spectral components, the method comprising:

applying RF pulses to a subject using said MRI system;

receiving RF signals from said first and second spectral components, in each segment of the phased array coil arrangement;

processing said received RF signals to provide two initial complex images;

the first of said two initial complex images ostensibly being a plus image;

processing said two initial complex images to provide a first processed complex image that is ostensibly a plus image and a second processed complex image that is ostensibly a minus image;

matching the ostensibly plus and ostensibly minus images; and

grouping the plus and minus image of each of the spectral components for further processing to provide resultant complex images of each of the spectral components.

17. The method of claim 16, wherein said spectral components are acquired using a two point Dixon method.

18. A magnetic resonance imaging system for acquiring and accumulating separate data contributions for images derived from first and second spectral components, the system comprising of:

a coil used in applying RF pulses;

a phased array coil arrangement comprising a plurality of segments for receiving RF signals;

each segment of the phased array coil acquiring data for two initial complex images;

analog to digital converters converting the data acquired to digital data;

image processors using the digital data to provide ostensibly plus and minus images;

a spectral component processor determining and combining the real plus and real minus images to provide a resultant complex image for each of the spectral components; and

a display for displaying the resultant complex images.

19. The system of claim 18 wherein an echo scan sequence is used for performing a two point Dixon method for acquiring said data, said two point Dixon method comprising:

shifting the 180°

pulse and the corresponding slice selective gradient of the echo scan sequence by an amount sufficient to assure that the first and second spectral components in-phase in a first scan, applying the 90°

RF pulse and the 180°

RF pulse in a second scan so that the spectral components are 180°

out-of-phase; and

providing two initial spectral complex images S1 and S2, wherein in S1 the spectral components are in-phase and in S2 the spectral components are out-of-phase.

20. The system of claim 19, comprising image processors per segment providing ostensible plus and minus complex images and, tables for testing the images to determine whether they are indeed plus or minus images.

21. The system of claim 20 including a circuit for adding a constant value for preventing any 360°

phase jumps in the center of an unwrapped phase map that is obtained after phase unwrapping.

22. The system of claim 21 including a circuit removing a constant value from the unwrapped phase map.

23. The system of claim 22 including a low pass filter used to filter the output of the unwrapped phase map.

24. The system of claim 23 where the filter comprises a Fermi filter.

25. The system of claim 24 wherein the spectral component processor determines the average noise level of the in-phase image to provide a weight mask using the average noise level as a threshold.

26. The system of claim 25 wherein the threshold is between 3 and 5 times the average noise level, wherein the in-phase image pixels having intensities level greater than the threshold are given a value of 1, all other pixels have a 0 value, and giving the pixels of the weight noise a 0 value mask if the pixel to pixel difference either in the X or in the Y direction is greater than a second threshold determined by the maximum BO in homogeneity.

27. The system of claim 26 including determining if the phase unwrapping procedure has introduced the factor of 2(2n−

1)π

to the unwrapped phase map where n is an integer and exchanging the sign of the resulting images if there is a 2(2n−

1)π

factor introduced to the unwrapped phase map.