Method to correct magnetic field/phase variations in proton resonance frequency shift thermometry in magnetic resonance imaging

• US 20040041563A1
• Filed: 05/14/2003
• Published: 03/04/2004
• Est. Priority Date: 05/15/2002
• Status: Active Grant

• Current Assignee: Case Western Reserve University
• Original Assignee: Case Western Reserve University
• Inventors: Lewin, Jonathan S., Duerk, Jeffrey L., Barkauskas, Kestutis J.
• Granted Patent: US 7,359,745 B2
• Time in Patent Office: Days
• US Class Current: 324/307
• CPC Class Codes: G01R 33/022 Measuring gradient
• CPC Class Codes: G01R 33/4804 Spatially selective measurement of temperature or pH
• CPC Class Codes: G01R 33/565 Correction of image distortions, e.g. due to magnetic field inhomogeneities
• CPC Class Codes: G01R 33/56563 caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0 G01R33/56509, G01R33/56518, G01R33/56536 take precedence

Abstract

PRF shift MRI data is acquired. The PRF shift MRI data may include signals affected by both a desired PRF shift and an undesired PRF shift. Thus, example systems and methods describe manipulating the PRF shift MRI data to make it substantially free of the effects of the undesired PRF shift, which facilitates displaying certain MRI images based on the desired PRF shift.

It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the application. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Claims

1. A system, comprising:

a data store for storing an input signal data received from an MRI or NMR system, where the input signal data includes two or more related sets of image data;

a first logic for processing the input signal data into a processed signal data, where the effects of one or more undesired phase shifting events on the input signal data are reduced in the processed signal data; and

a second logic for processing the processed signal data to quantify the effects of one or more desired phase shifting events.

2. The system of claim 1, where the undesired phase shifting events are one or more of a variation in a main magnetic field in the MRI or NMR system and a misaligning of echoes in the input signal data.

3. The system of claim 2, where the variation in the main magnetic field is one or more of a temporal variation, a spatial variation, and a field strength variation.

4. The system of claim 1, where a desired phase shifting event is a temperature change.

5. The system of claim 1, where the desired phase shifting event is one or more of a motion change, a velocity change, and an acceleration change.

6. The system of claim 1, where a set of image data is a set of k-space data that contains a peak k-space signal location.

7. The system of claim 6, where the first logic identifies the peak k-space signal location for two or more of the related sets of image data and aligns the two or more related sets of image data.

8. The system of claim 7, where the first logic employs an indirect interpolation algorithm to identify the peak k-space signal location.

9. The system of claim 1, where the second logic establishes a first set of processed signal data as a reference signal.

10. The system of claim 9, where the second logic compares one or more second sets of processed signal data to the reference signal to create one or more sets of difference data.

11. The system of claim 10, comprising a data store for storing the reference signal and one or more sets of difference data.

12. The system of claim 10, comprising a display for displaying an image developed from the reference signal and one or more sets of difference data.

13. The system of claim 10, where the second logic creates the one or more sets of difference data by determining one or more phase shifts between one or more elements of the processed signal data according to Δ

φ

=φ

_ref−

φ

₂.

14. The system of claim 13, where the second logic computes φ

_ref and φ

₂ according to φ

=tan<sup>−

1</sup>(I(x,y)/R(x,y) ).

15. The system of claim 1, where the first logic processes the input signal by computing a phase shift due to a variation in B₀ by processing k-space data centered around and local to a peak k-space signal location.

16. The system of claim 15, where the second logic quantifying the effects of one or more desired phase shifting events includes computing a phase shift due to a temperature change by processing k-space data in the absence of one or more contributions of data near and local to the peak k-space signal location.

17. The system of claim 1, where the first logic extracts a wrap free phase difference on a pixel by pixel basis from the input signal data.

18. The system of claim 10, comprising the second logic quantifying the effects of temperature change as a desired phase shifting event according to:

Δ

φ

(x,y,t)=γ

*B₀(x,y,t)*δ

*TE_eff*Δ

T(x,y,t).

19. The system of claim 18, where δ

is approximately −

0.01 ppm/C.°

.

20. The system of claim 18, where γ

is approximately 2π

*43 MHz/T.

21. The system of claim 1, where the first logic and the second logic are implemented in a single logic.

22. A computer readable medium storing computer executable components of the system of claim 1.

23. A system for correcting B₀ background phase variations to facilitate accurately reconstructing thermal profiles in proton resonance frequency (PRF) shift MR thermometry, comprising:

an MRI apparatus for generating a plurality of sets of image data;

an echo aligner for aligning the plurality of sets of image data with respect to a maximum k-space signal location in each of the sets of image data;

an image phase corrector for correcting background phase variations in the aligned sets of image data to form one or more phase corrected sets of image data;

a phase processor for forming a wrap free phase difference in the phase corrected sets of image data to form one or more wrap free phase difference data; and

an imaging apparatus for reconstructing a thermal profile from the one or more wrap free difference data.

24. The system of claim 23, where the background phase variations are induced by one or more of B₀ field inhomogeneity, temporal instability, and spatial instability.

25. A computer readable medium storing computer executable components of the system of claim 23.

26. A method, comprising:

centering a point of interest of an object to be imaged in a field of view;

receiving a first MRI data of the object;

establishing a reference image from the first MRI data;

receiving one or more second MRI data of the object, where the object has been subjected to one or more proton resonance frequency altering actions;

determining a maximum k-space amplitude location in one or more of the second MRI data;

manipulating the first MRI data to align the maximum k-space amplitude locations with an assumed k-space center to within a tolerance;

phase correcting the first MRI data;

determining a wrap free phase change from the reference image on an element-by-element basis; and

producing a display data that can be displayed, where the display data facilitates displaying the results of a desired phase shift event on the object to be imaged.

27. The method of claim 26, where determining a maximum k-space amplitude location comprises iteratively bisecting a given search space about an initial guess where one or more linear phase offsets in an image domain are employed to selectively interpolate midpoints between one or more known k-space amplitudes.

28. The method of claim 26, where phase correcting the first MRI data comprises employing the phase of a low resolution image of an N×

M mask of low-frequency Fourier co-efficients as a phase correction map, where N and M refer to the respective number of rows and columns of data contained within the mask, M and N being integers.

29. The method of claim 28, where N is less than 5 and where M is less than 5.

30. The method of claim 28, where N and M are related to one or more of the FOV and the size of the expected site of phase variation.

31. The method of claim 26, where the tolerance is about {fraction (1/128)}th of a cycle/FOV.

32. The method of claim 26, where determining the wrap free phase change comprises examining one or more relationships between two phasors over the range 0 to 2π

.

33. The method of claim 26, where the element is a pixel.

34. A method for calculating a temperature change in an MR image in the presence of temporally and/or spatially dependent variations in B₀, comprising:

receiving an MRI signal;

accurately aligning echoes in a signal;

phase correcting the signal;

extracting wrap free phase differences from the signal on a unit by unit basis; and

calculating the temperature change from the signal.

35. The method of claim 34, where accurately aligning echoes in a signal comprises:

digitizing the signal;

sampling the signal at time intervals to produce a sampled data;

indirectly interpolating the sampled data to find a peak signal location; and

applying a phase shift to one or more sets of data encoded in the signal to align the one or more sets of data.

36. The method of claim 35, where the time intervals are uniform.

37. The method of claim 34, where phase correcting the signal comprises:

identifying the phase change due to an undesired phase changing event, where the phase change effect will be localized near the center of k-space; and

manipulating the data to suppress the undesired phase change.

38. The method of claim 37, where manipulating the data to suppress the undesired phase change comprises creating an opposite effect of what is seen at the center of k-space.

39. A computer readable medium storing computer executable instructions operable to perform computer executable portions of the method of claim 35.

40. A set of application programming interfaces embodied on a computer readable medium for execution by a computer component in conjunction with an application program that processes PRF phase shift data, comprising:

a first interface that facilitates communicating image data;

a second interface that facilitates communicating alignment data; and

a third interface that facilitates communicating phase difference data where the phase difference data is produced from the image data and the alignment data.

41. A system for processing PRF shift MRI data, comprising:

means for acquiring a PRF shift MRI data;

means for distinguishing a desired PRF shift from an undesired PRF shift in the PRF shift MRI data; and

means for manipulating the PRF shift MRI data to reduce the effects of the undesired PRF shift.

42. A system for processing PRF shift MRI data, comprising:

a PRF shift data corrector; and

a magnetic resonance imager.

43. The system of claim 42, where the magnetic resonance imager comprises:

a polarizing magnetic field generator for generating a polarizing magnetic field in an examination region;

an RF generator for generating an excitation magnetic field that produces transverse magnetization in nuclei subjected to the polarizing magnetic field;

a sensor for sensing a magnetic resonance signal produced by the transverse magnetization;

a gradient generator for generating a magnetic field gradient to impart a read component into the magnetic resonance signal, where the read component indicates a location of a transversely magnetized nuclei along a first projection axis, the gradient generator generating subsequent magnetic field gradients to impart subsequent read components into the magnetic resonance signal that indicates subsequent locations of the transversely magnetized nuclei along one or more subsequent projection axes;

a pulse controller operably coupled to the RF generator, the gradient generator, and the sensor, the pulse controller conducting a scan in which a series of data points are acquired at read points along a radial axis to form a magnetic resonance data view, one or more subsequent magnetic resonance data views defining a magnetic resonance data set;

a data store for storing the magnetic resonance data set; and

a processor for reconstructing an image for a display from the stored magnetic resonance data set.

44. The system of claim 43, the PRF shift data corrector comprising:

a first logic for processing the magnetic resonance data set to mitigate the effects of an undesired phase shifting event in the magnetic resonance data set; and

a second logic for quantifying the effects of a desired phase shifting event after the processing performed by the first logic.

45. The system of claim 44, where the first logic and the second logic are embodied in a single logic.