Magnetic resonance method and system for quantification of anisotropic diffusion

• US 20030160612A1
• Filed: 01/15/2003
• Published: 08/28/2003
• Est. Priority Date: 01/16/2002
• Status: Active Grant

• Current Assignee: Washington University In St Louis
• Original Assignee: Washington University In St Louis
• Inventors: Yablonskiy, Dmitriy A., Conradi, Mark S., Sukstanskii, Alexander L.
• Granted Patent: US 7,078,897 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/56341 Diffusion imaging

Abstract

An MR method and system of determining elements of the apparent diffusion coefficient tensor in a material with plurality of anisotropic structural units that can be too small to be resolved by direct imaging. MR data is acquired with MR system including pulse sequences, the sequences including imaging or spectroscopy pulse sequences with a series of embedded diffusion-sensitizing gradient waveforms with different gradient strength applied to the material. A nonlinear function of a b-value corresponding to the pulse sequence is defined and the acquired MR data is processed according to defined nonlinear function. Images/maps of the components of the tensor of apparent diffusion coefficients, corresponding to anisotropic structural units, based on the processed MR data, are created. A method of evaluating of the geometrical parameters of lung airways is also described.

Claims

1. A method of acquiring MR images of a plurality of anisotropic structural units (that can be too small to be resolved by direct imaging) using an MRI system, said method comprising the steps of:

generating MR pulse sequences via the MRI system to generate MR data corresponding to the structural units, the sequences including imaging or spectroscopy pulse sequences with a series of embedded diffusion-sensitizing gradient waveforms of varying gradient strength;

acquiring the MR data after each gradient waveform;

defining a nonlinear function of a b-value of the MR data;

fitting the MR data for each imaging unit (pixel or voxel) to the nonlinear function to determine elements of the apparent diffusion coefficient tensor of structural units for each of the fitted imaging units; and

creating images based on the determined elements of the apparent diffusion coefficient tensor of structural units for each of the fitted imaging units.

2. The method of claim 1 further comprising the step of conducting a diagnostic evaluation of geometrical parameters of structural units based on the MR data.

3. The method of claim 1 wherein the MR (spin echo, gradient echo or spectroscopy) pulse sequence includes either a two-dimensional multi-slice multi-gradient pulse sequence, or a three-dimensional multi-gradient pulse sequence, or spectroscopy pulse sequence.

4. The method of claim 1 wherein the b-value is determined by a time-dependent field gradient waveform G(t) such that,

5. The method of claim 1 wherein the gradient waveform comprises a trapezoidal gradient waveform and wherein a general expression for the b-value is

6. The method of claim 1 wherein the nonlinear function F(b) defines signal attenuation in the presence of diffusion-sensitizing gradients such that

7. The method of claim 1 wherein the nonlinear function F(b) for a material having homogeneous orientation distribution of the structural units is reduced to

8. The method of claim 1 further comprising reducing the nonlinear function F(b) for materials including similar structural units of uniaxial anisotropy (D_x=D_y) to:

9. The method of claim 1 wherein the step of creating images based on the determined elements comprises the steps of creating an image corresponding to a mean diffusivity D_AV(x, y, z) based on the fitted MR data, creating an image corresponding to an out-of-plane anisotropy diffusivity D_AN(x, y, z) based on the fitted MR data, and creating an image corresponding to an in-plane anisotropy diffusivity d_AN(x, y, z) based on the fitted MR data.

10. The method of claim 1 wherein the structural units comprise a gas in small airways such as small airways of lungs of humans and animals (that can be too small to be resolved by direct imaging).

11. An MRI apparatus for acquiring MR images of a material comprising:

a magnetic resonance imaging system having a magnetic assembly including a magnet to impress a polarizing magnetic field on the material, a plurality of gradient coils positioned about a bore of a magnet to impress a logical inhomogeneous magnetic field on the material, a RF transceiver system having an RF coil assembly, and an RF switch for controlling the RF transceiver system to transmit and receive RF signals to and from the material to acquire MR signals representative of the MR images of the material;

a processor for;

acquiring multiple sets of MR data from the RF transceiver system;

determining a signal intensity for each MR data;

fitting the determined signal intensity for each MR data to a nonlinear function;

determining elements of the apparent diffusion coefficient tensor for the fitted MR data; and

reconstructing MR images based on the determined elements of the apparent diffusion coefficient tensor; and

a display for displaying the reconstructed images as the acquired MR images of the material.

12. The system of claim 11 further comprising:

a digitizer for digitizing the RF signals sensed up by the RF coil and received by the RF transceiver system; and

a memory module for storing the digitized MR signals in an array of raw k-space data; and

wherein the processor rearranges the raw k-space data into separate k-space data arrays for each image to be reconstructed;

wherein each separate k-space data array is processed to reconstruct images; and

wherein the display displays the reconstructed images.

13. The system of claim 11 wherein the material comprises a gas in small airways such as small airways of lungs of humans and animals (that can be too small to be resolved by direct imaging).

14. A system for acquiring MR images of a plurality of anisotropic structural units (that can be too small to be resolved by direct imaging) using an MRI apparatus, said system comprising:

an MR pulse sequence generator, the MRI apparatus being able to acquire MR data corresponding to the structural elements, the sequences including imaging or spectroscopy pulse sequences with a series of embedded diffusion-sensitizing gradient waveforms of varying gradient strength;

a memory storing the MR data acquired after each gradient waveform;

a processor fitting each imaging unit (pixel or voxel) of the acquired MR data to a nonlinear function to determine elements of the apparent diffusion coefficient tensor for structural units; and

a display displaying images based on the determined elements of the apparent diffusion coefficient tensor for each structural unit in each imaging unit (pixel or voxel).

15. The system of claim 14 wherein the MR (spin echo, gradient echo or spectroscopy) pulse sequence includes either a two-dimensional multi-slice multi-gradient pulse sequence, or a three-dimensional multi-gradient pulse sequence, or spectroscopy pulse sequence.

16. The system of claim 14 wherein the non-linear function is a nonlinear function of a b-value of the MR data and wherein the b-value is determined by a field gradient waveform G(t) such that,

17. The system of claim 14 wherein the gradient waveform comprises a trapezoidal gradient waveform and wherein a general expression for the b-value is

18. The system of claim 14 wherein the nonlinear function F(b) defines signal attenuation in the presence of diffusion-sensitizing gradients such that

19. The system of claim 14 wherein the nonlinear function F(b) for materials having homogeneous orientation distribution of the structural units is reduced to

20. The system of claim 14 wherein the nonlinear function F(b) for materials including similar structural units of uniaxial anisotropy (D_x=D_y) is reduced to:

21. The system of claim 14 wherein the step of creating images based on the determined elements comprises the steps of creating an image corresponding to a mean diffusivity D_AV(x, y, z) based on the fitted MR data, creating an image corresponding to an out-of-plane anisotropy diffusivity D_AN(x, y, z) based on the fitted MR data, and creating an image corresponding to an in-plane anisotropy diffusivity d_AN(x, y, z) based on the fitted MR data.

22. The system of claim 14 wherein the material comprise a gas in small airways such as small airways of lungs of humans and animals (that can be too small to be resolved by direct imaging).

23. An MRI method for acquiring MR images of a material using a magnetic resonance imaging system having a magnetic assembly including a magnet to impress a polarizing magnetic field on the material, a plurality of gradient coils positioned about a bore of a magnet to impress a logical inhomogeneous magnetic field on the material, a RF transceiver system having an RF coil assembly, and an RF switch responsive to a pulse module for controlling the RF transceiver system to transmit and receive RF signals to and from the material to acquire MR signals representative of the MR images of the material;

the MRI method comprising the steps of;

acquiring multiple sets of MR data from the RF transceiver system;

determining a signal intensity for each MR data;

fitting the determined signal intensity for each MR data to a nonlinear function;

determining elements of the apparent diffusion coefficient tensor for the fitted MR data; and

reconstructing MR images based on the determined elements of the apparent diffusion coefficient tensor; and

displaying the reconstructed images as the acquired MR images of the material.

24. The method of claim 23 further comprising the steps of:

digitizing the RF signals sensed up by the RF coil and received by the RF transceiver system; and

storing the digitized MR signals in a memory in an array of raw k space data;

rearranging the raw k-space data into separate k-space data arrays for each image to be reconstructed;

processing each separate k-space data array to reconstruct images; and

displaying the reconstructed images.

25. The method of claims 23-24 wherein the material comprises a gas in small airways such as small airways of lungs of humans and animals (that can be too small to be resolved by direct imaging).