Process for imaging by nuclear magnetic resonance

• US 4,780,674 A
• Filed: 12/24/1986
• Issued: 10/25/1988
• Est. Priority Date: 01/29/1986
• Status: Expired

• Current Assignee: Thomson-Cgr
• Original Assignee: Thomson-Cgr
• Inventors: Le Bihan, Denis, Breton, Eric
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Fecs, Lawrence G.
• Application Number:
  US06/946,034
• Time in Patent Office: 671 Days
• Field of Search: 128/653, 324/306, 324/307, 324/308, 324/309, 324/310, 324/311, 324/312, 324/313, 324/314, 324/300
• US Class Current: 324/309
• CPC Class Codes: G01R 33/56341 Diffusion imaging

Abstract

The invention proposes an imaging process by nuclear magnetic resonance in which a calculated image is given corresponding to the molecular diffusion value at each point of an investigated medium. The diffusion image is obtained by comparing images relative to same sections of the medium and acquired during two excitation sequences, whereof one is not very sensitive to the diffusion and the other is particularly sensitive thereto. These two images undergo comparison in order to produce a diffusion image.

Claims

1. A process for imaging by nuclear magnetic resonance, wherein it comprises the following stages for giving an image of the molecular diffusion of an investigated body:

the body is placed in a constant magnetic field B_o ;

the thus positioned body is subject to a first plurality of first spin echo excitation sequences in the presence of first field gradient sequences, said first spin echo sequences having an integral number N equal to or greater than one excitations where the magnetic moments of the nuclei of the body are flipped by 180°

following an excitation in which said moments have been flipped by 90°

to obtain in this way sequences with N slightly diffusing echoes;

the magnetic resonance signals are recorded at the end of these first sequences and a first image is calculated corresponding to echo N of these signals by allocating to each point of the image a value corresponding to the magnetic resonance signal of the point corresponding thereto in the body;

the thus positioned body is subject to a second plurality of second spin echo excitation sequences in the presence of second field gradient sequences, said second spin echo sequences having at least one 180°

excitation following a 90°

excitation for forming diffusing sequences with at least one echo, the total echo durations of the second excitation sequences being equal to the total echo durations of the first excitation sequences;

the magnetic resonance signals are recorded at the end of said second excitation sequences and a second image is calculated corresponding to the echoes of these signals by allocating to each image point a value corresponding to the magnetic resonance signal of the point corresponding thereto in the body;

there is a point-by-point comparison of the values allocated for the first image with the values allocated for the second image, in order to produce a third image representing the molecular diffusion at each point of the body.

2. A process according to claim 1, wherein the second sequences have longer and/or more powerful field gradient pulses out of the presence of spin echo excitations and oriented according to one axis in order to form field gradient supplements.

3. A process according to claim 2, in which the field gradient sequences incorporate field gradient pulses oriented along three axes X, Y, Z, the axis Z of the constant B_o or selection axis, as well as two axes X, Y orthogonal to said axis and respectively called phase coding axis Y and reading axis X, in which the method for calculating the images is of the 2 DFT type, wherein the axis of the supplements is the reading axis.

4. A process according to claim 2, in which the field gradient sequences incorporate field gradient pulses oriented along three axes X, Y, Z, the axis Z of the constant field B_o or selection axis, as well as two axes X, Y, orthogonal to said axis and respectively called phase coding axis Y and reading axis X, in which the method for calculating the images is of the 2 DFT type, wherein the axis of the supplements is the selection axis.

5. A process according to claim 2, in which the field gradient sequences incorporate field gradient pulses oriented in accordance with three axes X, Y, Z, the axis Z of the constant field B_o or selection axes, as wall as two axes X, Y, orthogonal to said axis and respectively called the phase coding axis Y and the reading axis X, in which the method for calculating the images is of type 2 DFT, wherein the axis of the supplements is the phase coding axis.

6. A process according to the claim 1, wherein it is performed on a number of occasions for forming third multisection images of the studied body.

7. A process according to the claim 2, wherein the application times of the gradient supplements are, within a same sequence, as timely spaced as possible before and respectively after the 180°

radio frequency excitation time.

8. A process according to the claim 1, wherein a standard body is placed alongside the body to calibrate the calculations of the third image.

9. A process according to the claim 1, wherein the durations of the echo time T_E, of the first spin echo sequences are all equal to one another.

10. A process according to the claim 1, wherein N is equal to four.

11. A process according to claim 2, wherein the stages relative to the second image are modified with gradient supplements oriented along another axis in order to produce another third image for determining the nature of the imaged regions.

12. A process according to the claim 11, wherein the logarithm of the ratio of the values is calculated for comparison purposes.

13. A process according to claim 1, wherein the thus positioned body is subject to a third plurality of third spin echo excitation sequences in the presence of third field gradient sequences, the third spin echo sequences having at least one 180°

excitation following the 90°

excitation for producing diffusing sequences with at least one echo, the total echo durations of the third excitation sequences being equal to the total echo durations of the first excitation sequences, the third field gradient sequences differing from the second field gradient sequences;

the magnetic resonance signals are read at the end of said third excitation sequences and a fourth image is calculated corresponding to the echoes of these signals by attributing to each image point a value corresponding to the magnetic resonance signal of the point corresponding thereto in the body;

the values attributed for the first image are compared point-by-point with the values attributed for the fourth image for producing a fifth image representing the molecular diffusion at each point in the body;

and then the values attributed for the third image are compared point-by-point with the values attributed for the fifth image for producing a sixth image representing the true molecular diffusion in the body and which is free from micro-circulation interference.

14. A process according to claim 1, wherein the thus positioned body is subject to a third plurality of third spin echo excitation sequences in the presence of third field gradient sequences, the third spin echo sequences having at least one 180°

excitation following the 90°

excitation for producing diffusing sequences with at least one echo, the total echo durations of the third excitation sequences being equal to the total echo durations of the first excitation sequences, the third field gradient sequences differing from the second field gradient sequences;

the magnetic resonance signals are read at the end of these third excitation sequences and a fourth image is calculated which corresponds to the echoes of these signals by attributing to each image point a value corresponding to the magnetic resonance signal of the point corresponding thereto in the body;

the values attributed for the first image are compared point-by-point with the values attributed for the fourth image for producing a fifth image representing the molecular diffusion at each point of the body;

and the values attributed for the third image are compared point-by-point with the values attributed for the fifth image in order to produce a seventh image representing a perfusion phenomenon in the body.

15. A process according to claim 13 further comprising a step wherein the values attributed for the third image are compared point-by-point with the values attributed for the fifth image in order to produce a seventh image representing a perfusion phenomena in the body and wherein the sixth and seventh images are simultaneously produced.

16. A process according to any one of the claims 1 to 14, wherein the diffusing sequences are sequences with a single spin echo.

17. A process according to any one of the claims 11, 13 or 14 wherein the effect of the speed of moving parts of the body created by so-called interfering field gradient sequences is modulated by applying, before the record of the signal, a compensating magnetic field sequence, whose integral calculated on its duration is zero and whose history and value are a function of the history and value of the interfering fields.

18. A process according to claim 17, wherein the sequence of the interfering magnetic field incorporates magnetic field pulses along orthogonal axes X, Y, Z, and wherein the sequence of the compensating magnetic field incorporates magnetic field pulses along these three same axes, in order to modulate one by one the effects of the speed of the moving parts of the body along these three same axes.

19. A process according to claim 18, wherein the compensating magnetic field pulses are determined a priori in a form, in a duration, and in a position and wherein their amplitude λ

is evaluated to obtain the sought modulation.

20. A process according to claim 17, wherein the sequence of the compensating magnetic fields incorporates bipolar pulses pairs.

21. A process according to the claim 17, wherein the sequence of the compensating magnetic fields has pulse pairs, each pulse of a pair having a value, shape, duration and signal equal to the other pulse of the pair, said pulses being respectively timely located before and after a second high frequency pulse.

22. A process according to the claim 17, wherein the integral of the product of the value of the interfering field pulses by the time separating them from the recording of the emitted signal as compensated by the integral of the same product obtained with compensating field pulses in lieu of the interfering field pulses.

23. A process according to claim 20, wherein the pulses of the pulse pairs comprise pulses which are as timely spaced as possible from one another.

24. A process according to the claim 17, wherein the effect of the speed of the moving parts of the body produced solely in the second field gradient sequences is modulated.