Optimized high-speed magnetic resonance imaging method and system using hyperpolarized noble gases

• US 7,174,200 B2
• Filed: 04/12/2002
• Issued: 02/06/2007
• Est. Priority Date: 04/13/2001
• Status: Active Grant

• Current Assignee: University of Virginia Patent Foundation (University of Virginia)
• Original Assignee: University of Virginia Patent Foundation (University of Virginia)
• Inventors: Brookeman, James R., Mugler, John P. III, Salerno, Michael
• Primary Examiner(s): Casler; Brian L.
• Assistant Examiner(s): Cheng; Jacqueline
• Application Number:
  US10/474,571
• Publication Number:
  US 20040260173A1
• Time in Patent Office: 1,761 Days
• Field of Search: 600/420, 600/431, 600/407, 424/9.3
• US Class Current: 600/420
• CPC Class Codes: G01R 33/5601 involving use of a contrast agent for contrast manipulation, e.g. a paramagnetic, super-paramagnetic, ferromagnetic or hyperpolarised contrast agent
• CPC Class Codes: G01R 33/56341 Diffusion imaging

Abstract

A system and method for using hyperpolarized noble gases together with an appropriately designed and optimized magnetic resonance imaging pulse sequence to rapidly acquire static or dynamic magnetic resonance images. The strong magnetic resonance signal from hyperpolarized gases, combined with the present magnetic resonance imaging technique, presents the opportunity for the imaging of gases with both high spatial and high temporal resolution. One potential application for such a method is the direct, dynamic visualization of gas flow, which would be extremely useful for characterizing a variety of fluid systems. In the medical field, one such system of substantial importance is the lung. The system and method provides for visualizing regional ventilatory patterns throughout the respiratory cycle with high temporal and high spatial resolution. The low sensitivity to susceptibility artifacts permits good image quality to be obtained in various orientations. Depending on the application, temporal resolution can be traded for anatomical coverage. Such application of dynamic imaging of the lung using hyperpolarized gases will provide unique information on the physiology and pathophysiology of the lung, and has the potential for many clinically-relevant applications.

Claims

1. A method for generating a pulse sequence for operating a magnetic resonance imaging system for imaging a region of an object, wherein at least a portion of the region contains hyperpolarized noble gas for at least a portion of the time required to apply said pulse sequence, said method comprising:

a) selecting of spatial-encoding magnetic-field gradients to generate spatial-frequency-space trajectories that;

i) permit the data corresponding to one complete image to be acquired using at most one-half the number of spatial-frequency-space trajectories that would be required for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

ii) for at least one-half of said spatial-frequency-space trajectories, begin at approximately zero spatial frequency along at least two spatial-frequency axes;

iii) for at least one-half of the spatial-frequency-space trajectory duration, provide motion-induced phase shifts that are less than those corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

iv) provide motion-induced phase shifts that vary smoothly along said spatial-frequency-space trajectories;

v) sample approximately the same total extent of spatial-frequency space and approximately the same proportions of low, middle and high spatial frequencies;

vi) provide diffusion-induced signal attenuation that is less than that corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution; and

vii) use a data-sampling period that is chosen based on the application, said object and strength of a main magnet system of said magnetic resonance imaging system to yield a pre-determined or desired level of magnetic field inhomogeneity-induced image artifacts;

b) selecting of excitation radio-frequency pulse flip angles wherein said flip angles are specifically chosen to use a fraction of the non-equilibrium hyperpolarized magnetization, said fraction determined based on the total number of images to be acquired from said region of said object;

c) generating of magnetic resonance signals from said object by applying radio-frequency pulses to excite nuclear magnetization with said flip angles and by applying said spatial-encoding magnetic-field gradients; and

d) reconstructing of a magnetic resonance image from the generated magnetic resonance signals.

2. The method of claim 1, wherein, for said conventional rectilinear-trajectory gradient-echo pulse sequence, the number of spatial-frequency-space trajectories corresponding to each of the phase-encoded directions is approximately equal to the maximum spatial extent of said object along the given phase encoding directions divided by the desired voxel dimension in said direction.

3. The method of claim 1, wherein data is acquired corresponding to at least one of at least two magnetic resonance images and at least two regions within said object that at some time during the acquisition comprise hyperpolarized noble gas.

4. The method of claim 3, wherein, for at least one of any region within said object for which the data corresponding to at least two magnetic resonance images is acquired, said data is further processed to yield at least one additional image, said additional image reconstructed from some combination of the spatial-frequency data from the individual unprocessed images corresponding to said imaged-region so as to depict, with higher temporal resolution than that provided by the unprocessed images, any movement of the hyperpolarized gas within said object during the acquisition of said data.

5. The method of claim 1, wherein said object is the lung of an animal or of a human.

6. The method of claim 1, wherein the temporal order of data collection for the spatial-frequency-space trajectories corresponding to at least one magnetic resonance image for at least one region within said object is specifically chosen to reduce motion-induced artifacts.

7. The method of claim 3, wherein, for acquisitions comprising at least two regions within said object, the data acquisition order comprises collecting at least one spatial-frequency-space trajectory corresponding to one magnetic resonance image for one region, and at most the complete set of spatial-frequency-space trajectories corresponding to said image, then proceeding to acquire the equivalent spatial-frequency-space trajectory or set of trajectories from a different region within said object, and finally repeating this process for any remaining regions, and then for any remaining spatial-frequency-space trajectories, until all images for all regions have been acquired.

8. The method of claim 7, wherein for at least two magnetic resonance images for each region within said object that therefore comprise a temporal series for each region, said data acquisition order is repeated a number of times equal to the number of images in the temporal series for each region.

9. The method of claim 1, wherein diffusion-sensitization magnetic-field gradient pulses are applied before said spatial-encoding magnetic-field gradients and along at least one spatial axis.

10. The method of claim 9, wherein data is collected corresponding to at least two different values of said diffusion-sensitization magnetic-field gradient pulses, and spatial maps of the apparent diffusivity are calculated from the images corresponding to the different diffusion sensitizations.

11. The method of claim 1, wherein said flip angles for said excitation radio-frequency pulses are all equal.

12. The method of claim 1, wherein said flip angles for said excitation radio-frequency pulses vary between pulse-sequence repetitions and are chosen to yield a specific temporal evolution of the magnetic resonance signals.

13. The method of claim 1, wherein, in addition to the magnetic resonance signals that comprise the image data, a radio-frequency receiver system of the magnetic resonance imaging system is used to receive, for at least one spatial-frequency-space trajectory, a navigator magnetic resonance signal that is used to correct for signal intensity variations between pulse-sequence repetitions.

14. The method of claim 13, wherein the image and navigator magnetic resonance signals are acquired during the same data-sampling period.

15. The method of claim 1, wherein a spatial-frequency-space trajectory measurement method is used to map the actual spatial-frequency-space trajectories generated by said spatial-encoding gradients, and this data is used for at least one of modification of the applied spatial-encoding gradients and correction of the spatial-frequency data during reconstruction.

16. The method of claim 1, wherein said magnetic resonance images are corrected for the effects off-resonance induced phase shifts.

17. The method of claim 1, wherein at least one spatial-frequency-space trajectory for at least one magnetic resonance image corresponding to at least one region within said object is initiated by a trigger signal to synchronize the pulse sequence with at least one of at least one external temporal event and at least one internal temporal event.

18. The method of claim 17, wherein said external and internal events comprise at least one of at least one voluntary action, at least one involuntary action, at least one respiratory cycle and at least one cardiac cycle.

19. The method of claim 1, wherein for all said spatial-frequency-space trajectories the durations of the data-sampling periods are equal.

20. The method of claim 1, wherein for at least one said spatial-frequency-space trajectory the duration of the corresponding data-sampling period is different from that for at least one other spatial-frequency-space trajectory.

21. The method of claim 1, wherein all said radio-frequency pulses are at least one of non-spatially selective, non-chemically selective, spatially selective and chemically selective.

22. The method of claim 1, wherein at least one of the radio-frequency pulses is at least one of spatially selective in one of one, two and three dimensions, chemically selective, and adiabatic.

23. The method of claim 1, wherein, for at least one repetition of the pulse sequence for at least one region within said object, magnetic field gradient-based spoiling of the transverse magnetization is used.

24. The method of claim 1, wherein, for at least one magnetic resonance image for at least one region within said object, radio-frequency spoiling of the transverse magnetization is used.

25. The method of claim 1, wherein a two-dimensional plane of spatial-frequency space is sampled.

26. The method of claim 1, wherein a three-dimensional volume of spatial-frequency space is sampled.

27. The method of claim 26, wherein, for said three-dimensional volume of spatial-frequency space, the third spatial dimension is encoded using conventional phase-encoding gradients.

28. The method of claim 1, wherein said spatial-encoding magnetic-field gradients are configured so as to collect data, following each of at least one of said excitation radio-frequency pulses, along one of a spiral spatial-frequency-space trajectory and a twisted radial spatial-frequency-space trajectory, each trajectory of which is contained in one of two dimensions and three dimensions, and each trajectory of which passes approximately through one of a single point in spatial-frequency space and a single line in spatial-frequency space.

29. The method of claim 28 wherein the single point in spatial-frequency space is about zero spatial frequency.

30. The method of claim 28 wherein the single line in spatial-frequency space passes through about zero spatial frequency.

31. The method of claim 1, wherein said spatial-encoding magnetic-field gradients are configured, for at least one magnetic resonance image for at least one region within said object, so that an incomplete spatial-frequency space data set is acquired based on the desired field of view, spatial resolution and temporal resolution.

32. The method of claim 31, wherein said incomplete spatial-frequency space data set is reconstructed using an appropriate algorithm that accounts for effects of the missing data based on at least one of spatial interpolation, temporal interpolation and Hermitian symmetry.

33. The method of claim 1, wherein, for at least one magnetic resonance image for at least one region within said object, the temporal order in which the spatial-frequency-space data is collected is based on achieving predetermined properties of the corresponding point spread function.

34. The method of claim 3, wherein, for at least one magnetic resonance image for at least one region within said object, the temporal order in which the spatial-frequency-space data is collected is different from that for at least one other magnetic resonance image.

35. The method of claim 3, wherein, for at least one magnetic resonance image for at least one region within said object, the extent of spatial-frequency-space data that is collected is different from that for at least one other magnetic resonance image.

38. A computer readable media carrying encoded program instructions for causing a programmable magnetic resonance imaging system to perform the method of claim 1.

36. A magnetic resonance imaging system for generating a pulse sequence for operating the system for imaging a region of an object, wherein at least a portion of the region contains hyperpolarized noble gas for at least a portion of the time required to apply said pulse sequence, the system comprising:

main magnet system for generating a steady magnetic field in at least a region of the object to be imaged;

gradient magnet system for generating temporary magnetic-field gradients in at least a region of the object to be imaged;

radio-frequency transmitter system for generating radio-frequency pulses in at least a region of the object to be imaged;

radio-frequency receiver system for receiving magnetic resonance signals from at least a region of the object to be imaged;

reconstruction system for reconstructing an image of at least a region of the object from the received magnetic resonance signals; and

control system for generating signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequency receiver system, and the reconstruction system, wherein the control system generates signals causing;

a) selecting of spatial-encoding magnetic-field gradients to generate spatial-frequency-space trajectories that;

i) permit the data corresponding to one complete image to be acquired using at most one-half the number of spatial-frequency-space trajectories that would be required for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

ii) for at least one-half of said spatial-frequency-space trajectories, begin at approximately zero spatial frequency along at least two spatial-frequency axes;

iii) for at least one-half of the spatial-frequency-space trajectory duration, provide motion-induced phase shifts that are less than those corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

iv) provide motion-induced phase shifts that vary smoothly along said spatial-frequency-space trajectories;

v) sample approximately the same total extent of spatial-frequency space and approximately the same proportions of low, middle and high spatial frequencies;

vi) provide diffusion-induced signal attenuation that is less than that corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution; and

vii) use a data-sampling period that is chosen based on the application, said object and strength of said main magnet system to yield a pre-determined of desired level of magnetic field inhomogeneity-induced image artifacts;

b) selecting of excitation radio-frequency pulse flip angles wherein said flip angles are specifically chosen to use a fraction of the non-equilibrium hyperpolarized magnetization, said fraction determined based on the total number of images to be acquired from said region of said object;

c) generating of magnetic resonance signals from said object by applying radio-frequency pulses to excite nuclear magnetization with said flip angles and by applying said spatial-encoding magnetic-field gradients; and

d) reconstructing of a magnetic resonance image from the generated magnetic resonance signals.

37. A magnetic resonance imaging system for generating a pulse sequence for operating the system for imaging a region of an object, wherein at least a portion of the region contains hyperpolarized noble gas for at least a portion of the time required to apply said pulse sequence, the system comprising:

main magnet means for generating a steady magnetic field in at least a region of the object to be imaged;

gradient magnet means for generating temporary magnetic-field gradients in at least a region of the object to be imaged;

radio-frequency transmitter means for generating radio-frequency pulses in at least a region of the object to be imaged;

radio-frequency receiver means for receiving magnetic resonance signals from at least a region of the object to be imaged;

reconstruction means for reconstructing an image of at least a region of the object from the received magnetic resonance signals; and

control means for generating signals controlling the gradient magnet means, the radio-frequency transmitter means, the radio-frequency receiver means, and the reconstruction means, wherein the control means generates signals causing;

a) selecting of spatial-encoding magnetic-field gradients to generate spatial-frequency-space trajectories that;

i) permit the data corresponding to one complete image to be acquired using at most one-half the number of spatial-frequency-space trajectories that would be required for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

ii) for at least one-half of said spatial-frequency-space trajectories, begin at approximately zero spatial frequency along at least two spatial-frequency axes;

iii) for at least one-half of the spatial-frequency-space trajectory duration, provide motion-induced phase shifts that are less than those corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

iv) provide motion-induced phase shifts that vary smoothly along said spatial-frequency-space trajectories;

v) sample approximately the same total extent of spatial-frequency space and approximately the same proportions of low, middle and high spatial frequencies;

vi) provide diffusion-induced signal attenuation that is less than that corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution; and

vii) use a data-sampling period that is chosen based on the application, said object and strength of said main magnet means to yield a pre-determined of desired level of magnetic field inhomogeneity-induced image artifacts;

b) selecting of excitation radio-frequency pulse flip angles wherein said flip angles are specifically chosen to use a fraction of the non-equilibrium hyperpolarized magnetization, said fraction determined based on the total number of images to be acquired from said region of said object;

c) generating of magnetic resonance signals from said object by applying radio-frequency pulses to excite nuclear magnetization with said flip angles and by applying said spatial-encoding magnetic-field gradients; and

d) reconstructing of a magnetic resonance image from the generated magnetic resonance signals.

39. A computer program product comprising a computer useable medium having computer program logic for enabling at least one processor in a magnetic resonance imaging apparatus to generate a pulse sequence, said computer program logic comprising:

a) selecting of spatial-encoding magnetic-field gradients to generate spatial-frequency-space trajectories that;

i) permit the data corresponding to one complete image to be acquired using at most one-half the number of spatial-frequency-space trajectories that would be required for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

ii) for at least one-half of said spatial-frequency-space trajectories, begin at approximately zero spatial frequency along at least two spatial-frequency axes;

iii) for at least one-half of the spatial-frequency-space trajectory duration, provide motion-induced phase shifts that are less than those corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution;

iv) provide motion-induced phase shifts that vary smoothly along said spatial-frequency-space trajectories;

v) sample approximately the same total extent of spatial-frequency space and approximately the same proportions of low, middle and high spatial frequencies;

vi) provide diffusion-induced signal attenuation that is less than that corresponding to the frequency-encoding magnetic-field gradient for a conventional rectilinear-trajectory gradient-echo pulse sequence with equivalent spatial resolution; and

vii) use a data-sampling period that is chosen based on the application, said object and strength of a main magnet system of said magnetic resonance imaging system to yield a pre-determined or desired level of magnetic field inhomogeneity-induced image artifacts;

b) selecting of excitation radio-frequency pulse flip angles wherein said flip angles are specifically chosen to use a fraction of the non-equilibrium hyperpolarized magnetization, said fraction determined based on the total number of images to be acquired from said region of said object;

c) generating of magnetic resonance signals from said object by applying radio-frequency pulses to excite nuclear magnetization with said flip angles and by applying said spatial-encoding magnetic-field gradients; and

d) reconstructing of a magnetic resonance image from the generated magnetic resonance signals.