Method and apparatus for spin-echo-train MR imaging using prescribed signal evolutions

Abstract

A magnetic resonance imaging “MRI” method and apparatus for lengthening the usable echo-train duration and reducing the power deposition for imaging is provided. The method explicitly considers the t1 and t2 relaxation times for the tissues of interest, and permits the desired image contrast to be incorporated into the tissue signal evolutions corresponding to the long echo train. The method provides a means to shorten image acquisition times and/or increase spatial resolution for widely-used spin-echo train magnetic resonance techniques, and enables high-field imaging within the safety guidelines established by the Food and Drug Administration for power deposition in human MRI.

We claim: 
     
       1. A method for generating a spin echo pulse sequence for operating a magnetic resonance imaging apparatus for imaging an object that permits at least one of lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the tissue signal evolutions, said method comprising:
 a) providing contrast-preparation, said contrast-preparation comprising generating at least one of at least one radio-frequency pulse, at least one magnetic-field gradient pulse, and at least one time delay, whereby said contrast preparation encodes the magnetization with at least one desired image contrast;   b) calculating flip angles and phases of refocusing radio-frequency pulses that are applied in a data-acquisition step, wherein said calculation provides desired prescribed signal evolution and desired overall signal level, said calculation comprises:
 i) selecting values of T1 and T2 relaxation times and selecting proton density; 
 ii) selecting a prescribed time course of the amplitudes and phases of the radio-frequency magnetic resonance signals that are generated by said refocusing radio-frequency pulses; and 
 iii) selecting characteristics of said contrast-preparation step, said data-acquisition step and a magnetization-recovery step, with the exception of the flip angles and phases of the refocusing radio-frequency pulses that are to be calculated; and 
   c) providing said-data acquisition step based on a spin echo train acquisition, said data-acquisition step comprises:
 i) an excitation radio-frequency pulse having a flip angle and phase; 
 ii) at least two refocusing radio-frequency pulses, each having a flip angle and phase as determined by said calculation step; and 
 iii) magnetic-field gradient pulses that encode spatial information into at least one of said radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses; 
   d) providing magnetization-recovery, said magnetization-recovery comprises a time delay to allow magnetization to relax; and   e) repeating steps (a) through (d) until a predetermined extent of spatial frequency space has been sampled.   
     
     
       2. The method of  claim 1 , wherein said calculation of the flip angles and phases is generated using an appropriate analytical or computer-based algorithm. 
     
     
       3. The method of  claim 1 , wherein said calculation of the flip angles and phases is generated to account for, the effects of multiple applications of: said contrast-preparation, said data-acquisition and said magnetization-recovery steps, which are required to sample the desired extent of spatial-frequency space. 
     
     
       4. The method of  claim 1 , wherein a two-dimensional plane of spatial-frequency space is sampled. 
     
     
       5. The method of  claim 1 , wherein a three-dimensional volume of spatial-frequency space is sampled. 
     
     
       6. The method of  claim 1 , wherein at least one of said contrast-preparation and magnetization-recovery steps is omitted. 
     
     
       7. The method of  claim 1 , wherein said calculation step is performed once before one of said first contrast-preparation step and said first data-acquisition step. 
     
     
       8. The method of  claim 1 , wherein at least one of at least one said contrast-preparation step, at least one said data-acquisition step and at least one said magnetization-recovery step is initiated by a trigger signal to synchronizes the pulse sequence with at least one of at least one external temporal event and at least one internal temporal event. 
     
     
       9. The method of  claim 8 , 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. 
     
     
       10. The method of  claim 1 , wherein at least one of at least one radio-frequency pulse and at least one magnetic-field gradient pulse is applied as part of at least one of at least one said magnetization-preparation step and at least one said data-acquisition step is for the purpose of stabilizing the response of at least one of magnetization related system and said apparatus related hardware system. 
     
     
       11. The method of  claim 1 , wherein time duration varies between repetitions for at least one of at least one said contrast-preparation step, at least one said data-acquisition step and at least one said magnetization-recovery step. 
     
     
       12. The method of  claim 1 , wherein the time periods between consecutive refocusing radio-frequency pulses applied during said data-acquisition steps are all of equal duration. 
     
     
       13. The method of  claim 1 , wherein time periods between consecutive refocusing radio-frequency pulses applied during said data-acquisition steps vary in duration amongst pairs of refocusing radio-frequency pulses during at least one said data-acquisition step. 
     
     
       14. The method of  claim 1  wherein all the radio-frequency pulses are at least one of non-spatially selective and non-chemically selective. 
     
     
       15. 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. 
     
     
       16. The method of  claim 1 , wherein during each said data-acquisition step, the phase difference between the phase for the excitation radio-frequency pulse and the phases for all refocusing radio-frequency pulses is about 90 degrees. 
     
     
       17. The method of  claim 1 , wherein during each data-acquisition step, the phase difference between the phase for any refocusing radio-frequency pulse and the phase for the immediately subsequent refocusing radio-frequency pulses is about 180 degrees, and the phase difference between the phase for the excitation radio-frequency pulse and the phase for the first refocusing pulse is one of about 0 degrees and about 180 degrees. 
     
     
       18. The method of  claim 17 , wherein the flip angle for the excitation radio-frequency pulse is about one-half of the flip angle for the first refocusing radio-frequency pulse. 
     
     
       19. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during each said data-acquisition step are configured so as to collect data, following each of at least one of the refocusing radio-frequency pulses, for one line in spatial-frequency space which is parallel to all other lines of data so collected, so as to collect the data using a magnetic resonance imaging technique selected from the group consisting of rapid acquisition with relaxation enhancement (RARE), fast spin echo (FSE), and turbo spin echo (TSE or TurboSE). 
     
     
       20. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during each said data-acquisition step are configured so as to collect data, following each of at least one of the refocusing radio-frequency pulses, for two or more lines in spatial-frequency space which are parallel to all other lines of data so collected, so as to collect the data using a magnetic resonance imaging technique selected from the group consisting of gradient and spin echo (GRASE) and turbo gradient spin echo (TGSE or TurboGSE). 
     
     
       21. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during each said data-acquisition step are configured so as to collect data, following each of at least one of the refocusing radio-frequency pulses, for one or more lines in spatial-frequency space, each of which pass through one of a single point in spatial-frequency space and a single line in spatial-frequency space, so as to collect the data using a magnetic resonance imaging technique selected from the group consisting of radial sampling or projection-reconstruction sampling. 
     
     
       22. The method of  claim 21 , wherein the single point in spatial-frequency space is about zero spatial frequency. 
     
     
       23. The method of  claim 21 , wherein the single line in spatial-frequency space includes zero spatial frequency. 
     
     
       24. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during each said data-acquisition step are configured so as to collect data, following each of at least one of the refocusing radio-frequency pulses, along a spiral trajectory in spatial-frequency space, each trajectory of which is contained in one of two dimensions and three dimensions, and each trajectory of which passes through one of a single point in spatial-frequency space and a single line in spatial-frequency space. 
     
     
       25. The method of  claim 24 , wherein the single point in spatial-frequency space is about zero spatial frequency. 
     
     
       26. The method of  claim 24 , wherein the single line in spatial-frequency space includes zero spatial frequency. 
     
     
       27. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during at least one of said data-acquisition steps are configured to collect sufficient spatial-frequency data to reconstruct at least two image sets, each of which exhibits contrast properties different from the other image sets. 
     
     
       28. The method of  claim 27 , wherein at least some of the spatial-frequency data collected during at least one of said data-acquisition steps is used in the reconstruction of more than one image set, whereby the data is shared between image sets. 
     
     
       29. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during at least one of said data-acquisition steps are configured so that, for the echo following at least one of the refocusing radio-frequency pulses, at least one of the first moment, the second moment and the third moment corresponding to at least one of the spatial-encoding directions is approximately zero. 
     
     
       30. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during at least one of said data-acquisition steps are configured so that, following at least one of the refocusing radio-frequency pulses, the zeroth moment measured over the time period between said refocusing radio-frequency pulse and the immediately consecutive refocusing radio-frequency pulse is approximately zero for at least one of the spatial-encoding directions. 
     
     
       31. The method of  claim 1 , wherein during all said data-acquisition steps the duration of all data-sampling periods are equal. 
     
     
       32. The method of  claim 1 , wherein during at least one of said data-acquisition steps at least one of the data-sampling periods has a duration that differs from the duration of at least one other data-sampling period. 
     
     
       33. The method of  claim 1 , wherein the spatial-encoding magnetic-field gradient pulses applied during said data-acquisition steps are configured so that the extent of spatial-frequency space sampled along at least one of the spatial-encoding directions is not symmetric with respect to zero spatial frequency, whereby a larger extent of spatial-frequency space is sampled to one side of zero spatial frequency as compared to the opposite side of zero spatial frequency. 
     
     
       34. The method of  claim 33  wherein said spatial-frequency data is reconstructed using a partial-Fourier reconstruction algorithm. 
     
     
       35. The method of  claim 1 , wherein during at least one of said data-acquisition steps the temporal order in which spatial-frequency space data is collected for at least one of the spatial-encoding directions is based on achieving at least one of selected contrast properties in the image and selected properties of the corresponding point spread function. 
     
     
       36. The method of  claim 1 , wherein during at least one of said data-acquisition steps the temporal order in which spatial-frequency space data is collected is different from that for at least one other data-acquisition step. 
     
     
       37. The method of  claim 1 , wherein during at least one of said data-acquisition steps the extent of spatial-frequency space data that is collected is different from that for at least one other data-acquisition step. 
     
     
       38. The method of  claim 1 , wherein during at least one of said data-acquisition steps spatial encoding of the radio-frequency magnetic resonance signal that follows at least one of the refocusing radio-frequency pulse is performed using only phase encoding so that said signal is received by the radio-frequency transceiver in the absence of any applied magnetic-field gradient pulses and hence contains chemical-shift information. 
     
     
       39. The method of  claim 1 , wherein at least one navigator radio-frequency pulse is incorporated into the pulse sequence for the purpose of determining the displacement of a portion of the object. 
     
     
       40. A magnetic resonance imaging apparatus generating a spin echo pulse sequence in order to operate the apparatus in imaging an object that permits at least one of lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the tissue signal evolutions, the apparatus comprising:
 a main magnet system generating a steady magnetic field;   a gradient magnet system generating temporary gradient magnetic fields;   a radio-frequency transmitter system generating radio-frequency pulses;   a radio-frequency receiver system receiving magnetic resonance signals;   a reconstruction unit reconstructing an image of the object from the received magnetic resonance signals; and   a control unit generating signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequency receiver system, and the reconstruction unit, wherein the control unit generates signals causing:
 a) providing contrast-preparation, said contrast-preparation comprising generating at least one of at least one radio-frequency pulse, at least one magnetic-field gradient pulse, and at least one time delay, whereby said contrast preparation encodes the magnetization with at least one desired image contrast; 
 b) calculating flip angles and phases of refocusing radio-frequency pulses that are applied in a data-acquisition step, wherein said calculation provides desired prescribed signal evolution and desired over-all signal level, said calculation comprises:
 i) selecting values of T1 and T2 relaxation times and selecting proton density; 
 ii) selecting a prescribed time course of the amplitudes and phases of the radio-frequency magnetic resonance signals that are generated by said refocusing radio-frequency pulses; and 
 iii) selecting characteristics of said contrast-preparation step, said data-acquisition step and a magnetization-recovery step, with the exception of the flip angles and phases of the refocusing radio-frequency pulses that are to be calculated; and 
 
 c) providing said-data acquisition step based on a spin echo train acquisition, said data-acquisition step comprises:
 i) an excitation radio-frequency pulse having a flip angle and phase, 
 ii) at least two refocusing radio-frequency pulses, each having a flip angle and phase as determined by said calculation step, and 
 iii) magnetic-field gradient pulses that encode spatial information into at least one of said radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses; 
 
 d) providing magnetization-recovery, said magnetization-recovery comprises a time delay to allow magnetization to relax; and 
 e) repeating steps (a) through (d) until a predetermined extent of spatial frequency space has been sampled. 
   
     
     
       41. A magnetic resonance imaging apparatus generating a spin echo pulse sequence in order to operate the apparatus in imaging an object that permits at least one of lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the tissue signal evolutions, the apparatus comprising:
 main magnet means generating a steady magnetic field;   gradient magnet means generating temporary gradient magnetic fields;   radio-frequency transmitter means generating radio-frequency pulses;   radio-frequency receiver means receiving magnetic resonance signals;.   reconstruction means reconstructing an image of the object from the received magnetic resonance signals; and   control means 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) providing contrast-preparation, said contrast-preparation comprising generating at least one of at least one radio-frequency pulse, at least one magnetic-field gradient pulse, and at least one time delay, whereby said contrast preparation encodes the magnetization with at least one desired image contrast; 
 b) calculating flip angles and phases of refocusing radio-frequency pulses that are applied in a data-acquisition step, wherein said calculation provides desired prescribed signal evolution and desired over-all signal level, said calculation comprises:
 i) selecting values of T1 and T2 relaxation times and selecting proton density; 
 ii) selecting a prescribed time course of the amplitudes and phases of the radio-frequency magnetic resonance signals that are generated by said refocusing radio-frequency pulses; and 
 iii) selecting characteristics of said contrast-preparation step, said data-acquisition step and a magnetization-recovery step, with the exception of the flip angles and phases of the refocusing radio-frequency pulses that are to be calculated; 
 
 c) providing said-data acquisition step based on a spin echo train acquisition, said data-acquisition step comprises:
 i) an excitation radio-frequency pulse having a flip angle and phase, 
 ii) at least two refocusing radio-frequency pulses, each having a flip angle and phase as determined by said calculation step, and 
 iii) magnetic-field gradient pulses that encode spatial information into at least one of said radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses; 
 
 d) providing magnetization-recovery, said magnetization-recovery comprises a time delay to allow magnetization to relax; and 
 e) repeating steps (a) through (d) until a predetermined extent of spatial frequency space has been sampled. 
   
     
     
       42. A computer readable media carrying encoded program instructions for causing a programmable magnetic resonance imaging apparatus to perform the method of  claim 1 . 
     
     
       43. A computer program provided on a computer useable readable medium having computer program logic enabling at least one processor in a magnetic resonance imaging apparatus to generate a spin echo pulse sequence that permits at least one of lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the tissue signal evolutions, said computer program logic comprising:
 a) providing contrast-preparation, said contrast-preparation comprising generating at least one of at least one radio-frequency pulse, at least one magnetic-field gradient pulse, and at least one time delay, whereby said contrast preparation encodes the magnetization with at least one desired image contrast;   b) calculating flip angles and phases of refocusing radio-frequency pulses that are applied in a data-acquisition step, wherein said calculation provides desired prescribed signal evolution and desired overall signal level, said calculation comprises:
 i) selecting values of T1 and T2 relaxation times and selecting proton density; 
 ii) selecting a prescribed time course of the amplitudes and phases of the radio-frequency magnetic resonance signals that are generated by said refocusing radio-frequency pulses; and 
 iii) selecting characteristics of said contrast-preparation step, said data-acquisition step and a magnetization-recovery step, with the exception of the flip angles and phases of the refocusing radio-frequency pulses that are to be calculated; and 
   c) providing said-data acquisition step based on a spin echo train acquisition, said data-acquisition step comprises:
 i) an excitation radio-frequency pulse having a flip angle and phase; 
 ii) at least two refocusing radio-frequency pulses, each having a flip angle and phase as determined by said calculation step; and 
 iii) magnetic-field gradient pulses that encode spatial information into at least one of said radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses; 
   d) providing magnetization-recovery, said magnetization-recovery comprises a time delay to allow magnetization to relax; and   e) repeating steps (a) through (d) until a predetermined extent of spatial frequency space has been sampled.   
     
     
       44. The method of  claim 40 , wherein at least one of said contrast-preparation and magnetization-recovery steps is omitted. 
     
     
       45. The method of  claim 41 , wherein at least one of said contrast-preparation and magnetization-recovery steps is omitted. 
     
     
       46. The method of  claim 43 , wherein at least one of said contrast-preparation and magnetization-recovery steps is omitted. 
     
     
       47. A method of generating a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with varying flip angles, said method comprising:
 generating, via a control unit, a three-dimensional T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence used in operating a magnetic resonance imaging apparatus that images tissues of a human subject, the generated pulse sequence having varying flip angles that vary among a majority of refocusing radio-frequency pulses of an echo train by decreasing to a minimum value and later increasing, and   wherein the varying flip angles result in a reduced power deposition compared to the power deposition that is obtained using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees;   applying the pulse sequence to a radio-frequency transmitter coil of the magnetic resonance imaging apparatus to generate radio-frequency pulses in an examination zone that includes tissues of the human subject and receiving resulting magnetic resonance signals from tissues of the human subject, using a radio-frequency receiver coil of the magnetic resonance imaging apparatus; and   reconstructing magnetic resonance images from the resulting magnetic resonance signals from the tissues of the human subject,   wherein the reconstructed magnetic resonance images have a T2-weighted contrast, and   wherein the T2-weighted contrast is substantially the same as contrast in T2-weighted magnetic resonance images generated from a fast-spin-echo or turbo-spin-echo pulse sequence using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees.   
     
     
       48. The method of claim 47, wherein the pulse sequence has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches. 
     
     
       49. The method of claim 47, wherein the varying flip angles are calculated to provide a prescribed signal evolution. 
     
     
       50. The method of claim 49, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that are arbitrarily chosen. 
     
     
       51. The method of claim 50, wherein the pulse sequence has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches. 
     
     
       52. A magnetic resonance imaging (MRI) apparatus that images tissues of a human subject and is configured to generate a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with varying flip angles, the apparatus comprising:
 a main magnet system that generates a steady magnetic field;   a gradient magnet system that generates temporary gradient magnetic fields;   a radio-frequency transmitter system that generates radio-frequency pulses;   a radio-frequency receiver system that receives magnetic resonance signals;   a reconstruction unit that reconstructs magnetic resonance images of the subject from the received magnetic resonance signals from the tissues of the human subject; and   a control unit that generates signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequency receiver system, and the reconstruction unit, wherein the control unit further provides signals that generate:   a three-dimensional T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence used in operating the MRI apparatus, the generated pulse sequence having varying flip angles that vary among a majority of refocusing radio-frequency pulses of an echo train by decreasing to a minimum value and later increasing, and   wherein the varying flip angles result in a reduced power deposition compared to the power deposition that is obtained using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees,   wherein the reconstructed magnetic resonance images have a T2-weighted contrast, and   wherein the T2-weighted contrast is substantially the same as contrast in T2-weighted magnetic resonance images generated from a fast-spin-echo or turbo-spin-echo pulse sequence using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees.   
     
     
       53. The apparatus of claim 52, wherein the pulse sequence that is generated has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches. 
     
     
       54. The apparatus of claim 52, wherein a control unit executes a software program that performs calculations and is configured to calculate the varying flip angles to provide a prescribed signal evolution. 
     
     
       55. The apparatus of claim 54, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that are arbitrarily chosen. 
     
     
       56. The apparatus of claim 55, wherein the pulse sequence that is generated has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches. 
     
     
       57. A non-transitory computer readable medium having computer program logic that when implemented enables one or more processors in a magnetic resonance imaging apparatus that images tissues of a human subject to generate a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with varying flip angles, said computer program logic comprising:
 logic for generating a three-dimensional T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence used in operating the magnetic resonance imaging apparatus, the generated pulse sequence having varying flip angles that vary among a majority of refocusing radio-frequency pulses of an echo train by decreasing to a minimum value and later increasing, and   wherein the varying flip angles result in a reduced power deposition compared to the power deposition that is obtained using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees; and   logic for reconstructing magnetic resonance images from magnetic resonance signals received from tissues of the human subject as a result of applying the generated pulse sequence,   wherein the reconstructed magnetic resonance images have a T2-weighted contrast, and   wherein the T2-weighted contrast is substantially the same as contrast in T2-weighted magnetic resonance images generated from a fast-spin-echo or turbo-spin-echo pulse sequence using refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees.   
     
     
       58. The non-transitory computer readable medium having computer program logic as defined in claim 57, wherein the pulse sequence that is generated has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches. 
     
     
       59. The non-transitory computer readable medium having computer program logic as defined in claim 57, wherein the computer program logic also calculates the varying flip angles to provide a prescribed signal evolution. 
     
     
       60. The non-transitory computer readable medium having computer program logic as defined in claim 59, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that are arbitrarily chosen. 
     
     
       61. The non-transitory computer readable medium having computer program logic as defined in claim 60, wherein the pulse sequence that is generated has a usable echo-train duration for spin-echo train imaging that is lengthened substantially beyond that achievable with constant, low-flip-angle or pseudosteady-state approaches.