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 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. 
   
     
     
       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 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; 
 
 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 spin-echo-train pulse sequence used in operating a magnetic resonance imaging apparatus configured for imaging an object, said method comprising:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a conventional spin-echo pulse sequence, 
 wherein an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an echo time of said conventional spin-echo pulse sequence, and 
 wherein: said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least on the order of 300 milliseconds; and/or the duration of said spin-echo trains with said signal evolutions of said substances is at least on the order of 600 milliseconds; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; and 
   repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled.   
     
     
       48. The method of claim 47, wherein a three-dimensional volume of spatial-frequency space is sampled. 
     
     
       49. The method of claim 53, wherein said flip angles of said refocusing radio-frequency pulses decrease, within the first approximately 15% of the total number of echoes, down to a value that is no more than approximately one-third of the initial flip angle of said refocusing radio-frequency pulses. 
     
     
       50. The method of claim 54, wherein the number of refocusing radio-frequency pulses following at least one said excitation radio-frequency pulse is greater than 50. 
     
     
       51. The method of claim 50, wherein said echo time of said conventional spin-echo pulse sequence has a value typical in T2-weighted clinical magnetic resonance imaging of the brain. 
     
     
       52. The method of claim 51, wherein: said duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence is less than 300 milliseconds; and/or said effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence has a value typical in T2-weighted clinical magnetic resonance imaging of the brain. 
     
     
       53. The method of claim 48, wherein said signal evolutions result in T2-weighted contrast in the corresponding image(s) that is substantially the same as T2-weighted contrast that would be provided by imaging said object by using a turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence that has refocusing radio-frequency pulses with constant flip angles of 180 degrees, and wherein: said duration of said spin-echo trains with said signal evolutions of said substances is at least twice the duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; and/or said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence. 
     
     
       54. The method of claim 49, wherein said reduced power deposition is lower by at least 30% compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees. 
     
     
       55. The method of claim 52, wherein, for at least one of said signal evolutions of said substances, the signal amplitude decreases, within the first approximately 20% of the total number of echoes, down to a value that is no more than approximately two-thirds of the initial value of the signal evolution, and the signal amplitude is then substantially constant, up to at least approximately 50% of the total number of echoes. 
     
     
       56. The method of claim 55, wherein said flip angles and phase angles of the refocusing radio-frequency pulses are calculated using an appropriate analytical or non-transitory computer-based algorithm, either prior to or substantially simultaneous with the execution of the pulse sequence. 
     
     
       57. A magnetic resonance imaging (MRI) apparatus that is configured to generate a spin-echo-train pulse sequence used in imaging an object, the apparatus comprising:
 a main magnet system that is operable in order to generate a steady magnetic field;   a gradient magnet system that is operable in order to generate temporary gradient magnetic fields;   a radio-frequency transmitter system that is operable in order to generate radio-frequency pulses;   a radio-frequency receiver system that is operable in order to receive magnetic resonance signals;   a reconstruction unit that is operable in order to reconstruct an image of the object from the received magnetic resonance signals; and   a control unit that is operable in order to generate signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequency receiver system, and the reconstruction unit, wherein the control unit is further operable to generate signals that enable:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a conventional spin-echo pulse sequence, 
 wherein an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an echo time of said conventional spin-echo pulse sequence, and 
 wherein: said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least on the order of 300 milliseconds; and/or the duration of said spin-echo trains with said signal evolutions of said substances is at least on the order of 600 milliseconds; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; and 
 
 repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled. 
   
     
     
       58. The MRI apparatus of claim 57, wherein said apparatus is operable in order to sample a three-dimensional volume of spatial-frequency space. 
     
     
       59. The MRI apparatus of claim 63, wherein said flip angles of said refocusing radio-frequency pulses decrease, within the first approximately 15% of the total number of echoes, down to a value that is no more than approximately one-third of the initial flip angle of said refocusing radio-frequency pulses. 
     
     
       60. The MRI apparatus of claim 64, wherein the number of refocusing radio-frequency pulses following at least one said excitation radio-frequency pulse is greater than 50. 
     
     
       61. The MRI apparatus of claim 60, wherein said echo time of said conventional spin-echo pulse sequence has a value typical in T2-weighted clinical magnetic resonance imaging of the brain. 
     
     
       62. The MRI apparatus of claim 61, wherein: said duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence is less than 300 milliseconds; and/or said effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence has a value typical in T2-weighted clinical magnetic resonance imaging of the brain. 
     
     
       63. The MRI apparatus of claim 58, wherein said signal evolutions result in T2-weighted contrast in the corresponding image(s) that is substantially the same as T2-weighted contrast that would be provided by imaging said object by using a turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence that has refocusing radio-frequency pulses with constant flip angles of 180 degrees, and wherein: said duration of said spin-echo trains with said signal evolutions of said substances is at least twice the duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; and/or said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence. 
     
     
       64. The MRI apparatus of claim 59, wherein said reduced power deposition is lower by at least 30% compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees. 
     
     
       65. The MRI apparatus of claim 62, wherein, for at least one of said signal evolutions of said substances, the signal amplitude decreases, within the first approximately 20% of the total number of echoes, down to a value that is no more than approximately two-thirds of the initial value of the signal evolution, and the signal amplitude is then substantially constant, up to at least approximately 50% of the total number of echoes. 
     
     
       66. The MRI apparatus of claim 65, wherein said flip angles and phase angles of the refocusing radio-frequency pulses are calculated using an appropriate analytical or non-transitory computer-based algorithm, either prior to or substantially simultaneous with the execution of the pulse sequence. 
     
     
       67. A method of generating a spin-echo-train pulse sequence used in operating a magnetic resonance imaging apparatus configured for imaging an object, said method comprising:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence that has constant flip angles, with values of 180 degrees, for the refocusing radio-frequency pulses, and 
 wherein: the duration of said spin-echo trains with said signal evolutions of said substances is at least twice the duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; and/or an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; and 
   repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled.   
     
     
       68. The method of claim 67, wherein a three-dimensional volume of spatial-frequency space is sampled. 
     
     
       69. The method of claim 68, wherein the number of refocusing radio-frequency pulses following at least one said excitation radio-frequency pulse is greater than 50. 
     
     
       70. The method of claim 69, wherein said flip angles of said refocusing radio-frequency pulses decrease, within the first approximately 15% of the total number of echoes, down to a value that is no more than approximately one-third of the initial flip angle of said refocusing radio-frequency pulses. 
     
     
       71. The method of claim 70, wherein: said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least on the order of 300 milliseconds; and/or said duration of said spin-echo trains with said signal evolutions of said substances is at least on the order of 600 milliseconds. 
     
     
       72. The method of claim 70, wherein said reduced power deposition is lower by at least 30% compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees. 
     
     
       73. A magnetic resonance imaging (MRI) apparatus that is configured to generate a spin-echo-train pulse sequence used in imaging an object, the apparatus comprising:
 a main magnet system that is operable in order to generate a steady magnetic field;   a gradient magnet system that is operable in order to generate temporary gradient magnetic fields;   a radio-frequency transmitter system that is operable in order to generate radio-frequency pulses;   a radio-frequency receiver system that is operable in order to receive magnetic resonance signals;   a reconstruction unit that is operable in order to reconstruct an image of the object from the received magnetic resonance signals; and   a control unit that is operable in order to generate signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequency receiver system, and the reconstruction unit, wherein the control unit is further operable to generate signals that enable:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence that has constant flip angles, with values of 180 degrees, for the refocusing radio-frequency pulses, and 
 wherein: the duration of said spin-echo trains with said signal evolutions of said substances is at least twice the duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; and/or an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; and 
 
 repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled. 
   
     
     
       74. The MRI apparatus of claim 73, wherein said apparatus is operable in order to sample a three-dimensional volume of spatial-frequency space. 
     
     
       75. The MRI apparatus of claim 74, wherein the number of refocusing radio-frequency pulses following at least one said excitation radio-frequency pulse is greater than 50. 
     
     
       76. The MRI apparatus of claim 75, wherein said flip angles of said refocusing radio-frequency pulses decrease, within the first approximately 15% of the total number of echoes, down to a value that is no more than approximately one-third of the initial flip angle of said refocusing radio-frequency pulses. 
     
     
       77. The MRI apparatus of claim 76, wherein: said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least on the order of 300 milliseconds; and/or said duration of said spin-echo trains with said signal evolutions of said substances is at least on the order of 600 milliseconds. 
     
     
       78. The MRI apparatus of claim 76, wherein said reduced power deposition is lower by at least 30% compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees. 
     
     
       79. A non-transitory computer readable medium having computer program logic that when implemented causes and enables at least one processor in a magnetic resonance imaging apparatus to generate a spin-echo-train pulse sequence, said computer program logic comprising:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a conventional spin-echo pulse sequence, 
 wherein an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an echo time of said conventional spin-echo pulse sequence, and 
 wherein: said effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least on the order of 300 milliseconds; and/or the duration of said spin-echo trains with said signal evolutions of said substances is at least on the order of 600 milliseconds; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; 
   repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled; and   reconstructing an image of the object from data received from said data-acquisition step.   
     
     
       80. A non-transitory computer readable medium having computer program logic that when implemented causes and enables at least one processor in a magnetic resonance imaging apparatus to generate a spin-echo-train pulse sequence, said computer logic comprising:
 providing a data-acquisition step based on said spin-echo-train pulse sequence, said data-acquisition step comprises:
 providing an excitation radio-frequency pulse; 
 providing at least two refocusing radio-frequency pulses, each having a flip angle and phase angle,
 wherein, in order to permit during said data-acquisition step lengthening usable echo-train duration, reducing power deposition and incorporating desired image contrast into the signal evolutions, said flip angle is selected to vary, among a majority of the total number of said refocusing pulses applied during the echo train, by decreasing to a minimum value and later increasing in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one first substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, and in order to yield a signal evolution pertaining to the associated train of spin echoes of at least one second substance of interest in said object, with corresponding T1 and T2 relaxation times and a spin density of interest, 
 wherein said varying flip angle results in a reduced power deposition compared to the power deposition that would be achieved by using refocusing radio-frequency pulses with constant flip angles of 180 degrees, 
 wherein said signal evolutions result in a T2-weighted contrast in the corresponding image(s) that is substantially the same as a T2-weighted contrast that would be provided by imaging said object by using a turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence that has constant flip angles, with values of 180 degrees, for the refocusing radio-frequency pulses, and 
 wherein: the duration of said spin-echo trains with said signal evolutions of said substances is at least twice the duration of a spin-echo train associated with said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; and/or an effective echo time corresponding to said spin-echo trains with said signal evolutions of said substances is at least twice an effective echo time of said turbo-spin-echo pulse sequence or fast-spin-echo pulse sequence; 
 
 providing magnetic-field gradient pulses that perform at least one of encoding spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of said refocusing radio-frequency pulses and dephasing transverse magnetization associated with undesired signal pathways in order to reduce or eliminate a contribution of said transverse magnetization into sampled signals; and 
 providing data sampling, associated with magnetic-field gradient pulses that perform spatial encoding; 
   repeating said data-acquisition step until a predetermined extent of spatial frequency space has been sampled; and   reconstructing an image of the object from data received from said data-acquisition step.