Method and apparatus for spin-echo-train MR imaging using prescribed signal evolutions
DC CAFCFirst 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.
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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.
66 Citations
80 Claims
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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:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 42)
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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:
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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. - View Dependent Claims (44)
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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:
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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. - View Dependent Claims (45)
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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:
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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. - View Dependent Claims (46)
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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:
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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. - View Dependent Claims (48, 49, 50, 51, 52, 53, 54, 55, 56)
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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:
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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. - View Dependent Claims (58, 59, 60, 61, 62, 63, 64, 65, 66)
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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:
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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. - View Dependent Claims (68, 69, 70, 71, 72)
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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:
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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. - View Dependent Claims (74, 75, 76, 77, 78)
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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:
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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.
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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:
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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.
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Specification