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.

Description

provides a graph of normalized signal amplitude versus echo number (total number of echoes for signal evolution=160) that shows an example of a prescribed signal evolution for gray matter that can be used to generate T2-weighted MR images of the brain. The evolution consists of the following: exponential decay during the first 20 echoes with decay constant of 114 ms, constant for 66 echoes, and exponential decay during the remaining echoes with decay constant of 189 ms. FIG. 3 shows the corresponding variable-flip-angle series that was derived using the present invention methods as described herein. Using an interactive computer-based (Ultra-60 workstation; Sun Microsystems, Inc.) theoretical model, and the prescribed signal evolution for brain gray matter, at 1.5 Tesla (see FIG. 2), the four-step process described above was used to derive the corresponding variable-flip-angle series depicted in FIG. 3. The pulse-sequence parameters included an echo train length of 160, an echo spacing of 4.1 ms (fixed), a total echo-train duration of 656 ms, a repetition time of 2750 ms and an effective echo time (i.e., the time period from the excitation RF pulse to the collection of data corresponding to substantially zero-spatial frequency (the center of k space)) of 328 ms.

Example No. 1

FIGS. 4B-4F show an example of MR brain images obtained at 1.5 Tesla using the variable-flip-angle series of FIG. 3 in a “turbo-SE” type spin-echo-train pulse sequence; collectively. In particular, the T2-weighted two-dimensional and three-dimensional SE images of FIGS. 4(A) and 4(B)-4(C), respectively, were obtained from a 59 year old volunteer for demonstrating age-related non-specific white-matter lesions. As can be observed, arrows mark several of these lesions. The adjacent 1-mm thick 3D images, as shown in FIGS. 4B-4D, correspond to the single 3-mm thick 2D image in FIG. 4A. In the 3D images, the phase-encoding direction corresponding to the 160-echo train is left-to-right in FIGS. 4B-4D and 4F. No image artifacts secondary to this very long spin-echo train are apparent. Pulse sequence parameters for the 10 minute 3D acquisition included the following: repetition time/effective echo time, 2750/328 ms; matrix, 256×160×216; field of view, 25.6×16.0×21.6 cm; voxel size, 1.0×1.0×1.0 mm; echo spacing, 4.1 ms; echo train length, 160; and full-Fourier acquisition. Pulse sequence parameters for the 14.8 minute 2D acquisition included the following: repetition time/first echo time/second echo time, 2750/20/80 ms; matrix, 256×160; field of view, 25.6×16.0 cm; section thickness, 3.0 mm; number of sections, 54; full-Fourier acquisition; first-order flow compensation; and reduced bandwidth on second echo.

In summary, using the variable-flip-angle series of FIG. 3, the T2-weighted 3D images were obtained at 1.5 Tesla from the brain of a healthy volunteer, and were compared to images from a 2D conventional-SE pulse sequence (see FIG. 4). The images in FIG. 4 exhibit two important features: (1) the very long spin-echo-train images (FIGS. 4B-4F) display high contrast between the age-related lesions in the brain of this volunteer and surrounding normal appearing white matter, indicating that this echo train shall provide clinically useful contrast characteristics that appear very similar to those for conventional T2-weighted SE images (FIG. 4A); and (2) the thin 1-mm sections provide an improved definition of lesion location and extent; the lesions seen in the 2D image appear, to varying degrees, in three adjacent 1-mm sections. Furthermore, the overall image quality for the very long spin-echo-train and conventional-SE images is similar, despite the much thinner sections of the former.

Finally, referring to FIGS. 4B-4F, such figures depict the largest lesion in sagittal and coronal orientations, respectively, this demonstrates the capability of the 3D acquisition to provide high-quality images in arbitrary orientations.

Example No. 2

Next, referring to FIGS. 5A-5C, using the same pulse-sequence parameters as described above in FIGS. 2-4, T2-weighted images were also obtained at 3 Tesla from the brain of a healthy volunteer. The three 1-mm thick images were all reconstructed from the same 3D acquisition. These images appeared similar to those obtained at 1.5 Tesla, but exhibited higher signal-to-noise ratios. Of particular importance, the partial-body and local values for the specific absorption rate (SAR) were 1.29 W/kg and 3.16 W/kg, respectively, compared to the FDA limits for partial-body and local SAR of 3.0 and 8.0 W/kg, respectively. The SAR values at 3 Tesla were much less than the FDA limits, indicating that there remains substantial latitude in the pulse-sequence design from the perspective of power deposition, including the possibility for even more refocusing RF pulses per excitation. Thus, according to the present invention, although the use of spin-echo-train methods has been restricted at high fields, such as 3 Tesla, due to power deposition limits, very long spin-echo trains based on prescribed signal evolutions permit high-quality brain images to be acquired at 3 Tesla with power deposition well below the FDA limits.

Example No. 3

Referring to FIG. 6, as the final example, FIG. 6 shows a T2-weighted sagittal image of the cervical spinal cord obtained at 1.5 Tesla from a healthy volunteer, again using a 160-echo train. The quality of cervical-spine images from T2-weighted MRI techniques is often compromised by artifacts arising from the pulsatile motion of the CSF surrounding the cord. One potential solution to this problem is to use FLAIR imaging. See Hajnal et al. While this technique can completely suppress the signal from CSF, there remains some concern about its ability to depict the full range of clinically-relevant lesions. See Hittmair et al. and Keiper et al. As illustrated in FIG. 6, an alternative is to use a T2-weighted technique with a very long spin-echo train based on a prescribed signal evolution, as provided by the present invention. The signal from CSF is uniformly suppressed without generating motion artifacts. The combination of the long echo train and the relatively low flip angles of the refocusing RF pulses results in suppression of even slowly moving fluid. The “dark-CSF” image in FIG. 6 differs from a FLAIR image, among other things, in an important way. With FLAIR, the CSF is suppressed based on its long T1. Hence, the signals from any other tissues with relatively long T1s will be diminished. This is one potential explanation for the problems in depicting certain lesions with FLAIR—these lesions may have long T1 components. In contrast, turning to the present invention, the CSF suppressed in FIG. 6 is solely due to its motion; long T1 lesions in the cord will be unaffected.

An advantage of the present invention is that it provides a method, apparatus, and computer useable medium (readable media) to extend the usable duration of the echo train in magnetic resonance imaging pulse sequences such as RARE, turbo-spin-echo, fast-spin-echo or GRASE, substantially beyond that obtainable with conventional methods. This increase in the echo-train duration can be used to decrease the image acquisition time and/or increase the spatial resolution. The power deposition achieved with this technique is much less than that for conventional spin-echo-train pulse sequences, and thus the invention shall be especially useful, among other things, for human imaging applications at high magnetic field strengths.

Another advantage of the present invention is that it improves the imaging of various objects and zones, including the brain. The present invention is also applicable to other regions of the body such as the spinal cord or joints. In particular, the present invention enables high-resolution 3D imaging of the brain with clinically-reasonable acquisition times, which is useful for quantitative imaging of disseminated diseases such as multiple sclerosis. For these diseases, high-resolution 3D imaging provides a valuable tool for monitoring disease progression and response to therapy during treatment or drug trials. The present invention is also useful for non-human applications of magnetic resonance, such as imaging of materials (e.g., plants or food products) or animal models of disease at high field.

Further yet, an advantage of the present invention is that it provides a means to shorten image acquisition times and/or increase spatial resolution for widely-used spin-echo-train magnetic resonance imaging techniques. Such improvements will in turn make it feasible to obtain images with certain valuable combinations of resolution and image contrast which have not been practical heretofore. In addition, the present invention permits spin-echo-train methods to be used for high-field imaging that would not otherwise meet the safety guidelines established by the Food and Drug Administration for power deposition in human MRI.

Finally, another advantage of the present invention method and apparatus is that it explicitly considers the T1 and T2 relaxation times for the tissues of interest and thereby permits the desired image contrast to be incorporated into the tissue signal evolutions corresponding to the long echo train. Given the considerable role that spin-echo-train methods already play in MR imaging, the present invention methodology will be of significant importance.

All US patents and US patent applications cited herein are incorporated herein by reference in their entirety and for all purposes to the same extent as if each individual patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced herein.

Claims

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 T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with varying flip angles, said method comprising:

calculating varying flip angles for refocusing radio-frequency pulses of an echo train of a pulse sequence that is generated, the varying flip angles providing a prescribed signal evolution;

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 at least the following defining characteristics:

(i) the calculated varying flip angles, wherein the varying flip angles vary among a majority of the refocusing radio-frequency pulses by decreasing to a minimum value and later increasing,

(ii) an echo-train duration that is longer than the longest typical echo-train duration for a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees, and

(iii) an effective echo time providing images with T2-weighted contrast, 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

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, for subsequent reconstruction of magnetic resonance images,

wherein the reconstructed magnetic resonance images have T2-weighted contrast that 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 varying flip angles decrease from an initial value of no more than approximately 180 degrees.

49. The method of claim 47, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that do not correspond to a specific material or biological tissue.

50. The method of claim 47, wherein the effective echo time is 328 ms. and the echo-train duration is 656 ms.

51. 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 computer system that performs calculations and is configured to calculate varying flip angles for refocusing radio-frequency pulses of an echo train of a pulse sequence that is generated, the varying flip angles providing a prescribed signal evolution;

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 images of the subject from the received magnetic resonance signals; 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 at least the following defining characteristics:

(i) the calculated varying flip angles, wherein the varying flip angles vary among a majority of the refocusing radio-frequency pulses by decreasing to a minimum value and later increasing,

(ii) an echo-train duration that is longer than the longest typical echo- train duration for a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees, and

(iii) an effective echo time providing images with T2-weighted contrast, 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

wherein the reconstructed magnetic resonance images have T2-weighted contrast that 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.

52. The MRI apparatus of claim 51 wherein the varying flip angles decrease from an initial value of no more than approximately 180 degrees.

53. The MRI apparatus of claim 51, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that do not correspond to a specific material or biological tissue.

54. The MRI apparatus of claim 51, wherein the effective echo time is 328 ms. and the echo-train duration is 656 ms.

55. 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 calculating varying flip angles for refocusing radio-frequency pulses of an echo train of a pulse sequence that is generated, the varying flip angles providing a prescribed signal evolution;

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 at least the following defining characteristics:

(i) the calculated varying flip angles, wherein the varying flip angles vary among a majority of the refocusing radio-frequency pulses by decreasing to a minimum value and later increasing,

(ii) an echo-train duration that is longer than the longest typical echo-train duration for a T2-weighted fast-spin-echo or turbo-spin-echo pulse sequence having refocusing radio-frequency pulses with constant flip angles of approximately 180 degrees, and

(iii) an effective echo time providing images with T2-weighted contrast, 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 T2-weighted contrast that 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.

56. The non-transitory computer readable medium having computer program logic as defined in claim 55, wherein the varying flip angles decrease from an initial value of no more than approximately 180 degrees.

57. The non-transitory computer readable medium having computer program logic as defined in claim 55, wherein the prescribed signal evolution is associated with T1 and T2 relaxation times that do not correspond to a specific material or biological tissue.

58. The non-transitory computer readable medium having computer program logic as defined in claim 56, wherein, wherein the effective echo time is 328 ms. and the echo-train duration is 656 ms.