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Method and apparatus for spin-echo-train MR imaging using prescribed signal evolutions
RE44644 Method and apparatus for spin-echo-train MR imaging using prescribed signal evolutions
Patent Drawings:

Inventor: Mugler, III, et al.
Date Issued: December 17, 2013
Application:
Filed:
Inventors:
Assignee:
Primary Examiner: Koval; Melissa
Assistant Examiner: Fetzner; Tiffany
Attorney Or Agent: Decker; Robert J.
U.S. Class: 324/307; 324/309; 324/314; 324/318; 600/410; 600/413; 600/425; 600/428
Field Of Search: ;324/300; ;324/301; ;324/302; ;324/303; ;324/304; ;324/305; ;324/306; ;324/307; ;324/308; ;324/309; ;324/310; ;324/311; ;324/312; ;324/313; ;324/314; ;324/315; ;324/316; ;324/317; ;324/318; ;324/319; ;324/320; ;324/321; ;324/322; ;600/407; ;600/408; ;600/409; ;600/410; ;600/411; ;600/412; ;600/413; ;600/414; ;600/415; ;600/416; ;600/417; ;600/418; ;600/419; ;600/420; ;600/421; ;600/422; ;600/423; ;600/424; ;600/425; ;600/426; ;600/427; ;600/428; ;600/429; ;600/430; ;600/431; ;600/432; ;600/433; ;600/434; ;600/435; ;382/128; ;382/129; ;382/130; ;382/131
International Class: G01V 3/00; G01R 33/50; A61B 5/055
U.S Patent Documents:
Foreign Patent Documents:
Other References: Hennig, J., "RARE Imaging: A Fast Imaging Method for Clinical MR", Magnetic Resonance in Medicine, 1986, vol. 3, p. 823-833. cited byapplicant.
Mulkern, R.V., "Contrast Manipulation and Artifact Assessment of 2D and 3D Rare Sequences", Magnetic Resonance Imaging, 1990, vol. 8, p. 557-566. cited by applicant.
Melki, P.S., "Partial RF Echo Planar Imaging with the Faise Method. I. Experimental and Theoretical Assessment of Artifact", Magnetic Resonance in Medicine, 1992, vol. 26, p. 328-341. cited by applicant.
Jones, K.M., "Fast Spin-Echo MR Imaging of the Brain and Spine: Current Concepts", AJR 1992, 158, p. 1313-1320. cited by applicant.
Feinberg, D.A., "Grase (Gradient--and Spin-Echo) MR Imaging: A New Fast Clinical Imaging Technique", Radiology, 1991, vol. 181, No. 2, p. 597-602. cited by applicant.
Oshio, K., "GRASE (Gradient--and Spin-Echo) Imaging: A Novel Fast MRI Technique", Magnetic Resonance in Medicine, 1991, vol. 20, p. 344-349. cited by applicant.
Hajnal, J.V., "Use of Fluid Attenuated Inversion Recovery (FLAIR) Pulse Sequences in MRI of the Brain", Journal of Computer Assisted Tomography, 1992, vol. 16, No. 6, p. 841-844. cited by applicant.
Constable, R.T., "The Loss of Small Objects in Variable TE Imaging: Implications for FSE, RARE, and EPI", Magnetic Resonance in Medicine, 1992, vol. 28, p. 9-24. cited by applicant.
Ortendahl, D.A., "Analysis of Hybrid Imaging Techniques", Magnetic Resonance in Medicine, 1992, vol. 26, p. 155-173. cited by applicant.
Hennig, J., "Multiecho Imaging Sequences with Low Refocusing Flip Angles", Journal of Magnetic Resonance, 1988, vol. 78, p. 397-407. cited by applicant.
Alsop, D.C., "The Sensitivity of Low Flip Angle Rare Imaging", Magnetic Resonance in Medicine, 1997, vol. 37, p. 176-184. cited by applicant.
Hittmair, K., "Spinal Cord Lesions in Patients with Multiple Sclerosis: Comparison of MR Pulse Sequences", American Journal of Neuroradiology, 1996, vol. 17, p. 1555-1565. cited by applicant.
Keiper, M.D., "The Low Sensitivity of Fluid-Attenuated Inversion-Recovery MR in the Detection of Multiple Sclerosis of the Spinal Cord", American Journal of Neuroradiology, 1997, vol. 18, p. 1035-1039. cited by applicant.
Haacke, E.M., "Magnetic Resonance Imaging: Physical Principles and Sequence Design", John Wiley & Sons, New York, 1999, p. 51-64. cited by applicant.
Forsythe, G.E., "Computer Methods for Mathematical Computations", Prentice-Hall, Englewood Cliffs, N. J., 1977, p. 156-177. cited by applicant.
Le Roux, P., "Stabilization of Echo Amplitudes in FSE Sequences", Magnetic Resonance in Medicine, 1993, vol. 30, p. 183-190. cited by applicant.
Hennig, J., Medicine, "Easy Improvement of Signal-to-Noise in RARE Sequences with Low Refocusing Flip Angles", Magnetic Resonance in Medicine, 2000, vol. 44, p. 983-985. cited by applicant.
Hennig, J., "Hyperechoes", Magnetic Resonance in Medicine, 2001, vol. 46, p. 6-12. cited by applicant.
Hennig, J., "Multiecho Sequences with Variable Refocusing Flip Angles: Optimization of Signal Behavior Using Smooth Transitions Between Pseudo Steady States (TRAPS)", Magnetic Resonance in Medicine, 2003, vol. 49, p. 527-535. cited by applicant.
Glover, G.H., "Reduction of Non-Equilibrium Effects in Rare Sequences", Works in Progress Book of Abstracts, Tenth Annual Scientific Meeting and Exhibition, Society of Magnetic Resonance in Medicine, 1991, San Francisco. p. 1242. cited by applicant.
Schaffter, T., "PSF Improvements in Single Shot GRASE Imaging", Proceedings of the Society of Magnetic Resonance, 1994. Second Meeting, San Francisco. p. 27. cited by applicant.
Mugler, III, J.P., "T1-Weighted and T2-Weighted 3D Spin-Echo-Based Imaging of the Whole Brain: 1 mm3 Resolution in Under 10 Minutes", Proceedings of the International Society for Magnetic Resonance in Medicine, 1998. Sixth Meeting, Sydney. p. 1959.cited by applicant.
Mugler, III, J.P., "Single-Slab Three-Dimensional FLAIR Imaging of the Brain", Proceedings of the International Society for Magnetic Resonance in Medicine, 1999. Seventh Meeting, Philadelphia. p. 8. cited by applicant.
Mugler, III, J.P., "Three-Dimensional Spin-Echo-Train Proton-Density-Weighted Imaging Using Shaped Signal Evolutions", Proceedings of the International Society for Magnetic Resonance in Medicine, 1999. Seventh Meeting, Philadelphia. p. 1631. citedby applicant.
Mugler, III, J.P., "Motion-Artifact-Free T2-Weighted 3D Imaging of the Cervical Spine", Proceedings of the International Society for Magnetic Resonance in Medicine, 2000. Eighth Meeting, Denver. p. 402. cited by applicant.
Mugler, III, J.P., "Three-Dimensional T2-Weighted Imaging of the Brain Using Very Long Spin-Echo Trains", Proceedings of the International Society for Magnetic Resonance in Medicine, 2000. Eighth Meeting, Denver. p. 687. cited by applicant.
Hennig, J., "Hyperechoes--Basic Principles and Applications", Proceedings of the International Society for Magnetic Resonance in Medicine, 2001. Ninth Meeting, Glasgow. p. 437. cited by applicant.
Mugler, III, J.P., "T2-Weighted 3D Spin-Echo Train Imaging of the Brain at 3 Tesla: Reduced Power Deposition Using Low Flip-Angle Refocusing RF Pulses", Proceedings of the International Society for Magnetic Resonance in Medicine, 2001. NinthMeeting, Glasgow. p. 438. cited by applicant.
Hennig, J., "Hyperechoes in RARE(TSE, FSE )--Sequences", Proceedings of the International Society for Magnetic Resonance in Medicine, 2001. Ninth Meeting, Glasgow. p. 1769. cited by applicant.
Busse, R.F., "Reducing SAR in Real-Time SSFSE Imaging with Variable-Flip Hard Refocusing RF Pulses", Proceedings of the International Society for Magnetic Resonance in edicine, 2001. Ninth Meeting, Glasgow. p. 1790. cited by applicant.
Elster, et al., "Questions and Answers in Magnetic Resonance Imaging", 2001, 2 ed., p. 102-103. cited by applicant.
Kallmes, et al., "Suppression of Cerebrospinal Fluid and Blood Flow Artifacts in Flair MR Imaging with a SingleSlab Three-dimensional Pulse Sequence: Initial Experience", Radiology 2001, vol. 221, No. 1, p. 221-255. cited by applicant.
Mugler III, et al., "Optimized Single-Slab Three-dimensional Spin-Echo MR Imaging of the Brain", Radiology 2000, vol. 216, No. 3, p. 891-899. cited by applicant.
Tkach et al., article "A comparison of fast spin echo and gradient field echo sequences" Magnetic Resonance Imaging (Jul.-Aug. 1988) vol. 6, No. 4, p. 373-89. cited by examiner.









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.
Claim:
Description: FIELD OF INVENTION

The present invention relates to a pulse sequence for use in operating a magnetic resonance imaging apparatus, and in particular for lengthening the usable echo-train duration and reducing the power deposition for spin-echo-train magneticresonance imaging.

BACKGROUND OF INVENTION

Over the past twenty years, nuclear magnetic resonance imaging (MRI) has developed into an important modality for both clinical and basic-science imaging applications. A large portion of MRI techniques are based on spin-echo (SE) acquisitionsbecause they provide a wide range of useful image contrast properties that highlight pathological changes and are resistant to image artifacts from a variety of sources such as radio-frequency or static-field inhomogeneities.

Spin-echo-based methods can be subdivided into two categories, including those that generate one spin echo for each desired image contrast following each excitation radio-frequency (RF) pulse, and those that generate more than one spin echo foreach desired image contrast following each excitation RF pulse. The first category includes, but is not limited thereto, the techniques commonly referred to as "conventional SE" imaging. The second category includes, but is not limited thereto, amethod called "RARE" (See Hennig J., Nauerth A., Friedburg H., "RARE Imaging: A Fast Imaging Method for Clinical MR", Magn. Reson. Med. 1986, 3:823-833; and Mulkern R. V., Wong S. T. S., Winalski C., Jolesz F. A., "Contrast Manipulation and ArtifactAssessment of 2D and 3D RARE Sequences", Magn. Reson. Imaging 1990, 8:557-566, of which are hereby incorporated by reference in their entirety) and its derivatives, commonly referred to as "turbo-SE" or "fast-SE" imaging (See Melki P. S., Jolesz F. A.,Mulkern R. V., "Partial RF Echo Planar Imaging with the FAISE Method. I Experimental and Theoretical Assessment of Artifact", Magn. Reson. Med. 1992, 26:328-341 and Jones K. M., Mulkern R. V., Schwartz R. B., Oshio K., Barnes P. D., Jolesz F. A.,"Fast Spin-Echo MR Imaging of the Brain and Spine: Current Concepts", AJR 1992, 158:1313-1320, of which are hereby incorporated by reference in their entirety). For the purposes of this disclosure, we are primarily interested in the generalized form oftechniques in the second category; however the present invention is applicable to the first category as well. The term "generalized" refers to the form of the spatial-encoding gradients that are applied following any given refocusing RF pulse. Forexample, RARE imaging encodes one line of spatial-frequency space (k-space) data following each refocusing RF pulse using a constant, frequency-encoding magnetic field gradient. In contrast, "GRASE" imaging (See Feinberg D. A., Oshio K. "GRASE(Gradient- And Spin-Echo) MR Imaging: A New Fast Clinical Imaging Technique", Radiology 1991, 181: 597-602; and Oshio K., Feinberg D. A. "GRASE (Gradient- And Spin-Echo) Imaging: A Novel Fast MRI Technique", Magn. Reson. Med. 1991, 20:344-349, ofwhich are hereby incorporated by reference in their entirety) encodes three or more lines of k-space data following each refocusing RF pulse using an oscillating, frequency-encoding gradient waveform. This oscillating gradient waveform collects one lineof k-space data that includes the spin echo, and one or more additional lines of k-space data before the spin echo and after the spin echo. One skilled in the art would appreciate that there exist an infinite number of possibilities for spatiallyencoding the MR signal following each refocusing RF pulse. For the purpose of this disclosure, we define the term "spin-echo-train" imaging to encompass all of these possibilities, including, but not limited thereto, RARE, turbo-SE, fast-SE and GRASEimaging, because the present invention deals with, among other things, the RF-pulse history during the echo train, not the details of the spatial encoding.

In general, one of the major goals of technique development for MRI has been to increase the amount of k-space data sampled per unit time, under the constraints of obtaining the desired image contrast and maintaining image artifacts at atolerable level. Increases in the data rate are typically traded for a decrease in the image acquisition time and/or an increase in the spatial resolution. In this respect, spin-echo-train methods have played an important role; fast-SE imaging isroutinely and widely used in clinical MRI.

For instance, the echo trains used in clinical fast-SE imaging generally employ high flip angles (>100.degree.) for the refocusing RF pulses, and their durations are typically less than the T2 relaxation times of interest for short effectiveecho times (e.g., T1 or proton-density weighting.Iadd.; effective echo time is the time period from the excitation RF pulse to the collection of data corresponding to substantially zero-spatial frequency (the center of k space.Iaddend.) or less than twoto three times these T2 values for long effective echo times (e.g., T2 weighting or "FLAIR"; see Hajnal J. V., Bryant D. J., Kasuboski L., Pattany P. M., De Coene B., Lewis P. D., Pennock J. M., Oatridge A., Young I. R., Bydder G. M., "Use of FluidAttenuated Inversion Recovery (FLAIR) Pulse Sequences in MRI of the Brain", J. Comput. Assist. Tomogr. 1992, 16:841-844, of which is hereby incorporated by reference in its entirety). For example, considering brain imaging at 1.5 Tesla, these limitstranslate to echo-train durations of <100 ms and <300 ms for short and long effective echo times, respectively. When high flip angles are used for the refocusing RF pulses, echo-train durations that are longer than these limits can substantiallydegrade image contrast and introduce artifacts such as blurring (See Mulkern et al.; Melki et al.; Constable R. T., Gore J. C., "The Loss of Small Objects in Variable TE Imaging: Implications for FSE, RARE and EPI", Magn. Reson. Med. 1992, 28:9-24;and Ortendahl D. A., Kaufman L., Kramer D. M., "Analysis of Hybrid Imaging Techniques", Magn. Reson. Med. 1992, 26:155-173, of which are hereby incorporated by reference in their entirety).

Nonetheless, if it were possible to substantially lengthen echo-train durations beyond these limits, while achieving the desired image contrast and limiting artifacts, it would represent a useful and widely applicable advance.

Preliminary studies with the goal of lengthening the echo-train duration in spin-echo-train-based acquisitions have been performed by other researchers. Over a decade ago, Hennig (See Hennig J., "Multiecho Imaging Sequences with Low RefocusingFlip Angles", J. Magn. Reson. 1988, 78:397-407, of which is hereby incorporated by reference in its entirety) proposed the use of constant, low-flip-angle refocusing RF pulses to introduce a T1 dependence to the evolution of the echo train and therebylengthen its usable duration. More recently, this concept was extended by Alsop, who derived variable flip-angle series based on the "pseudosteady-state" condition of a constant signal level when T1 and T2 relaxation are neglected (See Alsop D. C., "TheSensitivity of Low Flip Angle RARE Imaging", Magn. Reson. Med. 1997; 37:176-184, of which is hereby incorporated by reference in its entirety). Alsop also found that the echo-train performance was improved by using a signal evolution that decreasedfor the first few echoes and was then constant, instead of being constant for the complete echo train. Using these evolutions, artifact-free human brain images with T2-weighting were acquired by Alsop. An 80-echo train with a duration of 400 ms andasymptotic flip angles ranging from 17.degree. to 90.degree. were used.

Turning to the present invention, a method and related apparatus is provided for lengthening the usable echo-train duration for spin-echo-train imaging substantially beyond that achievable with the constant, low-flip-angle or pseudosteady-stateapproaches. The present invention method and apparatus explicitly consider the T1 and T2 relaxation times for the tissues of interest and thereby permit the desired image contrast to be incorporated into the tissue signal evolutions corresponding to thelong 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.

SUMMARY OF THE INVENTION

This present invention comprises the methodology, related apparatus, and .Iadd.non-transitory .Iaddend.computer useable medium (readable media) for using a series of refocusing RF pulses with variable flip angles and, optionally, variable phaseangles, in a spin-echo-train MRI pulse sequence wherein the flip-angle series is specifically designed to achieve a prescribed signal evolution during the echo train for selected T1 and T2 relaxation times. By employing such a series of refocusing RFpulses, the usable duration of the echo train can be extended 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 spatialresolution.

In one aspect, the present invention features a method for generating a pulse sequence for operating a magnetic resonance imaging apparatus for imaging an object, the method comprising: a) providing contrast-preparation, the contrast-preparationcomprising 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 the 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 the calculation provides desired prescribed signal evolution and desired overall signal level, the 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 the refocusing radio-frequencypulses; and ii) selecting characteristics of the contrast-preparation step, the 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 the data acquisition step based on a spin echo train acquisition, the 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 havinga flip angle and phase as determined by the calculation step; and iii) magnetic-field gradient pulses that encode spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of the refocusingradio-frequency pulses; d) providing magnetization-recovery, the 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 beensampled.

In a second aspect, the present invention features a magnetic resonance imaging apparatus for generating a pulse sequence for operating the apparatus for imaging an object, the apparatus comprising a main magnet system for generating a steadymagnetic field; a gradient magnet system for generating temporary gradient magnetic fields; a radio-frequency transmitter system for generating radio-frequency pulses; a radio-frequency receiver system for receiving magnetic resonance signals; areconstruction unit for reconstructing an image of the object from the received magnetic resonance signals; and a control unit for generating signals controlling the gradient magnet system, the radio-frequency transmitter system, the radio-frequencyreceiver system, and the reconstruction unit, wherein the control unit generates signals causing: a) providing contrast-preparation, the contrast-preparation comprising generating at least one of at least one radio-frequency pulse, at least onemagnetic-field gradient pulse, and at least one time delay, whereby the 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 appliedin a data-acquisition step, wherein the calculation provides desired prescribed signal evolution and desired overall signal level, the calculation comprises: i) selecting values of T1 and T2 relaxation times and selecting proton density; ii) selecting aprescribed time course of the amplitudes and phases of the radio-frequency magnetic resonance signals that are generated by the refocusing radio-frequency pulses; and ii) selecting characteristics of the contrast-preparation step, the data-acquisitionstep 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 the data acquisition step based on a spin echo train acquisition, thedata-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 the calculation step, and iii) magnetic-fieldgradient pulses that encode spatial information into at least one of the radio-frequency magnetic resonance signals that follow at least one of the refocusing radio-frequency pulses; d) providing magnetization-recovery, the magnetization-recoverycomprises 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.

In a third aspect, the present invention features a .Iadd.non-transitory .Iaddend.computer readable media carrying encoded program instructions for causing a programmable magnetic resonance imaging apparatus to perform the method discussed abovein the first aspect of the invention. Similarly, the invention features a .Iadd.non-transitory .Iaddend.computer program product comprising a .Iadd.non-transitory .Iaddend.computer useable medium having .Iadd.non-transitory .Iaddend.computer programlogic for enabling at least one processor in a magnetic resonance imaging apparatus to generate a pulse sequence, the .Iadd.non-transitory .Iaddend.computer program logic comprising the method discussed above in the first aspect of the invention.

Because the flip angles for the refocusing RF pulses that are derived with this method are typically much less than 180.degree. for a substantial portion of the total number of RF pulses, the power deposition is much less than thatcorresponding to 180.degree. RF pulses, which are commonly used in conventional spin-echo-train pulse sequences. This feature is particularly important for high field MRI (>1.5 Tesla), wherein power deposition is a critical pulse-sequence designfactor for human applications. The present invention permits long, closely-spaced echo trains 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 inhuman MRI.

Another potentially useful feature of the present invention is that, for specific forms of the encoding-gradient waveforms, signals from moving or flowing materials are strongly attenuated, even when the velocities are relatively low. Aspecific example of this behavior is the attenuation of the signal from cerebrospinal fluid (CSF) surrounding the cervical spinal cord due to its oscillatory motion, which can be used to generate CSF-suppressed T2-weighted MR images of the spinal cordwithout requiring inversion-nulling of the CSF signal. Studies have indicated that the full range of clinically-relevant cord lesions may not be adequately detected using inversion-nulling of the CSF signal (i.e., FLAIR) (See Hittmair K., Mallek R.,Prayer D., Schindler E. G., Kollegger H., "Spinal Cord Lesions in Patients with Multiple Sclerosis: Comparison of MR Pulse Sequences", AJNR 1996, 17:1555-1565; and Keiper M. D., Grossman R. I., Brunson J. C., Schnall M. D., "The Low Sensitivity ofFluid-Attenuated Inversion-Recovery MR in the Detection of Multiple Sclerosis of the Spinal Cord", AJNR 1997, 18:1035-1039, of which are hereby incorporated by reference in their entirety).

These and other objects, along with advantages and features of the invention disclosed herein, will be made more apparent from the description, drawings and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the present invention, as well as the invention itself, will be more fully understood from the following description of preferred embodiments, when read together with the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of a general spin-echo-train MRI pulse sequence. This is an exemplary type of MRI pulse sequence to which the invention applies. The present invention method can be applied to various types of pulsesequences.

FIG. 2 shows an example of a prescribed signal evolution that can be used to generate T2-weighted MR images of the brain or spine.

FIG. 3 shows the variable-flip-angle series corresponding to FIG. 2 that was derived using the present invention methods as described herein.

FIGS. .[.4-6.]. .Iadd.4A-4F, 5A-5C, and 6 .Iaddend.show example MR images obtained using the variable-flip-angle series of FIG. 3 in a "turbo-SE" type spin-echo-train pulse sequence; collectively, FIGS. .[.4-6.]. .Iadd.4A-6 .Iaddend.provideexamples of the potential utility of the present invention. In particular, showing brain images obtained at 1.5 Tesla, FIGS. 4(A) and 4(B)-4(C) compare T2-weighted two-dimensional and three-dimensional SE images, respectively. Further, showing brainimages obtained at 3 Tesla, FIGS. 5(A)-5(C) show T2-weighted sagittal, coronal, and axial images, respectively, reconstructed from the same three-dimensional acquisition. Finally, FIG. 6 shows a sagittal image of the cervical spinal cord obtained at 1.5Tesla.

FIG. 7 illustrates a simplified exemplary embodiment of a MRI apparatus for practicing the present invention. The present invention method can be applied to various commercially available MRI apparatuses.

FIG. 8 is an exemplary flowchart for a simplified preferred implementation of the methods of the present invention.

FIGS. 9A-9B is an exemplary flowchart for a simplified preferred implementation of the calculation methods of the present invention, step 200.

FIG. 10 is an exemplary flowchart for a simplified preferred implementation of the contrast-preparation methods of the present invention.

FIG. 11 is an exemplary flowchart for a simplified preferred implementation of the data-acquisition methods of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, first presented is an exemplary embodiment of a MR apparatus for practicing the MR methods of the present invention for imaging an object, moving or stationary. Following are descriptions of preferred and alternativeembodiments of the methods of the present invention, including their exemplary implementation as .Iadd.non-transitory .Iaddend.computer hardware, firmware, and/or software.

An Exemplary MR-Apparatus of the Present Invention

FIG. 7 illustrates a simplified schematic of a MR apparatus 1 for practicing the present invention. The MR apparatus 1 includes a main magnet system 2 for generating a steady magnetic field in an examination zone(s) of the MR apparatus. Thez-direction of the coordinate system illustrated corresponds to the direction of the steady magnetic field generated by the magnet system 2.

The MR apparatus also includes a gradient magnet system 3 for generating temporary magnetic fields G.sub.x, G.sub.y and G.sub.z directed in the z-direction but having gradients in the x, y or z directions, respectively. With this magneticgradient system, magnetic-field gradients can also be generated that do not have directions coinciding with the main directions of the above coordinate system, but that can be inclined thereto, as is known in the art. Accordingly, the present inventionis not limited to directions fixed with respect to the MR apparatus. In this application, for ease of description, the directions x, y and z (and the gradients along these directions) are used for the read direction, the phase-encode direction andslice-selection direction (or second phase-encode direction for 3D imaging), respectively.

Also, while traditional commercial methods provide linear gradients in the x, y, or z directions it is also possible not to utilize all three of these linear gradients. For example, rather than using a linear z gradient, one skilled in the artcan use a z-squared dependence or some other spatial dependence to provide desired results.

The magnet systems 2 and 3 enclose an examination zone(s) which is large enough to accommodate a part of an object 7 to be examined, for example a part of a human patient. A power supply means 4 feed the gradient magnet system 3.

The MR apparatus also includes an RF transmitter system including RF transmitter coil 5, which generates RF pulses in the examination zone and is connected via transmitter/receiver circuit 9 to a RF source and modulator 6. The RF transmittercoil 5 is arranged around the part of body 7 in the examination zone. The MR apparatus also comprises an RF receiver system including an RF receiver coil which is connected via transmitter/receiver circuit 9 to signal amplification and demodulation unit10. The receiver coil and the RF transmitter coil 5 may be one and the same coil. The MR apparatus also includes an amplification and demodulation unit 10, which, after excitation of nuclear spins in a part of the body placed within the examinationspace by RF pulses, after encoding by the magnetic-field gradients and after reception of the resulting MR signals by the receiver coil, derives sampled phases and amplitudes from the received MR signals. An image reconstruction unit 12 processes thereceived MR imaging signals to, inter alia, reconstruct an image by methods well-known in the art, such as by Fourier transformation. It should be appreciated by one skilled in the art that various reconstruction methods may be employed besides theFourier Transform (FT) depending on factors such as the type of signal being analyzed, the available processing capability, etc. For example, but not limited thereto, the present invention may employ Short-Time FT (STFT), Discrete Cosine Transforms(DCT), or wavelet transforms (WT). By means of an image processing unit 13, the reconstructed image is displayed, for example, on monitor 14. Further, the image reconstruction unit can optionally process MR navigator signals to determine thedisplacement of a portion of the patient.

The MR apparatus also includes a control unit 11 that generates signals for controlling the RF transmitter and receiver systems by means of a modulator 6, the gradient magnetic field system by means of the power supply means 4, an imagereconstruction unit 12 and an image processing unit 13. In a preferred embodiment, the control unit 11 (and other control elements in the MR apparatus) are implemented with .Iadd.non-transitory .Iaddend.programmable elements, such as one or moreprogrammable signal processors or microprocessors, communicating over busses with supporting RAM, ROM, EPROM, EEPROM, analog signal interfaces, control interfaces, interface to .Iadd.non-transitory .Iaddend.computer-readable media and so forth. Theseprogrammable elements are commanded by software or firmware modules loaded into RAM, EPROM, EEPROM or ROM, written according to well-known methods to perform the real-time processing required herein, and loaded from .Iadd.non-transitory.Iaddend.computer-readable media (or .Iadd.non-transitory .Iaddend.computer useable medium), such as magnetic disks or tapes, or optical disks, or network interconnections, removable storage drives, or so forth. The present invention may be implementedusing hardware, software or a combination thereof and may be implemented in one or more .Iadd.non-transitory .Iaddend.computer systems or processing systems, such as personal digit assistants (PDAs), for various applications, e.g., remote care andportable care practices.

In a less preferred embodiment, the control unit that directs a MR apparatus for practicing the present invention can be implemented with dedicated electronic components in fixed circuit arrangements. In this case, these dedicated componentsare arranged to carry out the method described above. For example, the invention is implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits(ASICs). Implementation of the hardware statemachine to perform the functions described herein will be apparent to persons skilled in the relevant art(s).

In particular, the control unit commanded by its loaded software causes the generation of MR signals by controlling the application of MR pulse sequences, which comprise RF-pulses, time delays and temporary magnetic-field gradient pulses. Thesepulse sequences are generated according to the methods of the present invention as subsequently described, and generally include 2D and 3D imaging pulse sequences and optionally navigator pulse sequences for determining the displacement of the patient ormaterial.

Furthermore, according to alternate embodiments of the present invention, the MR apparatus also optionally includes various other units (not illustrated) from which the state of motion of the part of the patient being imaged can be measured. These can include sensors directly indicating the instantaneous state of motion of the part of the patient being imaged, such as a chest belt for directly indicating chest displacement during respiration, or MR-active micro-coils whose position can betracked, or optical means, or ultrasound means, or so forth. These units can also include sensors indirectly indicating the instantaneous state of motion of the part of the patient being imaged. For example, electrocardiogram and peripheral pulsesensors measure the temporal progress of the cardiac cycle, and permit inference of the actual state of motion of the heart from knowledge of cardiac displacements associated with each phase of the cardiac cycle. When these sensors are present tomeasure the state of motion, the control unit need not generate navigator pulse sequences.

Moreover, the control unit 11 may also include a communications interface 24. The communications interface 24 allows software and data to be transferred between and among, via communication path (i.e. channel) 28, the control unit 11,reconstruction unit 12, image processing unit 13, and monitor 14 and external devices. Examples of the communications interface 24 may include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc.Software and data transferred via communications interface 24 are in the form of signals which may be electronic, electromagnetic, optical or other signals capable of being received by communications interface 24. The signals are provided tocommunications interface 24 via the communications path (i.e., channel) 26. The channel 26 carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, a RF link, IR link and other communicationschannels.

The preferred embodiments of the present invention may be implemented as .Iadd.non-transitory .Iaddend.software/firmware/hardware with various MR apparatuses, and methods, as one skilled in the art would appreciate. Other exemplary apparatusesand methods, but not limited thereto, are disclosed in the following U.S. patents, of which are hereby incorporated by reference in their entirety herein: U.S. Pat. No. 6,230,039 B1--Staber et. al.; U.S. Pat. No. 5,749,834--Hushek; and U.S. Pat. No. 5,656,776--Kanazawa.

The Methods of the Present Invention

Turning now to FIG. 1, pertaining to the general methods of this invention, first in a preferred embodiment, the present invention applies to magnetic resonance imaging MRI) using a "spin-echo-train" MRI pulse sequence, which is a pulse sequencethat generates more than one spin echo for each desired image contrast following each excitation RF pulse. Any form of the applied spatial-encoding gradient waveforms, variations in the spacing between refocusing RF pulses, and/or any combination ofnon-selective, spatially-selective, and spectrally-selective RF pulses are applicable to the present invention methods as long as their effects on the magnetization are appropriately considered in the derivation of the variable-flip-angle series. Acontrast preparation phase, such as an inversion RF pulse followed by a time delay, may precede the acquisition phase of the pulse sequence.

Still referring to FIG. 1, there is shown a schematic representation of a general spin-echo-train MRI pulse sequence. This is an exemplary type of MRI pulse sequence to which the invention applies. The representation is of a generalspin-echo-train MRI pulse sequence showing the excitation RF pulse (.alpha.) and the first three (.beta..sub.1, .beta..sub.2, .beta..sub.3) of n refocusing RF pulses, where n>1. The RF pulse waveforms are drawn as rectangular for simplicity, but theymay be amplitude and/or phase modulated as appropriate for the desired application. The echo spacing (ESP) may be fixed or may vary between echoes. The contrast preparation module 20 denotes the optional use of additional RF pulses, gradient pulsesand/or time delays (e.g., an inversion pulse followed by a time delay) to permit additional control over the image contrast. The boxes on the G.sub.select axis, referenced as 31, 32, 33 and 34, symbolically denote the optional use of magnetic-fieldgradient waveforms for spatial and/or spatial-spectral selection. The boxes on the G.sub.encode axis, referenced as 41, 42, and 43, symbolically denote the magnetic-field gradient waveforms used for spatial encoding. The contrast preparation (if any)and the echo train are repeated as necessary to collect the desired k-space data. The timing parameters and the number of echoes may vary between repetitions.

For a spin-echo-train pulse sequence, an object of the present invention is to derive a series of refocusing RF pulses with variable flip angles, and, optionally, variable phase angles, that yields a specifically prescribed signal evolutionduring the echo train for selected T1 and T2 relaxation times. To achieve this, a mathematical model of the pulse sequence, incorporating the specific timing, gradient and RF parameters of choice, is used to calculate the signal evolution during theecho train. This model would typically be implemented in the form of a .Iadd.non-transitory .Iaddend.computer program that is based on the established mathematical equations that describe the behavior of the magnetization during a pulse sequence. SeeHaacke E. M., Brown R. W., Thompson M. R., Venkatesan R., "Magnetic Resonance Imaging: Physical Principles and Sequence Design", John Wiley & Sons, New York, 1999, of which is hereby incorporated by reference in its entirety. Other exemplaryspin-echo-train MR imaging methods are disclosed in the following U.S. patents, of which are hereby incorporated by reference in their entirety herein: U.S. Pat. No. 5,680,045--Feinberg; U.S. Pat. No. 5,612,619--Feinberg; U.S. Pat. No.5,541,511--Henning; U.S. Pat. No. 5,315,249--Le Roux et al.; U.S. Pat. No. 5,270,654--Feinberg et al.; U.S. Pat. No. 4,901,020--Ladebeck et al. and U.S. Pat. No. 4,818,940--Henning et al.

Given such a computer-based calculation tool, the process for deriving this flip-angle series can be generally summarized in the following four steps (steps I-IV) briefly discussed below. Firstly, STEP I, the pulse sequence timing parameters(e.g., repetition time, echo spacing(s), other time delays), the pulse sequence magnetic-field gradient configuration, the desired shape of the prescribed signal evolution during the echo train, the T1 and T2 relaxation parameters and the proton densityfor the "target" tissue, and a target signal intensity are chosen. The signal evolution may assume any physically-realizable shape. Some examples, but not limited thereto, include: a constant; a linear decay; an exponential decay; a linear orexponential decay for the initial portion and a constant for the remainder; and a linear or exponential decay for the initial portion, a constant for the second portion and a linear or exponential decay for the remainder. The T1, T2 and proton densityfor the target tissue may equal those for a specific biological tissue (e.g., brain gray matter) or material, or they may be arbitrarily chosen. The target signal intensity is the desired signal intensity corresponding to a specific echo in the echotrain (e.g., the first or the middle echo).

Secondly, STEP II, the flip angle .beta..sub.i (see FIG. 1) which yields the desired signal intensity for the i.sup.th echo interval is determined, where i ranges from 1 to the number of refocusing RF pulses in the echo train. This flip anglecan be calculated using any appropriate method such as a "brute-force" search or interval bisection and interpolation. See Forsythe G. E., Malcolm M. A., Moler C. B., "Computer Methods for Mathematical Computations", Prentice-Hall, Englewood Cliffs,1977, of which is hereby incorporated by reference in its entirety.

Thirdly, STEP III, the pulse number i is incremented and the second step is repeated until all flip angles for a given echo train are calculated. If, for any value of i, the desired signal intensity for the i.sup.th echo interval cannot beachieved, the target signal intensity is reduced and the calculation process is restarted.

Fourthly, STEP IV, if the pulse sequence under consideration requires more than one repetition of the echo train to acquire the desired k-space data, the second and third steps are repeated as necessary until a steady state of the magnetizationis reached.

After a given series of variable flip angles are derived, the target signal intensity can be incremented until the maximum value for which the prescribed signal-evolution shape can be realized is reached, thus allowing determination of themaximum signal and/or contrast values that can be obtained for a specific pulse sequence configuration and signal evolution.

Next, exemplary .Iadd.non-transitory .Iaddend.hardware, firmware and software implementations of the methods of the present invention are discussed.

FIG. 8 illustrates a preferred method for practicing the invention as implemented by, for example, .Iadd.non-transitory .Iaddend.software loaded into the control unit of the MR apparatus. Once the process starts and initializes, at step 200,the contrast-preparation is provided by generating at least one of a RF pulse, magnetic-field gradient pulse, and/or time delay. The contrast-preparation encodes the magnetization with at least one desired image contrast.

During step 300, flip angles and phases are calculated for refocusing RF pulses that are applied in subsequent data-acquisition steps so as to yield--for selected values of T1 and T2 relaxation times and proton density--a prescribed time coursefor the amplitudes and phases of the RF magnetic resonance signals that are generated by the refocusing RF pulses.

During step 400, data-acquisition is achieved based on an echo-train acquisition, comprising the following: i) an excitation RF pulse having a flip angle and phase; ii) at least two refocusing RF pulses, each having a flip angle and phase asdetermined by the calculation step; and iii) magnetic-field gradient pulses that encode spatial information into at least one of the RF magnetic resonance signals that follow at least one of the at least two refocusing RF pulses.

Also, in step 500, magnetization-recovery is provided whereby the magnetization-recovery comprises a time delay to allow magnetization to relax. Finally, as illustrated by step 550, the aforementioned method is repeated until a predeterminedextent of spatial-frequency space has been sampled.

It is important to appreciate that the various steps discussed herein need not be performed in the illustrated order, and in fact it may be preferred to perform the steps, at least in part, simultaneously or omit some of the illustrated steps,at least in part.

Next, turning to FIGS. 9A-9B, an exemplary method for the calculating step 300 is provided. In step 310, T1, T2, and proton density are chosen, and in step 320, a desired prescribed signal evolution which describes the time course of the signalamplitudes and phases, is also chosen. Turning to steps 330 and 340, the characteristics of the contrast-preparation and data-acquisition, respectively, are chosen (additional details shall be discussed with FIGS. 10-11). Provided in step 350, thecharacteristics of the magnetization-recovery period are chosen. Accordingly, at step 360, the process of calculating the individual flip angles starts and is initialized with thermal equilibrium magnetization. At step 370, the magnetization, M.sub.1,is calculated that exists immediately after the contrast-preparation and excitation RF pulse are applied. At step 380, starting with M.sub.1 which is the input magnetization for the first refocusing RF pulse, the flip angle and phase are calculated forthe current refocusing RF pulse that yields the desired corresponding signal amplitude and phase, and this process is repeated until the last refocusing RF pulse is achieved, step 385. If the last pulse is achieved, then step 390 permits relaxationduring the magnetization-recovery period. In steps 392 and 394, the process checks for a single-shot pulse sequence method and whether steady state has been achieved. This accounts for the effects of multiple applications of the contrast-preparation,data-acquisition and magnetization-recovery steps if required to sample the desired extent of spatial-frequency space. As used herein, the "steady state" of magnetization is a state created by certain fast (with repeat times short compared to relaxationtimes) imaging pulse sequences during which both the longitudinal and the transverse components of the nuclear magnetization exhibit a steady temporal state. Once steady state is satisfied the process checks whether the target signal has been achieved,step 396, so as to increment across the overall signal level.

Turning to the exemplary contrast-preparation process as shown in FIG. 10, at step 331, the following are chosen: flip angle, phase, waveform and the time of application of any RF pulses. In step 332, the following are chosen: strength,duration, time-dependence, axis and time of application of any magnetic-field gradient pulses. Moreover, in step 333, the following are chosen: duration and time of application of any time delays.

Turning to the exemplary data-acquisition process as shown in FIG. 11, at step 341, the following are chosen: the flip angle and phase of the excitation RF pulse. Also, at steps 342, 343, and 344, the times between all RF pulses; number ofrefocusing RF pulses; and configuration of spatial-encoding magnetic-field gradient pulses are chosen, respectively.

EXAMPLES

Specific implementations of the present invention methodology are useful to illustrate its nature. These examples are non-limiting and are offered as exemplary only. For this purpose, set forth herein are experimental studies in which thepresent invention method was used to generate variable-flip-angle series for three-dimensional (3D) T2-weighted MR imaging of the human brain and cervical spine using a "turbo-SE" type (i.e., RARE-as set forth in Henning et al., Magn. Res. Med. 1986,3:823-833) spin-echo-train pulse sequence, of which is hereby incorporated by reference in its entirety. Brain studies were performed at 1.5 Tesla and 3 Tesla; spine studies were performed at 1.5 Tesla. MR images were obtained using a 1.5-Teslacommercial whole-body imager (MAGNETOM SYMPHONY, Siemens Medical Systems, Iselin, N.J.) or a 3-Tesla commercial whole-body imager (MAGNETOM ALLEGRA, Siemens Medical Systems, Iselin, N.J.). The standard head RF coil supplied with the imager was used. Informed consent was obtained from all subjects prior to imaging.

Turning to FIGS. 2-3, FIG. 2 .Iadd.provides a graph of normalized signal amplitude versus echo number (total number of echoes for signal evolution=160) that .Iaddend.shows an example of a prescribed signal evolution for gray matter that can beused 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 withdecay 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.) theoreticalmodel, 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 includedan echo train length of 160, an echo spacing of 4.1 ms (fixed), a total echo-train duration of 656 ms .Iadd.(i.e., on the order of 600 ms).Iaddend., a repetition time of 2750 ms and an effective echo time .Iadd.(i.e., the time period from the excitationRF pulse to the collection of data corresponding to substantially zero-spatial frequency (the center of k space) .Iaddend.of 328 ms .Iadd.(i.e., on the order of 300 ms).Iaddend..

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 andthree-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 adjacent1-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 artifactssecondary 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.times.160.times.216; field of view,25.6.times.16.0.times.21.6 cm; voxel size, 1.0.times.1.0.times.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/firstecho time/second echo time, 2750/20/80 ms; matrix, 256.times.160; field of view, 25.6.times.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.]. .Iadd.4A.Iaddend.). The images in .[.FIG. 4.]. .Iadd.FIGS. 4B-4F .Iaddend.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 andsurrounding normal appearing white matter, indicating that this echo train shall provide clinically useful contrast characteristics that appear very similar to those for conventional T2-weighed SE images (FIG. 4A); and (2) the thin 1-mm sections providean 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 issimilar, despite the much thinner sections of the former.

Finally, referring to FIGS. 4E-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.]. .Iadd.2-4F.Iaddend., T2-weighted images were also obtained at 3 Tesla from the brain of a healthy volunteer. The three 1-mm thickimages 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 specificabsorption 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 remainssubstantial 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-trainmethods 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 theFDA 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-weightedMRI 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 thesignal 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 longspin-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 ofthe 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 TI. Hence, the signalsfrom 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 .Iadd.non-transitory .Iaddend.computer useable medium (readable media) to extend the usable duration of the echo train in magnetic resonance imaging pulsesequences 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 spatialresolution. 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 magneticfield 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. Inparticular, 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 ofmaterials (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 willin 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 imagingthat 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 tissuesignal 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 indicatedto 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 theinvention 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 beembraced herein.

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