US007164268B2
`
`(12) United States Patent
`Mugler, III et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,164,268 B2
`Jan. 16, 2007
`
`(54) METHOD AND APPARATUS FOR
`SPIN-ECHO-TRAIN MR IMAGING USING
`PRESCRIBED SIGNAL EVOLUTIONS
`
`(58) Field of Classi?cation Search .............. .. 324/309,
`324/307, 311, 318, 322, 300; 600/410, 407
`See application ?le for complete search history.
`
`(75) Inventors: John P. Mugler, III, Charlottesville, VA
`(Us); Jam‘? R- BYOOkeman,
`Charlottesville, VA (US)
`
`(56)
`
`(73) Assignee: University of Virginia Patent
`Foundation, Charlottesville, VA (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U-S-C~ 154(b) by 0 days-
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`9/1987 Kramer et a1.
`4,695,800 A
`4,703,271 A 10/1987 Loef?er et a1~
`4,769,603 A
`9/1988 OPP‘?!t et 31'
`4,818,940 A
`4/1989 Henmg et al.
`_
`(comlnued)
`OTHER PUBLICATIONS
`
`(21) Appl. N0.:
`
`10/451,124
`
`(22) PCT Filed:
`
`Dec. 21, 2001
`
`(86) PCT N0.:
`
`PCT/US01/50551
`
`§ 371 (OX1),
`(2), (4) Date?
`
`Jun- 19’ 2003
`
`Tkach et al., article “A comparison of fast spin echo and gradient
`?eld echo sequences” Magnetic Resonance Imaging (Jul-Aug.
`1938) V01~ 6, N0~ 4, P~ 373-39~*
`
`(Commued)
`Primary ExamineriDiego Gutierrez
`Assistant ExamineriTiiTany A. FetZner
`(74) Attorney, Agent, or FirmiRobert J . Decker
`
`(87) PCT Pub. N0.: W002/50574
`
`(57)
`
`ABSTRACT
`
`PCT Pub' Date: Jun‘ 27’ 2002
`Prior Publication Data
`
`(65)
`
`Us 2004/0051527 A1
`
`Man 18, 2004
`_
`_
`Related U's' Appheatlon Data
`(60) Provisional application No. 60/257,l82, ?led on Dec.
`21, 2000
`
`(51) Int_ CL
`(200601)
`G01V 3/00
`(2006,01)
`A61B 5/055
`(52) US. Cl. .................... .. 324/307; 600/410; 600/413;
`600/428; 324/309; 324/318
`
`A magnetic resonance imaging “MRI” method and appara
`tus for lengthening the usable echo-train duration and reduc
`ing 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 corre
`Spondmg to the 10% echo @112; The method Preludes a
`means to shorten image acqu1s1t1on t1mes and/or mcrease
`spatial resolution for Widely-used spin-echo train magnetic
`resonance techniques, and enables high-?eld imaging Within
`the safety guidelines established by the Food and Drug
`Administration for poWer deposition in human MRI.
`
`46 Claims, 8 Drawing Sheets
`
`@
`
`PROVIDE CONTRAST-PREPARATION THE CONTRAST-PMPARATION COMPRISING GENERATING
`AT LEAST ONE OF AT LEAST ONE RADIOFREOUENDY PULSE AT LEAST ONE MAGNETIC
`FIELD GRADIENT PULSE, AND AT IEAST ONE TIME DELAY VIHEREBY SAID CONTRAST
`PREPARATION ENCODES TIIE MAGNETIZATION WITH AT LEASTONE DESIRED IMAGE CONTRAST
`
`/ 200
`
`I
`
`CALCULATE FLIP ANGLES AND PHASES FOR REFOCUSING RADIOFREDUBICY PULSES THAT ARE
`ENT DATA-AODUISITION STEPS THE CALCULATION COIAPRISES.
`AP
`PLIED IN SUBSEOU
`TIMESAND SELECTING PROTON DENSITY 80 AS TO
`SELECTING VALUES OF TI AND T') RELAXATION
`PRESCRIBED TIME COURSE OF TH
`MPLITUDES AND PHASES OF THE RADIO
`EA
`PROVIDE A
`FREQUENCY MAGNETIC RESONANCE SIGNALS THAT ARE GENERATED BY THE REFOOUSING
`RADIOFREDUENCY PULSES.
`I
`PROVIDE THE DATAACDUISTTION STEP BASED ON AN ECHOTRAIN ACQUISITION. THE DATA
`ACOUISITI
`STEP C
`PULSE
`FLIP
`
`AS DETERMINED BY SAID CALCULATION
`OF D
`GRADIENT PULSES THAT ENOODE SPATIAL INFORMATION INTO ATLEAST
`RADIOFREOUENCY MAGNETIC RESONANCE SI GNALS THAT FOLLOWAT LEAST ONE OF
`SAID REFOCUSING RADIOFREDUENCY PULSES.
`
`I
`
`PROVIDE MAGNETIZATION-RECOVERY THE MAGNETIZATION-RECOVERY
`COMPRISES ATIME DELAY TO ALLOW NIAGNETIZATION TO I‘ELAX,
`
`HA
`XTENT
`EDE
`PREDETERMIN
`FRED
`OF SPATIAL
`UENCY SPACE
`"A
`DEE
`PLED
`
`General Electric Co. 1013 - Page 1
`
`

`
`US 7,164,268 B2
`Page 2
`
`5,680,045 A 10/1997 Feinberg
`5,749,834 A
`5/1998 Hushek
`6,020,739 A *
`2/2000 Meyer et al. ............. .. 324/309
`6,230,039 B1
`5/2001 Stuberetal.
`6,404,194 B1* 6/2002 Irarrazabal etal. ....... .. 324/307
`2004/0051527 A1* 3/2004 Mugler, 11161111. ...... .. 324/309
`
`US. PATENT DOCUMENTS
`
`2/1990 Laclebeck er 91-
`4,901,020 A
`3/1991 Male/r er 91-
`5,001,428 A
`5/1993 Harm? eta1~ ------------- -~ 324/309
`5,214,382 A
`8/1993 Delmllng
`5235280 A
`9/1993 Mugler et al. ............ .. 324/309
`5,245,282 A
`5,256,967 A 10/1993 FOO et al. ................. .. 324/311
`5’270’654 A 12/1993 Felnberg et a1‘
`5,304,929 A
`4/1994 Fang et al.
`
`5’3l5’249 A
`5,347,216 A
`
`5,391,990 A
`
`5/1994 LeROuX et 31'
`9/1994 FOO .......................... .. 324/309
`.
`2/1995 Schmitt et al.
`
`3/1995 Pauly et al. .............. .. 324/307
`5,402,067 A
`.
`7/1996 Hennig
`5,541,511 A
`7/1996 Heid et a1‘
`5541514 A
`8/1996 Foo .......................... .. 324/309
`5,545,992 A
`5,565,776 A 10/1996 Kanazawa
`5,612,619 A
`3/ 1997 Feinberg
`
`.
`
`OTHER PUBLICATIONS
`Kallmes, et al., “Suppression of Cerebrospinal Fluid and Blood
`.
`.
`.
`.
`.
`Flow Artifacts in FLAIR MR Imaging With a Single-Slab Three
`dimensional Pulse Se uence' Initial EX erience ” Radiolo
`2001
`q
`'
`P
`'
`gy
`’
`vol. 221, No. 1, pp. 251-255.
`Mugler III, et al., Optimized Signle-Slab Three dimensional Spin
`“
`.
`.
`-
`-
`»
`-
`Echo MR Imaging of the Brain. Radiology 2000, vol. 216, No. 3,
`89l_899
`Ellnster et al. “Questions and Answers in Magnetic Resonance
`Ima i’n
`,, 2’ed
`102 103
`g g’
`" p‘
`’
`'
`* cited by examiner
`
`.
`
`.
`
`.
`
`General Electric Co. 1013 - Page 2
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 1 0f 8
`
`US 7,164,268 B2
`
`ESP1/2 ESP1l2
`B1
`\
`
`0‘
`
`EsP2/2
`B2
`\-
`
`ESP2/2
`
`ESPg/Z
`
`B3
`
`_
`
`_
`
`20
`\
`
`RF-—
`Gselect
`
`/31-/32
`- CONTRAST 1:
`
`-/33
`
`'34'
`-
`
`'
`-
`
`PREPARATION
`
`_
`
`-
`
`eencode"
`_
`
`
`
`NORMALIZED SIGNAL
`
`0.35
`5
`02;
`0.1;
`0
`
`0'0:
`
`/
`41
`
`/
`42
`
`/ REPEATASREQlIRED
`43
`
`w .
`
`.
`
`.
`
`.
`
`.
`
`. .
`
`.
`
`. j .
`
`.
`
`.
`
`.
`
`.
`
`_
`160
`
`I
`
`2;; 80.
`E 60
`2 40'
`g 20'.
`160
`120
`so
`40
`0
`REFOCUSlNGRF-PULSENUMBER
`FIG. 3
`
`“- 0' .
`
`.
`
`.
`
`.
`
`. . , .
`
`.
`
`.
`
`.
`
`. .
`
`.
`
`. ..
`
`.
`
`.
`
`.
`
`120
`60
`40
`ECHONUMBER
`FIG. 2
`
`General Electric Co. 1013 - Page 3
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 2 6f 8
`
`US 7,164,268 B2
`
`General Electric Co. 1013 - Page 4
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 3 6f 8
`
`US 7,164,268 B2
`
`General Electric Co. 1013 - Page 5
`
`

`
`U.S. Patent
`
`Jan. 16,2007
`
`Sheet 4 of8
`
`US 7,164,268 B2
`
`1
`
`POWER
`
`SUPPLY
`/
`4
`
`11
`\
`_ CONTROL
`
`26
`
`CHANNEL
`
`1/1,’
`28/1;
`COMMUNICATIONS
`INTERFACE
`
`10/
`
`AMP. & DEMOD.
`UNIT
`
`28
`13 y @\28
`Z
`\
`K4
`RECONSTRUCTION __IMAGE PROC _ MONITOR
`UNIT
`1 12/
`UNIT
`
`3
`
`)%
`
`\
`2/4—
`
`77 \
`
`-~
`
`2;;
`
`f3 5
`7/4
`(ll
`\
`U0
`
`/
`
`‘
`
`P]
`
`44
`
`3
`
`9
`\
`XMIT/RCV
`
`6
`\
`
`28
`
`Y
`
`FIG. 7
`
`/
`14
`
`General Electric Co. 1013 - Page 6
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 5 6f 8
`
`US 7,164,268 B2
`
`< START
`
`>
`
`V
`PROVIDE CONTRAST-PREPARATION. THE 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.
`
`'
`
`\200
`
`V
`CALCULATE FLIP ANGLES AND PHASES FOR REFOCUSING RADIO-FREQUENCY PULSES THAT ARE
`APPLIED IN SUBSEQUENT DATA-ACQUISITION STEPS. THE CALCULATION COMPRISES:
`SELECTING VALUES OF T1 AND T2 RELAXATION TIMES AND SELECTING PROTON DENSITY SO AS TO \ 300
`PROVIDE A PRESCRIBED TIME COURSE OF THE AMPLITUDES AND PHASES OF THE RADIO
`FREQUENCY MAGNETIC RESONANCE SIGNALS THAT ARE GENERATED BY THE REFOCUSING
`RADIO-FREQUENCY PULSES.
`
`V
`PROVIDE THE DATA-ACQUISITIONISTEP BASED ON AN ECHO-TRAINACOUISITION. THE DATA
`ACQUISITION STEP COMPRISES: %AN EXCITATION RADIO-FREQUENCY PULSE HAVINGAFLIP
`ANGLEANDPHASE'QATLEASTTW REFOCUSING RADIO-FREOUENCYPULSHES, EACH HAVINGA
`FLIPANGLEANDPH SEASDETERMINEDBYSAIDCALCULATIONSTEP;ANDIIBMAGNETIC-FIELD \
`GRADIENTPULSESTHATENCODESPATIALINFORMATIONINTOATLEAST NEOFSAID
`400
`RADIO-FREQUENCYMAGNETIC RESONANCESIGNALSTHATFOLLOWATLEASTONE OF
`SAIDREFOCUSINGRADIO-FREQUENCYPULSES.
`
`V
`PROVIDE MAGNETIZATION-RECOVERY. THE MAGNETIZATION-RECOVERY
`COMPRISES ATIME DELAY TO ALLOW MAGNETIZATION TO RELAX.
`
`\SOO
`
`HAS
`PREDETERMINED EXTENT
`OF SPATIAL FREQUENCY SPACE
`BEEN SAMPLED
`
`FIG. 8
`
`General Electric Co. 1013 - Page 7
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 6 6f 8
`
`US 7,164,268 B2
`
`@ ’
`
`CHOOSE m2, AND PROTON DENSITY
`T
`CHOOSE DESIRED PRESCRlBED SIGNAL EVOLUTION WHICH DESCRIBES THE TIME COURSE OF THE
`SIGNAL AMPLITUDES AND PHASES.
`T
`CHOOSE CHARACTERISTICS OF CONTRAST-PREPARATION
`J
`CHOOSE CHARACTERISTICS OF DATA-ACQUISITION
`r
`CHOOSE CHARACTERISTICS OF MAGNETIZATION-RECOVERY
`+
`START WITH THERMAL EQUILIBRIUMMAGNETIZATION
`$
`_ CALCULATEMAGNETIZATION(M1)THATEXISTSJUSTAFTERTHEC0NTRAST-PREPARAT!ONAND
`'
`EXCITATION RADIO-FREQUENCY PULSE ARE APPLIED
`
`\ 310
`
`\320
`
`\ 330
`
`\
`340
`\350
`
`\360
`
`\m
`
`+
`w
`CALCULATE THE FLIPANGLE AND PHASE FOR THE CURRENT REFOCUSING RADIO-FREQUENCY PULSE
`THATYIELDS THE DESIRED CORRESPONDING SIGNALAMPLITUDE AND PHASE.
`\380
`
`NO
`
`:
`
`ALLOW RELAXATION DURINGTHE MAGNETIZATION-RECOVERY STEP
`
`\390
`
`FIG. 9A
`
`General Electric Co. 1013 - Page 8
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 7 6f 8
`
`US 7,164,268 B2
`
`TARGET SIGNAL
`ACHIEVED?
`
`FIG. 9B
`
`General Electric Co. 1013 - Page 9
`
`

`
`U.S. Patent
`
`Jan. 16, 2007
`
`Sheet 8 6f 8
`
`US 7,164,268 B2
`
`CHOOSE FLIP ANGLE, PHASE, WAVEFORM AND TIME OF
`APPLICATION OF ANY RADIO-FREQUENCY PULSES
`
`\ 331
`
`TV
`CHOOSE STRENGTH, DURATION, TIME-DEPENDENCE, AXIS AND
`TIME OF APPLICATION OF ANY MAGNETIC-FIELD GRADIENT PULSES \ 332
`
`TI
`
`CHOOSE DURATION AND TIME OF APPLICATION OF ANY TIME DELAYS \ 333
`
`FIG. 10
`
`CHOOSE FLIP ANGLE AND PHASE OF EXCITATION RADIO-FREQUENCY PULSE \ 341
`
`TI
`CHOOSE TIMES BETWEEN ALL RADIO-FREQUENCY PULSES
`
`\ 342
`
`TI
`CHOOSE NUMBER OF REFOCUSING RADIO-FREQUENCY PULSES
`
`V
`CHOOSE CONFIGURATION OF SPATIAL-ENCODING MAGNETIC-FIELD GRADIENT PULSES \ 344
`
`FIG. 11
`
`General Electric Co. 1013 - Page 10
`
`

`
`US 7,164,268 B2
`
`1
`METHOD AND APPARATUS FOR
`SPIN-ECHO-TRAIN MR IMAGING USING
`PRESCRIBED SIGNAL EVOLUTIONS
`
`RELATED APPLICATIONS
`
`This application is a national stage ?ling of International
`Application No. PCT/US01/50551, ?led 21 Dec. 2001,
`Which claims bene?t under 35 U.S.C. Section 119(e) from
`US. Provisional Application Ser. No. 60/257,182, ?led on
`Dec. 21, 2000, entitled “Spin-Echo-Train MR Imaging
`Using Prescribed Signal Evolutions,” the entire disclosure of
`Which is hereby incorporated by reference herein. The
`present application is related to US. Pat. No. 5,245,282,
`?led on Jun. 28, 1991, entitled “Three-dimensional Mag
`netic Resonance Imaging,” the entire disclosure of Which is
`hereby incorporated by reference herein.
`
`GOVERNMENT SUPPORT
`
`Work described herein Was supported by Federal Grant
`Number NS-35142, aWarded by the National Institutes of
`Health. The United States Government possesses certain
`rights in and to this invention.
`
`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 magnetic
`resonance 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)
`acquisitions because 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-?eld inhomogene
`ities.
`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 for each desired image contrast folloWing each
`excitation RF pulse. The ?rst 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, a method called “RARE” (See
`Hennig 1., Nauerth A., Friedburg H., “RARE Imaging: A
`Fast Imaging Methodfor ClinicalM ”, Magn. Reson. Med.
`1986, 3z823i833; 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, 8z557i566, of Which are hereby
`incorporated by reference in their entirety) and its deriva
`tives, commonly referred to as “turbo-SE” or “fast-SE”
`imaging (See Melki P. S., JolesZ F. A., Mulkem R. V.,
`“Partial RFEcho Planar Imaging with the FAISE Method.
`I Experimental and Theoretical Assessment of Artifact”,
`Magn. Reson. Med. 1992, 26z328i34l and Jones K. M.,
`Mulkem R. V., SchWartZ R. B., Oshio K., Barnes P. D.,
`JolesZ F. A., “Fast Spin-Echo MR Imaging ofthe Brain and
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`Spine: Current Concepts”, AIR 1992, l58tl3l3il320, of
`Which are hereby incorporated by reference in their entirety).
`For the purposes of this disclosure, We are primarily inter
`ested in the generaliZed form of techniques in the second
`category; hoWever the present invention is applicable to the
`?rst category as Well. The term “generalized” refers to the
`form of the spatial-encoding gradients that are applied
`folloWing any given refocusing RF pulse. For example,
`RARE imaging encodes one line of spatial-frequency space
`(k-space) data folloWing each refocusing RF pulse using a
`constant, frequency-encoding magnetic ?eld 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, 20z344i349, of Which 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
`line of 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 in?nite number of possibilities
`for spatially encoding the MR signal folloWing each refo
`cusing RF pulse. For the purpose of this disclosure, We
`de?ne the term “spin-echo-train” imaging to encompass all
`of these possibilities, including, but not limited thereto,
`RARE, turbo-SE, fast-SE and GRASE imaging, 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 develop
`ment for MRI has been to increase the amount of k-space
`data sampled per unit time, under the constraints of obtain
`ing the desired image contrast and maintaining image arti
`facts at a tolerable 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 is routinely and Widely used in clinical
`MRI.
`For instance, the echo trains used in clinical fast-SE
`imaging generally employ high ?ip angles (>100°) for the
`refocusing RF pulses, and their durations are typically less
`than the T2 relaxation times of interest for short effective
`echo times (e.g., T1 or proton-density Weighting) or less
`than tWo to three times these T2 values for long effective
`echo times (e.g., T2 Weighting or “FLAIR”; see Hajnal J. V.,
`Bryant D. 1., Kasuboski L., Pattany P. M., De Coene B.,
`LeWis P. D., Pennock J. M., Oatridge A., Young I. R., Bydder
`G. M., “Use of Fluid Attenuated 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 limits translate
`to echo-train durations of <100 ms and <300 ms for short
`and long effective echo times, respectively. When high ?ip
`angles are used for the refocusing RF pulses, echo-train
`durations that are longer than these limits can substantially
`degrade 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, 28z9i24; and Ortendahl D. A., Kaufman
`L., Kramer D. M., “Analysis of Hybrid Imaging Tech
`
`General Electric Co. 1013 - Page 11
`
`

`
`US 7,164,268 B2
`
`3
`niques”, 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 1., “Multiecho Imaging Sequences
`with Low Refocusing Flip Angles”, J. Magn. Reson. 1988,
`78:397*407, of Which is hereby incorporated by reference in
`its entirety) proposed the use of constant, loW-?ip-angle
`refocusing RF pulses to introduce a T1 dependence to the
`evolution of the echo train and thereby lengthen its usable
`duration. More recently, this concept Was extended by
`Alsop, Who derived variable ?ip-angle series based on the
`“pseudosteady-state” condition of a constant signal level
`When T1 and T2 relaxation are neglected (See Alsop D. C.,
`“The Sensitivity ofLow Flip Angle RARE Imaging”, Magn.
`Reson. Med. 1997; 37:176*184, of Which is hereby incor
`porated by reference in its entirety). Alsop also found that
`the echo-train performance Was improved by using a signal
`evolution that decreased for the ?rst 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 and asymptotic ?ip
`angles ranging from 17° to 900 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-?ip-angle or pseu
`dosteady-state approaches. 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 the long echo train.
`Given the considerable role that spin-echo-train methods
`already play in MR imaging, the present invention method
`ology Will be of signi?cant importance.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`SUMMARY OF THE INVENTION
`
`This present invention comprises the methodology,
`related apparatus, and computer useable medium (readable
`media) for using a series of refocusing RF pulses With
`variable ?ip angles and, optionally, variable phase angles, in
`a spin-echo-train MRI pulse sequence Wherein the ?ip-angle
`series is speci?cally 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
`RF pulses, the usable duration of the echo train can be
`extended substantially beyond that obtainable With conven
`tional methods. This increase in the echo-train duration can
`be used to decrease the image acquisition time and/or
`increase the spatial resolution.
`In one aspect, the present invention features a method for
`generating a pulse sequence for operating a magnetic reso
`nance imaging apparatus for imaging an object, the method
`comprising:
`a) providing contrast-preparation, the contrast-preparation
`comprising generating at least one of at least one radio
`frequency pulse, at least one magnetic-?eld gradient
`pulse, and at least one time delay, Whereby the contrast
`preparation encodes the magnetiZation With at least one
`desired image contrast;
`
`45
`
`50
`
`55
`
`65
`
`4
`b) calculating ?ip 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
`frequency pulses; and
`ii) selecting characteristics of the contrast-preparation
`step, the data-acquisition step and a magnetization
`recovery step, With the exception of the ?ip 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 ?ip angle
`and phase;
`ii) at least tWo refocusing radio-frequency pulses, each
`having a ?ip angle and phase as determined by the
`calculation step; and
`iii) magnetic-?eld gradient 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
`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.
`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 steady magnetic ?eld; a gradient magnet system
`for generating temporary gradient magnetic ?elds; a radio
`frequency transmitter system for generating radio-frequency
`pulses; a radio-frequency receiver system for receiving
`magnetic resonance signals; a reconstruction unit for recon
`structing 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-frequency receiver 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 one magnetic-?eld gradient
`pulse, and at least one time delay, Whereby the contrast
`preparation encodes the magnetiZation With at least one
`desired image contrast;
`b) calculating ?ip 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
`frequency pulses; and
`ii) selecting characteristics of the contrast-preparation
`step, the data-acquisition step and a magnetization
`
`General Electric Co. 1013 - Page 12
`
`

`
`US 7,164,268 B2
`
`5
`recovery step, with the exception of the ?ip 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 ?ip angle
`and phase,
`ii) at least two refocusing radio-frequency pulses, each
`having a ?ip angle and phase as determined by the
`calculation step, and
`iii) magnetic-?eld gradient 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
`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.
`In a third aspect, the present invention features a computer
`readable media carrying encoded program instructions for
`causing a programmable magnetic resonance imaging appa
`ratus to perform the method discussed above in the ?rst
`aspect of the invention. Similarly, the invention features a
`computer program product comprising a computer useable
`medium having computer program logic for enabling at least
`one processor in a magnetic resonance imaging apparatus to
`generate a pulse sequence, the computer program logic
`comprising the method discussed above in the ?rst aspect of
`the invention.
`Because the ?ip angles for the refocusing RF pulses that
`are derived with this method are typically much less than
`1800 for a substantial portion of the total number of RF
`pulses, the power deposition is much less than that corre
`sponding to 1800 RF pulses, which are commonly used in
`conventional spin-echo-train pulse sequences. This feature
`is particularly important for high ?eld MRI (>1.5 Tesla),
`wherein power deposition is a critical pulse-sequence design
`factor for human applications. The present invention permits
`long, closely-spaced echo trains to be used for high-?eld
`imaging that would not otherwise meet the safety guidelines
`established by the Food and Drug Administration for power
`deposition in human MRI.
`Another potentially useful feature of the present invention
`is that, for speci?c forms of the encoding-gradient wave
`forms, signals from moving or ?owing materials are
`strongly attenuated, even when the velocities are relatively
`low. A speci?c example of this behavior is the attenuation of
`the signal from cerebrospinal ?uid (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 cord without requiring inversion
`55
`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 of Fluid-Attenuated Inversion-Recovery MR in
`the Detection of Multiple Sclerosis of the Spinal Cord”,
`AJNR 1997, 18:1035*1039, of which are hereby incorpo
`rated by reference in their entirety).
`
`45
`
`50
`
`60
`
`65
`
`20
`
`25
`
`30
`
`35
`
`40
`
`6
`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 accom
`panying drawings, 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
`pulse sequences.
`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-?ip-angle series corresponding
`to FIG. 2 that was derived using the present invention
`methods as described herein.
`FIGS. 4*6 show example MR images obtained using the
`variable-?ip-angle series of FIG. 3 in a “turbo-SE” type
`spin-echo-train pulse sequence; collectively, FIGS. 4*6 pro
`vide examples of the potential utility of the present inven
`tion. 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, respec
`tively. Further, showing brain images 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 sag
`ittal image of the cervical spinal cord obtained at 1.5 Tesla.
`FIG. 7 illustrates a simpli?ed exemplary embodiment of
`a MRI apparatus for practicing the present invention. The
`present invention method can be applied to various com
`mercially available MRI apparatuses.
`FIG. 8 is an exemplary ?owchart for a simpli?ed pre
`ferred implementation of the methods of the present inven
`tion.
`FIGS. 9Ai9B is an exemplary ?owchart for a simpli?ed
`preferred implementation of the calculation methods of the
`present invention, step 200.
`FIG. 10 is an exemplary ?owchart for a simpli?ed pre
`ferred implementation of the contrast-preparation methods
`of the present invention.
`FIG. 11 is an exemplary ?owchart for a simpli?ed pre
`ferred implementation of the data-acquisition methods of the
`present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`In the following, ?rst presented is an exemplary embodi
`ment 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 alter
`native embodiments of the methods of the present invention,
`including their exemplary implementation as computer
`hardware, ?rmware, and/or software.
`An Exemplary MR-Apparatus of the Present Invention
`FIG. 7 illustrates a simpli?ed schematic of a MR appa
`ratus 1 for practicing the present invention. The MR appa
`ratus 1 includes a main magnet system 2 for generating a
`
`General Electric Co. 1013 - Page 13
`
`

`
`US 7,164,268 B2
`
`20
`
`25
`
`30
`
`7
`steady magnetic ?eld in an examination Zone(s) of the MR
`apparatus. The Z-direction of the coordinate system illus
`trated corresponds to the direction of the steady magnetic
`?eld generated by the magnet system 2.
`The MR apparatus also includes a gradient magnet system
`3 for generating temporary magnetic ?elds Gx, Gy and G2
`directed in the Z-direction but having gradients in the x, y or
`Z directions, respectively. With this magnetic gradient sys
`tem, magnetic-?eld 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
`invention is not limited to directions ?xed 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 and slice-selection direction (or second phase
`encode direction for 3D imaging), respectively.
`Also, While traditional commercial methods provide lin
`ear 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 art can
`use a Z-squared dependence or some other spatial depen
`dence 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
`transmitter coil 5 is arranged around the part of body 7 in the
`examination Zone. Th