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WO2004008169A1 - Methode de resonance magnetique (mr) faisant appel a des impulsions rf multidimensionnelles - Google Patents

Methode de resonance magnetique (mr) faisant appel a des impulsions rf multidimensionnelles Download PDF

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Publication number
WO2004008169A1
WO2004008169A1 PCT/IB2003/003058 IB0303058W WO2004008169A1 WO 2004008169 A1 WO2004008169 A1 WO 2004008169A1 IB 0303058 W IB0303058 W IB 0303058W WO 2004008169 A1 WO2004008169 A1 WO 2004008169A1
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WIPO (PCT)
Prior art keywords
trajectory
space
pulse
time
density
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/IB2003/003058
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English (en)
Inventor
Peter Boernert
Bernd Aldefeld
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Philips Intellectual Property and Standards GmbH
Koninklijke Philips NV
Original Assignee
Philips Intellectual Property and Standards GmbH
Koninklijke Philips Electronics NV
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Application filed by Philips Intellectual Property and Standards GmbH, Koninklijke Philips Electronics NV filed Critical Philips Intellectual Property and Standards GmbH
Priority to AU2003247011A priority Critical patent/AU2003247011A1/en
Publication of WO2004008169A1 publication Critical patent/WO2004008169A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/446—Multifrequency selective RF pulses, e.g. multinuclear acquisition mode
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5611—Parallel magnetic resonance imaging, e.g. sensitivity encoding [SENSE], simultaneous acquisition of spatial harmonics [SMASH], unaliasing by Fourier encoding of the overlaps using the temporal dimension [UNFOLD], k-t-broad-use linear acquisition speed-up technique [k-t-BLAST], k-t-SENSE

Definitions

  • MR magnetic resonance
  • RF pulses are designated to be "multi-dimensional” if they act on an examination zone simultaneously with at least two gradient fields with temporally and spatially different gradients.
  • the nuclear magnetization can be excited in a spatially limited part of the examination zone by means of such multi-dimensional RF pulses.
  • the nuclear magnetization is not excited in the remaining part of the examination zone, even though the RF pulse and the magnetic gradient fields also act on this remaining part.
  • Multi-dimensional RF pulses of this kind are subject to limitations in that on the one hand the RF pulse may have only a given duration and in that on the other hand the capacity of the gradient system is limited (or a given speed of change of the gradient may not be exceeded for medical reasons).
  • an MR method in accordance with the invention which includes the steps of: generating at least one RF pulse which acts on an examination zone, and generating at least two gradient magnetic fields with gradients which vary differently in time and in space, which act on the examination zone simultaneously with the RF pulse, and which vary in time in such a manner that during the RF pulse a trajectory with a spatially varying density is followed in the k space, that is, a trajectory which notably has a density which is higher in a central zone of the k space than in the zones outside this central zone.
  • a trajectory which exhibits a variable density in the k space instead of the customary constant density.
  • the invention is based on the recognition of the fact that not all k values or all spatial frequency components are of equal importance for the excitation of a desired imaging zone. For most objects the central zone of the excitation k space which is associated with the low spatial frequencies is most important, because the energy is concentrated essentially in the central zone, of the k space.
  • the trajectory should preferably have a density which is higher in the central zone of the k space (that is, at low spatial frequencies) than in zones situated outside this central zone.
  • the examination zone may also contain (for example, periodic) structures which could be excited better in a spatial frequency domain outside the center.
  • the excitation with a higher density of the trajectory in the central zone of the k space is the optimum approach.
  • Claim 2 discloses an advantageous further embodiment of the invention. Granted, when the density is higher at the center of the k space than outside the center of such a spiral-shaped trajectory, the number of turns of the spiral-shaped trajectory increases (in comparison with a spiral-shaped trajectory with a constant spacing of the turns), but the duration of the RF pulse is not prolonged to the same extent as a result thereof. This because during the scanning of the central zone of the k space the magnetic gradient fields still have a very small gradient only, so that at a given maximum rate of variation of the gradient said inner turns can be traversed significantly faster than the outer turns.
  • the trajectory may also follow a different course in the k space; for example, in conformity with claim 3 it may comprise a set of parallel straight lines, as in the cited US patent application 09/728111 and analogous to the EPI sequence during the reading out of MR signals, whose spacing is less at the center of the k space than in the zones outside the center.
  • the density of the trajectory may vary in steps (claim 4) or continuously (claim 5).
  • the invention is also suitable for use in the case of so-called "Transmit Sense" methods in which the multi-dimensional RF pulses generated by a plurality of RF coils act simultaneously on the examination zone.
  • the RF pulses may then exhibit a different variation in time.
  • Claim 8 discloses an MR apparatus which is suitable for carrying out the method in accordance with the invention and claim 9 defines a computer program for the control unit of an MR apparatus of this kind.
  • Fig. 1 shows the block diagram of an MR apparatus which is suitable for carrying out the invention
  • Fig. 2 shows the variation in time of a sequence with a two-dimensional RF pulse
  • Fig. 3a shows a spiral-shaped trajectory in the k space with a constant density
  • Fig. 3b shows the associated intensity profile
  • Fig. 4a shows a spiral-shaped trajectory in accordance with the invention
  • Fig. 4b shows the associated intensity profile
  • the reference numeral 1 in Fig. 1 denotes a diagrammatically represented main field magnet which generates a steady, essentially uniform magnetic field of a strength of, for example, 1.5 Tesla which extends in the z direction in an examination zone (not shown). The z direction then extends in the longitudinal direction of an examination table (not shown) on which a patient is accommodated during an MR examination.
  • a gradient coil system 2 which includes three coil systems which are capable of generating gradient magnetic fields G x , G y and G z which extend in the z direction and have a gradient in the x direction, the y direction and the z direction, respectively.
  • the currents for the gradient coil system 2 are supplied by a gradient amplifier 3. Their variation in time is controlled by a waveform generator 4, that is, separately for each direction.
  • the waveform generator 4 is controlled by an arithmetic and control unit 5 which calculates the variation in time of the magnetic gradient fields G x , G y , G z required for a given examination method and loads these values into the waveform generator 4.
  • the control unit 5 also acts on a workstation 6 which includes a monitor 7 for the display of MR images. Entries can be made via a keyboard 8 or an interactive input unit 9.
  • the nuclear magnetization in the examination zone can be excited by RF pulses from an RF coil 10 which is connected to an RF amplifier 11 which amplifies the output signals of an RF transmitter 12.
  • the (complex) envelopes of the RF pulses are modulated with the carrier oscillations which are supplied by an oscillator and whose frequency corresponds to the Larmor frequency (approximately 63 MHz in the case of a main field of 1.5 Tesla).
  • the arithmetic and control unit loads the complex envelope into a generator 14 which is coupled to the transmitter 12.
  • a plurality of RF coils which comprise a respective RF transmission channel, each of which is provided with an RF coil.
  • the MR signals generated in the examination zone are picked up by a receiving coil 20 and amplified by an amplifier 21.
  • the amplified MR signal is demodulated in a quadrature demodulator 22 by way of two 90° mutually offset carrier oscillations of the oscillator 13, so that two signals are generated which may be considered as the real part and the imaginary part of a complex MR signal.
  • These signals are applied to an analog-to-digital converter 23 which forms MR data therefrom.
  • the MR data is subjected to various processing operations in an evaluation unit 24, that is, inter alia a Fourier transformation. It is also possible to provide a plurality of RF receiving channels for a plurality of receiving coils.
  • FIG. 2 shows the variation in time of a sequence which includes a two- dimensional RF pulse.
  • An RF pulse RF 0 then acts on the examination zone, the envelope of said RF pulse being loaded into the generator 14 by the control unit 5, and simultaneously with the RF pulse two magnetic gradient fields G x0 and G y o, act on the examination zone, their variation in time being imposed by the waveform generator 4 under the control of the control unit 5.
  • the magnetic gradient fields G x o and G y0 are formed by oscillations with an amplitude which decreases in time and with a distance between the zero-crossings which also decreases. For the zero-crossing of one oscillation G x0 or G y0 the respective other oscillation always exhibits a relative maximum.
  • the variation in time of the envelopes of the RF pulse RFo is tuned to the variation in time of the magnetic gradient fields G x0 and G y0 in such a manner that the nuclear magnetization is excited in a zone which is bounded in space in the x direction and the y direction.
  • the subsequent reading out of the spatial distribution of the excited nuclear magnetization takes place in the form of a so-called EPI sequence.
  • the magnetic gradient field G x is then generated with a periodic variation whose polarity continuously reciprocates between a positive value and a negative value.
  • the magnetic gradient field G y is active in the form of short pulses ("blips") which occur each time at the zero-crossings of the magnetic gradient field.
  • the signal received by the receiving coil 20, and subsequently demodulated and digitized is acquired by the evaluation unit 24.
  • An image of the nuclear magnetization distribution in the previously excited slice, bounded in the x direction and the y direction can be derived from the totality of MR signals received.
  • the letter G represents the magnetic gradient field resulting from the superposition of G x0 and G y0 .
  • the variation of the magnetic gradient fields G x0 and G y o as shown in Fig. 2 results in a spiral-shaped trajectory which is followed inwards from the outside.
  • Fig. 3 a shows such a trajectory with a constant density of the spiral turns.
  • a pronounced maximum of the transverse magnetization can be recognized at the center, the width of this maximum being co-determined by the envelope of the RF pulse RF 0 -
  • secondary maxima of lower amplitude are situated to both sides of this main maximum.
  • Fig. 4a shows the course of the trajectory of a multi-dimensional RF pulse in accordance with the invention.
  • the density of the spiral turns is doubled in a central zone which corresponds to the low k values or the low spatial frequencies, whereas outside this central region it corresponds to the density of the spiral turns in Fig. 3a.
  • Fig. 2 shows the variation in time of the magnetic gradient fields corresponding to Fig. 3a, it is necessary for such a trajectory that as from a given instant the envelope of the magnetic gradient fields G x0 or G y0 decreases more slowly than shown in Fig.
  • Fig. 4a It will be recognized that at the locations in which pronounced secondary maxima are present in the excitation profile shown in Fig. 3b, secondary maxima are still present; however, they have a significantly reduced amplitude. Such secondary maxima are caused by the higher spatial frequency components where the trajectory passes through the k space with the same density as in fig. 3 a. The maxima imposed by the increased (but still finite) density of the trajectory in the range of the lower spatial frequencies have been pushed further outwards, so that they are no longer visible in the representation of Fig. 4b. Undesirable aliasing effects are thus suppressed practically completely.
  • the invention can be used not only for an imaging sequence as shown in Fig. 2, but also for the generating of navigator pulses.
  • the width of the main maximum in the excitation profile is then even significantly smaller than shown in Fig. 3b or Fig. 4b, and no slice-selective pulse (RFj, G zl , Fig. 2) is then required.
  • only one MR signal is acquired.
  • the invention can also be used for the so-called Transmit Sense, where a plurality of RF coils, having different spatial sensitivities, simultaneously generate multidimensional pulses, for each RF coil there being provided a separate variation in time of the RF pulse RF (see Katscher et al., Proc. ISMRM 2002, page 189).
  • Transmit Sense enables a reduction of the duration of the RF pulses while maintaining the spatial resolution of the excitation profile.
  • the application of the invention to Transmit Sense again yields a reduction of backfolding or aliasing artifacts in conformity with the described principle.
  • a further advantage of the use of the invention in conjunction with Transmit Sense consists in that the complex calculation of the individual RF time functions can be dispensed with if the same time function is used for each individual transmission coil. This is because in accordance with the invention the sub-sampling occurring in the case of Transmit Sense is canceled at least for the central zone of the k space. As a result, aliasing artifacts which would otherwise occur within the excitation profile in this case are also minimized in this manner.
  • the invention can be used not only for multi-dimensional excitation pulses, but also for multi-dimensional focusing pulses.

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  • Physics & Mathematics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)

Abstract

L'invention concerne une méthode MR faisant appel à des impulsions RF multidimensionnelles. Dans cette méthode, la variation de temps de champs magnétique à gradient qui sont simultanément actifs avec les impulsions RF est choisie de sorte qu'une trajectoire présentant une densité variant dans l'espace est suivie dans l'espace k. Le profil d'excitation peut ainsi être considérablement amélioré, tout en maintenant le même taux maximal de changement de gradient, et en acceptant seulement une prolongation très faible de la durée d'impulsion.
PCT/IB2003/003058 2002-07-17 2003-07-08 Methode de resonance magnetique (mr) faisant appel a des impulsions rf multidimensionnelles Ceased WO2004008169A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2003247011A AU2003247011A1 (en) 2002-07-17 2003-07-08 Mr method utilizing multi-dimensional rf pulses

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE10232342.9 2002-07-17
DE2002132342 DE10232342A1 (de) 2002-07-17 2002-07-17 MR-Verfahren mit mehrdimensionalen Hochfrequenzimpulsen

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7404636B2 (en) 1999-07-02 2008-07-29 E-Vision, Llc Electro-active spectacle employing modal liquid crystal lenses
NL1030695C2 (nl) * 2004-12-20 2009-03-09 Gen Electric Werkwijze en systeem voor ruimtelijke-spectrale excitatie door middel van parallelle RF uitzending.
US7853060B2 (en) 2006-04-13 2010-12-14 Siemens Aktiengesellschaft Method and MR system for generating MR images
CN104337516A (zh) * 2013-07-30 2015-02-11 西门子公司 磁共振控制序列的确定

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5105152A (en) * 1990-03-22 1992-04-14 The Board Of Trustees Of The Leland Stanford Junior University Magnetic resonance imaging and spectroscopy using a linear class of large tip-angle selective excitation pulses

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5105152A (en) * 1990-03-22 1992-04-14 The Board Of Trustees Of The Leland Stanford Junior University Magnetic resonance imaging and spectroscopy using a linear class of large tip-angle selective excitation pulses

Non-Patent Citations (8)

* Cited by examiner, † Cited by third party
Title
BORNERT P ET AL: "On spatially selective RF excitation and its analogy with spiral MR image acquisition", MAGNETIC RESONANCE MATERIALS IN PHYSICS, BIOLOGY AND MEDICINE, DEC. 1998, ELSEVIER, NETHERLANDS, vol. 7, no. 3, pages 166 - 178, XP009020065, ISSN: 1352-8661 *
HARDY C J ET AL: "Correcting for nonuniform k-space sampling in two-dimensional NMR selective excitation", JOURNAL OF MAGNETIC RESONANCE, ACADEMIC PRESS, LONDON, GB, vol. 87, no. 3, May 1990 (1990-05-01), pages 639 - 645, XP002187825, ISSN: 0022-2364 *
KATSCHER U. ET AL: "Theory and experimental verification of transmit SENSE", PROCEEDINGS OF THE INTERNATIONAL SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE, TENTH SCIENTIFIC MEETING AND EXHIBITION, 18 May 2002 (2002-05-18), Honolulu, HI, USA, XP002259673 *
MEYER C H ET AL: "Square-Spiral Fast Imaging", BOOK OF ABSTRACTS OF THE INTERNATIONAL SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE 8TH. ANNUAL MEETING AND EXHIBITION. (IN COLLABORATION WITH THE SIXTH ANNUAL CONGRESS OF THE EUROPEAN SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE AND BIOLOGY AND THE SEVE, vol. 1 MEETING 8, 12 August 1989 (1989-08-12), pages 362, XP002114115 *
SCHRÖDER CHRISTOPH ET AL: "Spatial excitation using variable-density spiral trajectories.", JOURNAL OF MAGNETIC RESONANCE IMAGING: JMRI. UNITED STATES JUL 2003, vol. 18, no. 1, July 2003 (2003-07-01), pages 136 - 141, XP001155957, ISSN: 1053-1807 *
SPIELMAN D M ET AL: "Magnetic Resonance Fluoroscopy using spirals with variable sampling densities", MAGNETIC RESONANCE IN MEDICINE, ACADEMIC PRESS, DULUTH, MN, US, vol. 34, no. 3, 1 September 1995 (1995-09-01), pages 388 - 394, XP000527023, ISSN: 0740-3194 *
STENGER V.A. ET AL: "Variable Density Spiral 3D Tailored RF PULSES", PROCEEDINGS OF THE INTERNATIONAL SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE, TENTH SCIENTIFIC MEETING AND EXHIBITION, 18 May 2002 (2002-05-18), Honolulu, HI, USA, XP002259672 *
TSAI C-M ET AL: "Reduced aliasing artifacts using variable-density k-space sampling trajectories", MAGNETIC RESONANCE IN MEDICINE, ACADEMIC PRESS, DULUTH, MN, US, vol. 43, no. 3, March 2000 (2000-03-01), pages 452 - 458, XP002179553, ISSN: 0740-3194 *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7404636B2 (en) 1999-07-02 2008-07-29 E-Vision, Llc Electro-active spectacle employing modal liquid crystal lenses
NL1030695C2 (nl) * 2004-12-20 2009-03-09 Gen Electric Werkwijze en systeem voor ruimtelijke-spectrale excitatie door middel van parallelle RF uitzending.
US7853060B2 (en) 2006-04-13 2010-12-14 Siemens Aktiengesellschaft Method and MR system for generating MR images
CN104337516A (zh) * 2013-07-30 2015-02-11 西门子公司 磁共振控制序列的确定
US10012713B2 (en) 2013-07-30 2018-07-03 Siemens Aktiengesellschaft Method and device for determination of a magnetic resonance control sequence

Also Published As

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DE10232342A1 (de) 2004-01-29
AU2003247011A1 (en) 2004-02-02

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