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WO2016124397A1 - Imagerie par résonance magnétique (rm) avec cartographie b1 - Google Patents

Imagerie par résonance magnétique (rm) avec cartographie b1 Download PDF

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Publication number
WO2016124397A1
WO2016124397A1 PCT/EP2016/051069 EP2016051069W WO2016124397A1 WO 2016124397 A1 WO2016124397 A1 WO 2016124397A1 EP 2016051069 W EP2016051069 W EP 2016051069W WO 2016124397 A1 WO2016124397 A1 WO 2016124397A1
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Prior art keywords
pulses
magnetic field
during
signals
slice
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English (en)
Inventor
Kay Nehrke
Peter Boernert
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Koninklijke Philips NV
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Koninklijke Philips NV
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    • 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
    • G01R33/24—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/246—Spatial mapping of the RF magnetic field B1

Definitions

  • the invention relates to the field of magnetic resonance (MR) imaging. It concerns methods of MR imaging of at least a portion of a body.
  • the invention also relates to a MR device and to a computer program to be run on a MR device.
  • Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
  • the body of the patient to be examined is arranged in a strong, uniform magnetic field (Bo field) whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based.
  • the magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength. These energy levels can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as Bi field) of defined frequency (so-called Larmor frequency, or MR frequency).
  • RF field electromagnetic alternating field
  • Bi field defined frequency
  • the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
  • the precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle.
  • the magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse.
  • 90° pulse the spins are deflected from the z axis to the transverse plane (flip angle 90°).
  • the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant Ti (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T 2 (spin- spin or transverse relaxation time).
  • Ti spin lattice or longitudinal relaxation time
  • T 2 spin- spin or transverse relaxation time
  • the decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing).
  • the dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
  • the signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body.
  • the MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data.
  • the k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
  • Bi mapping is an essential pre-requisite for multi-coil transmit applications like RF shimming or accelerated multi-dimensional RF pulses.
  • the recently introduced DREAM Bi mapping approach allows the acquisition of a Bi map in a short time interval.
  • the DREAM approach includes a stimulated echo-type preparation phase in which locally effective flip angles of the RF pulses of the preparation phase are encoded into longitudinal magnetization. This flip angle information is obtained in a subsequent acquisition phase in which a FID signal and a stimulated echo signal are acquired.
  • a Bi map indicating the spatial distribution of the local flip angle i.e. the local amplitude of the RF pulses of the preparation phase
  • a method of MR imaging of an object placed in the examination volume of a MR device comprises the steps of: - subjecting the object to an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a stimulated echo sequence including:
  • one or more reading RF pulses are applied during the acquisition period of the stimulated echo sequence, wherein the FID signals and the stimulated echo signals are acquired quasi- simultaneously.
  • a MR image can be reconstructed from the FID signals and another MR image can be reconstructed from the stimulated echo signals.
  • the Bi map can be derived from the voxel-wise intensity ratio of the two MR images reconstructed from the FID and stimulated echo signals, respectively.
  • a plurality of FID signals and stimulated echo signals with appropriate phase encoding need to be acquired for generating a complete Bi map.
  • Efficient sampling schemes like EPI, parallel imaging or compressed sensing can be advantageously applied for this purpose in combination with the invention.
  • the slice-selection magnetic field gradients applied during the preparation period and during the acquisition period are of equal strength, resulting in a congruent shift of the slices excited during the preparation period and the acquisition period respectively.
  • the Bi encoding mechanism of the method remains valid, yielding proper Bi maps.
  • the off-resonance will result in a small spatial shift of the derived Bi map. Considering the smoothness of typical Bi maps, this can be expected to be a minor problem.
  • the proposed approach can be used for parallel transmit applications, wherein the RF pulses are radiated toward the portion of the body via two or more RF coils (or two or more sets of RF coils) in parallel.
  • a Bi map may be derived from the acquired FID and stimulated echo signals for each RF coil or set of RF coils, each Bi map indicating the spatial distribution of the RF field of the RF pulses irradiated via the respective RF coil or set of RF coils.
  • Bi mapping is performed for multiple RF transmit elements (RF coils) of the used MR device to map their corresponding transmit sensitivities.
  • An individual mapping scan may be performed according to the method of the invention for each individual RF transmit element (or for a combination of them).
  • a small spatial distortion of the Bi map caused by a small spatial shift of the derived Bi map is identical for each RF coil, and, hence, does not affect the RF shimming optimization problem.
  • the Bi maps obtained according to the invention can advantageously be used in subsequent imaging scans for RF shimming.
  • RF shimming refers to the spatial
  • the RF shim settings can be optimized in a patient specific way.
  • one or more Bi maps are derived from the FID and stimulated echo signals in the above-described fashion and optimal RF shim sets can be derived.
  • FID and stimulated echo MR signals are generated by means of a plurality of consecutive reading RF pulses, each having a flip angle of less than 90°, preferably less than 45°, most preferably less than 30°.
  • a train of reading RF pulses having small flip angles can be used to achieve a fast readout of multiple FID and stimulated echo signals.
  • echo times can be used in order to minimize T 2 relaxation.
  • the method of the invention described thus far can be carried out by means of a MR device including at least one main magnet coil for generating a uniform steady magnetic field within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing MR images from the received MR signals.
  • the method of the invention is preferably implemented by a corresponding programming of the reconstruction unit and/or the control unit of the MR device.
  • the methods of the invention can be advantageously carried out in most MR devices in clinical use at present. To this end it is merely necessary to utilize a computer program by which the MR device is controlled such that it performs the above-explained method steps of the invention.
  • the computer program may be present either on a data carrier or be present in a data network so as to be downloaded for installation in the control unit of the MR device.
  • Figure 1 schematically shows a MR device for carrying out the methods of the invention
  • Figure 2 shows a schematic diagram illustrating an imaging sequence according to the invention.
  • a MR device 1 comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field Bo is created along a z-axis through an examination volume.
  • the device further comprises a set of (1 st , 2 nd , and - where applicable - 3 rd order) shimming coils 2', wherein the current flow through the individual shimming coils of the set 2' is controllable for the purpose of minimizing Bo deviations within the examination volume.
  • a magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.
  • a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume.
  • a digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume.
  • a typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance.
  • the RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume.
  • the MR signals are also picked up by the body RF coil 9.
  • a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging.
  • the array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.
  • the array RF coils 1 1, 12, 13 may also be used for RF transmission, for example for the purpose of RF shimming.
  • the resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11 , 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown).
  • the receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.
  • a host computer 15 controls the current flow through the shimming coils 2' as well as the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of MR imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like.
  • EPI echo planar imaging
  • the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse.
  • a data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.
  • the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such like SENSE or SMASH.
  • the MR image may represent a planar slice through the patient, an array of parallel planar slices, a three- dimensional volume, or the like.
  • the image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.
  • Figure 2 shows a diagram illustrating an imaging sequence according to the invention.
  • the depicted imaging sequence is a stimulated echo sequence which is subdivided into a preparation period 21 and an acquisition period 22.
  • Two preparation RF pulses having a flip angle of a are applied during the preparation period 21 in the presence of a slice- selection magnetic field gradient Gsi.
  • the two preparation RF pulses are separated by a time interval T E .
  • a de-phaser magnetic field gradient G mc2 is applied between the two preparation RF pulses.
  • a sequence of reading RF pulses having flip-angle ⁇ are generated during the acquisition period 22 in the presence of a slice-selection gradient Gs 2 -
  • An FID signal Ii and a stimulated echo signal I 2 are acquired after each reading pulse as gradient-recalled echoes.
  • Applied phase-encoding magnetic field gradients are not shown in Figure 2 for
  • M z i and M z2 denote the un-prepared (i.e. in-phase) and the stimulated echo-prepared (i.e. de-phased) longitudinal magnetization, respectively.
  • Both the FID signal Ii generated from M z i and the stimulated echo signal I 2 generated from M z2 are acquired at different points in time T E i and ⁇ ⁇ ⁇ + ⁇ , respectively.
  • the delay ⁇ between the two echoes I ls I 2 is determined by the relation:
  • a mc2 denotes the gradient-time area of the de-phaser gradient G mc2 and G m denotes the strength of the readout magnetic field gradient.
  • I 2 S - C(T El + AT - T E )s ( )M z2
  • S represents a complex system constant, which is equal for both echo signals Ii and I 2 and which is determined e.g. by transmit and receive coil sensitivities for a given voxel
  • ⁇ is the nominal flip angle of the reading RF pulses.
  • C describes the static signal de -phasing for a given voxel due to susceptibility and chemical shift effects:
  • the de-phasing term C is identical for both echo signals, apart from the mirrored phase.
  • the imaging sequence shown in Fig. 2 is actually a basic version of the known DREAM Bi mapping sequence (Magnetic Resonance in Medicine, 68, 1517-1526, 2012).
  • Gsi is typically several times smaller than Gs 2 in the conventional DREAM technique. This implies that the slice encoded by the preparation RF pulses (a) and the slice encoded by the reading RF pulses ( ⁇ ) will shift non-congruently in the presence of B 0 inhomogeneity, which results in wrong estimates of the Bi map.
  • the slice-selection magnetic field gradients Gsi applied during the preparation period and the slice-selection magnetic field gradients Gs 2 applied during the acquisition period are all of equal strength.

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

Abstract

L'invention concerne un procédé d'imagerie par Résonance Magnétique (RM) d'un objet (10), l'objet (10) étant soumis à une séquence d'imagerie (IMG) d'impulsions radiofréquence (RF) et des gradients de champ magnétique commutés, la séquence d'imagerie (IMG) étant une séquence d'écho stimulé comprenant : i) au moins deux impulsions RF de préparation (α) rayonnées vers l'objet (10) en présence d'un gradient de champ magnétique de sélection de tranche (GS) pendant une période de préparation (21), et ii) une ou plusieurs impulsion(s) RF de lecture (β) rayonnée(s) vers l'objet (10) en présence d'un gradient de champ magnétique de sélection de tranche (GS) pendant une période d'acquisition (22) temporellement ultérieure à la période de préparation (21), les gradients de champ magnétique de sélection de tranche (GS) appliqués pendant la période de préparation (21) et pendant la période d'acquisition (22) étant d'intensité égale. Un signal ou plusieurs signaux FID (I1) et un signal ou plusieurs signaux d'échos stimulés (I2) sont acquis pendant la période d'acquisition (22). Au moins une carte B1 indiquant la distribution spatiale du champ RF des impulsions RF dans l'objet (10) est dérivée des signaux FID (I1) et d'écho stimulé (I2) acquis. L'invention concerne en outre un dispositif à RM (1) et un programme informatique pour un dispositif à RM (1).
PCT/EP2016/051069 2015-02-02 2016-01-20 Imagerie par résonance magnétique (rm) avec cartographie b1 Ceased WO2016124397A1 (fr)

Applications Claiming Priority (2)

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EP15153416.1 2015-02-02
EP15153416 2015-02-02

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WO2016124397A1 true WO2016124397A1 (fr) 2016-08-11

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120139539A1 (en) * 2010-12-01 2012-06-07 Mathias Nittka Method for generating a pulse sequence to acquire magnetic resonance data, and operating method and magnetic resonance system employing the generated pulse sequence
US20120235684A1 (en) * 2011-03-16 2012-09-20 Alto Stemmer Method to determine the actual flip angle and method to adjust the transmitter voltage in a magnetic resonance apparatus
WO2013105006A1 (fr) * 2012-01-12 2013-07-18 Koninklijke Philips N.V. Imagerie par résonance magnétique avec cartographie b1
US20140218022A1 (en) * 2013-02-01 2014-08-07 Michael Koehler Magnetic resonance system and method to acquire mr data and to determine a b1 magnetic field

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120139539A1 (en) * 2010-12-01 2012-06-07 Mathias Nittka Method for generating a pulse sequence to acquire magnetic resonance data, and operating method and magnetic resonance system employing the generated pulse sequence
US20120235684A1 (en) * 2011-03-16 2012-09-20 Alto Stemmer Method to determine the actual flip angle and method to adjust the transmitter voltage in a magnetic resonance apparatus
WO2013105006A1 (fr) * 2012-01-12 2013-07-18 Koninklijke Philips N.V. Imagerie par résonance magnétique avec cartographie b1
US20140218022A1 (en) * 2013-02-01 2014-08-07 Michael Koehler Magnetic resonance system and method to acquire mr data and to determine a b1 magnetic field

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
AKOKA S ET AL: "Radiofrequency map of an NMR coil by imaging", MAGNETIC RESONANCE IMAGING, ELSEVIER SCIENCE, TARRYTOWN, NY, US, vol. 11, no. 3, 1 January 1993 (1993-01-01), pages 437 - 441, XP002466125, ISSN: 0730-725X, DOI: 10.1016/0730-725X(93)90078-R *
FINSTERBUSCH J ET AL: "Single-shot 3D gradient and stimulated echo imaging", PROCEEDINGS OF THE INTERNATIONAL SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE, 14TH SCIENTIFIC MEETING AND EXHIBITION, SEATTLE, WASHINGTON, USA, 6-12 MAY 2006, 22 April 2006 (2006-04-22), pages 2424, XP040597941 *
KAY NEHRKE ET AL: "DREAM-a novel approach for robust, ultrafast, multislice B 1 mapping", MAGNETIC RESONANCE IN MEDICINE, vol. 68, no. 5, 1 November 2012 (2012-11-01), pages 1517 - 1526, XP055056287, ISSN: 0740-3194, DOI: 10.1002/mrm.24158 *

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