WO2007004123A2 - Séquence de navigateur d'irm à magnétisation restaurée - Google Patents

Séquence de navigateur d'irm à magnétisation restaurée Download PDF

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Publication number
WO2007004123A2
WO2007004123A2 PCT/IB2006/052116 IB2006052116W WO2007004123A2 WO 2007004123 A2 WO2007004123 A2 WO 2007004123A2 IB 2006052116 W IB2006052116 W IB 2006052116W WO 2007004123 A2 WO2007004123 A2 WO 2007004123A2
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WO
WIPO (PCT)
Prior art keywords
navigator
sequence
imaging
signals
volume
Prior art date
Application number
PCT/IB2006/052116
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English (en)
Other versions
WO2007004123A3 (fr
Inventor
Gabriele M. Beck
Gerrit H. Van Ijperen
Original Assignee
Koninklijke Philips Electronics N.V.
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2007004123A2 publication Critical patent/WO2007004123A2/fr
Publication of WO2007004123A3 publication Critical patent/WO2007004123A3/fr

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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
    • G01R33/44—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
    • G01R33/567—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution gated by physiological signals, i.e. synchronization of acquired MR data with periodical motion of an object of interest, e.g. monitoring or triggering system for cardiac or respiratory gating
    • G01R33/5676—Gating or triggering based on an MR signal, e.g. involving one or more navigator echoes for motion monitoring and correction

Definitions

  • the invention relates to a method for magnetic resonance (MR) imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field.
  • MR magnetic resonance
  • the invention relates to a device for MR imaging and to a computer program for an MR device.
  • pulse sequences consisting of RF and magnetic field gradient pulses are applied to an object (a patient) to generate magnetic resonance signals, which are scanned in order to obtain information therefrom and to reconstruct images of the object. Since its initial development, the number of clinical relevant fields of application of MRI has grown enormously. MRI can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body.
  • the pulse sequence which is applied during an MRI scan determines completely the characteristics of the reconstructed images, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of a MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.
  • MRI methods can be employed for magnetic resonance angiography (MRA), particularly for coronary MRA.
  • MRA magnetic resonance angiography
  • respiratory motion of the heart can severely deteriorate the image quality of cardiac MR imaging.
  • Gating and image correction based on MR navigator signals was introduced to reduce these artifacts.
  • the position of the diaphragm can be monitored and used as an input for an appropriate gating algorithm.
  • the information of the navigator signal may be used to perform motion correction to improve image quality.
  • the afore-described navigator technique can generally be applied in different fields of MR imaging in order to detect a specific change in imaging conditions.
  • a further example is the triggering of an imaging sequence after the bolus arrival of a contrast agent at a specific organ of interest. Nevertheless, the most prominent use of the navigator technique is the detection of a breathing state. It is applied whenever the breathing motion may have an adverse influence on image detection.
  • 2D RF pulses For registering the MR navigator signals, so-called 2D RF pulses may be used. These excite a spatially restricted volume, for example of pencil beam shape, which is read out using a gradient echo. This allows to monitor motions of the examined portion of the body along one direction.
  • the navigator volume In coronary MRA, the navigator volume is usually localized at the dome of the right hemidiaphragm such that the motion of the diaphragm can be observed by the image contrast between the liver and the lung.
  • the above-mentioned 2D RF pulses consist of shaped RF pulses which are irradiated in combination with fast magnetic field gradient switching. It has been shown, that this technique facilitates the excitation of arbitrarily shaped profiles in two dimensions.
  • An alternative method for generating MR navigator signals in a spatially restricted volume is to excite nuclear magnetization by means of two subsequent slice- selective RF pulses. The slices acted upon by the two RF pulses are selected such that they are crossing each other along the desired beam-shaped volume. The MR navigator signal may then easily be detected as a spin echo in the presence of a read out gradient along the direction of the line of intersection of the two slices. Motions of the examined portion of the body along this direction can be monitored in this way.
  • spin echoes are measured as MR imaging signals for reconstructing an MR image therefrom, for example by 2D Fourier transformation.
  • a (TVcontrast enhanced) MRA procedure of the type specified above is for example described in the document WO 2004/034075 Al.
  • a T2-preparation sequence is applied in order to obtain the desired contrast between blood and muscle.
  • the patient is subjected to a 2D navigator sequence and the MR navigator signal is measured.
  • a series of MR imaging signals is subsequently generated by a corresponding imaging sequence and an MR image is reconstructed therefrom.
  • the essential feature of the method disclosed in the mentioned document is that a 2D navigator restore sequence is applied after the T2-preparation sequence and prior to the 2D navigator sequence.
  • the 2D navigator restore sequence of the known method is applied in order to avoid that the initial T 2 -preparation disturbs the generation and registration of the MR navigator signal.
  • the 2D navigator restore sequence compensates for the (adverse) effect the T2-preparation sequence has on the MR navigator signal. An improved functioning of the navigator is achieved in this way.
  • the restricted navigator volume is ideally placed over the interface (localized at the dome of the right hemidiaphragm) between the liver and the lung in order to detect the breathing state of the examined patient. This is because of the high MR signal contrast between the lung and the liver.
  • a problem of this known technique is that the nuclear magnetization within the restricted navigator volume remains saturated after measuring the MR navigator signals. Because of this, the known navigator methods are difficult to apply for MR imaging of the liver, the kidneys, or the renal arteries. It cannot be avoided that the navigator volume is (at least partly) superimposed upon the respective regions of interest with the negative consequence that the navigator volume appears as a saturated region in the reconstructed MR images.
  • a method for magnetic resonance imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field comprises the following steps: exciting nuclear magnetization within a spatially restricted navigator volume by subjecting the portion to a navigator sequence; measuring MR navigator signals from the navigator volume; transforming the nuclear magnetization within the navigator volume back into longitudinal magnetization by subjecting the portion to a navigator unlabeling sequence; generating a series of MR imaging signals by subjecting the portion to an imaging sequence and measuring the MR imaging signals for reconstructing an MR image from these signals.
  • the present invention suggests to apply a navigator unlabeling sequence prior to generating the actual imaging sequence.
  • the effect of the navigator unlabeling sequence is that the nuclear magnetization within the restricted navigator volume is converted back into longitudinal magnetization. In this way the effect of the preceding navigator sequence is largely neutralized and the actual image acquisition starts without disturbance by the navigator. Since there remains essentially no saturated magnetization within the navigator volume, imaging can be performed without restrictions with regard to the location of the region of interest. Imaging is possible even if the navigator volume and the region of interest are fully or partially overlapping.
  • 2D sequences comprising at least two shaped RF pulses and at least one gradient pulse being switched during irradiation of said shaped RF pulse are well suited both as a navigator sequence and as a navigator unlabeling sequence in accordance with the present invention.
  • the 2D navigator unlabeling sequence is applied for selectively transforming magnetization within the navigator volume back into longitudinal magnetization before the actual imaging sequence starts.
  • the navigator sequence and the navigator unlabeling sequence may each comprise at least two spatially selective RF pulses for first exciting and then transforming back nuclear spins within two crossing slice-shaped regions within the body of the examined patient.
  • consecutive slice-selective RF pulses may be used in order to prepare the desired state of the spins in the beam- shaped crossing region forming the navigator volume.
  • the navigator sequence comprises a slice-selective 90° RF pulse and an also slice-selective 180° RF refocusing pulse, which refocuses the magnetization only in the crossing region of the two slices. This signal is measured as the MR navigator signal in the presence of a read-out gradient.
  • the navigator unlabeling sequence comprises two inverse 180° and 90° RF pulses in order to compensate for the effect of the two RF pulses of the navigator sequence.
  • the MR navigator signal of the present invention can advantageously be employed for gating of the imaging sequence and/or for adjusting the parameters of said imaging sequence and/or for correction of said MR image.
  • good results are obtained if both gating and adaptive motion correction of the imaged volume (so-called slice-tracking) are performed. It is easily possible to incorporate the method of the present invention in a dedicated device for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field.
  • Such an MRI scanner comprises means for establishing the main magnetic field, means for generating gradient magnetic fields superimposed upon the main magnetic field, means for radiating RF pulses towards the body, control means for controlling the generation of the gradient magnetic fields and the RF pulses, means for receiving and sampling magnetic resonance signals generated by sequences of RF pulses and switched gradient magnetic fields, and reconstruction means for forming an image from said signal samples.
  • the control means which is usually a microcomputer with a memory and a program control, comprises a programming with a description of an imaging procedure according to the above-described method of the invention.
  • a computer program with instructions for carrying out the imaging procedure of the invention can advantageously be implemented on any common computer hardware, which is presently in clinical use for the control of magnetic resonance scanners.
  • the computer program can be provided on suitable data carriers, such as CD-ROM or diskette. Alternatively, it can also be downloaded by a user from an Internet server.
  • Fig.l shows a diagram of a pulse sequence in accordance with a first embodiment of the present invention
  • Fig.2 shows a diagram of a pulse sequence in accordance with a second embodiment of the present invention
  • Fig.3 shows an embodiment of an MRI scanner according to the invention.
  • FIG.l A first sequence design in accordance with the method of the present invention is depicted in Fig.l.
  • the diagram shows the temporal succession of radio frequency pulses RF and of magnetic field gradient pulses GX, GY, GZ in three orthogonal directions.
  • a patient placed in a stationary and substantially homogeneous main magnetic field is subjected to these pulses during the MRI procedure of the invention.
  • the sequence begins with a 2D navigator sequence NAV.
  • the navigator sequence NAV might be applied after an initial T 2 preparation sequence (not depicted) for obtaining the desired contrast.
  • the sequence NAV comprises a 2D pulse consisting of a shaped RF pulse ⁇ , during which gradients GX and GY are switched rapidly.
  • a restricted two-dimensional spatial profile as for example a pencil beam shaped navigator volume at the dome of the right diaphragm of the patient, is excited by these pulses.
  • an MR navigator signal is measured in the presence of a read-out gradient GZ, thereby enabling the reconstruction of a one-dimensional image of the navigator volume. This image is used to monitor the position of the patient's diaphragm during respiration.
  • the sequence UNLBL comprises an inverse 2D pulse consisting of a shaped RF pulse - ⁇ and rapidly switched gradients GX and GY.
  • the inverse 2D pulse selectively transforms the nuclear spins within the navigator volume back into longitudinal magnetization such that there remains essentially no saturated nuclear magnetization within the region of interest before the actual image acquisition starts.
  • Two spatially non-selective 180° RF pulses are used as part of the sequence UNLBL before and after the inverse 2D pulse in order to compensate for dephasing of transverse nuclear magnetization due to inhomogeneities of the main magnetic field B 0 .
  • the final 180° RF pulse is used to re-invert the nuclear magnetization which was not excited by both ⁇ and - ⁇ pulses. It is possible to dispense with the final non-selective 180° RF pulse if the duration of the sequence UNLBL should be limited.
  • a series of MR imaging signals is generated by subjecting the patient to a turbo field echo sequence TFE. These signals are measured and used for reconstruction of a diagnostic MR image, for example of the kidneys or the renal arteries of the patient.
  • the navigator signals which had been measured during the sequence NAV, are used for gating of the imaging sequence TFE and for correction of the reconstructed MR image.
  • the navigator sequence NAV comprises a first slice-selective RF pulse ⁇ which is generated in the presence of a first slice selection gradient GX.
  • a second 180° RF pulse refocuses the magnetization excited by the RF pulse ⁇ in the presence of a second slice selection gradient GY.
  • the result is a spin echo emanating from the beam shaped crossing region of the two orthogonal slices which is measured as an MR navigator signal in the presence of a read-out gradient GZ.
  • a one- dimensional image of the navigator volume is reconstructed from the spin echo signal. This image is used to monitor the position of the patient's diaphragm during respiration.
  • the navigator unlabeling sequence UNLBL comprises a slice-selective RF pulse - ⁇ and a slice-selective -180° RF pulse in reverse order.
  • the - ⁇ RF pulse acts on the same slice as the initial RF pulse ⁇ and the slice selection of the -180° RF pulse corresponds to the 180° RF pulse of the sequence NAV.
  • the effect of the two inverse RF pulses of the sequence UNLBL is that the spins within the navigator volume are transformed back into the longitudinal direction such that essentially no saturated state of magnetization remains before the actual image acquisition starts with the sequence TFE.
  • Fig.2 shows that the unlabeling sequence UNLBL further includes a slice- selective 180° RF pulse which is irradiated before the two inverse RF pulses.
  • This 180° RF pulse acts on the same slice as the RF pulse ⁇ of the sequence NAV and is used as a further refocusing pulse in order to avoid problems due to field inhomogeneity effects.
  • spatially non-selective 180° RF pulses may be generated before and/or after the slice selective inverse RF pulses of the sequence UNLBL.
  • a magnetic resonance imaging device 1 is diagrammatically shown.
  • the apparatus 1 comprises a set of main magnetic coils 2 for generating a stationary and homogeneous main magnetic field and three sets of gradient coils 3, 4 and 5 for superimposing additional magnetic fields with controllable strength and having a gradient in a selected direction.
  • the direction of the main magnetic field is labelled the z- direction, the two directions perpendicular thereto the x- and y-directions.
  • the gradient coils are energized via a power supply 11.
  • the apparatus 1 further comprises a radiation emitter 6, an antenna or coil, for emitting radio frequency (RF) pulses to a body 7, the radiation emitter 6 being coupled to a modulator 8 for generating and modulating the RF pulses.
  • RF radio frequency
  • a receiver for receiving the MR-signals can be identical to the emitter 6 or be separate. If the emitter and receiver are physically the same antenna or coil as shown in Fig.3, a send-receive switch 9 is arranged to separate the received signals from the pulses to be emitted.
  • the received MR-signals are input to a demodulator 10.
  • the modulator 8, the emitter 6 and the power supply 11 for the gradient coils 3, 4 and 5 are controlled by a control system 12 to generate the above-described sequence of RF pulses and a corresponding sequence of magnetic field gradient pulses.
  • the control system is usually a microcomputer with a memory and a program control.
  • the demodulator 10 is coupled to a data processing unit 14, for example a computer, for transformation of the received echo signals into an image that can be made visible, for example on a visual display unit 15.
  • a data processing unit 14 for example a computer
  • an input means 16 e.g. an appropriate keyboard, connected to the control system 12, which enables an operator of the device to interactively adjust the parameters of the imaging procedure.

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  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Cardiology (AREA)
  • Radiology & Medical Imaging (AREA)
  • Physiology (AREA)
  • Power Engineering (AREA)
  • Pulmonology (AREA)
  • General Health & Medical Sciences (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
  • Biophysics (AREA)
  • Signal Processing (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • High Energy & Nuclear Physics (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 (IRM) d'au moins une partie d'un corps placé dans un champ magnétique principal fixe et sensiblement homogène. Selon ce procédé, une magnétisation nucléaire est excitée à l'intérieur d'un volume de navigateur limité dans l'espace par exposition de ladite partie à une séquence de navigateur (NAV) à partir de laquelle des signaux de navigateur RM sont mesurés. La magnétisation nucléaire à l'intérieur dudit volume de navigateur est ensuite retransformée en magnétisation longitudinale par exposition de ladite partie à une séquence de suppression du navigateur (UNLBL). Pour finir, une série de signaux d'imagerie RM est générée par exposition de ladite partie à une séquence d'imagerie (TFE). Ces signaux d'imagerie RM sont mesurés pour reconstruire une image RM à partir desdits signaux.
PCT/IB2006/052116 2005-06-30 2006-06-27 Séquence de navigateur d'irm à magnétisation restaurée WO2007004123A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP05105892.3 2005-06-30
EP05105892 2005-06-30

Publications (2)

Publication Number Publication Date
WO2007004123A2 true WO2007004123A2 (fr) 2007-01-11
WO2007004123A3 WO2007004123A3 (fr) 2007-05-03

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008041060A1 (fr) * 2006-10-05 2008-04-10 Koninklijke Philips Electronics N.V. séquence de navigateur IRM avec magnétisation restaurée
CN102772209A (zh) * 2011-01-27 2012-11-14 美国西门子医疗解决公司 用于在所成像的组织类型之间进行区分的mr成像系统
US9223001B2 (en) 2009-11-05 2015-12-29 Koninklijke Philips N.V. MR imaging using navigators

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2004034075A1 (fr) * 2002-10-11 2004-04-22 Koninklijke Philips Electronics N.V. Procede par resonance magnetique et dispositif correspondant
JP2005021488A (ja) * 2003-07-04 2005-01-27 Hitachi Medical Corp 磁気共鳴イメージング装置

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2004034075A1 (fr) * 2002-10-11 2004-04-22 Koninklijke Philips Electronics N.V. Procede par resonance magnetique et dispositif correspondant
JP2005021488A (ja) * 2003-07-04 2005-01-27 Hitachi Medical Corp 磁気共鳴イメージング装置

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
STUBER M ET AL: "THREE-DIMENSIONAL HIGH-RESOLUTION FAST SPIN-ECHO CORONARY MAGNETIC RESONANCE ANGIOGRAPHY" MAGNETIC RESONANCE IN MEDICINE, ACADEMIC PRESS, DULUTH, MN, US, vol. 45, no. 2, February 2001 (2001-02), pages 206-211, XP001165735 ISSN: 0740-3194 *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008041060A1 (fr) * 2006-10-05 2008-04-10 Koninklijke Philips Electronics N.V. séquence de navigateur IRM avec magnétisation restaurée
US9223001B2 (en) 2009-11-05 2015-12-29 Koninklijke Philips N.V. MR imaging using navigators
CN102772209A (zh) * 2011-01-27 2012-11-14 美国西门子医疗解决公司 用于在所成像的组织类型之间进行区分的mr成像系统

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Publication number Publication date
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