WO2011080693A1 - Procédé d'imagerie d'au moins deux espèces chimiques à l'aide de l'imagerie par résonance magnétique - Google Patents

Procédé d'imagerie d'au moins deux espèces chimiques à l'aide de l'imagerie par résonance magnétique Download PDF

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WO2011080693A1
WO2011080693A1 PCT/IB2010/056083 IB2010056083W WO2011080693A1 WO 2011080693 A1 WO2011080693 A1 WO 2011080693A1 IB 2010056083 W IB2010056083 W IB 2010056083W WO 2011080693 A1 WO2011080693 A1 WO 2011080693A1
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echo
partial
chemical species
echoes
image data
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PCT/IB2010/056083
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English (en)
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Miha Fuderer
Holger Eggers
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Koninklijke Philips Electronics N.V.
Philips Intellectual Property & Standards Gmbh
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Publication of WO2011080693A1 publication Critical patent/WO2011080693A1/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
    • G01R33/4828—Resolving the MR signals of different chemical species, e.g. water-fat imaging

Definitions

  • the present invention relates to a method of imaging at least two chemical species using magnetic resonance imaging (MRI) with signal separation for the at least two chemical species, a computer program product and a magnetic resonance imaging apparatus for imaging at least two chemical species with signal separation for the at least two chemical species.
  • MRI magnetic resonance imaging
  • 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 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 which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency).
  • RF field electromagnetic alternating field
  • Larmor frequency Larmor 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 (also referred to as longitudinal 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.
  • 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 Tl (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time).
  • Tl spin lattice or longitudinal relaxation time
  • T2 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 signal data obtained via the receiving 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 an MR image by means of Fourier transformation.
  • Dixon imaging usually relies on the acquisition of at least two echoes to separate water and fat signal.
  • water and fat images are generated by either addition or subtraction of the "in-phase” and "out-of-phase” data sets. This separation is possible because of the precessional frequency difference of hydrogen in fat and water.
  • One drawback of Dixon imaging is that the acquisition of at least two echoes results in a substantial prolongation of the overall scan time.
  • a method of imaging at least two chemical species using magnetic resonance imaging with signal separation for the at least two chemical species comprises acquiring dual partial echoes at different echo times, wherein the partial echoes of the respective echo times are located in opposite regions of k-space.
  • the processing of the partial echoes comprises a Dixon reconstruction technique.
  • the two chemical species are water and fat.
  • the dual partial echoes are acquired by means of a balanced fast field echo pulse sequence (BFFE sequence).
  • BFFE sequence balanced fast field echo pulse sequence
  • the processing of the partial echoes comprises removing the effect of main field inhomogeneity of the main magnetic field used for imaging the chemical species. This ensures that with respect to the acquired data the main field inhomogeneity excluding chemical shift is only varying smoothly over space. As a consequence, state of the art partial echo reconstruction can be performed which relies on the assumption that all phase variations over space are smooth.
  • the echo times are chosen such that a phase shift due to chemical shift is large compared to the phase errors induced by the partial echo reconstruction.
  • the phase shift induced by the partial echo reconstruction will be at most ⁇ /2 while the phase shift due to the chemical shift is between ⁇ /2 and ⁇ such that it will dominate over any phase errors induced by the partial echo reconstruction process.
  • a more precise determination of desired phase shifts due to a chemical shift depending on the water/fat fraction in the voxel (three dimensional pixel) and the difference in echo times can be performed. Further, said desired phase shift can be distinguished over undesired phase shifts due to partial echo
  • the echo times are chosen such that their differences in phase shift due to chemical shift is either 0 or ⁇ , independent of the water/fat fraction. This also ensures in a similar manner, as described above, that the possible differences between phase shifts due to the chemical shifts are constrained and a separation of both effects, namely desired and undesired phase shifts, becomes feasible and a clear separation of water and fat signal is obtained.
  • a central k-space area is covered by each partial echo. This allows deriving low resolution estimates of water signal, fat signal, and field strength from two such echoes without relying on a partial echo reconstruction. This information can then be used to remove the influence of the main field inhomogeneity from the data and additionally to optionally constrain subsequently derived high resolution estimates.
  • the estimate on the field strength may be initialized using a priori information, for instance from results for adjacent slices or for models of the field distortion. Otherwise, the estimate from the previous iteration on the low resolution level may be interpolated to the current resolution level.
  • An appropriate starting point may be selected, usually based on criteria such as the signal strength, the estimate of the field strength at the spatial position, and the fit error.
  • the water signal, the fat signal and the field strength are calculated along a predefined path covering the field of view, for instance with a least squares estimation.
  • Dependent on the obtained results and the signal- to-noise ratio (SNR) the estimates are either marked as reliable or unreliable.
  • the calculation is preferably performed in several passes, wherein in this way pixels for which the estimates are initially considered as unreliable may benefit from the availability of more reliable information in their vicinity in later passes. These steps may be repeated for two or more resolution levels.
  • Constraining the deviation of the field strength from the estimate found on a coarser resolution level prevents to a large extend the local swapping of water and fat signal on finer resolution levels.
  • the allowed maximum deviation may be made dependent on the local signal strength and the spatial position, for instance to account for larger variations in the field strength at tissue air interfaces and towards the edges of the homogeneity volume.
  • a multi path strategy or a path covering the field of view may be extended to 3D.
  • the invention in another aspect, relates to a magnetic resonance imaging apparatus for imaging at least two chemical species with signal separation for the at least two chemical species, wherein the apparatus comprises a magnetic resonance imaging scanner for acquiring magnetic resonance image data, a controller adapted for controlling a scanner operation of acquiring dual partial echoes at different echo times, wherein the partial echoes of the respective echo times are located in opposite regions of k-space.
  • the apparatus further comprises a data reconstruction system adapted for processing the partial echoes for reconstruction of a first and second image data set, wherein the first and second image data set comprises separate image data of the first and second chemical species.
  • the data reconstruction system may be further adapted for processing of the partial echoes by means of a Dixon reconstruction technique.
  • the apparatus may be adapted in order to carry out any of the method steps described above.
  • the method 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. Therefore, the invention also relates to a computer program product comprising computer executable instructions to perform the method as described above.
  • Fig. 1 shows an MR device for carrying out the method of the invention
  • Figs. 2a,2b illustrate a pulse sequence diagram for Dixon imaging
  • Figs. 3a,3b show a pulse sequence diagram employing a combination of a fast spin echo sequence with partial echo imaging
  • Fig. 4 illustrates a pulse sequence diagram in which a fast spin echo sequence with interleaving of echo times by RF pulses is used
  • Fig. 5 illustrates a pulse sequence diagram in which a GRASE like fast spin echo sequence with three echoes is used in combination with partial echo imaging
  • Fig. 6 illustrates a pulse sequence diagram in which a balanced fast field echo in combination with dual partial echoes is used
  • Figs. 7a,7b show a pulse sequence diagram employing a combination of a fast spin echo sequence with partial echo imaging and slightly asymmetric echo time formation.
  • a MR device 1 With reference to Figure 1, a MR device 1 is shown.
  • the device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an 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
  • a RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a whole-body volume RF coil 9 to transmit RF pulses into the examination volume.
  • a typical 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 whole-body volume 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 resultant MR signals are picked up by the whole body volume 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 gradient pulse amplifier 3 and the transmitter
  • 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 analogue-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.
  • 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.
  • the host computer 15 and the reconstruction processor 17 comprise a programming by which they are enabled to execute the above-described MR imaging method of the invention.
  • FIG. 2 illustrates a pulse sequence diagram of a state of Dixon imaging technique.
  • the phase encoding and slice selection gradients are not shown in this diagram of a fast spin echo sequence. Only shown are excitation and refocusing pulses, as well as frequency encoding gradients and acquired signals.
  • the sequence starts in Figure 2a with an excitation pulse 100, followed by a frequency encoding gradient of area b. Subsequently, a refocusing pulse 102 is applied which leads to the formation of an echo 110.
  • a frequency encoding gradient 106 is centered on the centre 112 of the echo 110 and has an area which equals 2b.
  • This may be repeated with one or more subsequent identical refocusing pulses, interleaved by respective frequency encoding gradients centered on the echo 110 having an area 2b.
  • a second fast spin echo sequence is applied starting again with an excitation pulse 100 and followed by a frequency encoding gradient 118 of area b.
  • the frequency encoding gradient 118 is followed by a refocusing pulse 120 in such a manner that this time the measurement samples the echo at a slightly different point in time than in the previous measurement illustrated in Figure 2a.
  • one set of measurements e.g. the top one
  • Another set of measurements e.g. the bottom row
  • T WF - In Dixon imaging, the multiple images acquired at different echo times are jointly processed to produce water and fat selective images.
  • the pulse sequence shown in Figure 2a,b has the disadvantage that in case a large T WF is required in order to improve the visualization of the difference between water and fat protons, the interval between the refocusing pulses has to be increased. This also increases the repetition time, which is often undesirable.
  • FIG. 3a,b a fast spin echo sequence is shown which consists of two measurements as illustrated in Figure 3 a and Figure 3b.
  • both measurements sample only partial echoes, wherein the echoes are partial on opposite sides of k-space.
  • T WF is increased for a given time interval between the refocusing pulses.
  • the time interval between subsequent refocusing pulses may be decreased which allows shortening the repetition time when carrying out the pulse sequence.
  • the first measurement starts with an excitation pulse 100, followed by a frequency encoding gradient 200 of area a.
  • This gradient is followed by a refocusing pulse 102 and a frequency encoding gradient 202, wherein the gradient 202 is dimensioned in such a manner that a partial echo 204 is formed.
  • the frequency encoding gradient 202 consists of an inverted part with area b-a and a positive part of area b+a.
  • a further refocusing pulse 104 may be applied, followed by a further frequency encoding gradient of similar type. This may be repeated one or several times.
  • the reason for using a combination of an inverted and positive frequency encoding gradient is that the application of a fast spin echo sequence requires keeping the frequency encoding gradient area between two refocusing pulses constant.
  • the second measurement shown in Figure 3b also starts with an excitation pulse 100, followed by an frequency encoding gradient 206 of area a. Subsequently, a refocusing pulse 120 is applied which is followed by a frequency encoding gradient 208. While in Figure 3 a the frequency encoding gradient 202 was arranged for an echo formation at a time later than the RF echo time, in Figure 3b the gradient 208 is arranged to permit an echo formation at a time earlier than the RF echo time to obtain a controlled dephasing between water and fat protons. As a consequence, a partial echo 210 which builds up in sequence 3b is pushed to an earlier formation time compared to the echo 204 in Figure 3 a.
  • the two echo formation times should not be symmetric with respect to the RF echo time, since this makes the separation difficult. Therefore, as shown with respect to Figure 7a,b (which is basically identical to Figure 3), a slightly asymmetric setup is also desired.
  • the frequency encoding gradient may be followed by one or more further refocusing pulses 126, 128, as well as further frequency encoding gradients 210.
  • FIG. 4 illustrates a pulse sequence diagram in which a fast spin echo sequence with interleaving of echo times by RF pulses is used.
  • the sequence starts with an excitation pulse 100, followed by a frequency encoding gradient 400 of area a. This is followed by a refocusing pulse 102 and subsequently a frequency encoding gradient 402.
  • This frequency encoding gradient 402 is similar to the frequency encoding gradient 202 which was described with respect to Figure 3 a. Instead of using two sets of different measurements as discussed with respect to Figures 3a and 3b, in Figure 4 only one set of measurement is used. Therefore, the frequency encoding gradient 402 which leads to the formation of the partial echo 406 is followed by a refocusing pulse 104 and subsequently by a further frequency encoding gradient 404.
  • This frequency encoding gradient 404 is similar to the frequency encoding gradient 208 as discussed with respect to Figure 3b.
  • two different echoes appearing at different points in time than the RF echo time are obtained and can be combined in order to obtain information with respect to the relative contribution of the two dominant chemical species like water and fat.
  • FIG. 5 illustrates a GRASE like fast spin echo sequence.
  • GRASE is the abbreviation of "Gradient and Spin Echo” and stands for a hybrid sequence with a combination of a gradient and spin echo sequences.
  • the GRASE like fast spin echo sequence in Figure 5 comprises three echoes 502, 504 and 506, wherein the echoes 502 and 506 are partial echoes on opposite sides of k- space.
  • the sequence starts with an excitation pulse 100, followed by a frequency encoding gradient 508 of area a. This is followed by a refocusing pulse 102, a frequency encoding gradient 500 of area a+b which leads to the formation of a partial echo 502, an inverted frequency encoding gradient of area 2b which leads to the formation of the inverted echo 504, as well as a frequency encoding gradient 500 of length b+a which leads to the formation of the partial echo 506.
  • the principle of dual partial echoes may equally be applied advantageously to balanced fast field echo (BFFE) sequences, as shown in the embodiment of Figure 6.
  • the sequence starts with a pulse a, denoted by the reference numeral 600.
  • a subsequent frequency encoding gradient 606 leads to the formation of two consecutive partial echoes 610 and 612, which are both partial on opposite sides of k-space.
  • a further pulse -a is applied, as denoted by the reference numeral 604.
  • This pulse 604 is again followed by the frequency encoding gradient 606 which similarly as before leads to the formation of two partial echoes 610 and 612 which are formed at different points in time than the RF echo time.
  • the sequence may then be continued by again a further pulse 600.
  • this information can be used to remove the influence of main magnetic field inhomogeneity from the data and in additional to optionally constrain subsequently derived high resolution estimates.
  • the resulting complex images contain two kinds of phase information, namely desired phase shifts due to chemical shift, depending on the water/fat fraction in the voxel and the difference in echo times, and undesired phase shifts due to the partial echo reconstruction.

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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 porte sur un procédé d'imagerie d'au moins deux espèces chimiques à l'aide de l'imagerie par résonance magnétique avec séparation de signaux pour lesdites au moins deux espèces chimiques, le procédé consistant à : acquérir des échos partiels doubles (204, 210, 406, 408, 502, 506, 10, 612) à différents temps d'écho, des échos partiels aux temps d'écho respectifs étant situés dans des régions opposées de l'espace k, traiter les échos partiels pour la reconstruction d'un premier ensemble de données d'image et d'un second ensemble de données d'image, les premier et second ensembles de données d'image comprenant des données d'image séparées des première et seconde espèces chimiques.
PCT/IB2010/056083 2009-12-30 2010-12-27 Procédé d'imagerie d'au moins deux espèces chimiques à l'aide de l'imagerie par résonance magnétique WO2011080693A1 (fr)

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WO2018114554A1 (fr) * 2016-12-20 2018-06-28 Koninklijke Philips N.V. Imagerie rm à séparation eau/graisse de type dixon
EP3511725A1 (fr) * 2018-01-11 2019-07-17 Koninklijke Philips N.V. Imagerie par résonance magnétique dixon à double résolution
DE102018208569A1 (de) * 2018-05-30 2019-12-05 Siemens Healthcare Gmbh Aufnahme zweier Magnetresonanz-Bilder
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US10234523B2 (en) 2013-12-19 2019-03-19 Koninklijke Philips N.V. MRI with dixon-type water/fat separation with estimation of the main magnetic field variations
WO2015091956A1 (fr) * 2013-12-19 2015-06-25 Koninklijke Philips N.V. Irm avec séparation eau/graisse de type dixon avec estimation des variations du champ magnétique principal
CN105934683A (zh) * 2013-12-19 2016-09-07 皇家飞利浦有限公司 具有对主磁场变化的估计的dixon型水/脂肪分离的mri
CN105934683B (zh) * 2013-12-19 2019-05-28 皇家飞利浦有限公司 具有对主磁场变化的估计的dixon型水/脂肪分离的mri
KR101863893B1 (ko) 2014-07-31 2018-06-01 지멘스 악티엔게젤샤프트 자기 공명 데이터를 취득하기 위한 방법 및 이를 위한 수단
US10379185B2 (en) 2014-07-31 2019-08-13 Siemens Aktiengesellschaft Method and apparatus for acquiring magnetic resonance data
KR20160016672A (ko) * 2014-07-31 2016-02-15 지멘스 악티엔게젤샤프트 자기 공명 데이터를 취득하기 위한 방법 및 이를 위한 수단
US10895619B2 (en) 2016-11-24 2021-01-19 Koninklijke Philips N.V. MR imaging with Dixon-type water/fat separation
WO2018114554A1 (fr) * 2016-12-20 2018-06-28 Koninklijke Philips N.V. Imagerie rm à séparation eau/graisse de type dixon
US11280865B2 (en) 2018-01-11 2022-03-22 Koninklijke Philips N.V. Dual resolution Dixon magnetic resonance imaging
EP3511725A1 (fr) * 2018-01-11 2019-07-17 Koninklijke Philips N.V. Imagerie par résonance magnétique dixon à double résolution
WO2019137932A1 (fr) * 2018-01-11 2019-07-18 Koninklijke Philips N.V. Imagerie par résonance magnétique de dixon à double résolution
CN111542762A (zh) * 2018-01-11 2020-08-14 皇家飞利浦有限公司 双分辨率Dixon磁共振成像
CN111542762B (zh) * 2018-01-11 2024-03-15 皇家飞利浦有限公司 双分辨率Dixon磁共振成像
DE102018208569A1 (de) * 2018-05-30 2019-12-05 Siemens Healthcare Gmbh Aufnahme zweier Magnetresonanz-Bilder
US11067655B2 (en) 2018-05-30 2021-07-20 Siemens Healthcare Gmbh Method and apparatus recording two magnetic resonance images
JP2021528221A (ja) * 2018-09-04 2021-10-21 パースペクトゥム リミテッド 磁気共鳴画像撮影装置の画像の分析方法
JP7016993B2 (ja) 2018-09-04 2022-02-07 パースペクトゥム リミテッド 磁気共鳴画像撮影装置の画像の分析方法
WO2021052882A1 (fr) * 2019-09-16 2021-03-25 Koninklijke Philips N.V. Imagerie par résonance magnétique (rm) à séparation eau/graisse de type dixon
EP3792647A1 (fr) * 2019-09-16 2021-03-17 Koninklijke Philips N.V. Irm avec séparation eau/graisse de type dixon
CN113835058A (zh) * 2020-06-24 2021-12-24 通用电气精准医疗有限责任公司 采集和处理mr数据的方法、mri系统和方法、存储介质

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