EP0463009A1 - Procede de spectroscopie ou de tomographie par resonance magnetique dans une zone preselectionnee d'un materiau et application de ce procede - Google Patents

Procede de spectroscopie ou de tomographie par resonance magnetique dans une zone preselectionnee d'un materiau et application de ce procede

Info

Publication number
EP0463009A1
EP0463009A1 EP90904246A EP90904246A EP0463009A1 EP 0463009 A1 EP0463009 A1 EP 0463009A1 EP 90904246 A EP90904246 A EP 90904246A EP 90904246 A EP90904246 A EP 90904246A EP 0463009 A1 EP0463009 A1 EP 0463009A1
Authority
EP
European Patent Office
Prior art keywords
spin
pulse
lock
area
magnetization
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
EP90904246A
Other languages
German (de)
English (en)
Inventor
Eberhard Rommel
Rainer Kimmich
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.)
Bruker Biospin MRI GmbH
Original Assignee
Bruker Medizintechnik GmbH
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 Bruker Medizintechnik GmbH filed Critical Bruker Medizintechnik GmbH
Publication of EP0463009A1 publication Critical patent/EP0463009A1/fr
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

Definitions

  • the invention relates to a method for magnetic resonance spectroscopy or tomography in a preselectable area of a material.
  • the method is particularly suitable for examining biological tissue in magnetic resonance imaging.
  • Measurement methods derived from the aforementioned method e.g. relaxometry, diffusion measurements, flow measurements are also counted towards the invention.
  • a method for spatially determining the relaxation time TIP in the rotating reference system is known from US Pat. No. 4,743,850 (Sepponen).
  • a ⁇ / 2 pulse is first irradiated to produce a transverse magnetization and then a spin-lock pulse is irradiated onto the sample at an angle of 90 °, both in the absence of gradients. Then a volume is selected using the applied gradients and finally the signal evaluation.
  • a method according to claim 1 for magnetic resonance spectroscopy or tomography in a preselectable area of a material After a transverse magnetization in the material has been excited or already exists, the preselectable area of the material is irradiated with a high-frequency pulse (spin-ock pulse) which, in conjunction with a weak magnetic field gradient, is applied to spin - Ocking of the transverse magnetization is suitable in the area of interest, while in the other material areas the gradient mentioned destroys the coherences.
  • spin-ock pulse high-frequency pulse
  • the coherence in the area of interest which is ultimately measured as free induction decay, is protected against phase decay with the help of the spin-lock pulse, which is discussed in detail below. All other spins are dephased by the weak field gradient.
  • the invention is well compatible with numerous methods for editing (eg suppression of unwanted signals) and for further processing the spin coherences.
  • a focal point of the invention is in the field of resonance spectroscopy, in which, in contrast to tomography, the main interest is not in image generation, but in the acquisition of measured values.
  • the transverse magnetization of the material is generated by irradiation of a hard (i.e. a broadband) high-frequency pulse or a composite pulse or by means of another excitation method, the angle between Magnetic field and magnetization vector is preferably 90 ° This angle (flip angle) can also deviate from 90 °.
  • the spin coherences to be spin-locked are prepared in preceding preparation and evolution intervals, for example for homo- or heteronuclear editing or line suppression purposes.
  • Line suppression pulses can also be applied between or after the layer selection measures.
  • further homo- or heteronuclear echo pulse sequences can be connected, for example for editing or polarization transfer purposes.
  • a thin slice is selected as the region of interest of the material sample, the thickness of which can be adjusted by varying the amplitude of the spin-lock pulse and / or the strength of the field gradient. It is particularly advantageous to specify the field gradient and keep it constant during the method, and to determine the thickness of the disk only by adjusting the amplitude of the spin-lock pulse.
  • a hard high-frequency pulse or a composite pulse or another spin excitation method can be used so that the selected layer magnetization is transferred in the direction of the magnetic field or another direction.
  • two slice selections are carried out sequentially with the aid of gradients in two different spatial directions, so that the magnetization of a rod-shaped area is transferred in the direction of the magnetic field or another direction.
  • a well-defined volume element can be selected in that three slice selections are carried out sequentially with the aid of gradients in three different spatial directions, so that the magnetization of a parallelepiped-shaped volume area is transferred in the direction of the magnetic field or another direction.
  • the magnetization of the selected area can be converted into the transverse direction by a high-frequency pulse or another spin excitation method and can be read out via a free induction decay.
  • the field gradient is designed to be self-refocusing after the end of the spin-lock pulse, so that directly readable or further evolving spin coherences of the selected region are produced (SLISE).
  • the spin coherences generated in a layered or rod-shaped region are transferred according to intervals or pulse sequences effective for spectral editing or polarization transfer by a high-frequency pulse in the direction of the magnetic field or in another direction and then transferred with the aid a further slice-selective operation in spin coherences of a rod-shaped or parallelepiped-shaped region is transferred. It is also possible to connect further homo- or heteronuclear echo pulse sequences after the area selection, for example for editing purposes or polarization transfer purposes.
  • a spin coupling is carried out before, between or after the layer selection measures. It can also happen the so-called dynamic polarization is applied between or after the slice selection measures.
  • the area selection is linked to the determination of the spin-lattice relaxation time or the transversal relaxation time or the dipolar relaxation time or quadrupolar relaxation time or the diffusion coefficient or another measurement variable suitable for material characterization.
  • the present method has the particular advantage that no complicated pulse shapes are required for the spin-lock pulse, and that the pulse amplitude, especially in in-vivo applications, is sufficiently small to reliably avoid a dangerous level of high-frequency radiation.
  • the method is intended for magnetic resonance imaging and imaging of biological tissue, it can also be used in conventional magnetic resonance spectroscopy, e.g. in electron spin resonance spectrometers or tomographs, on organic and inorganic samples.
  • transverse magnetization used here is defined as a component perpendicular to the direction of a constant magnetic field Bo.
  • the transverse magnetization is generated by a high-frequency pulse or by an incomplete fast adiabatic resonance passage, or it is already present from a previous sequence (e.g. editing, relaxation time measurement, measurement of the diffusion coefficient, etc.).
  • magnetic resonance spectroscopy leads to a spectrum, tomography to an image generation, relaxometry to relaxation times, diffusion and flow measurement techniques to diffusion and flow parameters, e.g. the speed of a liquid flow.
  • Composite pulse is understood to be a composite high-frequency pulse that generates certain flip angles and is only weakly influenced by incorrect settings.
  • spectrum editing effective interval e.g. understood an interval in which multi-quantum coherences for multi-quantum filtering were generated.
  • polarization transfer effective interval means e.g. an interval as it occurs in the known polarization transfer methods (INEPT, DEPT, CYCLPOT, etc.). Such intervals are part of specific pulse sequences.
  • the features of claim 3 relate to preceding pulse sequences which serve for the suppression or filtering ("editing") of certain lines.
  • line suppression pulses selectively excite the frequency of the line to be suppressed. Every spectrally resolved line can be suppressed.
  • area selection is used as a generic term for a layer, rod or volume selection (parallelepiped selection).
  • filtering filtering
  • polarization transfer which is used for amplification or also for editing certain signals.
  • the purpose of the measures described in claims 8 to 10 is to selectively generate magnetization in a predetermined direction from a rod-shaped or parallelepiped-shaped area.
  • the spin coupling is of interest if a multiplet splitting or the resulting evolution of the coherences is to be avoided.
  • the spin coupling is carried out by selectively irradiating an intense high frequency onto the coupled spin group.
  • the timing is not arbitrary, but refers to either an evolution or a detection interval. With dynamic polarization, the time of application is before the signal detection.
  • the features of claim 17 can be concretized in that measurement methods known per se are linked to the volume selection method according to the invention by means of measurement variables which are linked to the magnetic resonance, so that volume-selective measurement is carried out.
  • the method characterized in claim 1 can follow an excitation pulse ( ⁇ / 2 pulse) or an inversion pulse (a ⁇ pulse) after a variable interval.
  • the detected signal is then dependent on the spin-lattice relaxation time.
  • two slice selections using the method according to the invention with a subsequent one slice-selective excitation can be combined by a SINC pulse in the presence of a field gradient.
  • FIG 1 shows the pulse scheme of the spin-lock pulse according to the present invention for the slice-selective excitation of spin coherences.
  • FIG 3 shows the variation of the spin lock amplitude taking into account the disk thickness according to an embodiment of the invention.
  • FIG. 4 shows the pulse scheme for three-dimensional spin locking according to a further embodiment of the invention.
  • FIG. 5 shows two sample examples that were used in test series to test the volume-selective method according to an embodiment of the invention.
  • FIG. 6 shows spectral curves recorded from the samples according to FIG. 5.
  • the direction of the constant magnetic field Bo is commonly referred to as the z direction.
  • FIG. A preferred form of the high-frequency part of the pulse scheme is shown in FIG. followed by a spin-lock pulse (SD with a phase rotation of 90 °, ie in the y direction in the example.
  • SD spin-lock pulse
  • a broadband pulse is known as “hard”.
  • the magnetic field gradient (G) (in the example in the x direction ) is applied simultaneously with the ⁇ / 2 pulse and the spin lock pulse
  • This gradient causes a change in the constant magnetic field Bo in the direction of the X axis, which is perpendicular to the vector of the magnetic field Bo.
  • the amplitude Bi of the spin In this exemplary embodiment, the lock pulse and the absolute value G of the field gradient determine the thickness 4J x of a slice to be selected, while the position and orientation of the slice is determined by the carrier frequency of the spectrometer or the direction of the field gradient:
  • the amplitude Bi of the (relatively long in time) spin-lock pulse is considerably smaller than that of the (relatively short in time) ⁇ / 2 pulse, so that high doses of high-frequency radiation can be avoided in person examinations.
  • the phase-coupled spins relax in the rotating reference system in accordance with their longitudinal relaxation time TIP. All other spins lose the fixed phase relationship after a time constant T2 *, which is characteristic of the current Bo field distribution.
  • T2 * which is characteristic of the current Bo field distribution.
  • this time constant is short compared to the longitudinal relaxation time in the rotating reference system. If the duration of the spin lock interval t s 1 the condition
  • the field gradient G shown in FIG. 1 a can be applied in a special direction, for example in the x direction.
  • a slice profile is then scanned.
  • the slice thickness is proportional to the amplitude of the spin-lock pulse and inversely proportional to the field gradient.
  • the thickness of the disk can therefore be adjusted either by simultaneously varying both parameters or by keeping one parameter constant when the other is varied.
  • the gradient is preferably kept constant and the amplitude of the spin-lock pulse is varied.
  • the shape of the spin-lock pulse does not have to be changed, and within certain limits the thickness of the disk is independent of the length of the spin-lock pulse, in contrast to the conventional narrowband pulse techniques. As will be explained, satisfactory results can be achieved with a pulse-lock pulse duration of 3 to 20 ms, while a range of 5 to 10 ms is preferred.
  • the cross-magnetization phase-coupled by spin-locking has a relaxation time T ⁇ P during the spin-lock interval ts i.
  • the coherence and spin state change echo, which is recorded after the application of the complete pulse train, is therefore with the proportionality factor
  • the impulse for the slice selection can be expanded to three dimensions (see below).
  • 1b shows a further embodiment of the invention, in which the pulse sequence begins with a hard ⁇ / 2 pulse in the absence of a gradient.
  • the one after the ⁇ / 2 pulse The beginning of the gradient has a reversal of direction, so that a gradient echo arises which reaches its maximum exactly when the field gradient plateau begins.
  • the spin lock pulse is started at this time.
  • the position of the slice to be examined is determined by the offset of the transmission frequency.
  • the optimum frequency switchover point lies exactly at the maximum of the echo, since then there are no problems with phase adjustments. Otherwise, the phase difference between the high-frequency excitation and the spin precession would have to be taken into account.
  • the gradient is switched off (reversal of direction again) so that a further gradient echo appears.
  • the advantage of this embodiment is that the transverse magnetization is generated in the absence of the field gradient.
  • the bandwidth or excitation bandwidth of the ⁇ / 2 pulse can thereby be made smaller than in the exemplary embodiment according to FIG.
  • the position and width of the preselectable or selected area is determined as in the exemplary embodiment according to FIG.
  • the amplitudes (Bi) of the spin lock pulse were calibrated using the length of a ⁇ / 2 test pulse.
  • the duration of the hard ⁇ / 2 pulse was 90 ⁇ s.
  • the spin lock period was varied, with satisfactory results for t 8 I values in the range of 3 to 20 ms, with a preferred range of 5 to 10 ms. could be achieved.
  • the phase relaxation time T2 * was significantly shorter than the duration ts 1 of the spin-lock pulse.
  • the left part of FIG. 2 shows the profiles obtained by Fourier transforming a Hahn echo (spin echo) of the free induction decay. The profiles were recorded with the relatively weak Bo gradient of 0.33 G / cm. For comparison, the profiles were calculated using the formula
  • Ms 1 (x) Mo • cos 2 [arctan (x • G x / B_)]
  • Ms 1 is the phase-coupled magnetization
  • Mo is the magnetization at the beginning of the spin-lock pulse
  • x is the coordinate with the origin given by the resonance condition.
  • FIG. 3 shows the results of experiments in which the amplitude Bi of the spin lock pulse was varied according to FIG. It can be seen that the layer thickness depends linearly on the amplitude. This linear relationship has the advantage of easily adjusting the desired thickness without the need for strong field gradients or complicated pulse shapes.
  • the method can be expanded to three dimensions, so that the material area of interest can be selected as a volume element.
  • the spin-lock pulse is preferably applied in periods in which the magnetic field gradients are constant.
  • a preferred pulse scheme is shown in FIG. 4.
  • the three separate radio frequency pulse groups (first three lines of Fig. 4) become consecutive applied, a field gradient (G, GY, G_) being present in one of the three special coordinate directions x, y and z during each high-frequency pulse group.
  • Each pulse group consists of a hard ⁇ / 2 pulse, a spin-lock pulse (SD and a final in-phase or opposite-phase hard ⁇ / 2 pulse. The latter brings the phase-coupled coherence back into the z direction.
  • the gradients are kept constant during the spin-locking pulses. Without these measures, the adiabatic magnetization transmission to or from directions that lie outside the resonance of the effective field would hinder the full utilization of the dephasia effect.
  • the volume element can be easily adjusted by adjusting the amplitude of the three spin-lock pulses accordingly. 4 is preferred for volume selection. A modification based on the pulse of Fig. 1b can also be used.
  • the present invention can be put into practice in a comparatively simple manner and is particularly well suited for magnetic resonance imaging. No complicated pulse shapes have to be produced and the area of interest in the examination object can be easily adjusted by controlling the high-frequency amplitude of the spin-lock pulse.
  • the selection of very thin slices is possible without creating strong field gradients. In this case (disc with a thickness of 1 mm), a field gradient of 0.33 G / cm would require an amplitude of the spin-lock pulse of approximately 0.015 G (taking into account the full width at half maximum).
  • the field gradients are preferably reduced instead of increasing the amplitudes of the spin lock pulses.
  • Another advantage of the invention lies in the possibility of obtaining information about the spin-lattice relaxation time in the rotating reference system. By directly determining this Size gives a further parameter which characterizes the tissue to be examined or, for example, certain organs or organ parts.
  • the recording of tip-weighted images can be of interest when assessing material conditions, in particular with biological material.
  • the method according to the invention can be used in magnetic resonance tomography.
  • a certain preselectable area of a sample e.g. a disk or a volume element is selectively excited by phase-coupling the transverse magnetization of the desired region by spin-locking, while the spin coherences in the remaining spatial region are destroyed by phase relaxation.
  • the thickness of the desired disk or the dimensions of the desired volume element can be varied by adjusting the amplitude of the spin-lock pulse while the magnetic field gradient is kept constant. Comparatively low magnetic field gradients can be used. Even with such low magnetic field gradients, good spatial resolution can be achieved while reducing the energy of the high-frequency excitation. Because of the relatively low energy radiation in the material to be examined, the method is particularly suitable for in vivo applications.

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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

Procédé susceptible d'applications en tomographie par résonance magnétique. Une zone déterminée et présélectionnée d'un échantillon, par exemple une coupe ou un élément d'un volume, est excitée sélectivement par un couplage en phase de l'aimatation transversale de la zone à étudier par blocage de spin, tandis que les cohérences de spin de la zone restante sont détruites par relaxation de phase. Il est possible de faire varier l'épaisseur de la coupe ou les dimensions de l'élément de volume à étudier en réglant l'amplitude de l'impulsion de blocage de spin, tout en maintenant constants les gradients de champ magnétique. Il est possible d'employer des gradients de champ magnétique relativement faibles. Même avec ces faibles gradients de champ magnétique, on peut obtenir une bonne résolution spatiale en réduisant l'énergie de l'excitation radiofréquence. Le procédé convient en particulier pour les applications in vivo, en raison d'un rayonnement énergétique relativement faible dans le matériau à examiner.
EP90904246A 1989-03-15 1990-03-14 Procede de spectroscopie ou de tomographie par resonance magnetique dans une zone preselectionnee d'un materiau et application de ce procede Ceased EP0463009A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE3908392 1989-03-15
DE3908392A DE3908392A1 (de) 1989-03-15 1989-03-15 Verfahren zur lokalisierten magnetischen resonanz-spektroskopie (losy) und zur schichtselektiven anregung von transversalmagnetisierungen (slise)

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EP0463009A1 true EP0463009A1 (fr) 1992-01-02

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US (1) US5317264A (fr)
EP (1) EP0463009A1 (fr)
DE (1) DE3908392A1 (fr)
WO (1) WO1990010878A1 (fr)

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US5404882A (en) * 1992-08-05 1995-04-11 Wisconsin Alumni Research Foundation Adiabatic off-resonance spin-locking for enhanced tumor contrast in NMR imaging
US6528997B2 (en) * 2000-03-20 2003-03-04 University Of Rochester Intermolecular multiple-quantum coherence MR imaging in humans
US6373250B1 (en) * 2000-05-19 2002-04-16 Ramot University Authority For Applied Research And Industrial Development Ltd. Method of magnetic resonance imaging
US6836114B2 (en) * 2002-03-15 2004-12-28 The Trustees Of The University Of Pennsylvania Pulse imaging sequences and methods for T1p-weighted MRI
AT412253B (de) * 2002-07-26 2004-12-27 Ropele Stefan Dipl Ing Dr Verfahren und einrichtung zur messung der makromolekularen protonendichte
US7705596B2 (en) * 2007-05-18 2010-04-27 The Trustees Of The University Of Pennsylvania System and method for minimizing MRI-imaging artifacts
JP5971677B2 (ja) * 2010-11-26 2016-08-17 東芝メディカルシステムズ株式会社 磁気共鳴診断装置、および、磁気共鳴スペクトロスコピーのデータ収集方法
JP6084392B2 (ja) * 2011-09-22 2017-02-22 東芝メディカルシステムズ株式会社 磁気共鳴イメージング装置
US9720128B2 (en) * 2013-02-15 2017-08-01 Schlumberger Technology Corporation Rotating frame pulsed nuclear magnetic resonance spectroscopy
US11280867B2 (en) * 2019-11-08 2022-03-22 The Chinese University Of Hong Kong System and method for quantitative magnetization transfer imaging based on spin-lock

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Also Published As

Publication number Publication date
DE3908392C2 (fr) 1993-04-15
US5317264A (en) 1994-05-31
WO1990010878A1 (fr) 1990-09-20
DE3908392A1 (de) 1990-09-20

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