GB2246637A - ECHO-VOLUMAR IMAGING USING 180ì RF PULSES - Google Patents

ECHO-VOLUMAR IMAGING USING 180ì RF PULSES Download PDF

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GB2246637A
GB2246637A GB9116410A GB9116410A GB2246637A GB 2246637 A GB2246637 A GB 2246637A GB 9116410 A GB9116410 A GB 9116410A GB 9116410 A GB9116410 A GB 9116410A GB 2246637 A GB2246637 A GB 2246637A
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pulses
gradient
producing
magnetic resonance
modulated
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Peter Mansfield
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National Research Development Corp UK
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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
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02G—HOT GAS OR COMBUSTION-PRODUCT POSITIVE-DISPLACEMENT ENGINE PLANTS; USE OF WASTE HEAT OF COMBUSTION ENGINES; NOT OTHERWISE PROVIDED FOR
    • F02G1/00—Hot gas positive-displacement engine plants
    • F02G1/04—Hot gas positive-displacement engine plants of closed-cycle type
    • F02G1/043—Hot gas positive-displacement engine plants of closed-cycle type the engine being operated by expansion and contraction of a mass of working gas which is heated and cooled in one of a plurality of constantly communicating expansible chambers, e.g. Stirling cycle type engines
    • F02G1/044—Hot gas positive-displacement engine plants of closed-cycle type the engine being operated by expansion and contraction of a mass of working gas which is heated and cooled in one of a plurality of constantly communicating expansible chambers, e.g. Stirling cycle type engines having at least two working members, e.g. pistons, delivering power output
    • F02G1/0445—Engine plants with combined cycles, e.g. Vuilleumier
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02G—HOT GAS OR COMBUSTION-PRODUCT POSITIVE-DISPLACEMENT ENGINE PLANTS; USE OF WASTE HEAT OF COMBUSTION ENGINES; NOT OTHERWISE PROVIDED FOR
    • F02G2250/00—Special cycles or special engines
    • F02G2250/18—Vuilleumier cycles
    • 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
    • G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
    • G01R33/482—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a Cartesian trajectory
    • G01R33/4822—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a Cartesian trajectory in three dimensions
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G01R33/5618—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using both RF and gradient refocusing, e.g. GRASE

Abstract

A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process including subjecting the object to an initial selection process to select a thick slice within the object to produce an active volume of spin magnetisation subjecting the active volume so defined to a combination of 180 DEG RF pulses and suitably modulated x, y and z gradients to produce an echo volumar image.

Description

Echo-Volumar Imagine using 1800 RF Pulses The present invention relates to magnetic resonance imaging systems and, more particularly, to Echo Volumar Imaging (EVI) using 1800 RF pulses.
Magnetic Resonance imaging using the echo-volumar imaging approach (P. Mansfield, A.M. Howseman and R.J.
Ordidge, Volumar Imaging (EVI) at 0.1 T, J. Phys. E. 22, 324-330 (1989) allows the simultaneous examination of several planes within a defined thick slice to be observed in the same time as would be necessary for the examination of a single plane in echo-planar imaging (P. Mansfield, A.M. Howseman and R.J. Ordidge, Volumar Imaging using NMR Spin Echoes, J. Phys. C. 10, L55 (1977) EPI. Following an initial thick slice selection process, the spin system is encoded along three principal axes, using suitably modulated x, y and z linear magnetic gradients. This type of echo-volumar imaging works well for homogenous specimens at high field and for less homogenous specimens at low field. The operating field strength comes into question since bulk susceptibility artefacts are induced, causing a degradation of the static field homogeneity.If this static field homogeneity is degraded too far, EVI as described above becomes impractical with the gradient strengths and switching rates currently available.
It is an object of the present invention to provide a means of overcoming the induced local field effects arising from susceptibility effects. The EVI modification uses 1800 RF pulses instead of gradient reversals. A similar modification has been described for use with Echo Planar Imaging EPI in British Patent Application 8918105.1.
The present invention therefore provides a method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process including subjecting the object to an initial selection process to select a thick slice within the object to produce an active volume of spin magnetisation subjecting the active volume so defined to a combination of 1800 RF pulses and suitably modulated x, y and z gradients to produce an echo volumar image.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a timing diagram for the initial thick slice selection phase. Also included are the pre-phasing pulses for Gx, Gy and Gz (shaded); Figure 2 shows a timing diagram for the modified EVI sequence using 1800 RF pulses. (Note the gradient Gx is monopolar with small gaps or notches allowing a window for the application of the non-selective RF selective pulses and also the application of Gyt and G2 gradient blips).
Figure 3(a) shows the k-space trajectory for the modified EVI sequence shown in Figure 2. (Note that the effect of the 1800 RF pulse is to take the k-trajectory to a conjugate point in k-space); Figure 3(b) shows an alternative scanning procedure to Figure 3(a) in which the scan starts in the central z = 0 and steps out subsequently through pairs of conjugal planes finishing at the +z plane.
Figure 4(a) shows a timing diagram for a modified hybrid EVI sequence in which occasional 1800 pulses are used on conjunction with a bipolar Gx gradient; Figure 4(b) shows an alternative timing diagram for a modified hybrid EVI sequence using 1800 RF pulses.
Figure 5 shows the k-space trajectory for the hybrid EVI sequence shown in Figures 4(a) and 4(b), a) conjugate points Q b) conjugate points R The modified EVI sequence employs short 1800 RF pulses instead of gradient reversal. In one version described below, the 1800 RF pulses are non-selective, but variants of the new sequence are possible in which short selective 1800 pulses are used instead. In this section we concentrate on the use of non-selective 1800 pulses.
The timing diagram for the initial thick slice selection is shown in Figure 1. Here a shaped RF pulse is used to selectively excite a thick slice in the presence of a gradient Gz Also applied in this time frame are prephasing pulses Gx, Gy and Gz (shaded). It is emphasised that the initial slice selection process described is a particular example of a range of initial slice selection or volumar selection processes, which could precede the volumar encoding phase, in which the Rf shape is modulated in the presence of one or more gradients to define a more general volume of active spins.
Following the initial slice selection phase, RF pulses and gradient pulses are applied as indicated in Figure 2.
Unlike standard EVI, the gradient Gx is monopolar with small gaps or windows as indicated, in which the RF pulses are applied. In this version of the experiment the gradient Gy is applied in the form of short blips which are arranged to fall in alternate Gx gaps, but so as not to coincide with the RF pulses. In the example given, the Gy blips precede the RF pulses. The G2 gradient is also applied in the form of blipped pulses, but less frequently as indicated. These also precede the RF pulses in the example given.
The complex three-dimensional path through k-space is indicated in Figure 3(a). The scan starts at A and moves to B under the action of Gx (see Figure 2). The application of a 1800 RF pulse then takes the locus of k-space to the conjugate point C and then on to D, etc. Following the scan of conjugate planes at +z the k-locus ends up at point E. The first Gz blip is then applied taking the scan to P.
A 1800 pulse then moves the scan to the conjugate point Q.
Under the action of Gx the scan proceeds to R. Another 1800 pulse then takes the scan to S and then on to P, etc. In this way pairs of conjugate z-planes are scanned ending up on the z = 0 plane. However, the procedure could be reversed by starting the scan in the z = 0 plane and hopping to pairs of conjugate planes ending up at the +z planes as indicated in the k-trajectory of Figure 3(b). In this example, the initial z = 0 plane is scanned using +Gy blips to reach point P. A +Gz blip takes the scan to Q.
The conjugate +z planes are scanned using -Gy blips and the sign of Gy is reversed in the alternate sets of conjugate planes. Gx is monopolar with windows for the 1800 RF pulses and blipped gradients.
The effect of this rather complex trajectory through k-space means that the signal data obtained in such a sequence are properly ordered so that a Fourier transform can reconstitute the three-dimensional image set from the 3-D k-space data set.
The use of 180 RF pulses means that any spin dephasing due to local inhomogeneity effects is reversed and, therefore, is effectively removed as an artefact from the final image.
Naturally the mean RF power deposition in a patient would be greater for this type of EVI, but in imaging applications where power deposition is no problem, i.e. the study of inanimate spin distributions, such a sequence could prove of value. In medical imaging applications an alternative approach in which occasional 1800 RF pulses are used could be beneficial.
Occasional 1800 RF pulses In this version of EVI, a combination of occasional 1800 RF pulses in conjunction with gradient reversal is proposed. Two versions of the pulse timing diagram following initial slice selection are indicated in Figure 4. In both scans Gx is modulated for a short period to provide spatial encoding along the x-axis. At the same time the gradient Gy is applied in short blips at points corresponding to the zero crossings of Gx The difference in 4(a) and 4(b) lies in the way the conjugate z scans are terminated. Looking at the k-space trajectory for these sequences, Figure 5, the path taken in this initial phase of the experiment corresponds to scanning k-space in a given z-plane determined by the size of the initial Gz prepulse. Let the scan start at point P.Following this plane scan, a 1800 RF pulse is applied taking the ktrajectory from point Q to a conjugate point Q' in the kspace diagram, (see Figure 4(a), or from R to R' (see Figure 4(b). At this point the gradient Gx must continue with the same phase to scan the conjugate plane. At the end of this scan, (either points S or T), a Gz blip is applied taking the k-locus to either S' or T' in order to scan an adjacent z-plane and its corresponding conjugate plane. This is then followed by a 1800 RF pulse taking the k-locus to either point U or V. This process is repeated with a series of Gz blips until the scan finishes in the z = 0 plane.
In a modification which uses less RF power, alternate RF pulses are omitted and the Gy blips between missing RF pulse pairs are reversed in sign. In a further modification, the whole scanning process starts in the z = 0 plane and steps out in conjugate pairs of planes finishing in the planes +z.
With these sequences it is possible to scan the whole of 3-D k-space in a single pass in times of the order of 100 ms. During the k-trajectory scan, data are sampled.
With suitable data reordering and by Fourier transformation, a three-dimensional image of the object can be produced.
The use of occasional RF pulses as outlined above will work only if the loss of signal due to inhomogeneous spin dephasing is small in the time TR. This could well be the case in medical imaging applications where the susceptibility difference between tissues and organs in for example the abdomen or the h- > ad, (excluding gaseous or bony cavities), is small. In this situation the use of occasional RF pulses as indicated will dramatically reduce the RF power deposition within the patient, making the method acceptable clinically and at the same time more robust and less dependent on high static magnetic field homogeneity.
Although we have emphasised the use of blipped or short duration Gy and Gz gradients in the above description, it is pointed out that EVI images may also be obtained where Gy is applied in the form of a square or trapezoidally modulated waveform, or as a constant low level waveform and Gz is applied as a constant low level waveform. It is also possible to use combinations of the above described waveforms with blipped waveforms.
We have presented modifications to echo-volumar imaging (EVI) which incorporate the use of 1800 RF pulses to either completely replace bipolar gradient modulation or to partially replace bipolar gradient modulation. In both cases the use of 1800 RF pulses has the effect of effectively removing induced local inhomogeneities due to susceptibility effects of the sample at high magnetic field strengths. Such inhomogeneity effects are common in patient imaging and spectroscopy at high static field strengths, and in such cases the use of occasional RF pulses is a means of overcoming the deleterious effects of induced inhomogeneity while at the same time subjecting the patient to an acceptable RF power deposition.

Claims (12)

1. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process including subjecting the object to an initial selection process to select a thick slice within the object to produce an active volume of spin magnetisation subjecting the active volume so defined to a combination of 1800 RF pulses and suitably modulated x, y and z gradients to produce an echo volumar image.
2. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 1 in which the combination of the suitably modulated x, y and z gradients (GX,Gy,Gz) and the 1800 RF pulses produces a complex trajectory in k-space which ensures that signal data obtained in such a sequence is properly ordered so that a Fourier transform is able to reconstitute a three dimensional image set from the three dimensional k-space data set.
3. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 2 in which the x gradient (Gx) is monopolar with small gaps (or windows) within which the RF pulses are applied.
4. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 3 in which the modulated y gradient (Gy) comprises a series of short blips which are arranged to fall in alternate Gx gaps but not to coincide with the RF pulses.
5. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 4 in which the Gy blips precede the RF pulses within the Gx gaps.
6. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 5 in which the modulated z gradient (Gz) is applied as a series of blipped pulses at predetermined intervals the Gz pulses being less frequent than the Gy pulses.
7. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 6 in which the Gz pulses precede the RF pulses within the gaps in the Gx pulses.
8. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 2 in which the x gradient (Gx) is modulated for a short period to provide spatial encoding along the x-axis and in which the y gradient (Gy) is applied in short blips at points corresponding to the zero crossings of Q.
9. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 8 in which the modulated z gradient (Gz) pulses are applied during periods in which the Gx gradient is zero; the Gz pulses being applied less frequently than the Gy pulses.
10. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 2 in which the modulated y gradient Gy is applied in the form of a square or trapezoidally modulated waveform and in which the modulated z gradient (Gz) is applied in the form of a constant low level waveform.
11. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process as claimed in claim 2 in which the modulated y gradient (Gy) is applied in the form of a low level waveform and in which the modulated z gradient Gz is applied in the form of a constant low level waveform.
12. A method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process substantially as described with reference to the accompanying drawings.
GB9116410A 1990-07-31 1991-07-30 Echo-volumar imaging using 180ì RF pulses Expired - Fee Related GB2246637B (en)

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EP (1) EP0541616B1 (en)
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US5684398A (en) * 1993-09-02 1997-11-04 Hitachi Medical Corporation Fluid measurement method using nuclear magnetic resonance imaging and apparatus therefor
US5532595A (en) * 1994-04-18 1996-07-02 Picker International, Inc. Three-dimensional spiral echo volume imaging
DE4423806C2 (en) * 1994-07-06 2000-08-24 Siemens Ag Process for MR imaging with a sequence of individual measurements
US5928271A (en) * 1998-02-25 1999-07-27 Medtronic, Inc. Atrial anti-arrhythmia pacemaker and method using high rate atrial and backup ventricular pacing
DE102014203432B3 (en) * 2014-02-26 2015-07-30 Siemens Aktiengesellschaft Magnetic resonance spectroscopy with phase rotation
US11857306B1 (en) 2018-06-15 2024-01-02 Unm Rainforest Innovations Concurrent MRSI and fMRI

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US4698592A (en) * 1985-08-16 1987-10-06 The Regents Of The University Of California MRI of chemical shift spectra within limited inner volume
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US4799013A (en) * 1987-01-26 1989-01-17 Kabushiki Kaishi Toshiba Method for detecting and processing magnetic resonance signals
US4818940A (en) * 1984-09-18 1989-04-04 Bruker Medizintechnik Gmbh Method for measuring nuclear magnetic resonance

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JPH0763455B2 (en) * 1986-03-31 1995-07-12 株式会社東芝 Magnetic resonance imager
GB8918105D0 (en) * 1989-08-08 1989-09-20 Nat Res Dev Echo planar imaging using 180grad pulses
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EP0091008A2 (en) * 1982-04-05 1983-10-12 General Electric Company Method of three-dimensional NMR imaging using selective excitation
US4716369A (en) * 1984-06-20 1987-12-29 Hitachi, Ltd. High speed imaging method with three-dimensional NMR
US4818940A (en) * 1984-09-18 1989-04-04 Bruker Medizintechnik Gmbh Method for measuring nuclear magnetic resonance
US4698592A (en) * 1985-08-16 1987-10-06 The Regents Of The University Of California MRI of chemical shift spectra within limited inner volume
US4799013A (en) * 1987-01-26 1989-01-17 Kabushiki Kaishi Toshiba Method for detecting and processing magnetic resonance signals

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DE69131619T2 (en) 2000-03-16
DE69131619D1 (en) 1999-10-21
GB9116410D0 (en) 1991-09-11
GB2246637B (en) 1994-11-23
GB9016803D0 (en) 1990-09-12
US5363036A (en) 1994-11-08
IL98977A (en) 1995-03-30
JP2989892B2 (en) 1999-12-13
JPH06500166A (en) 1994-01-06
EP0541616A1 (en) 1993-05-19
EP0541616B1 (en) 1999-09-15
WO1992002829A1 (en) 1992-02-20
IL98977A0 (en) 1992-07-15

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