WO2010097094A1

WO2010097094A1 - Magnetic resonance imaging with chemical species selectivity

Info

Publication number: WO2010097094A1
Application number: PCT/EP2009/001344
Authority: WO
Inventors: Dimo Ivanov; Robert Turner
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 2009-02-25
Filing date: 2009-02-25
Publication date: 2010-09-02
Anticipated expiration: 2011-08-25

Abstract

A method of magnetic resonance imaging of an object (1), which includes at least two chemical species, e. g. water and fat, comprises the steps of arranging the object (1) in a stationary magnetic field, subjecting the object (1) to an excitation slice-selection gradient and an excitation pulse, subjecting the object (1) to a refocussing slice-selection gradient and a refocussing pulse, collecting magnetic resonance signals created in the object (1), and reconstructing an image of the object (1) based on the magnetic resonance signals, wherein the excitation and refocussing pulses and their respective gradients have different durations and the excitation and the refocussing slice selection gradients have different gradient strengths such that the chemical species are excited by the first pulse, but only one of the chemical species is refocussed by the second pulse, so that the magnetic resonance signal thereof is created, while the magnetic resonance signals from the other species are eliminated. Furthermore, an imaging device for magnetic resonance imaging of an object is described.

Description

Magnetic resonance imaging with chemical species selectivity

Technical field

The present invention relates to a method of magnetic resonance (MR) imaging of an object, in particular to an MR imaging method with slice selection of distinct chemical species. Furthermore, the invention relates to an MR imaging device adapted for implementing the MR imaging method. Preferred applications of the invention are in the field of MR imaging with chemical-shift artefact suppression or MR imaging of specific MR-observable chemical species.

Technical background of the invention

For magnetic resonance imaging (MRI), an object region to be investigated (region of investigation, ROI) is arranged in a stationary magnetic field and subjected to a succession (MR imaging sequence) of at least one electromagnetic radio frequency (RF) excitation pulse and at least one following refo- cussing pulse, while magnetic field gradients are applied in different spatial directions. The radio frequency excitation of magnetic spin states and the relaxation thereof induces detectable nuclear magnetic resonance signals (NMR signals, echoes) which include information as to the magnetic condition and the geometric distribution of the excited species, in particular protons thereof, in the ROI. Adjusting the mag- netic field gradients allows a resonance signal collection with local resolution providing a contrast image of the ROI.

Main MRI applications are in the field of medical imaging, but also in non-destructive material testing. As an example for medical imaging, functional brain activity is routinely investigated using functional MRI. For this purpose single- shot echo-planar imaging (EPI) is the sequence of choice, due to its great acquisition speed, its ease of implementation and its robustness. The fast acquisition speed of EPI, which eliminates artefacts associated with head movement, is vital for imaging of water diffusion. However, EPI acquisition has excessive sensitivity to chemical-shift artefacts. Such artefacts can occur because the hydrogen protons in tissue de- tected with the MR imaging may have different resonance frequencies (chemical shifts) . The most serious artefact (fat- shift artefact) comes from the often abundant fat protons, whose resonance frequency differs by 3.35 parts per million from that of water protons. It is most severe when very fast MR acquisition methods are used, such as the EPI method.

Chemical-shift artefacts appear in the phase-encoding direction of the EPI image acquisition, due to the comparatively low bandwidth in that direction, and consist of an image e.g. of the fat distribution in the object, displaced by several pixels from the desired water proton image.

Several methods have been developed to deal with the fat- shift artefact. For example, the short Tl inversion-recovery method inserts a RF inversion pulse at the beginning, and image acquisition is initiated when the longitudinal magnetization of the fat protons passes through zero (see G. M. Bydder et al. in "J. Comput . Assist. Tomogr." vol. 9, 1985, p. 659 - 675) . This relies on the fat protons having a distinctly dif- ferent Tl from the water protons. One disadvantage is that the water signal may itself be somewhat reduced.

The most common fat suppression method is to apply a spectrally-selective ^λfat-sat' RF pulse with a narrow bandwidth, centered on the fat proton frequency (see P. J. Keller et al . in "Radiology" vol. 164, 1987, p. 539 - 541; B. R. Rosen et al. in "J. Comput. Assist. Tomogr." vol. 8, 1984, p. 813 - 818; and A. Haase et al. in "Phys. Med. Biol." vol. 30, 1985, p. 341 - 344) followed by spoiler gradients, prior to the main RF excitation pulse. This pulse nutates the fat proton magnetization to the transverse plane, while the water magnetization remains unaffected along the longitudinal axis. The spoiler gradients then dephase and thus nullify the fat signal before image acquisition. Combination of these two techniques is also possible, and known as spectral inversion recovery (see E. Kaldoudi et al. in "Magn. Reson. Imaging" vol. 11, 1993, p. 341 - 355) . As essential disadvantages, these fat suppression methods all require additional radio frequency and gradient pulses, increasing the specific absorption rate (SAR) and lengthening the acquisition time.

Furthermore, the conventional methods using a fat-saturation pulse are extremely demanding of static magnetic field homo- geneity, because such pulses do not suppress signal from off- resonance fat protons. Moreover, functional or diffusion scans are often long, with a high-duty cycle, which can lead to drifts of the centre frequency of the magnet over the experiment. This causes both the fat and the water signal to shift in frequency, making the spectrally selective fat- saturation fat suppression less effective and possibly suppressing the water signal.

Another approach, the slice-selection gradient reversal method, requires no additional pulses, provided that the sequence already contains at least one 180° refocussing pulse (H. W. Park et al. in "Magn. Reson. Med." vol. 4, 1987, p. 526 - 536; A. VoIk et al. In "J. Magn. Reson." vol. 71, 1987, p. 168 - 174; or J. M. Gomori et al. in "Radiology" vol. 168, 1988, p. 493 - 495) . The crucial aspect of this method is to use slice-selection gradients of opposite polarity for the excitation and refocussing pulses. Without fat saturation, the 90° pulse excites both fat and water. However, it excites fat in a slice that is displaced relative to the water slice, due to the 3.35 ppm chemical shift of fat. If a gradient of the same polarity and same magnitude is used both the excited fat and water signal are refocussed, and thus the fat is still detectable during an acquisition. If the gradient ap- plied during the refocussing pulse is in the reversed direction, however, the position where the fat signal would be refocussed is displaced in the opposite direction. Thus only the fat signal from the region of slice overlap will be present in the image. Careful attention to RF pulses duration and gradient strength can minimize this overlap. Recently, the idea has been demonstrated using two refocussing pulses (see Z. Nagy et al. in "Magn. Reson. Med." vol. 60, 2008, p. 1256 - 1260) for imaging water diffusion in the brain at a magnetic field of 3 Tesla. Again, this latter fat suppression method implementation suffers from additional radio frequency and gradient pulses, which increases the SAR and lengthens the acquisition time.

Objective of the invention

It is an objective of the present invention to provide an improved MR imaging method with a separation of chemical species, in particular with chemical-shift artefact reduction, which is capable of avoiding the disadvantages of the conven- tional techniques. In particular, it is an objective of the invention to provide such MR imaging method without increasing the SAR and/or the acquisition time. Furthermore, the objective of invention is to provide an improved MR imaging de- vice being capable of the separation of chemical species and avoiding disadvantages of conventional MR imaging devices.

These objectives are solved with methods and devices as de- fined in the independent claims. Advantageous embodiments and applications of the invention are defined in the dependent claims .

Summary of the invention

According to a first aspect of the invention, a method of magnetic resonance imaging of an object including two or more chemical species is provided, wherein slice-selective excitation and refocussing pulses and their respective gradients of the MR imaging sequence have different time durations and the excitation and the refocussing slice selection gradients have different gradient strengths (magnitudes) such that all chemical species are excited by the excitation pulse, but only one of them is refocussed by the refocussing pulse. The magnetic resonance signal of the excited and refocussed species is generated and collected for image reconstruction. The magnetic resonance signal of the at least one other species, which is excited but not refocussed, is eliminated due to the missing refocussing effect.

The durations of the excitation and refocussing pulses and the durations and magnitudes of their respective slice- selection gradients are selected such that the excited and refocussed slice profiles for the chemical species of choice coincide. With increasing pulse duration, the slice-selection gradient strength is reduced, and vice versa. Thus, changing the duration of the excitation or refocussing pulses and the slice-selection gradient strengths during excitation and refocussing pulses relative to each other, while maintaining the slice profile and position of the chosen species, results in different displacements of the selected slices of the other species relative to it during the excitation and refo- cussing steps. The changed durations and gradients strengths are set such that the slices of all of the other species during the excitation and refocussing steps do not overlap.

The inventive method utilizes a similar physical effect like the aforementioned conventional slice-selection gradient re- versal method with two refocussing pulses, but does not require two refocussing pulses for achieving the chemical-shift artefact suppression. When the strength of the gradient applied during the excitation pulse is sufficiently different from the gradient strength applied during refocussing, a com- plete spatial mismatch is achieved between e.g. the fat slice excited and the fat slice refocussed.

As the main advantage, an MR imaging slice selection of distinct chemical species is provided with the invention. The slice selection yields a simple method for chemical-shift artefact removal. As the preferred application of the invention, chemical-shift artefact suppression (in particular fat suppression) can be achieved by introducing the spatial mismatch between e.g. the fat slices excited and refocussed. The separation between these can be controlled by increasing the duration of either the excitation or the refocussing pulse. As a particular advantage, multiple species can be suppressed simultaneously. Additionally, the radiofrequency power deposition, in particular in spin-echo type MRI pulse sequences is decreased. In particular, increased pulse duration results in decreased pulse amplitudes and decreased values of SAR.

According to a preferred application of the invention, the object contains two chemical species to be separated, e.g. a first species to be imaged and a second species to be suppressed in the image, such as water and fat.

Since the physical effect utilized with the invention re- quires only reasonable static magnetic field homogeneity within the object under study, it can be applied for imaging any region of the object, e.g. a human or animal body. Conventional methods using a fat-saturation pulse require extremely homogeneous static magnetic fields (see above) . Ad- vantageously, the inventive method, is insensitive to this problem.

According to a second aspect of the invention, an imaging device for magnetic resonance imaging of the object is pro- vided, which includes two or more chemical species. The MR imaging device comprises a main magnetic device arranged for creating the stationary magnetic field, a slice-selection gradient device creating the excitation slice-selection gradient or the refocussing slice-selection gradient, a trans- mitter device creating an excitation pulse or at least one refocussing pulse, a receiver device collecting magnetic resonance signals created in the object, a control device controlling the slice-selection gradient device and the transmitter device, and an image reconstructing circuit ar- ranged for reconstructing the image of the object based on the magnetic resonance signals. According to the invention, the control device is adapted for adjusting different pulse durations of the excitation and refocussing pulses and their respective gradients and different gradient strengths of the excitation and the refocussing slice selection gradients such that the above excitation and refocussing of exclusively one of all present chemical species is obtained. With further details, the object is arranged in the stationary magnetic field of the MR imaging device, like an MR scanner. The object is represented by any body including solid and/or liquid material, like e.g. biological material, in particular tissue or an organ, or non-biological material, with at least two chemical species including magnetically excitable nuclei, such as protons. The object is subjected to the excitation slice-selection gradient and the excitation pulse, and to the refocussing slice-selection gradient and the at least one refocussing pulse. Typically, the pulses have a duration in the range of 500 μs to 12 ms . Magnetic resonance signals created in the object are collected and an image of the object based on the resonance signals is reconstructed as it is known from conventional MR imaging.

The MR imaging sequence including the pulses and gradients is selected in dependency on the requirements of the particular application. Preferably, echo-planar imaging (EPI) is provided. However, the invention is not restricted to this se- quence, but rather possible with other schemes. Certain important MRI sequences use an excitation pulse followed by a series of refocussing pulses, such as turbo spin echo (TSE) and gradient and spin echo (GRASE) imaging.

According to a first preferred embodiment of the invention, the duration of the refocussing pulse is extended compared with the excitation pulse. Because the refocussing pulse (or pulses) is normally greater in amplitude than the excitation pulse, the SAR benefit from increasing its duration is gener- ally larger than from lengthening the excitation pulse.

With the first embodiment of the invention, the pulse durations r_exc and r_ref (in s) of the excitation and refocussing pulses preferably fulfill

wherein δ is the chemical shift between the most closely spaced resonance lines of the chemical species to be dis- criminated (in ppm) , B₀ is the strength of the stationary magnetic field (in Tesla) , γ is the gyromagnetic ratio of the nucleus under study, e.g. proton (in MHz/T) , G_exc is the excitation slice-selection gradient strength (in mT/m) , and G_ref is the refocussing slice-selection gradient strength (in mT/m) . As a preferred example, if the sine-shaped excitation pulse has a duration in the range of 1 ms to 5 ms, the at least one sine-shaped refocussing pulse should have a duration 2.5 times as long as the excitation pulse.

According to a second preferred embodiment of the invention, the duration of the excitation pulse is extended compared with the refocussing pulse. This embodiment may have advantages if lengthening the refocussing pulse would increase the minimal echo time achievable, which for certain applications, such as diffusion imaging, could represent a disadvantage. In such cases increasing the duration of the excitation pulse might be the preferred way of achieving chemical-shift artefact suppression by the inventive method.

With the second embodiment of the invention, the pulse durations T_exc and τ_ref (in s) of the excitation and refocussing pulses preferably fulfill τ > ^Pexc^Gref^Tref

7δBj G_ref -G_a ^Vref ^~ Prefiexc

As a preferred example, if the at least one sine-shaped refo^¬ cussing pulse has a duration in the range of 1 ms to 5 ms, the sine-shaped excitation pulse at 7 Tesla should be 2.5 times as long as the refocussing pulse.

If the initial excitation pulse is lengthened and a weaker slice-selection gradient is used, as described here, the fat signal, that would cause a chemical-shift artefact in a water image, will not be refocussed, but instead destroyed by the first refocussing pulse, and it will thus not appear in the image. This demonstrates the general applicability of the method for sequences involving at least one slice-selective refocussing pulse.

The inventive method can be applied throughout the entire object, such as e.g. a patient's body provided that the mag- netic field distribution across the volume of interest is reasonably homogeneous. Although the method can be used at any field strength, it becomes practical in its current implementation at high magnetic field. According to a further preferred embodiment of the invention, the stationary mag- netic field of the MR imaging method and device is at least 3 T, in particular at least 4 T, e.g. 5 T or more. The invention has been demonstrated e.g. with a stationary magnetic field of 6 T or more, in particular 7 T or more.

The virtue of the proposed chemical-shift artefact suppression, in particular fat suppression method becomes very pronounced at 7 T or higher field strengths, where in medical imaging the admissible SAR restricts the number of image slices acquired per unit time. Achieving chemical-shift arte- fact suppression without any additional RF or gradient pulses is preferred. If the signals of the one or more species to be suppressed is not refocussed, it decays with a time constant T2* of the species at the corresponding field strength. High field strength is again beneficial, since T2* of e.g. scalp fat is then extremely short.

If according to a further preferred embodiment of the inven- tion, the refocussing pulse is surrounded by spoiler gradient pulses, as it is known in spin-echo MR imaging sequences, the signal to be eliminated will be crushed, and so will not appear in the image, whatever the value of T2*.

Another advantage of the invention is given by the fact that there are no restrictions with regard to the chemical species to be separated in the MR image. Preferably, for most of the imaging tasks, the MR image represents the contrast of protons of water in the object, however the invention is not re- stricted to hydrogen nuclei and can be applied to other MR- observable nuclei such as carbon-13 or fluorine-19. In the former case, the chosen species to be imaged is water, while the species to be eliminated in the image may be any artefact creating substance, such as e.g. fat, N-acetyl-aspartate and/or choline. On the other hand, if another substance is to be used for MR imaging like N-acetyl-aspartate, the species to be eliminated in the image will be substances such as e.g. water, fat.

The proposed method for e.g. fat suppression depends only on the spatial mismatch between the fat slices excited and refo- cussed. The separation between these can be controlled by increasing the duration of the excitation or the refocussing pulse. This leads to a significant overall decrease in SAR, both by omitting the RF pulse normally used for fat suppression, and by decreasing the amplitude of one of the other RF pulses used. According to a further advantageous embodiment of the invention, at least one of the duration of the excitation pulse, the duration of the refocussing pulse, the gradi- ent strength of the excitation slice selection gradient and the gradient strength of the refocussing slice selection gradient can be adjusted such that the RF power deposition into the object is minimized. Minimization comprises selecting the above parameters in dependency on the imaging conditions (imaging sequence, main field strength, acquisition speed) such that the SAR has a minimum value. Preferably, the RF power deposition is reduced by more than 40% (compared with the process without fat suppression) by increasing the duration of the refocussing pulse to the required value for fat suppression. In case the excitation pulse is prolonged to the extent required for fat suppression the RF power deposition will be reduced by more than 20% (compared with the process without fat suppression) .

Brief description of the drawings

Further details and advantages of the invention are described in the following with reference to the attached drawing, which show in:

Figure 1: schematic illustrations of the inventive slice selection of distinct chemical species (A, B) in comparison with the con- ventional technique (C) ;

Figure 2: pulse sequences of the inventive MR method for slice selection of distinct chemical species (A, B) in comparison with the conventional technique (C) ;

Figure 3: a schematic illustration of a preferred embodiment of the inventive MR imaging device; and Figures 4 and 5: photographs illustrating experimental results obtained with the inventive MR imaging method with fat suppression.

Preferred embodiments of the invention

Preferred embodiments of the invention are described in the following with reference to the MR imaging slice selection of distinct chemical species by the inventive adjustment of the excitation and refocussing pulses and slice selection gradient magnitudes. Details of MR imaging sequences, collecting magnetic resonance signals and controlling an MR scanner as well as details of the construction of an MR scanner are not described as far as they are known from conventional MR imaging techniques.

Exemplary reference is made to slice-selection gradients directed in x-direction, with the z-direction corresponding to the vector of the main magnetic field. The implementation of the invention is not restricted to this direction selection, but rather possible with spatial encoding gradients having other directions. Furthermore, exemplary reference is made to the fat suppression, i.e. the species of interest is water, while the undesired species is only one -(fat) . The implementation of the invention is not restricted to this example, but rather possible with other substances (see below), the contributions of which are to be separated in the MR image.

MR imaging method

The essential step of the inventive MR imaging method is the control of the excitation and refocussing pulses and their respective gradients such that they have different durations and the excitation and the refocussing slice selection gradients have different gradient strengths. In the following, the physical basis of this inventive control of the pulse sequence parameters is described. Reference is made to Figure 1, which schematically illustrates the dependency of the slice-selection gradients on the geometric x-coordinate. Due to the relationship between the frequency bandwidth of the RF pulses and the gradient strength (see below), the gradients are shown in a coordinate system with a frequency axis (f [a.u.]) and a spatial coordinate axis (x [a.u.]) .

Figure IA illustrates the excitation of the magnetic spins in the ROI. Due to the different chemical environments, the effective magnetic field B_eff experienced by the spins of each species, when a magnetic field gradient is applied is different. This leads to a difference of the resonance frequencies of hydrogen protons (the nucleus under study) in different tissues, so that the excitation with a RF pulse having a frequency bandwidth Δω_exc occurs in different slices perpendicu- lar to the x-direction. The slices of the first species sp.l and the second species sp.2 are displaced by an amount D_exc-

If the refocussing pulse and refocussing slice-selective gradient have the same duration and magnitude as those of the excitation pulse and the excitation slice-selective gradient, as it is the case with conventional techniques without chemical-shift artefact suppression, the illustration of Figure 1C results. The refocussing of the first and second species sp.l, sp.2 occurs in different slices with a displacement D_ref being identical or slightly different compared with the excitation displacement D_exc- According to Figure 1C, a chemical artefact suppression cannot be obtained as both species are simultaneously excited and refocussed for obtaining a collectable magnetic resonance signal. On the contrary, with the inventive method, a refocussing displacement D_ref is adjusted such that the excitation and refocussing slices of the second species do not longer overlap (Figure IB) .

For both excitation and refocussing conditions, the displacement D (D_exc or D_ref) iⁿ nun of the fat slice relative to the water slice is given by

Z> = ^^> (D

where δ is the chemical shift in parts per million (ppm) , Bo is the main magnetic field in Tesla (T) , and G_s is the slice- selection gradient strength in mT/m. In general, the profiles of the fat slice excited and refocussed may differ in thickness, d (mm), so equation (2) has to hold in order for complete fat-signal suppression to be achieved:

I Dexc - Dref I > <d_exc + d_lef) /2 ( 2 )

When the same RF pulse shape is used for excitation and refocussing, the right-hand side of equation (2) is replaced by the nominal slice thickness chosen. However, the success of the inventive chemical artefact suppression method does not depend on the particular RF pulse shape used, provided that the slice profiles of the fat signal excited and fat signal refocussed do not overlap.

A frequency-selective radiofrequency pulse shape is characterized by a dimensionless number, the bandwidth-time product, P. As can be seen from equation (1), G_s is the key parameter for changing the displacement of both fat slices relative to the water slice, and hence relative to each other. One way of achieving a smaller slice-selection gradient strength, while keeping the same slice profile, is to increase the duration of the RF pulse, as can be seen from equation (3) .

Δω = γdG_s = P/τ (3)

Δω is the pulse bandwidth in Hertz (Hz) ; d is the slice thickness in mm; γ is the gyromagnetic ratio of the nucleus under study, e.g. protons in MHz/T; and r is the duration of the RF pulse in seconds. Lengthening the RF pulse results in a pulse of smaller bandwidth. Once the desired slice thickness is chosen, the slice-selection gradient magnitude is ad- justed by the control device 50 in Figure 3 to appropriately match the bandwidth of the pulse. Using (1) and (3) in (2) gives the following expression:

Further, if the duration of a RF pulse is scaled by a factor c according to expression (5)

τ→cτ, (5)

and the flip angle as well as the slice thickness are kept constant then

G_s→GJc E→ EIc, (6)

where E is the energy deposited by the RF pulse in the sample. Therefore the factor c in expression (5) and (6) deter- mines the SAR and the slice selection gradient decrease achieved by lengthening any of the RF pulses.

Equation (7) gives an expression for the slice-selection gra- dient strength difference required for fat suppression. From this it can be seen that for efficient fat suppression it is irrelevant which of the two RF pulses is lengthened, as Figure 5 also confirms.

Figure IB illustrates the effect of a decreased refocussing slice-selection gradient applied in combination with a lengthened refocussing pulse. While the RF pulse with the frequency bandwidth Δω_ref yields refocussing of species 1

(sp.l) in a slice corresponding to the excitation slice, refocussing of species 2 (sp.2) occurs in an essentially shifted slice. The displacement D_ref differs from the excitation displacement D_exc • For refocussing the second species in a slice corresponding to the excitation slice of the second species (sp.2*), a pulse with a frequency Δω^* _ref would be necessary. However, as the refocussing effect of the refocussing pulse occurs in the shifted slice for the second species (sp.2), a magnetic resonance signal from it for image recon- struction can not be obtained.

Equation (8) provides an expression for the duration of the excitation pulse required to suppress the fat signal, in terms of the refocussing pulse duration and the slice select gradients:

The gradient G can take negative values in these equations. In particular, the gradient during the refocussing pulse can be equal in magnitude and with opposite sign to the gradient during the excitation pulse.

Lengthening the excitation pulse beyond the duration derived using equation (8) will not further improve fat suppression, but will clearly help in decreasing the overall SAR of the sequence. If the duration of the other pulse is increased in addition, there will still be a SAR benefit; however, the fat signal suppression will be compromised as can be seen from equation (8) . The formula for the preferred minimal duration of the refocussing pulse can be obtained by exchanging the indices of the refocussing and excitation pulse in equation (8) .

It is important to note that the main magnetic field strength enters the denominator on the right-hand side of (7), which means that the inventive method is better suited for use at higher fields. At fields lower than 3 T, the RF pulses necessary to achieve complete fat-signal suppression are quite long, so that practical use in a pulse sequence is provided in exceptional cases only.

Equation (9) provides an expression for the duration of the refocussing pulse required to suppress the fat signal, in terms of the slice thickness desired and the slice select gradients':

τ > ^Pref^Gexc _{/ Q χ}

^Tref ~

2δB₀γ G_ref -G_exc - γdG_excG_ref Thus, the smallest possible duration of the refocussing pulse required for fat suppression for a chosen slice profile and thickness depends on the magnitude of the slice-selection gradient applied during the excitation pulse. According to equation (3) with increasing slice selection gradient, the duration of the excitation pulse decreases. The magnitude of the maximum possible gradient is a technical characteristic of the gradient system and it will set the lower limit to the duration of the excitation pulse. Another hardware constraint that could also limit the extent to which the excitation pulse can be shortened is the maximum peak voltage that the transmitter RF system can deliver. Necessarily, the duration of the shortest possible excitation pulse will determine the duration of the shortest possible refocussing pulse that is required for fat suppression.

Equation (9) and the discussion following it remain true if the excitation pulse and the excitation slice selection gradient are substituted respectively by the refocussing pulse and refocussing slice selection gradient and vice versa.

Although having both pulses as short as possible, along with fulfilling the condition for fat suppression, has an essential advantage, it is not always realizable in practice due to SAR constrains, in particular at fields as high as 7 Tesla.

Another effect which the current calculation does not consider, important for very long pulse durations, is T2* decay during the application of the RF pulse. Although this effect will not degrade the fat suppression achieved, it may affect the overall image quality. At the field strength of 7 T, the duration of one of the RF pulses needs to be increased to about 6 ms to achieve 100 % fat suppression, for the case that the other RF pulse has duration of around 2.5 ms . Such RF pulse durations give no reduction in image quality (Figure 5) for brain imaging.

Figures 2A and 2B illustrate examples of the inventive modifications to MR imaging sequences. Figure 2A shows the preferred embodiment with a lengthened refocussing pulse and a lengthened duration of its slice-selection gradient. On the contrary, Figure 2B illustrates the alternative embodiment with an excitation pulse having a longer duration compared with the refocussing pulse. The duration of the slice- selection gradient during the refocussing pulse is shortened accordingly. Furthermore, Figures 2A and 2B show that with a lengthened refocussing pulse, the magnitude of the slice- selection gradient is reduced, while with a shortened refocussing pulse the magnitude of the slice selection gradient is increased. For comparison purposes, Figure 2C shows the conventional case of excitation and refocussing pulses having the same duration.

MR imaging device

Figure 3 schematically illustrates an embodiment of the inventive imaging device 100 including a main magnetic device 10 creating a stationary magnetic field, a magnetic field gradient device 20 creating an excitation slice-selection gradient or a refocussing slice-selection gradient and possibly further spatial gradients waveforms for manipulating the proton magnetization, a transmitter device 30 creating an ex- citation RF pulse or at least one refocussing RF pulse, a receiver device 40 for collection of the NMR signals created in the object 1, a control device 50 controlling the slice- selection gradient device 20 and the transmitter device 30, and an image reconstructing circuit 60 reconstructing a MR image of the object 1 based on resonance signals collected with the receiver device 40. The components 10-40 and 60 are constructed as it is known from conventional MR scanners. In particular, the components 10 to 40 comprise coils, which are arranged around a space accommodating the object 1, which is typically arranged on a carrier 70.

The control device 50 is connected with the coils of the slice-selection gradient device 20 and the transmitter device 30. The control device 50 includes an electrical circuit 51, which is adapted for adjusting the pulse durations of the excitation and refocussing pulses and the respective gradients thereof as well as the magnitudes of the slice-selection gradients. Furthermore, the control device 50 includes a setting device 52. Preferably, the setting device 52 is adapted for adjusting the pulse duration only. The associated gradient's strength for creating the RF pulses with different durations but keeping constant excitation or refocussing slices can be provided using a calculating unit 53.

Accordingly, to allow the use of longer RF pulses for excitation and/or refocussing requires only a slight modification of the relevant MRI sequence code is necessary. As an example, the code for the Siemens 7T whole-body MR scanner (type: MAGNETOM 7T) product Spin-Echo EPI sequence was modified for obtaining the results illustrated in Figures 4 and 5.

Implementation and Results

Figure 4 and Figure 5 show spin-echo EPI images acquired at the same position in the head at 3 mm isotropic resolution with identical acquisition parameters. In Figure 4, SE EPI images were acquired with (A) 2.56 ms excitation pulse, 3.84 ms refocussing pulse and fat saturation pulse, (B) 2.56 ms excitation pulse, 3.84 ms refocussing pulse and no fat saturation pulse, (C) 3.84 ms excitation pulse, 2.56 ms refocussing pulse and no fat saturation pulse, and (D) 3.84 ms excitation pulse, 2.56 ms refocussing pulse and fat satura- tion pulse. In Figure 5, SE EPI images were acquired with (A) 2.56 ms excitation pulse, 6.40 ms refocussing pulse and fat saturation pulse, (B) 2.56 ms excitation pulse, 6.40 ms refocussing pulse and no fat saturation pulse, (C) 6.40 ms excitation pulse, 2.56 ms refocussing pulse and no fat saturation pulse, and (D) 6.40 ms excitation pulse, 2.56 ms refocussing pulse and fat saturation pulse.

Figure 4B and Figure 4C demonstrate clearly the fat signal present in the images if a fat-sat pulse is not applied. Fig- ure 4A and Figure 4D show images when the fat-sat pulse is employed, in which the fat artefact is no longer visible. It is worth noting that the appearance of the artefact in Figure 4B and Figure 4C is quite similar, as one would expect from (5) since only the region of overlap between the fat slice excited and the fat slice refocussed will appear in the image .

Figure 5 shows images obtained from the inventive sequence, which allows longer RF pulse durations with correspondingly weaker slice-selection gradients. Figure 5C and Figure 5D demonstrate that an RF pulse as long as 6.40 ms is sufficient for achieving complete fat signal suppression, provided that the other RF pulse has 2.56 ms duration. Fig 5A and Figure 5D confirm that adding the standard fat-sat pulse gives no fur- ther improvement.

Comparison of the images in Figure 4 and Figure 5 shows that increasing the duration of either of the RF pulses within the given range does not affect the overall image quality. Further Applications

The ease with which protons at specific frequencies can be selected with the inventive method suggests very useful applications at high magnetic field for imaging fat distributions in tissue, or even those of particular molecules, such as N-acetyl-aspartate or choline, which have distinct spectral lines well separated from water. In such cases the scan- ner resonance frequency would be set to the resonance frequency of the desired molecule. Here the water and fat signal would be dispersed by the crusher gradients surrounding the refocussing pulse. Furthermore, chemical species of other MR- observable nuclei, such as helium-3, carbon-13, oxygen-17, fluorine-19, sodium-23 and phosphorus-31 can also be easily selected by the inventive method. Thus the inventive method can be used for imaging the distribution of particular molecules containing any of the aforementioned nuclei and so applied to the fields of metabolic and/or contrast agent imag- ing.

The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination for the realization of the invention in its various embodiments.

Claims

1. Method of magnetic resonance imaging of an object (1), which includes at least two chemical species, comprising the steps of:

- arranging the object (1) in a stationary magnetic field,

- subjecting the object (1) to an excitation slice-selection gradient and an excitation pulse,

- subjecting the object (1) to a refocussing slice-selection gradient and a refocussing pulse,

- collecting magnetic resonance signals created in the object (1), and - reconstructing an image of the object (1) based on the magnetic resonance signals, characterized in that

- the excitation and refocussing pulses and their respective gradients have different durations and the excitation and the refocussing slice selection gradients have different gradient strengths such that the chemical species are excited by the first pulse, but only one of the chemical species is refocussed by the second pulse, so that the magnetic resonance signal thereof is created, while the magnetic resonance signals from the other species are eliminated.

2. Method according to claim 1, wherein

- the object contains two chemical species, the first of which being refocussed while the magnetic resonance signals from the second species are eliminated.

3. Method according to claim 1, wherein

- the refocussing pulse has a longer duration than the excitation pulse.

4. Method according to one of the foregoing claims, wherein - the pulse durations r_exc and τ_ref (in s) of the excitation and refocussing pulses fulfill

wherein δ is the chemical shift between the most closely spaced resonance lines of the chemical species to be discriminated (in ppm) , B₀ is the strength of the stationary magnetic field (in Tesla) , y is the gyromagnetic ratio of the nucleus under study (in MHz/T) , G_exc is the excitation slice- selection gradient strength (in mT/m) , and G_ref is the refocussing slice-selection gradient strength (in mT/m) .

5. Method according to claim 1, wherein - the excitation pulse has a longer duration than the refocussing pulse.

6. Method according to one of the foregoing claims, wherein - the pulse durations τ_exc and τ_ref (in s) of the excitation and refocussing pulses fulfill

wherein δ is the chemical shift between the most closely spaced resonance lines of the chemical species to be discriminated (in ppm) , B₀ is the strength of the stationary magnetic field (in Tesla) , γ is the gyromagnetic ratio of the nucleus under study (in MHz/T) , G_exc is the excitation slice- selection gradient strength (in mT/m) , and G_ref is the refocussing slice-selection gradient strength (in mT/m) .

7. Method according to one of the foregoing claims, comprising the step of - setting the stationary magnetic field to be at least 3 T.

8. Method according to one of the foregoing claims, comprising the step of

- creating spoiler gradient pulses surrounding the refocussing pulse.

9. Method according to one of the foregoing claims, wherein the chemical species refocussed comprises any of the following: water, fat, N-acetyl-aspartate or choline, and the species eliminated comprise one or more of the rest of the aforementioned chemical species.

10. Method according to one of the foregoing claims, comprising the step of - setting at least one of the duration of the excitation pulse, the duration of the refocussing pulse, the gradient strength of the excitation slice selection gradient and the gradient strength of the refocussing slice selection gradient such that the RF power deposition is minimized.

11. Method according to one of the foregoing claims, wherein the RF power deposition is reduced by more than 40%.

12. Imaging device (100) for magnetic resonance imaging of an object (1), which includes at least two chemical species, comprising the steps of:

- a main magnetic device (10) arranged for creating a stationary magnetic field,

- a slice-selection gradient device (20) arranged for creating an excitation slice-selection gradient or a refocussing slice-selection gradient,

- a transmitter device (30) arranged for creating an excitation pulse or a refocussing pulse, - a receiver device (40) arranged for collecting resonance signals created in the object,

- a control device (50) arranged for controlling the slice- selection gradient device (20) and the transmitter device (30), and

- an image reconstructing circuit (60) arranged for reconstructing an image of the object based on the magnetic resonance signals, characterized in that - the control device (50) is adapted for adjusting different pulse durations of the excitation and refocussing pulses and their respective gradients and different gradient strengths of the excitation and the refocussing slice selection gradients such that the chemical species are excited by the first pulse, but only one of the chemical species is refocussed by the second pulse, so that the magnetic resonance signal thereof is created, while the magnetic resonance signals from the other species are eliminated.