MAGNETIC RESONANCE IMAGING METHOD AND DEVICE

Abstract

A method for generating a magnetic resonance imaging, MRI, image of a subject including applying a magnetic field B.sub.0 to the subject, applying a sequence of electromagnetic pulses to the subject, applying further magnetic field gradients in addition to the magnetic field B.sub.0, the magnetic field including first gradients and at least one second gradient; measuring signal echoes produced by the object in response to the electromagnetic pulses and the first and second magnetic field gradients; acquiring image data at a first spatial resolution from the signal echo produced by the object in response to the electromagnetic pulses and the first and second magnetic field gradients, and combining the image data acquired from signal echoes in order to produce at least one image of the object at a second spatial resolution.

Claims

1. A method for generating a magnetic resonance imaging, MRI, image of a subject, the method comprising: applying a magnetic field B.sub.0 to the subject; applying a sequence of electromagnetic pulses to the subject; applying further magnetic field gradients in addition to the magnetic field B.sub.0, the magnetic field gradients comprising a plurality of first gradients and at least one second gradient; measuring signal echoes produced by the object in response to a plurality of the electromagnetic pulses and the first and second magnetic field gradients; acquiring image data at a first spatial resolution from the signal echo produced by the object in response to the electromagnetic pulses and the first and second magnetic field gradients; and combining the image data acquired from signal echoes in order to produce at least one image of the object at a second spatial resolution, the second spatial resolution being higher than the first spatial resolution, wherein the first gradients are fully rewound in the interval between successive electromagnetic pulses, while the at least one second gradient has a non-zero gradient-time integral between successive electromagnetic pulses, wherein the phase of at least one electromagnetic pulse in the sequence of electromagnetic pulses is different to another electromagnetic pulse in the sequence of electromagnetic pulses.

2. A method for generating a magnetic resonance image of a subject according to claim 1, wherein the sequence of electromagnetic pulses comprises at least a first set of electromagnetic pulses, wherein a first pulse of the first set has a first phase φ.sub.1, and each subsequent pulse of the first set has a phase which is incremented by a first interval.

3. A method for generating a magnetic resonance image of a subject according to claim 2, wherein the sequence of electromagnetic pulses comprises a second set of electromagnetic pulses, wherein a first pulse of the second set has a second phase φ.sub.2, and each subsequent pulse in the second set has a phase which is incremented by a second interval which is different to the first interval.

4. A method for generating a magnetic resonance image of a subject according to claim 1, wherein the sequence of electromagnetic pulses comprises at least a first set of electromagnetic pulses, wherein a first pulse of the first set has a first phase φ.sub.1, and each subsequent pulse of the first set has a phase which is incremented by a quadratically increasing interval.

5. A method for generating a magnetic resonance image of a subject according to claim 1, wherein the image data from more than one signal echo is measured in the interval between subsequent pulses in the sequence of electromagnetic pulses.

6. A method for generating a magnetic resonance image of a subject according to claim 1, wherein the first gradients comprise at least one of phase encoding gradients, slice select gradients and frequency encoding gradients arranged along a first axis, and wherein the gradient-time integral of the second gradient is oriented along the first axis.

7. A magnetic resonance imaging, MRI, device which comprises a controller, the controller being configured to control the MRI device to: apply a magnetic field Bo to a subject; apply a sequence of electromagnetic pulses to the subject; apply further magnetic field gradients in addition to the magnetic field B.sub.0, the magnetic field gradients comprising a plurality of first gradients and at least one second gradient; and measure signal echoes produced by the object in response to a plurality of the electromagnetic pulses and the first and second magnetic field gradients, the controller being further configured to: acquire image data at a first spatial resolution from the signal echo produced by the object in response to the electromagnetic pulses and the first and second magnetic field gradients; and combine the image data acquired from signal echoes in order to produce at least one image of the object at a second spatial resolution, the second spatial resolution being higher than the first spatial resolution, wherein the first gradients are fully rewound in the interval between successive electromagnetic pulses, while the at least one second gradient has a non-zero gradient-time integral between successive electromagnetic pulses, wherein the phase of at least one electromagnetic pulse in the sequence of electromagnetic pulses is different to another electromagnetic pulse in the sequence of electromagnetic pulses.

8. An MRI device according to claim 7, wherein the sequence of electromagnetic pulses comprises at least a first set of electromagnetic pulses, wherein a first pulse of the first set has a first phase φ.sub.1, and each subsequent pulse of the first set has a phase which is incremented by a first interval.

9. An MRI device according to claim 8, wherein the sequence of electromagnetic pulses comprises a second set of electromagnetic pulses, wherein a first pulse of the second set has a second phase φ.sub.2, and each subsequent pulse in the second set has a phase which is incremented by a second interval which is different to the first interval.

10. An MRI device according to claim 7, wherein the sequence of electromagnetic pulses comprises at least a first set of electromagnetic pulses, wherein a first pulse of the first set has a first phase φ.sub.1, and each subsequent pulse of the first set has a phase which is incremented by a quadratically increasing interval.

11. An MRI device according claim 7, wherein the image data from more than one signal echo is measured in the interval between subsequent pulses in the sequence of electromagnetic pulses.

12. An MRI device according to claim 7, wherein the first gradient comprises at least one of phase encoding gradients, slice select gradients and frequency encoding gradients arranged along a first axis, and wherein the gradient-time integral of the second gradient is oriented along the first axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Various aspects of the teachings of the present disclosure, and arrangements embodying those teachings, will hereafter be described by way of illustrative example with reference to the accompanying drawings, in which:

[0032] FIG. 1 is an example of a Magnetic Resonance Imaging device according to the present disclosure;

[0033] FIGS. 2 and 3 illustrates the magnetic field gradients that are applied by the MRI device;

[0034] FIG. 4 illustrates off resonance frequency profiles for a group of protons in the MRI device;

[0035] FIG. 5 illustrates the fractional shift in the off resonance frequency profile when applying sets of electromagnetic pulses with different linear phase increments;

[0036] FIGS. 6A-7B are example images from the MRI device. FIGS. 6A-6D illustrate the method through simulations while FIGS. 7A-7B is an example using real acquired data.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] FIG. 1 is a block diagram of a Magnetic Resonance Imaging (MRI) device 100 according to the present disclosure. The MRI device 100 includes a chamber 110 which holds the subject which is to be scanned, and which is surrounded by a magnetic array 120 which is arranged to induce magnetic fields in the chamber 110. The functioning of the magnetic array 120 is controlled by a controller 130. The controller 130 also controls a transmitter 140, which is arranged to broadcast radiofrequency pulses into the chamber 110. A receiver 150 is arranged to detect electromagnetic signals from the subject in the chamber 110, and provide these signals to the controller 130.

[0038] FIG. 2 is a diagram illustrating the operation of the MRI device 100 when used for image acquisition using a refocussed Steady State Free Precession (SSFP) pulse sequence. FIG. 2 illustrates the pulses and magnetic fields applied during one repetition time, TR.

[0039] In refocussed SSFP sequences a radiofrequency pulse 210, 220 is repeatedly applied, alongside magnetic field gradients which are fully rewound during the repetition time, TR. This means that the time integral of the magnetic field gradients applied to the subject during the interval between two radiofrequency pulses 210, 220 is zero.

[0040] At the beginning of a repetition time TR, a first radiofrequency pulse 210 is applied to the subject. The first radiofrequency pulse 210 has an intensity a and a phase φ. The first radiofrequency pulse 210 causes excitation of nuclei in the subject to be scanned, causing the net magnetisation vector of the nuclei in the subject to rotate into the transverse plane by a flip angle α. The nuclei may be those of hydrogen atoms. The magnetisation then precesses about the main magnetic field, B.sub.0, at a rate proportional to the local off-resonance frequency. The magnetisation of the subject relaxes with time constants T.sub.1 and T.sub.2.

[0041] FIG. 2 illustrates the magnetic field gradients that are applied during the repetition time as part of slice selection (SS), phase encoding (PE) and frequency encoding (FE). The magnetic field gradients applied for slice selection 231, 232, 233, 234 create a corresponding gradient in precession frequencies of the subject, so that a given slice of the subject can be selected for scanning. In particular, the magnetic field gradients applied for slice selection 231, 232, 233, 234 can be varied in intensity to select the desired slice width. Slice selection therefore allows for localisation along a first axis within the subject.

[0042] The magnetic field gradients applied for phase encoding (PE) 241, 242 cause phase dispersion in the precession of the nuclei. The magnetic field gradients applied for phase encoding 241, 242 may be also varied in intensity in order to localise along a second axis within the subject which is perpendicular to the first axis. In some embodiments, phase encoding is used on two axes, the second phase encoding PE2 complementing or replacing slice selection.

[0043] The magnetic field gradients applied for frequency encoding (FE) 251, 252, 253 cause a frequency dispersion in the precession of nuclei along a third axis which is perpendicular to the first axis and the second axis, in order to localise along this third axis within the subject.

[0044] As such, when signals are detected by the receiver 150, signals related to areas within the slice can be identified by their unique combination of frequency and phase encoding, and used to generate pixels for use in a resulting image of the slice, usually by the application of a Fourier transformation.

[0045] Signals are detected during each TR, centred at the echo time (TE) which is the time between the application of a radiofrequency pulse and the centre of the signal which is induced in the receiver 150. TE is typically equal to half of TR.

[0046] FIG. 3 is a diagram illustrating the operation of the MRI device 100 when used for image acquisition according to the present disclosure. In the operation, a similar data acquisition is employed to that used for refocussed SSFP acquisitions. In particular, a plurality of radiofrequency pulses 310, 320 are applied to the subject to be scanned while it is in the chamber 110. In each TR, magnetic field gradients 331, 332, 333, 334, 341, 342, 351, 352, 353 are applied to the subject in order to provide slice selection, phase encoding and frequency encoding. An additional magnetic field gradient is applied with a non-zero time integral in a specific direction. In the example shown in FIG. 3, the additional magnetic field gradient 343 is applied in the same direction as the phase encoding. The additional magnetic field gradient 343 is applied in each TR.

[0047] For a given material in a subject to be scanned, the steady-state magnetisation is strongly dependent on the off-resonance frequency Δf, which is the deviation from the frequency produced by an ideal homogeneous magnetic field. FIG. 4 illustrates the dependence of signal magnitude on off resonance frequency. In the limit of low flip angles, such as α=1°, the magnitude of the off-resonance profile approximates a damped comb function with a period of 1/TR. Following a radiofrequency pulse with a phase of φ, the next radiofrequency pulse has a phase of φ+Δφ. By linearly incrementing the phase of successive radiofrequency excitation pulses, the off-resonance profile is offset along the frequency dimension by Δφ/(2πTR) in the final image.

[0048] The off-resonance profile and the additional magnetic field gradient 343 result in a periodic modulation of the spatial magnetisation along the direction of the additional magnetic field gradient, where the position of the peaks in the periodic modulation depend upon the value of the phase increment of the radiofrequency pulse series. As such, when the phase of each radiofrequency pulse in a set is incremented by Δφ, the position of the peaks in the periodic modulation are shifted slightly with respect to the subject.

[0049] FIG. 5 illustrates the migration of peaks in the periodic modulation as the linear phase increment is changed between radiofrequency pulse sets, from Δφ=0 to Δφ=2π. The images which are derived from each set therefore correspond to different fractional shifts of this periodic modulation. The images from each set can then be merged to generate a super-resolution image.

[0050] FIGS. 6A, 6B, 6C and 6D show an example application of the approach illustrated in FIG. 3, where the MRI device 100 is used in creating a cross-sectional scan of a brain. One of the sets of image data is shown in FIG. 6A, and the underlying magnetisation profile in FIG. 6A is shown in FIG. 6B. If the phase increment between successive radiofrequency excitations is changed, the modulation pattern shifts in the resultant image as shown in FIG. 6C, thus containing different information about high-spatial frequencies. Seventeen sets of image data were simulated with different phase increments, including the image shown in FIG. 6A, and merged to generate the super-resolution image shown in FIG. 6D.

[0051] FIG. 7A shows a single 2D SSFP acquisition with a nominal spatial resolution of 0.3×1.8 mm, using a flip angle α=0.4° and including an additional magnetic field gradient as illustrated in FIG. 3, such that the periodicity of the modulation pattern is equal to 1.8 mm in the up-down direction. By interleaving six of these images voxel by voxel, where the spatial modulation pattern has been shifted in six equidistant steps, super-resolution is achieved in one dimension and a 0.3×0.3 mm image is produced, as shown in FIG. 7B.

[0052] The use of low flip angles allows for a simple deconvolution operation when merging the images, since most of the signal is generated within the narrow peak in the off-resonance profile, as can be seen for example in FIG. 6B, and there is minimal signal cancellation from magnetisation elsewhere. As a result, the voxel interleaving and deconvolution operations work effectively using only the magnitude of the complex images. However, the approach could equally be used at higher flip angles if combined with a deconvolution operation which accounts for the complex integral of the broader off-resonance profiles, which can be seen in FIG. 2. This may further increase the SNR efficiency of the proposed super-resolution approach. However, the use of low radiofrequency pulse flip angles can help to minimise tissue heating.

[0053] Although in FIG. 3 the additional magnetic field gradient is positive in the phase encode direction, it can take any shape provided it has the required non-zero time integral, and it can be equally applied in any direction. For example, the additional magnetic field gradient may be applied in the slice selection direction or the second phase encoding direction, and the additional magnetic field gradient may be applied in the frequency encoding direction.