WO2011015865A1

WO2011015865A1 - T1-weighted ir ss fse mri scanning of the fetal brain

Info

Publication number: WO2011015865A1
Application number: PCT/GB2010/051281
Authority: WO
Inventors: Joseph Vilmos Hajnal; Mary Ann Rutherford; Christina Malamateniou; Amy Kathleen Mcguinness
Original assignee: Imperial Innovations Limited
Priority date: 2009-08-03
Filing date: 2010-08-03
Publication date: 2011-02-10

Abstract

A method of T1-weighted magnetic resonance imaging of the fetal brain, comprising performing an inversion recovery (IR) preparation step followed by a rapid single shot (ss) fast spin echo (FSE) readout process.

Description

T1 -WEIGHTED IR SS FSE MRI SCANNING OF THE FETAL BRAIN

Field of the invention

The present invention relates to MRI scanning and in particular to the imaging of moving subjects. It has particular application in fetal imaging. Background to the invention

MRI scanning works broadly by applying a strong static magnetic field to a subject which tends to align the spin of subatomic particles, such as protons, in the subject, with the magnetic field, producing a longitudinal magnetization (Mz) that is parallel to the applied strong field, and then applying a radio-frequency (RF) magnetic field to the subject and monitoring the response. The RF magnetic field rotates the magnetization produced by the strong field by an angle is referred to as the flip angle, and the rate at which the net magnetization of the substance recovers due to re-alignment is defined by a time constant referred to as Tl. When the net magnetization is not aligned with the strong magnetic field it precesses around the direction of the strong magnetic field, having a transverse component which rotates with the precession producing a rotating magnetic field that can be detected by a RF coil to produce a signal that can be stored and processed. The detected signal decays over time with a time constant referred to as T2. As Tl and T2 vary in different ways between different types of tissue or material, images in which the evolution of the magnetization depends on Tl and T2 can distinguish between different types of material in different ways. Therefore it is desirable to be able to generate both Tl and T2 weighted images of a subject to be able to examine it as thoroughly as possible. For imaging fetuses and other moving subjects, rapid imaging methods that produce T2 weighted images have successfully been used, but Tl weighted images are more difficult to produce, and for fetal imaging have not been produced with satisfactory clarity. Specifically, in some cases, Tl weighted fetal brain imaging can suffer from low grey-white matter contrast and increased susceptibility to movement with some commonly applied gradient (field) echo breath-hold sequences.

Summary of the Invention

The present invention provides a method of magnetic resonance imaging of a subject, which may be moving, comprising applying a static magnetic field to the subject, applying a magnetization preparation step which may comprise an RF pulse of magnetic field to the subject to invert or otherwise alter the nuclear magnetization, in a slice slab or other region of the subject. The method may then comprise applying an excitation pulse. It may then comprise a rapid readout process over a period which is preferably short enough to freeze any motion of the subject. The readout process may comprise a single shot readout sequence, for example it may be a single shot fast spin echo sequence.

The magnetization preparation step is arranged to make the nuclear magnetization in the subject vary with a parameter that is to be used for the imaging. In Tl weighted imaging it is arranged to vary with Tl. In other cases the magnetization preparation step can take another form, for example it can be a flow sensitive magnetization preparation step in which the magnetization produced is dependent on the movement of a component of the subject being imaged. Magnetic transfer preparations can also be used.

The excitation pulse may be arranged to generate at least a degree of transverse magnetization in the subject. For example it may cause the magnetization to rotate by 90 degrees to the static field. The readout sequence may further comprise a series of re-focusing pulses, which may cause rotations of 180 degrees or some other angle. Each of the re-focusing pulses may be followed by a data acquisition period during which field gradients are applied to the subject.

Instead of fast spin echo, other rapid readout methods can be used, such as echo planar imaging or rapid acquired gradient echo. The field gradients, or other data acquisition steps may be arranged such that data is acquired for a series of lines or regions in k-space. These may be acquired in an order which starts with a low value of k, preferably k_x = k_y = 0. They are then preferably acquired in order of increasing magnitude of k. For example, if the lines are straight parallel lines, they may alternate between positive and negative values of k_x. The inversion time between the inversion pulse and the excitation pulse may be low. For example it may be less than Is. In some cases it may be less than 600 ms, for example between 400 and 600ms. This can allow a magnitude-only image reconstruction to be used. The pixel values of the reconstructed image can be multiplied by (-1), for example if the tissue TIs are such that Mz is still negative for all materials at the time of the excitation pulse.

The inversion pulse may be spectrally selective, for example allowing different frequencies of pulse to be used to select between different material types.

The present invention can in some cases be referred to as inversion recovery prepared single shot fast spin echo. Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings.

Brief description of the drawings

Figure 1 is a diagram of an MRI scanning system according to an embodiment of the invention; Figure 2 is a diagram showing recovery of longitudinal magnetization over time following inversion for a range of different materials; Figure 3 is a diagram showing the sequence of pulses and readings in a method according to an embodiment of the invention;

Figure 4 is a diagram showing the order of acquisition of data in k-space in the method of Figure 3. Description of the Preferred Embodiments

Referring to Figure 1, an MRI scanning system, which may be based for example on a Philips 1.5T MRI system using a 5-channel cardiac phased-array coil, comprises a primary coil 10 arranged to produce a strong magnetic field in a scanning volume 12, a set of gradient coils 14 arranged to produce a magnetic field gradient in the scanning volume, a set of transmitter coils 16 arranged to generate RF pulses of magnetic field in the scanning volume, and a set of receiver coils 18 arranged to detect RF signals. A controller 20 is arranged to control operation of the gradient and transmitter coils 14, 16 and to receive signals from the detector coils 18. The controller 20 includes a processor 20a, and memory 20b in which a number of scanning protocols are stored. User inputs 20c can be input via a user interface 22 to select one of the protocols, and the controller is arranged, in response to that, to perform a scan according to a method defined in the protocol.

Referring to Figure 2, as is known, if a subject is placed in a strong static magnetic field in the direction z, for example in the scanning volume 12, and then an inversion pulse, which is a RF pulse of magnetic field, is applied to the subject, for example using the transmitter coil 16, the spins of, for example, protons in the subject, can be inverted, i.e. turned through a flip angle of 180 degrees from their initial orientation aligned with the main field, defined as the positive z direction, so that their magnetization is anti-parallel with the main field, in the negative z direction, by the pulse. The spins will then gradually recover over time back to alignment with the direction of the static field. If the inversion pulse is at time t = 0 as shown in Figure 2, then the 'recovery' rate at which the spins re-align is exponential with a time constant referred to as Tl, which varies between material types. For example the lines shown represent the recovery of different tissues such as grey matter, white matter and cerebrospinal fluid (CSF), with different Tl values. In Figure 2, 110 has the shortest Tl followed by 112, 114 and 116 has the longest Tl.

If, during the recovery of the longitudinal magnetization, an excitation pulse of magnetic field is produced by the transmitter coil 16, this converts the longitudinal magnetization to a transverse magnetization. The level of transverse magnetization can be measured with one or more of the detector coils 18, and also decays with time over a time period T2. Therefore the initial level of transverse magnetization, together with the inversion time TI between inversion pulse and excitation pulse can give information about Tl, and the rate of decay of transverse magnetization after the excitation pulse can give information about T2.

Referring to Figure 3, in one embodiment of the invention, an inversion pulse is applied to the subject at time t = 0 which inverts the longitudinal magnetization in the subject. Then after an inversion time of TI to allow Tl processes to occur, resulting in differences in the amount of recovered magnetization in different tissues or fluids, an excitation pulse with a flip angle of 90 degrees is applied. This converts all the longitudinal magnetization to transverse magnetization, or signal. Therefore immediately after a 90 deg pulse, Mz = 0. It then starts to recover by a Tl process, in which the transverse magnetization reverts to longitudinal magnetization. Then, after a short delay τ a re- focusing RF pulse, which could produce a rotation of 180 degrees or another flip angle is applied. The re-focusing pulse is arranged to reverse gradual dispersion of the transverse spins so that they produce an 'echo' or repeat coherent state regenerating the transverse magnetization. Then a succession of re-focusing pulses is applied at time intervals of 2τ. After each re-focusing pulse magnetic gradient fields are applied to the subject and there is a data acquisition period. The gradients are arranged such that, in each period, data for a respective line in k-space is acquired. Referring to Figure 4, the field gradients are arranged such that the first line to be acquired is k=0. This is then followed, in order, by k=l, k= -1, k=2, k= -2 etc. up to k=kmax. The order therefore alternates in sign of k, and increases in magnitude of k up until sufficient data has been acquired for a full plane. This k space sampling order allows a short effective echo time which minimizes the T2 weighting.

Other acquisition methods can be used. If the aim is to minimize T2 effects in the image, it is preferable to start at the centre of k-space where k = 0, or at least near to that point. Another suitable method therefore is to acquire data for a series of points in k-space that move out from the centre of k-space in a spiral. If T2 effects are not to b e minimized, then still further methods of data acquisition can be used.

Gradients may also be applied during the RF pulses to make their action on the magnetization slice or slab selective.

Each refocusing pulse is designed to act on the transverse magnetization by rotating it to make it start to re-focus towards a coherent state, but it also rotates through the same angle, whatever longitudinal magnetization has built up. This keeps happening through the FSE part of the sequence so that at the end there is still almost no Mz, even though it may be half a second or more after the excitation pulse. For this reason it is advantageous to allow a significant delay after the end of the sequence before it can be re-started. Therefore, after the end of the last refocusing pulse and final data collection process, there is a long delay to allow the longitudinal magnetization of all tissues and fluids to substantially recover to equilibrium, then the whole process is repeated. During the long delay, the scanner is usually set to repeat the sequence of events using RF pulses set at a different centre frequency, so as to invert, excite and then readout other slices.

Referring back to Figure 2, a short TI is chosen, in this case e.g. 400 to 600 ms. At this time the magnetization in the z direction Mz is still negative for most tissue types, and in particular for the four tissue types of Figure 2. This means that in image reconstruction the phase of the magnetization can be ignored and the magnitude only measured and used. Although the phase is not measured, it is assumed to be negative for all pixels, and the magnitude values for each pixel are therefore multiplied by (-1) in the image reconstruction. This has the advantage that, at boundaries between regions of different tissue, there is no change of sign of the magnetization, which can otherwise lead to artifacts in the final image. Normally the choice of TI is determined in order to get optimal contrast between the different materials in the subject, but in this embodiment a short TI is chosen which provides improved image reconstruction as described above, specifically because it reduces the amount of movement of the subject between the inversion pulse and the image readout. In other embodiments, different image reconstruction methods can be used. For example phase corrected reconstruction can be used in which the final image retains, for each pixel, the sign of the longitudinal magnetization associated with that pixel at the time of excitation. Some embodiments of the invention allow a Tl weighted image of moving subjects, for example fetal images such as fetal brain images, to be obtained. Some embodiments can provide single-shot methods for Tl-weighted imaging using inversion recovery (IR) methods, which may be compatible with generating 3D reconstructions using snapshot imaging with volume reconstruction (SVR) as described, for example, in Jiang S, Xue H, Glover A, Rutherford M, Rueckert D, Hajnal JV. MRI of moving subjects using multi-slice Snapshot images with Volume Reconstruction (SVR): application to fetal, neonatal and adult brain studies. Medical Imaging, IEEE Transactions on 2007.

In some embodiments, the inversion pulse can be made spectrally selective, e.g. in the form of a spectro-spatial pulse. This allows the inversion to be targeted at specific materials, for example to select either water or fat.

As described above, in some embodiments an MRI scanner has a plurality of scanning protocols which can be selected to enable it to perform well in different applications. In this embodiment, the scanner has a fetal imaging protocol which, when selected, controls the MRI scanner to operate according to the method as described above.

In one example, 8 adult volunteers and 21 fetal patients (median 26/40, range 20.4/40 - 37.3/40) were scanned on a Philips 1.5T MRI system using a 5-channel cardiac phased-array coil using the method described above and compared to a standard Tl breath hold fast field echo (FFE) acquisition. SVR was applied to the Tl-weighted single-shot images.

Results: A TI of 400-600ms achieved Tl-weighted contrast while being robust to fetal motion between inversion and readout. Echo time was set to a minimum to avoid excessive T2- weighting.

Inversion and readout slices had equal thickness of 2.5mm. Using a resolution of 1.5mm x 1.5mm and SENSE factor of 2, fine anatomical detail of the full brain could be observed in about 2 mins. At longer TI, brain and cerebrospinal fluid signals can have opposite signs so real reconstruction is optimal, but less robust than magnitude reconstruction. For the selected TI of 400-600ms, all signals remain negative so magnitude reconstruction was used with inverted grey scale. Anatomical detail was well reconstructed into 3D by SVR.

Claims

1. A method of magnetic resonance imaging of a moving subject, comprising applying a magnetic field to a subject, performing a magnetization preparation step followed by a rapid readout process comprising an excitation step and a series of readout steps.

2. A method according to claim 1 wherein the readout steps are each arranged to generate an echo.

3. A method according to claim 1 or claim 2 wherein the readout process is carried out over a period short enough to freeze motion of the subject.

4. A method according to any forgoing claim wherein the magnetization preparation step results in Tl dependent contrast between different tissues or fluids.

5. A method according to any foregoing claim wherein the magnetization preparation step comprises applying an inversion pulse of magnetic field to the subject.

6. A method according to any foregoing claim wherein the readout step comprises single shot imaging step.

7. A method according to any foregoing claim wherein the readout step comprises a fast spin echo (FSE) sequence.

8. A method according to any foregoing claim wherein the readout steps comprise a series of re-focusing pulses.

9. A method according to claim 8 wherein each of the re-focusing pulses is followed by a data acquisition period during which field gradients are applied to the subject.

10. A method according to any foregoing claim wherein the readout steps are arranged such that data is acquired for a series of lines in k-space in an order which starts with a low absolute value of k and then moves outwards to larger absolute values of k so that the required region of k-space is covered.

11. A method according to any foregoing claim wherein the inversion time between the inversion pulse and the excitation pulse is less than Is.

12. A method according to any foregoing claim wherein a phase corrected reconstruction is used and the final image retains the sign of the longitudinal magnetization at the time of excitation.

13 A method according to any foregoing claim wherein magnitude-only image reconstruction is used.

14. A method according to claim 13 wherein the pixel values of the reconstructed image are multiplied by (-1).

15. A method according to any of claims 11 to 14 wherein the inversion time is selected such that the longitudinal magnetization is still negative for all or substantially all materials in the subject at the time of the excitation pulse.

16. A method according to any foregoing claim wherein the magnetization preparation is frequency selective.

17. A method according to claim 16 wherein magnetization preparation is arranged to be selective of one substance over another.

18. A method according to claim 17 wherein one of the substances is fat or water.

19. An MRI scanner having a plurality of protocols stored in memory, wherein one of the protocols is arranged to control the scanner to operate according to the method of any foregoing claim.

20. A method of scanning a moving subject comprising a method of any of claims 1 to 18.

21. A method according to claim 20 wherein the subject is a fetus.

22. A method according to claim 21 wherein the subject is a fetal brain.