GB2462416A

GB2462416A - Pre-polarising MRI Magnet

Info

Publication number: GB2462416A
Application number: GB0814201A
Authority: GB
Inventors: Marcel Jan Marie Kruip; Ian Nicholson
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-08-04
Filing date: 2008-08-04
Publication date: 2010-02-10
Anticipated expiration: 2028-08-04
Also published as: GB0814201D0; GB2462416B

Abstract

An arrangement of magnet coils for magnetic resonance imaging, comprising a first coil arrangemet (1a, 1b), which in use generates a first magnetic field in an imaging region (10), and a second coil arrangement (2, 2a), which in use generates a second magnetic field in the imaging region, perpendicular to the first magnetic field. The second field has higher strength and lower homogeneity than the first field, and is used as a pre- polarising field, which is switched off before imaging. The second coils may be superconductive and the first coils resistive.

Description

PRE-POLARISING MRI MAGNET WITH IMPROVED OPENNESS

The present invention provides equipment for MRI (magnetic resonance imaging) or NMR (nuclear magnetic resonance) imaging, using the prepolarising method.

The prepolarising method is known in itself, and allows MRI or NMR imaging with much lower background imaging field strength than is otherwise possible. A very strong magnetic field is required to align the proton spins in the target to be imaged. A very homogeneous magnetic field is required to enable effective imaging. Conventionally, a high-strength homogeneous magnetic field is used, which requires very large and expensive equipment.

The prepolarising method, descrthed in United States patent 4,906,931, uses a homogeneous field of relatively low strength for imaging, and a strong magnetic field of relatively low homogeneity for prepolarisation. Prior to imaging, a high-strength prepolarising field is applied to the target, in addition to the lower-strength, high-homogeneity imaging field. The protons in the target align to the combined imaging and prepolarising field. Just before imaging, the prepolarising field is removed, and the aligned protons move to align to the imaging field.

Imaging can then take place, while the protons are still aligned. The time during which such imaging is still possible is limited by the time which the protons take to diverge from their aligned orientations. This is characterised by a time constant commonly referred to as Ti, a characteristic of the magnetic field. Ti is also referred to as the relaxation time constant for axial magnetisation, and represents the time constant for

aligning protons in a magnetic field.

It is known to apply the prepolarising field in the same direction as the imaging field; or perpendicular to it. An advantage in applying the prepolarising field perpendicular to the imaging field is that any residual magnetic field remaining in the prepolarising field does not interfere with the homogeneity of the imaging field to the extent that it would if both

fields were oriented in the same direction.

European patent EP0895093 and United States patent U55,296,811 each descrthe apparatus for performing MRI imaging using a prepolarising method. Each of these documents describes an arrangement in which a magnet, either a permanent magnet or a magnet consisting of a block of superconducting material with electrical currents induced in it, is placed close to an imaging target, in each case a person's head, and once the target has been magnetised (prepolarised), the magnet is rapidly physically drawn away, leaving residual magnetism in the target sufficient to carry out an imaging step. In each case, the prepolarising field is

perpendicular to the imaging field.

In US5,296,811, a resistive electromagnet or permanent magnet provides a background field in the vertical direction. A gradient coil arrangement B provides a gradient field, in the horizontal direction, perpendicular to the background field. A prepolarising magnet D is provided. This is a relatively strong permanent magnet which is moveable between two locations. In a first, rest, location, the prepolarising magnet is located away from the imaging region P. In a second, active, location, the magnet is in close proximity to the imaging region. In operation, the prepolarising magnet is brought to its active location to perform prepolarisation on the object to be imaged. Just before a measurement is taken, the prepolarising magnet is rapidly moved away from the active location to the rest location. This may be achieved by various magnetic, mechanical or pneumatic arrangements.

EP0895093 is rather similar, in that a homogeneous vertical background field is produced by a magnet 1, while a relatively strong, relatively inhomogeneous horizontal prepolarising field is provided by a magnet 8 which is moveable between two locations. In a first, rest, location, the prepolarising magnet is located away from the imaging region. In a second, active, location, the magnet is in close proximity to the imaging region. In operation, the prepolarising magnet is brought to its active location to perform prepolarisation on the object to be imaged.

Just before a measurement is taken, the prepolarising magnet is rapidly moved away from the active location to the rest location. This may be achieved by various magnetic or mechanical arrangements. EP0895093 differs from USS,296,811 in that the system contemplated in EP0895093 is a whole-body MRI system, whereas the system contemplated by USS,296,811 is a small MRI system suitable for imaging only a part of the body. More importantly, the system of EP0895 093 employs blocks of high temperature superconductor as the magnets. These blocks must themselves be magnetised by passing them through a strong magnetic field, and must be maintained at a temperature below the critical temperature for the material.

The prior art arrangements of EP0895093 and USS,296,811 provide at least the following disadvantages. Eddy currents are induced by rapidly moving the prepolarising magnets, which may interfere with imaging sequences. Housings need to be placed very close to the target body, to house the prepolarising magnet and its range of motion, with noise and vibration of that housing being generated by the rapid motion of the prepolarising magnet, possibly to the alarm and discomfort of the patient.

A relatively complex arrangement results, particularly in the case of EP0895093, where the prepolarising magnet must be magnetised before use.

The present invention proposes the use of one or more coils for generation of the prepolarising field. The coils are permanently located near the imaging region, and do not move in operation. When it is necessary to cease application of the prepolarising field, current flowing in the prepolarising coils is turned off very rapidly. The coil(s) may be of high temperature superconductor.

The above, and further, objects, advantages and characteristics of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Figs. 1 and la respectively show an arrangement of coils in an embodiment of the present invention, in perspective and cross-sectional views; Fig. 2 shows a representation of electrical current flowing in the prepolarising coils during an imaging cycle; Fig. 3 shows a representation of magnetic field strength in the imaging region during an imaging cycle; and Fig. 4 shows a representation of magnetic field strength in the target during an imaging cycle.

As shown in Figs. 1, 1A, according to an embodiment of the invention, a pair of parallel coils la, ib, generate a homogenous imaging magnetic field E of relatively low strength in an imaging region 10. The coils are shown as essentially rectangular coils, but may be of any suitable shape. The rectangular coils illustrated may be preferred as they produce imaging magnetic field E in the imaging region 10 in a plane XZ, in a direction X which is transverse to the axis Z of the patient 12 undergoing imaging, and allow easier access to the patient for interventional procedures, and patient comfort, than more conventional solenoidal, or tunnel' magnets.

Preferably, the coils la, lb are preferably mounted on a former (not shown) of electrically non-conductive material, such as glass fibre reinforced plastic. Coils la and lb are symmetrically arranged with respect to a central plane XZ, which in this example is horizontal. The coils 1 a and lb generate imaging magnetic field E of high homogeneity, of the order of a few parts per million (ppm), sufficient for imaging. The imaging magnetic field produced by coils la and lb need not be very strong, just sufficient for imaging. Because there is no need for an imaging field of high field strength, the coils la, lb producing the imaging magnetic field can be significantly smaller than coils used to generate a magnetic field of same homogeneity but greater field strength. There is also no need to an active or passive magnetic field shielding system, further reducing the size and cost of the imaging magnetic field generating arrangement.

Furthermore, as only a low-strength imaging field id required, it may be possible to use inexpensive resistive coils, which are cheap to build and do not require the expense of cryogenic cooling associated with superconducting coils. The imaging magnetic field produced by coils la and lb will be parallel with the central plane XZ and, in the illustrated embodiment, perpendicular to an axis Z of patient 12.

Such an arrangement may also be preferred over the more conventional arrangements of solenoidal, or tunnel, magnets because a good quality field can be generated with a relatively narrow width of patient bed 14, enabling easy access to the patient.

According to an aspect of the invention, and as shown in Figs. 1 and 1A, a prepolarising coil 2 is placed close to the imaging region 10. A single coil may be used, or a pair of prepolarising coils 2, 2a may be used, preferably symmetrically disposed about the central plane XZ. The axis of the prepolarising coil(s) 2, 2a is perpendicular to the mid-plane XZ of the coils la, lb. The prepolarising coil(s) produce(s) a prepolarising magnetic field perpendicular to the imaging field Ejm* The prepolarising magnetic field is significantly stronger than the imaging field, but less homogeneous. Because the prepolarising field does not need to be very homogeneous, the prepolarising coil(s) used to produce the prepolarising field can be significantly smaller than a coil which generates a homogeneous field of the same field strength. The prepolarising field is applied in a pulsed fashion, with each "on" period lasting for at least a significant fraction of Ti, the relaxation time constant for axial magnetisation. While the prepolarisation field is on, a population of polarised protons is created, with the magnetisation direction being the direction of the combined prepolarisation and imaging fields.

As clearly illustrated in Fig. 1A, prepolarising coil 2 may be concealed below the patient table 14, so that the patient is unaware of the magnetic coil placed so close to their body. In other embodiments, the prepolarising coil 2 may be embedded within the patient bed. In embodiments using a pair of prepolarising coils 2, 2a, the upper coil 2a is more difficult to conceal from the patient.

Fig. 2 schematically illustrates the current I flowing in the prepolarising coil(s) over a cycle of prepolarisation and imaging. In this example, the cycle lasts 1 second, with about the first 500ms representing application of a current into the prepolarising coil(s) 2, 2a generating the prepolarising field Typically, the duration of application of the current in the prepolarising coils represents the relaxation time constant Ti. The current in the prepolarising coil(s) is then rapidly turned off. In alternative arrangements, the cycle may last, for example, for a time between 2Ti and liTi, with the allowed imaging period -the time that the prepolarising current is off -lasting from Ti to 1 OTi.

Fig. 3 shows the magnetic field strength Hin the imaging region 10 over a cycle of prepolarisation and imaging. About the first 500ms represents application of the prepolarising field and the imaging field Eim combined. Once the current I through the prepolarising field is turned off, the magnetic field strength H in the imaging region falls to the imaging field Ejm alone, and imaging is performed in the remaining SOOms.

Fig. 4 shows the magnetic field strength Ht in target region for imaging over a cycle of prepolarisation and imaging. In the first SOOms, application of the prepolarising field causes a gradual increase in the magnetisation of the target. Towards the end of the SOOms prepolarisation period, the magnetisation of the target nears completion, as a large proportion of the protons within the target align with the applied prepolarising field and imaging field Ejm combined. Once the prepolarising field is turned off, the magnetic field strength in the target begins to decay, as the protons begin to lose their alignment. However, the level of magnetisation remains sufficiently high to allow imaging for a significant period after the prepolarising field is turned off.

In order to obtain good imaging, it is important that no significant residual field from the prepolarising coils 2, 2a remains during imaging.

In a preferred embodiment, the prepolarising coils are isolated during imaging by an electronic switch, for example an IGBT (insulated-gate bipolar transistor) to achieve raid turn-off of the prepolarising coils.

Furthermore, eddy currents are preferably avoided by using electrically non-conductive materials for coil supports.

In order to generate the required strong magnetic fields, the prepolarising coil(s) 2, 2a are preferably superconducting coils, cooled within a cryostat provided for the purpose (not shown in the drawings). A so-called high-temperature superconductor is preferably used, and may be cooled in a cryostat using liquid neon, for example. The cryostat is preferably of an electrically non-conductive material to avoid the formation of eddy currents in the material of the cryostat. Such superconducting prepolarising coil(s) will be smaller than either a resistive coil or a low temperature superconductor cooled to helium temperature.

Potentially inconvenient eddy currents are further reduced by the relative orientation of the imaging field and the prep olarising field The pulsed magnetic field of the prepolarising coil(s) will induce eddy currents in conducting elements which are within the prepolarising field.

When the prepolarising field is switched off, eddy current are induced in these conducting elements, to generate a magnetic field predominantly in the same direction as the magnetic field which caused the eddy currents.

Any residual field generated by such eddy currents will, then, be

perpendicular to the imaging field.

The imaging process is sensitive to the absolute value of the magnetic field at any given position. Any disturbance due to a residual field will have much less impact on the imaging if it is perpendicular to the imaging field than if it is in the same direction as the imaging field. For example, assume that the imaging field has a relative strength of 1, and a perturbing field has a relative strength of 100x1ft6. If the perturbing field were in the same direction as the imaging field, the effect would be to increase the magnetic field strength to 1+100x106, a disturbance of 1 O0ppm. On the other hand, if the perturbing field is perpendicular to the imaging field, the resultant field will be sqrt(12 + (100x106)2), a disturbance of 5x109, or 0.OOSppm. This is well within the tolerable disturbance for an uncontrolled time dependent field for a typical MRI system.

The present invention accordingly provides at least some of the following advantages over the prior art. Patients will be more relaxed, and the magnet structure will be more open allowing improved access to the patient since the prepolarising coil may be hidden in the patient bed and no noise or mechanical vibration will be caused by moving prepolarisation magnets. The interference of eddy currents with imaging is reduced since no movement of magnets takes place, and the use of conductive materials is avoided in the structure of the apparatus. The structure is simple. -10-

Operation of the prepolarising magnet is controlled by the MRI control system, and is simply an electrical switching function.

Claims

-11 -CLAIMS1. An arrangement of magnet coils for magnetic resonance imaging, comprising a first coil arrangement (la, ib), which in use generates a first magnetic field (Birn) in an imaging region (10), and a second coil arrangement (2, 2a), which in use generates a second magnetic field (E) in the imaging region, perpendicular to the first magnetic field; the first magnetic field having a first field strength and a first homogeneity within the imaging region (10) and the second magnetic field having a second field strength and a second homogeneity within the imaging region (10), the second field strength being greater than the first field strength and the second homogeneity being less than the first homogeneity.
2. An arrangement according to claim 1, further comprising a timing arrangement arranged to supply electrical current to the second coil arrangement for a certain period of time, then to cease the current flowing in the second coil arrangement, while current continues to flow in the first coil arrangement.
3. An arrangement according to claim 2, comprising an electronic switch for switching the electrical current through the second coil arrangement.
4. An arrangement according to any preceding claim wherein the first coil arrangement comprises a pair of coils (la, ib) located on opposite sides of the imaging region.

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5. An arrangement according to claim 4 wherein each coil of the first coil arrangement is essentially rectangular.
6. An arrangement according to claim 4 wherein each coil of the first coil arrangement is resistive.
7. An arrangement according to any preceding claim wherein the second coil arrangement comprises a pair of coils (2, 2a) located on opposite sides of the imaging region.
8. An arrangement according to any preceding claim wherein the second coil arrangement comprises at least one superconducting coil (2, 2a).
9. An arrangement according to claim 8 wherein the superconductive coil is housed within a cryostat substantially formed of electrically non-conductive material.
10. An arrangement according to any preceding claims wherein the coils of the first and second coil arrangements are provided with supports substantially formed of electrically non-conductive material.
11. A magnetic resonance imaging system comprising an arrangement according to claim 2, further comprising imaging means arranged to perform imaging within a predetermined period of time following the cessation of current within the second coil arrangement.

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12. A magnetic resonance imaging system according to claim 11, further comprising a patient bed 14, the second coil arrangement being at least partially concealed below the patient bed.
13. A method for performing magnetic resonance imaging, comprising the steps of: -applying a first magnetic field of relatively low strength and relatively high homogeneity within an imaging region (10); -applying a second magnetic field of relatively high strength and relatively low homogeneity, perpendicular to the first magnetic field, within the imaging region for such duration (Ti) as to cause substantial proton alignment of a target within the imaging region with thecombined first and second magnetic fields;-ceasing application of the second magnetic field while maintainingthe first magnetic field; and-performing magnetic resonance imaging of the target within the imaging region within a predetermined time following the cessation of thesecond magnetic field,characterised in that the second magnetic field is generated by at least one magnetic coil placed in proximity to the imaging region.AMENDED CLAIMS HAVE BEEN FILED AS FOLLOWSCLAIMS1. An arrangement of magnet coils for magnetic resonance imaging, comprising a first coil arrangement (la, ib), which in use generates a first magnetic field (jm) in an imaging region (10), and a second coil arrangement (2, 2a), which in use generates a second magnetic field (E,) in the imaging region, perpendicular to the first magnetic field; the first magnetic field having a first field strength and a first homogeneity within the imaging region (10) and the second magnetic field having a second field strength and a second homogeneity within the imaging region (10), the second field strength being greater than the first field strength and the second homogeneity being less than the first homogeneity, Q wherein the first coil arrangement comprises a pair of essentially rectangular coils (la, ib) located on opposite sides of the imaging region, Q operable to produce the first magnetic field in the imaging region (10) in (Y) a direction (X) which is transverse to an axis (Z) of a patient bed (14).2. An arrangement according to claim 1, further comprising a timing arrangement arranged to supply electrical current to the second coil arrangement for a certain period of time, then to cease the current flowing in the second coil arrangement, while current continues to flow in the first coil arrangement.3. An arrangement according to claim 2, comprising an electronic switch for switching the electrical current through the second coil arrangement.4. An arrangement according to any preceding claim wherein each coil of the first coil arrangement is resistive.5. An arrangement according to any preceding claim wherein the second coil arrangement comprises a pair of coils (2, 2a) located on opposite sides of the imaging region.6. An arrangement according to any preceding claim wherein the second coil arrangement comprises at least one superconducting coil (2, 2a).7. An arrangement according to claim 6 wherein the superconductive coil is housed within a cryostat substantially formed of 0") electrically non-conductive material.8. An arrangement according to any preceding claims wherein the coils of the first and second coil arrangements are provided with C) supports substantially formed of electrically non-conductive material.9. A magnetic resonance imaging system comprising an arrangement according to claim 2, further comprising imaging means arranged to perform imaging within a predetermined period of time following the cessation of current within the second coil arrangement.10. A magnetic resonance imaging system according to claim 9, further comprising a patient bed (14), the second coil arrangement being at least partially concealed below the patient bed.