WO2014013257A1

WO2014013257A1 - Field-offset coil for use with an open mri scanner

Info

Publication number: WO2014013257A1
Application number: PCT/GB2013/051921
Authority: WO
Inventors: David John Lurie; Gareth Reynold DAVIES; Kerrin James PINE
Original assignee: University Court Of The University Of Aberdeen
Priority date: 2012-07-19
Filing date: 2013-07-18
Publication date: 2014-01-23
Also published as: GB201212813D0

Abstract

Apparatus for use with an open magnetic resonance imaging (MRI) scanner (1). The MRI scanner (1) generates a primary magnetic field (19) within its scanning region. The apparatus comprises an insert unit (9) comprising a magnetic coil (12) for generating a secondary magnetic field (20) and a support (3-6) for supporting the insert unit (9) in an elevated position within the scanning region of the MRI scanner (1). In use, the secondary magnetic field (20) alters the primary magnetic field (19) for generating an altered magnetic field region (21).

Description

FIELD-OFFSET COIL FOR USE WITH AN OPEN MRI SCANNER

[001] The present invention concerns apparatus for use with a magnetic resonance imaging (MRI) scanner and in particular, a device for enabling fast field-cycling (FFC) in an open MRI scanner .

[002] MRI scanners have become widely used in the field of medical imaging. During a scan, a strong magnetic field is created by the magnetic coil in an MRI scanner which causes hydrogen atoms in the water molecules of a patient's body to align with the field. An RF system is then used to apply a burst of weak radio waves, which results in some of the energy being absorbed, causing the hydrogen atoms to alter their alignment. After a time delay, called the spin-lattice, or Tl, relaxation time, the hydrogen atoms revert to their previous orientation and emit a nuclear magnetic resonance (NMR) signal, which is picked up by the RF receiver part of the RF system. The Tl relaxation time is very sensitive to the type of tissue being scanned. For example, the Tl of a kidney is different to the Tl of a leg muscle. Further, the Tl relaxation time also differs if the tissue is diseased. For example, a brain tumour has a longer Tl than normal brain tissue. Therefore, by repeating the testing process hundreds of times over the course of a few minutes, the NMR signals can be analysed by a computer to produce a cross sectional image of the patient, with variations in Tl providing contrast between tissues. [003] Commercially available MRI scanners come in two main types, categorised by the geometry of the magnet that they employ. The first and currently most common type is a cylindrical MRI scanner in which a patient is positioned inside the bore of a cylindrical high-field superconducting magnet. The cylindrical magnet is used to generate a fixed horizontal magnetic field, typically of 1.5 or 3 Tesla (T) . The second type of MRI scanner is an open MRI scanner, which has an open scanning region, rather than a closed bore. A common type of open MRI scanner employs a vertical magnetic field using a low to a medium field permanent or superconducting magnet. Such open MRI scanners typically operate at a fixed field of 0.2-0.4T, although some units are available which operate at 1.2T. [004] In many instances, open MRI scanners are preferred because their open geometry helps to reduce claustrophobia in patients, enables orthopaedic imaging in natural positions, and allows larger patients to be scanned. [005] As the use of MRI scanners has become more widespread, clinicians remain keen to explore additional ways in which the NMR response can be used to provide additional information concerning the tissues being analysed. [006] In this connection, recently some experimental studies have looked into the variation of Tl relaxation time at different magnetic field strengths. For example, it has been found that the Tl of liver tissue is longer when measured using a 1.5T magnet than it is when using 1.0T magnet, but that this variation is different in different tissues, and is also different in diseased tissues.

[007] For example, most body tissues give rise to a steady increase of Tl with increasing magnetic field strength. However, studies indicate that tissues containing immobile protein molecules exhibit pronounced reductions in Tl at three distinct field strengths (16 mT, 49 mT and 65 mT) . These are the magnetic field strengths at which the nuclear magnetic resonance (NMR) frequency is equal to the nuclear quadrupole resonance (NQR) frequency of nitrogen-14. These reductions in Tl are called "quadrupole dips" and they arise due to the presence of multiple hydrogen-nitrogen chemical bonds on a protein molecule's "backbone". This is important in bio-medical and clinical MRI because of the ubiquity of proteins throughout the body and since many diseases involve faulty proteins. For example, Parkinson's disease, Alzheimer's disease, multiple sclerosis and many others.

[008] Accordingly, by cycling the magnetic field to vary the field strength, the manner in which Tl changes as a function of the strength of a magnetic field, so-called "Tl dispersion", can be analysed. This technique has been dubbed Fast Field-Cycling MRI (FFC) . [009] Whilst there is increasing interest in using FFC and Tl dispersion to provide information to assist clinicians, at present, commercially available hospital MRI scanners cannot be used to derive this data. This is because they are designed to operate at a single fixed magnetic field strength and therefore their magnetic fields cannot be cycled or varied. Consequently, at present, the use of FFC has been limited to experimental set ups measuring Tl dispersion in very small samples (e.g. 1ml) . [0010] The present invention therefore seeks to address the above problems with the prior art.

[0011] According to an aspect of the present invention there is provided apparatus for use with an open magnetic resonance imaging (MRI) scanner, the MRI scanner for generating a primary magnetic field within its scanning region, the apparatus comprising: an insert unit comprising a magnetic coil for generating a secondary magnetic field; and a support for supporting the insert unit in an elevated position within the scanning region of the MRI scanner so that, in use, the secondary magnetic field alters the primary magnetic field for generating an altered magnetic field region.

[0012] In this way, the apparatus can be used in combination with an existing open MRI scanner to allow for fast field- cycling. That is, the present invention allows the insert to be positioned above a patient within the scanning region of a conventional open MRI scanner and, by controlling the power applied to the magnetic coil, the magnetic field strength within the altered magnetic field can be selectively varied. As such, the NMR measurements recorded by the RF coil within the altered magnetic field region can be taken under different field strengths. Consequently, by cycling the applied magnetic field, Tl dispersion can be analysed, for example. This analysis could not otherwise occur in a conventional MRI scanner because such a scanner is only able to provide a fixed magnetic field. Furthermore, the MRI scanner can revert back to its normal operation simply by retracting the insert from the scanning region.

[0013] Preferably, the magnetic coil is configured so that, in use, the altered magnetic field region is projected from a surface of the insert unit. In this way, the altered magnetic field region can project into the interior of a patient for performing analysis on internal tissues.

[0014] Preferably, the magnetic coil is configured so that the secondary magnetic field comprises a homogeneous region projected from a surface of the insert unit, in use the homogeneous region defining the altered magnetic field region. In this way, accurate Tl dispersion data can be derived .

[0015] Preferably, the secondary magnetic field has an opposing polarity to the primary magnetic field. In this way, the magnetic field is reduced within the altered magnetic field region, allowing Tl to be tested at a reduced field strength . [0016] Preferably, the support comprises one or more arms. In this way, the insert can be easily located in a suspended position within the MRI scanner's scanning region.

[0017] Preferably, the support further comprises a base, the one or more arms connecting between the base and insert unit.

[0018] Preferably, the base further comprises counter balancing means for countering forces applied to the insert unit during a scan. In this way, the counter balance acts to prevent the insert from moving during testing as a result of both its weight and the forces generated by the magnetic fields within the scanner.

[0019] Preferably, the balancing means comprises a counterweight.

[0020] Preferably, the support further comprises articulation means for moving the insert unit within the scanning region. In this way, the position of the volume/region being tested can be easily modified by adjusting the support.

[0021] Preferably, the articulation means is configured to facilitate movement of the insert unit laterally and/or vertically. In this way, scanning can be performed across and/or through a section of the patient's body.

[0022] Preferably, the apparatus further comprises a cooling system integrated into the insert unit for cooling the magnetic coil. This prevents the magnetic coil from overheating during testing, which could otherwise compromise the patient's comfort and/or the operation of components within the insert or scanner. [0023] Preferably, the support further comprises a coolant feed for feeding cooling fluid to the cooling system. In this way, the apparatus can be connected to an external coolant pumping system. [0024] Preferably, the support further comprises an electrical feed for feeding electricity to the insert unit. In this way, power and/or control signals can be easily fed into the apparatus from an external source. [0025] Preferably, the insert unit further comprises an outer casing, the outer casing being formed of an insulating material. In this way, the risk of injury to the patient by contacting the magnetic coil during testing is avoided. Furthermore, the casing allows the insert to be easily cleaned.

[0026] Preferably, the insert unit further comprises a radiofrequency (RF) system. In this way, the RF transmitter and receiver system is integrated into the apparatus, thereby avoiding the need to separately position these to effect the transmission and receipt of the NMR signals.

[0027] Preferably, RF system comprises one or more radiofrequency (RF) coils for transmitting and/or receiving electromagnetic signals.

[0028] Alternatively, the RF system comprises one or more sockets for receiving one or more radiofrequency (RF) coils for transmitting and/or receiving electromagnetic signals. In this way, different RF coils can be attached depending on the requirements of operation for allowing the measurements taken to be optimised.

[0029] Preferably, the one or more RF coils comprise a surface RF coil.

[0030] Preferably, the one or more RF coils are positioned in a plane parallel to the planar surface of the magnetic coil .

[0031] Preferably, the one or more RF coils are positioned so that, in use, they are located between the magnetic coil and a patient being scanned. In this way, detection of NMR signals from the altered magnetic field region is improved.

[0032] Preferably, the RF system comprises adjustment means for adjusting the position of one or more of the RF coils relative to the magnetic coil. In this way, the positioning of the one or more RF coils can be optimised depending on the operating parameters of the system to improve the delivery and receipt of NMR signals.

[0033] Preferably, the magnetic coil is a cylindrical pancake magnetic coil. In this way, a vertical secondary magnetic field can be generated having projected region with a high degree of homogeneity. At the same time, the pancake profile allows for a low profile insert to thereby provide sufficient space for the patient within the MRI scanner. [0034] Preferably, the apparatus further comprises a positioning system for indicating the location where the altered magnetic field region will be generated.

[0035] In an embodiment, the support comprises a mount for connection to the open MRI scanner. In this way, the MRI scanner itself acts as a counter weight base for supporting the insert.

[0036] Preferably, the apparatus further comprises a control means for adjusting the current applied to the magnetic coil for varying the distance by which the secondary magnetic field projects from the insert. In this way, by increasing the operating current, the secondary magnetic field can project further through the patient's body, allowing testing to be performed at deeper regions of the patient.

[0037] According to a further aspect of the present invention, there is provided an add-on unit for an open MRI scanner, the add-on unit comprising: a field offset magnetic coil for reducing at least a portion of the MRI scanner' s magnetic field when power is applied to the coil; a support for suspending the magnetic coil above a patent within the MRI scanner so that, in use, the MRI scanner's magnetic field strength within a region of the patent is reduced; and control means for cycling the power applied to the coil.

[0038] Illustrative embodiments of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows an FFC unit according to a first embodiment of the invention, being used in combination with an open MRI scanner;

Figure 2 shows a side cross-sectional schematic view of the FFC unit shown in Figure 1; and

Figure 3 shows a cross sectional view of the FFC unit in use.

[0039] Figure 1 shows an FFC add-on unit according to a first embodiment of the invention, together with a patient 8 and an open MRI scanner 1. The open MRI scanner 1 has a bottom section 7 which includes a patient platform and an upper section 2 which defines an opening 10 with the bottom section 7 into which a portion of the patient 8 is positioned during a scan. In use, the scanner 1 generates a vertical magnetic field in a scanning region between the bottom section 7 and the upper section 2, which passes through a portion of the patient 8 located in the opening 10. In one specific illustrative embodiment, an add-on unit is provided for use with permanent-magnet 59mT MRI scanner.

[0040] The FFC unit comprises an insert 9 supported by arms 6 which are connected via joints 5 to base 3. The joints 5 are configured to allow the insert 9 to be moved both up and down, and laterally into different positions within the scanner. The base 3 comprises a counterweight 4 which is used to balance the magnetic and gravitational forces applied to the insert 9 by the scanner 1. The base 3 is also provided with rollers on its bottom surface which allow the entire add-on unit to moved away from the scanner 1 when not in use, thereby withdrawing the insert 9 from opening 10.

[0041] Figure 2 shows a side cross-sectional view of the FFC unit shown in Figure 1. Within the housing of insert 9 is a circular pancake magnetic field offset coil 12 configured to generate, in use, a vertical magnetic field with a homogeneous region projected from the insert's lower surface so that it is formed beneath the insert 9. In one specific illustrative embodiment, the magnetic field offset coil 12 has a diameter of 38cm and a thickness of 6cm and generates a homogeneous region having a diameter of 5cm offset/projected from the bottom surface of the coil by 5cm.

[0042] An RF system 11 is provided on the lower surface of the insert 9 and, in this embodiment, comprises a surface RF coil transceiver. The RF system 11 is configured, through the RF coil, to transmit and receive NMR signals from a volume of the patient within the homogeneous region generated by the magnetic field offset coil 12. In particular, the RF coil 11 has a flat construction and is positioned between the magnetic coil 12 and the patient 8, with its RF magnetic field being substantially perpendicular to the main magnetic field generated by the scanner and parallel to the lower surface of the magnetic field offset coil 12. [0043] The base 3 of the FFC unit is further provided with keyed coolant and electrical connector ports 17 and 18 which connect the unit to coolant and power/control feeds, respectively. The coolant connector 17 feeds coolant through conduit 14 up through the base 3 and along the arms 6 to coolant ducts 13 provided within insert 9. The coolant ducts 13 are adjacent to the magnetic coil 12 and act to maintain the coil's operating temperature. Electrical connector port 18 feeds power and control signals to the magnetic field offset coil 12 and RF system 11 via connector 15 which pass up through the base 3 and along the arms 6 to insert 9.

[0044] The insert 9, arms 6, and base 3 are surrounded in an insulating casing for electrically insulating their components and allowing the unit to be wiped clean.

[0045] Figure 3 shows a cross sectional view of the FFC unit in use. As shown, a patient 8 is positioned in the scanning region of the open MRI scanner 1 defined between upper section 2 and bottom section 7. The insert 9 is also positioned in the opening 12 of the open MRI scanner 1 and above the patient 8.

[0046] The MRI scanner 1 is used to generate a primary magnetic field 19 over the scanning region which passes through the patient and has a generally homogeneous field strength. The magnetic field offset coil 12 within insert 9 is then powered to generate a secondary magnetic field 20 having an opposing polarity to the primary magnetic field 19, thereby acting to reduce the magnetic field strength within the scanning region. As the homogeneous region in this secondary magnetic field 20 is configured to be projected beneath the bottom surface of the insert 9, a region 21 having a substantially homogeneous magnetic field of a lower field strength than the primary magnetic field is generated within the patient 8.

[0047] Accordingly, within region 21, the scanner's magnetic field is altered/modified since it is partially cancelled or offset by the magnetic field generated by the insert 9's magnetic coil 12. Furthermore, by varying the power applied to magnetic coil 12, different cancelling/offset field strengths can be generated. As such, by turning the secondary magnetic field 20 on and off and/or varying its field strength, Tl can be measured at different magnetic field strengths. By repeating this process rapidly, fast field-cycling can be achieved to obtain Tl dispersion data for tissues within the altered magnetic field region 21.

[0048] The position of the altered magnetic field region 21, and consequently the volume being analysed, can be adjusted by moving the insert unit using arms 6 at joints 5.

[0049] Once testing has been completed, the insert 9 can be removed from opening 10 either by retracting the arms 6 or moving the entire FFC add-on unit away from the scanner. In this way, the scanner 1 can again be used for standard MRI imaging .

[0050] Accordingly, the present invention provides an apparatus which can be used with a standard, commercially available open MRI scanner to allow Tl dispersion data to be derived. As such, additional information regarding the tissues within the scanning region can be derived. [0051] It will be understood that the embodiment illustrated above shows applications of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

[0052] For example, the apparatus may further comprise a positioning system for assisting the operator in positioning the insert into an optimal position for measuring Tl dispersion in the patient 8.

[0053] Furthermore, whilst in the above embodiment, the apparatus is provided as a floor mounted unit, alternative configurations are also envisaged. For example, the apparatus may be supported by a mount which attaches to the MRI scanner itself. Such an arrangement may also be provided with one or more moveable arms which allow the insert to be moved in and out of the scanning region. These arrangements would allow the weight of the MRI scanner to function as a counter weight for balancing the forces applied to the insert unit during operation.

[0054] Furthermore, the RF system 11 may comprise a separate transmitter coil and/or a receiver coil. Furthermore, it may also allow the position of the RF coil to be adjusted relative to the magnetic coil 12 for optimising the operating depth (i.e. the position where the transmission and reception of RF emissions is optimal) . Furthermore, rather than including integrated RF coils, the system may comprise one or more sockets for allowing different RF coils to be attached depending on their intended function, such as intended operating depth.

[0055] Furthermore, the apparatus may further comprise a device for monitoring the temperature of the insert 9. This may be linked to a control system for controlling the coolant system.

[0056] Furthermore, the apparatus may further comprise means for altering the current applied to the magnetic coil 12, in order to vary the position of the homogenous region 21 of the secondary magnetic field 20. In this way, by increasing the operating current, the homogeneous region 21 can be moved downwardly so as to perform analysis on a deeper region of the patient 8.

Claims

1. Apparatus for use with an open magnetic resonance imaging (MRI) scanner, the MRI scanner for generating a primary magnetic field within its scanning region, the apparatus comprising:

an insert unit comprising a magnetic coil for generating a secondary magnetic field; and

a support for supporting the insert unit in an elevated position within the scanning region of the MRI scanner so that, in use, the secondary magnetic field alters the primary magnetic field for generating an altered magnetic field region.

2. Apparatus according to claim 1, wherein the magnetic coil is configured so that, in use, the altered magnetic field region is projected from a surface of the insert unit.

3. Apparatus according to claim 1 or 2, wherein the magnetic coil is configured so that the secondary magnetic field comprises a homogeneous region projected from a surface of the insert unit, in use the homogeneous region defining the altered magnetic field region.

4. Apparatus according to any preceding claim, wherein the secondary magnetic field has an opposing polarity to the primary magnetic field.

5. Apparatus according to any preceding claim, wherein the support comprises one or more arms.

6. Apparatus according to claim 5, wherein the support further comprises a base, the one or more arms connecting between the base and insert unit.

7. Apparatus according to claim 6, wherein the base further comprises counter balancing means for countering forces applied to the insert unit during a scan.

8. Apparatus according to claim 7, wherein the balancing means comprises a counterweight.

9. Apparatus according to any preceding claim, wherein the support further comprises articulation means for moving the insert unit within the scanning region.

10. Apparatus according to claim 9, wherein the articulation means is configured to facilitate movement of the insert unit laterally and/or vertically.

11. Apparatus according to any preceding claim, further comprising a cooling system integrated into the insert unit for cooling the magnetic coil.

12. Apparatus according to claim 11, wherein the support further comprises a coolant feed for feeding cooling fluid to the cooling system.

13. Apparatus according to any preceding claim, wherein the support further comprises an electrical feed for feeding electricity to the insert unit.

14. Apparatus according to any preceding claim, wherein the insert unit further comprises an outer casing, the outer casing being formed of an insulating material.

15. Apparatus according to any preceding claim, wherein the insert unit further comprises a radiofrequency (RF) system.

16. Apparatus according to claim 15, wherein RF system comprises one or more radiofrequency (RF) coils for transmitting and/or receiving electromagnetic signals.

5 17. Apparatus according to claim 15, wherein RF system comprises one or more sockets for receiving one or more radiofrequency (RF) coils for transmitting and/or receiving electromagnetic signals.

10 18. Apparatus according to claim 16 or 17, wherein the one or more RF coils comprise a surface RF coil.

19. Apparatus according to any one of claims 16 to 18 wherein the one or more RF coils are positioned in a plane

15 parallel to the planar surface of the magnetic coil.

20. Apparatus according to any one of claims 16 to 19, wherein the one or more RF coils are positioned so that, in use, they are located between the magnetic coil and a patient

20 being scanned.

21. Apparatus according to any one of claims 15 to 20, wherein the RF system comprises adjustment means for adjusting the position of one or more of the RF coils

25 relative to the magnetic coil.

22. Apparatus according to any preceding claim, wherein the magnetic coil is a cylindrical pancake magnetic coil.

30 23. Apparatus according to any preceding claim, further comprising a positioning system for indicating the location where the altered magnetic field region will be generated.

24. Apparatus according to any preceding claim, wherein

35 the support comprises a mount for connection to the open MRI scanner .

25. Apparatus according to any preceding claim, further comprising a control means for adjusting the current applied to the magnetic coil for varying the distance by which the

5 secondary magnetic field projects from the insert.

26. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

10 27. An add-on unit for an open MRI scanner, the add-on unit comprising:

a field offset magnetic coil for reducing at least a portion of the MRI scanner's magnetic field when power is applied to the coil;

15 a support for suspending the magnetic coil above a patent within the MRI scanner so that, in use, the MRI scanner' s magnetic field strength within a region of the patent is reduced; and

control means for cycling the power applied to the

20 coil.