WO2006010955A1

WO2006010955A1 - Nmr detection in the inhomogeneous fringe field of a magnet

Info

Publication number: WO2006010955A1
Application number: PCT/GB2005/003009
Authority: WO
Inventors: Jennifer Susan Gregory; Richard Malcolm Aspden; Hugh Charles Seton; Sanaa Faisal Rahmatallah; Yan Li
Original assignee: Aberdeen University
Priority date: 2004-07-30
Filing date: 2005-08-01
Publication date: 2006-02-02
Also published as: GB0703965D0; GB2431727B; GB2431727A; GB0417094D0

Abstract

A method of operating a magnetic resonance (“MR”) system to derive therefrom resolveable response signals from a plurality of locations distributed along a substantially lineal region of examination or test relative to a body or object. The method comprises the steps of (a) generating a radio frequency magnetic field (H1) in a direction substantially transverse to said lineal region; and (b) utilising a magnetic source (10) to generate, externally of the source, a non-homogeneous and static magnetic field (HO); the static magnetic field (i) being directed substantially parallel to said lineal region and extending therealong, thereby running substantially orthogonal to the field H1, and (ii) decreasing in amplitude with increasing distance from the magnetic source. The decrease in amplitude of said static magnetic field is selected to be of gradient sufficiently shallow whereby resolvable response signals can be derived from each of said plurality of said locations, substantially without adjustment of said radio frequency magnetic field.

Description

NMRDETECTION INTHEINHOMOGENEOUSFRINGEFIELDOFAMAGNET

[001] This invention relates to magnetic resonance detection utilizing magnetic resonance (^λλMR") methods. [002] Such methods have found broad application, particularly in the field of medical imaging, both as a supplement to, and as a replacement for, computed tomographic

("CT") systems, since MR systems provide images of similar definition to- those obtainable with CT but, advantageously, do not require the use of ionising radiation.

[003] Usually, for medical applications of MR, a high strength and highly homogeneous magnetic field (designated HO) is established in an examination zone, and a patient under examination is positioned so as to align an area of interest in the patient's body with the examination zone. Typically, a cylindrical magnet configuration (of circular cross section) is employed, with the examination zone existing within the cylinder; the diameter of the cylinder being sufficient to accommodate a patient. Means are provided to move the patient along the axis of the cylinder in order to effect the desired coincidence of the area of interest with the examination zone. [004] As is well known, the response signals for processing (which may be spin echo signals, but which can take other forms) are obtained by means of transceiver coils which both apply to and receive from the examination zone radio frequency electromagnetic fields (designated Hl) in a direction orthogonal to HO. The relationship between HO and Hl is controlled to select planar regions, within the body under examination, for which data are derived and processed to generate images of cross-sectional "slices" or laminae through the body; such images exhibiting a high degree of definition in both dimensions across the slice o'r lamina. [005] A drawback to the more general application of MR systems, therefore, is their substantial cost, since the requirements for images of high resolution dictates the use of dimensionally large and highly homogeneous magnetic fields (designated HO) at high magnetic field strengths, leading to 5 the need for massive, powerful magnets and associated cryogenic systems.

[006] Various attempts have been made to apply MR principles in areas which place lesser demands.upon the magnetic field requirements, by utilising techniques which are capable of 0 being implemented with greater economy, with the objective of opening up a range of applications for which MR technique's are

^■ not currently considered. For example, it would be useful to devise techniques which, whilst not capable of providing detailed images of planar slices of a body under examination, 5 could provide some indication of differing conditions at different depths within a body or object under examination or test. In a more general sense, such techniques could be used for differentiating or comparing the differing conditions of bodies, for example, differentiating between a healthy ankle 0 and one with damaged ligaments.

[007] In this respect, the most common proposals are currently based upon a construction which can be used to examine material disposed outside of its magnet system. One of these, for example, has been termed an NMR "MOUSE". 5 Typically, the magnets used in such applications have comprised U-shaped magnets. In some recent developments, however, solid bar magnets have been used to generate a magnetic field (HO) exhibiting a strong decay gradient (along the axis of the magnet) within an examination zone external 0 of the magnet. A body or article under examination is located within the examination zone and a radio frequency (RF) excitation field (Hl) is tuned to select specific levels or depths of interest in the article to be examined, see for example Macromol. Mater. Eng. 2003, 2003, 288, 312-317 Blϋmich et al. In order to examine regions at different depths of interest, the Hl field is generally re-tuned before the data is re-sampled. [008] An object of the present invention is to utilise MR principles in a method which is relatively inexpensive to implement and uses an external magnetic field to derive useful information from a plurality of locations at a range of different depths within a body or object under examination or test.

[009] According to one aspect of the present invention there is provided a method of operating a magnetic resonance ("MR") system to derive therefrom resolveable response signals from a plurality of locations distributed along a- substantially lineal region of examination or test relative to a body or object; the method comprising the steps of:

(a) generating a radio frequency magnetic field (Hl) in a direction substantially transverse to said lineal region; and ^■

(b) utilising a magnetic source to generate, externally of the source, a non-homogeneous and static magnetic field (HO) ; the static magnetic field

(i) being directed substantially ^• parallel to said lineal region and extending therealong, thereby running substantially orthogonal to the field Hl, and (ii) decreasing in amplitude with increasing distance from the magnetic source; wherein the decrease in amplitude of said static magnetic field is selected to be of gradient sufficiently shallow whereby resolvable response signals can be derived from each of said plurality of said locations, substantially without adjustment of said radio frequency magnetic field. [0010] In preferred embodiments, the gradient of the HO magnetic field over said lineal region is between 1 and 6 Tesla/metre, and at most 8 Tesla/metre.

[0011] It is further preferred that said lineal region extends over at least 2mm.

[0012] In one embodiment, said static field is generated by means of a permanent magnet and the lineal region is located a predetermined distance beyond a physical extremity of said magnet. Conveniently, the Hl field is generated by a coil arrangement disposed closely adjacent said extremity.

[0013] In preferred embodiments, the said lineal region is located at a point of inflexion in the axial magnetic field.

In this region, where the curvature of the H0_z field is zero, there is also no lateral curvature, which is advantageous for

1-D profiling.

[0014] A preferred coil arrangement comprises an electromagnetic coil arrangement configured in "figure-8" form and comprising first and second semi-elliptical multi-turn loops, disposed with their flat faces adjacent and connected in parallel.

[0015] The invention also encompasses such methods as aforesaid together with the further¹ steps of detecting the response signals and correlating and processing said response signals to provide information about a distribution of resonance data in the body or object along the direction of, or substantially parallel to, said lineal region. [0016] In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:-

[0017] Figures l(a) and 1 (b) show, in cross-sectional and plan views respectively, a hollow, circular cross section cylindrical magnet for generating an HO field for use in a method in accordance with one embodiment of the invention;

[0018] Figure 2 shows the HO field variation against axial position for the magnet of figure 1;

[0019] Figure 3 shows, in simplified plan view, a representation of a coil arrangement utilised in one example of the invention to generate the Hl field; [0020] Figures 4 (a) and 4 (b) show, in cross-sectional and plan views respectively, a hollow, conical magnet for generating the HO field for use in a method according to another embodiment of the invention; [0021] Figure 5 shows the HO field variation against axial position for the magnet of figure 4; and

[0022] Figure 6 shows certain basic components of a detection apparatus capable of utilising the methods of the invention.

[0023] Referring now to figure 1, there is shown a tubular magnet 10, in the form of a right cylinder, having a hollow bore and a height h. The magnet has an outer radius R₀ and an inner radius R₁ with an axis 12. The magnet 10 is magnetised radially (i.e. in a direction substantially transverse to the axis 12) with N along the axially facing surface. By way of example only, R₁ has a value of 28 mm, R₀ has a value of 50 mm, and h has a value of 80 mm.

[0024] Referring to figure 2, there is shown the magnetic field magnitude in milli Tesla taken along the axis 12 in mm. It can be seen that the peak field strength is external to the magnet, that is to say, it is beyond the end face or extremity of the magnet. Moreover, the decay of the field strength from the peak is relatively gradual or shallow compared with that normally associated with a solid bar magnet. The region just beyond the peak comprises the region of interest (ROI) 18 where a body to be examined is located. In practice, gradients of only 8 Tesla/metre or less; and preferably less than 6 Tesla/metre, are used. [0025] A magnetic device in the form of a radio frequency (RF) coil arrangement of any convenient design is used to generate the Hl field, in a direction transverse to the axis 12. Referring to figure 3, a "figure-of-eight" coil 16 is used to generate the Hl field and excite the spins as well as to collect the response signals. In this example, the coil arrangement 16 comprises two twelve-turn, semi-elliptical loops wound separately, disposed face-to-face and connected in parallel to form the "figure-of-eight". The coils are fed with equal currents of opposing polarity. Typically, the coils of the arrangement 16 are wound of wire of diameter 0.5 mm, have an inductance of 1.2 microHenry and a Q-factor of 79. [0026] In a static magnetic field, the radio frequency is directly proportional to the magnetic flux density HOo and is selected according to the following formula:-

where γ is the gyromagnetic ratio of the nucleus of interest. [0027] Figure 6 shows an example of a detection apparatus including the magnet of figure 1 and the coil of figure 3. In particular, the coil 16 is deposited on the underside of a glass sheet 42 which is suspended in any convenient manner adjacent an end surface 44 of the magnet 10. A first sample 46, for example rubber, is placed on top of the glass sheet 42. A further sheet of glass 48 covers the sample 46 to act as a spacer and support for a second sample 50 of rubber. Glass sheets are used for support and separation as they do not give rise to the generation of potentially confusing response signals. [0028] A circuit arrangement 52 of known kind is connected to the coil arrangement 16 to energise it to generate the Hl field and for receiving response signals. The received response signals are fed to a processor 54 of known kind, configured to process the response signals and to produce output signals indicative of the samples 46 and 50. The output signals so produced are applied to a monitoring and/or recording unit 56 of any convenient kind. [0029] With the arrangement described above, it has been found possible to distinguish between the responses of the two or more samples, which are spaced apart along a line of measurement coincident with (or parallel to) the magnet axis.

[0030] The method of the invention may be thus be configured in a so-called "1-D" mode to utilise the naturally shallow field gradient derivable, for example, from a hollow, tubular and radially magnetised magnet to provide comparative response signals at different depths of the body to be examined (i.e. along the line of axis 12, or parallel thereto) to effect spatial resolution within the region 14 (i.e. between locations separated along the line of axis 12) . This is effected without the need to re-tune the RF coil for each different measuring location along the line from which response signals are to be derived. Typically the lineal extent of the region 14 is at least 2mm.

[0031] Referring to the magnet 10, it will be appreciated /that regions of defined field strength and gradients can be created by selection of the material of which the magnet is constructed and the geometry of the magnet, typical parameters to be selected including the height, inner diameter and outer diameter. A typical material used for magnet construction in examples of the invention is neodymium iron boron (NdFeB) . [0032] The direction of magnetisation in this example was such as to configure the outer surface of the magnet as South and the inner surface as North, though the polarisation can be reversed. The magnet 10 may alternatively be magnetised axially (i.e. in a direction parallel with the axis 12) and in either direction. [0033] In embodiments of the invention such as that shown in figure 1, it is convenient to construct the magnet out of four sections, each comprising a 90 degree component of the magnet, glued together and housed within a tubular sleeve of

5 non-magnetic material.

[0034] Figures 4 (a) and 4 (b) show an example of an alternative magnetic element, in which a truncated conical magnet 20 is employed to generate a magnetic field distribution of the kind illustrated schematically in figure 0 5. The maximum value 24 of the magnetic field HO (also known as BO) measured along the axis 22 (Z-axis) in milli Tesla, its position along the axis 22 from the end of the magnet 20 in mm, and the field gradients can be varied by altering the dimensions of the magnet 20; i.e. the height (h) measured 5 along the axis 22, upper inner radius r, upper thickness w, base angles α and β and the angle of the magnetisation direction γ.

[0035] Specific values utilised in one specific embodiment were as follows:- 0- r = 20 mm; h = 35 mm; w = 30 mm; α=β = 75 deg. ; and Y = 0 deg. 5 [0036] The conical magnet may be magnetised at any uniform angle with respect to its base plane, from 0 degrees (radial) through 90 degrees (axial) and 180 degrees (anti-radial) to 360 degrees, (anti-axial). [0037] It will be apparent that the present invention is 0 capable of modification, the detailed embodiments of which

^• will be readily apparent to those skilled in the art. For example, alternative magnetic structures may be used to generate the HO field. In particular, the central bore of the magnet may be wholly or partially filled with a material chosen to influence a particular characteristic of the HO field and to extend the range of distances over which the decay gradient of the HO field is sufficiently shallow to achieve the objective of achieving depth resolution without the need for re-tuning the Hl field.

Claims

[0038] 1. A method of operating a magnetic resonance ("MR") system to derive therefrom resolveable response signals from a plurality of locations distributed along a substantially lineal region of examination or test relative to a body or object; the method comprising the steps of:

(a) generating a radio frequency magnetic field (Hl) in a direction substantially transverse to said lineal region; and (b) utilising a magnetic source to generate, externally of the source, a non-homogeneous and static magnetic field (HO) ; the static magnetic field

(i) being directed ^' substantially parallel to said lineal region and extending therealong, thereby running substantially orthogonal to the field Hl, and

(ii) decreasing in amplitude with increasing distance from the magnetic source; wherein the decrease in amplitude of said static magnetic field is selected to be of gradient sufficiently shallow whereby resolvable response signals can be derived from each of said plurality of said locations, substantially without adjustment of said radio frequency magnetic field. [0039] 2. A method according to claim 1 wherein the gradient of the HO magnetic field over said lineal region is at most 8 Tesla/metre.

[0040] 3. A method according to claim 1 or claim 2 wherein said lineal region extends over at least 2mm. [0041] 4. A method according to any preceding claim wherein said static field is generated by means of a permanent magnet and the lineal region is located a predetermined distance beyond a physical extremity of said magnet.

[0042] 5. A method according to claim 4 wherein the lineal region is located at the point of inflexion in the axial magnetic field

[0043] 6. A method according to claim 4 or 5 wherein the Hl field is generated by a coil arrangement disposed closely adjacent said extremity. [0044] 7. A method according to claim 6 wherein the coil arrangement comprises an electromagnetic coil arrangement configured in ^λλfigure-8" form.

[0045] 8. A method according to claim 7 wherein said coil configuration comprises first and second .semi-elliptical multi-turn loops, disposed with their flat faces adjacent and connected in parallel.

[0046] 9. A method according to any preceding claim further comprising the steps of detecting said response signals and correlating and processing said response signals to provide- information about a distribution of resonance data in the body or object along the direction of, or substantially parallel to, said lineal region.