CA2600990A1 - Probe, system and method suitable for unilateral nuclear magnetic resonance - Google Patents
Probe, system and method suitable for unilateral nuclear magnetic resonance Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/3808—Magnet assemblies for single-sided MR wherein the magnet assembly is located on one side of a subject only; Magnet assemblies for inside-out MR, e.g. for MR in a borehole or in a blood vessel, or magnet assemblies for fringe-field MR
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
- G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
- G01R33/34046—Volume type coils, e.g. bird-cage coils; Quadrature bird-cage coils; Circularly polarised coils
- G01R33/34053—Solenoid coils; Toroidal coils
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
- G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
- G01R33/341—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/383—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using permanent magnets
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
- G01N24/08—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using nuclear magnetic resonance
- G01N24/081—Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/445—MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/448—Relaxometry, i.e. quantification of relaxation times or spin density
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Abstract
A probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising; a static magnetic field generator; a radiofrequency magnetic field generator adjacent to the static magnetic field generator; a circuit controlling the frequency response of the radiofrequency magnetic field generator, adjacent to the static magnetic field generator; an input cable coupled to the frequency control circuit and the frequency control circuit coupled to the radiofrequency magnetic field generator.
Description
I'ItOB)s, SYSTEM AND METHOD SUITABLE FOR UNILATERAL NUCLEAR MAGNETIC RESONANCE
CROSS-REFERENCE TO RELATED APPLICATIONS
[00011 Not applicable.
MICROPICHb APPENDIX
CROSS-REFERENCE TO RELATED APPLICATIONS
[00011 Not applicable.
MICROPICHb APPENDIX
[0002] Not applicable.
'I't;CHN1CAL FIL="LD
100031 This application relates to nuclear magnetic resonancc techniques in general, and to a probe, system and method suitable for unilateral nuclear magnetic resonance, in particular.
BACKCfROUND OF THE INVENTIQN
[0004] Despite growing interest in magnetic resonance of porous materials such as soils and concrete, critical limitations exist in tenns of the types of nieasurements that can be made.
Botli spectroscopic and spatially resolved studies requiring superconducting magnets can only be carried out on samples of limited size. New advances in open, portable NMR instrumentation allow bulk relaxation and diffusion measurements to be made on arbitrarily large samples such as described in G. Eidmann, R. Savelsberg, P. Blilmler, B.
Blilmicit, J. Magn. Res. A 1996; 122:104-109.
However, experiments of this typa are limited by the penetratioA depth of Ho and B. This constraint has permitted higher field (10-20MHz) near surface studies (see Boguszynska, J. et al., Cem.
Concr, Res, 2005 35:2033-2040), along with lower field measurements at a greater, but still limited, depth.
100051 In many situations, it may be desirable to mcasure NMR parameters from deep within a sampl.e. Examples in the porous media regime could include larger concrete structures and soil 1'ormations. Prcvious work has used RF coils embedded within concrete samples in ordcr to aIlcviate the Bi penetration problem (sec Boguszynska, above). Extending this idea, a small, low cost NMR sensor suitable to be embcdded within a large sample has been developed. NdFeB disk magnets provide a local Bo field for a 1 H resonant frequency of between 6 -10 MHz depending on the design. A printed circuit board surface coil is located immediately above one face of the magnets, and tuned tp resonance with capacitors on the opposite face. The entire atrangement is connected to a 2.5 mm diametcr coaxial cable, and encased in epoxy.
The uihomogcneous BO and B I fields define a local sensitive spot in which bulk rclaxaLion time or diffusion measurements can be made.
[0006]
St7M1VfARY
100071 According to ono aspect of the present invention, there is provided a probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising; a magnet having a nonh and south pole; a radiofrequency coil adjacent one of the poles of the magnet; a capacitive tuning circuit adjaccnt the other pole of the magnct; a coaxial cable electrically connected to the tuning circuit and the tuning circuit electrically connected to the coil.
[0008] According to another aspect of the present invention, there is provided a probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising; a static magnetic field generator; a radiofrequency magnetic field generator adjacent to the static magnetic field generator;
a circuit controlling the frequency response of thc radiofrequency magnetic field generator, adjacent to the static magnetic field gcncrator; an input cable coupled to the frequency control circuit and the frequency control circuit coupled to the radiofrequency magnetic field generator.
100091 According to another aspect of the present invention, there is provided a method for magnetic resonance imaging of a sample comprising the step of embedding a probe in the sample to be imaged.
[00101 According to another aspect of the present invention, there is provided a system for use in unilatcrai nuclear magnetic resonance imaging comprising: a probe adapted to ba L-nbedded in a sample to be analysed, an RF supply module connected to probe suitable for generating an RF signal compatible with nuclear magnetic resonance.
100111 An RF coil suitable for nuclear magnetic resonance imaging wherein the coil in bowtie shaped. The coil may have multiple windings.
1001z1 Other aspects and features of the prese.nt invention will become apparent to those ordinarily skilled in rhe art upon review of the following description of spe=cific embodiments of a probe, system and method suitable for unilateral nuclear magnctic resonance in conjunction with the accompanying drawing figures.
BRtEF DESCRIPTION OF TNB DTtA WINCiS
[0013] Embodiments of the present invention will now be described, by way of example only, with refercnce to the accompanying drawing figures, wherein;
FIG. I is a scliatnatic diagram of an array of three diie magnets and an associated magnetic field of interest;
FIG. 2 is an image of the magnetic scalar potential for the array of magnets of FIG. 1;
FIG. 3 is a photograph of components of a probe according to the present invention;
FIG. 4 is photograph of an assembled probe according to the preseot invention;
FIG. 5 is a schematic diagram showing the interconnection of eomponents of the probe of f'ig. 4;
FIG. 6 is a plot of NMR measurements of the moisture eontcnt of a sand sample acquired aecording to the invention;
FTG. 71s a plot of signal amplitude as a funotion of time measured with an apparatus according to the invention embedded in curing cement;
FIG. 8 is a plot of apparnnt T2 measurod by Carr-Purcell-Meiboom-Gill (CPMG) method in a curing ASTM
Type I cement sample;
FIG. 9 is another embodiment of an apparatus according to the present invention;
FTG. 10 is an image of the magnetic scalar potential for the sensor of FTG. 9;
FIG. 11 is a block diagram of an embodiment of and NMR apparatus provided in accordance with the present invention;
FIG. 12 is a spiral coil;
FIG. 13 is a modified DD coil;
FIG. 14 is a"bowtie" coil compristtl of a single coil; and FIG. 15 is a "bowtie" coil comprised of multiple coils.
100141 Like reference numerals are used in diffcrent figures to denote similar elements.
DETAIT,$D DESCRIPTION OF THE DRAWINGS
10015] Referring to Figs. I and 2, the measured tnagnetic Geld magnitude over, a stack (atray) of three disk magnets is shown. About 3 mm above the magnets, the field is -2500 G (-10.5 MHz'H). The field is inhomogeneous, due to the single sided naturc of the device as well as inhomogencous magnetization in the low quality magnets.
[00161 Referring to Fig. 3, components of the probe (also commonly referred to as a sensor) are shown. From left to right the componca,ts of the top row are: two capacitors; two discs made of iron which serve as a yoke in the assembled sensor; two disc magnets made of NdFeB; and a radiofrequency (RF) coil milled on a printed circuit board. The RP coil is a modified "double-d" design. The printed circuit board in the assembled sensor sirs on top of the magnets to generate a field centred above the top of the magnet stack. The two Euro coin shown blow the row of components is to show scale and does not form part of the invention. The iron yoke which in the assembled sensor sits below the magnets, can be used to adjust the field strengtlt above the magnets. The RF coil is tuned to a frequency suitable for nuelear magnetic rosonance in a volume above the coil frequency with small, fixed value capacitors mounted to a PC board below the magnets, and fed through a thin coaxial cable. In some embodiments, ihe discs of magnets may be replaced by one magnet of a size comparable to the combined size of the two disc magnets. In some embodiments, more than two disc magnets may be used.
[0077] Refarring to Fig. 4, an assembled sensor according to the present invention is shown, The sensor includes the components shown in Fig. 3. The sensor is small relative to the dimensions of tha sample in which it is to be embedded (wet cement in this cxample) and is approximately 3 mm in diameter and 2 cm in thickness and includes the components shown in Fig. 2, '1'he sensor of Fig. 4 is encased in a water proof epoxy to protect the components in the wet cement environment.1'he nominal static field value above the sensor gives a Larmor frequency of 8.08 M)-[z'H.
[0018] lteferring to Fig, 5, the radiofrequency coil 2 is fixed atop the magnets 4. The coil is conneeted by wire leads (shown in dashed lines) to a capacitive tuning circuit 6 mounted on a circuit board and fixed to the bottom of the magnets. The tuned circuit is connected to a coaxial cable 8, which is attached to the RF supply inodule and signal detection module as described with rolcrence to Fig. 11.
[0019J Measurements were made using the sensor of Fig. 4, both of moisture content in sand, and signal amplitude /
relaxation times in curing concrete. In each case, a scnsor was immersed in sand / concrete and a Bruker Minispec spectrometer was used for RF supply, NMR signal detection and experimental control. These measurements are shown here as a proof of principle. In the case of tha moisture content measurements, the signal intensity correlates well with the mcasured moisture content. For the curing concrete, further study is required to correlate measurements with relevant material parameters.
10020] Referrimg to Fig. 6, NMR measurements of the moisture content of sand.
The sensor was immersed in wet sands with different weight percentages of water. A CPMG sequence was used to measure the moisture content, with 32 echoes, an echo time of 0.2 nis, and 512 scans for an acquisition tinne of 9 ntinutes per point. The echoes were co-added to obtain the signal. A linear relationship between the water content and 1v1R
signal intensity is observed.
[0021] Referring to Fig. 7, Signal amplitude as a function of time measured with a sensor embedded in curing ASTM
Type 1 cement, w/c ratio 0.45. The first two echoes in a CPMG sequence (TE =
0. 168 ms, 8192 scans) were averaged to give the signal, 100221 Referring to Fig. 8, apparent'f2 measurcd by CPMG (TE = 0.168 ms, 8192 scans) in a curing ASTM Type I
cemCnt sample. Because of the grossly inhoniogeneous field, the relaxation eonstant is a combination of T2 and diffusive attenuation.
J00231 Referring to p'ig. 9, an alternate magnet arrattgement producing a more homogeneous field a more homogeneous field than the embodhnent of p'ig. 1, at a frequency around 6 MHz.
A washer magnet is placed on top of a stack of disk magnets, giving a saddle point in the field. While the field is slightly lowcr in this arranl;ement, the saddle point ensures that signal can be obtained from a larger volume, increasing sensitivity. Furthermore, the gradient around the saddle point is low, mitigating diffusive attenuation.
100241 Altematively, it will be understood by those skilled in the art that other magnet designs may be employed depending upon the magnetic Pold dcsired. For example, a magnet with two north poles and two south poles of the type in illustrated in FIG. 15 could be used as part of a probe according to the invention.
f00251 FIG. I l is a block diagram of one embodiment of an NMR system provided in accordance with the present invention. The magnet (a) produces a suitable static magnetic field in a region of interest. The dynamic field generator (b) generates a dynamic radiofrequency magnetic field in the region of interest. '1'he RF supply module (c) is connected to the dynamic field generator and supplies an RF signal compatible with nuclear magnetic resonance to the dynamic field generator. The detection module (d) detects nuclear magnetic resonance signals induced in the dynamic field generator. The RF supply module is controlled with a computer (e), and the signals detected by the detection module are recorded and processed on the computer.
100261 Alternate pattcrns may be used for the printed circuit board for the probes according to the present invention and may have application for other NMR probes. Single-sided magt,etic resonance measurements require single-sided or surface coils for B, generation and signal detection. The fundamental constraint in coil design is that B, must be orthogonal to B. For the case where B, must be normal to the magnet surface, a circular surface coil is the clear choice.
J0027J In many cases, it is desirable to have Bo normal to the inagnet surface, and B, must therefore be generated parallel to the surface of the coil in order to meet the orthogonality condition necessary for magnetic resonance. This can be achieved in practice by a sheet of RF current, approximated by several parallel wires. However, in this arrangement, additional wire is necessary to form a closed loop of current. Along with increasing the resistance of the coil, this additional wire generates a spurious B, field. This field can be parallel to Bo, and therefore not useable for NMR
purposes, or outside the desired scnsitive volume of the coil. In the fvst oase, the field effectively reduces the filling factor and directly increases the measurement noise. In the second case, signal from uuwanted regions of a sample may be measured, affecting experimental results.
1002$1 In selecting a coil design for generating a field parallel to the surface of the coil, the goal is to generate the strongest possible field in a region of interest (ROI), while reducing spurious fields as much as possible.
10029] Fig. 12 depicts a"spiral" coil, Fig. 13 a "modified DD' eoil and Fig.
14. a "bowtie" coil.
100301 Coils using the spiral, modified DD and bowtie patterns were manufactured from wire epoxied to a thin substrate. 'Ihe coil pattems were first simulated using the Biot-Savart law, and the rransvarse field intensity Integrated over a 1 am deptli to give the plots shown. A 5 cm by 5 om by 1 cm thick rubber sample was then imaged at 8.3 MHz using the BPRiTB MRl technique using each coil.
10031] The spiral coil gives a relatively uniform field over a ring shaped region, and results in the highest signal intensity when tested. No signal, however, is derived from the centre of the sample. in many unilaterai applications, it is this region that is most importarat In terms of Bo characteristics, and the spiral coil may not be an appropriate choice.
[00321 The double-d coil features comparable signal 'uitensity to that of the spiral coil, but the image indicates that rnore of this signal originates from the center of the coil. 'I'here is a large amount of signal from the coil return paths, with an intensity of approximately half that of the center region.
[00331 '1'he bowtie coil has a lower total signal, however the 2D image indicates that this coil has superior spatial selectivity, and a higher sensitivity in the central region than either the spiral or double-d coils. It will be understood that the number of windings can be varied according to the desired magnetie field profile and/or coil performance.
[01001 The above-described embodiments af the present invention are intended to be examples only_ Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departina from the scope of the invention, which is set forth in the claims.
'I't;CHN1CAL FIL="LD
100031 This application relates to nuclear magnetic resonancc techniques in general, and to a probe, system and method suitable for unilateral nuclear magnetic resonance, in particular.
BACKCfROUND OF THE INVENTIQN
[0004] Despite growing interest in magnetic resonance of porous materials such as soils and concrete, critical limitations exist in tenns of the types of nieasurements that can be made.
Botli spectroscopic and spatially resolved studies requiring superconducting magnets can only be carried out on samples of limited size. New advances in open, portable NMR instrumentation allow bulk relaxation and diffusion measurements to be made on arbitrarily large samples such as described in G. Eidmann, R. Savelsberg, P. Blilmler, B.
Blilmicit, J. Magn. Res. A 1996; 122:104-109.
However, experiments of this typa are limited by the penetratioA depth of Ho and B. This constraint has permitted higher field (10-20MHz) near surface studies (see Boguszynska, J. et al., Cem.
Concr, Res, 2005 35:2033-2040), along with lower field measurements at a greater, but still limited, depth.
100051 In many situations, it may be desirable to mcasure NMR parameters from deep within a sampl.e. Examples in the porous media regime could include larger concrete structures and soil 1'ormations. Prcvious work has used RF coils embedded within concrete samples in ordcr to aIlcviate the Bi penetration problem (sec Boguszynska, above). Extending this idea, a small, low cost NMR sensor suitable to be embcdded within a large sample has been developed. NdFeB disk magnets provide a local Bo field for a 1 H resonant frequency of between 6 -10 MHz depending on the design. A printed circuit board surface coil is located immediately above one face of the magnets, and tuned tp resonance with capacitors on the opposite face. The entire atrangement is connected to a 2.5 mm diametcr coaxial cable, and encased in epoxy.
The uihomogcneous BO and B I fields define a local sensitive spot in which bulk rclaxaLion time or diffusion measurements can be made.
[0006]
St7M1VfARY
100071 According to ono aspect of the present invention, there is provided a probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising; a magnet having a nonh and south pole; a radiofrequency coil adjacent one of the poles of the magnet; a capacitive tuning circuit adjaccnt the other pole of the magnct; a coaxial cable electrically connected to the tuning circuit and the tuning circuit electrically connected to the coil.
[0008] According to another aspect of the present invention, there is provided a probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising; a static magnetic field generator; a radiofrequency magnetic field generator adjacent to the static magnetic field generator;
a circuit controlling the frequency response of thc radiofrequency magnetic field generator, adjacent to the static magnetic field gcncrator; an input cable coupled to the frequency control circuit and the frequency control circuit coupled to the radiofrequency magnetic field generator.
100091 According to another aspect of the present invention, there is provided a method for magnetic resonance imaging of a sample comprising the step of embedding a probe in the sample to be imaged.
[00101 According to another aspect of the present invention, there is provided a system for use in unilatcrai nuclear magnetic resonance imaging comprising: a probe adapted to ba L-nbedded in a sample to be analysed, an RF supply module connected to probe suitable for generating an RF signal compatible with nuclear magnetic resonance.
100111 An RF coil suitable for nuclear magnetic resonance imaging wherein the coil in bowtie shaped. The coil may have multiple windings.
1001z1 Other aspects and features of the prese.nt invention will become apparent to those ordinarily skilled in rhe art upon review of the following description of spe=cific embodiments of a probe, system and method suitable for unilateral nuclear magnctic resonance in conjunction with the accompanying drawing figures.
BRtEF DESCRIPTION OF TNB DTtA WINCiS
[0013] Embodiments of the present invention will now be described, by way of example only, with refercnce to the accompanying drawing figures, wherein;
FIG. I is a scliatnatic diagram of an array of three diie magnets and an associated magnetic field of interest;
FIG. 2 is an image of the magnetic scalar potential for the array of magnets of FIG. 1;
FIG. 3 is a photograph of components of a probe according to the present invention;
FIG. 4 is photograph of an assembled probe according to the preseot invention;
FIG. 5 is a schematic diagram showing the interconnection of eomponents of the probe of f'ig. 4;
FIG. 6 is a plot of NMR measurements of the moisture eontcnt of a sand sample acquired aecording to the invention;
FTG. 71s a plot of signal amplitude as a funotion of time measured with an apparatus according to the invention embedded in curing cement;
FIG. 8 is a plot of apparnnt T2 measurod by Carr-Purcell-Meiboom-Gill (CPMG) method in a curing ASTM
Type I cement sample;
FIG. 9 is another embodiment of an apparatus according to the present invention;
FTG. 10 is an image of the magnetic scalar potential for the sensor of FTG. 9;
FIG. 11 is a block diagram of an embodiment of and NMR apparatus provided in accordance with the present invention;
FIG. 12 is a spiral coil;
FIG. 13 is a modified DD coil;
FIG. 14 is a"bowtie" coil compristtl of a single coil; and FIG. 15 is a "bowtie" coil comprised of multiple coils.
100141 Like reference numerals are used in diffcrent figures to denote similar elements.
DETAIT,$D DESCRIPTION OF THE DRAWINGS
10015] Referring to Figs. I and 2, the measured tnagnetic Geld magnitude over, a stack (atray) of three disk magnets is shown. About 3 mm above the magnets, the field is -2500 G (-10.5 MHz'H). The field is inhomogeneous, due to the single sided naturc of the device as well as inhomogencous magnetization in the low quality magnets.
[00161 Referring to Fig. 3, components of the probe (also commonly referred to as a sensor) are shown. From left to right the componca,ts of the top row are: two capacitors; two discs made of iron which serve as a yoke in the assembled sensor; two disc magnets made of NdFeB; and a radiofrequency (RF) coil milled on a printed circuit board. The RP coil is a modified "double-d" design. The printed circuit board in the assembled sensor sirs on top of the magnets to generate a field centred above the top of the magnet stack. The two Euro coin shown blow the row of components is to show scale and does not form part of the invention. The iron yoke which in the assembled sensor sits below the magnets, can be used to adjust the field strengtlt above the magnets. The RF coil is tuned to a frequency suitable for nuelear magnetic rosonance in a volume above the coil frequency with small, fixed value capacitors mounted to a PC board below the magnets, and fed through a thin coaxial cable. In some embodiments, ihe discs of magnets may be replaced by one magnet of a size comparable to the combined size of the two disc magnets. In some embodiments, more than two disc magnets may be used.
[0077] Refarring to Fig. 4, an assembled sensor according to the present invention is shown, The sensor includes the components shown in Fig. 3. The sensor is small relative to the dimensions of tha sample in which it is to be embedded (wet cement in this cxample) and is approximately 3 mm in diameter and 2 cm in thickness and includes the components shown in Fig. 2, '1'he sensor of Fig. 4 is encased in a water proof epoxy to protect the components in the wet cement environment.1'he nominal static field value above the sensor gives a Larmor frequency of 8.08 M)-[z'H.
[0018] lteferring to Fig, 5, the radiofrequency coil 2 is fixed atop the magnets 4. The coil is conneeted by wire leads (shown in dashed lines) to a capacitive tuning circuit 6 mounted on a circuit board and fixed to the bottom of the magnets. The tuned circuit is connected to a coaxial cable 8, which is attached to the RF supply inodule and signal detection module as described with rolcrence to Fig. 11.
[0019J Measurements were made using the sensor of Fig. 4, both of moisture content in sand, and signal amplitude /
relaxation times in curing concrete. In each case, a scnsor was immersed in sand / concrete and a Bruker Minispec spectrometer was used for RF supply, NMR signal detection and experimental control. These measurements are shown here as a proof of principle. In the case of tha moisture content measurements, the signal intensity correlates well with the mcasured moisture content. For the curing concrete, further study is required to correlate measurements with relevant material parameters.
10020] Referrimg to Fig. 6, NMR measurements of the moisture content of sand.
The sensor was immersed in wet sands with different weight percentages of water. A CPMG sequence was used to measure the moisture content, with 32 echoes, an echo time of 0.2 nis, and 512 scans for an acquisition tinne of 9 ntinutes per point. The echoes were co-added to obtain the signal. A linear relationship between the water content and 1v1R
signal intensity is observed.
[0021] Referring to Fig. 7, Signal amplitude as a function of time measured with a sensor embedded in curing ASTM
Type 1 cement, w/c ratio 0.45. The first two echoes in a CPMG sequence (TE =
0. 168 ms, 8192 scans) were averaged to give the signal, 100221 Referring to Fig. 8, apparent'f2 measurcd by CPMG (TE = 0.168 ms, 8192 scans) in a curing ASTM Type I
cemCnt sample. Because of the grossly inhoniogeneous field, the relaxation eonstant is a combination of T2 and diffusive attenuation.
J00231 Referring to p'ig. 9, an alternate magnet arrattgement producing a more homogeneous field a more homogeneous field than the embodhnent of p'ig. 1, at a frequency around 6 MHz.
A washer magnet is placed on top of a stack of disk magnets, giving a saddle point in the field. While the field is slightly lowcr in this arranl;ement, the saddle point ensures that signal can be obtained from a larger volume, increasing sensitivity. Furthermore, the gradient around the saddle point is low, mitigating diffusive attenuation.
100241 Altematively, it will be understood by those skilled in the art that other magnet designs may be employed depending upon the magnetic Pold dcsired. For example, a magnet with two north poles and two south poles of the type in illustrated in FIG. 15 could be used as part of a probe according to the invention.
f00251 FIG. I l is a block diagram of one embodiment of an NMR system provided in accordance with the present invention. The magnet (a) produces a suitable static magnetic field in a region of interest. The dynamic field generator (b) generates a dynamic radiofrequency magnetic field in the region of interest. '1'he RF supply module (c) is connected to the dynamic field generator and supplies an RF signal compatible with nuclear magnetic resonance to the dynamic field generator. The detection module (d) detects nuclear magnetic resonance signals induced in the dynamic field generator. The RF supply module is controlled with a computer (e), and the signals detected by the detection module are recorded and processed on the computer.
100261 Alternate pattcrns may be used for the printed circuit board for the probes according to the present invention and may have application for other NMR probes. Single-sided magt,etic resonance measurements require single-sided or surface coils for B, generation and signal detection. The fundamental constraint in coil design is that B, must be orthogonal to B. For the case where B, must be normal to the magnet surface, a circular surface coil is the clear choice.
J0027J In many cases, it is desirable to have Bo normal to the inagnet surface, and B, must therefore be generated parallel to the surface of the coil in order to meet the orthogonality condition necessary for magnetic resonance. This can be achieved in practice by a sheet of RF current, approximated by several parallel wires. However, in this arrangement, additional wire is necessary to form a closed loop of current. Along with increasing the resistance of the coil, this additional wire generates a spurious B, field. This field can be parallel to Bo, and therefore not useable for NMR
purposes, or outside the desired scnsitive volume of the coil. In the fvst oase, the field effectively reduces the filling factor and directly increases the measurement noise. In the second case, signal from uuwanted regions of a sample may be measured, affecting experimental results.
1002$1 In selecting a coil design for generating a field parallel to the surface of the coil, the goal is to generate the strongest possible field in a region of interest (ROI), while reducing spurious fields as much as possible.
10029] Fig. 12 depicts a"spiral" coil, Fig. 13 a "modified DD' eoil and Fig.
14. a "bowtie" coil.
100301 Coils using the spiral, modified DD and bowtie patterns were manufactured from wire epoxied to a thin substrate. 'Ihe coil pattems were first simulated using the Biot-Savart law, and the rransvarse field intensity Integrated over a 1 am deptli to give the plots shown. A 5 cm by 5 om by 1 cm thick rubber sample was then imaged at 8.3 MHz using the BPRiTB MRl technique using each coil.
10031] The spiral coil gives a relatively uniform field over a ring shaped region, and results in the highest signal intensity when tested. No signal, however, is derived from the centre of the sample. in many unilaterai applications, it is this region that is most importarat In terms of Bo characteristics, and the spiral coil may not be an appropriate choice.
[00321 The double-d coil features comparable signal 'uitensity to that of the spiral coil, but the image indicates that rnore of this signal originates from the center of the coil. 'I'here is a large amount of signal from the coil return paths, with an intensity of approximately half that of the center region.
[00331 '1'he bowtie coil has a lower total signal, however the 2D image indicates that this coil has superior spatial selectivity, and a higher sensitivity in the central region than either the spiral or double-d coils. It will be understood that the number of windings can be varied according to the desired magnetie field profile and/or coil performance.
[01001 The above-described embodiments af the present invention are intended to be examples only_ Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departina from the scope of the invention, which is set forth in the claims.
Claims
1. A probe suitable for use in unilateral nuclear magnetic resonance imaging and adapted to be embedded in a sample to be analysed, the probe comprising;
a static magnetic field generator;
a radiofrequency magnetic field generator adjacent to the static magnetic field generator;
a circuit controlling the frequency response of the radiofrequency magnetic field generator, adjacent to the static magnetic field generator, an input cable coupled to the frequency control circuit and the frequency control circuit coupled to the radiofrequency magnetic field generator.
a static magnetic field generator;
a radiofrequency magnetic field generator adjacent to the static magnetic field generator;
a circuit controlling the frequency response of the radiofrequency magnetic field generator, adjacent to the static magnetic field generator, an input cable coupled to the frequency control circuit and the frequency control circuit coupled to the radiofrequency magnetic field generator.
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| CA 2600990 CA2600990C (en) | 2007-09-10 | 2007-09-10 | Probe, system and method suitable for unilateral nuclear magnetic resonance |
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| CA 2600990 CA2600990C (en) | 2007-09-10 | 2007-09-10 | Probe, system and method suitable for unilateral nuclear magnetic resonance |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2010003237A1 (en) * | 2008-07-08 | 2010-01-14 | University Of New Brunswick | Magnetic field gradient monitor apparatus and method |
| US8791695B2 (en) | 2009-08-31 | 2014-07-29 | University Of New Brunswick | Magnetic resonance apparatus and method |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2010003237A1 (en) * | 2008-07-08 | 2010-01-14 | University Of New Brunswick | Magnetic field gradient monitor apparatus and method |
| US8717022B2 (en) | 2008-07-08 | 2014-05-06 | University Of New Brunswick | Magnetic field gradient monitor apparatus and method |
| US8791695B2 (en) | 2009-08-31 | 2014-07-29 | University Of New Brunswick | Magnetic resonance apparatus and method |
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| CA2600990C (en) | 2014-06-03 |
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