SONDE, SYSTEME ET METHODE APPROPRIES POUR LA RESONANCE MAGNETIQUE NUCLEAIRE UNILATERALE

PROBE, SYSTEM AND METHOD SUITABLE FOR UNILATERAL NUCLEAR MAGNETIC RESONANCE

Abrégé français

Une sonde appropriée pour une utilisation en imagerie par résonance magnétique nucléaire unilatérale est adaptée pour être intégrée dans un échantillon à analyser; la sonde comprend un générateur de champ magnétique statique; un générateur de champ magnétique de radiofréquence, adjacent au générateur de champ magnétique statique; un circuit contrôlant la réponse en fréquence du générateur de champ magnétique de radiofréquence; un câble d'entrée couplé au circuit de contrôle de réponse en fréquence et le circuit de contrôle de fréquence couplé au générateur de champ magnétique de radiofréquence.

Abrégé anglais

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.

Revendications

Claims:
1. A unilateral nuclear magnetic resonance imaging probe adapted to be
embedded in a
material sample to be analyzed wherein the probe is enclosed firmly in the
sample, the probe
comprising: a permanent magnet; a radiofrequency coil adjacent to a pole of
the permanent
magnet; a frequency control circuit that controls the frequency response of
the radiofrequency
coil, the frequency control circuit being adjacent to the permanent magnet and
also coupled to
the radiofrequency coil; and an input cable coupled to the frequency control
circuit.
2. The probe of claim 1, wherein the radiofrequency coil has at least one
winding.
3. The probe of claim 1, wherein the pole of the permanent magnet is not
adjacent to a pole
of another permanent magnet.
4. The probe of claim 3, wherein the permanent magnet comprises at least
two permanent
magnets where the poles of all of the magnets are collinear on an axis passing
through all of the
magnets.
5. The probe of claim 4, wherein one of the magnets is washer-shaped.
6. The probe of claim 2, wherein the radiofrequency coil comprises a bowtie-
shaped coil.
7. The probe of claim 2, wherein the radiofrequency coil comprises a double-
d shaped coil.
8. The probe according to claim 1, wherein the sample comprises a non-human
sample.
9. The probe according to claim 8, wherein the sample is selected from the
group consisting
of cement, concrete, a structure and soil.
10. A method for in-situ measurement nuclear magnetic resonance parameters
of a sample
comprising the steps of: embedding the probe of claim 1 in the sample wherein
the probe is
enclosed firmly in the sample; connecting the probe to a spectrometer suitable
for nuclear
magnetic resonance measurement; generating B0 and B1 fields in order to define
a local sensitive
spot; taking NMR measurements in the sensitive spot, selected from the group
comprising bulk
relaxation time and diffusion measurements.
7

11. The method of claim 10, wherein the sample is selected from the group
consisting of
cement, concrete, a structure and soil.
12. The method of claim 10, wherein in the step of embedding the probe, the
probe is
permanently or semi-permanently embedded in the sample.
13. A unilateral nuclear magnetic resonance imaging system comprising: a
probe adapted to
be embedded in a non-human sample to be analyzed wherein the probe is enclosed
firmly in the
sample, the probe comprising: a permanent magnet; a radiofrequency coil; a
radiofrequency
supply module connected to the radiofrequency coil, capable of generating a
radiofrequency
signal compatible with nuclear magnetic resonance.
14. The system of claim 13, further comprising a computer that controls the
radiofrequency
supply module and the recording and processing signals detected by the module.
15. The system of claim 13, wherein the sample is selected from the group
consisting of
cement, concrete, a structure and soil.
16. The system of claim 14, wherein the radiofrequency coil has multiple
windings.
17. The system of claim 16, wherein the coil is bowtie shaped.
18. The system of claim 16, wherein the radiofrequency coil is double-d
shaped.
8

Description

CA 02600990 2009-03-25
PROBE, SYSTEM AND METHOD SUITABLE FOR UNILATERAL NUCLEAR
MAGNETIC RESONANCE
CROSS-REFERENCE TO RELA1b1.) APPLICATIONS
Not applicable.
MICROFICHE APPENDIX
Not applicable.
TECHNICAL FIELD
This application relates to nuclear magnetic resonance techniques in general,
and to
a probe, system and method suitable for unilateral nuclear magnetic resonance,
in
particular.
BACKGROUND OF THE INVENTION =
Despite growing interest in magnetic resonance of porous materials such a:s
soils
and concrete, critical limitations exist in terms of the types of measurements
that can
be made. Both spectroscopic and spatially resolved studies requiring
superconducting magnets can only be carried out on samples of limited size.
New
advances in open, portable MYER instrumentation allow bulk relaxation and
diffusion measurements to be made on arbitrarily large samples such as
described in
G. Eidrnann, R. Savelsberg, P. Bliimler, B. Blamler, J. /vIagn, Res. A 1996;
122:104-109.
However, experiments of this type are limited by the penetration depth of 1 o
and
131. 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.
In many situations, it may be desirable to measure NIvIR parameters from deep
within a sample. Examples in the porous media regime could include larger
concrete
structures and soil formations. Previous work has used RF coils embedded
within

CA 02600990 2009-03-25
concrete samples in order to alleviate the Bi, penetration problem (see
Boguszynska,
above)_
SUMMARY
Extending this idea, a small, low cost NMR=sensor suitable to be embedded
within a
large sample has been developed. NcIFeB disk magnets provide a local Bo field
for a
1H resonant frequency of between 640 MHz depending on the design. A printed
circuit board surface coil is located immediately above one face of the
magnets, and
tuned to resonance with capacitors on the opposite face. The entire
arrangement is
connected to a 2.5tiarn diameter coaxial cable, and encased in epoxy. The
inhomogeneous Bo and Bi fields define a local sensitive spot in which bulk
relaxation
time or diffusion measurements can be made.
According to one 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
north and south pole; a radiofrequency coil adjacent one of the poles of the
magnet;
a capacitive tuning circuit adjacent the other pole of the magnet; a coaxial
cable
electrically connected to the tuning circuit and the tuning circuit
electrically
connected to the coil.
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 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.

CA 02600990 2009-03-25
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.
According to another aspect of the present invention, there is provided a
system for
use in unilateral nuclear magnetic resonance imaging comprising: a probe
adapted
to be embedded in a sample to be analysed, an RE supply module connected to
probe suitable for generating an RF signal compatible with nuclear magnetic
resonance.
An RE coil suitable for nuclear magnetic resonance imaging wherein the coil in
bowtie shaped. The coil may have multiple windings.
Other aspects and features of the present invention will become apparent to
those
ordinarily skilled in the art upon review of the following description of
specific
embodiments of a probe, system and method suitable for unilateral nuclear
magnetic resonance in conjunction with the accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of example
only, with reference to the accompanying drawing figures, wherein:
FIG. 1 is a schematic diagram of an array of three disc 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;

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FIG. 4 is photograph of an assembled probe according to the present
invention;
FIG. 5 is a schematic diagram showing the interconnection of components of
the probe of Fig. 4;
FIG. 6 is a plot of NMR measurements of the moisture content of a sand
sample acquired according to the invention;
FIG. 7 is a plot of signal amplitude as a function of time measured with an
apparatus according to the invention embedded in curing cement;
FIG. 8 is a plot of apparent T2 measured by Carr-Purcell-Meiboom-Gill
(CP-MG) method in a curing ASTM Type 1 cement sample;
FIG. 9 is another embodiment of an apparatus according to the present
invention;
FIG. 10 is an image of the magnetic scalar potential for the sensor of FIG. 9;
FIG. 11 is a block diagram of an embodiment of and /`..IMR apparatus
provided in accordance with the present invention;
FIG. 12 is a spiral coil;
FIG_ 13 is a modified DID coil;
FIG. 14 is a "bowtie" coil comprised of a single coil; and
FIG. 15 is a "bowtie" coil comprised of multiple coils.
like reference numerals are used in different figures to denote similar
elements.
=

CA 02600990 2009-03-25
DETAILED DESCRIPTION OF THE DRAWINGS
=
Referring to Figs. 1 and 2, the measured magnetic field magnitude over a stack
(array) of three disk magnets is shown_ About 3mm above the magnets, the field
is
¨2500 G (-10.5 Is4I-LeH). The field is inhomogeneous, due to the single sided
nature
of the device as well as inhornogeneous magnetization in the low quality
magnets.
Referring to Fig. 3, components of the probe (also commonly referred to as a
sensor)
are shown. From left to right the components 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 RF coil is a modified "double-d" design. The printed circuit board in the
assembled sensor sits 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
strength
above the magnets. The RF coil is tuned to a frequency suitable for nuclear
magnetic
resonance 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, the discs of magnets may be replaced by one magnet of a size
comparable to the combined si7e of the two disc magnets. In some embodiments,
more than two disc magnets may be used.
Referring 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 the sample in which it is to be embedded (wet cement in this
example) and is approximately 3 mm in diameter and 2 cm in thickness and
includes
the components shown in Pig. 2. The sensor of Fig. 4 is encased in a water
proof
epoxy to protect the components in the wet cement environment, The nominal
static
field value above the sensor gives a Larmor frequency of 8.08 MHz1H.

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Referring to Fig. 5, the radiofrequency coil 2 is fixed atop the magnets 4.
The coil is
connected 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
module
and signal detection module as described with reference to Fig. 11.
Measurements were made using the sensor of Pig. 4, both of moisture content in
sand, and signal amplitude/relaxation times in curing concrete. In each case,
a
sensor 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 the
moisture
content measurements, the signal intensity correlates well with the measured
moisture content. For the curing concrete, further study is required to
correlate
measurements with relevant material parameters.
Referring to Pig. 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
0.2ms, and 512 scans for an acquisition time of 9 minutes per point. The
echoes were
co-added to obtain the signal. A linear relationship between the water content
and
MR signal intensity is observed.
Referring to Fig. 7, Signal amplitude as a function of time measured with a
sensor
embedded in curing ASTM Type 1 cement, wjc ratio 0,45. The first two echoes in
a
CPMG sequence (TE = 0.168 ms, 8192 scans) were averaged to give the signal.
Referring to Fig. 8, apparent T2 measured by CPMG (TE = 0.168 ms, 8192 scans)
in a
curing ASTM Type 1 cement sample. Because of the grossly inhornogeneous
the relaxation constant is a combination of T2 and diffusive attenuation.

CA 02600990 2009-03-25
Referring to Fig. 9, an alternate magnet arrangement producing a more
homogeneous field a more homogeneous field than the embodiment of Fig. 1, at a
frequency around 6 /ski-lz. 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 lower
in this
arrangement, 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.
Alternatively, it will be understood by those skilled in the art that other
magnet
designs may be employed depending upon the magnetic field desired. 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.
FIG. 11 is a block diagram of one embodiment of an NMR system provided in
accordance with the present invention. The magnet 100 produces a suitable
static
magnetic field in a region of interest. The dynamic field generator 110
generates a
dynamic radiofrequency magnetic field in the region of interest. The RE supply
module 120 is connected to the dynamic field generator and supplies an RE
signal
compatible with nuclear magnetic resonance to the dynamic field generator. The
detection module 130 detects nuclear magnetic resonance signals induced in the
dynamic field generator. The RF supply module is controlled with a computer
140,
and the signals detected by the detection module are recorded and processed on
the
computer.
Alternate patterns 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 magnetic resonance measurements require single-sided or surface
coils
for Si generation and signal detection. The fundamental constraint in coil
design is
that B1 must be orthogonal to Bo. For the case where B1 must be normal to the
magnet surface, a circular surface coil is the clear choice.

CA 02600990 2009-03-25
In many cases, it is desirable to have Bo normal to the magnet surface, and Bi
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 l3-1 field. This field can be parallel to Bo, and therefore not
useable for NMR
purposes, or outside the desired sensitive volume of the coil. In the first
case, the
field effectively reduces the filling factor and directly increases the
measurement
noise. In the second case, signal from unwanted regions of a sample may be
measured, affecting experimental results,
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.
Fig, 12 depicts a "spiral" coil, Pig. 13 a "modified DD" coil and Fig. 14, a
"bowtie"
coil.
Coils using the spiral, modified DD and bowtie patterns were manufactured from
wire epoxied to a thin substrate. The coil patterns were first simulated using
the
Biot-Savart law, and the transverse field intensity integrated over a 1 cm
depth to
give the plots shown. A 5 cm by 5 cm by 1 cm thick rubber sample was then
imaged
at 8.3 MHz using the SPRITE MRI technique using each coil.
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. hi many unilateral applications, it is this region that
is most
important in terms of Bo characteristics, and the spiral coil may not be an
appropriate choice.

CA 02600990 2009-03-25
The double-d coil features comparable signal intensity to that of the spiral
coil, but
the image indicates that more of this signal originates from the center of the
coil.
There is a =large amount of signal from the coil return paths, with an
intensity of
approximately half that of the center region.
The 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 magnetic field profile and/or coil
performance.
The above-described embodiments of 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 departing from the scope of
the
invention, which is set forth in the claims.

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