Note: Descriptions are shown in the official language in which they were submitted.
MRI METAMATERIAL LINER
TECHNICAL FIELD
[0001] Metamaterial Liners for Magnetic Resonance Imaging.
BACKGROUND
[0002] MRI scanners use a strong, uniform static magnetic field,
conventionally
denoted Bo, to cause protons (hydrogen nuclei) to be subject to a resonance at
a particular
radio frequency (RF), known as the Larmor frequency, which depends on the
field strength:
fo=713o, y=42.6MHz/T. In that formula fo is the Larmor frequency and Bo is the
static
magnetic field, with y being a proportionality factor (gyromagnetic ratio).
Near-field coils
are used to excite protons at the Larmor frequency and to detect a signal from
the oscillating
protons. The main static field is typically aligned with the length of the
scanner's bore. The
RF excitations applied to excite the protons include a magnetic field
conventionally denoted
B1, typically transverse to the main static magnetic field and circularly
polarized. The
excitations typically have a bandwidth less than 100 kHz. These excitations at
the Larmor
frequency cause the spins of the protons, which initially are on average
aligned with the
main static field, to change orientation so that the spins on average point in
a different
direction. The spins precess about the main static field, causing the average
orientation of the
spins to also precess, producing a detectable signal. Typically, to encode
position
information in the signal to construct an image, gradient fields are applied
that change the
strength of the main static magnetic field depending on position. The RF
excitations are
typically applied in pulses of duration in the range of 100 s ¨ 3ms and with a
repetition rate
in the range of 5ms to several seconds. Preferably, the amplitude of the
excitations is
uniform to achieve uniform image intensity and contrast, and high sensitivity
(high
Bimagnetic field per unit voltage excitation or power). For safety reasons,
there are local and
whole body specific absorption rate (SAR) constraints (IEC 60601-2-33).
[0003] Due to difficulties in producing a strong uniform magnetic field
over a large
volume, MRI scanners typically have narrow bores, which can lead to
claustrophobia in
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patients. At the typical magnetic field strengths, the Larmor frequency for
protons is
sufficiently low and the bore sufficiently narrow that electromagnetic waves
at the Larmor
frequency cannot propagate through the bore. To produce the excitations,
antennas known as
a body coil or "birdcage" coil are provided within the bore. The birdcage coil
30 (an example
shown in Fig. 1) is a resonant ladder network excited in CP (quadrature) mode.
"Rungs" 32
each have a capacitor 34 to cause resonance. The static magnetic field Bo is
indicated by
arrow 36. Electrical connections to drive the birdcage coil are not shown. An
array of
separate receiving coils/loops (not shown) is used for maximum signal-to-noise
ratio (SNR)
in reception. The birdcage coil can also be used to receive, but the SNR will
be low. The
birdcage coil typically takes up significant space in the bore, increasing
claustrophobia.
Simply extending the birdcage too close to a conductive bore to reduce
claustrophobia would
create image currents on the bore, reducing efficiency. In Fig. 2, 40 shows an
example
magnetic field (uniform birdcage mode) represented by field lines 38, the
magnetic field
represented being the magnetic field 131 induced in the space within a
conductive bore 40 at a
point in time by a birdcage coil 30 within the bore.
[0004] Traditional body coils do not require typically subject-specific
adjustments at
lower Bo field strengths, but at high frequency they are highly sensitive to
dielectric loading.
These higher Bo fields and Larmor frequencies can be useful to improve signal-
and contrast-
to-noise ratios, allowing higher resolution. Body coils are also costly to
build because they
contain expensive components like high-voltage capacitors, and the use of a
small number of
localized elements requires tuning and balancing on the bench for optimal
operation. Simply
increasing the number of rungs to distribute the capacitance would increase
the number of
paths, and lead to a cluttered mode spectrum.
[0005] Travelling wave (TW) MRI has been one proposal to deal with the
claustrophobia issue. Like a waveguide, the TW MRI bore has a cutoff frequency
for
propagating waves, and because of the size of bore required to accommodate the
body of the
patient, this cutoff frequency is in the order of several hundred MHz. For
example, a typical
MRI bore may be 58 cm in diameter and have a natural frequency cutoff of the
TEll mode
of approximately 300 MHz. This natural cutoff frequency of the MRI bore
prevents waves
having a frequency below the natural cutoff frequency from propagating through
the MRI
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bore. In a typical travelling wave MRI, a stronger magnetic field is used to
increase the
Larmor frequency for protons to above the cutoff for the bore. Antennas are
then placed
outside the bore to produce and detect excitations that can propagate through
the bore.
However, travelling wave MRI has developed a reputation for a lower image SNR
than
conventional MRI.
[0006] US patent 9,529,062 describes a metamaterial liner for MRI for
lowering a
cutoff frequency of the MRI bore for use in travelling-wave magnetic resonance
imaging.
Metamaterials are periodic structures that can provide effective bulk
permeability and
permittivity responses beyond those found in nature. The lowering of the
cutoff frequency
allows travelling-wave MRI to be used in an MRI scanner with conventional
field strength,
so that a conventional MRI scanner can be retrofitted as a travelling-wave MRI
scanner.
[0007] A metamaterial for lowering the cutoff frequency of a bore, using
radial
inductors and circumferential capacitors, was disclosed in Justin Pollock and
Ashwin K.
lyer, "Experimental Verification of Below-Cutoff Propagation in Miniaturized
Circular
Waveguides Using Anisotropic ENNZ Metamaterial Liners", IEEE Transactions On
Microwave Theory and Techniques, vol. 64, no. 4, 2016.
[0008] The phrase "lowering the cutoff frequency" is used to mean that a
new
passband is introduced by the metamaterial liner in the otherwise below-cutoff
frequency
region. This passband is different than that above cutoff in an empty circular
waveguide. The
passband introduced by the metamaterial has a cutoff frequency below which
propagation is
permitted.
SUMMARY
[0009] There is provided a liner for a bore of an MRI scanner, the liner
having an
annular shape and being formed of a metamaterial having a relative electrical
permittivity
that is negative and near zero at a working frequency of the MRI scanner
corresponding to a
Larmor frequency in a magnetic field of the MRI scanner. The liner may extend
less than a
full length of the bore.
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[0010] In various embodiments, there may be included any one or more of
the
following features: the liner may extend through an intermediate portion of
the bore between
two ends of the bore and not extend to either of the two ends. The static
magnetic field of the
MRI scanner may have any strength, but particularly including a strength of
greater than 1.5
Tesla or greater than 3 Tesla and less than 7 Tesla. The static magnetic field
of the MRI
scanner may also have a strength of less than 0.5 Tesla. The liner may be
formed as a
cylindrical arrangement of a 2D metamaterial. The metamaterial may comprise
plural
annular metamaterial segments. Each annular metamaterial segment may have an
inner
circumferential conductor and an outer circumferential conductor, capacitors
on at least one
of the inner and outer conductors, and inductors connecting the inner and
outer conductors.
Each annular metamaterial segment may be configured to be removable from the
bore. Each
annular metamaterial segment may be part of a metamaterial module, the
respective
metamaterial modules being sized and shaped to be interchangeable. The
metamaterial
modules may be configured to connect to adjacent metamaterial modules. Each
metamaterial
module may comprise an additional bore segment, the additional bore segments
forming an
additional bore within the scanner bore when the modules are in place within
the bore.
Additional modules sized and shaped to be interchangeable with the
metamaterial modules
may be placed within the bore to provide further additional bore segments to
cause the
additional bore to extend the full length of the scanner bore. Electromagnetic
excitations may
be produced at the working frequency by an antenna located within the bore
adjacent to the
liner and connected to a feed line. The antenna may be in an antenna module,
the antenna
module being sized and shaped to be interchangeable with metamaterial modules.
Electromagnetic excitations may be produced at the working frequency by a
metamaterial
segment connected to a feed line. Any feed line may be multiple feed lines.
Where there is
an additional bore, the feed line may extend through a gap between the
additional bore and
the scanner bore. The feed line may comprise coaxial cable having a sheath
grounded to the
bore. The sheath may be removably grounded to the bore.
[0011] There is also provided a liner for a bore of an MRI scanner
having a static
magnetic field strength of less than 7 Tesla and greater than 1.5 Tesla, the
liner having an
annular shape and being formed of a metamaterial having a relative electrical
permittivity
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that is negative and near zero at a working frequency of the MRI scanner
corresponding to a
Larmor frequency in the magnetic field of the MRI scanner.
[0012] In various embodiments, there may be included any one or more of
the
following features: the static magnetic field may be greater than 3 Tesla and
less than 7
Tesla. The liner may be formed as a cylindrical arrangement of a 2D
metamaterial. The
metamaterial may comprise plural annular metamaterial segments. Each annular
metamaterial segment may have an inner circumferential conductor and an outer
circumferential conductor, capacitors on at least one of the inner and outer
conductors, and
inductors connecting the inner and outer conductors. Each annular metamaterial
segment
may be configured to be removable from the bore. Electromagnetic excitations
may be
produced at the working frequency by an antenna located adjacent to the bore
or within the
bore adjacent to the liner. A metamaterial segment may be electrically
connected to a power
source to produce the electromagnetic excitations at the working frequency.
[0013] There is also provided a method of magnetic resonance imaging.
The method
includes supplying an MRI scanner having a scanner bore, positioning a first
number of
annular metamaterial segments within the bore, to form a metamaterial liner
extending a first
length within the scanner bore and having a relative electrical permittivity
that is negative
and near zero at a working frequency of the MRI scanner corresponding to a
Larmor
frequency in the magnetic field of the MRI scanner, producing first
excitations within the
first length of the scanner bore, detecting first NMR signals resulting from
the first
excitations within the first length of the scanner bore to produce a first
image, removing one
or more of the annular metamaterial segments from the scanner bore to reduce
the
metamaterial liner to a second length within the scanner bore smaller than the
first length,
producing second excitations within the second length of the scanner bore, and
detecting
second NMR signals resulting from the second excitations within the second
length of the
bore to produce a second image.
[0014] In various embodiments, there may be included any one or more of
the
following features: Each of the metamaterial segments may be part of a
metamaterial
module, the respective metamaterial modules being sized and shaped to be
interchangeable.
The metamaterial modules may be configured to connect to adjacent metamaterial
modules.
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Each metamaterial module may comprise an additional bore segment, the
additional bore
segments forming an additional bore within the scanner bore when the modules
are in place
within the bore. Additional modules sized and shaped to be interchangeable
with the
metamaterial modules may be placed within the bore to provide further
additional bore
segments to cause the additional bore to extend a full length of the scanner
bore. The first
excitations may be produced by an antenna within the scanner bore and
connected to a feed
line. The antenna may be in an antenna module, the antenna module being sized
and shaped
to be interchangeable with the metamaterial modules. The first excitations may
be produced
by a metamaterial segment connected to a feed line. Any feed line may be
multiple feed
lines. Where there is an additional bore, the feed line may extend through a
gap between the
additional bore and the scanner bore. The feed line may comprise coaxial cable
having a
sheath grounded to the bore. The sheath may be removably grounded to the bore.
[0015] These and other aspects of the device and method are set out in
the claims.
BRIEF DESCRIPTION OF THE FIGURES
[0016] Embodiments will now be described with reference to the figures,
in which
like reference characters denote like elements, by way of example, and in
which:
[0017] Fig. 1 is a schematic isometric view of a prior art birdcage
coil.
[0018] Fig. 2 is a schematic cross section showing a magnetic field
produced by a
prior art birdcage coil.
[0019] Fig. 3 is a plot of the transverse (i.e., non-axial) vector
components of electric
and magnetic fields produced in lined and unlined bores at their respective
cutoff
frequencies.
[0020] Fig. 4 is a pair of drawings showing an infant model used in a
simulation,
showing a transverse slice of the head on the left and a transverse slice of a
body portion on
the right.
[0021] Fig. 5 is an isometric view of the infant model in a large
birdcage antenna.
[0022] Fig. 6 is a drawing, not to scale, of an annular piece from which
a
metamaterial lining may be constructed.
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[0023] Fig 7 is a perspective drawing, not to scale, of annular pieces
as shown in Fig.
6, but without feed lines, within a bore.
[0024] Fig. 8 is a schematic cross section, not to scale, showing a test
configuration
of a metamaterial as shown in Fig. 6, but without feed lines, within an
interior portion of a
bore and arranged between two antennas.
DETAILED DESCRIPTION
[0025] Immaterial modifications may be made to the embodiments described
here
without departing from what is covered by the claims.
[0026] A metamaterial liner is provided for a MRI scanner. As compared
to a
birdcage coil, a metamaterial has a larger number of lumped elements,
distributing elements
at extremely sub-wavelength intervals. Each distributed circuit element can
have a smaller
effect on the overall characteristics of the metamaterial than an element of a
birdcage coil,
allowing the use of less precise, cheaper elements (although more total
elements are
required).
[0027] A metamaterial lining of a conductive bore may be formed as a
cylindrical
version of a 2D metamaterial. The metamaterial may be formed using a
Transmission Line
(TL) model. Although the metamaterial does not eliminate image currents on a
conductive
bore which the metamaterial lines, the TL currents produced in the
metamaterial and the
image currents in the bore can be used to engineer the field distribution and
modes supported
in the bore volume.
[0028] Although a metamaterial may support an infinite number of modes,
which of
these are excited (the so-called "dominant" modes) may be restricted through
proper choice
of an excitation/antenna/source and by limiting the frequency range.
Therefore, although the
metamaterial may generally support multiple modes, careful design of the
metamaterial,
selection of the source, and restriction of the frequency range may allow the
support of a
single dominant mode. This can alleviate the multiple mode problem that can
arise from
increasing the number of rungs of a birdcage coil. It is also possible to
exploit different
modes occurring at the same frequency by encoding different streams of
information into
each. This is known as "RF shimming".
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[0029] In order to generate a suitable excitation for MRI, it is desired
that a relatively
homogeneous transverse magnetic field be created. This relatively homogeneous
transverse
magnetic field is associated, in the interior of the bore, with a homogeneous
transverse
electric field, E. However, a conductive bore cannot have tangential electric
fields at its
surface; any electric fields must be normal to a conductive bore's surface. A
thin liner may
be constructed with an electrical permittivity selected to form a transition
between the
homogenous transverse electric field in the interior of the bore encircled by
the liner and a
radial electric field within the liner itself. For an annular liner we have
[0030] E,
tan 02 = tan 01 ¨
Ei
[0031] where 01 is the angle between a radial direction and the electric
field in the
interior of the bore encircled by the liner, this interior portion having
permittivity ci, and 02
is the angle between the radial direction and the electric field within the
liner itself, the liner
having permittivity 62. If the interior is vacuum, si = 1, and for air or
typical dielectrics ci
will be higher than but not many times higher than one. 01 will depend on the
circumferential
position within the bore at any given time, varying between 90 degrees and
zero degrees.
Thus, to make the field within the liner nearly radial (i.e. 02 consistently
close to zero) we
can use 62 near zero, i.e., an epsilon near zero (ENZ) metamaterial can be
used. The liner
fields will be very strong, but the bore fields weaker, at a proportionality
of about 6I/ 62. The
permittivity must also have a dispersion (variation with frequency) that is
causal. It turns out
that 62 must also be <0, so an Epsilon-Negative-Near-Zero (ENNZ) metamaterial
may be
used.
[0032] In general, the permittivity of a metamaterial is frequency-
dependent. A
metamaterial may be selected to have a negative and near zero permittivity at
a working
frequency of an MRI scanner, typically the Larmor frequency for a magnetic
field strength of
the MRI scanner.
[0033] Transverse components of magnetic and electric fields were
plotted as shown
in Fig. 3 for a linearly polarized TEii propagating waveguide mode in a
circular waveguide
bounded by a perfect electric conductor at the respective cutoff frequencies
for an unlined
bore and a bore lined with an ENNZ metamaterial. The field directions in the
transverse
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plane are represented by arrays of triangles. As expected, the E fields were
normal and H
fields tangential at the perfect electric conductor. The field magnitude
variations were strong
in the unlined (vacuum filled) bore and relatively uniform in the metamaterial-
lined case. As
expected, electric fields in the liner itself were strong.
[0034] Simulations have been conducted on a metamaterial lined bore of
an MRI
scanner at a working frequency of 200 MHz (corresponding to a proton Larmor
frequency
for an MRI scanner with a magnetic field of 4.7 Tesla). According to the
simulations at 200
MHz, the liner offers performance comparable to that of birdcage coils in
terms of field
homogeneity, sensitivity and SAR, while allowing operation at frequencies
beyond those
where birdcage coils operate reliably. The frequency of 200 MHz (4.7 T) chosen
for this
example is beyond the preferred range of suitability of traditional body-sized
resonators
(<3T), and below frequencies at which their performance would suffer greatly
(>7T), where
travelling-wave effects can also become dominant.
[0035] In general, higher fields allow greater signal to noise ratio,
but high fields
result in technical difficulties. The range between 1.5T and 7T corresponds to
RF (Larmor)
frequencies between 64MHz and 300MHz. Human-body-size scanners will still be
below
cutoff in this frequency range, and can therefore benefit from the
retrofitting of metamaterial
liners to enable travelling-wave excitation. It is believed that the range of
1.5 T to 7T, and
especially 3T to 7T is therefore a particularly useful range of field
strengths for MRI. 4.7 T
could be a particular "sweet spot" for body imaging, low enough to avoid the
major
challenges of 7T, but high enough to get better SNR than 3T. The ENNZ
metamaterial liner
also allows lower field strengths to be used, including (potentially very) low
static Bo filed
strengths far below the unlined waveguide cutoff frequency, such as below 0.5
T, while still
allowing excitation using a travelling wave. This could be useful to reduce
the costs of
producing the magnetic field, at the expense of SNR.
[0036] Three geometries were simulated in Ansys HFSS, a finite-element-
method
full-wave electromagnetic simulator:
[0037] 1. A small hybrid birdcage (12 sections, 41.5 cm long and 36.4 cm
in
diameter), made resonant at 200 MHz using 46.2 pF end-ring capacitors and 2X50
pF
capacitors on each rung, and driven in quadrature using two lumped ports at
each end ring.
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[0038] 2. A large hybrid birdcage (16 sections, 42 cm long and 52 cm in
diameter),
made resonant at 200 MHz using 28.5 pF end-ring capacitors and 2X100 pF
capacitors on
each rung, and driven in quadrature using four lumped ports at each end ring.
[0039] 3. The metamaterial liner was 160 cm in length, 2 cm in
thickness, and its
negative permittivity observes a Drude dispersion attaining values of
approximately -0.118
at the frequency of interest (200 MHz).
[0040] A driven modal solution was used with discrete, high accuracy
sweep, and
meshing was adaptive with a maximum segment length of 3 cm. All models
included a
cylindrical perfect electrical conductor boundary 160 cm in length and 56 cm
in diameter.
This has an unmodified cutoff frequency of 313.93 MHz. The empty bore is thus
below
cutoff. For the birdcage coils the ends of the cylinder were also perfect
electrical conductor,
while for the metamaterial liner they were wave ports (TE mode, circularly
polarized), one of
which was driven.
[0041] For the metamaterial case, conducting hollow cylindrical sections
80 cm in
length and 140 cm in diameter, which are above cutoff at 200MHz, were used
between the
lined bore and wave ports to illuminate the lined bore using a propagating
TEii mode and
couple to the lined bore's below-cutoff HEI I mode. An anthropomorphic model
of an infant
52 cm long (uniform c = 76, o- = 0.8 S/m) was included as a load. A larger
body was not
used because the large hybrid birdcage was too sensitive to such loads at this
frequency.
From the simulations, comparisons were made of SAR and B field homogeneity, as
well as
excitation efficiency and safety efficiency (defined in Table 1).
[0042] The simulations for each geometry included a simulation of fields
with no
infant model present, with the infant model present with the head of the
infant centrally
located with respect to the length of the bore, and a simulation with a body
portion of the
infant centrally located with respect to the length of the bore. The head was
modeled as
having a circular cross section 11.2 cm in diameter. The body portion was
modeled as having
a cross section 22.8cm across, as measured between two arm portions adjacent
to the body,
the cross section being 7 cm wide. Fig. 4 shows the head-centered and body
centered cross
sections of the infant model 50. The cross sections represent the midpoint of
the length of the
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bore in the head-centered and body-centered simulations respectively. Fig. 5
shows the
infant model 50 within the simulated large hybrid birdcage 52.
[0043] Table 1
Head-centered Body-Centered
MTM LBC SBC MTM LBC SBC
Mean field variation (%) 17 17 18 24 23 21
Excitation Efficiency 151 [IITNW] 0.66 0.69 0.70 0.56 0.63 0.65
Safety Efficiency [en 0.54 0.51 0.51 0.40 0.48 0.55
Max.SARiog
[ T/4(W/kg)]
SARmax/SARmean 5.61 7.94 8.10 6.22 5.25 5.87
[0044] Field homogeneity achieved by the metamaterial liner is
equivalent to that
achieved by the birdcage coils both in the empty bore and with a load. The SAR
performance
is also comparable, with the metamaterial liner outperforming the birdcage
coils in the head-
centered case (lower SAR hotspots and higher safety efficiency).
[0045] While it is a considerable challenge to operate a large birdcage
at 200 MHz
with the same stability with respect to loading achieved at lower frequencies,
the
metamaterial liner is robust and can be readily implemented using inexpensive
printed circuit
board (PCB) techniques and retrofitted into an existing scanner bore.
[0046] Our simulations confirm that a metamaterial bore liner could
function as a
high-field transmit (TX) body coil. While similar to travelling-wave MRI, the
liner does not
suffer from low sensitivity. When compared to standard birdcage coils the
liner offers better
or comparable field homogeneity, lower SAR hotspots and higher safety
efficiency in the
head-centered position. Transmit efficiency is also comparable, while offering
69% more
clear space in the bore than the small birdcage.
[0047] Thus instead of using birdcage coils, an alternative is to line
the bore of the
MR system with a metamaterial that can be excited to produce fields that are
similar to those
produced by traditional body coils. The periodic arrangement of the
metamaterial facilitates
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modular construction using automated PCB techniques and the distributed nature
implies an
inherent general robustness to manufacturing tolerances.
[0048] As shown in Fig. 6 an example annular metamaterial segment 10 may
comprise an annular dielectric substrate 12 supporting an inner
circumferential conductor 14,
and outer circumferential conductor 16, capacitors 18 on the inner and outer
conductors, and
inductors 20 connecting the inner and outer conductors. In the embodiment
shown, the inner
and outer circumferential conductors 14 and 16 may be copper traces on the
dielectric
substrate. The capacitors 18 may be implemented by gaps in the copper traces,
as shown in
Fig. 6. Conventional capacitors may also be used. Fig. 6 is not to scale, and
of particular
note, the widths of the gaps are exaggerated for clarity. The inductors 20 may
be
conventional inductors, for example from a commercial supplier. In the
embodiment shown
each inductor 20 has a first terminal 22 connected to the inner conductor 14
and a second
terminal 24 connected to the outer conductor 16. The terminals are wires that
protrude from
the inductors and are soldered to portions of the inner and outer conductors
shaped to
accommodate the terminals.
[0049] Optionally, one or more metamaterial segments 10 can be directly
connected
to sources to produce excitations and could also be connected to detection
electronics to
detect excitation. Such electrical connections enable the segments 10 to act
as antennas. The
electrical connections may each comprise a feed network for quadrature drive.
Fig. 6
schematically shows electrical connections of feed lines 60 to segments at
each end of the
liner. An electrically connected segment could also be used to detect
excitations produced by
a conventional antenna or vice versa.
[0050] Electrical connections of sources may be applied in parallel
with, or instead
of, one or more of the radial inductive loading elements or the azimuthal
capacitive gaps of a
metamaterial segment, which would generate the appropriate electric-field
pattern. Since
each of these sources could be driven with different phases, a quadrature
drive and other
driving scenarios could be realized. The wires delivering the source to a
specific inductor or
gap could be discreetly integrated with the wires already constituting the
metamaterial liner.
[0051] For example, the metamaterial segment 10 may be connected via
feed lines 60
to a source (not shown) to drive the metamaterial segment 10 so that the
metamaterial
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segment 10 acts as an antenna. In Fig. 6, four coaxial cable feed lines 60 are
shown, each one
connecting across one of the inductors 20. Different numbers of feed lines may
be used. Feed
lines could also connect across capacitors 18; which of the inductors or
capacitors is
connected across may be chosen depending on the impedance seen at the
different electronic
components. It would also be possible to have some feed lines connecting
across inductors
and other feed lines connecting across capacitors. Also, a single feed line
could connect
between any two points in the segment, including across multiple components.
In the
embodiment shown, the top and bottom feed lines could connect to a first port
(not shown)
and the right and left feed lines could connect to a second port (not shown).
The first and
second ports may be driven 90 degrees out of phase of each other, for example
using a quad
hybrid, to produce circular polarization. Different numbers of ports may be
used. Although
two ports should be enough, and are typical for birdcage antennas, four ports
are common for
driving larger structures.
[0052] The feed lines in Fig. 6 are schematically shown as leaving the
metamaterial
segment 10 near where they connect across an electronic component. The feed
lines may be
run down the scanner bore separately. The feed lines may also follow the
metamaterial
segment to a particular circumferential location so that the multiple feed
lines may exit the
bore axially at a collocated circumferential position in the bore. Where the
feed lines meet,
multiple feed lines associated with the same port may be combined into one.
Where the feed
lines follow the metamaterial segment, they may be integrated into the
metamaterial
segment, the integrated feed line portions being configured to attach to an
external feed line
where the feed line leaves the segment. Integrated feed line portions may
comprise coaxial
cable but could also use other conductors such as conductive traces on the
substrate 12.
[0053] Normally, it is not desired to drive more than one or two
metamaterial
segments using feed lines to act as antennas. Other metamaterial segments thus
may have no
feed lines. Optionally, other metamaterial segments may be configured to allow
feed lines to
attach, including if applicable integrated feed line portions, so that driven
segments may be
interchangeable with non-driven segments. Antennas may also be used instead of
driven
metamaterial segments. Where such antennas are used, optionally no
metamaterial segments
may be configures to connect to feed lines 60.
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[0054] As shown in Fig. 7, a metamaterial liner 26 may be formed, for
example, by
multiple annular metamaterial segments 10. Fig. 7 is a not-to-scale
perspective view of a
bore 28 lined with plural annular metamaterial segments 10 forming a
metamaterial liner 26.
Inductors 20 are represented schematically by cylinders on the substrates 12
and other
electromagnetic elements are not shown.
[0055] A metamaterial liner 26 similar to that shown in Fig. 7, at
approximately 20x
smaller scale than a size suitable for an MRI bore, was disclosed in
"Experimental
Verification of Below-Cutoff Propagation in Miniaturized Circular Waveguides
Using
Anisotropic ENNZ Metamaterial Liners". As disclosed in that paper, a
metamaterial liner
was placed in a small bore with a shielded loop antenna adjacent to each end
of the
metamaterial liner. Measurements were conducted at one antenna of
electromagnetic waves
originating from the other antenna and propagating through the metamaterial-
lined bore. The
shielded loop antennas used in this test configuration are King type shielded
loop antennas.
[0056] The bore in the test configuration disclosed in that paper
continued some
distance beyond the ends of the metamaterial liner and terminates in a closed
end. This
enabled more convenient simulation as the simulation could deal with only a
finite volume
with the perfect conductor as a boundary condition. A standard excitation of a
circular
waveguide such as this for simulation and testing involves antennas backed by
closed
conductive ends, typically at a distance of one-quarter of a wavelength at the
nominal
operating frequency. Otherwise, the simulation domain would need to be
terminated in some
sort of radiation boundary condition or a perfectly absorbing boundary, which
would be less
convenient. The wavelength used was such that the electromagnetic waves
produced could
not propagate significant distances through unlined portions of the bore.
[0057] An arrangement of a metamaterial liner between antennas may also
be used in
an MRI scanner. Fig. 8 is a schematic cross section, not to scale, showing an
arrangement of
annular metamaterial segments 10 between King-type shielded loop antennas 30.
In an
example, the liner 26 may be formed of annular segments 10 arranged at regular
intervals,
each occupying an axial length of the bore. The antennas 30 may for example be
positioned
at an axial length of the bore at a same regular interval from an end annular
segment of the
annular segments 10 as adjacent metamaterial annular segments are positioned
from each
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other, so that the antenna occupies a position that an additional metamaterial
segment would
occupy if it were added to the liner.
[0058] In the embodiment shown, each metamaterial segment and antenna
is part of
a respective removable module 70. The lined length of the MRI bore may be
adjusted by
removing and replacing annular metamaterial segments, so that where imaging of
a small
length of the bore is required the lining may be restricted to that length of
the bore only to
improve SNR. Antennas may also be made movable or removable, and as shown, may
themselves be part of removable modules 70. To facilitate removability, the
annular
metamaterial segments 10 as shown may each be self-contained, with no need for
electrical
connections to external objects or the bore. The annular metamaterial segments
may
comprise respective RF-transparent support elements such as respective blocks
of
polystyrene foam 72 in which the electronic components and substrate 12 are
embedded, to
ease handling and protect electronic components.
[0059] The metamaterial segments may also be made different from one
another.
This could be useful to adjust the uniformity of the fields in the
longitudinal direction.
[0060] Running unshielded feed lines to a metamaterial segment or
antenna through
the interior of the bore from an open end of the bore can create a TEM line
supporting a
TEM mode, which has no cutoff frequency and will end up propagating across all
frequencies. Penetrating the bore 28 is one way to provide shielding, but may
make moving
an annular segment connected to the wires inconvenient. Also, in an MR1
scanner typically
the gradient coil sits right outside the bore, further making such penetration
of the bore 28
inconvenient.
[0061] One option to provide shielding for feed lines, shown in Fig. 8,
is to provide
an additional bore inside the main scanner bore, for example concentric within
the scanner
bore. The feed lines can then run outside the additional bore. Where an
additional inner bore
is used, the inner bore may be made modular. For example, as shown in Fig. 8,
each annular
metamaterial segment 10, and each antenna 30, may have its own associated
length 74 of
inner bore radially outside the electronic components of the annular
metamaterial segment or
antenna, but radially inward of the scanner bore 28. Source wires may connect
to a
metamaterial segment or antenna by penetrating through the length 74 of inner
bore
CA 3001217 2018-04-12
associated with the metamaterial segment or antenna and exiting the scanner
axially through
a gap 76 between the outer bore 28 and inner bore lengths 74. Where the lined
portion of the
bore is only a portion of the full length of the bore, additional removable
module 78 may be
added having no metamaterial or antenna but providing additional lengths of
inner bore.
These additional modules 78 and the modules 70 may be sized and shaped to be
interchangeable. The modules 70 and 78 may be configured to connect together
with
adjacent modules 70 or 78. Thus, by adding these additional modules 78, the
inner bore
formed by lengths 74 may be extended as desired to provide additional
shielding for the
wires, for example for the full length of the scanner bore. This may be
combined with
shielding the feed lines in other ways such as via a shielded conduit or
coaxial cables with
grounded sheaths. Such additional shielding may also be made modular, with a
length of
shielded conduit (not shown) connected to each module 70, or grounding points
and supports
(not shown) for coaxial cable connected to each module 70. A removed module 70
and
removed additional module 78 are shown to the right of the scanner bore 28.
[0062] Alternatively or in addition to the additional bore, to reduce
excitation of a
TEM mode the wires may be made of coaxial cable, and may use a grounded sheath
(e.g.
grounded to the bore) to reduce radiation. The coaxial cable sheath may be
grounded to the
bore at regular intervals or continuously. The wires may also, in addition or
as an alternative,
be shielded in other ways, e.g. by going through a shielded conduit.
[0063] The feed lines in an embodiment may be movable so that the
metamaterial
segment or antenna to which they connect may also be moved. Thus by connecting
sources
to movable metamaterial segments, the movable segments may act as antennas
with
adjustable positions. Other antennas may also be connected movably in the same
manner.
The movable wires may be coaxial cable with sheaths periodically grounded to
the bore in a
removable manner. The movable wires may, in addition or as an alternative, go
through a
movable shielded conduit.
[0064] The metamaterial segments or modules could also each have one or
more
notches to support coaxial lines.
[0065] Another approach is to supply shielded feed lines running a
partial or full
length of the scanner bore, with discrete points at which antennas or
metamaterial segments
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can be connected to them. For example, such discrete points may be positioned
at spacing
corresponding to a spacing of the modules 70. The discrete points may also be
positioned
less frequently and with movable shielded feed line segments connecting to the
discrete
points.
[0066] Other antenna designs may also be used than the King type
shielded loop
antennas shown in Fig. 8, including metamaterial segments that are connected
to feed lines,
as shown in Fig 6. The shielded loop antennas shown in Fig. 8 produce linearly
polarized
excitations. It is believed that these antennas may be adapted for multi-port
excitation, e.g.
using two linearly polarized ports with a phase shift created externally, to
realize circular
polarization. A 1-port antenna could also be designed that would create
circular polarization.
Standard excitation methods such as stubs should also work, as the
metamaterial lined bore
when excited has strong E fields to couple to. Coupling stubs are short posts
that are
connected on one end to a coaxial cable and the other end protrudes (radially)
into the
waveguide. Similarly you can bend the stub and connect it to the waveguide
wall to obtain a
coupling loop.
[0067] Separate antennas could be used for transmit and receive, or a
single antenna
could be used for both, e.g. using a circulator. A transmit antenna as
described above could
also be used with a conventional receive antenna.
[0068] The simulation above using waveports should be applicable to
excitations
using an antenna, as the waveport excitation is essentially a boundary
condition on the
simulation domain that assumes a field distribution corresponding to certain
propagating
modes. The boundary condition will faithfully mimic a real excitation (e.g. an
antenna) that
is known to produce the same fields.
[0069] Where the metamaterial lining covers only a portion of the
length of the
bore, this allows improved SNR as compared to conventional traveling wave MRI,
due to the
waves only propagating through the lined portion of the bore. It is believed
that the lower
image quality of conventional travelling wave MRI results at least in part
from the travelling
wave passing through the entire length of the bore rather than through a more
localized part
of the bore, reducing signal to noise ratio for any particular part.
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[0070] The lined portion of the bore may extend through an intermediate
portion of
the bore between two ends of the bore and not extend to either of the two
ends. As the waves
reflect off the unlined portion of the bore, this leads to standing waves
within the lined
portion. Where the lined portion extends to one or both ends of the bore, the
waves will also
reflect off the end of the bore, along with radiating from the end. In either
case of reflecting
off an unlined portion or reflecting off an end of the bore the standing waves
are not "pure"
because the evanescent fields permitted outside the lined region, or the
radiation pattern from
an open end, provide a boundary condition causing the mode to reflect which is
not a hard
boundary condition. In fact, our simulations using the annular metamaterial
segments show
that the fields are still very homogeneous across the length of the lined
bore. The fields die
down as they leave the lined portion of the bore.
[0071] As described above and shown in Fig. 8, antennas 30 located at
the inner
diameter of the bore adjacent to the metamaterial liner 26 may be used to
produce and/or
detect excitations for MRI. The antennas produce the excitations at a working
frequency of
the MRI scanner which is typically the Larmor frequency for protons at the
magnetic field
strength in the bore. In order to keep the antenna from reducing the available
cross section of
the liner, the antenna may be for example loop shaped and positioned within a
range of radial
positions with respect to the bore that is also occupied by the liner, so that
the antenna does
not further reduce the available cross section of the bore. As shown, it can
be in a module 70
that is sized and shaped to be interchangeable with other modules containing
metamaterial
liner segments. The statement that modules are sized and shaped to be
interchangeable
means that the modules can be physically moved to swap positions, and does not
mean that
they have the same size and shape in dimensions that are inessential to this
swap of
positions, e.g. interior diameter.
[0072] Larger antennas may also be used and may still reduce
claustrophobia relative
to a birdcage coil. For an MRI with a working frequency below a cutoff
frequency of the
unlined bore, which is the case for typical MRI scanners, the antennas may be
positioned
close enough to the metamaterial liner that the liner that the bore at the
position of the
antenna has a passband including the working frequency. The antenna may also
be
positioned more distantly from the liner, so long as the evanescent waves of
the antenna
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substantially reach the liner to energize the liner with good efficiency.
Proximity to the liner
also helps smooth out the response of the excitations to the input from the
antenna. The
antennas could also be placed radially inward of the metamaterial segments or
between
annular metamaterial segments.
[0073] One can also have an adjustable length of lined bore with a fixed
antenna, by
having an antenna at one end of the lined length of the bore and adjusting the
other end of
the lined length of the bore, for example using modules 70 as described above.
Feed lines
may be connected to the antenna fixedly with fixed shielding. In an
embodiment, the lined
length of the bore may extend to the end of the bore at the end with the
antenna, and the
antenna may be positioned outside of the bore, alleviating the shielding
requirements for the
feed lines.
[0074] While conventional MRI uses surface coils within the bore to
detect NMR
signals, routing cables to surface coils in the bore in travelling wave MRI
can change the
propagation characteristics of the bore by introducing TEM modes. To deal with
this, the
cables can be shielded in any of the same ways as described above for feed
lines, or where
the lined portion extends to an end of the bore, an antenna may be placed at
the end of the
bore.
[0075] In the claims, the word "comprising" is used in its inclusive
sense and does
not exclude other elements being present. The indefinite articles "a" and "an"
before a claim
feature do not exclude more than one of the feature being present. Each one of
the individual
features described here may be used in one or more embodiments and is not, by
virtue only
of being described here, to be construed as essential to all embodiments as
defined by the
claims.
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