Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02676510 2014-04-01
COIL DECOUPL1NG FOR AN RF COIL ARRAY
This invention relates to an RF coil array for use in a magnetic
resonance system for detecting MR signals.
BACKGROUND OF THE INVENTION
The following relates to phased arrays of radio frequency (RF) coils for
magnetic resonance imaging, and will be described with particular reference
thereto.
It finds application more generally in conjunction with magnetic resonance
imaging,
magnetic resonance spectroscopy, and other magnetic resonance applications.
Many techniques increasingly employ arrays of surface coils to act as
a transmit and receive antenna. Each surface coil of the array typically
includes a
conductive RF loop, and required electronic components providing required
features
including frequency tuning the magnetic resonance frequency and matching to
required impedance (e.g. 50 ohm); a pre-amplifier for amplifying the received
signal
from the subject in the magnet; coil detuning during the transmit phase; coil
decoupling from adjacent and non-adjacent coil elements of the phased array
coils.
The required electronics are typically positioned close to the conductive RF
loop.
Normally, each element of the phased array coil includes the
conductive coil loop, a preamplifier decoupling network and a preamplifier,
typically
the preamplifier includes a transistor amplifier preceded by conditioning
circuit.
The standard preamplifier decoupling network consists of capacitors
and inductors which form a parallel resonant circuit with the conditioning
circuit of
the preamplifier and the output capacitor of the coil loop and blocks current
from
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flowing in the surface coil. This circuit de-couples the coil elements,
especially non-
adjacent elements. As is well known, adjacent coil elements are typically de-
coupled by different methods, including geometry, capacitive, and inductive de-
coupling, but it is not always possible to de-couple by these methods
therefore the
parallel resonant circuit with lower input impedance preamplifier in each coil
is
necessary to de-couple from non-adjacent coils of the array.
However, using the traditional decoupling techniques of the prior art,
either a good impedance matching with the pre-amplifier or a good decoupling
(high
decoupling impedance) may be achieved. However, simultaneous improvement of
both of these parameters is very difficult and often impossible to obtain.
Thus
improvement in one comes generally at a deterioration in the other leading to
unacceptable design compromises to obtain the best SNR available.
SUMMARY OF THE INVENTION
It is one object of the invention to provide an RF coil array which will
decrease the compromise between de-coupling and matching by providing a high
level of decoupling as well as an improved level of matching.
According to one aspect of the invention there is provided an RF coil
array for use in a magnetic resonance system for detecting MR signals from a
subject comprising:
a plurality of separate RF coil elements arranged in an array for
receiving RF signals from the subject, each of the coil elements having a
conductive
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loop with a plurality of capacitors and with a pair of ends across one of the
capacitors at which the signal is extracted;
at least one of the coil elements having a pre-amplifier circuit for
receiving the signal from the pair of ends of the coil element for
amplification of the
signal to submit to a signal analysis system;
the pre-amplifier circuit including a transistor and conditioning
components in advance of the transistor arranged to determine a required
impedance for the pre-amplifier circuit;
the conditioning components including an inductance and a
capacitance;
said at least one of the coil elements having a preamplifier decoupling
network connected across the pair of ends at said one of the capacitors for
generating a high impedance across the pair of ends so as to inhibit coupling
in the
coil from signals in adjacent and non-adjacent coils and coil elements of the
array;
wherein the decoupling circuit comprises a capacitance including said
one of the capacitors which co-operates with the inductance of the
conditioning
circuit components to form a parallel resonant circuit to generate said high
impedance;
and wherein the capacitance and the coil element are arranged with
impedance values selected to define an output impedance which substantially
matches the required impedance of the pre-amplifier circuit.
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Preferably the values of the capacitance and the inductance are
selected first such that the parallel resonant circuit maximizes the impedance
across
the pair of ends so as to inhibit coupling in the coil from signals in
adjacent and non-
adjacent coils of the array while allowing a certain range of difference
between the
output impedance of the coil element and the required impedance of the pre-
amplifier circuit.
The primary characteristic in design of the parallel resonant circuit
components is that the values produce maximized impedance, and these
component values also provides a matching impedance as close as possible to
that
of the required impedance (e.g. 50 Ohm) of the amplifier. The tradition design
of the
parallel resonance circuit uses a inductor between the coil element and the
preamplifier. The new design replaces the inductor with a trimmer capacitor.
This
change reduces the resistance and increases the Q factor of the parallel
resonant
circuit, thereby increasing the impedance without effecting the matching of
the coil
output to the required impedance of preamplifier.
The functions of the conditioning circuit element are as follows:
The conditioning circuit includes an inductor and capacitors to form a
network which transforms the output coil loop impedance at input of the
preamplifier
to the required impedance at the input of the transistor.
Preferably the conditioning circuit element includes an inductor
connected between lines from the ends of the coil carrying the signal to the
preamplifier and defining said inductance.
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According to a second aspect of the invention there is provided an RF
coil array for use in a magnetic resonance system for detecting MR signals
from
subject comprising:
a plurality of separate RF coil elements arranged in an array for
5 receiving RF signals from the subject, each of the coil elements having a
conductive
loop with a plurality of capacitors and with a pair of ends across one of the
capacitors at which the signal is extracted;
at least one of the separate coil elements having a pre-amplifier circuit
for receiving the signal from the pair of ends of the coil element for
amplification of
the signal to submit to a signal analysis system;
the pre-amplifier circuit including a transistor and a conditioning circuit
element in advance of the transistor arranged to determine a required
impedance for
the pre-amplifier circuit;
the conditioning circuit element including an inductance;
said at least one of the coils having a parallel resonant circuit across
the pair of ends at said one of the capacitors for generating a high impedance
across the pair of ends so as to inhibit coupling in the coil from signals in
adjacent,
non-adjacent coils and coil elements of the array,
wherein the parallel resonant circuit is defined by a capacitance
including said one of the capacitors and the inductance of the conditioning
circuit
element of the pre-amplifier circuit;
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and wherein an output impedance of the coil element is arranged to
substantially match the required impedance of the pre-amplifier circuit.
According to a third aspect of the invention there is provided an RF coil
array for use in a magnetic resonance image system for detecting MR signals
from a
subject comprising:
a plurality of separate RF coil elements arranged in an array for
receiving RF signals from the subject, each of the coil elements having a
conductive
loop with a pair of ends across which the signal is applied;
at least one of the separate coil elements having a pre-amplifier circuit
for receiving the signal from the pair of ends of the coil element for
amplification of
the signal to submit to a signal analysis system;
the pre-amplifier circuit including a transistor and a conditioning circuit
element in advance of the transistor arranged to determine a required
impedance for
the pre-amplifier circuit, the conditioning circuit element including an
inductor;
said at least one of the coil elements having in the coil elements a first
capacitor across which the pair of ends is connected and a second capacitor
connected between one of the ends and the input of the pre-amplifier circuit,
the first
and second capacitors and the inductor from the preamplifier forming
components of
a parallel resonant circuit for generating a high impedance across the pair of
ends so
as to inhibit coupling in the coil from signals in adjacent and non-adjacent
coil
elements of the array coil;
and wherein an output impedance of the coil element is arranged to
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substantially match the required impedance of the pre-amplifier circuit.
Preferably the values of capacitance and the inductance in the parallel
resonant circuit are selected such that the parallel resonant circuit
maximizes the
impedance across the pair of ends so as to inhibit coupling in the coil from
signals in
adjacent and non-adjacent coil elements of the array while the values selected
allow
some difference between the output impedance of the coil element and the input
impedance of the pre-amplifier circuit.
Preferably the inductor of the parallel resonance circuit from the
conditioning circuit of the preamplifier is connected between lines from the
ends of
the coil element carrying the signal to the preamplifier.
Preferably the parallel resonant circuit includes at least one further
capacitor in the conditioning circuit element of the pre-amplifier circuit.
Preferably the parallel resonant circuit consists solely of the first and
second capacitors, at least one further capacitor in the conditioning circuit
element of
the pre-amplifier circuit and the inductor in the conditioning circuit element
of the pre-
amplifier circuit.
Preferably said at least one further capacitor in the conditioning circuit
element of the pre-amplifier circuit is variable.
Preferably the second capacitor is variable.
Preferably the first capacitor is fixed.
Preferably the values of the first and second capacitors are selected
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such that the parallel resonant circuit maximizes the impedance across the
pair of
ends so as to inhibit coupling in the coil from signals in adjacent and non-
adjacent
coil elements of the array while allowing some difference between the output
impedance of the coil element and the input impedance of the pre-amplifier
circuit.
The new design therefore includes using the existing built in inductor of
the pre-amplifier conditioning circuit, which results in the pre-amplifier
being part of
the decoupling network. This new design improves both the matching and the
decoupling simultaneously thereby producing better signal to noise ratio
Using the built in inductor of the pre-amplifier simplifies the decoupling
network, increases the power of preamplifier decoupling, reduces the loss of
signal
by eliminating an external inductor, and improves coil matching, therefore
produce
better images.
The pre-amplifier decoupling network is conventionally complicated to
build and test. Using the preamplifier inductor simplifies the circuit
construction.
The matching and the pre-amplifier decoupling of the phased array coil
design is in conflict, where good decoupling would result in poor matching,
and
similarly, good matching would result in poor decoupling. This method produces
good decoupling and significantly improved matching, both of which produce
better
images. Using fewer components, especially the inductors, is very important
for the
phased array coil design (larger number channel array coil), due to the
limitation of
space inside the coil enclosure.
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The new design is more efficient and takes less space than traditional
arrangements for de-coupling adjacent and non-adjacent coil elements by
reducing
the resistance of parallel resonant circuit of preamplifier decoupling network
which
uses the built in inductor of preamplifier, eliminating number of components
and
improving the SNR. This efficiency and reduced space requirements are even
more
beneficial in a coil with larger number of elements (e.g. 32 or even more
Channel
coil), specifically in combined imaging using a multiple phased array coils.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described in conjunction
with the accompanying drawings in which:
Figure 1 is a schematic of one PRIOR ART coil element of a phased
coil array with preamplifier decoupling circuit where the remaining coils are
identical.
Figure 2 is a schematic of one coil element of a phased coil array with
preamplifier decoupling circuit according to the present invention.
In the drawings like characters of reference indicate corresponding
parts in the different figures.
DETAILED DESCRIPTION
In Figure 1 is shown a single coil of an RF phased coil array for use in
a magnetic resonance system for detecting NMR signals from a subject. The
array
includes a series of RF coil elements arranged in an array for receiving RF
signals
from the subject, each of the coil elements having a conductive loop 10 with a
pair of
ends 11 and 12 across which the signal is applied. The coil includes a number
of
=
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components in series as indicated at 13, 14 and 15 arranged for frequency
tuning to
the magnetic resonance frequency. The components 13, 14 and 15 are well known
to persons skilled in the art of coil design so that further description is
not required.
The coil element has a pre-amplifier circuit 16 for receiving a signal
5 from the pair of ends 11 and 12 of the coil element 10 for amplification
of the signal
to submit to a signal analysis system.
There is also provided a preamplifier decoupling circuit 18 which is
arranged to provides a high impedance across the ends 11 and 12 to inhibit
coupling
between the adjacent and non-adjacent coils. The preamplifier decoupling
network
10 consists of capacitors, inductor 17 and preamplifier arranged to form a
parallel
resonant circuit which is tuned to the magnetic resonance frequency and
generates
a very high impedance in the coil element which is seen by adjacent and non-
adjacent coil elements so as to reduce as far as possible the coupling in this
coil
element from signals in the adjacent and non-adjacent coil elements. In
addition the
preamplifier decoupling network is arranged to provide an output impedance of
the
coil circuit is matched as closely as possible to the required impedance of
the pre-
amplifier circuit. As is well known an optimum matching of the impedance
maximizes the SNR.
Turning now to Figure 2, the arrangement of the present invention is
shown. The pre-amplifier can be of the type manufactured by any suitable
preamplifier manufacturer such as SIEMENS (trade mark) and comprises an
amplifier 20 defined by transistors such as FET transistor and a conditioning
circuit
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element 21 in advance of the transistor 20 arranged to determine a required
impedance for the pre-amplifier circuit.
The conditioning circuit element includes a capacitor C2 and an
inductor L.
The decoupling circuit element 18 connected across the pair of ends
11 and 12 is arranged for generating a high impedance across the pair of ends
11,
12 so as to inhibit coupling in the coil from signals in adjacent and non-
adjacent coils
of the array. The decoupling circuit element 18 comprises a capacitance
defined by
a first capacitor C3 connected across the ends 11 and 12, a capacitor Cl and
the
conditioning circuit of the preamplifier. The capacitance defined by the
capacitors
Cl and C3 co-operates with the inductance of the conditioning circuit element
defined by the capacitor C2 and inductor L to form a parallel resonant circuit
to
generate the required high impedance.
The capacitance and the coil element are arranged with impedance
values arranged to define an output impedance which substantially matches the
required impedance of the pre-amplifier circuit.
The values of the capacitance and the inductance are selected such
that the parallel resonant circuit maximizes the impedance across the pair of
ends so
as to inhibit coupling in the coil from signals in adjacent and non-adjacent
coil
elements of the array while allowing some difference between the output
impedance
of the coil element and the required impedance of the pre-amplifier circuit.
That is
the value of the capacitance and the inductance of the parallel resonance
circuit is
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selected to work with the preamplifier to produce attenuation larger than
20dB.
The pre-amplifier circuit including the transistor and the conditioning
,
circuit element is defined as a separate component carried on a circuit board
to
which the coil and the decoupling circuit element are attached. Any suitable
preamplifier, such as FET preamplifier with the conditioning circuit, is
acceptable.
The parallel resonant circuit is thus defined by the capacitance and the
inductance of the conditioning circuit element of the pre-amplifier circuit.
The first
and second capacitors and the inductor thus form components of the parallel
resonant circuit for generating the high impedance across the pair of ends so
as to
inhibit coupling in the coil from signals in adjacent and non-adjacent coil
elements of
the array.
The capacitor C2 in the conditioning circuit element of the pre-amplifier
circuit is variable. The second capacitor Cl also is variable. The first
capacitor C3
is fixed.
,
'
