U.S. patent application number 13/856562 was filed with the patent office on 2013-08-29 for magnetic resonance signal detection using remotely positioned receive coils.
The applicant listed for this patent is IMRIS INC.. Invention is credited to Mehran Fallah-Rad, Michael Lang, Labros Petropoulos, John Saunders, Wayne Schellekens, Haoqin Zhu.
Application Number | 20130221966 13/856562 |
Document ID | / |
Family ID | 47020810 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130221966 |
Kind Code |
A1 |
Zhu; Haoqin ; et
al. |
August 29, 2013 |
Magnetic Resonance Signal Detection Using Remotely Positioned
Receive Coils
Abstract
The receive coil arrangement includes an inner local volume coil
adjacent the part to be imaged so as to maximize the received MR
signal and an outer coil, which may be the built in body coil of
the magnet, connected by cable to the signal processing system.
Both the coils are individually tuned to the common resonant
frequency and the local volume coil include an arrangement to halt
current flow therein during the transmit stage. The local volume
coil has no cable and is arranged to communicate the MR signal
therein to the signal processing system through the outer coil by
inductive coupling to the outer coil. Despite inherent losses by
interfering with the tuning of the loops and in the inductive
coupling this magnifies the MR signal and makes the local volume
coil wireless.
Inventors: |
Zhu; Haoqin; (Winnipeg,
CA) ; Petropoulos; Labros; (Winnipeg, CA) ;
Schellekens; Wayne; (Winnipeg, CA) ; Saunders;
John; (Winnipeg, CA) ; Lang; Michael;
(Winnipeg, CA) ; Fallah-Rad; Mehran; (Winnipeg,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMRIS INC.; |
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US |
|
|
Family ID: |
47020810 |
Appl. No.: |
13/856562 |
Filed: |
April 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13231004 |
Sep 13, 2011 |
8487615 |
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13856562 |
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13090816 |
Apr 20, 2011 |
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13231004 |
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Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/3635 20130101;
G01R 33/3642 20130101; G01R 33/34076 20130101; G01R 33/3692
20130101; G01R 33/34 20130101; G01R 33/34007 20130101; G01R
33/34084 20130101; G01R 33/3664 20130101; G01R 33/3657 20130101;
G01R 33/3415 20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01R 33/34 20060101
G01R033/34 |
Claims
1. Apparatus for MR imaging of a subject comprising a MR magnet
with gradient coil operable to generate a variable magnetic field
to be applied to the subject; an RF transmit arrangement for
generating an RF pulse in a transmit stage to be applied to the
subject to be imaged such that the subject generates an MR signal
in response to the magnetic field and the RF pulse applied; a
receive coil arrangement for acquiring the MR signal in a receive
stage; and a signal processing system for receiving the MR signal
for carrying out signal processing by which an image is generated;
the receive coil arrangement comprising: a birdcage coil configured
to surround the subject so as to receive the MR signal; at least
one receive coil; said at least one receive coil having at least
one signal communication cable connected to the signal processing
system for transferring the MR signal therein to the signal
processing system; said at least one receive coil and said birdcage
coil being individually tuned to a common resonant frequency for
receiving said MR signal; all coil loops of said birdcage coil and
said at least one receive coil which act only in the receive stage
and do not transmit the applied RF pulse in the transmit stage
having therein an arrangement to halt current flow therein at the
resonant frequency during the transmit stage so as to prevent the
presence of said all coil loops from interfering with the RF pulse
during the transmit stage; said birdcage coil being arranged to
communicate the MR signal therein to the signal processing system
through said at least one receive coil by inducing the MR signal
onto said at least one receive coil.
2. The apparatus according to claim 1 wherein said at least one
receive coil is located at a spacing from said birdcage coil such
that: the signal from said birdcage coil is induced onto said at
least one receive coil at an efficiency of induction sufficient
that the MR signal on said at least one receive coil is greater
than the MR signal which would be generated in the absence of said
birdcage coil; and mutual inductance between said birdcage coil and
said at least one receive coil is insufficient to change the tuned
common resonant frequency of the volume coil and the receive coil
sufficiently to reduce the MR signal at said at least one receive
coil to a value which is less than the MR signal which would be
generated in the absence of said birdcage coil.
3. The apparatus according to claim 1 wherein said birdcage coil is
free from a wired cable carrying the MR signal to the signal
processing system.
4. The apparatus according to claim 1 wherein said birdcage coil is
arranged to be located as close as physically possible to the
subject.
5. The apparatus according to claim 1 wherein said birdcage coil
includes a plurality of loops and each loop includes an addressable
switch operable remotely to halt flow of current in the loop so
that each loop can be activated in turn.
6. The apparatus according to claim 1 wherein the signal processing
system includes a plurality of channels for individual processing
of separate MR signals and wherein there is provided an arrangement
for generating the separate MR signals for the separate channels
from the signal induced onto said at least one receive coil.
7. The apparatus according to claim 6 wherein said birdcage coil
includes a plurality of separate first loops and wherein each first
loop includes an addressable switch operable remotely to halt flow
of current in the first loop so that each first loop can be
activated in turn, wherein said at least one receive coil comprises
a single second loop and wherein there is provided a signal
dividing system arranged to receive the signal from the single
second loop and to calculate the separate MR signals for the
separate channels from the signal induced onto said single second
loop.
8. The apparatus according to claim 6 wherein said at least one
receive coil comprises a plurality of separate loops each providing
a signal to a respective one of the channels.
9. The apparatus according to claim 1 wherein the arrangement to
halt current flow in the loops comprises an arrangement to
temporarily de-tune the loop from the resonant frequency.
10. The apparatus according to claim 1 wherein said at least one
receive coil comprises a body coil carried on to the magnet.
Description
[0001] This application is a divisional application of application
Ser. No. 13/231,004 filed Sep. 13, 2011 which is a continuation in
part application of application Ser. No. 13/090,816 filed Apr. 20,
2011.
[0002] This invention relates to an RF receive coil assembly for MR
imaging.
BACKGROUND OF THE INVENTION
[0003] As is well known, MR imaging uses an RF receive coil to
receive the signals emitted by the subject under test in response
to excitation of a selected volume of the subject which is
generated by a RF transmit coil, such as the built in body coil.
Thus the Gradient coils generate controlled variations in the main
magnetic field (B0) magnetic field to produce selected spatial
excitation volume and the signal emitted by that selected volume is
picked up by the receive coil arrangement and transmitted to a
signal processing system.
[0004] The receive coil arrangement can comprises a single coil
loop or element or it can include a series of loops arranged in a
pattern around the part of the subject to be imaged.
[0005] MR systems provide a built in body coil in the magnet
construction and this can operate as both the transmit coil and the
receive coil.
[0006] However in some cases the body coil does not provide an
image of sufficient quality to meet the requirements and hence
local coils must be used. These are typically volume coils which
are configured to at least partially or completely surround the
region of interest of the subject so as to receive the MR signal
and include a plurality of connected conductors.
[0007] Some current volume coils consist of coil loops, phased
array, birdcage, TEM, all of which could be single frequency or
dual frequency coils. These require matching networks,
preamplifiers, decoupling networks, cables and connectors.
[0008] There are a number of challenges with the current standard
volume coil designs:
[0009] a) The number of channels is limited to the number of
receivers in the system.
[0010] b) A large diameter cable bundle, such as an eighteen
channel phased array coil which require 18 channel cables,
containing 18 coaxial cables and at least 25 control wires, would
be much too large to enable construction of the conventional cable
trap in the cable.
[0011] c) It is difficult to build because the electrical
components, such as the circuit board baluns and preamps, are
complicated and time consuming to assemble by a skilled and
experienced technician. These components require significant effort
during design and construction to produce high quality images and
to reduce the crosstalk between components.
[0012] d) The required mechanical components, such as the long
cables, cable traps, and connector interface also increase the
overall size and weight of the coil.
[0013] e) The large size and weight of the coils increases
complexity of workflow for customer and complexity of the workflow
design.
[0014] f) Long cables are heavy and cumbersome to position.
[0015] g) There are patient positioning and surgical access issues
due to the inflexibility of the current design and the
ever-changing surgical requirements and surgeon's preferences.
[0016] h) Coil cables have the possibility of patient burns
resulting from skin-to-cable contact, resulting in increased space
between cables, magnet bore and patient. This provides less in less
patient space for nursing staff to properly position the patient
before scanning.
[0017] i) In an inter-operative suite, there are safety issues
related to OR staff forgetting to unplug the coils and increased OR
workflow due to the additional patient safety checkpoint.
[0018] Normally, each individual loop or loops of the MRI receive
coil arrangement are connected to a single receiver of the signal
processing system via preamplifier and other components with a
cable.
[0019] Such receive coil arrangements can therefore use the so
called "built in body coil" carried on the magnet as receive coil
which is connected by cable to the signal processing system. In
this case the so called "built in body coil" is also used as
transmit coil
[0020] Such receive coil arrangements can therefore comprise a
single loop which is connected by a single wire to a single channel
of the signal processing system. In this case the system can use
the so called "built in body coil" carried on the magnet as
transmit coil. This signal loop receive coil then supplies the
received signal collected around the subject, typically a lying
patient, and communicates it to the single channel for processing
using conventional systems well known to persons in this art.
[0021] Such receive coil arrangements can therefore comprise a
multiple loop arrangement including a so-called "phased array" of
loops each of which is connected by a respective wire to a separate
one of a plurality of channels of the signal processing system.
[0022] In this case the system typically uses a portable coil
assembly arranged to wrap around the body part of the patient but
each loop must have its own set of processing components and its
own wire connecting the signal to the separate channel for
processing.
[0023] However in recent developments not yet widely adopted, the
"built in body coil" carried on the magnet as the receive coil
arrangement is separated into individual loop components for
supplying a separate signal to the separate channels.
[0024] It is well known that there are parallel imaging techniques
to reduce the time necessary to obtain a complete scan of the part
of the patient by using the signals from the separate channels to
carry out various calculations and extrapolations, thus avoiding
the necessity to obtain image results at each location in the image
space or in K-space. Some of these parallel imaging techniques are
known as SMASH and SENSE and GRAPPA.
[0025] To obtain better images, the preamplifiers are located as
close to the coil elements as possible. Although the size of MR
preamplifier is greatly reduced recently, it still takes much space
of overall array coil. In addition the area of coil enclosure at
preamplifier must be rigid.
[0026] The coil cable, as is well known, consists of multi-coaxial
cable and signal control wires and outer shield. Common mode
current or shield current will be generated on outer surface of the
shield during transmit phase by the high RF field generated by the
transmit coil. To prevent the patient from being overheated
dangerously by shield current, cable traps are required for the
coil cable assembly. Longer cable with more cable traps is required
for the clinic applications, such as intra-operative MR imaging on
a moving magnetic system.
SUMMARY OF THE INVENTION
[0027] According to the invention there is provided an apparatus
for MR imaging of a subject comprising
[0028] a MR magnet with gradient coil operable to generate a
variable magnetic field to be applied to the subject;
[0029] an RF transmit arrangement for generating an RF pulse in a
transmit stage to be applied to the subject to be imaged such that
the subject generates an MR signal in response to the magnetic
field and the RF pulse applied;
[0030] a receive coil arrangement for acquiring the MR signal in a
receive stage;
[0031] and a signal processing system for receiving the MR signal
for carrying out signal processing by which an image is
generated;
[0032] the receive coil arrangement comprising: [0033] a birdcage
coil configured to surround the subject so as to receive the MR
signal; [0034] at least one receive coil; [0035] said at least one
receive coil having at least one signal communication cable
connected to the signal processing system for transferring the MR
signal therein to the signal processing system; [0036] said at
least one receive coil and said birdcage coil being individually
tuned to a common resonant frequency for receiving said MR signal;
[0037] all coil loops of said birdcage coil and said at least one
receive coil which act only in the receive stage and do not
transmit the applied RE pulse in the transmit stage having therein
an arrangement to halt current flow therein at the resonant
frequency during the transmit stage so as to prevent the presence
of said all coil loops from interfering with the RF pulse during
the transmit stage; [0038] said birdcage coil being arranged to
communicate the MR signal therein to the signal processing system
through said at least one receive coil by inducing the MR signal
onto said at least one receive coil.
[0039] Preferably the birdcage coil is therefore wholly free from a
wired cable carrying the MR signal to the signal processing
system.
[0040] Preferably the birdcage coil is arranged to be located as
close as physically possible to the subject.
[0041] In one arrangement the birdcage coil can be arranged to be
located inside the body of a patient forming the subject.
[0042] Preferably the at least one receive coil is located at a
spacing from said birdcage coil such that firstly the signal from
said birdcage coil is induced onto said at least one receive coil
at an efficiency of mutual induction sufficient that that the MR
signal on said at least one receive coil is greater than the MR
signal which would be generated in the absence of said birdcage
coil; and secondly mutual inductance between said birdcage coil and
said at least one receive coil is insufficient to change the tuned
common resonant frequency of the birdcage coil and the receive coif
sufficiently to reduce the MR signal at said at least one receive
coil to a value which is less than the MR signal which would be
generated in the absence of said birdcage coil.
[0043] That is the receive coil should be spaced sufficiently far
from the birdcage coil so that the MR signal of the subject
received by the birdcage is transmitted through mutual inductance
to the receive coil and this space is also sufficiently large to
not significantly detune both the birdcage coil and the receive
coil, and therefore not effect either coil performance.
[0044] Preferably the birdcage coil includes a plurality of loops
and each loop includes an addressable switch operable remotely to
halt flow of current in the loop so that each loop can be activated
in turn. This is called active decoupling, needing a control signal
to turn the coil on and off.
[0045] Preferably the birdcage coil includes a plurality of loops
and each loop includes a passive decoupling circuit to halt the
current in the loop during transmit stage and automatically
activated during receive stage. This is called passive decoupling,
which does not need a control signal and can be switched on and off
automatically by body coil. When the body coil transmits, the
birdcage coil is off, and when the body coil receives the birdcage
coil is on.
[0046] Preferably the signal processing system includes a plurality
of channels for individual processing of separate MR signals and
wherein there is provided an arrangement for generating the
separate MR signals for the separate channels from the signal
induced onto said at least one receive coil.
[0047] For this purpose the birdcage coil can include a plurality
of separate first loops wherein each first loop includes an
addressable switch operable remotely to halt flow of current in the
first loop so that each first loop can be activated in turn, and
the receive coil comprises a single second loop. There is then
provided a signal dividing system arranged to receive the signal
from the single second loop and to calculate the separate MR
signals for the separate channels from the signal induced onto the
single second loop.
[0048] Alternatively the receive coil comprises a plurality of
separate loops each providing a signal to a respective one of the
channels.
[0049] Preferably the arrangement to halt current flow in the loops
comprises an arrangement to temporarily de-tune the loop from the
resonant frequency.
[0050] The arrangement to halt current flow therein at the resonant
frequency during the transmit stage can be both active and passive,
or active only, or passive only.
[0051] The term "loop" herein is used for one component or element
of a complex receive coil arrangement and this term is not intended
to limit the shape or structure of the individual elements defined
by this term. Typically each loop is a single loop with a
conductive wire or other conductive material so that current flows
around the loop in response to the signal. Different materials can
be used for the conductive material and certainly the terms used
herein are not limited to specific materials which can be used.
[0052] For example such a "loop" can be formed by a complex volume
coil which surrounds a part to be imaged.
[0053] The intention in the above arrangement is that said the
first coil is free from a wired cable carrying the MR signal to the
signal processing system. This can provide a number of significant
advantages.
[0054] The arrangement provided herein therefore consists of a
cable-less volume coil, which works by coupling with the built in
body coil of the MR magnet. This volume coil does not have as many
components as a conventional MR imaging coil. The design can be
defined by a birdcage resonator and is used as a volumetric
coil.
[0055] This arrangement can provide one or more of the following
features and advantages:
[0056] a) Inductive volume coils can achieve equal or better images
compared with commercial phased array volume coils. The coils
herein can provide highly uniform images with good SNR numbers;
[0057] b) there is no limitation to the number of channel
regardless of the number of receivers in the system.
[0058] c) No cables with external cable traps are required to
connect the coil to the system.
[0059] d) It is significantly easier to build as the coil contains
only passive elements.
[0060] e) There is no need for internal baluns, preamps, connection
cables, cable traps, or external connector blocks, or extension
cables.
[0061] f) The coil has smaller physical dimensions (size, weight)
compared with similar (same field of view) phased array volume
coil.
[0062] g) the cable-less volume coil can improved hospital
workflow.
[0063] h) patient positioning and surgical access is significantly
improved.
[0064] j) The possibility of patient burns resulting from patient
skin-to-coil cable contact is completely eliminated.
[0065] k) Increased patient safety.
[0066] l) Passive decoupling is provided for eliminating crosstalk
between the inductive wireless coils to the built-in body coil
during the transmit phase. Therefore, B1 distortion, coil heat and
image non uniformity caused by B1 distortion is eliminated. B1 is
RF field generated by the built in body coil.
[0067] Many different arrangements of the cable-less volume coil
can be provided. These include: [0068] Standard style birdcage
include high pass, low pass and band pass volume coils. [0069] Half
birdcage volume coil, either top half, bottom half, or both halves
working together. [0070] Spiral style birdcage [0071] Split Volume
style [0072] Head and neck combination coil [0073] Radiolucent
Volume coil [0074] Dual frequency for either 1.5T or 3T, and
multiple nuclei imaging and spectroscopy [0075] iPAT style fast
imaging
[0076] Both 1.5T and 3T coil imaging is comparable to the existing
commercial phased array Head Coil and provide very good image
uniformity and high SNR.
[0077] A number of possible arrangements can be used within this
broad definition.
[0078] Firstly the second coil can be a built in body coil carried
on the magnet. Such body coils are typically available on magnet
systems.
[0079] The second coil or body coil can act as the transmit coil or
another dedicated coil can be separately used.
[0080] There can be only two coils using the inductive coupling to
transfer the signal to the processor or there may be a stack of
three coils or even more.
[0081] In this arrangement, the first coil can be located inside
the body of a patient and the second coil is arranged outside the
body of the patient. Typically in this arrangement, the second coil
is as close as possible to the exterior of the patient and this in
turns communicates inductively to the body coil (or other coil)
around the patient.
[0082] The first coil is arranged to be located as close as
physically possible to the subject and the second coil is arranged
to be located at a position spaced from the subject greater than
that of the first coil so as to receive the signal inductively and
transfer it to the processing unit.
[0083] The arrangement herein is predicated on the discovery that
providing a first coil as close as possible to the part to be
imaged and covering as small a volume as possible generates a
signal which has significantly greater signal to noise ratio than a
second coil located at a spacing from the part. Then the signal
picked up by the first coil is communicated inductively to the
second coil even though there are significant losses in so doing.
It has been found that the signal from the first coil is induced
onto said at least one second loop at an efficiency of induction
(less than 100%) sufficient that that the MR signal on second coil
is greater than the MR signal which would be generated in the
absence of the first coil. This includes the possibility of a
catheter coil being used which increases the signal to the surface
coil. That is there is a magnifying effect by providing the first
coil close to the subject and then communicating the signal to the
second coil despite the losses in the inductive coupling.
[0084] Another issue which arises is that mutual inductance between
the coils can change the tuned common resonant frequency of the
loops to reduce the MR signal unacceptably. Typically therefore it
would be considered that the problems of mutual inductance changing
the tuned frequency would at least balance and more likely outweigh
the advantages obtained by providing the additional first coil.
However this has been found not to be so. Provided the distances
are carefully managed by experiments to determine what distances
provide an advantage without adversely affecting the tuning to a
situation where the MR signal is at a value which is less than the
MR signal which would be generated in the absence of said at least
one first loop, significant advantages can be obtained.
[0085] One issue which arises and is addressed herein is that of
how to generate separate signals for separate channels of the
signal processing unit in order to take advantage of the high speed
imaging which can be obtained by using parallel channels such as by
SENSE or SMASH or other more recent techniques. Preferably each
loop includes an addressable switch operable remotely to halt flow
of current in the loop so that each loop can be activated in
turn.
[0086] In a first embodiment to overcome this difficulty, the first
coil includes a plurality of separate loops and there is provided
an arrangement for generating the separate MR signal for the
separate channels from the signal induced onto the second coil.
[0087] In one arrangement, each first loop includes an addressable
switch operable remotely to halt flow of current in the first loop.
In this way each first loop can be activated in turn. In this
arrangement using conventional MRI equipment where the body coil
has a single output. In this arrangement, the individual element
sensitivity profiles can be obtained to perform parallel imaging. A
signal processing system is arranged to receive the signal from the
single channel, and along with the sensitivity profiles will
separate the combined single channel into its individual elements
for processing by the scanner. The individual signals from coils
can be determined by measuring what are known as the Sensitivity
Profile and Noise Correlation Matrix of the coil using those
factors to determine the individual signals for the separate
channels. In this arrangement, the sensitivity profile and possibly
Noise Correlation Matrix of the single second coil can be
determined by operating the switch to turn off each of the first
coils. After this is determined, the sensitivity profile and Noise
Correlation Matrix of each of the first loops can be determined by
activating only each one in turn with the others turned off and
then by subtracting the signal obtained from single second coil
from the total signal obtained by the second coil and the activated
one of the first loops. The Sensitivity Profile and possibly the
Noise Correlation Matrix are then used to determine from the single
output of the single second receive coil the required individual
signals required for the separate channels of the processing
system. For the parallel imaging, a base image is obtained with RF
body coil only. Utilizing the switching of the individual loops, an
image for each of the inductive loops is obtained in succession as
well as any possible combination of them. Thus, by a subtraction of
images from the body coil base image, a picture of the sensitivity
fields and correlation matrices between coils is obtained. Once
this arrangement is obtained an under sampling during the parallel
imaging can be unfolded. This technique can be extended in space
and time domain as well with methods like GRAPPA.
[0088] In a second arrangement applicable to arrangements with a
body coil which has separate loops connected to separate channels,
the arrangement of the body coil has been found to provide the
required signal to each respective one of the channels.
[0089] In accordance with another important aspect of the
invention, the coil is provided with a switch which acts to
deactivate the coil after a period of time. Thus the switch can be
moved to open circuit when a time period after first activation has
elapsed. In this way, the active life of the coil can be
controlled. This can be limited for example to a number of hours so
that the coil is a one time use product. Thus the switch is
activated on receipt of the first RF pulse and then has a timing
circuit which times out to operate the switch to open circuit
preventing further ruse of the coil assembly. In another
arrangement, the switch may act in response to sterilization so
that it allows a certain number of sterilizing actions before
moving to open circuit. In yet another arrangement, the total
allowable lifetime of the coil can be predetermined by the
manufacturer and then actively enforced against users who may try
to use the product beyond its life. This arrangement allows the
coil to be a one time use product requiring it to be discarded
after the one time use with this protocol being fully enforced
against users wanting to ignore it.
[0090] In order to make the product disposable, components can be
provided to control the operation of the loops which avoids the use
of higher cost components such as transistors and variable
elements. This can be achieved by using de-tuning of the coil to
switch the coil when it is not required to respond to the RF
signal. Thus de-tuning of the coil to a resonant frequency
sufficiently different from the RF frequency is equivalent or
achieves the same result as switching the loop to open circuit.
This can be achieved in many ways and in particular by moving of a
cooperating coil to a position close to the coil to change the
tuning.
[0091] In order to ensure the separate loops are de-coupled so as
to avoid interfering with the resonant tuning, conventional
de-coupling techniques can be used including geometric arrangements
of the loops, capacitive de-coupling, inductive decoupling and the
use of a separate additional loop which acts to inductively couple
between two of the separate loops to provide the necessary current
cancelling actions necessary to provide the de-coupling between the
two separate loops. All of these techniques are known to persons
skilled in the art.
[0092] The coil size (with built in preamplifiers) and cable are
the primary issues that affect coil performance, workflow,
sterilization and safety. This new design described herein can
greatly improve coil performance, workflow, sterilization and
safety, since it does not include any of these components.
[0093] In the arrangement where the first coil is a phased array
including a plurality of separate loops, one or more loops of the
phased array coil are without preamplifiers and no cables, no
physical connection to the scanner, thus providing a so called
"wireless coil". These wireless coil elements are resonators and
tuned at MR scanner working frequency. These wireless coil elements
or loops are decoupled from each other using conventional
techniques by coil loop overlap, capacitive techniques including
shared conductor, inductive and geometry (such as quadrature)
methods. These wireless coil elements can be transverse
electromagnetic (TEM) coil and receive only coils with good
decoupling between coil elements by using current technology
without cable and preamplifier.
[0094] These wireless coil elements are inductively coupled in the
receive stage to the built in RF body coil. In a multiple system
using additional coils, these wireless coils can couple with each
other in a successive manner to larger and/or smaller coils that
consequently couple to the built in RF body coil. These coils are
passively detuned from the Transmit portion of the TX/RX Whole Body
RF coil or other transmit coil during the transmit stage.
[0095] The frequency of operation covers the entire spectrum of RF.
The wireless coil elements combination can be inductive coupled
multi loops along the magnet axis or off axis.
[0096] The coil elements are passively decoupled from transmit coil
during the transmit stage. The transmit coil can be the built in
body coil in the MR scanner or can be a local transmit coil or
transmit phased array. Or a transceiver coil can work with a multi
transmitter system. The wireless coil elements size can be as large
as head or body coil and as small as intra-cardiac coil (diameter
<10 mm).
[0097] The sensitivity of the wireless coil elements can be
adjusted by detune, insert impedance and other methods to eliminate
coil crosstalk and optimize signal to noise ratio.
[0098] The distance between wireless coil elements and pickup coils
can be adjusted for optimized SNR bearing in mind the competing
requirements of reducing mutual inductance to prevent de-tuning and
maximizing signal transfer efficiency.
[0099] The distance between wireless coil elements and subject to
be imaged can be adjusted for optimized SNR bearing in mind the
competing requirements of reducing load and keep the Q factor
higher of each coil elements, so that each coil element can get the
maximum MR signal from the subject to be imaged.
[0100] The arrangement described herein may have one or more of the
following advantages or features:
[0101] The wireless coil elements can be rigid, flexible or any
combination
[0102] The wireless coil elements can be sterilized, reusable,
limited reusable and disposable. The wireless coil can be shaped to
match required operations such as with openings at the required
locations of the coil.
[0103] Can be manufactured very cheaply so as to be disposable.
[0104] Have no wires so that they can be left in place during the
whole procedure such as intra-operative neurosurgery.
[0105] Very flexible so can conform more accurately to the patient
body.
[0106] These wireless coil elements can be made radiolucent for use
with X-ray or radiation treatment using such techniques as aluminum
coil material and the material of atomic number Z.ltoreq.30 with or
without Gold and silver plate.
[0107] The geometry of these wireless coil elements can be any
shape such as, but not limited to: round, rectangular, butterfly,
microstrip-based coil, or microstrip transmit line (MTL coil),
birdcage, half birdcage and other volume coil.
[0108] In arrangements where the magnet is movable for
intra-operative procedures, as there are no cables, the first coil
can be left in place.
[0109] In such arrangements the cables are typically very long so
that the absence of a cable at all is of increased advantage.
[0110] As the first coil can be much smaller, the signal to noise
ratio which is dependent on the amount of noise generated within
the volume of the coil is much reduced.
[0111] The signals generated by the first close coil and by the
more remote second coil are added at the second coil since the
second coil remains responsive to the signal directly from the
subject. While this in many cases has been found not to be a large
effect, it still adds the quality of the resultant signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0112] One embodiment of the invention will now be described in
conjunction with the accompanying drawings in which:
[0113] FIG. 1 is a schematic illustration of an MRI system
including a first embodiment of the present invention.
[0114] FIG. 2 is a schematic illustration of an MRI system
including a second embodiment of the present invention.
[0115] FIG. 3 is a schematic illustration of the head coil of FIG.
2 along the lines 3-3.
[0116] FIG. 4 is a schematic plan illustration of a top flexible
portion of the head coil of FIG. 2.
[0117] FIG. 5 is a schematic illustration of an MRI system
including a third embodiment of the present invention.
[0118] FIG. 6 is a schematic illustration of an MRI system
including a fourth embodiment of the present invention.
[0119] FIG. 7 is a schematic illustration of an MRI system
including a fifth embodiment of the present invention.
[0120] FIG. 8 is a schematic illustration of an MRI system
including a sixth embodiment of the present invention.
[0121] FIG. 9 is a schematic illustration of a first arrangement of
the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a high pass birdcage coil.
[0122] FIG. 10 is a schematic illustration of an arrangement of the
volume coil for use in the construction of FIG. 1, where the volume
coil comprises a low pass birdcage coil.
[0123] FIG. 11 is a schematic illustration of a first arrangement
of the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a band pass birdcage coil.
[0124] FIG. 12 is a schematic illustration of a first arrangement
of the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a high pass birdcage coil constructed for
dual frequency.
[0125] FIG. 13 is a schematic illustration of a first arrangement
of the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a high pass birdcage coil with dual rungs
between capacitors.
[0126] FIG. 14 is a schematic illustration of an arrangement of the
volume coil for use in the construction of FIG. 1, where the volume
coil comprises a high pass birdcage coil of the Litzcage type.
[0127] FIG. 15 is a schematic illustration of an arrangement of the
volume coil for use in the construction of FIG. 1, where the volume
coil comprises a high pass birdcage coil designed for use on the
head and neck of the patient.
[0128] FIG. 16 is a schematic illustration of an arrangement of the
volume coil for use in the construction of FIG. 1, where the volume
coil comprises a half birdcage coil.
[0129] FIG. 17 is a schematic illustration of a first arrangement
of the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a birdcage coil of the type formed by two
halves which are split.
[0130] FIG. 18 is a schematic illustration of a first arrangement
of the volume coil for use in the construction of FIG. 1, where the
volume coil comprises a birdcage coil of the Alderman-Grant
type.
[0131] In the drawings like characters of reference indicate
corresponding parts in the different figures.
DETAILED DESCRIPTION
[0132] The apparatus for MR imaging of a subject includes a
conventional cylindrical MR magnet 10 operable by a field control
system to generate a variable magnetic field to be applied to the
subject.
[0133] The MR system includes an RF transmit arrangement 12 for
generating RF pulses in a transmit stage to be applied to the
subject to be imaged and a receive arrangement for acquiring the MR
signal in a receive stage with a signal processing system 13 for
receiving the MR signal for carrying out signal processing by which
an image is generated. As is well known, the subject generates an
MR signal in response to the magnetic field and the RF signal
applied which is detected and processed to generate an image. The
arrangement is well known and a suitable system is available from
Siemens.
[0134] Typically the magnet 10 carries an RF coil known as a body
coil 14 which is mounted on the cylindrical magnet housing so as to
surround the patient. This is usually used as the transmit coil.
However separate transmit can be used. The body coil can also
operate as the receive coil. However again separate receive coils
can be used. The transmit and receive coils can be the same coils
or can be provided by separate coils.
[0135] In the first embodiment shown in FIG. 1, the transmit coil
is defined by the body coil 14. The receive coil arrangement
comprises an innermost coil loop 15 located inside the body of the
patient. This is inserted by a suitable support 15A which moves the
coil 15 to the required location within the patient, for example
within the heart or other organ to be imaged. The receive coil
arrangement further comprises a first outer coil 16 located
adjacent to the innermost coil 15 but outside the body. The coil 16
can be formed by a single loop but more preferably by a phased
array of loops. The receive coil arrangement further comprises a
second outer coil defined by the body coil 14 surrounding the coil
16. In other arrangements a separate coil can be used for the
second outer coil. In any case, the second outer coil has a signal
communication cable 14A connected to the signal processing system
13 for transferring the MR signal therein to the signal processing
system.
[0136] In FIG. 4, a plan view is provided on the coil 16 which in
this arrangement is formed by a phased array of loops 16A, 16B, 16C
etc. All of the loops of the coils 15, 16 and 14 are individually
tuned by a tuning component such as capacitors schematically
indicated at 17 to a common resonant frequency for receiving said
MR signal using conventional tuning devices well known to a person
skilled in the art.
[0137] All of the coil loops of the coils 15 and 16 which act only
in the receive stage and do not transmit the applied RF pulses in
the transmit stage have therein an arrangement schematically
indicated at 18, such as a passive block circuit with capacitors,
inductor and pin diodes, to halt current flow therein during the
transmit stage so as to prevent the presence of said all coil loops
from interfering with the RF pulses during the transmit stage.
Devices of this type are known so that explanation of the operation
is not necessary.
[0138] The loop of the innermost coil 15 is arranged to communicate
the MR signal therein to the signal processing system through the
loops of the first outer coil 16 and through the second outer coil
14 by inducing the MR signal onto the coil 15 and therefrom onto
the coil 14.
[0139] The intention in the above arrangement is that said the
coils 15 and 16 are free from a wired cable carrying the MR signal
to the signal processing system.
[0140] Typically in this arrangement, the coil 16 is as close as
possible to the exterior of the patient and this in turns
communicates inductively to the body coil (or other coil) around
the patient.
[0141] Thus the coil 16 is arranged to be located as close as
physically possible to the subject and the second coil or body coil
14 is located at a position spaced from the subject greater than
the that of the coil 16 so as to receive the signal inductively and
transfer it to the processing unit.
[0142] The coil 15 is as close as possible to the part to be imaged
and covers or surrounds as small a volume as possible so as to
receive noise from as small a volume as possible and so as to
receive as much signal as possible, bearing in mind that the signal
falls rapidly as is passes through the tissue. This therefore
generates a signal which has significantly greater signal to noise
ratio than a second coil located at a greater spacing from the
part. Then the signal picked up by the coil 15 is communicated
inductively to the coil 16 even though there are significant losses
in the inductive communication. The signal from the coil 15 is
induced onto the coil 16 at an efficiency of induction (less than
100%) but sufficient that that the MR signal on coil 16 is greater
than the MR signal which would be generated on coil 16 in the
absence of the coil 15. That is there is a magnifying effect by
providing the coil 15 close to the subject and then communicating
the signal to the coil 16 despite the losses in the inductive
coupling. The same effect occurs at the second inductive stage
between the coil 16 and the coil 14.
[0143] It will be appreciated that the coils 16 and 14 also receive
signals directly from the part being imaged which signals are added
to the signals communicated inductively. However in each case, the
inductively coupled signal is much greater than the directly
detected signal.
[0144] Another issue which arises is that mutual inductance between
the coils 14, 16 and 15 can change the tuned common resonant
frequency of the loops to reduce the MR signal unacceptably. Thus
the spacing between them must be sufficient such that the amount of
mutual inductance does not change the tuning frequency sufficiently
to interfere with the tuning to a level where the acquisition of
the signal is degraded. This is of course a trade off and the
actual distance spacing between the particular coils of a specific
embodiment must be determined by simple experimentation to move the
coils to the required position to obtain the best signal having the
best signal to noise ratio.
[0145] In the second embodiment shown in FIG. 2, there are only two
coils defined by the body coil 14 and the coil 16. Thus the signal
is obtained primarily by the coil 16 and is transferred inductively
to the coil 14 for cable transmission to the signal processing
system 13.
[0146] In this embodiment, the coil 16 is a head coil including a
bottom section 16X underneath the head and a top section 16Y on top
of the head. One or both sections can be flexible since each is
formed simply by a carrier substrate 16Z and the conductive loops.
In this embodiment 6 loops 16A to 16F are shown but different
numbers can be used. Each loop includes circuit elements defining
the tuning component 17 and the switch 18.
[0147] In FIG. 5 is shown a further embodiment wherein the body
coil is absent or is not used where there is a separate transmit
coil 20 and the coil 23 is connected to the signal processing
system by a cable 13A.
[0148] Turning now to the embodiment shown in FIG. 6 there is shown
an arrangement in which the body coil 14 is a single channel coil
generating a single MR signal (compare to multi channel array coil
which generate multiple signals) on a cable 14A. In order to take
advantage of the high speed imaging which can be obtained by using
parallel channels such as by SENS or SMASH or other more recent
techniques, it is necessary to generate separate signals for
separate channels 13B to 13G of the signal processing unit 13.
[0149] In this embodiment the coil 16 includes a plurality of
separate loops 16A to 16F. As shown in FIG. 4, each circuit of the
loops 16A to 16F includes an addressable switch 19 operable
remotely to halt flow of current in the loop so that each loop can
be activated in turn.
[0150] In general, there is provided an arrangement in this
embodiment defined by a signal separation system 21 for generating
the separate MR signals for the separate channels from the signal
induced onto the separate loops 16A to 16F of the second coil
16.
[0151] In this arrangement, each loop includes an addressable
switch operable remotely by a wired or wireless activation system
schematically indicated at 22 to halt flow of current in the loop.
In this way each loop can be activated in turn with the other loops
turned off. A signal separation system 21 arranged to receive the
signal from the body coil 14 and to calculate the separate MR
signals, emulating the multi-channel signal 13A for the separate
channels from the signal induced onto said body coil 14.
[0152] More particularly the signal dividing system is arranged in
imaging calibration sequence to use the addressable switch 22 to
determine the individual effects of each of the first loops so as
to determine the sensitivity profile and possibly the Noise
correlation Matrix for each received MR loop signal. The
calibration sequence selects each loop individually to obtain the
sensitivity profile. Once the individual sensitivity profiles are
known, the system will perform processing on the combined MR signal
14A and present separate outputs 13B through 13G which emulate a
standard multi-channel phased array coil. Turning now to FIG. 7,
there is shown an arrangement where the body coil 14, is replaced
by a multichannel transmit and receive coil 24 which has separate
loops 24B to 24G connected to separate channels 13B to 13G of the
signal processing system 13. The coil 16 is also a coil formed by a
plurality of loops as previously described, phase array body coil
used for both transmit and receive coil. The communication of the
signals inductively from the coil 16 to the coil 14 provides the
required separate signal to each respective one of the channels and
communicates the separate signals to the signal processing system
13 to generate imaging by standard parallel image methods.
[0153] Turning now to the embodiment shown in FIG. 8 there is shown
an arrangement in which the body coil 14 is a quadrature birdcage
coil which generates RF pulses to the subject to be imaged and is
used for transmit coil only In order to take advantage of the high
speed imaging which can be obtained by using parallel channels such
as by SENSE or SMASH or other more recent techniques, it is
necessary to add a phased array receive only coil in the magnet
which is as close to body coil as possible, there is shown an
arrangement where the phased array coil 25 which has separate loops
25B to 25G connected to separate channels 13B to 13G of the signal
processing system 13. The coil 16 inductively couples the signals
from the coil 16 to the coil 25. The coil 25 provides the required
separate signal to each respective one of the channels and
communicates separate signal to the signal processing system 13 to
generate imaging by standard parallel image methods.
[0154] Turning now to FIGS. 9 to 18, the arrangement provided
herein therefore consists of a cable-less volume coil, which works
by coupling with the built in body coil. The inductive coupling
with the built in body coil 14 provides SNR and image uniformity
comparable with other local volume coils. This coil has no
preamplifiers, cables, cable traps, baluns or connectors. It is in
this embodiment the design is defined by a birdcage resonator and
is used as a volumetric coil.
[0155] In FIG. 9 the volume coil 16 comprises a high pass birdcage
coil. This is of the conventional birdcage shape with two end-rings
161 and 162 connected by a series of parallel conductors 163. The
coil can be cylindrical in which case the end-rings 161 and 162 are
of the same diameter. The coil can be conical in which case the
end-ring 161 is larger than the end-ring 162. The coil can be
barrel shaped in which case the end-rings 161 and 162 are of the
same diameter and the conductors 163 are arched.
[0156] In the end-rings 161 and 162 between each conductor 163 and
the next is provided a circuit component 17/18 which acts to carry
out the functions of the components 17 and 18 described above. Thus
each individual loop formed by part of the end-rings 161 and 162
together with two parallel conductors 163 is individually switched
by the circuit elements 17/18 (for convenience merely illustrated
as a gap between the conductors where the components are located)
in the arrangements and for the functions described above.
[0157] In this arrangement the Inductive volume coil 16 can achieve
equal or better images compared with commercial phased array volume
coils. The coils herein can provide highly uniform images with good
SNR numbers without any preamplifiers and cables. There is no
limitation to the number of channel regardless of the number of
receivers in the system. No cables are used to connect the coil 16
to the system. The coil contains only passive elements defined by
the circuit components 17/18. The inductive volume coil 16 contain
no baluns, preamps, cables, cable traps, connector blocks, or
extension cables. The birdcage coil has smaller physical dimensions
(size, weight) compared with similar (same field of view) phased
array volume coil.
[0158] Patient positioning and surgical access is significantly
improved due the lack of the long connecting cable and the location
of the connectors. The length of the cable and the location of the
connectors occasionally restrict the angles and/or orientation of
the coils to fit the patient and imaging. The smaller size and
weight of the wireless coil will assist with patient positioning by
allowing more freedom. Surgical access is also improved because the
coil is physically smaller than conventional coils and the
conventional coil cable usually restricts or blocks surgical
access. Patient safety is improved, as there are no cables, the
possibility of patient burns resulting from patient skin-to-coil
cable contact is completely eliminated.
[0159] The circuit components 17/18 provide tuning and passive
decoupling Passive decoupling eliminates crosstalk between the
inductive wireless coils to the built-in body coil during the
transmit phase. Therefore, B1 distortion, coil heat and image non
uniformity caused by B1 distortion is eliminated. B1 is RF field.
The local coil, if not decoupled from the built in body coil, will
generate a local B1 field at the region of interest at the same
time as the B1 field from the built in body coil is present, at the
same time and in the same region. Both of these B1 field combine to
create a distorted effective B1 field, and therefore, causes
distorted images.
[0160] In FIG. 10 the volume coil 16 comprises the volume coil
comprises a low pass birdcage coil. All the tuning and decoupling
components 17/18 of the high pass birdcage coils are located on the
end rings 161 and 162, and the tuning and decoupling components on
a low pass birdcage coil are located on the rungs. The choice of
high pass or low pass coils depends a variety of factors, such as
field strength, coil size, patient load, and intended use of the
coil for best imaging performance.
[0161] In FIG. 11 the volume coil 16 comprises a band pass birdcage
coil. All the tuning and decoupling components 17/18 of the band
pass birdcage coils are located on the end rings 161 and 162, and
are also located on the rungs. The choice of band pass coils
depends a variety of factors, such as field strength, coil size,
patient load, and intended use of the coil for best imaging
performance.
[0162] In FIG. 12 the volume coil 16 comprises a high pass dual
frequency birdcage coil, consisting of two individual coils 16A and
16B separated by proper spacing in one housing, constructed for
dual frequency, such as for use on dual nuclei, for example proton
(1H) and phosphorous (31P) images or, dual field strength, for
example 1.5 Tesla and 3 Tesla imaging systems. In this arrangement
the end-rings 161 and 162 are each divided into separate end-ring
components 165, 166 and 167, 168 tuned to the separate frequencies
of the dual frequencies, each of which also includes the components
17/18.
[0163] In FIG. 13 the volume coil 16 comprises a high pass birdcage
coil of the dual rung type. In this arrangement each rung of each
section in the normal birdcage coil has been replaced by dual
parallel rungs 163A and 163B and all the rungs are equal space so
that it provide uniform RF field. The end-rings 161 and 162 also
includes the components 17/18 for coil tuning and decoupling from
body coil during transmit phase.
[0164] In FIG. 14 the volume coil 16 comprises a high pass birdcage
coil of the Litzcage type. In this arrangement each rung 163 of
each section in the normal birdcage coil has been replaced by two
parallel rungs 163C, 163D with an insulated crossover 163E at the
center, all the rungs are equal space so that it provide uniform RF
field. The end-rings 161 and 162 also includes the components 17/18
for coil tuning and decoupling from body coil during transmit
phase.
[0165] In FIG. 15 the volume coil 16 comprises a birdcage coil of
the type used for head and neck. This is constructed in the manner
of FIG. 9 but the end-ring 162 is shaped to form lobes 171 and 172
which extend longitudinally along the axis of the coil to a greater
length so that some of the connecting rungs 163 are longer than
others. The extended lobes provide imaging are for both the head
and upper cervical neck area.
[0166] In FIG. 16 the volume coil 16 comprises the volume coil
comprises a half birdcage coil. All the tuning and decoupling
components 17/18 of the high pass birdcage coils are located on the
end rings 161 and 162. This style of coil provides increased
positioning flexibility and superior surgical access.
[0167] In FIG. 17 the volume coil 16 comprises a birdcage coil of
the type formed by two halves which are split. The two halves are
inductive coupled to resonate at the frequency that can produce a
uniform B1 RF field. The split birdcage coil provides increased
patient positioning flexibility and the upper coil can be
completely removed and leave the bottom coil in place during the
surgery. The upper coil could be put back to work with the bottom
coil during imaging.
[0168] In FIG. 18 the volume coil 16 comprises a volume coil of the
Alderman-Grant type. In this arrangement, two rungs are formed from
solid elements connected in two layers, at two end rings. The two
inner shield rings are located at the end rings of the volume coil
and are continuous, but are insulated from the outer rungs and both
end rings. The outer rungs are comprised of two separate large
rungs connected to both end rings. The end-rings also includes the
components 17/18 for coil tuning and decoupling from body coil
during transmit phase.
[0169] Thus as shown above, many different arrangements of the
cable-less volume coil can be provided. These include: [0170]
Standard style birdcage include high pass, low pass and band pass
volume coils. [0171] Dual frequency high pass birdcage coil for
either 1.5T or 3T, and dual nuclei imaging and spectroscopy. [0172]
High pass dual rung birdcage coil. [0173] Litzcage high pass volume
coil. [0174] High pass head and neck combination coil. [0175] Half
birdcage volume coil. [0176] Split high pass birdcage style. [0177]
Radiolucent Volume coil, used for CT/MRI hybrid systems, such as
for X-ray/MR and MR guided radiation therapy. [0178] iPAT style
fast imaging with inductive coil.
[0179] Both 1.5T and 3T coil imaging is comparable to the existing
commercial phased array head coil and provide very good image
uniformity and high SNR.
* * * * *