Note: Descriptions are shown in the official language in which they were submitted.
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FLOATING SEGMENTED SHIELD CABLE ASSEMBLY
This invention relates to coaxial cables for use in high RF fields where
currents induced in the shield of the cable can have deleterious effects. The
invention is particularly applicable to such cable when used in the high RF
fields
used in Magnetic Resonance Imaging but can relate to other cables. The
invention
also includes a jacket arrangement which can be applied on to a conventional
coaxial cable to obtain the advantageous construction described herein.
BACKGROUND OF THE INVENTION
Many coaxial cables are required to be used in the high RF fields used
in MRI. These include primarily the cables to the receive coil array but also
other
cables that must enter the high RF field such as those used in pacemakers, ECG
testing, electrophysiology and EEG monitoring, and Deep Brain Stimulation
systems
COBS).
Common mode signals or shield currents on coil cables are often
caused by the coil itself or by an external source such as a surrounding
transmit
body coil during transmit phase. Electromagnetically induced currents by an
external source, such as those produced by the body transmit coil, are
responsible
for the majority of the shield current. and therefore heat, on the surface of
the cable.
These currents, and the resulting heat produced, can cause serious patient
heating
or burns. Common mode currents also degrade the image quality by affecting
coil
tuning, coil-to-coil coupling in phased array coils..
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In addition the generation of currents in the shield of cables within the
coil, especially cables close to or crossing the individual coil loops in a
phased array
coil, in the magnetic field of the MR scanner can interfere with the creation
of the
homogenous RF field generated by the transmit body coil. This inhomogeneity of
the RF field can generate artifacts within the image obtained.
The advantageous use of coaxial cables having an inner axially
oriented elongated conductor separated from an annular electrically conductive
shield by a dielectric material has long been known. Such coaxial cables have
been
used in magnetic resonance imaging, as well as numerous other uses.
Among the important safety concerns related to magnetic resonance
imaging technology are the possible bums and excessive heat due to the induced
RF currents on the electrical cables. To reduce the risk of such localized
heating or
burns, the users of the MR scanners are instructed to minimize patient contact
with
cables. Such contact, however, is unavoidable in many cases such as when using
ECG cables, surface coils, or intra-cavity coils.
To minimize localized heating or burns and induced currents on the
cables, some commercial MR coils, such as the magnetic resonance coils of GE
Medical Systems, for example, use patient safety modules. This design
decreases
the unbalanced currents on the coaxial cable. In addition to patient safety,
this
design effects reduction in radiation losses and common mode noise in the
coil.
Similar and more serious problems exist for the coils that are inserted
inside the body such as endorectal, esophageal, and intravascular RE probes.
As
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these devices are closer to the body, the risk of localized heating or burning
a
patient is increased. Also, the wavelength of the RF signal in the body is
approximately nine times shorter as compared with the wavelength in the air.
As a
result, current induction on short cables is possible. There remains,
therefore, a
need for an improved coaxial cable which will perform effectively for its
intended
purpose while resisting the generation of high currents in the shield which
can cause
undesired excessive heating or burning of a patient and which can cause
interference with the homogeneity of the RF field thus generating artefacts.
Typically the effect of the generation of currents in the shield of the
coaxial cable is reduced by using cable traps which are placed in the cable at
spaced positions along the length of the cable. These act to reduce the
generation
of the current.
This is particularly exacerbated where the cable must be very long to
accommodate various movements, such as in the system described in US Patent
5,735,278 (Hoult et al) issued April 7th 1998 in which is disclosed a medical
procedure where a magnet is movable relative to a patient and relative to
other
components of the system. The moving magnet system allows intra-operative MRI
imaging to occur more easily in neurosurgery patients, and has additional
applications for liver, breast, spine and cardiac surgery patients. In this
case the
high number of cable traps required in the intra-operative MRI coil signal
transmission cable in conjunction with the great length of the cable makes the
cable
particularly unwieldy.
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One type of cable traps typically involve an inductor formed from the
cable shield braid by wrapping the cable around a helical support so that the
shield
forms a helical inductor. At one end the copper conductor is electrically
connected
to the cable shield braid and at the other end one or more capacitors are
connected
in parallel to the inductor between the copper conductor and the shield to
form a
tank circuit which acts to attenuate the unwanted shield current on the cable.
In the cable trap arrangement, the shield braid is continuous along the
cable and has formed at points along its length the tank circuit defined by
the
inductor portion of the shield, the copper conductor, and the capacitors.
The cable traps improve the coil performance by eliminating or
reducing the shield current along the cable shield. The cable trap is designed
to
reduce the shield current, but the helical inductor formed from the cable
shield of the
cable trap also effectively acts as an antenna, to receive RF power from the
transmit
body coil and contributes unexpected current in the cable.
Experiments have shown that the copper conductor contributed
additional heat. This type of cable trap increases the overall coil and cable
weight
and is not convenient for handling in a surgical setting
The generation of the shield current is proportional to the geometry of
the cable. A larger surface cable generates more shield current than a smaller
surface area cable. For example, a longer cable with a larger diameter
produces
more current than a shorter cable with a smaller diameter.
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The generation of the shield current is also proportional to the system
RF power. For example, the power from a 3.0 Tesla system will be four times
the
power from the 1.5 Tesla system, and much higher power for a 7.0 Tesla or
higher
system. The required number of cable traps for a 3.0 Tesla system will be
approximately doubled compared to the 1.5T system, with closer spacing between
cable traps. A 7.0 Tesla or higher system would require even more cable traps
with
closer spacing.
Also the additional length of the raw cable required, when wrapped
helically, to form a cable trap negatively affects the RF chain.
A number of cable designs have previously been proposed as follows:
US Patent 6,284,971 (Atalar) issued to Johns Hopkins University on
September 4th, 2001 discloses a co-axial cable for probes used in MRI, which
has an
outer dielectric layer with high dielectric constant, between inner shield
portion and a
segmented outer shield portion of outer conductor so as to inhibit induced
radio
frequency current. Thus the arrangement disclosed connects the one end of a
segmented shield to the cable shield braid and use the free end of the
segmented
shield as a 1/4 wave cable trap.
US Patent 7,123,013 (Gray) issued to Biophan technologies on
October 17th, 2006 discloses an arrangement in which a voltage compensation
unit
reduces the effects of induced voltages upon a device having a single wire
line
having balanced characteristic impedance. The voltage compensation unit
includes
a tuneable compensation circuit connected to the wire line which applies
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supplemental impedance to the wire line and causes the characteristic
impedance of
the wire line to become unbalanced, thereby reducing the effects of induced
voltages.
US Patent 7,205,768 (Schulz) issued to Phillips on April 17th, 2007 .
discloses a lead for use in an MRI device which has an auxiliary electrical
device
connecting to the lead with sections with inductive coupling element of
limited length
not equal to integral multiple of the half wavelength.
US Patent 7,294,785 (Uutela) issued to GE Healthcare on November
13th, 2007 discloses a lead for use in an MRI device where, in order to
eliminate the
risk of thermal injuries without compromising the signal-to-noise ratio more
than
what is required for patient safety, the lead comprises two successive cable
elements having different resistance characteristics. The second cable
element,
which is connected by the first cable element to the patient, has a total
resistance
increased from a normal high-conductivity resistance value of a patient cable
to
suppress antenna resonances in the second cable element. The first cable
element,
which is connected to the electrodes on the skin of the patient, has a total
resistance
substantially greater than that of the second cable element to prevent
electromagnetically induced currents from flowing to the patient and to
prevent
excessive heating of the cable by electromagnetic induction.
SUMMARY OF THE INVENTION
It is one object of the invention to provide a cable for communicating
signals in an RF field where the creation of currents in the cable by the RF
field is
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reduced.
According to one aspect of the invention there is provided a shielded
cable for use in an apparatus for transmitting signals in an RF field
comprising a
source of RF signals to be transmitted and a circuit ground, comprising:
an inner conductor construction extending axially along the cable and
providing electrical connection for transmission of signals between opposite
ends of
the cable;
an axially extending outer shield conductor disposed in spaced
surrounding relationship around the inner conductor construction, the outer
shield
conductor extending continuously between the opposite ends of the cable;
the outer shield conductor being connected to the circuit ground;
the outer shield conductor fully enveloping the inner conductor
construction so as to shield the inner conductor construction from said RF
field;
the inner conductor construction being electrically insulated from the
outer shield conductor by dielectric material interposed between;
a plurality of segmented shield conductor portions each surrounding
the outer shield conductor and each having a length less than that of the
cable;
the segmented shield conductor portions being arranged at locations
along the cable;
the segmented shield conductor portions being electrically separated
each from the others such that the segmented shield conductor portions float
electrically relative to the other segmented shield conductor portions;
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the segmented shield conductor portions being electrically separated
from the outer shield conductor such that the segmented shield conductor
portions
float electrically relative to the outer shield conductor;
and a cable jacket enclosing the segmented shield conductor portions
and the outer shield conductor.
In most cases the conductor portions are annular, that is they fully
surround the cable, but this is not an essential requirement provided the
portions
carry out their shielding action.
In one example the conductor portions are formed from braid but it is
often preferred that they are formed from an annular or spiral wrapped non-
magnetic
metal foil tape since the foil tape avoids the intervening holes between the
wires in
the braid which can reduce the shielding effect. A combination of braid and
solid
conductors is also possible.
In one example the conductor portions may be axially spaced, that is
the end of one may be spaced from the adjacent end of the next, so as to leave
portions of the outer shield conductor which are not covered by the conductor
portions. However where a high level of protection is required, the conductor
portions may be arranged such that the ends of each are overlapped with
corresponding ends of next adjacent conductor portions such that the outer
shield
conductor is wholly covered by the conductor portions. In this case there will
be
applied a dielectric material between the outer surface of one portion and the
overlapping inner surface of the next adjacent portion to ensure electrical
separation.
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This can be formed by a wrapped tape such as a Teflon"' tape.
In one embodiment there is provided a continuous jacket formed of a
dielectric material surrounding the outer shield conductor over which the
conductor
portions are engaged. However the separation of the conductor portions from
the
outer shield can be formed by other material such as an annular or spiral
wrapped
non-magnetic metal foil tape.
In particular in one important feature, the conductor portions are
shaped and arranged to reduce heating of the cable in an RF field and
particularly
the conductor portions are shaped and arranged to reduce heating of the cable
in an
RF field of a Magnetic Resonance Imaging system to a temperature less than
that
sufficient to cause injurious burns to human tissue.
Preferably the conductor portions for 1.5 Tesla systems or higher have
a length less than a maximum 10 inches and preferably of the order of 0.5 to
2.0
inches which is a practical dimension for manufacture while ensuring the
reduction in
induced current in the shielding conductor and in the portions themselves to a
level
which enhances the operation of the cable.
The above defined cable can be formed as an integral construction
where the conductor portions are engaged around an intermediate dielectric
layer
with the cable jacket engaged over the whole construction. However as an
alternative the construction can be formed using any existing conventional
cable,
including coaxial cable enclosed by a cable jacket where the conductor
portions are
carried on an inner hollow sleeve member which is engaged by sliding over the
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cable jacket with a second outer jacket which surrounds the inner sleeve
member
and the conductor portions. This technique avoids the manufacture of a
complete
cable and allows the use of existing cable constructions as part of the
construction,
which are inexpensive due to high volume manufacture.
According to a second aspect of the invention there is provided a
method of communicating signals in an RF field comprising:
connecting the signals to be communicated to an elongate axially
extending inner conductor construction of a communication cable;
providing an axially extending shield conductor of the cable disposed in
spaced surrounding relationship around the inner conductor construction, the
shield
conductor extending continuously between the opposite ends of the cable and
being
connected to a circuit ground for shielding the inner conductor construction
from
external fields;
the inner conductor construction being electrically insulated from the
shield conductor by dielectric material interposed between;
and providing a cable jacket enclosing the shield conductor;
wherein the inner conductor construction and the outer shield
conductor are located in an RF field of sufficient intensity and over time
period and
of a wavelength which would generate heat therein;and reducing the amount of
heat generated by:
providing a plurality of segmented shield conductor portions,
each surrounding the shield conductor and each having a length less than that
of the
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cable;
the segmented shield conductor portions being arranged at
locations along the cable;
the segmented shield conductor portions being electrically
separated each from the others such that the segmented shield conductor
portions
float electrically relative to the other segmented shield conductor portions;
and the segmented shield conductor portions being electrically
separated from the shield conductor such that the segmented shield conductor
portions float electrically relative to the shield conductor of the cable;
where the segmented shield conductor portions act to shield the shield
conductor to reduce the heating of the shield conductor in the RF field while
the
electrical separation of the segmented shield conductor portions each from the
others reduces the generation of a current along the segmented shield
conductor
portions.
This method can be applied to either a single or multiple conductor
cable, and can be used with the second aspect of this invention of the outer
jacket
surrounding the inner sleeve member and conductor portions.
The above method is particularly applicable where the RF field is
generated by an RF transmit coil in a Magnetic Resonance Imaging system.
However the method and the cable can be used in to the situation where a high
RF
field would otherwise generate deleterious currents in the outer shield
conductor of a
coaxial cable.
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For example, where the inner conductor construction and the outer
shield conductor are located in an RF field of sufficient intensity and time
period and
of a wavelength which would act to generate heat to a temperature sufficient
to
cause injurious burns to human tissue, the plurality of segment shield
conductor
portions are shaped, arranged and dimensioned relative to the outer shield
conductor so as to reduce the heating to a temperature less than that
sufficient to
cause such burns.
Thus the conductor portions act to shield the outer shield conductor to
reduce the heating thereof in the RE field while the electrical separation of
the
conductor portions each from the others reduces the generation of a current
along
the portions.
In one particular example, the method is used in a Magnetic
Resonance Imaging system for communication of signals from the RF receive
coil.
In this arrangement, the inner conductor construction includes at least one
conductor
connected to the RF receive coil of the Magnetic Resonance Imaging system. The
plurality of conductor portions are arranged relative to the outer shield
conductor so
as to reduce currents in the cable from interfering with the homogeneity of
the RF
transmit field and thereby causing artifacts in the image. The conductor
portions act
to shield the outer shield conductor to reduce generation of a current in the
cable
caused by the transmit RF field while the electrical separation of the
conductor
portions each from the others reduces the generation of a current along the
portions.
In another particular example, the method is used where the RF
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receive coil construction has therein a plurality of receive coil sections. In
this
example, the inner conductor construction includes a plurality of conductor
elements
each for communication with a respective one of the individual coil loops in
the -
receive coil.
The conductor elements, either separate single wires or multiple
coaxial cables, are combined into the cable connected from the receive coil
construction to the MRI system. The conductor elements are branched off at the
receive coil into separate paths and each path includes an axially extending
outer
shield conductor of the path disposed in spaced surrounding relationship
around the
inner conductor element and there is provided a plurality of the conductor
portions
as described above surrounding the outer shield conductor.
Preferably the length of each conductor portion is less than A/4 where A
is the wavelength of the RF field and more preferably the length of each
conductor
portion is less than A/8 where A is the wavelength of the RE field.
The present method thus isolates the segmented shield conductor
formed by the conductor portions from the outer cable braid shield with an
insulator
so that each piece of the segmented shield prevents the continuous current on
the
cable braid. The segmented shield produces a negligible current; with the
smaller
the segment, the smaller the current produces.
The floating segmented shield is different from the prior art patents,
especially the John Hopkins patent, in that these patents accept the shield
current
and then try to attenuate or reduce the current by some method of blocking the
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current flow. The present method prevents the shield current from generating
on the
cable shield.
Experimental testing, has shown that cable heating can be significantly
reduced through the use of the segmented and floating supplemental shielding
as
described herein. Thus the addition of a floating segment shield outside the
primary
continuous shield can prevent or reduce the common mode current in the primary
shield of the cable by preventing the power from the RF transmit coil from
reaching
the primary shield. The gaps in the segmented supplemental shield prevent the
current flow in the segmented shield.The floating segmented shield cable
design can be used to reduce the
heating of a wide variety of cables. Applications include cables used for
communication with coils used for catheters, ECG, Deep Brain Stimulation
(DBS);
and even pacemakers could benefit from this invention to make them MR safe.
Any
conductive electrical wires, including those with a outer continuous shield
can be
protected by this invention.
The arrangement described herein can provide some of all of the
following features in an MRI coil embodiment:
A greatly reduced shield current in the RX cable caused by the body
transmit coil and therefore reduced cable heating and increased patient
safety;
Increased overall imaging performance by reduced shield current and
increased the image quality by improving coil tuning, coil-to-coil coupling in
phased
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array coils, image uniformity by reducing distortion in the RF B1 field, and
image
SN R.
Reduce raw cable length, nearly 4 feet shorter based on 3 cable traps
for 1.5T and nearly 8 feet for 3T, and therefore reduce the weight of the
overall coil
and cable assembly;
The floating segmented shield concept may be used in conjunction
with current coil design;
Increased ease of manufacture due to innovative design;
The cable jacket (or cable hose) material can be selected to be water
proof and sterilized for intra-operative coils used in clinical surgery.
This is a cost effective and more efficient method to reduce the shield
current on the cable braid, which will increase patient safety in multiple
applications.
The arrangement described herein can be used to replace
conventional cable traps thus significantly reducing the total weight of the
cable
Alternatively the arrangement can be used with the conventional cable
traps located at spaced positions along the shielded cable so that the
shielded cable
is used in conjunction with the cable traps to further reduce the heating
effect and to
reduce the number of cable traps required in a predetermined length of the
cable.
In this case, the housing of the cable trap itself can be used as another
one of the conductor portions where the housing is coated on an inner surface
with a
non-magnetic conductive material so as to surround that portion of the cable
at the
cable trap, the conductive material on the housing being electrically
separated from
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the other conductor portions of the cable and from the outer shield conductor
within
the cable trap.
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 illustration of a communication cable according
to the present invention having a single conductor.
Figure 2 is a schematic illustration of a communication cable according
to the present invention having a plurality of conductors.
Figure 3 is a schematic illustration of a shielding sleeve for a
communication cable according to the present invention.
Figure 4 is a cross sectional view of a communication cable according
to the present invention having a single conductor similar to that of Figure 1
but
including overlapping conductor portions. Figure 5 is a cross sectional
view of a communication cable according
to the present invention including a cable trap.Figure 6 is a schematic
illustration of an MRI system using the cable of
Figure 1. Figure 7 is a schematic
illustration of a receive coil for the MR system
of Figure 6 including a plurality of coil loops.In the drawings like
characters of reference indicate corresponding
parts in the different figures.
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DETAILED DESCRIPTION
In Figure 6 is shown schematically a magnetic resonance imaging
system which includes a magnet 10 having a bore 11 into which a patient 12 can
be
inserted on a patient table 13. The system further includes an RF transmit
body coil
14 which generates a RF field within the bore.
The system further includes a receive coil system generally indicated
at 15 which is located at the isocenter within the bore and receives signals
generated from the human body in conventional manner. A RF control system 17
acts to control the transmit body coil 14 and to receive the signals from the
receive
coil 15. The cable 16 must be draped into the bore alongside the patient to
connect
to the received coil assembly within the bore.
In figure 7 is shown schematically the receive coil assembly 15 which
in this arrangement includes a plurality of received coil loops 15A, 15B, 150
and
15D. Each of these loops is connected to a signal transmit coaxial cable and
control
wire bundle portion 16A, 16B, 160 and 16D so that the received signal from
that
loop can be transmitted through a larger, multiple coaxial cable and control
wire
bundle 16 to the RF control system 17.
Thus within the receive coil assembly 15 is located a plurality of
conductors which pass through the construction forming the receive coil
assembly to
various locations within the receive coil assembly for connection to the
individual
receive coil loops. The arrangement shown is of a very simple nature and it
will be
appreciated that such receive coil assemblies are often quite complicated
involving
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the connection of overlapping sections so that the wiring of the signal
communication cable portions is relatively complex through the structure. Each
receive coil loop is connected to a respective preamplifier 18 located as
close as
possible to the loops and its respective communication cable bundle.
In an arrangement such as that described previously in U.S. patent
5,735,278, the magnet is moved relative to the patient on the table and this
requires
in many cases a particularly long cable 16 since the cable is required to
accommodate the moving magnet system, and the draping required during surgery.
The particular problem which arises in relation to the above MRI
system is that any cable located within the high power RF field generated by
the
transmit coils can receive induced currents on the external metallic shield of
the
cable. These are typically of such a magnitude which is sufficient to cause
unacceptable heating. In addition the induced currents can be communicated to
the
receive coil thus generating extraneous RF fields which will interfere with
the
homogeneity of the transmit field and thus generate artefacts within the
image.
This problem is of course well known and the solution typically
employed is to provide so called cable traps at spaced positions along the
cable.
The number of such cable traps required is dependent upon the RF field and so
that
for a particular RF field there is required a certain spacing between the
cable traps.
Thus in a situation where the field is increased due to increased power in the
MRI
system or in a situation where the cable length is increased, the cable
carrying the
cable traps is increased in weight and difficulty to handle. The cable also
must carry
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a thick insulating layer to protect the patient from close encounter with the
heated
conductor. All of these requirements significantly increase the weight and
structure
of the cable to a situation where it becomes unwieldy.
In Figure 1 is shown a construction according to the present invention
which can be used to reduce currents induced in the outer conductive shield so
as to
reduce heating and artefacts as described above. Figure 1 therefore shows a
cable
21 with a single inner conductor 20 surrounded by a dielectric layer 22 and an
outer
braided, non-magnetic metal shield conductor 23. In addition to these
conventional
elements is provided an additional cylindrical surrounding dielectric layer 24
which is
covered by a series of spaced non-magnetic metal conductor portions 25 at
spaced
positions along the cable. Around the conductor portions 25 is provided an
outer
jacket 26 of a conventional construction. The outer jacket 26 may be simply of
a
dielectric material for providing surrounding protection or it may include a
foam
insulating layer to reduce heat transfer.
The cylindrical outer shielding conductor 23 is continuous along the
cable so that it can be connected to a circuit ground for grounding currents
in the
conductor 23. This coaxial cable is connected to the coil so that the signals
received
are transmitted along the cable to the RF system control and are shielded from
RF
noise effects by the continuous shield 23.
The conductor portion 25 in the embodiment shown in Figure 1 are
spaced so that the end of one conductor portion is axially spaced from the
adjacent
end of the next adjacent conductor portion leaving a bare area 25A between the
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conductor portions.
As previously described, the conductor portions act to shield the outer
shielding conductor 23 from electromagnetically induced current therealong.
Thus
the outer conductor portion 25 is electrically separated from the conductor 23
by the
layer 24. The outer conductor portion 25 acts as a shield to effectively
reduce the
current on the braided conductor 23. Also the conductor portions 25 are
electrically
separated each from the next and each from the others so that any current
generated is negligible in each conductor portion and therefore the amount of
heat
created is reduced.
In Figure 2 is shown an embodiment similar to that of Figure 1 in which
the single central conductor 20 is replaced by a plurality 20A of individual
conductor
elements 20B, comprised of individual coaxial cables and control wires. This
of
course produces an internal diameter which is larger than that of the cable 21
so that
the cable 21A in Figure 2 includes a larger diameter inner dielectric layer
22A, which
is surrounded by the shield 23A, another dielectric layer 24, and by the
individual
conductor portions 25. A jacket 26A surrounds the structure as previously
described.
In Figure 3 is shown an alternative arrangement which is used in
conjunction with conventional cables utilizing the construction in which one
or more
individual inner conductors is surrounded by a dielectric layer which in turn
is
surrounded by the outer shielding layer and an outer jacket. In the embodiment
of
Figure 3, is provided an inner sleeve 27 which carries a plurality of
conductor
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21
portions 25 at spaced positions along its length. The sleeve portion and the
conductor portions are covered by an outer jacket 26B. The sleeve portion 27
has
an inner surface 27A which can slide over the conventional jacket as a sliding
fit so
that the inner surface 27A surrounds the cable. This surface may also be
coated
with heat activated adhesive to permanently affix the sleeve to the jacket of
the
coaxial cable or wires to be shielded. In this way a conventional cable can be
used
and can be supplemented in its shielding effect by the provision of the
construction
shown in Figure 3 provided by the inner sleeve, the conductor portions and the
outer
jacket.
Turning now to Figure 4, there is shown in cross section a construction
similar to that of Figure 1 including the central conductor 20, the dielectric
layer 22,
the outer shield 23 and the jacket 26.
In this embodiment, the conductor portions 25 are supplemented by
additional conductor portions 25B which overlap with the conductor portions
25.
Thus there are no open or bare portions 25A since the whole of the length of
the
outer shield in conductor 23 is covered by the conductor portions 25 and 25B.
Outside the conductor portions 25 is provided an insulating or dielectric
layer 28
which separates the conductor portions 25B from the conductor portions 25 so
that
all the conductor portions are electrically separated from one another and
electrically
separated from the common shielding layer 23. Thus as shown each conductor
portion 25B has ends 25C and 25D which overlap the ends 25E and 25F of the
adjacent conductor portions 25. It will be appreciated that the overlap may be
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reduced to a very small amount or to where the ends are approximately directly
overlying with the intention that the whole of the shielding conductor 23
shielded by
the conductor portions while minimizing the amount of conductor portions
utilized.
The shielding conductor 23 is typically a braid but can be formed from
helically wrapped non-magnetic metal foil. The dielectric layers are typically
extruded jackets but also can be formed from a wrapped tape such as Teflon TM
tape.
The Teflon 1." tape has the advantage that it is slippery and hence allows a
sliding
action where required.
The dielectric layer 24 is shown as a continuous cylindrical sleeve but
it will be appreciated that it can be formed in portions since its function is
primarily to
separate the segmented shield conductor portions 25 from the underlying
shielding
layer 23 and hence the dielectric layer 24 need be located only underneath the
segmented shield conductor portions in the arrangement shown in Figures 1, 2
and
3.
Turning now to Figure 5, there is shown an alternative arrangement
which utilizes both the above shielding arrangements and also the conventional
cable trap which are used in combination to further reduce the generation of
currents
in the shielding layer.
Thus in Figure 5 is shown cable portions 121 and 221 which are of the
construction shown in Figures 1 or 2. Thus the cable portions 121 and 221
include a
shielding layer 123 and 223 which is covered by a dielectric layer 124 and
224.
Around this is provided the segmented shield conductor portions 125 and 225
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together with the jackets 126 and 226. A cable trap 30 is located between
these
cable portions. The cable trap includes an outer housing 31 which is clamped
onto
the ends of the jackets 126 and 226 and acts to bridge an area between these
jackets. Inside the housing 31 the jacket is stripped away and the portion of
the
cable defined by the inner conductor and the shield 123 is coiled around a
support
32 to form a helical portion 33 of the stripped portion of cable. This helical
wrapping
of the stripped cable portion forms the outer shield 123 into a helical coil
defining an
inductor. Around the outside of this inductor is provided a non-magnetic metal
conductor 34 which is located inside the housing 31. On the inside of the
housing is
provided a cylindrical shielding layer 35. This shielding layer can be formed
by a
spray coating of a non-magnetic metallic substance which is conductive. The
shielding layer 35 is maintained separate from the conductor 34 so as to be
electrically separated therefrom. In general this is achieved by mounting the
conductor on the support 32 so that it is held spaced at a radial separation
from the
housing 31 and its inside layer 35. The conductor 34 is electrically attached
at one
end to the shielding layer 123 by a soldered joint 37. At the other end of the
conductor is provided a capacitor 38 which is also attached to the conductor
and to
the shield by a soldered joint 39, 40. In this way the inductor defined by the
coiled
shielding layer and the capacitor 40 form a tank circuit which acts to define
a high
impedance to currents tending to formed along the continuous shielding layer
123,
223.
The conductive layer 35 is electrically separated from the shield 123
CA 02636936 2010-08-26
24
and electrically separated from the segmented shield conductor portions 125,
225 so
that it also acts as another of the conductive portions surrounding that part
of the
cable trap between the ends of the cable portions 121 and 221.
Turning now to Figure 7, the cable 16 is of the construction described
above formed solely by the construction of Figures 1, 2, 3 or 4 or including
cable
traps shown in Figure 5. In this arrangement the cable is a multiple conductor
cable
of the type shown in Figure 2. At the location where the cable enters the
receive coil
structure 15, the cable shielding material is opened and removed to expose the
individual conductor portion 16A, 16B, 16C and 16D. These conductors are then
connected to either the pre-amplifiers for each individual coil loop, or
directly to the
coil loop. Around the outer structure is provided a jacket or a covering to
prevent
inadvertent electrical connection. Thus each of the cable portions 16A through
16D
is itself of the construction shown in Figure 1 or Figure 2.
In this way the presence of these cable portions inside the received
coil structure avoids the generation of currents on control wires or the
shielding
conductors of these coaxial cables. While the heating effect is of lesser
importance
in this area, the presence of currents on the shielding conductor would
otherwise
provide extraneous RF fields at the coil portions 15A through 15D which would
interfere with the RF field from the transmit body coil and thus generate
artefacts.
Thus the individual cable portions are shielded using the same concept as
described
herein to reduce the currents in the conductors thereof using the same
concepts and
arrangements.
CA 02636936 2010-08-26
25
Since various modifications can be made in my invention as herein
above described, and many apparently widely different embodiments of same made
within the spirit and scope of the claims without department from such spirit
and
scope, it is intended that all matter contained in the accompanying
specification shall
be interpreted as illustrative only and not in a limiting sense.
