U.S. patent application number 09/849217 was filed with the patent office on 2002-12-05 for magnetic resonance imaging coil arrays with separable imaging coil elements.
Invention is credited to Vij, Kamal.
Application Number | 20020180442 09/849217 |
Document ID | / |
Family ID | 25305329 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020180442 |
Kind Code |
A1 |
Vij, Kamal |
December 5, 2002 |
MAGNETIC RESONANCE IMAGING COIL ARRAYS WITH SEPARABLE IMAGING COIL
ELEMENTS
Abstract
An apparatus for assembling a magnetic resonance imaging (MRI)
coil array using separable imaging coil elements and a cable holder
for connecting the imaging coil elements to the MRI scanner. Each
imaging coil element has a plug shaped connector, which is used for
separably connecting to a mating receptacle interface on the cable
holder. Each cable holder is pre-formed for a particular anatomy
and has receptacle interfaces that are located for optimal
placement of imaging coil elements. Ultra-low input impedance
preamplifiers are used for minimizing interaction between imaging
coil elements. This gives users greater freedom in optimizing
position of individual imaging coil elements, as overlapping of
adjacent coil elements is not required. Imaging coil elements from
one coil array can be easily removed and used in assembling a
different coil array.
Inventors: |
Vij, Kamal; (Chandler,
AZ) |
Correspondence
Address: |
KAMAL VIJ
5791 WEST GAIL DRIVE
CHANDLER
AZ
85226
US
|
Family ID: |
25305329 |
Appl. No.: |
09/849217 |
Filed: |
May 4, 2001 |
Current U.S.
Class: |
324/322 ;
324/318 |
Current CPC
Class: |
G01R 33/3621 20130101;
G01R 33/3415 20130101; G01R 33/341 20130101 |
Class at
Publication: |
324/322 ;
324/318 |
International
Class: |
G01V 003/00 |
Claims
I claim:
1. An apparatus for assembling magnetic resonance imaging (MRI)
coil arrays for receiving radio frequency MRI signals, said
apparatus comprising: a) at least one imaging coil element, wherein
said imaging coil element has a plug interface, including
conductive connectors; b) at least one cable holder, wherein said
cable holder has: i) at least one receptacle interface, including
conductive connectors, that mates with said plug interface; and ii)
means for connecting at least one said imaging coil element to the
MRI scanner; whereby a coil array is assembled by separably
connecting at least one said imaging coil element to said cable
holder across said plug interface and said receptacle
interface.
2. The apparatus of claim 1 further comprising: means, including
low input impedance preamplifier, for causing said imaging coil
element to operate in close proximity, with minimal interaction, of
another imaging coil element without necessitating any overlapping
between the two.
3. The apparatus of claim 1 further comprising: means, including
adding circuits, for combining MRI signals received by two or more
imaging coil elements.
4. The apparatus of claim 1, wherein said imaging coil element is
fabricated from flexible materials thereby allowing said imaging
coil element to closely conform to the patient anatomy.
5. The apparatus of claim 1, wherein said imaging coil element is
selected from the group comprising loop type imaging coils, figure
eight shaped imaging coils, saddle shaped imaging coils and
combinations thereof.
6. A MRI coil array for receiving radio frequency MRI signals
comprising: a plurality of imaging coil elements where in at least
one of the elements is a separable imaging coil element; a cable
holder having means for connecting said imaging coil elements to
the MRI scanner; a connection interface, including mating
conductive connectors, separably connecting said separable imaging
coil element to said cable holder across a predetermined joint
area.
7. The MRI coil array of claim 6 further comprising: means,
including low input impedance preamplifiers, for causing said
imaging coil elements to operate in close proximity, with minimal
interaction with each other, without necessitating overlapping of
adjacent imaging coil elements.
8. The MRI coil array of claim 6 further comprising: means,
including adding circuits, for combining MRI signals received by
two or more of said imaging coil elements.
9. The MRI coil array of claim 6, where in said imaging coil
elements are fabricated from flexible materials thereby allowing
said imaging coil elements to closely conform to the patient
anatomy.
10. The MRI coil array of claim 6, where in said imaging coil
elements are selected from the group comprising loop type imaging
coils, figure eight shaped imaging coils, saddle shaped imaging
coils and combinations thereof.
11. A method for assembling MRI coil arrays for receiving radio
frequency MRI signals from a shared set of common parts thereby
minimizing number of dedicated parts used for assembling each coil
array, said method comprising the steps of: providing a collection
of separable imaging coil elements, each having a plug interface,
including conductive connectors; providing a collection of cable
holders, each having at least one receptacle interface, including
conductive connectors, that is capable of mating with said plug
interface; joining at least one of said separable imaging coil
elements to one of said cable holders across a predetermined said
receptacle interface using said plug interface to form a coil
array; whereby it is possible to remove one or more of said imaging
coil elements from one coil array and use them in assembling
another coil array.
12. Method of claim 11, further comprising the step of: providing
means, including low input impedance preamplifiers, for causing
said separable imaging coil elements to operate in close proximity,
with minimal interaction, of each other without necessitating any
overlapping between adjacent elements.
13. Method of claim 11, further comprising the step of providing
means, including adding circuits, for combining MRI signals
received by two or more of said separable imaging coil
elements.
14. Method of claim 11, further comprising the step of: fabricating
said separable imaging coil elements from flexible materials such
that they can closely conform to the patient anatomy.
15. Method of claim 11, further comprising the step of: selecting
said separable imaging coil elements from the group comprising loop
type imaging coils, figure eight shaped imaging coils, saddle
shaped imaging coils and combinations thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable
STATEMENT REGARDING FEDERALLY SPONSERED RESEARCH OR DEVELOPMENT
[0002] Not applicable
[0003] 1. Field of Invention
[0004] This invention relates to magnetic resonance imaging coils,
specifically to such coils that are arrays comprising of multiple
smaller coils.
[0005] 2. Description of Prior Art
[0006] In a typical magnetic resonance imaging (MRI) examination
activity, an imaging coil is placed over the anatomy of interest
and then the patient is placed inside a large magnet. The static
field of the magnet aligns the protons in the patient's body in the
direction of its magnetic field. A large whole body coil is used to
generate high-energy (pea power 5-15KW) radio frequency (RF) pulses
that excite protons in the patient. These protons relax back to
their original state after the excitation is switched off. At this
time the scanner automatically switches `ON` the imaging coil and
turns the body coil `OFF`. As the protons relax they induce minute
RF signals in the imaging coils. These signals are picked up by the
imaging coil and fed to a signal processing system from where the
information is transferred to a computer for image reconstruction
and display.
[0007] Signal to noise ratio (SNR) and field of view (FOV) are the
two most important parameters for characterizing a MRI imaging
coil. SNR is a measure of the sensitivity of the imaging coil and
its intrinsic electric resistance. Higher the sensitivity of a coil
greater is the signal voltage induced in the coil. A coil with
higher intrinsic resistance generates more noise than the one with
lower intrinsic resistance. FOV is a measure of the region over
which the coil is sensitive enough to pick-up acceptable SNR
signals. The ultimate MRI imaging coil can be defined as the one
with the largest FOV and the highest SNR over that FOV.
[0008] Single loop imaging coil is the most basic MRI coil. For a
single loop coil there is the diameter of the loop determines the
SNR and the FOV of the coil. Larger the diameter of the coil larger
is its FOV. However large diameter coils have higher intrinsic
resistance because the induced currents have to travel over a
longer conductor path along the circumference in comparison to a
smaller diameter loop coil. Therefore, in the past, designers have
had to optimize the diameter of such loop coils depending on the
application and make a choice between SNR and FOV.
[0009] U.S. Pat. No. 4,825,162 describes a method for
simultaneously receiving NMR signals from a plurality of
closely-spaced imaging coils. This technique overcomes the tradeoff
between SNR and FOV by using multiple small sized coils for
simultaneous data acquisition. Such coils are commonly referred to
as phased array coils or coil arrays. Depending on the anatomy
several designs exist for these coil arrays. The most well known
coil array is the four element spine array. As the name suggests
this is made of four identical circular loops placed side by side
in a line with adjacent loops slightly overlapping each other.
Signals from each of these loops is acquired simultaneously and
digitally recombined, such that a long FOV is generated. Such a FOV
can be generated using a single loop coil with a much larger
diameter. However the SNR would suffer as the large loop has lower
sensitivity than each of the smaller loop coils. Thus, by
recombining signals from an array of smaller coils it is possible
to get high SNR as well as a large FOV.
[0010] When MRI coils that are tuned to the same radio frequency
are brought together in close proximity as in phased array coils,
they inductively couple to each other and detune each other. This
results in image quality that is worse than what can be achieved
when each coil is used by itself Phase array coil technology
overcomes this problem by slightly overlapping adjacent coils and
by using specially designed preamplifiers that dampen the current
flow in the individual coils.
[0011] In order to get the best image quality coil arrays are
placed as close as possible to the anatomy under examination. The
array housing is specially contoured to fit a particular part of
the body such that the anatomy of interest lies well within the
area of highest sensitivity of individual coil elements. Since the
FOV needed for every application is different, the number and the
type of elements used in a array coil vary from one array to the
other. This leads to an MRI clinic investing in a unique array for
every application, for example, a special spine array with a long
FOV is used for spine imaging while a special knee array with much
smaller FOV is used for knee imaging. A typical spine array has six
imaging coil elements while a knee coil has only four elements
[0012] During an MRI scan a patient can get burnt if the coaxial
cable of the imaging coil loops over itself. Large RF current can
get induced in the loop from the RF energy transmitted by the body
coil. Also, the cable it self can act like an antenna and have RF
standing waves form on the outer shield of the coaxial line. All
these can result in RF burns to the patient. These issues have been
overcome by mechanically making cables less prone to looping, by
using Baluns and RF traps to prevent any RF currents from flowing
on the cable shield.
[0013] Cable burn issues are an even greater concern with phased
arrays since there are as many cables as there are imaging coil
elements. Care is taken to anchor the cable in special jackets to
prevent them from looping. Additionally the coils them selves are
anchored in preset location inside coil housing to minimize their
interaction with each other and their cables. Individual coil
overlap is preset during manufacture of the coil array and cannot
be changed. Also, since all the elements of the array are together
if one element or the cable assembly fails then the whole array is
sent back to the factory for repair.
[0014] Phased array coils such as those described in U.S. Pat. Nos.
6,084,411 and 5,905,378 restrict a user to coil arrangements in
which adjacent coils elements have to be in a fixed predetermined
overlap position in order to minimize inter-coil coupling locations
and minimize the loss in SNR due to coupling. Coils that are not
overlapped have to be physically separated at a minimal distance
such that they do not couple to each other. This limits the
configurations the device can be used in. For example, these coils
cannot be used in a geometrically opposed manner in which two coils
face each other without significantly degrading SNR. Further, the
coils are isolated from the cables through tuned Baluns. Another
drawback of using this approach is that the tuned Balun introduces
an insertion loss that adds to the overall noise in the system.
Also, this approach does not eliminate burns that can occur from a
cable looping over it-self
[0015] All the phased array coil heretofore known suffer from a
number of disadvantages:
[0016] (1) they are all application specific coils with dedicated
coil elements,
[0017] (2) adjacent coil elements have to be overlapped in order to
minimize interactions with each other,
[0018] (3) coil elements are not removable or exchangeable between
array configurations,
[0019] (4) users have limited options for optimizing array
configurations to match their needs and
[0020] (5) if an individual coil element fails then the whole array
assembly has to be sent for repairs.
[0021] The forgoing illustrates limitations known to exist in
present phased array coils. It is apparent that it would be
advantageous to provide an improved phased array coil, such as an
array coil with separable imaging coil elements, and which is
directed to overcoming one or more of the limitations set forth
above. Accordingly, a suitable alternative is provided including
features more fully disclosed hereinafter.
SUMMARY
[0022] The present invention creates a paradigm shift in the
current art of MRI imaging coil arrays. In one aspect of the
present invention, a user can assemble different array coils from a
set of basic building blocks--imaging coil elements and cable
holders. These imaging coil elements can be used over and over to
form array coils of various shapes and sizes. Moreover, since these
coil elements are not dependent on overlap for minimizing
interaction with other coils, the users have considerable freedom
in placing coil elements to better match each individual
application. The ability to move the elements relative to each
other allows the MRI technicians to determine an optimal position
to get the best image quality. These coil elements can be
configured in various planar as well as three-dimensional
arrangements to form arrays that are optimized for SNR and FOV.
[0023] In one embodiment, individual coil elements can be directly
connected via a quick disconnect connector to a pre-formed cable
holder to form a coil array. The same coil elements can be reused
over and again on a different cable holder to form a different
array.
[0024] It is another purpose of this invention to improve patient
safety and minimize the possibility of cable burns by restricting
the movement of cables used for connecting individual coil elements
to the MRI scanner.
[0025] It is another purpose of this invention to allow a user to
make a cost effective investment in a few standard sized coil
elements that can be reconfigured to form new coil arrays versus
investing in several dedicated application specific arrays.
[0026] It is another objective of this invention to produce modular
arrays that can be easily field serviced and maintained. Defective
modules that are easy to identify and can be exchanged in the field
instead of the entire array being sent back to the factory for
repair.
BRIEF DESCRIPTION OF DRAWING FIGURES
[0027] The foregoing summary, as well as the following detailed
description of a preferred embodiment of the invention will be
better understood when read in conjunction with the appended
drawings. For purpose of illustrating the invention, there is shown
in the drawings an embodiment, which is presently preferred. It
should be understood, however, that the invention is not limited to
the precise arrangement and instrumentality shown. In the
drawings:
[0028] FIG. 1a illustrates the equivalent electrical schematic
representation of a typical imaging coil element of the present
invention.
[0029] FIG. 1b illustrates the equivalent electrical schematic
representation of a typical low impedance preamplifier of the
present invention.
[0030] FIG. 2a illustrates the antenna geometry of a prior art loop
antenna used in the loop shaped imaging coil elements of the
present invention.
[0031] FIG. 2b illustrates the antenna geometry of a prior art
figure eight shaped antenna used in the figure eight shaped imaging
coil element of the present invention.
[0032] FIG. 2c illustrates the antenna geometry of a prior art
saddle shaped antenna used in the saddle shaped imaging coil
element of the present invention.
[0033] FIG. 3a illustrates an isometric view of a set comprising of
three loop shaped imaging coil elements--small, medium and
large.
[0034] FIG. 3b illustrates an isometric view of a set comprising of
three figure eight shaped imaging coil elements--small, medium and
large.
[0035] FIG. 3c illustrates an isometric view of a set comprising of
three saddle shaped imaging coil elements--small, medium and
large.
[0036] FIG. 4a illustrates an isometric view of a four-channel
cable holder in which the mating connectors for individual coil
elements are distributed along one dimension.
[0037] FIG. 4b illustrates an isometric view of the four-channel
cable holder populated with saddle shaped imaging coil elements and
setup for imaging a patient's leg.
[0038] FIG. 5a illustrates an isometric view of a four-channel
pelvic array cable holder.
[0039] FIG. 5b illustrates an isometric view of a six-channel spine
array cable holder.
[0040] FIG. 6a illustrates an isometric view of a four-channel
shoulder coil array cable holder.
[0041] FIG. 6b illustrates the shoulder coil array cable holder
populated with loop coil imaging elements and setup on the shoulder
of a patient.
[0042] FIG. 7 illustrates the block diagram of a four channel array
of the present invention with two system connector termination
options--(1) four channel option and (2) single channel option.
DETAILED DESCRIPTION OF THE INVENTION
[0043] A coil array of the present invention consists of multiple
separable imaging coil elements. Each imaging coil elements is
capable of independently receiving MRI radio frequency signals. A
typical imaging coil element of the present invention can be
closely modeled using an equivalent electrical circuit shown in
FIG. 1. In this circuit capacitors C1 27, C2 26 and C3 22 resonate
the imaging coil antenna inductance L.sub.13 Coil 23 at the
resonant frequency of protons. Resonant frequency of protons is 64
MHz in a 1.5 Tesla magnet. Patient loading loss introduced in the
imaging coil is modeled as a resistor R_Patient 24 while the
component losses are lumped together as R_Coil 25.
[0044] Inductor L1 28 has a dual function. During the MRI
examination the scanner biases diode D1 29 `ON` whenever the body
coil transmits. When PIN diode D1 29 is biased `ON` it becomes
close to a short circuit. Value of inductor L1 28 is selected such
that it then resonates the capacitor C1 27 at the resonant MRI
frequency. This results in high impedance across points A and B
shown in the circuit diagram. Imaging coil element is thus open
circuited when the D1 29 is biased `ON`. When the imaging coil
element is used for receiving MRI signals from the patient this
diode is biased `OFF` i.e. reverse biased. During receive mode
inductor L1 28 also helps in matching the coil impedance to fifty
ohms at the input of the preamplifier 31 shown by points C and
D.
[0045] Preamplifier 31 has a ultra-low input impedance which is
designated by resistor R_Preamp 30. This impedance, R_Preamp 30,
has a very small value--typically one and a half ohms. This low
impedance results in the inductor L1 28 resonating capacitor C1 27
and creating high impedance across point A and B even when D1 29 is
biased `OFF`. This impedance helps in minimizing current flow in
the coil thereby reducing the effect of coupling of noise from one
coil element to the other element in the array.
[0046] It is well known to those skilled in the art that for a
single element the noise picked up by the element is due to the
thermal noise from R_Patient+R_Coil. When a second identical
imaging coil is brought close to the first element, noise from the
second element couples into the first element. In such a case the
thermal noise in the first imaging coil element increases to
R_Patient+R_Coil+R12, where R12 is the extra noise resistance given
by the following equation: 1 R12 = 2 L_Coil 2 K 2 R_Patient +
R_Coil + XC1 2 R_Pr eamp ( 1 )
[0047] Where,
[0048] .omega.=2.pi.*(frequency of resonance)
[0049] K--is the coupling coefficient between two imaging coil
elements
[0050] XC1--is the impedance of capacitor C1 at the resonant
frequency
[0051] In order to minimize R12 adjacent coils are slightly
overlapped in an array. Overlapping reduces the coupling
coefficient K to nearly zero.
[0052] Alternatively, minimizing preamplifier impedance R_Preamp 30
can also minimize R12. Theoretically, if R_Preamp 30 is zero then
R12 will also be zero. In this case, inter-coil coupling
coefficient K can have larger values without any coupling of noise
from one coil to the other.
[0053] Present invention allows the end users considerable freedom
in relative positioning of the coil elements of the array. It
relies on R_Preamp 30 to be small enough to minimize R12 effects
while the inter-coil coupling coefficient K can have relatively
higher values. R_Preamp 30 is designed to be less than 1.5 ohms.
Each element of the present invention phased array coil has its own
ultra-low input impedance preamplifier 31 which is mounted adjacent
to the coil. Mounting the preamplifier 31 so close to the coil
requires a small size, high stability and the use of nonmagnetic
components. However, the advantage is that there is no additional
cable loss which would otherwise increase the R_Preamp 30. As long
as R_Preamp 30 is small the coil elements can tolerate higher
inter-coil coupling without significant loss in SNR
performance.
[0054] FIG. 1b illustrates the electrical schematic for the
ultra-low input impedance preamplifier circuit used in present
invention. Signal amplification is achieved using a low noise radio
frequency transistor like a Gallium Arsenide Field Effect
Transistor (GaAs FET) 42. Since the input impedance of the
preamplifier is critical for de-coupling the imaging coil element,
all the elements at the input of the transistor 42 have an ultra
low resistive loss. The input matching inductor 43 is made from a
low loss materials like oxygen free 16AWG copper or silver wire.
The matching inductor 43 design is optimized for minimal series
resistance by adjusting the diameter and length of the inductor. A
conductive shield 41 is used for isolating the sensitive
preamplifier 31 electronics from external sources of noise. This
conductive shield 41 which surrounds the components is kept at a
minimal distance from the matching inductor 43. Typically, this
distance is at least two times the diameter of the input inductor
43. Matching capacitor 44 is a low loss ceramic capacitor with a
minimal Quality Factor of one thousand. Radio frequency choke 45
which provides a bypass path for the bias for the PIN diode 29,
typically, has a minimal impedance of five kilo-ohms at the MRI
signal frequency. The GaAs FET 42 is carefully selected for the
highest input impedance, gain and stability at the MRI frequency.
Rest of the components of the preamplifier are fairly standard and
are well known to those skilled in the art.
[0055] Once inter-coil coupling is minimized and the weak signals
picked by the coil elements amplified by the low input impedance
preamplifier 31 there are several options for transmitting the
signals to the main receiver. The simplest method is via a coaxial
cable. Coil element shown in FIG. 1a is connected to the scanner
using a quick disconnect plug shaped connector 5 that has a coaxial
contact and a single pin. Pin 1 brings in the DC bias for the
preamplifier while pin 2 which is the center pin of the coaxial
connector brings in the center conductor of the coaxial cable. Pin
3, the outer shell of the coaxial connector, is connected to the
system ground via the shield of the coaxial cable. Pin 2 carries
both the RF output from the preamplifier 31 as well as the DC
switching bias for the PIN diode 29 on the coil element. Since, the
first stage of signal amplification is on the coil subsequent
stages like the connector and the cable can have higher RF
insertion loss without affecting the image SNR.
[0056] Most array coils are a combination of several closely placed
loop antennas 51 or figure eight shaped antennas 52 or saddle
shaped antennas 53. The individual geometry of the loop antenna,
figure eight shaped antenna and saddle shaped antenna is
illustrated in FIG. 2a, FIG. 2b and FIG. 2c respectively. These
three antenna shapes have a different imaging volume associated
with each of them. The loop antenna 51 produces flux that is
perpendicular to the face of plane containing the loop. Loop
antenna 51 has the highest sensitivity in the center of the loop
and its sensitivity drops as one moves away from the loop. As a
rule of thumb, for a circular loop, the useful imaging volume is
restricted to a cylindrical volume defined by the perimeter of the
loop and bound on either side of the plane containing the loop at a
distance equal to the radius of the loop. A figure eight shaped
antenna 52 produces flux that is parallel to the plane containing
the antenna. This field is the strongest close to the coil but dies
rapidly away from the coil. In conjunction with a loop antenna 51,
the figure eight shaped antenna 52 is used to produce a quadrature
field. A saddle shaped antenna 53 has a very homogenous imaging
volume inside the cylindrical shape enclosed by the antenna. This
makes this particular shape very useful for volume imaging
application, example: wrist and knee imaging, where a homogenous
field is desired over the entire enclosed cylindrical shaped
volume. Thus, most of the phased array coil designs reported in
literature till date are closely spaced clusters of various sized
antennas made from above mentioned basic three antenna shapes.
These basic shapes also form the building blocks of the present
invention.
[0057] FIG. 3a shows three different sizes--small 1, medium 2 and
large 3 of the preferred embodiment of the loop type imaging coil
elements that are used in building the array coils of the present
invention. The antenna used in the coil element is made from
flexible conductive materials such as flexible circuit boards. The
antenna is then molded in a soft polyurethane foam housing. All the
antenna electronics shown in FIG. 1a, including the preamplifier,
are enclosed within this flexible housing. Electrical connection to
the imaging coil element is made through a short, 3 to 4 inch,
cable assembly 4 that comes out of one side of the housing and is
terminated into a quick-disconnect plug-shaped connector 5.
Additional types of imaging coil elements used are shown in FIG. 3b
and FIG. 3c. These are the figure eight shape and saddle shape
imaging coil elements. Just like the loop coil elements they are
also available in different sizes--small figure eight 6, medium
figure eight 7, large figure eight 8, small saddle 9, medium saddle
10 and large saddle 11 shaped imaging coil element.
[0058] In one embodiment of the present invention, special cable
holders are used for managing the various cables that connect the
arrays to the MRI scanner. It is important for patient safety that
the cables do not loop. Cable looping can cause RF burns. FIG. 4a
illustrates a linear four channel cable holder. This holder has
four mating receptacle-shaped connectors 13 for joining the
plug-shaped connector 5 of an imaging coil element. The cable
holder has a flexible flat cable section 14 and a stiffer
semi-rigid front end 15. Semi-rigid front end 15 prevents the cable
assembly from accidentally looping over it self Since the different
cables for the individual coil elements emanate from this end it is
designed to provide strain relief to rest of the cable assembly The
cable holder connects to the MRI receiver through a system
connector box 19.
[0059] The linear four channel cable holder can be used to assemble
a lower extremity array coil using 4 saddle shaped imaging coil
elements. FIG. 4b illustrates the lower extremity array setup on a
patient's leg. This array comprises of a large saddle shaped
imaging coil element 11 to go over the thigh, two medium saddle
shaped imaging coil elements 10 to go over the knee and the calf
area and a small saddle shaped imaging coil element 9 to go over
the area between the calf and the ankle. Other different sized
saddle shaped imaging coil element can be used to optimize this
array for a given patient size. The same coil array could also be
used with a loop type imaging coil element 3 and three saddle
shaped imaging coil elements 11 to image the femoral artery from
the pelvis to the knee. Similarly, the opposed pair cable holder
shown in FIG. 5a can be used for forming a four channel pelvic
array using four large loop coil elements 3.
[0060] FIG. 5b shows a spine array cable holder 17 which has an
integrated soft pad 17b on which the patient can lie down on her
back for a spine exam. This holder is used to make a six-channel
spine array coil using six small loop coil elements 1. Before using
this coil each element is first connected to it own mating location
on the cable holder 13 and then dropped into the matching slot
built into the cable holder. A flat cable 14 comes out of one end
of the coil and carries the signals from the six elements to the
system connector box 19.
[0061] Four small loop coil elements 1 from the spine array can
also be used to form a four-channel shoulder array coil. FIG. 6a
shows the cable holder 18 for a four-channel shoulder array coil.
Two elements are dropped in bottom pad while the upper two elements
connect to the cables that come out of a semi rigid post 33. For
shoulder exam using this array the patient first lies down on the
imaging table and then the lower pad of the array is inserted under
the shoulder, see FIG. 6b. Then the upper loop coil elements 1 are
connected to the cable holder. The upper two coils can be freely
moved two to four inches by the radiology technician for optimal
placement over the region of interest. Elastic straps 32 and other
similar means can be used to hold the upper coils elements in place
after their location has been finalized.
[0062] The cable holders connect to the MRI scanner via the system
connector box 19. Depending on the receiver design of the MRI
scanner there are at least two options for the system connector box
19. FIG. 7 illustrates the two options in the case of a array coil
that has four imaging coil elements of the present invention. The
cable connections are shown by dotted line going to both types of
system connector 19 boxes. Depending on the MRI scanner one of the
two options will be used at a given time.
[0063] Some scanners have several receivers for handling signals
emanating from each element of the array. If a scanner has such a
multi-channel receiving capability then the system connector box 19
has the appropriate mating connectors 20 for transferring signals
from each imaging coil element directly to the MRI scanner. This
multi-channel output is called option 1 in FIG. 7.
[0064] If the MRI scanner can only receive a single signal channel
then the outputs of the individual imaging coil elements are
combined in an analog fashion using Wilkinson hybrid combiners 21
in the system connector box 19. Such hybrid combiners 21 are well
known to those skilled in the art. This single-channel output is
called option 2 in FIG. 7.
[0065] Accordingly, the reader will see that phased array coils of
the present invention can be assembled easily by joining imaging
coil elements to the cable holder specified for the desired array
configuration. The elements can just as easily be removed from one
array configuration and joined to another cable holder to form a
second array configuration that may be totally different from the
first one. This ability to share imaging coil elements allows the
user to make several different array configurations from a few
common parts. This is more desirable than investing in several
different application specific phased arrays with dedicated imaging
coil elements. Further, since the coils are de-coupled effectively
by the low impedance preamplifier they do not have to rely on
overlapping for minimizing inter-coil coupling. This allows a user
considerable freedom in individual imaging coil placement without
SNR degradation. All the improvements mentioned earlier make the
imaging coil element extremely rugged and portable. It can be
joined in various array configurations with consistent SNR
performance from one array configuration to the other.
[0066] While several presently preferred embodiments and teachings
of the present invention have been described in detail herein, many
modifications and variations will now become obvious to those
skilled in the art. It is my intent therefore to be limited only by
the scope of appending claims and not by the specific detail
presented by way of illustration.
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