Switchable MRI RF coil array with individual coils having different and overlapping fields of view

An array of plural magnetic resonance imaging (MRI) RF coils is provided having different and overlapping fields of view. Controllable switches are connected with each individual coil of the array and are capable of selectively conditioning any one of the coils for individual usage in an MRI procedure. Either mechanical or electrical (e.g., PIN diode) switching control may be utilized. Preferably, controllable electrical switches are located at points having approximately zero RF potential. Distributed capacitance is also preferably employed for reducing terminal inductance, preventing the establishment of spurious magnetic fields and facilitating the use of electrical switching diodes and/or varactor capacitance elements. Such distributed capacitances are also dimensioned so as to cause the terminal inductance of each coil to be within the tuning/matching range of a common tuning/matching RF circuit.

This invention relates generally to magnetic resonance imaging (MRI) 
utilizing nuclear magnetic resonance (NMR) phenomena. It is particularly 
directed to method and apparatus associated with a selectively switchable 
array of MRI RF receiving coils having different and overlapping fields of 
view. 
Magnetic resonance imaging (MRI) systems are now in extensive commercial 
usage. In general, a human body or other object to be imaged is situated 
within a static magnetic field onto which may be superimposed a 
predetermined sequence of magnetic gradients and RF excitation pulses 
which cause NMR responses to be elicited from certain nuclei. These 
characteristic NMR RF responses are coupled to a suitable RF coil, 
detected and processed by any one of a number of suitable MRI algorithms 
and thus used to create a digitized electronic image of selected two or 
three dimensional portions of the body or object under examination. 
The design, physical realization and production of suitable RF coils for 
optimally coupling RF signals into and out of a human body (or other 
object) during MRI procedures turns out to be a very complex and difficult 
task. And it has also turned out to be one of the important subsystems of 
an overall MRI system with substantial effect on the imaging capability of 
that system. 
For example, image quality can be improved by improving the achievable 
signal-to-noise ratio (SNR) of the system. It is well understood that such 
improvements in SNR may be achieved by limiting the volume to which a 
given RF coil is effectively coupled--e.g., its effective field of view 
(FOV). At the same time, if one has a very limited FOV, then it will be 
impossible to obtain a large scale screening FOV image with that same 
coil--and the positioning of a small FOV coil with respect to the patient 
then becomes critical because its FOV must clearly be spatially 
overlapping the particular body region of interest. Thus, if a single coil 
of limited FOV is utilized, it may be necessary to remove the patient and 
associated RF coil assembly from the patient bed and to reposition the 
coil with respect to the patient one or more times during an extended MRI 
procedure. This can be inconvenient for the patient--and it is also time 
consuming in an environment where time is very valuable due to the very 
high capital investment and/or costs of operating personnel and the like. 
For example, when imaging the spine with MRI, it may be desirable to have 
an initial screening study over a long portion of the spine followed by a 
higher resolution study over a smaller portion of the spine. Assuming that 
electrical loading of the RF coil by the patient's body is the dominant 
source of image noise, noise levels obtained with any one coil will vary 
roughly in inverse proportion to the square root of the FOV of that coil. 
Thus, for a surface coil having a generally rectangular configuration, the 
SNR will decrease about 40% if the width is maintained constant and the 
longitudinal FOV is doubled. Furthermore, as spatial resolution is 
increased, the FOV of the longer coil may exceed that of the imaging 
sequence in the phase encoded axis. While there are techniques to reduce 
this particular problem, they increase the complexity of the imaging 
sequence and operation, can limit flexibility and may not everywhere be 
generally available. 
We have concluded that to optimize the SNR of a given MRI study, the coil 
size should be chosen so as to have an FOV which matches the FOV of real 
imaging interest. In accordance with this conclusion, at least two RF 
receiving coils would be needed to perform a screening study followed by a 
more detailed higher resolution study. 
In the past, different such coils have been utilized by physically 
replacing one coil with the other. Typically, this has required the 
patient to be removed from the patient bed for the coil swapping 
procedure. Clearly, this impairs operating efficiency--and the 
repeatability of body/center of FOV coil positioning is compromised if the 
patient moves while the coils are being physically swapped one for the 
other. 
We have discovered a convenient alternative to such a physical coil 
swapping procedure. In particular, we have discovered a convenient and 
practical arrangement utilizing a switchable MRI RF coil array having a 
different and overlapping field of view for the individual coils in the 
array. 
Prior to out conception and actual reduction to practice of this invention, 
we can recall no suggestion from others in the art of any type of switch 
selected MRI RF coils having different but overlapping fields of view 
which may be optimally switch selected so as to not necessitate physically 
repositioning or swapping RF coils during MRI studies. However, there 
recently have been some efforts by others along these general lines 
described during the Sixth Annual meeting and Exhibition of the Society of 
Magnetic Resonance and Medicine, Aug. 17-21, 1987 held in New York City. 
In addition to applicants' own abbreviated publication thereat, there were 
possibly related publications by others as noted below: 
"Switched Array Coils: A new multi-purpose tool in MRI" by Hequardt et al, 
page 408. 
"A New Revolution in Surface Coil Technology: The Array Surface Coil" by 
Boscamp, page 405. 
"A Dual Cervical/Thoracic Spine Surface Coil: Clinical Throughput 
Considerations" by Totterman, et al. page 4. 
"Practical Aspects of the Concentric Pair Surface Coil Design for 
Localizing Nuclear Magnetic Resonance Spectra from Human Organs" by 
Vaughan et at, page 849. 
"Arrays of Mutually Coupled Local Coils for High Resolution MR Imaging" by 
Wright et al, page 96. 
We have discovered a practical RF MRI coil array (e.g., having at least two 
individually usable and switch selectable coils which have differently 
dimensioned overlapping fields of view) and arrangements of controllable 
RF switches (e.g., either mechanically or electrically) actuated which may 
be connected with individual respectively associated coils and made 
capable for selective conditioning of any one of the coils for 
individualized use in an MRI procedure. 
Preferably, each coil includes at least one distributed capacitance element 
as well as at least one controllable switch element serially disposed 
therealong. If the switch element is electrically operated, then each 
switch element has at least one control circuit electrically connected 
thereto so as to permit selective electrical actuation of its associated 
coil. Such control circuits typically include a parallel resonance RF trap 
circuit having an approximate parallel resonance maximum RF impedance at 
the intended MRI RF operating frequency of the coil (e.g., 15 MHz) while 
still having a relatively small DC impedance for the pulses of DC control 
currents used to control the switch (e.g., an array of parallel-connected 
PIN diodes). 
The distributed capacitance elements are useful for simultaneously 
performing several important functions. For example, they may serve as the 
requisite DC blocking element so as to permit necessary DC bias voltages 
to be applied to PIN switching diodes and/or to varactor diode capacitance 
elements or the like (e.g., as used to tune/match the coil) while also 
preventing the flow of DC currents about the coil structure (which would 
produce spurious magnetic fields). 
In addition, by distributing capacitance elements along the length of at 
least the larger coil structures, the effective terminal inductive 
reactance of such coils may be maintained at fairly low levels thus 
decreasing the magnitude of voltage swings and also making it possible to 
use a common RF tuning and/or impedance matching network for any one of 
the array coils that is selected for actual use at a given time. 
Our preferred embodiment also locates the RF circuit switch elements at 
points having an approximately zero RF potential. By so doing, adverse 
effects upon the Q of the coil are minimized--and there is less likelihood 
of adverse RF coupling to the switch control circuits (which must 
necessarily be connected to the RF circuit in the case of PIN diodes). 
Although we have discovered a minimum switch control circuit configuration 
wherein a return circuit is shared by both the control circuit and an RF 
circuit, this arrangement requires an unbalanced RF coupling circuit. With 
somewhat more complexity in the control circuitry, a more desirable 
balanced RF coupling circuit may still be utilized. 
In one exemplary embodiment, each of two surface coils are made of 
conductive strip material having interruptions in continuity at periodic 
intervals which are bridged by capacitance elements. One of the surface 
coils has an enclosed approximately rectangular area approximately twice 
that of the other coil--and they have approximately coincident centroids. 
Such a surface coil arrangement is particularly useful in MRI studies of 
the spine, as previously mentioned. 
In an embodiment using a manually actuated switch for the selection of a 
given array coil, the exemplary embodiment includes an array of first, 
second and third fixed electrical conductive areas connected respectively 
to the first coil, to an RF input/output port (e.g., connected to the RF 
matching and tuning network) and to the second RF coil. A movable bridging 
contact having spring-loaded contact fingers is slidable into and out of 
bridging RF circuit contact between the common second contact area and 
either the first or third contact areas. It is mechanically coupled to an 
elongated operating arm which is slidably mounted for effecting the just 
mentioned slidable moveable of the spring-loaded contact member.

The exemplary system shown in FIG. 1 includes the usual elements of a 
magnetic resonance imaging (MRI) system. For example, there is a static 
magnet structure 10 (e.g., a cyrogenic solenoidal magnet producing a 
static field in the Z direction) associated with a collection of RF 
transmitting coil(s), magnetic gradient coils, shims, etc., 12 which are 
capable of effecting gradients in the static magnetic field along any 
desired spatial direction in conjunction with the programmed sequence of 
RF pulses transmitted into a desired portion of a body 14 to be imaged. 
MRI responses are, in this exemplary embodiment, selectively coupled 
either to the large field of view (FOV) RF receiving coil 16 or to a 
smaller FOV RF receiving coil 18. Selection of the desired active receive 
coil 16 or 18 may be achieved through manual switch actuation or through 
electrical control voltages via line 20 from MRI system control and 
processing circuits 22. Since the necessary electrical control signals 
simply constitute on/off bias voltages, it is not believed necessary to 
supply any detailed description as to how such bias voltages may be 
generated (e.g., by minor modification of conventional control and 
processing circuits 22) so as to turn "on" a desired coil by forward 
biasing the proper PIN diode switch). 
In the exemplary embodiment, a common RF matching and tuning circuit 24 is 
used for either of the selected active receive coils 16, 18 so as to match 
their impedance to a suitable RF transmission line (e.g., a 50 ohm coaxial 
cable) 26 which feeds the NMR RF responses to the MRI system control and 
processing circuits 22 for conventional processing into a MRI display 28. 
As will be appreciated, such systems typically are under operator control 
via keyboard 30 and the control and processing circuits 22 typically also 
actively control the magnetic gradient coils and RF transmitting coil, 
etc., via conventional control/RF transmission lines 32. 
The physical structure of exemplary surface coils 16, 18 is depicted 
schematically at FIG. 2a (and on the reverse side at FIG. 2b). Each 
surface coil 16, 18 is formed of a relatively wide strip of relatively 
thin copper (e.g., 1 to 3 inches wide and 0.020 inch thick). In the 
exemplary embodiment, each of the surface coils has a width dimension of 
approximately 20 cm while their length dimensions differ by a factor of 2. 
In particular, the large FOV coil 16 has a length of about 40 cm while the 
smaller FOV coil 18 has a length of about 20 cm. As depicted in the FIGS., 
the center of the field of view of each coil is approximately coincident. 
A suitable thin layer of insulation 50 (e.g., a layer of insulating tape 
or the like) electrically insulates coil 16 from coil 18. 
As depicted in FIG. 2athe right-hand terminal leg of each coil 16, 18 is 
commonly connected to one side of RF tuning/matching circuit 24 
(comprising parallel capacitance C.sub.p and balanced series capacitances 
C.sub.s) used to tune the inductive coil reactance to resonance at a 
desired operating frequency (e.g., 15 MHz) and to match its impedance to 
that of the transmission line 26 (e.g., 50 ohm coaxial cable). The other 
input terminal of the RF matching/tuning circuit 24 is connected to an 
electrical contact 52 located intermediate spaced apart left-hand terminal 
leg portions of coil 16, 18 and is capable of being selectively connected 
to either one of the proximate coil legs so as to selectively utilize 
either the large FOV coil 16 or the small FOV coil 18. 
As may be seen in FIGS. 2a and 2b, the continuity of the conductive portion 
of coil 16 has periodic interruptions 60, 62 and 64 which are bridged by 
distributed capacitance elements (e.g., fixed capacitances) C.sub.D1. 
Similarly, the small FOV coil 18 has its continuity interrupted 
periodically at 70 and 72 which interruptions are bridged by distributed 
capacitance elements C.sub.D2. 
Such distributed capacitance elements automatically break the DC continuity 
of the coil structures thus preventing the flow of currents which might 
disturb the static/gradient magnetic field while also facilitating use of 
electrically biased varactor capacitance C.sub.p and/or the series 
matching/coupling capacitances C.sub.s within the tuning/matching circuit 
24. 
As will be appreciated by those in the art, a given tuning/matching circuit 
24 is only capable of tuning/matching inductive reactances within some 
predetermined range extending from some minimum LA to some maximum LB. To 
accommodate inductive reactances outside this normal matching/tuning 
range, additional fixed capacitors or the like might have to be connected 
so as, in effect, to reconfigure or re-dimension the tuning/matching 
circuit 24. The use of the distributed capacitance elements C.sub.D also 
provide a convenient way to permit usage of but a single common 
tuning/matching circuit 24 for any one of the array coils that may be 
selected for use. For example, the terminal inductive reactance L1 of coil 
16 and L2 of coil 18 presented to the tuning/matching range LA to LB. That 
is, the terminal inductive reactance of any given coil will represent the 
sum of the reactive inductances of the various conductive coil sections 
minus the capacitive reactances of the distributed capacitance elements 
C.sub.D. Accordingly, by properly dimensioning the distributed 
capacitances, the terminal inductive reactance of even the larger coil 16 
may be reduced so as to be approximately similar to that of the smaller 
coil 18 
In one exemplary embodiment, the conductive strap elements are one inches 
wide, the gaps in continuity are formed at the mid-points of the 
respective legs as depicted in FIG. 2a and 2b and capacitances C.sub.D1 
are 230 pf while capacitances C.sub.D2 are 580 pf thus causing the 
terminal inductance of coil 16 to be 0.72 microhenries and the terminal 
inductance L2 of coil 18 to be 0.42 microhenries. Thus, the resulting 
inductive reactance presented at the terminal of either of the selected 
coils is within the operating range of the tuning/matching circuit 24 
(e.g., having parallel capacitance of 420-470 pf and balanced series 
capacitance C.sub.s of 25-100 pf (with respect to a 50 ohm coaxial cable 
26). 
Since a balanced RF feed is used in the embodiment of FIGS. 2a and 2b, the 
switch element 52 will be at a point which experiences a considerable RF 
voltage swing. Accordingly, the RF performance of the switch element is 
important--as is the ability to conveniently and easily actuate the switch 
element so as to select a desired one of the coils without necessitating 
movement of the patient and/or major coil structure with respect to the 
patient. An exemplary embodiment of a mechanically actuated RF electrical 
switch suitable for such purposes is depicted at FIG. 3. Here, the common 
RF input/output terminal 52 as well as the terminal leg portions of the 
coil 16 and 18 are all brought to a common plane within an insulating 
substrate 80. A relatively wide (e.g., 1.0 inches) spring-loaded 
conductive finger element 82 (e.g., made of beryllium-copper) is 
mechanically attached (e.g., by epoxy) to one end of a manual operator 
actuating rod 84 which is constrained for sliding movement by suitable 
guides 86a and 86b. These guides also hold the actuator rod 84 and hence 
conductive switch element 82 so as to exert a downwardly directed contact 
force on the conductive fingers at contact element 82 thus insuring that 
it is always in firm slidable contact with the conductive surfaces of the 
elements 16, 52 and 18 as depicted in FIG. 3. A stop element 88 may be 
affixed to the actuator rod 84 between guides 86a and 86b so as to limit 
sliding movement between a first position as shown in FIG. 3 (i.e., 
selecting the large FOV coil 16 for use) and an opposite second position 
(i.e., selecting the small FOV coil 18 for use). 
The presently preferred exemplary embodiment is schematically depicted at 
FIG. 4. Here, rather than manually actuated RF switch elements, 
electrically actuated switch elements (e.g., PIN diodes) are serially 
interposed within each of the coils 16 and 18. Here, a desirable balanced 
RF feed circuit is utilized as in FIG. 2a. As depicted in FIG. 4, this 
implies that there will be substantial voltage swings at the input 
terminals of the coil structures. However, it also implies that there is a 
substantial zero RF potential at the approximate mid-point of each of the 
coils 16, 18. Accordingly, it is possible to locate a PIN diode switch 
(typically a plurality of parallel connected PIN diodes) while minimizing 
adverse lowering of the coil Q and also facilitating the connection of the 
necessary control circuit connections for electrical actuation of such 
switch elements with minimum adverse RF coupling problems to such control 
circuits. 
As depicted in FIG. 4, a PIN diode switch 90 is located at the approximate 
zero RF potential point within coil 18 while a similar PIN diode switch 92 
is located at a similar point in coil 16. One side of each such switch 
structure is interconnected by an appropriate parallel resonant RF trap 
94, 96 to a common control circuit return path 98 while the other end of 
these switch structures 90, 92 is individually connected via a further 
appropriate parallel resonant RF trap 100 and 102 to separate coil select 
control circuits 104 and 106. As will be appreciated, the parallel 
resonant RF traps present an approximately maximum impedance at the 
intended operating frequency of the coil (e.g., 15 MHz) while presenting a 
very low DC impedance (e.g., the equivalent series resistance of the coil 
portion of the resonant circuit) to bias control voltages applied so as to 
switch the PIN diode switches 90, 92 either "on" or "off." 
As will be appreciated, it may be necessary to re-dimension the distributed 
capacitances C.sub.D in the FIG. 4 embodiment so as to provide terminal 
inductive reactances required so that a common tuning and matching circuit 
24 may still be utilized. 
As may be seen in FIG. 4, this preferred embodiment utilizes 4 parallel 
resonant trap circuits. In addition, if the control circuit connections 
and PIN diodes are to be located at a substantially zero RF potential 
point, then there are a number of components and connections which must be 
made in a fairly small physical space. 
Accordingly, an alternative exemplary embodiment is depicted at FIG. 5. 
Here, only two parallel resonant trap circuits (100 and 102) are required 
since the control circuit return in shared in common with the RF return in 
an unbalanced tuning/matching circuit 24. Such an unbalanced 
tuning/matching arrangement is required so as to again place the 
electrically switchable diode elements 90, 92 at approximately zero RF 
potential points (thus minimizing adverse Q effects on the coil and/or 
adverse RF coupling to the switch control circuit). 
It will be noted that in these exemplary embodiments, the non-selected 
coil(s) need not be actively detuned (e.g., so as to avoid adverse 
coupling to the selected coil and/or transmit RF coil) since it is 
effectively disconnected (e.g., so as to no longer constitute an RF coil). 
This may be useful in avoiding image artifact. 
As should be appreciated, although surface coils have been depicted in the 
exemplary embodiments, other types of RF coils (either transmitting and 
receiving or coils used commonly for both transmitting and receiving) may 
similarly be utilized. Furthermore, although the exemplary embodiments 
only depict two coils within the switch selectable coil array, additional 
individual coils could also be included within such a switch selectable 
array. 
While only a few exemplary embodiments have been disclosed in detail, those 
skilled in the art will recognize that many variations and modifications 
may be made in these exemplary embodiments while yet retaining many of the 
novel features and advantages of this invention. Accordingly, all such 
variations and modifications are intended to be included within the scope 
of the appended claims.