Double-sided RF shield for RF coil contained within gradient coils used in high speed NMR imaging

A magnetic resonance system includes a cylindrical shield disposed between a quadrature RF coil and surrounding gradient coils. The shield includes a pair of copper sheets separated by an insulating dielectric sheet. A pattern formed by cuts in one copper sheet aligns with the currents induced in the shield by one of the RF quadrature fields, and a pattern formed by cuts in the other copper shield aligns with the other RF quadrature field. The copper sheets are shorted together to prevent voltage breakdowns and short cuts are made in each to prevent gradient induced eddy currents.

BACKGROUND OF THE INVENTION 
The present invention relates to radio-frequency (RF) shields and, more 
particularly, to a novel double-sided RF shield for placement between an 
RF body coil and a set of gradient coils in a nuclear-magnetic resonance 
(NMR) imaging device. 
An NMR imaging device typically utilizes a set of three gradient coils to 
obtain spatially-selective information. Each of these gradient coils 
generally contain a multiplicity of turns of conductive wire, with total 
lengths of up to several hundred meters. RF fields lose a significant 
portion of their energy if these fields impinge upon the conductive wires 
of the gradient coils; while the loss mechanism is not fully understood, 
it is probably associated with high current resonances exciting the 
gradient structure and producing associated high losses. Any RF power 
loss, in the gradient coils or otherwise, appears as a lowering of the 
quality factor Q of the RF coil and consequently appears as a lowering of 
the signal-to-noise ratio (SNR) attainable in the imaging device. 
Accordingly, it is highly desirable to prevent penetration of the RF field 
into the surrounding gradient coils and a shield is typically placed 
between the RF coil and the gradient coils to accomplish this. The RF 
shield must, however, be substantially transparent to the gradient 
magnetic fields and therefore must prevent inducement of any significant 
shield currents at gradient frequencies (typically less than about 10 KHz) 
to prevent temporally-dependent and/or spatially-dependent magnetic field 
inhomogeneities from appearing and having an adverse affect on the 
resulting image. 
Hitherto, the most commonly used RF shield has been a double-sided shield 
using a copper-dielectric-copper laminate sheet. The pattern of the 
conductive paths in each copper sheet is generally an approximation of the 
current paths induced in a solid shield due to the field produced by the 
RF coil. One such structure is disclosed in U.S. Pat. No. 4,879,515, which 
issued on Nov. 7, 1989 and is entitled "Double-Sided RF Shield For RF Coil 
Contained Within Gradient Coils Of NMR Imaging Device". 
To prevent the flow of eddy currents in the copper sheets due to the 
gradient magnetic field pulses, the conductive patterns are cut, or 
open-circuited, such that no closed loops are formed. However, because the 
copper patterns are identical on both sides of the dielectric layer 
(except for the placement of the cuts) they form a capacitor which offers 
a very low impedance at the Larmor frequency of the RF coil. This 
capacitance effectively short circuits the cuts and maintains the 
integrity of the shield at RF frequencies. 
High speed imaging ("HSI") and echo planar imaging ("EPI") employ very high 
speed gradient pulses with slew rates up to 230 T/m/s. The standard 
gradient coils, gradient amplifiers, RF coils and shielding components on 
a whole body MR imaging system have had limited success in performing 
these protocols. The primary reason is the undesirable reactions between 
the RF shield and the gradient amplifiers, leading to gradient amplifier 
instability or saturation as well as RF shield heating. The transitional 
environment between the RF coil and the gradient coils must be improved if 
such protocols are to be used. 
Another challenge to the design of a successful RF shield is its cost. The 
RF coils used in state-of-the-art MR imaging systems produce two RF fields 
oriented 90.degree. from each other as measured in a plane perpendicular 
to the direction of the polarizing field. As a result, prior systems using 
such quadrature fields employ two separate shields (i.e. copper 
sheet-dielectric-copper sheet) with their conductive patterns separately 
aligned to coincide with the respective quadrature fields produced by the 
RF coil. The use of two such shields is costly. 
SUMMARY OF THE INVENTION 
The present invention is an RF shield for a quadrature RF field coil which 
does not significantly alter the performance of the gradient field 
subsystem or the performance of the RF coil. More specifically, the 
invention includes a dielectric sheet disposed around the quadrature RF 
field coil, a first copper sheet mounted on one surface of the dielectric 
sheet and having conductive paths formed therein which coincide with the 
currents induced by one of the fields produced by the quadrature RF field 
coil, and a second copper sheet mounted on the other surface of the 
dielectric sheet and having conductive paths formed therein which coincide 
with the currents induced by the other of the fields produced by the 
quadrature RF field coil, wherein the conductive paths formed in said 
first and second copper sheets include a plurality of closed loops, and 
cuts are made to open circuit each of these closed loops. 
A general object of the invention is to provide a shield for a quadrature 
RF coil. It has been discovered that a single copper-dielectric-copper 
shield can be used for a quadrature coil by shifting the conductive 
patterns 90.degree. with respect to each other so that each coincides with 
one of the quadrature RF fields. Since the conductors on both sides of the 
dielectric sheet no longer align with each other, the capacitance needed 
to short circuit the cuts in the closed loops is reduced. This is offset 
by reducing the thickness of the dielectric sheet, and by staggering the 
cuts to maintain balance between the performance (i.e. Q) of each 
quadrature coil mode. 
A more specific object of the invention is to prevent voltage breakdown 
events from occurring between the two copper sheets and between adjacent 
conductive paths on the same copper sheet. This is accomplished by short 
circuiting the two copper sheets at a plurality of locations around the 
circumference of the shield and by providing a short circuit path in the 
copper sheets between these locations to form a conductive ring around the 
circumference of the shield. The conductive ring insures that gradient 
induced voltages between sheets and between adjacent conductive paths in 
each sheet does not reach a level which can cause a breakdown through or 
across the dielectric material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1, a radio-frequency (RF) shield 10, in 
accordance with the present invention, is a hollow cylindrical conductive 
member situated between a RF whole body coil 11 and a set of magnetic 
field gradient coils 2, within the bore 4 of a magnet (not shown) in a 
nuclear magnetic resonance (NMR) imaging device. By convention, the static 
main magnetic field B.sub.0 of the main NMR system magnet (produced by the 
magnetic means formed about bore 4) is aligned with the z axis of a 
Cartesian coordinate system. The RF coil 11 forms an RF magnetic field 
B.sub.1 within the coil 11 bore responsive to a RF signal. Field B.sub.1 
is typically in the X-Y plane. A significant RF magnetic field is also 
present outside of coil 11, and, as is well known in the art, will impinge 
upon gradient coils 2 unless the RF shield 10 is present and effectively 
operates as an RF short circuit. Shield 10 must be substantially 
transparent to the magnetic fields from gradient coils 2, allowing those 
gradient magnetic fields to enter into the bore of RF coil 11 and place 
spatial-encoding information upon the volume therein. 
A typical whole body RF coil 11 is shown in FIG. 2 this high-pass 
"birdcage" coil, formed upon a cylindrical base 12 of insulative material, 
comprises first and second spaced-apart end rings 13 and 14, each having a 
plurality of conductive segments (here 8 segments) joined to one another 
by capacitive elements 16. Each of the end rings 13 and 14 is thus 
substantially in the X-Y plane, as is the B.sub.1 RF field produced. A 
like number of axial conductors 15 extend in the z direction between one 
of the conductive segments of first end ring 13 and a like-positioned 
conductive segment of the second conductive end ring 14. Thus, a first 
elongated conductive element 15 is positioned at an angle .theta.=0 and 
each of the remaining seven elongated conductive members 15 are placed at 
successively greater angles around the periphery. Because of the 
cylindrical symmetry of the coil, the RF magnetic fields and currents are 
analyzed utilizing cylindrical coordinates, of the form R, .theta., where 
.theta. is the angle of revolution with respect to that plane formed 
through the z axis and one of the conductive members 15. The axial center 
of coil 11 is placed at the z=0 coordinate, and it is assumed that the 
birdcage coil currents are confined to a very thin layer and tend to flow 
through the areas of the end rings, defined by inner end ring dimension 
z.sub.1 and outer end ring dimension z.sub.2. The assumed coil 
distribution contours, parallel to the lines of current flow, are shown in 
FIG. 3, for an unwrapped coil laid flat and having a single excited mode. 
The separations between contours are equally spaced in current, and the 
current flow is in the direction of arrows A. 
As explained in U.S. Pat. No. 4,879,515 issued on Nov. 7, 1989 and entitled 
"Double-Sided RF Shield For RF Coil Contained Within Gradient Coils Of NMR 
Imaging Device", a shield for the field producing the current contours of 
FIG. 3 is shown in FIG. 4. The disclosure of this patent is hereby 
incorporated by reference and teaches that a sheet of copper 20 is cut as 
indicated by contour lines 21 to form separate conductive paths 22 for 
currents I.sub.s induced by one of the quadrature fields produced by the 
RF coil 11. The resulting pattern includes numerous conductive loops 
encircling either of a pair of central conductive pads 23. These 
conductive loops will support eddy currents induced by the gradient fields 
unless they are open circuited. This is accomplished in the prior 
structure by making cuts indicated at 24 which extend from each conductive 
pad 23 to one edge of the copper sheet 20. While these cuts 24 stop eddy 
currents, they also impede the currents I.sub.s, and reduce the 
effectiveness as an RF shield. 
The solution disclosed in U.S. Pat. No. 4,879,515 is to construct an 
identical structure by making the same cuts in a second sheet of copper 20 
and forming the RF shield 10 by sandwiching a thin sheet of dielectric 
material 26 between the two copper sheets 20 as shown in FIG. 7. The 
conductive paths in each copper sheet 20 are aligned and form the plates 
of capacitors which offers low impedance connection of the respective 
conductive loops at the high RF frequencies, but not the lower frequencies 
of the changing gradient fields. Thus, the currents I.sub.s are allowed to 
flow in the copper sheets 20 to act as an RF shield, but the lower 
frequency eddy currents are blocked by the cuts 24. 
It is a teaching of the present invention that the same pattern of 
conductive paths 22 can be used in an RF shield 10 comprised of two sheets 
of copper and one dielectric layer to block both of the quadrature fields 
produced by the RF coil 11. This is accomplished by aligning the pattern 
formed on one of the copper sheets with one of the quadrature fields, and 
aligning the patterns on the other copper sheet with the other quadrature 
field. Such orthogonal patterns are shown in FIGS. 5 and 6, where FIG. 5 
illustrates the pattern on one sheet of copper 30 wrapped around the 
cylindrical RF coil 11, and FIG. 6 illustrates the location of the pattern 
on the other sheet of copper 31. As shown in FIG. 8, the copper sheets 30 
and 31 are disposed on the opposite sides of a dielectric sheet 32, and 
since their patterns are not aligned, the contour cuts 21 therein do not 
align and the total capacitance between the sheets 30 and 31 is reduced. 
This reduction is offset, however, by decreasing the thickness of the 
dielectric sheet 32, which in the preferred embodiment is a 
polytetraflourethylene ("PTFE") fiberglass laminant manufactured by Allied 
Signal Corporation having a thickness of 0.0032".+-.0.003" and a 
dielectric constant of 2.57. The copper sheets 30 and 31 are formed from 2 
oz copper having a thickness of 2.8 mils, and the contour cuts 21 that 
define the patterns therein have a width of approximately 20 mils. 
It has been discovered that this single three-part shield effectively 
blocks the two quadrature RF fields produced by the quadrature RF coil 11 
and prevents their interaction with the surrounding gradient coils 2. This 
is accomplished with a measured reduction in the SNR of less than 4% for 
the RF coil 11 when compared with the performance using a solid copper 
shield. 
Eddy currents induced by the gradient fields are blocked by open circuiting 
each conductive loop in the patterns formed in the copper sheets 30 and 
31. The pattern of cuts required to do this has been significantly altered 
to improve the RF performance of the shield. Referring to FIGS. 5 and 6, 
rather than extending a single cut from the center pads 34 to one edge, 
alternate conductive loops are open circuited by a series of short cuts 35 
that extend along the entire circumferential (.theta.) extent of each 
copper sheet 30 and 31. This alternating of the cuts in the conductive 
rings distributes the short cuts 35 equally in both directions from the 
center pads 34 and balances the performance of the shield 11 for both 
quadrature RF fields. 
Referring still to FIGS. 5-7, a number of measures are taken to eliminate 
the instances in which gradient induced voltage differences build up in 
the RF shield 11 and cause a breakdown producing noise. Such a breakdown 
can occur through the dielectric layer 32 between the sheets 30 and 31, or 
across contour cuts 21 on the same sheet 30 or 31. Excessive voltage 
between sheets 30 and 31 is prevented by shorting them together at three 
points indicated at 40, 41 and 42. This is accomplished by cutting slots 
through the dielectric layer 32, passing a conductive strap through each 
slot, and soldering the straps to each copper sheet 30 and 31. The 
shorting points 40, 41 and 42 are located at center pads 34 and they are 
located at the ends of conductive strips 44-47 which extend 
circumferentially through one-half of each conductive pattern. The 
shorting points 40, 41 and 42 join the conductive strips 44-47 together to 
form a continuous conductive ring that extends completely around the RF 
shield 11 at its center. However, this ring is open circuited at one point 
so as not to provide a path that will support eddy currents. Each 
conductive strip 44-47 short circuits all the conductive loops surrounding 
its associated center pad 34 so that excessive differential voltages 
cannot build up across the cuts 21 that form each conductive loop. Despite 
these shorting conductive strips 44-47 and examination of the pattern 
reveals that the alternated short cuts 35 still prevent a complete 
conductive loop from being formed and block gradient induced eddy 
currents. 
To further reduce gradient induced eddy currents, the copper regions 
located at the corners of each conductive pattern are also interrupted by 
straight cuts indicated at 49. These straight cuts 49 are spaced apart to 
break up the conductive regions surrounding each pattern so that circular 
current paths capable of supporting eddy currents are not present.