Self-shielded gradient coils are used to eliminate interactions between gradient coils and external structure. Interactions between gradient coils for different gradient axes are controlled by means of a balanced interconnection within each separate gradient coil set. One or both of the inner and outer coils within a gradient coil set are split such that for any localized capacitive current flow between coil sets there will be an equal and opposite current flow somewhere else between the coil sets.

BACKGROUND OF THE INVENTION 
The present invention relates in general to coils useful in nuclear 
magnetic resonance (NMR) apparatus, and more specifically to a balanced 
configuration for shelf-shielded gradient coils. 
Magnetic Resonance Imaging (MRI) systems are currently employed in forming 
tomographic images of the internal human anatomy. In such systems, a 
patient is placed in a static magnetic field and is subjected to 
radio-frequency electromagnetic pulses to excite nuclear spins. The 
magnetic resonance of the atomic nuclei of the patient is detected with a 
receiving coil to provide information from which an image of that portion 
of the patient containing these nuclei may be formed. The magentic field 
possesses gradients which are pulsed on during each detection sequence so 
that the position of resonating nuclei can be determined. These same 
phenomena are employed in magnetic resonance spectroscopy for analyzing 
properties and structures of substances. 
A shielded type of gradient coil is known from U.S. patent application Ser. 
No. 826,650, now U.S. Pat. No. 4,737,716, filed Feb. 6, 1986, which is 
hereby incorporated by reference. A shielded gradient coil is desired to 
reduce various interactions of the gradient fields with other structure 
such as the main field magnet. The interactions include field spatial and 
temporal distortions, energy dissipation and annoying audible sound. 
In the preferred embodiment of the previous invention, concentric inner and 
outer coils are employed for each gradient axis. The inner and outer coils 
for each axis are connected in series. The surface current distributions 
of each coil set result in a gradient magnetic field inside the coil set 
and a substantially zero field outside the coil set. 
Typical MRI systems employ gradient magnetic fields along three orthogonal 
axes in the x, y and z directions. The z-axis is usually defined as 
coinciding with the direction of the main static magnetic field (which is 
usually along the axis of the main cylindrical magnet) and the x- and 
y-axes are perpendicular to the static field. The gradient coil set for 
each axis has its own respective gradient amplifier which is under control 
of the imaging system. 
The coil sets must be operated in close proximity to each other. However, 
it has been found that problems can occur during pulsed operations of the 
gradient coils resulting from interactions between separate coil sets. For 
example, the gradient amplifiers may become unstable. 
Accordingly, it is a principal object of the present invention to improve 
the operation of gradient apparatus in MRI systems. 
It is another object to reduce or compensate for interactions between coil 
sets in close proximity. 
It is yet another object of the invention to avoid unstable operation of 
gradient amplifiers used in conjunction with self-shielded gradient coils. 
SUMMARY OF THE INVENTION 
These and other objects are achieved in a coil set for producing a magnetic 
field in a magnetic resonance apparatus which comprises an inner coil, an 
outer coil and particular interconnect means between the outer and inner 
coils. Further coil sets can be provided for other gradient axes. 
In particular, the outer coil is coaxially disposed from the inner coil. 
The co-action of the inner and outer coils provides a predetermined 
magnetic field within the coil set and a substantially zero magnetic field 
outside the coil set. The interconnect means provides an electrical 
interconnection between the inner and outer coils such that energizing 
current for the coil set flows through a portion of one of the inner and 
outer coils, through at least a portion of the other coil and then through 
a further portion of the one coil. In this way, the impedance of the coil 
set is symmetrically balanced and interactions can be avoided.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a pair of coils includes an inner coil 10 and a 
concentric outer coil 13, each of which is cylindrical and aligned with 
the z-axis. The coil set is arranged longitudinally with respect to the 
main static field B.sub.o in order to provide a gradient magnetic field in 
an imaging volume within its interior. 
Inner coil 10 comprises a cylindrical substrate 11 carrying gradient 
windings, a portion of which is seen as 12a. Likewise, outer coil 13 
comprises a substrate 14 and gradient windings including 15a and 15b. The 
substrates may typically be formed of a flexible printed circuit board 
material, and the windings may typically be etched conductors that may be 
on one or both sides of the circuit board and are covered with an 
insulating material. The windings on each circuit board resemble four 
fingerprints (i.e. spiral) interconnected to provide orthogonal gradient 
fields (see also FIG. 5). Each spatial location on the coils can be 
specified by its x, y and z coordinates or by cylindrical coordinates z, r 
and .theta. as shown in FIG. 1. 
FIG. 2 shows an x-gradient coil set 20 for providing the x-gradient 
.differential.B.sub.z /.differential. x and a concentric y-gradient coil 
set 21 for providing the y-gradient .differential.B.sub.z /.differential. 
y. Each coil set is connected to a separate power supply so that the 
separate gradients can be pulsed independently. For example, in the 
conventional spin warp imaging pulse sequence, the y-gradient may be 
pulsed during a spatial encoding step and then the x-gradient may be 
pulsed later during an NMR signal readout step. It is also common for the 
x- and y-gradients to be pulsed simultaneously. 
FIG. 3 shows a typical power supply connection for the coil sets of FIG. 2. 
The x-gradient set 20 includes an inner coil 23 and outer coil 24 
connected in series with an x-gradient master amplifier 25 and an 
x-gradient slave amplifier 26. Likewise, y-gradient coil set 21 includes 
an inner coil 30 and an outer coil 31 connected in series with a 
y-gradient master amplifier 32 and a y-gradient slave amplifier 33. The 
junction between each pair of amplifiers is connected to ground. 
Master amplifiers 25 and 32 receive respective current commands from an NMR 
system (not shown). In order to ensure that the commanded current actually 
flows to the gradient coils, master amplifiers 25 and 32 employ 
closed-loop feedback control using a current sensor, such as a 
current-sense resistor. Slave amplifiers 26 and 33 directly follow their 
respective master amplifier, but are controlled open-loop by the master. 
Each slave amplifier provides voltage in the opposite direction to its 
master. Thus, the amplifiers are stacked current-wise resulting in a 
desirably increased rail voltage. 
Because of the large amount of conductor area used to form each coil set 
and because of the close proximity between coil sets, a large amount of 
capacitance results between coil sets. This capacitance is shown as an 
inherent capacitance 35 in FIG. 3. A low impedance path 36 including 
capacitance 35 from one coil set to another can cause instabilities of the 
gradient amplifiers during gradient pulsing or other problems. 
By design, each outer coil 24 and 31 links no net flux (this is required 
for good shielding). Thus, each outer coil is essentially at constant 
potential even during a current pulse except for a small resistive voltage 
drop. If an outer coil is connected to one side of the master-slave 
amplifier combination, it will rise to the full potential of that side of 
the amplifier when pulsed (almost all the voltage drops across the inner 
coil). Consequently, a capacitive current flows from the pulsed outer coil 
to the windings of the other coil set and through the master or slave 
amplifier of the other coil set to ground. This current is sensed by the 
master amplifiers of both gradient axes which leads to the instability 
between amplifiers, each trying to correct the current excited by the 
other. 
Ideally, no uncommanded current should flow through master amplifiers 25 or 
32 since they operate with current feedback. However, a typical amplifier 
cannot prevent the capacitive current flow. Thus, the current can flow 
through both master and slave (i.e., path 36 is just one of several 
possible paths). 
To eliminate the problems associated with interactions between coil sets, 
the present invention substantially removes capacitive current flow by 
connecting the gradient coils as shown in FIG. 4. In the x-gradient coil 
set, inner coil 23 is split into two halves 23a and 23b. Outer coil 24 is 
connected between inner coil halves 23a and 23b. Other split 
configurations of either or both coils are also possible so long as the 
configuration is electrically symmetrical such that when the coil set 
receives an applied voltage from the gradient amplifier, every point on 
each coil of the set has a corresponding unique point on the same coil 
with equal and opposite potential. Thus, no net current will flow between 
separate gradient axes since for any localized current there will be an 
equal and opposite current somewhere else between coil sets. 
Although one can split either the inner or outer gradient coils, or both, 
it is preferred to split inner coils 23 and 30 as shown in FIG. 4. There 
is usually a greater amount of space for added connections on the inner 
coil since the outer coil is usually located very close to other 
structures such as the shimming coils. Also, the outer coil is larger and 
has more capacitance, so that having the entire outer coil at or near zero 
potential (due to the split power supply amplifier configuration) gives 
less overall localized current flow. 
FIG. 5 shows the winding configuration for the embodiment of FIG. 4 in more 
detail. Thus, a series path is provided from the master amplifier, through 
one-half (two fingerprints) of inner coil 23, through all of outer coil 
24, through the second half of inner coil 23 and to the slave amplifier. 
Thus, a symmetrically balanced coil set is provided which reduces or 
eliminates interactions between coil sets and which avoids gradient 
amplifier instabilities. 
While preferred embodiments of the invention have been shown and described 
herein, it will be understood that such embodiments are provided by way of 
example only. Numerous variations, changes and substitutions will occur to 
those skilled in the art without departing from the spirit of the 
invention. Accordingly, it is intended that the appended claims cover all 
such variations as fall within the spirit and scope of the invention.