MRI QD RF coil having diode switched detuning circuit producing reduced artifact

A detuning/decoupling arrangement for a Magnetic Resonance Imaging (MRI) system quadrature RF coil arrangement (of the type using the nuclear magnetic resonance, or NMR, phenomenon) uses switching diodes to selectively connect and disconnect portions of a segmented RF coil in response to a DC control signal. The DC control signal selectively forward biases and reverse biases the switching diodes. The DC control current flows through the RF coil itself, through the diodes, and then through the center conductor of a semi-rigid transmission line disposed in proximity to the RF coil. Because the DC current flowing through the transmission line is equal and opposite to the DC current flowing through the RF coil, the net magnetic field generated by the DC current flow is approximately zero--eliminating artifacts in the MRI image that would otherwise be generated due to continuous DC current flowing whenever the coil is being used to transmit or receive RF.

This invention is related to the field of magnetic resonance imaging (MRI) 
utilizing nuclear magnetic resonance (NMR) phenomenon. It is particularly 
related to an advantageous selectively detunable RF coil for an MRI 
system. 
This application is generally related to the following commonly-assigned 
patents of Crooks et al; U.S. Pat. Nos. 4,297,637; 4,318,043; 4,471,305 
and 4,599,565. These prior issued patents disclose MRI systems which 
produce images using the spin echo nuclear magnetic resonance phenomenon. 
Quadrature ("QD") RF excitation and reception in NMR (nuclear magnetic 
resonance) imaging applications is generally known (see, e.g., 
commonly-assigned copending application Ser. No. 893,889 of Arakawa et al 
filed Aug. 6, 1986). Such quadrature arrangements provide many advantages, 
including reduced levels of RF power deposition for a given transmitted RF 
pulse duration, and improved signal-to-noise ratio. 
It may be desirable to use different quadrature (or other) RF coils for 
receive and transmit. For example, a large "body" coil may be used to 
generate a RF field in and around the entire body of a subject to be 
imaged, and smaller coils (e.g., coils surounding the head or other body 
portion to be imaged) may then be used to receive. Such arrangements 
provide great flexibility, since the same quadrature body coil may always 
be used for transmit, and different receive coils may then be used 
depending upon the type of images desired (e.g., the body coil can be used 
to receive if an overall image of a large portion of the subject is 
desired, a smaller "head coil" surrounding the head of a subject may be 
used for detailed imaging of the subject's head, or a "surface coil" may 
be used for imaging other specific body portions such as the chest area). 
It is generally known to detune the transmit body coil during the time 
another coil receives the resulting RF echo pulses. See, for example, 
Misic et al, "Quadrature RF Coil And Decoupling Systems For Mid Field NMR 
Imaging," Vol. 1 Book of Abstracts, Society of Magnetic Resonance in 
Medicine 183 (Aug. 19, 1986). It has been recognized in the past that when 
separate quadrature coils are used for transmit and receive, it is 
necessary to decouple the receive coil system during the transmit portion 
of the pulse sequence, and to decouple the transmit coil during the 
receive portion of the sequence. Detuning the transmit coil during 
reception by a head or surface coil reduces, but does not entirely 
eliminate, artifacts and signal loss during receive. It is therefore known 
to actively open-circuit each loop of the body coil using PIN diodes 
whenever a head or surface coil is being used for reception (see the Misic 
et al paper cited above). Misic et al have determined that the results 
obtained from this type of arrangement compare favorably with the results 
obtained from independent single coil probe quadrature head and body 
coils. 
We have discovered, however, that significant artifacts may be generated by 
the magnetic fields produced by the DC control current used to activate 
such PIN switching diodes. Although this DC control current may have an 
amplitude of only a few amperes, the current flow generates a magnetic 
field (e.g., in the z axis) which at least locally may interfere with the 
gradient magnetic field needed for proper NMR imaging. More particularly, 
this DC magnetic field typically generates a so-called "artifact" 
disturbance in the image which degrades image quality and may render 
positions of the image unclear. In addition, undesirable eddy currents may 
flow over the DC current path in response to the strong pulsating gradient 
magnetic fields. These eddy currents in turn produce magnetic fields which 
add to the artifacts created by the DC control current field. 
We have now developed a unique coil decoupling arrangement using switching 
diodes which produces virtually no DC magnetic field and associated 
artifacts. 
The present invention provides an RF coil having a discontinuity or "gap," 
with one or more switching devices (e.g., diodes) connected across the 
gap. The diodes provide a conduction path for RF when they are forward 
biased by a DC control signal, and open-circuit the RF coil loop when 
reverse-biased. The DC control signal flows through the RF coil to the 
diodes, and returns through a DC path within a section of transmission 
line in proximity to the RF coil. Because equal amounts of DC current flow 
through the RF coil and through the transmission line, balanced (equal and 
opposite) DC magnetic fields are generated which cancel one another to 
leave substantially no net DC magnetic field (thus reducing or entirely 
eliminating image artifacts which would otherwise be generated by the DC 
current flow). RF traps provided close to each end of the transmission 
line prevent RF from being conducted within the line. 
Flow of DC control currents through the transmission line(s) used in the 
preferred embodiment of the present invention does not produce undesirable 
image artifacts due to spurious magnetic fields because generated magnetic 
fields are fully balanced and cancelled by equal and opposite magnetic 
fields generated by DC currents flowing through the nearby RF coil. In 
addition, the DC conductors within the transmission lines are isolated 
from RF signals, providing good isolation between the DC and RF circuits 
even though both DC and RF signals flow though the RF coil--and 
eliminating the need for additional RF traps or other isolation devices. 
The transmission line outer conductors are conductively bonded to the RF 
coil surfaces in the preferred embodiment in order to minimize mechanical 
vibration and provide mechanical strength as well as low resistance 
electrical connections. Some gaps in the RF coil of the preferred 
embodiment of the present invention have switching diodes connected across 
them--these gaps allow coil detuning/decoupling in response to a DC 
control signal. Other coil gaps in the preferred embodiment have DC 
blocking/RF coupling capacitors connected across them, these gaps limiting 
the DC control signal path and also reducing eddy currents which might 
flow through the coil in response to the pulsating gradient magnetic 
fields.

FIG. 1 is a schematic diagram of the presently preferred exemplary 
embodiment of a MRI system 50 in accordance with the present invention. 
MRI system 50 includes an RF front end 52 and a quadrature body coil 
arrangement 100. 
As will be explained, coil arrangement 100 includes switching devices which 
complete a resonant RF circuit (and thus make the coil arrangement 
operational) in response to a DC control signal S provided by front end 
52. When this positive DC control signal S is present, coil arrangement 
100 may be used for radiating RF signals provided by front end 52. 
Alternatively or in addition, coil arrangement 100 provides received RF 
signals to front end 52 when the DC control signal S is present. 
During times when front end 52 does not provide a positive DC control 
signal to body coil 100, the body coil is detuned so as not to resonate at 
the RF frequency of operation, and is virtually invisible to other, 
resonant RF coils operating nearby--for example, receive coil 200 and 
associated receiver 250 (since coil arrangement 100 does not provide a 
complete RF circuit in the preferred embodiment when the coil arrangement 
is detuned). 
In the preferred embodiment, front end 52 includes an RF section 54 and a 
DC controller section 56. RF section 54 includes an RF transmitter 58, an 
RF receiver 62, a coaxial relay switch 64, a PIN diode T/R switch 
("transmit/receive") 65, an RF/DC combiner circuit 66, a directional 
coupler 60, and a tuning and matching display circuit 61. 
RF transmitter 58 produces high-power RF signals (having a frequency of 
about 15 MHz in the preferred embodiment) which pass through T/R switch 65 
to the normally closed contact NC of relay 64. RF receiver 62 detects RF 
signals present on the relay 64 normally closed contact NC and passed to 
the receiver via T/R switch 65, and provides an output signal OUT which is 
further processed (using conventional MRI signal processing techniques) to 
produce images. 
T/R switch 65 causes a signal path to be created between RF transmitter 58 
and RF/DC combiner circuit 66 (via the relay 64 normally closed contact 
NC) whenever system 50 is in a transmit mode. During receive, T/R switch 
65 creates a signal path between RF/DC combiner circuit 66 and the RF 
input of receiver 62 (also via the relay normally closed contact NC). 
The relay 64 normally open (NO) contact is connected to a directional 
coupler 60 and associated tuning and matching display circuit 61--which 
are used to initially tune (resonate) coil arrangement 100 prior to 
detecting image signals. Relay coil 68 is energized during initial system 
setup so that coupler 60 and display circuit 61 can be used for initial 
tuning of coil arrangement 100--and is then de-energized during normal 
system operation. 
DC controller 56 selectively produces DC control signal S which combiner 
circuit 66 multiplexes with RF signals and applies to a coaxial RF cable 
70 (type RG-214 in the preferred embodiment) connecting front end 52 with 
coil arrangement 100. RF cable 70 in the preferred embodiment includes an 
RF trap circuit 70a (a "coaxial shielded choke") of the type described in 
commonly assigned issued U.S. Pat. No. 4,682,125 to Harrison et al to 
prevent unwanted spurious secondary RF fields from propagating over and 
transmitting from the outside (outer conductor) of the cable. 
Controller 56 in the preferred embodiment alternately provides either +2 
VDC or -24 VDC to combiner circuit 66--depending upon the setting of a 
selection switch 72 and also upon whether system 50 is transmitting or 
receiving. If coil arrangement 100 is being used for both transmit and 
receive, controller 56 produces a constant +2 VDC--controlling coil 
arrangement 100 to continuously provide a resonant RF circuit. If coil 
arrangement 100 is being used for transmit only (and coil 200 is being 
used for receive), on the other hand, controller 56 produces -24 VDC at 
all times except when RF transmitter 58 actually produces an RF signal. 
In the preferred embodiment, controller 56 includes MOSFETs 74, 76, bipolar 
junction transistors 78, 80, and resistors 82, 84, 86, 87. The drain of 
MOSFET 74 is connected to +5 VDC in the preferred embodiment, and the 
source of this MOSFET is connected through series resistor 86 to the drain 
of MOSFET 76. The source of MOSFET 76 is connected to -24 VDC in the 
preferred embodiment. Control line S is connected to the srouce of MOSFET 
74 through a current limiting resistor 87. 
The gate of MOSFET 74 is connected through series resistor 82 to the 
collector of driver transistor 78, and the gate of MOSFET 76 is connected 
through resistor 84 to the collector of driver transistor 80. Transistors 
78, 80 form part of a conventional level converter/logic circuit (not 
shown) which alternately turns on MOSFETs 74, 76 in response to the 
control signal level L at the output of switch 72. 
In particular, when signal level L is at logic level 1, MOSFET 74 turns ON 
and MOSFET 76 turns OFF--causing approximately +2 VDC to appear on DC 
control signal line S. When signal level L is at logic level 0, on the 
other hand, MOSFET 74 is turned OFF and MOSFET 76 is turned ON to cause 
approximately -24 VDC to appear on DC control signal line S. RF choke 88 
prevents RF signals produced by transmitter 58 or received by coil 
arrangement 100 over cable 70 from flowing into controller 56, but allows 
the DC control signal S to flow into combiner circuit 66 and over the 
cable. DC blocking capacitor 90 prevents the DC control signal S from 
flowing into relay switch 64. 
In the preferred embodiment, body coil arrangement 100 defines both a 
resonant RF path and a DC path--these two paths being co-extensive for 
part but not all of their lengths. 
RF produced by RF transmitter 58 flows through the switch 65, relay 64, DC 
blocking capacitor 90 and coaxial cable 70 into coil arrangement 100, and 
then flows through a series tuning capacitor C.sub.s and a parallel tuning 
capacitance C.sub.p into RF coil 102 (the connections between series 
tuning capacitor C.sub.s, parallel tuning capacitance C.sub.p, and the RF 
coil are shown schematically by a dotted line in FIG. 1). A return 
coupling capacitor C.sub.R RF couples the outer conductor of connector 71 
to RF coil 102, but prevents DC current from flowing from the coil 102 
into the connector outer conductor. 
As will be understood by those skilled in the art, the inductive reactance 
of RF coil 102 and the capacitive reactances of tuning capacitors C.sub.s 
and C.sub.p form a resonant RF circuit at the common operating frequency 
of RF transmitter 58 and RF receiver 62. RF signals received by this 
resonant circuit flow back over cable 70 and through combiner circuit 66, 
the T/R switch 65, and relay 64 to receiver 62. 
In the preferred embodiment, RF coil 102 includes discontinuities or gaps 
104a, 104b defined between an RF coil section 106 and coll sections 108a, 
108b. A bank of PIN diodes 111a (type UM-4902C or KS-1001 in the preferred 
embodiment) is connected across discontinuity 104a, with the anodes of 
each diode being connected to first coil portion 106 and the cathodes of 
each diode being connected to second coil portion 108a. 
Diodes 111a are used to selectively connect and disconnect coil portions 
106, 108 in the preferred embodiment. When diodes 111a are back-biased, 
they act as open circuits to RF (and DC) and cause coil portion 106 to be 
electrically disconnected from coil portion 108. When diodes llla are 
forward-biased, on the other hand, RF can flow through the diodes, coil 
portions 106, 108 are electrically connected together, and discontinuity 
104 is effectively eliminated. 
In the preferred embodiment, RF coil 102 and tuning capacitors C.sub.s and 
C.sub.p form a resonant RF circuit only when diodes 111 are 
forward-biased. When diodes 111 are reverse-biased, no RF can flow from RF 
coil portion 106 over discontinuity 104 to RF coil portion 108, and the RF 
coil 102 becomes non-resonant (and thus virtually invisible to other 
nearby resonant RF coils such as receive coil 200). 
The DC current path through RF coil arrangement 100 will now be described. 
The center conductor of RF coaxial cable 70 is connected (through a 
conventional RF connector 71 in the preferred embodiment) to one end of an 
RF trap 110 in addition to being connected to series tuning capacitor 
C.sub.s and parallel tuning capacitor C.sub.p. DC control signal S is 
blocked by tuning capacitors C.sub.s, C.sub.p but passes easily through 
trap 110 and flows into RF coil first section 106. RF trap 110 presents a 
very high impedance to RF signals present on tuning capacitors C.sub.s, 
C.sub.p and RF cable 70 center conductor, however, preventing RF from 
flowing through the trap into coil portion 106. 
If the voltage of DC control signal S exceeds the diode cut-in voltage 
(e.g., about 0.7 VDC), the diodes 111a become forward biased and conduct 
RF as well as DC currents. Consequently, when the voltage of DC control 
signal S is in the positive state (about +2 VDC is preferred), the DC 
control signal forward-biases the diodes in bank 111a and DC current 
(several amperes) flows through those diodes into second coil RF section 
108a. The DC current flows from coil second section 108a through RF trap 
112 into semi-rigid transmission line section 114; flows through the 
center conductor 148a of transmission line section 114 into trap 116; and 
flows from trap 116 into the outer conductor of transmission cable 70. 
Current flowing through similarly forward-biased diode bank 111b between RF 
coil sections 106, 108b flows through a further RF trap 117, through 
another semi-rigid transmission line section 118 center conductor 148b, 
and through trap 116 to the ground conductor of RF cable 70. 
In the preferred embodiment, semi-rigid transmission line 114 includes a 
center conductor 148a surrounded by dielectric insulation, the insulation 
being encased within a tubular semi-rigid copper outer conductor 150a. 
Simulairly, semi-rigid transmission line 118 includes a center conductor 
148b encased by dielectric and outer conductor 150b. The center conductors 
148a, 148b of semi-rigid transmission lines 114, 118 are connected 
together at node 120, and trap 116 is also connected to that node. Traps 
112, 116, 117 prevent RF signals from entering semi-rigid lines 114, 
118--so that only the DC control signal flows through the transmission 
line center conductors 148a, 148b. 
FIG. 2 is a more detailed schematic diagram of coil arrangement 100. In the 
preferred embodiment, RF coil 102 is a quadrature saddle type circular RF 
coil having an upper portion (band) 122 and a lower portion (band) 124 
(see copending commonly-assigned application Ser. No. 893,889, now U.S. 
Pat. No. 4,740,752, of Arakawa entitled "Non-overlapping QD MRI RF Coil" 
filed Aug. 6, 1986 for a detailed discussion of one exemplary body coil 
arrangement)--these upper and lower portions forming axially spaced apart 
annular members lying about the circumference of a common cylinder. Upper 
portion 122 is shown linearly in FIG. 2, but in the preferred embodiment 
is actually wrapped around in a circle (annulus) so that coil end 126 is 
mechanically and electrically connected to coil end 128. Upper coil 
section 122 defines a gap 130, and a series DC blocking/coupling capacitor 
132 couples RF current across the gap--permitting RF to flow in a 
continuous loop around coil upper portion 122 but preventing DC currents 
from flowing around the loop (and also tending to reduce eddy currents 
otherwise induced into the structure at audio frequencies due to pulsating 
magnetic gradient fields present within typical magnetic resonance imaging 
systems). As will be explained, DC blocking/coupling capacitor 132 (and 
other similar capacitors connected across other gaps in the coil 102) 
direct DC control signal S to flow over a predetermined paths--thereby 
enabling the magnetic fields generated from such DC current flow to be 
cancelled. 
In a similar manner, lower coil portion end 134 is electrically and 
mechanically connected to lower coil portion end 136 so that the lower 
coil portion forms an essentially contiuous loop with gaps or 
discontinuities 104a, 104b, 146 breaking the loop. Diode bank 111a is 
connected across discontinuity 104a and diode-bank 111b is connected 
across discontinuity 104b to permit RF signals to flow between coil 
section 106 and coil sections 108a, 108b whenever (only when) the diodes 
are forward-biased by DC control signal S. A DC blocking/coupling 
capacitor 144 is connected across gap 146 to permit RF but not DC (or eddy 
current) to flow around the loop. 
Trap 110 is directly connected to coil section 106 at a node or terminal 
138. The anodes of diodes llla are connected to coil section 106 at node 
140. When the negative (-24 VDC) DC control signal level is present, the 
diodes 111a are reverse biased and no DC or RF currents may flow through 
them. This -24 VDC level is used in the preferred embodiment because it is 
not so negative as to cause shock hazards to human operators, but is 
sufficiently negative to ensure isolation through the diodes (the 
"barrier" or "transition" capacitance of a back-biased diode decreases 
with increasing reverse voltage), and to ensure reverse biasing despite 
spurious differences in potential which may exist between coil sections 
106, 108a, 108b due to RF pickup, eddy current flow, or other effects. 
Whenever the positive (+2 VDC) DC control signal is present, DC current 
flows from trap 110 into coil section 106 at node 138, flows through RF 
coil section 106 between nodes 138, 140, and then flows through diodes 
111a (the positive DC control signal voltage creates a positive voltage 
differential across gap 104a to forward-bias the diodes). 
Even though diodes 111a are directly connected to RF coil section 108a, the 
DC current flowing through diodes 111a is prevented from flowing from coil 
section 108a into further sections of the coil (e.g., coil section 142) by 
DC blocking/coupling capacitor(s) 144 connected across gap 146 between the 
coil sections. Diodes 111a are unidirectional devices and therefore do not 
permit DC current to flow in a reverse direction from coil section 108a to 
coil section 106. Due to the structure of coil 102 and to the placement of 
DC blocking/coupling capacitor 144, the only return path for the DC 
current is through trap 112 connected to the cathodes of diodes 111a 
(which presents a relatively small resistance to DC current) and into the 
center conductor 148a of semi-rigid transmission line 114. 
DC blocking/coupling capacitor 144 in the preferred embodiment may be six 
parallel-connected 3000 pF type UFP1-302J capacitors--providing a 
capacitance large enough to pass higher frequency RF signals but small 
enough to prevent lower frequency eddy currents and DC control currents 
from flowing between coil sections 108a, 142. In addition, return 
capacitor C.sub.R connected between coil section 106 and the outer 
conductor of input connector 71 prevents the DC control signal S from 
simply flowing from cell section 106 directly into the cable 70 outer 
conductor (while still providing good RF coupling between coil section 106 
and the cable outer conductor). 
Semirigid transmission line 114 outer conductive copper shield portion 150a 
is soldered (or otherwise conductively bonded) to RF coil section 106 
substantially along the direct line path between nodes 138, 140. That is, 
transmission line 114 is physically disposed on the surface of coil 
section 106 on the same path along which DC current flows through the coil 
between trap 110 and diodes 111a. Because the transmission line outer 
conductor 150a is electrically bonded to the surface of coil section 106, 
some of the DC current flowing between nodes 138, 140 actually may flow 
through the transmission line outer conductor (and, of course, RF currents 
flow through the transmission line outer conductor whenever RF is present 
on the RF coil). The magnetic field produced by the DC current flowing 
through RF coil section 106 (and semi-rigid line 114 outer conductor 150a) 
between nodes 138, 140 surrounds the semi-rigid line inner conductor 148a 
(the semi-rigid line, being made out of copper, is non-magnetic). 
Transmission line inner conductor 148a is electrically insulated from the 
line outer conductor 150a by dielectric (air or any other insulator can be 
used). No electrical connections exist between traps 112, 116 and coil 
section 106 in the preferred embodiment. Since equal amounts of DC current 
must flow into and out of diodes 111a, the DC current flowing through 
semi-rigid line center conductor 148a has a magnitude exactly equal 
(neglecting losses due to DC resistance) to the magnitude of DC current 
flowing through coil section 106 (and semi-rigid line outer conductor 
150a) between nodes 138, 140, and flows in a direction which is opposite 
to the direction in which that current flows between nodes 138, 140. 
The current flowing through semi-rigid line 114 center conductor 148a 
therefore produces a magnetic field which is substantially equal in 
magnitude and opposite in direction to the magnetic field produced by the 
DC current flowing through said section 106 between nodes 138, 140, and 
these two equal and opposite (that is, balanced) magnetic fields exist in 
essentially the same space. The equal and opposite magnetic fields 
virtually completely cancel one another to leave a zero net magnetic 
field--even though several amperes of DC current may be continuously 
flowing through diodes 111a trap 112, semi-rigid line 114 center conductor 
148a, and trap 116. 
Additional gap 104b and associated switching diodes 111b are provided in 
coil 102 in the preferred embodiment because coil 102 is a quadrature coil 
and therefore has two resonant sections (yellow and blue in the preferred 
embodiment) which must be disabled. In a manner similar to that described 
above, DC current flows from node 138 through RF coil section 106 to node 
152 connected to diodes 111b. When the DC control signal S has a positive 
level, DC current flows from node 152 into the anodes of forward-biased 
diodes 111b and through the diodes. 
Although the cathodes of diodes 111b are directly connected to coil section 
108b, this coil section is not DC coupled to any other structure except 
for trap 117 (DC blocking/coupling capacitor 144 prevents DC current from 
flowing through coil section 108b around the coil loop into coil section 
108a). Therefore, all of the DC current flowing through diodes 111b flows 
into trap 117. The other side of the trap 117 is connected to semi-rigid 
transmission line section 118 center conductor 148b, the other end of the 
center conductor being connected to trap 116 (and also to the center 
conductor 148a of transmission line section 114) at node 154. Semi-rigid 
line 11 outer conductor 150b is soldered to coil section 106, and line 118 
is physically and mechanically located on the surface 160 of RF coil 
section 106 substantially along the direct line current path between nodes 
138, 152 in the preferred embodiment. The current flowing through 
transmission line 118 center conductor 148b produces a magnetic field 
having a magnitude which is approximately equal to the magnitude of the 
magnetic field generated by the DC current flowing between nodes 138 and 
152, but which is opposite in direction to that other magnetic field. 
These two magnetic fields cancel one another to leave a net magnetic field 
of approximately zero. 
FIG. 3 is a detailed perspective and schematic diagram of coil arrangement 
100 showing the mechanical and electrical connections between semi-rigid 
transmission line sections 114, 118 and RF coil section 106. Since coil 
section 106 is curved (i.e., a part of an annulus) in the preferred 
embodiment, transmission line sections 114, 118 are also preferably curved 
so as to conform in shape with the coil section outer surface 160. The 
solder bond 162 between transmission line outer conductors 150a, 150b and 
coil section outer surface 160 prevents the transmission line sections 
114, 118 from vibrating at audio frequencies due to the pulsating NMR 
gradient magnetic fields, and also establishes a low resistance DC path 
between the cable outer conductors and the coil section surface 160. 
The RF shielding effect provided by transmission line outer conductors 
150a, 150b prevents the transmission line center conductors 148a, 148b 
from being exposed to RF even when large RF currents are flowing through 
coil 102. In particular, RF currents tend to flow along the outer surface 
of semi-rigid line outer conductors 150 when transmitter 58 supplies RF to 
coil arrangement 100, but due to the so-called "skin effect" and other 
well-known transmission line characteristics, very little RF reaches the 
center conductors 148a, 148b. The use of semi-rigid transmission line 
(generally used for conducting RF signals, not DC signals) for conducting 
and RF shielding DC control signal S eliminates the need for additional 
traps, shielding, DC blocking or RF bypass capacitors, and the like which 
might otherwise be required to prevent RF from flowing in DC conductors. 
In the preferred embodiment, traps 112, 116, 117 are positioned as close as 
possible to the ends of semi-rigid line sections 114, 118--shortening the 
length of unshielded, exposed portions of line center conductors 148a, 
148b. Traps 110, 112, 116 and 117 are tuned to the body coil resonant 
frequency (15 MHz) in the preferred embodiment. Body coil 102 generates 
fields perpendicular to the Z axis, and traps 110, 112, 116, 117 are each 
oriented so that the fields they produce are in the same direction as the 
Z axis--minimizing spurious coupling to the main magnetic field generated 
by the body coil. 
FIGS. 4A and 4B are perspective and cross-sectional views, respectively, of 
an exemplary RF trap design used in the preferred embodiment for traps 
110, 112, 116 and 117. In the preferred embodiment, these traps are each 
made by winding 22 turns of 18 gauge wire 173 onto an acrylic tube 172 
having an outside diameter of about 1/2 inch and a length of about 13/4 
inches. A small threaded copper rod member 174 having a length of about 
1/4 inches and an outside diameter of 0.375 inches is advanced into an end 
176 of tube 172 and used to tune the trap 112 to the desired resonant RF 
frequency (i.e., 15 MHz). 
Tube 172 includes a number 50 drill hole 184a drilled into the tube end 176 
which is used to start winding 173, and a further number 50 drill hole 
184b drilled through the tube at a further end 186 which is used to end 
the winding. The resulting RF choke has an inductive reactance of about 
2.4 microhenries and a DC resistance of only 0.043 ohms. A capacitor 190 
of about 47 pF in the preferred embodiment is connected across winding 173 
to provide a parallel RF resonance circuit. The resulting trap 112 can be 
precisely tuned to the desired radio frequency by advancing member 174 
further into or retreating the number from tube end 176. 
The present invention provides a detuning/decoupling MRI RF coil 
arrangement using switching diodes to selectively connect and disconnect 
portions of a segmented RF coil in response to a DC control signal. The DC 
control signal selectively forward biases and reverse biases the switching 
diodes. The DC control current flows through the RF coil itself (and thus 
shares a path with RF currents), through the diodes, and then through the 
center conductor of a semi-rigid transmission line disposed in proximity 
to the RF coil surface. Because the DC current flowing through the 
transmission line is equal and opposite to the DC current flowing the RF 
coil, the net magnetic field generated by the DC current flow is 
approximately zero--reducing or eliminating artifacts in the image that 
would otherwise be generated due to DC bias current flow. 
Although the DC control current flows through the RF coil and the center 
conductor of an unbalanced transmission line conductively bonded to the RF 
coil surface in the preferred embodiment, in other applications it might 
be desirable to use a balanced transmission line (e.g., open wire ladder 
line connected directly across the switching diodes) instead. In addition, 
although two semi-rigid lines and two diode banks are used in the 
quadrature coil design of the preferred embodiment, a single line and 
associated diode bank can be used to decouple/detune a single loop 
non-quadrature type coil. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiments, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiments, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.