Optical control of radio frequency antennae in a magnetic resonance imaging system

A local RF antenna assembly for a magnetic resonance imaging (MRI) system includes a conducting RF antenna structure. First and second capacitors are connected in series with the inductance of the RF antenna structure to form a circuit that resonates at a Larmor frequency. An inductor has a first terminal connected to a node between the first and second capacitors and has a second terminal. A photosensitive first semiconductor switch is connected between the second terminal of the inductor and one terminal of the RF antenna inductance. A receive coil control selectively provides illumination that places the photosensitive first semiconductor switch in a conductive state. When the photosensitive first semiconductor switch is conductive, the inductor disables resonance of the RF antenna circuit at the Larmor frequency. This action inhibits the RF receive antenna from interacting with other antennae in the MRI system.

FIELD OF THE INVENTION 
The field of the invention is magnetic resonance imaging (MRI) systems; and 
more particularly, radio frequency antenna assemblies utilized in such 
systems. 
BACKGROUND OF THE INVENTION 
When a sample of a substance such as human tissue is subjected to a uniform 
magnetic field (polarizing field B.sub.0, defining the Z axis of a 
Cartesian coordinate system), the individual magnetic moments of the spins 
in tissue nuclei attempt to align with this polarizing field, but precess 
about it in random order at the characteristic Larmor frequency of the 
nuclei. If the substance is subjected to a magnetic field (excitation 
field B.sub.1) which is in the X-Y plane of the Cartesian coordinate 
system and which oscillates near the Larmor frequency, the net magnetic 
moment of the sample aligned along the Z axis, M.sub.z, may be rotated, or 
"tipped", into the X-Y plane to produce a net transverse magnetic moment 
M.sub.t. A magnetic resonance signal is emitted by the excited sample 
after the excitation signal B.sub.1 is terminated. This signal may be 
modified by application of additional magnetic fields and may be received 
and processed to form an image. This process known as magnetic resonance 
imaging (MRI) may be used for medical diagnosis as well as non-medical 
purposes. 
When utilizing these magnetic resonance signals to produce images, magnetic 
field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, 
the region to be imaged is subjected to a sequence of measurement cycles 
in which these gradients vary according to the particular localization 
method being used. The polarizing field B.sub.0 and the magnetic field 
gradients (G.sub.x, G.sub.y, and G.sub.z) typically are produced by 
relatively large electromagnetic coils around the patient being imaged. 
The B.sub.1 field may be transmitted into the object to be imaged by an 
antenna, which may be large or small compared with the object, and may or 
may not also be used to receive the subsequent magnetic resonance signal. 
Much smaller receive-only antennae, known as "local coils" or "surface 
coils", commonly are placed in close proximity to the portion of the 
patient to be imaged in order to better receive the magnetic resonance 
signals. The resulting set of received signals are digitized and processed 
to create the image using one of many well-known reconstruction 
techniques. 
Both the transmit and receive antennae are normally resonant at the 
frequency which is determined by the nuclear species and the static 
magnetic field strength, to maximize their efficiency. Unfortunately, 
these antennae can interact with each other via inductive and/or 
capacitive coupling, changing each other's effective resonant frequency 
and distorting each other's radio frequency field spatial distribution. 
This is ordinarily prevented by disabling the receive antenna during the 
transmit phase of the imaging process, and disabling the transmit antenna 
resonance during the receive phase. 
FIG. 1 shows a technique for enabling and disabling an MRI antenna with 
series RF switching using a PIN diode activated by DC current pulses. The 
antenna circuit consists of inductance 10 in series with capacitances 13 
and 14 and PIN diode 15, creating a series resonance which has a peak 
response at the resonant frequency, selected to be the Larmor frequency of 
the sample nuclei. Capacitance 13 may be a discrete capacitor, a plurality 
of discrete capacitors in series, or a distributed capacitance. Inductance 
10 normally is not a discrete inductor but the distributed inductance of a 
conducting structure, such as a loop of wire, which receives the magnetic 
resonance signal. Capacitance 14 is normally a discrete capacitor chosen 
to provide an impedance match between the antenna circuit and the 
transmission line, consisting of conductors 11 and 12, which carries the 
RF signal to the system's receiver. As illustrated the antenna circuit is 
completed when a DC current 17 is sent through the PIN diode 15, causing 
PIN diode 15 to conduct at RF frequencies. PIN diodes are utilized because 
of their high on/off conductance ratio. The circuit is open (disabled) 
when DC current is not applied to the PIN diode 15. This approach is less 
than optimum due to significant RF signal loss in PIN diode 15. 
In the alternative circuit shown in FIG. 2 the series-resonant antenna 
circuit consists of inductance 10 and capacitances 18 and 22. Inductance 
16, capacitance 18, and PIN diode 20 form a blocking resonant loop coupled 
through capacitor 18 to the antenna circuit. The DC disable signal is 
applied to terminals 24 and 26, which also serve as the RF signal 
terminals. Note that the polarity of PIN diode 20 must be such that the DC 
disable signal produces forward current through the diode. When DC current 
flows through PIN diode 20 inductance 16 is placed in parallel with 
capacitance 18, creating a parallel resonance which has a minimum response 
at the resonant frequency, selected to be the Larmor frequency of the 
sample nuclei. By adjusting inductance 16 it is possible to substantially 
null the response of the antenna circuit at the resonant frequency. This 
disables the antenna and minimizes its effect on the other antennae in the 
MRI system. In this embodiment, the antenna's resonant circuit does not 
include PIN diode 20 and thus the loss of that diode does not produce 
unwanted signal loss. 
When transmit or receive antennae are placed in close proximity to the 
patient; they present a potential source of hazardous DC or low frequency 
AC voltage and current in the event that one or more components fail. The 
potential for electric shock has become of greater concern with the advent 
of interventional procedures performed within an MRI system. In such 
procedures conductive fluids such as normal saline and body fluids from 
the patient often are present, and it may not be possible to guarantee 
complete isolation of these fluids from the conducting components 
associated with the antenna. 
For safety and shock prevention, it is possible to electrically isolate the 
RF signal path by AC coupling the system receiver to the antenna. When 
blocking techniques such as that shown in FIG. 2 are used, the blocking 
loop must be kept physically small to avoid inducing RF currents which can 
turn off or adversely heat the PIN diode 20. PIN diode 20 and inductor 16 
must therefore be located at the antenna, so a DC current path must exist 
from the system electronics to the antenna and complete isolation is not 
possible. 
SUMMARY OF THE INVENTION 
The present invention is embodied in an antenna assembly that includes a 
resonant circuit tuned to a Larmor frequency of the substance being 
examined by the MRI system. A photosensitive semiconductor switch 
selectively connects a reactive electrical device to the resonant circuit 
in response to impingement of light thereby nulling the response of the 
resonant circuit at the Larmor frequency. A light emitter is coupled, 
preferably by an optical fiber, to illuminate the photosensitive 
semiconductor switch. 
In the preferred embodiment, the resonant circuit comprises a pair of 
capacitances connected in series with the inductance of the conducting 
structure in which the magnetic resonance signals induce an electric 
current. The reactive electrical device is an inductor that is connected 
across one of the capacitances by the photosensitive semiconductor switch. 
The antenna circuit is altered by changing the conductive state of the 
photosensitive semiconductor switch. This nulls the response of the 
resonant antenna circuit at the Larmor frequency, preventing it from 
adversely affecting operation of other antennae of the MRI system during 
operating modes when it is not being used. During those modes the 
operation of the photosensitive semiconductor switch also electrically 
blocks current induced in the antenna from being conducted into sensitive 
electronics connected to the antenna. By optically controlling the 
photosensitive semiconductor switch, a DC electrical path, which could 
present a potential shock hazard for a patient being examined by the MRI 
system, is not required.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIG. 3, there is shown the major components of a 
preferred MRI system, which incorporates the present invention. The 
operation of the system is controlled from an operator console 100, which 
includes a keyboard and control panel 102 and a display 104. A separate 
display (not shown) may be located near the magnet system 103 for viewing 
by a physician attending the subject of a magnetic resonance imaging (MRI) 
scan. The console 100 communicates through a link 116 with a computer 
system 107 that enables an operator to control the production and display 
of images on the screen 104. The computer system 107 includes a number of 
modules, which communicate with each other through a backplane. These 
include an image processor module 106, a CPU module 108 and a memory 
module 113, known in the art as a frame buffer for storing image data 
arrays. The computer system 107 is linked to disk storage 111 and a data 
archive device 112 for storage of image data and programs, and 
communicates with a separate system control 122 through a high-speed link 
115. 
The system control 122 includes a set of modules connected together by a 
backplane. These include a CPU module 119 and a pulse generator module 
121, which connects to the operator console 100 through a data link 125. 
The pulse generator module 121 operates the system components to carry out 
the desired scan sequence. It produces data indicating the timing, 
strength and shape of the radio frequency (RF) pulses intended to be 
produced, and the timing of and length of the data acquisition periods. 
The pulse generator module 121 also connects to a set of gradient 
amplifiers 127, to indicate the timing, strength and shape of the gradient 
pulses to be produced during the scan. The pulse generator module 121 also 
may receive patient data from a physiological acquisition controller 129 
that receives signals from a number of different sensors connected to the 
patient, such as ECG signals from electrodes or respiratory signals from a 
bellows. 
The gradient waveforms produced by the pulse generator module 121 are 
applied to a gradient amplifier system 127 comprised of G.sub.x, G.sub.y 
and G.sub.z amplifiers. Each gradient amplifier excites a corresponding 
gradient coil in the magnet system 103 to produce the magnetic field 
gradients used for position encoding acquired signals. A transceiver 
module 150 in the system control 122 produces RF pulses which are 
amplified by an RF amplifier 151 and coupled to an RF transmit antenna 
(not shown) in the magnet assembly 103, optionally by a transmit switch 
153. The RF transmit antenna may be a fixed assembly integrated into the 
magnet system 103, a removable assembly with a predetermined position 
relative to the patient, or a local assembly placed as required against 
the patient. The RF transmit antenna may be used solely for transmitting 
RF into the subject or, optionally, may also be used to receive magnetic 
resonance signals from the subject. 
During a receive mode, the resulting magnetic resonance signals radiated by 
excited nuclei in the patient 145 are sensed by a RF receive antenna 
assembly 154, which optionally may also be the RF transmit antenna. The RF 
receive antenna assembly 154 is coupled through cable 158 to a 
preamplifier 156. The RF receive antenna assembly 154 is enabled by a 
receive antenna control 155 via an optical fiber 164. The transmit switch 
153 and receive antenna control 155 respond to a transmit/receive signal 
on line 157 from the pulse generator module 121 by electrically connecting 
the RF amplifier 151 to the RF transmit antenna only during the transmit 
mode, and by connecting the RF receive antenna assembly 154 to the 
preamplifier 156 only during the receive mode of operation. 
The magnetic resonance signals picked up by the RF receive antenna in 
assembly 154 are demodulated, filtered and digitized by the receiver 
section of transceiver module 150 and transferred to a memory module 160 
in the system control 122. A reconstruction processor 161 converts the 
received data into an array of image data. This image data is conveyed 
through link 115 to the computer system 107 where it is stored in memory 
module 113 and/or disk memory 111. In response to commands received from 
the operator console 100, this image data may be archived on the data 
archive device 112. Optionally, it may be further processed by the image 
processor 106 and conveyed to the operator console 100 and presented on 
the display 104. 
As noted previously, the resonant circuit in the RF receive antenna 
assembly 154 can interfere with the proper production of the excitation 
field during the transmit mode. Therefore, a receive antenna control 155 
converts a transmit/receive signal on line 157 into an optical signal by 
any of a plurality of well-known means. This optical signal is transmitted 
via optical fiber 164 or other transmission means to receive antenna 
assembly 154, where it disables an RF receive antenna assembly 154 during 
transmit mode and enables an RF receive antenna assembly 154 during 
receive mode. 
Referring to FIG. 4, the antenna assembly 154 comprises an inductance 170 
connected to form a resonant circuit 175 with a pair of series connected 
capacitances 172 and 174 with an intermediate node 176 between the 
capacitances. The resonant circuit 175 is tuned to the Larmor frequency of 
the substance being examined, (e.g. human tissue). One conductor of the 
signal cable 158 is connected via terminal 177 to the intermediate node 
176 by an inductor 178 or other reactive electrical device. The other 
conductor of the signal cable 158 is connected via terminal 179 to a node 
180 between the second capacitance 174 and inductance 170. A 
photosensitive semiconductor device, such as photodiode 182, is connected 
between the two conductors of the signal cable 158 without regard to diode 
polarity. Alternatively, the photosensitive device could comprise a 
PIN-type photodiode, a phototransistor, a photodarlington transistor pair, 
a light-activated SCR or a photo-FET. The end of the optical fiber 164 is 
positioned to illuminate the active surface of photodiode 182. 
With additional reference to FIG. 3, when the signal on line 157 indicates 
that the system is in the transmit mode, the receive antenna control 155 
responds by producing a light beam which is sent through the optical fiber 
164. This light beam illuminates the photodiode 182 in the receive antenna 
assembly 154, hereby rendering the photodiode conductive. This causes the 
blocking loop 187 formed by the photodiode 182, input inductor 178, and 
the second capacitance 174 to be parallel resonant at the Larmor 
frequency. The blocking loop 187 is coupled to the resonant circuit 175. 
This blocking loop parallel resonance substantially nulls the response of 
the resonant circuit 175 at the Larmor frequency, thereby preventing the 
receive antenna from affecting the performance of the other antennae 
during the transmit mode. During the receive mode, the receive antenna 
control 155 does not produce illumination of the photodiode 182 so that 
the blocking loop 187 does not form a complete parallel resonant circuit 
and has no effect on the resonant circuit 175. When resonant at the Larmor 
frequency, blocking loop 187 also presents a high impedance between the 
preamplifier 156 and resonant circuit 175 which electrically isolates the 
two components during the transmit mode. Thus any signal induced in the RF 
receive antenna, due to the intense transmit fields, will be attenuated 
before reaching the preamplifier and other electronics in the control 
system 122. 
In this embodiment, the electrical current and voltage required to disable 
resonant circuit 175 are remote from the RF antenna assembly 154, being 
isolated by the optical fiber 164. The DC path required for the prior 
antenna disabling techniques has been eliminated. As a result, potentially 
hazardous electrical voltages and currents arising from any source in the 
system, which might be transmitted to the subject by an electrically 
conducting signal path intended for disabling the RF receive antenna, are 
isolated from the RF antenna assembly 154 by the optical fiber. As a 
consequence, the patient being examined is not subjected to an electrical 
shock potential in the event of a failure of components connected to the 
receive antenna, provided that the RF path is also isolated by any of a 
plurality of well-known means, and that no additional electrically 
conducting paths are introduced. 
FIG. 5 illustrates an alternative embodiment 254 of the present optical 
technique for disabling an RF antenna. This embodiment is similar to that 
of FIG. 4 but has been modified with the addition of a semiconductor 
switch 188 in parallel with the photosensitive device 190, but with the 
opposite polarity (i.e. an anti-parallel connection with photosensitive 
device 190). In such a configuration, the normal forward current between 
terminals 191 and 192 through semiconductor switch 188 is opposite that of 
normal forward current between terminals 191 and 192 through photodiode 
190. Semiconductor switch 188 may, for example, be a PIN type diode, 
transistor, FET or SCR. The current produced by the photodiode or other 
type of photosensitive device 190, when illuminated, will flow through and 
partially turn on semiconductor switch 188 thereby reducing the net RF 
impedance between terminals 191 and 192. This will reduce the on-state 
impedance in blocking loop 194, increasing the degree to which the 
parallel resonance of blocking loop 194 nulls the response of resonant 
circuit 195 comprising inductance 196 and two capacitances 197 and 198. 
As a variation of the second embodiment, the semiconductor switch 188 may 
also be a photodiode or other type of photosensitive device. In this case, 
best operation will be obtained if provision is made to adequately 
illuminate both photosensitive devices 190 and 188 in order to render 
those devices conductive. 
FIG. 6 illustrates an alternative third embodiment 354 of the present 
optical technique for disabling an RF antenna. This embodiment has a 
parallel resonant blocking loop 201, comprised of photosensitive 
semiconductor switch 214, inductor 212, and capacitance 204 rather than 
capacitance 206 corresponding to capacitances 174 and 198 in FIGS. 4 and 
5, respectively, and optionally semiconductor switch 216. This can be done 
because there is no need to provide an electrically conducting path to 
photosensitive semiconductor device 214 as is the case for PIN diode 20 in 
FIG. 2. Device 214 may be connected without regard to diode polarity, and 
may be a photodiode, a PIN-type photodiode, a phototransistor, a 
photodarlington transistor pair, a light-activated SCR or a photo-FET. If 
semiconductor switch 216 is omitted the circuit operation is identical to 
that of the first embodiment in FIG. 4, while offering an additional 
option for physical placement of the components of blocking loop 201. The 
circuit of FIG. 6 offers the further advantage that photosensitive 
semiconductor switch 214 and inductance 212 are not in the signal path 
between the resonant circuit 208 and the signal cable 158 connected to 
terminals 218 and 219, and therefore do not attenuate the received signal 
in receive mode. 
As a variation of the third embodiment, the modifications of the second 
embodiment shown in FIG. 5, that is, the addition of a semiconductor 
switch 216 anti-parallel with the photosensitive semiconductor device 214, 
may be applied to the circuit of FIG. 6. This will reduce the on state 
impedance in blocking loop 201, increasing the degree to which the 
parallel resonance of blocking loop nulls the response of resonant circuit 
208. As a further variation of the third embodiment, the anti-parallel 
semiconductor switch 216 may also be a photodiode or other type of 
photosensitive device or any semiconductor activated by photodiode 214. 
The foregoing description was primarily directed to a preferred embodiment 
of the invention. Although some attention was given to various 
alternatives within the scope of the invention, it is anticipated that one 
skilled in the art will likely realize additional alternatives that are 
now apparent from disclosure of embodiments of the invention. For example, 
the invention may be used to disable a transmit antenna rather than a 
receive antenna, and may be used in systems other than MRI systems where 
similar functionality is desirable. Accordingly, the scope of the 
invention should be determined from the following claims and not limited 
by the above disclosure.