Patent Grant
12183990
Magnetic Resonance Imaging (MRI) is used to image biological tissue by creating an environment where a nuclear magnetic resonance (NMR) signal is generated. To do this, a sample, e.g., a patient or animal, is first placed in a uniform magnetic field (B0), which is oriented along the Z axis in x/y/z Cartesian space, to create a net magnetic moment parallel to the field of the sample's nuclear magnetic spins. While in the B0 field, a high-power radio frequency (RF) excitation pulse of energy is applied to create Larmor precession of protons in the x-y plane. This is known as the transmit mode of an MRI scan, with the applied RF excitation pulse, commonly referred to as the transmit pulse, as the transmit field, or as the B1+ field. This transmit pulse is applied at a frequency known as the Larmor frequency, determined by the field strength (B0) of the scanner and the gyromagnetic ratio of the sample's atomic nuclei of interest following the equation below, where γ is a known constant for a given element:
flarmor=B0*(γ/2π)
Due to the abundance of water in the human body and the nuclear properties of hydrogen, the majority of clinical MR images are acquired using the signals from hydrogen nuclei (γ=42.6 MHz/Tesla). Therefore, for clinical MRI at 1.5 T and 3 T, flarmor is approximately 64 MHz and 128 MHz, respectively.
Once the transmit field is turned off, the excited nuclei will gradually relax back to their resting state, oriented parallel to the B0 field. This period of gradual relaxation is known as the RF receive mode of an MRI scan. The transmit field excitation pulse(s) cause the spins of the nuclei to be “tipped” creating localized changing magnetic moments at the nuclei. The relaxation causes this created magnetic signal to decay. Differences in decay time between different materials is how MRI achieves contrast between different tissues in the body. The localized changing magnetic moments, can be converted into an electrical signal, referred to as a receive signal, using Faraday's law of induction, by placing a loop of wire near the sample. Loops of wire known as receive antennas, channels, elements, or antennas can be placed at different locations at a sample, e.g., overlaying different portions of a patient's body, to capture magnetic energy emanating from such different locations. In practice other RF antennas that are sensitive to the magnetic and electric fields at the Larmor frequency can also be used as received antennas (e.g. dipole, stripline, birdcage, patch antenna, etc).
Images are acquired in MRI by manipulating the field during the transmit mode of the scan with gradient antennas, which manipulate B0 in the x, y, or z direction in order to encode the spins with spatial information. During the RF receive mode, the resulting signals produced by the nuclei of interest and transformed into electrical RF receive signals in the local receive antennas, which then can be passed to MRI scanner processing circuitry, decoded, and reconstructed into cross-sectional or volumetric images of the sample.
Because the receive signal acquired by the receive antenna is converted directly into images, the quality of the receive signal directly impacts the clinical image quality. To help increase the amount and quality of the receive signal, receive antennas, generally are placed as close to the surface of a sample volume of interest as possible, to increase the amount of Faraday induction that occurs. Receive antennas typically are tuned to be resonant at the Larmor frequency, flarmor, by adjusting the amount of inductance and/or capacitance in the receive loop to further increase their sensitivity to the frequency of interest.
However, in order to place a receive antenna as close as possible to the region of interest, a receive antenna ordinarily must be placed directly within the high power transmit field, B1+. Because the transmit pulse is applied at the Larmor frequency, it is capable of driving a significant current on a receive antenna that is resonant at the Larmor frequency. This current could be capable of distorting the transmit field, destroying the resulting image and presenting a significant safety risk to the patient and to the MRI system. Therefore, to function correctly a surface antenna must be sensitive to the lower power magnetic energy present during relaxation of excited nuclei back to their resting state during the receive portion of the scan and invisible to the high power magnetic energy present during the transmit portion of the scan.
In practice this is generally accomplished by the use of active and passive detuning circuits, often referred to as decoupling or blocking circuits. U.S. Pat. No. 6,747,452, entitled, “Decoupling Circuit for Magnetic Resonance Imaging Local Coils”, discloses a local receive antenna operatively coupled to an active decoupling circuit and a passive decoupling circuit. An active detuning circuit typically is switched on and off by an external DC signal supplied through a wire to one or more PIN diodes in a decoupling circuit connected to the surface receive antenna. The PIN diodes act like a solid-state switch for this circuit, which is designed to present a high impedance which effectively detunes the antenna. The DC signal is turned on during the transmit mode of the scan to activate the active detuning circuit to prevent the transmit field from driving large currents onto the receive loop, which could potentially damage sensitive electronics, ruin the image, and/or harm the patient. The DC signal is turned off during the RF receive mode to deactivate the active detuning circuit to permit the loop tuned to the Larmor frequency, to resonate in response to magnetic energy emanating from the sample. A passive detuning circuit typically is used to detune the antenna during the transmit mode when the active detuning circuit fails to operate. Passive detuning often is accomplished using a cross-coupled diode pair, which is biased by a high-power voltage induced on the antenna during the transmit mode of the scan in the absence of an operating active detuning circuit. Thus, the combination of an active detuning circuit and a passive detuning circuit can provide a relatively failsafe mechanism to ensure that a receive antenna remains not resonant during the transmit mode. The active detuning circuit generally acts as the primary mechanism to detune the receive antenna during the transmit mode and the passive detuning circuit generally acts as a backup that automatically detunes the antenna if the active detuning circuit fails to do so during the transmit mode.
Modern MRI receive arrays are often multichannel (i.e., having more than one antenna in close proximity to the patient) to take advantage of modern image acceleration techniques allowing for faster exam times. In multichannel arrays it is necessary to ensure that the channels are isolated from one another so as to minimize RF energy shared between them. More particularly, it is necessary to ensure that current induced on one antenna loop is not parasitically shared in other nearby antenna loops of coils. To prevent this, amplifiers are commonly used in conjunction with receive circuits to present a high impedance at the antennas during the receive mode of an MRI scan. A high impedance at a receive antenna during the receive mode limits current flow within that antenna, which in turn, reduces RF electromagnetic energy transmitted by that antenna, which thereby limits inducing by that antenna of a parasitic current in a nearby antenna.
Both active, passive and amplifier detuning circuits generally are constructed of relatively large discrete components (e.g., inductors, capacitors, and diodes), resulting in relatively large, often fragile circuits. To protect the detuning circuits from mechanical damage, they often are placed in plastic or metal enclosures, which can be relatively large, bulky, heavy, and inflexible. In contrast, because receive antennas ordinarily must be placed as close to the imaging region of interest as possible to maximize image quality, the individual loops themselves typically are constructed to be lightweight and flexible to allow them to easily conform to the sample's contours, e.g., to the contours of a patient's body. Unfortunately, detuning circuitry, which is coupled to a receive antenna and housed in a bulky protective enclosure, can compete for space near a patient's anatomy, limiting the flexibility in placement of the receive antenna and therefore, potentially limiting clinical image quality.
In one aspect, a radio frequency (RF) circuit is provided for use with a magnetic resonance imaging (MRI) scanner to transmit an RF receive signal to an amplifier circuit. The RF circuit includes a transmission line including a first end portion and a second end portion, an antenna electrically connected in parallel with the transmission line at the first end portion of the transmission line, and an impedance transformation circuit. The RF circuit further includes an impedance matching and detuning circuit electrically connected between the transmission line and the impedance transformation circuit, in parallel with the transmission line at the second end portion of the transmission line, and in parallel with the impedance transformation circuit. The impedance matching and detuning circuit includes multiple reactive impedance elements and two or more switches. The two or more switches are operable to controllably switch between configuring RF connections between the multiple reactive impedance elements, to RF connect a first set of the multiple reactive impedance elements to the second end portion of the transmission line to cause matching of overall impedance of the RF circuit to a prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during a receive mode of the MRI scanner, and to RF connect a second set of the multiple reactive impedance elements to the second end portion of the transmission line to cause an increase of impedance at the antenna, to reduce sensitivity of the antenna to RF signals at the prescribed frequency during an excitation mode of the MRI scanner.
In another aspect, a radio frequency (RF) circuit is for use with a magnetic resonance imaging (MRI) scanner to transmit an RF receive signal to an amplifier circuit. The RF circuit includes a transmission line including a first end portion and a second end portion, an antenna electrically connected in parallel with the transmission line at the first end portion of the transmission line, and an impedance transformation circuit. The RF circuit includes an impedance matching and detuning circuit electrically connected between the transmission line and the impedance transformation circuit, in parallel with the transmission line at the second end portion of the transmission line, and in parallel with the impedance transformation circuit. The impedance matching and detuning circuit includes: a first reactive impedance element, a second reactive impedance element, and a third reactive impedance element, an active switch that includes a PIN diode electrically connected in series with the third reactive impedance element and a first passive switch that includes a first cross-coupled PIN diode pair electrically connected between the first node and a third node. The series-connected PIN diode and third reactive impedance are electrically connected in parallel with the second end portion of the transmission line, between a first node at a signal conductor of the transmission line and a second node at a relative ground of the transmission line. The first reactive impedance is electrically connected between the first node and the third node. The second reactive impedance is electrically connected between the third node and the second node.
Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.
During receive mode operation, the receive circuit 202 behaves as a matched resonant circuit that presents an input impedance to the amplifier circuit 204 that matches a prescribed input impedance of the amplifier circuit 204 at a prescribed frequency to optimize the amplifier's amplification, that is, to optimize the amplifier's noise figure at the prescribed frequency. The impedance elements of the circuit 202 are selected to achieve a reactance to create resonance and to achieve a specific voltage-current relationship at the input to amplifier 204 at a prescribed frequency. The Larmor frequency is the prescribed frequency in an example receive circuit 202. An example amplifier has a noise-match input impedance of 50 ohms in which case a noise-matched resonant receive circuit 202 would present 50 ohms to the input of the amplifier 204 at the prescribed frequency. Reactive elements of the receive circuit 202 are selected or tuned such that the receive circuit 202 is resonant at a prescribed frequency. That is, a relationship phase between voltage and current of the signal is altered at various parts in the circuit to ultimately achieve a specific relationship at the amplifier 204. An example input to amplifier 204 has a real input impedance (i.e., R=V/I), with substantially no imaginary component, at the prescribed Larmor frequency. The reactive elements of an example receive circuit 202 are selected to transform voltage and current components of an RF receive signal at the Larmor frequency so that they are as in phase as possible at the input to the amplifier 204 in order to optimize amplifier performance at the Larmor frequency.
It is well known in the art that impedance represents relations between voltage and current (i.e., electric and magnetic fields). Impedance is specific to a given frequency. Resistance is the real part, which represents difference in the voltage-current measurement caused by loss to heat. Reactance is the imaginary part, which represents a change in timing of the peaks and valleys of the voltage and current measurements (i.e., phase difference).
The antenna 208, which comprises a loop or coil of conductive material, is positioned within the EM fields produced during MRI scans. The antenna is sensitive to variations in the EM fields at the Larmor frequency. During RF receive mode, magnetic flux from the precession of excited nuclei back to their resting state induces an RF receive signal at the receive signal frequency in the antenna 208. A receive signal at the Larmor frequency is transformed, as explained above, such that the receive circuit 202 is tuned to a specific impedance at the input to the amplifier 204.
An example antenna 208 includes discrete reactive antenna impedance elements 222 electrically connected in a loop configuration by conductors 221. In the example antenna 208, the reactive antenna impedance elements 222 include respective discrete capacitors 224, and the conductors 221, which may exhibit inherent inductive impedance. Alternative reactive antenna impedances (not shown) can include multi-capacitor circuits, inductor circuits, or circuits containing a combination of capacitors and inductors. Different ones of the reactive antenna impedance elements 222 may have different values, and one or more of the antenna reactive impedance elements 222 and/or inductances of conductor segments 221 may be adjustable to tune a resonant frequency of the antenna 208 to a prescribed receive signal frequency such as to a Larmor frequency used by the example MRI scanner 100.
An example first transmission line 210 comprises a coaxial cable. A respective one of the antenna reactive impedance elements 224 indicated within lines 222 is electrically connected in parallel with the first end portion 210-1 of the first transmission line 210, between a fifth node, labelled node A, at a first end portion of an inner coaxial signal conductor 226 (indicated by lines) and a sixth node, labelled node B, at a first end portion of an outer coaxial relative ground 228. An alternative example first transmission line (not shown) may comprise a twisted pair, microstrip, or strip line, for example.
An example impedance transformation circuit 215 includes reactive circuit elements configured to present an input impedance at the amplifier circuit 204 that matches a prescribed input impedance of the amplifier circuit 204 at a prescribed frequency to optimize the amplifier's amplification, that is, to optimize the amplifier's noise figure at the prescribed frequency. Moreover, an example impedance transform circuit 215 is configured to transform a small input impedance of the amplifier 204 to a higher impedance at the antenna 208 located at the first end portion 210-1 of the first transmission line. (e.g., an impedance presented by the amplifier 204 to the antenna 208) for amplifier decoupling, to minimize current in the antenna 208 to prevent the antenna 208 from producing parasitic RF signal coupling at one or more other nearby antennas (not shown). It will be appreciated that a configuration of reactive impedance elements to achieve impedance matching and transformation of amplifier impedance sufficient to block current in the antenna 208 will be understood by persons skilled in the art and depends upon specific physical and electrical implementation of the receive circuit 202 and characteristics of amplifier 204. During receive mode operation of the MRI scanner 100, certain reactive impedance elements of the impedance matching and detuning circuit 212 together with the impedance transformation circuit 215 are operative to match impedance of the receive circuit 202 to a prescribed impedance seen at the amplifier circuit 204 at a prescribed frequency, e.g., the Larmor frequency. During operation of the MRI scanner 100 in excitation mode, certain reactive impedance elements of the impedance matching and detuning circuit 212 are operative to detune the receive circuit 202 from resonance at the prescribed frequency. An example impedance matching and detuning circuit 212 can achieve RF detuning during the excitation mode through either active detuning or passive detuning. RF operation of the receive circuit is explained with reference to
Referring to
More particularly, the reactive impedance elements 222, 230, 232, along with transmission line 210, cooperate to transform the impedance of the antenna 208 such that the impedance presented by the receive circuit 202 between third node, labelled node D, and second node, labelled node E, as further transformed by impedance transformation performed by the impedance transformation circuit 215, is optimal for the amplifier 204. More particularly, the first and second reactive impedances 230, 232 are coupled in an example L-network between the first transmission line 210 and the impedance transformation circuit 215. Impedance values for of the first and second reactive impedance elements 230, 232 in an example L-network are selected to operate with the impedance transformation circuit 215 to present an optimal impedance for the amplifier 204. This combination of reactive impedances 222, 232, 230, transmission line 210, and the impedance transformation circuit 215, allow the antenna 208 to be located at a distance from the amplifier 204 while presenting the optimal input impedance to the amplifier 204.
Thus, during operation of the RF receive mode circuit 300 shown in
An example first reactive impedance element 230 and an example second reactive impedance element 232 can include respective single capacitors 231 and 233 as shown. An alternative first series reactive impedance element (not shown) can include a multi-capacitor circuit, an inductor circuit, or circuits containing a combination of capacitors and inductors. Similarly, an alternative second reactive impedance element (not shown) can include a multi-capacitor circuit, an inductor circuit, or circuits containing a combination of capacitors and inductors.
Referring to
The example third reactive impedance element 240 shown in
During the RF receive mode, a first switch 234 comprising the PIN diode 235 is turned off such that an effective RF active detuning circuit 400 discussed below with reference to
Referring to
Still referring to
It will be understood that the RF passive detuning circuit 500 is operational if the RF active detuning circuit 400 fails while the receive circuit 202 resonates at the prescribed frequency while during the excitation mode, which induces a sufficiently high voltage to turn on the first and second cross-coupled PIN diodes pairs 238, 239. However, if the RF active detuning circuit 400 is operative during the transmit excitation mode, then a voltage at the cross-coupled diode pairs 238, 239 will not be sufficient to turn them on. It will be clear to those skilled in the art that it is possible that in some cases only the first cross-coupled PIN diode pair, 236, may have a sufficiently high voltage to turn on, and that the resulting effective circuit would most closely resemble that in
Referring to
Referring to
Also, during operation of the RF active detuning circuit 400, the first blocking capacitor 250, which is operatively connected between the fourth node, labelled node H, and the first cross-coupled PIN diode switch 238, blocks DC current from turning on either diode of the first cross-coupled PIN diode switch 238. Similarly, a second blocking capacitor 252, which is operatively connected between the second cross-coupled PIN diode switch 239 and third node, labelled node D, blocks DC current from turning on either diode of the second cross-coupled PIN diode switch 239.
Referring to
More particularly, during an RF excitation mode in the absence of turn-on of the switch 234, an excitation signal is generated across the second and third switches 236, 237 that has a voltage level that is sufficiently high to turn on the cross-coupled PIN diode switches 238, 239. The first and second blocking capacitances 250, 252 have capacitance values selected such that they behave as short circuit elements to an RF signal. Also, as explained above, the first blocking inductor 244 behaves as open circuit element to an RF signal. The first and second passive tuning elements 242, 243 are tunable to tune a frequency characteristics of the passive detuning circuit 500. The tuning inductors 242, 243 can be used for example to compensate for small innate capacitances and inductances of a circuit board (not shown) in which receive circuit 202 are formed. Thus, turn on of the second and third switches 236, 237 activates the passive detuning circuit 500 of
The example RF passive detuning circuit 500 includes the first, second, and third reactive impedance elements 230, 232, 240, the antenna reactive impedance 222 connected between fifth node, labelled node A, and sixth node, labelled node B, the first transmission line 210, and first and second tuning inductors 242, 243. It should be noted that the tuning inductors, 242,243, may be replaced with other reactive elements, such as capacitors. The first reactive impedance element 230 is operatively connected in series with the transmission line 210 between first node, labelled node C, and third node, labelled node D. A first RF sub-circuit 255′ comprising the third reactive impedance element 240 and the first tuning inductor 242, connected in series, is operatively connected between first node, labelled node C, and second node, labelled node E, in parallel with the second end portion 210-2 of the first transmission line 210 and node E. A second RF sub-circuit 257′ comprising the second reactive impedance element 232 and the second tuning inductor 243, connected in series, is operatively connected between first node, labelled node C, and second node, labelled node E, in parallel with the second end portion 210-2 of the first transmission line 210. Second tuning inductor 243 is also operatively connected between first node, labelled node C, and third node, labelled node D. The RF passive detuning circuit 500 is operative to detune sensitivity of the receive circuit 202 by creating a high impedance between fifth node, labelled node A, and sixth node, labelled node B, at the antenna 208 located at first end portion 210-1 of the first transmission line 210 to block current flow on antenna 208. This blocking is accomplished primarily by the third reactive impedance element 240 acting in conjunction with the first transmission line 210 and with the first and second reactive impedance elements 230 and 232. Blocking is defined as creating a high resistance at the antenna portion 208, so that the antenna is less sensitive to RF signals. The reduced sensitivity of the antenna 208 results in reduced risk of injury to a patient or damage to circuitry due to excitation mode energy. Blocking is achieved by creating a high resistance at the Larmor frequency at the first transmission line portion 210-1 from the antenna 208 as shown in
Referring to
Examples of the resonant trap can include:
Example 1 includes a radio frequency (RF) circuit for use with a magnetic resonance imaging (MRI) scanner to transmit an RF receive signal to an amplifier circuit, the RF circuit comprising: a transmission line including a first end portion and a second end portion; an antenna electrically connected in parallel with the transmission line at the first end portion of the transmission line; an impedance transformation circuit; an impedance matching and detuning circuit electrically connected between the transmission line and the impedance transformation circuit, in parallel with the transmission line at the second end portion of the transmission line, and in parallel with the impedance transformation circuit, wherein the impedance matching and detuning circuit includes: multiple reactive impedance elements; and two or more switches operable to controllably switch between configuring RF connections between the multiple reactive impedance elements to, RF connect a first set of the multiple reactive impedance elements to the second end portion of the transmission line to cause matching of overall impedance of the RF circuit to a prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during a receive mode of the MRI scanner, and RF connect a second set of the multiple reactive impedance elements to the second end portion of the transmission line to cause an increase of impedance at the antenna, to reduce sensitivity of the antenna to RF signals at the prescribed frequency during an excitation mode of the MRI scanner.
Example 2 can include the subject matter of Example 1, wherein the two or more switches include: an active switch controllable based upon a DC control signal to switch between, a first active switch configuration in which the active switch RF disconnects at least one of the multiple reactive impedance elements from the second end portion of the transmission line to configure the first set of reactive impedance elements contribute to matching overall impedance of the RF circuit to the prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during a receive mode of the MRI scanner, and a second active switch configuration in which first switch RF connects the at least one of the multiple reactive impedance elements to the second end portion of the transmission line to configure the second set of reactive impedance elements to increase impedance at the antenna, to reduce sensitivity of the antenna to RF signals during an excitation mode of the MRI scanner; and a passive switch controllable based upon a voltage level of the RF receive signal transmitted within the RF circuit between, a first passive switch configuration in which the second switch RF disconnects at least one of the multiple reactive impedance elements from the second end portion of the transmission line in absence of an RF signal transmitted within the RF receive circuit having a voltage at or above a threshold voltage to configure the first set of reactive impedance elements to contribute to matching overall impedance of the RF circuit to a prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during a receive mode of the MRI scanner, and a second passive switch configuration in which the second switch RF connects at least one of the multiple reactive impedance elements to the second end portion of the transmission line in response to an occurrence of an RF signal transmitted within the RF receive circuit having a voltage at or above the prescribed threshold voltage to configure the second set of reactive impedance elements to increase impedance at the antenna, to reduce sensitivity of the antenna to RF signals during an excitation mode of the MRI scanner.
Example 3 can include the subject matter of Example 1, wherein the multiple reactive impedance elements include: a first reactive impedance element; a second reactive impedance element; and a third reactive impedance element; wherein the first set includes the first reactive impedance element and the second reactive impedance element; and wherein the first set includes the first reactive impedance element, the second reactive impedance element, and the third reactive impedance element.
Example 4 can include the subject matter of Example 3, wherein the two or more switches operable to RF connect the first reactive impedance element and the second reactive impedance element in an L-configuration with the first reactive impedance element RF connected in series with the transmission line at the second end portion of the transmission line and the second reactive impedance element RF connected in parallel with the transmission line at the second end portion of the transmission line after the first reactive impedance element, to cause matching of overall impedance of the RF circuit to a prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during a receive mode of the MRI scanner; and wherein the two or more switches operable to RF connect the third reactive impedance element RF connected in parallel with the transmission line at the second end portion of the transmission line, to cause an increase of impedance at the antenna, to reduce sensitivity of the antenna to RF signals at the prescribed frequency during an excitation mode of the MRI scanner.
Example 5 can include the subject matter of Example 3, wherein the first reactive impedance element and the second reactive impedance element are operable to be RF connected to contribute to matching overall impedance of the RF circuit to a prescribed input impedance seen at the amplifier circuit at a prescribed RF frequency during the receive mode of the MRI scanner; and wherein the two or more switches include, at least one active switch controllable based upon a DC signal to RF connect the third reactive impedance to the second end portion of the transmission line in response to an occurrence of the DC signal, to increase impedance at the antenna, to reduce sensitivity of the antenna to RF signals during the excitation mode of the MRI scanner; and at least one passive switch controllable based upon a voltage level of an RF signal transmitted within the RF receive circuit to RF connect the third reactive impedance to the second end portion of the transmission line in response to an occurrence of an RF signal within the RF receive circuit having a voltage at or above a threshold voltage, to increase impedance at the antenna, from the matching of overall impedance, to reduce sensitivity of the antenna to RF signals during the excitation mode of the MRI scanner.
Example 6 can include the subject matter of Example 5 further including: a fourth RF impedance element; wherein the at least one passive switch includes a first passive switch controllable based upon the voltage level of an RF signal transmitted within the RF receive circuit to RF connect the third reactive impedance in series with the fourth reactive impedance and in parallel with the second end portion of the transmission line in response to an occurrence of an RF signal within the RF receive circuit having a voltage at or above the threshold voltage, to cause an increase of impedance at the antenna, to reduce sensitivity of the antenna to RF signals.
Example 7 can include the subject matter of Example 6, further including: a fifth RF impedance element; wherein the at least one passive switch includes a second passive switch controllable based upon the voltage level of an RF signal transmitted within the RF receive circuit to RF connect the fifth reactive impedance in parallel with the second end portion of the transmission line and in series with the second reactive impedance, in response to an occurrence of an RF signal within the RF receive circuit having a voltage at or above the threshold voltage, to cause an increase of impedance at the antenna, to reduce sensitivity of the antenna to RF signals.
Example 8 can include the subject matter of Example 7, wherein the first passive switch and the second passive switch are operable switch independently of one another in response to an occurrence of an RF signal within the RF receive circuit having a voltage at or above the threshold voltage.
Example 9 can include the subject matter of Example 7, wherein the first passive switch and the second passive switch are operable switch independently of one another in response to an occurrence of an RF signal within the RF receive circuit having a voltage at or above the threshold voltage; and wherein fourth reactive impedance is operable to cause an increase of impedance at the antenna, from the matching of overall impedance, to reduce sensitivity of the antenna to RF, in an event of a failure of the third reactive impedance.
Example 10 can include the subject matter of Example 1, wherein the RF receive circuit is resonant at the prescribed RF frequency having a prescribed frequency band and a prescribed center frequency, when the two or more switches RF connect the first set of the multiple reactive impedance elements to the second end portion of the transmission line; and wherein a failure of the circuit, that causes electrical disconnection of one or more of the impedance matching and detuning circuit elements or of the entire impedance matching and detuning circuit from the transmission line connecting the circuit to the antenna, causes a shift in resonant frequency of the RF receive circuit to a shifted resonant frequency, having a shifted frequency band and a shifted center frequency different from the respective prescribed frequency band and the prescribed center frequency.
Example 11 can include the subject matter of Example 1, wherein the transmission line is a differential transmission line that includes a signal conductor and a relative ground conductor; wherein the antenna is electrically connected across the signal conductor and the relative ground conductor at the first end portion of the transmission line; and wherein the increase of impedance at the antenna to, from the matching of overall impedance, reduce sensitivity of the antenna to RF signals occurs at a location of the RF circuit where antenna is electrically connected across the signal conductor and the relative ground conductor at the first end portion of the transmission line.
Example 12 can include the subject matter of Example 1, wherein the impedance transformation circuit is configured to transform an input impedance of the amplifier at the antenna to a higher impedance at the antenna during transmission of an RF receive signal within the RF circuit, to minimize current in the antenna to prevent the antenna from producing parasitic RF signal coupling at one or more other antennas.
Example 13 can include the subject matter of Example 1, wherein the prescribed frequency is a Larmor frequency.
Example 14 can include the subject matter of Example 2, wherein the predetermined voltage threshold is an MRI RF excitation signal voltage level.
Example 15 can include the subject matter of Example 3, wherein the two or more switches include an active switch and a first passive switch; wherein the active switch includes a PIN diode electrically connected in series with the third reactive impedance element; wherein the series-connected PIN diode and third reactive impedance are electrically connected in parallel with the second end portion of the transmission line, between a first node at a signal conductor of the transmission line and a second node at a relative ground of the transmission line; and wherein the first passive switch includes a first cross-coupled PIN diode pair electrically connected between the first node and a third node; wherein the first reactive impedance is electrically connected between the first node and the third node; and wherein the second reactive impedance is electrically connected between the third node and the second node.
Example 16 can include the subject matter of Example 15 further including: a first blocking capacitor electrically connected in series between the first passive switch and the third node; wherein the first blocking capacitor behaves as a short circuit to RF signals.
Example 17 can include the subject matter of Example 16, further including: a first reactive tuning element electrically connected in series between the first passive switch and the first blocking capacitor.
Example 18 can include the subject matter of Example 15, further including: a first blocking inductor electrically connected in parallel with the first reactive impedance; wherein the first blocking inductor behaves as an open circuit to RF signals.
Example 19 can include the subject matter of Example 15, further including: wherein the two or more switches include a second passive switch; wherein the second passive switch includes a second cross-coupled PIN diode pair electrically connected between the first node and a fourth node; wherein the third reactive impedance is electrically connected between the fourth node and the third node.
Example 20 can include the subject matter of Example 19, further including: a first blocking capacitor electrically connected in series between the first passive switch and the third node; and a second blocking capacitor electrically connected in series between the second passive switch and the fourth node; wherein each of the first blocking capacitor and the second blocking capacitor behaves as a short circuit to RF signals.
Example 21 can include the subject matter of Example 20, further including: a first reactive tuning element electrically coupled in series between the first passive switch and the first blocking capacitor; and a second reactive tuning element electrically coupled in series between the second passive switch and the second blocking capacitor.
Example 22 can include the subject matter of Example 21, further including: a first blocking inductor electrically connected in parallel with the first reactive impedance; and a second blocking inductor electrically connected in parallel with the third reactive impedance: wherein each of the first blocking inductor and the second blocking inductor behaves as an open circuit to RF signals.
Example 23 can include the subject matter of Example 1, further including: a reactive element electrically connected in parallel with the first end portion of the transmission line, between a fifth node at the signal conductor of the transmission line and a sixth node at a relative ground of the transmission line.
Example 24 can include a radio frequency (RF) circuit for use with a magnetic resonance imaging (MRI) scanner to transmit an RF receive signal to an amplifier circuit, the RF circuit comprising: a transmission line including a first end portion and a second end portion; an antenna electrically connected in parallel with the transmission line at the first end portion of the transmission line; an impedance transformation circuit; an impedance matching and detuning circuit electrically connected between the transmission line and the impedance transformation circuit, in parallel with the transmission line at the second end portion of the transmission line, and in parallel with the impedance transformation circuit, wherein the impedance matching and detuning circuit includes: a first reactive impedance element; a second reactive impedance element; and a third reactive impedance element; an active switch includes a PIN diode electrically connected in series with the third reactive impedance element; a first passive switch that includes a first cross-coupled PIN diode pair electrically connected between the first node and a third node; wherein the series-connected PIN diode and third reactive impedance are electrically connected in parallel with the second end portion of the transmission line, between a first node at a signal conductor of the transmission line and a second node at a relative ground of the transmission line; wherein the first reactive impedance is electrically connected between the first node and the third node; and wherein the second reactive impedance is electrically connected between the third node and the second node.
Example 25 can include the subject matter of Example 24, further including: a first blocking capacitor electrically connected in series between the first passive switch and the third node; wherein the first blocking capacitor behaves as a short circuit to RF signals.
Example 26 can include the subject matter of Example 25, further including: a first tuning inductor electrically connected in series between the first passive switch and the first blocking capacitor.
Example 27 can include the subject matter of Example 24, further including: a first blocking inductor electrically connected in parallel with the first reactive impedance; wherein the first blocking inductor behaves as an open circuit to RF signals.
Example 28 can include the subject matter of Example 24, further including: wherein the two or more switches include a second passive switch; wherein the second passive switch includes a second cross-coupled PIN diode pair electrically connected between the first node and a fourth node; wherein the third reactive impedance is electrically connected between the fourth node and the third node.
Example 29 can include the subject matter of Example 28, further including: a first blocking capacitor electrically connected in series between the first passive switch and the third node; and a second blocking capacitor electrically connected in series between the second passive switch and the fourth node; wherein each of the first blocking capacitor and the second blocking capacitor behaves as a short circuit to RF signals.
Example 30 can include the subject matter of Example 29, further including: a first tuning inductor electrically coupled in series between the first passive switch and the first blocking capacitor; and a second tuning inductor electrically coupled in series between the second passive switch and the second blocking capacitor.
Example 31 can include the subject matter of Example 30, further including: a first blocking inductor electrically connected in parallel with the first reactive impedance; and a second blocking inductor electrically connected in parallel with the third reactive impedance; wherein each of the first blocking inductor and the second blocking inductor behaves as an open circuit to RF signals.
Example 32 can include the subject matter of Example 24, further including: a reactive element electrically connected in parallel with the first end portion of the transmission line, between a fifth node at the signal conductor of the transmission line and a sixth node at a relative ground of the transmission line.
The above description is presented to enable any person skilled in the art to create and use an MRI RF receive circuit. Various modifications to the examples will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention. In the preceding description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the examples in the disclosure might be practiced without the use of these specific details. In other instances, well-known processes are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Identical reference numerals are used in some places to represent different views of the same or similar items in different drawings. Thus, the foregoing description and drawings of embodiments and examples are merely illustrative of the principles of the invention. Therefore, it will be understood that various modifications can be made to the embodiments by those skilled in the art without departing from the spirit and scope of the invention, which is defined in the appended claims.
The present application claims the benefit of U.S. Provisional Patent Application, Ser. No. 63/310,867, filed Feb. 16, 2022, entitled, “Failsafe Remote Matching and Blocking Circuit for Magnetic Resonance Imaging Surface Coils,” which is hereby incorporated by reference in its entirety.
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