Birdcage magnetic resonance imaging (MRI) coil with open shield for single tune MRI coil and multi-tune MRI coil

Embodiments relate to birdcage coils with in-plane open RF shielding capable of operating at 7T and higher field strength. One example embodiment comprises a first birdcage circuit and second birdcage circuit, each comprising two rings, N rungs that electrically connect the two rings of that circuit, a plurality of capacitors in the first birdcage circuit to form a first birdcage coil, and an optional plurality of capacitors in the second birdcage circuit to form a second birdcage coil when included or a non-resonant RF shield when omitted, wherein the first birdcage circuit is electrically isolated from the second birdcage circuit, wherein the first birdcage circuit and the second birdcage circuit have a common cylindrical axis, and wherein the N rungs of the second birdcage circuit are azimuthally rotated through a first angle relative to the N rungs of the first birdcage circuit.

BACKGROUND

Magnetic resonance imaging (MRI) involves the transmission and receipt of radio frequency (RF) energy. RF energy may be transmitted by a coil. Resulting magnetic resonance (MR) signals may also be received by a coil. In early MRI, RF energy may have been transmitted from a single coil and resulting MR signals received by a single coil. Later, multiple receivers may have been used in parallel acquisition techniques. Similarly, multiple transmitters may have been used in parallel transmission (pTx) techniques.

RF coils create the B1field that rotates the net magnetization in a pulse sequence. RF coils may also detect precessing transverse magnetization. Thus, RF coils may be transmit (Tx) coils, receive (Rx) coils, or transmit and receive (Tx/Rx) coils. An imaging coil should be able to resonate at a selected Larmor frequency. Imaging coils include inductive elements and capacitive elements. The inductive elements and capacitive elements have been implemented according to existing approaches using two terminal passive components (e.g., capacitors). The resonant frequency, f, of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit according to equation (1):

Imaging coils may need to be tuned. Tuning an imaging coil may include varying the value of a capacitor. Recall that frequency: f=ω/(2π), wavelength in vacuum: λ=c/f, and λ=4.7 m at 1.5 T. Recall also that the Larmor frequency: f0=γ B0/(2π), where (for1H nuclei) γ/(2π)=42.58 MHz/T; at 1.5 T, f0=63.87 MHz; at 3 T, f0=127.73 MHz; at 7 T, f0=298.06 MHz. Basic circuit design principles include the fact that capacitors add in parallel (impedance 1/(jCω)) and inductors add in series (impedance jLω).

DETAILED DESCRIPTION

The present disclosure will now be described with reference to the attached drawing figures, wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures and devices are not necessarily drawn to scale.

Embodiments described herein can be implemented in a MRI (Magnetic Resonance Imaging) system using any suitably configured hardware and/or software. Referring toFIG. 1, illustrated is an example MRI apparatus100that can be configured with example MRI RF coils, coil elements, coil arrays, or circuitry according to one or more embodiments described herein. Apparatus100includes basic field magnet(s)110and a basic field magnet supply120. Ideally, the basic field magnets110would produce a uniform B0field. However, in practice, the B0field may not be uniform, and may vary over an object being imaged by the MRI apparatus100. MRI apparatus100can include gradient coils135configured to emit gradient magnetic fields like Gx(e.g., via an associated gradient coil135x), Gy(e.g., via an associated gradient coil135y) and G, (e.g., via an associated gradient coil135z). The gradient coils135can be controlled, at least in part, by a gradient coils supply130. In some examples, the timing, strength, and orientation of the gradient magnetic fields can be controlled, and thus selectively adapted during an MRI procedure.

MRI apparatus100can include a primary coil165configured to generate RF pulses. The primary coil165can be a whole body coil. The primary coil165can be, for example, a birdcage coil. The primary coil165can be controlled, at least in part, by an RF transmission unit160. RF transmission unit160can provide a signal to primary coil165.

MRI apparatus100can include a set of RF antennas150(e.g., one or more RF antennas1501-150N, which can be as described herein). RF antennas150can be configured to generate RF pulses and to receive resulting magnetic resonance signals from an object to which the RF pulses are directed. In some embodiments, RF antennas150can be configured to inductively couple with primary coil165and generate RF pulses and to receive resulting magnetic resonance signals from an object to which the RF pulses are directed. In other embodiments, RF antennas150can be electrically coupled to a power source (e.g., RF Tx unit160) that can drive RF antennas150to generate RF pulses, and RF antennas can also be configured to receive resulting magnetic resonance signals from an object to which the RF pulses are directed. In one embodiment, one or more members of the set of RF antennas150can be fabricated from flexible coaxial cable, or other conductive material. The set of RF antennas150can be connected with an RF receive unit164.

The gradient coils supply130and the RF transmission units160can be controlled, at least in part, by a control computer170. The magnetic resonance signals received from the set of RF antennas150can be employed to generate an image, and thus can be subject to a transformation process like a two dimensional fast Fourier transform (FFT) that generates pixilated image data. The transformation can be performed by an image computer180or other similar processing device. The image data can then be shown on a display199. RF Rx Units164can be connected with control computer170or image computer180. WhileFIG. 1illustrates an example MRI apparatus100that includes various components connected in various ways, it is to be appreciated that other MRI apparatus can include other components connected in other ways, and can be employed in connection with various embodiments discussed herein.

In one embodiment, MRI apparatus100includes control computer170. In one example, a member of the set of RF antennas150can be individually controllable by the control computer170. A member of the set of RF antennas150can be an example MRI RF coil array including, for example, MRI RF coil arrays as described herein. In various embodiments, the set of RF antennas150can include various combinations of example embodiments of MRI RF coil arrays, elements or example embodiments of MRF RF coil arrays, including single-layer MRI RF coil elements or single-layer MRI RF coil arrays, according to various embodiments described herein.

An MRI apparatus can include, among other components, a controller (e.g., control computer170) and an RF coil (e.g., primary coil165) operably connected to the controller. The controller can provide the RF coil with a current, a voltage, or a control signal. The coil can be a whole body coil. The coil can inductively couple with an example MRI RF coil element, or MRI RF coil array, as described herein. Control computer170can provide a DC bias current, or control a DC bias control circuit to control the application of a DC bias current to MRI RF coil arrays or elements that can be part of antennas150.

Radio frequency (RF) shielding can be used in magnetic resonance imaging (MRI) coils to reduce coil coupling to other electronics and to reduce radiation. For example, cylindrical RF shields have been used between a MRI scanner gradient coil and the whole body coil (WBC) since the beginning of MRI. In this approach, the RF shield is always larger than the WBC in diameter, regardless of the shape of the WBC, which can be, for example, cylindrical or elliptical. Analysis of the RF shield can be realized using an image method.

Another kind of RF shield is non-cylindrical and solid. One example of non-cylindrical and solid shielding is a concentric shield outside a loop element, which decreases RF coil coupling from other RF coil loop elements. The coupling is reduced because the RF shield re-directs magnetic flux from reaching too far, constraining any stray magnetic fields that can be present.

Recently, MRI scanners having magnetic field strengths of 7T and higher have started to enter the MR market and are being used in research and clinical study. One of the major challenges associated with such systems is to build a cylindrical birdcage-like transmitter/receiver (Tx/Rx) coil. One existing approach, similar to a WBC shield approach, built a 16-rung birdcage coil with the RF shield as the transmitter coil. In that approach, the coil diameter was 30 cm and the shield diameter was 37.5 cm. The whole birdcage coil and its shield were enclosed in a plastic former. However, as expected, using this size of coil can be challenging for any human subjects with a claustrophobic condition.

Another approach used a transverse electromagnetic (TEM) resonator at 7T and higher as a transmitter. A TEM coil also requires an RF shield as its return path and for shielding. Existing TEM coils have the same disadvantages as the birdcage with shield approach, particularly with respect to claustrophobic conditions. In summary, existing approaches to RF shielding for birdcage coils at 7T and higher field strengths employ larger diameter shields with no large openings in the shield except for slots to break gradient field heating. Thus, existing approaches are confining and not suited for patients with claustrophobic conditions.

In contrast to existing systems, various embodiments discussed herein can provide an open shield in the same plane as a birdcage coil, even in MRI systems operation with a B0field at 7T or higher. Various embodiments discussed herein can comprise or employ a coil that is significantly thinner than existing systems, and can facilitate the inclusion of mechanically created openings, which can reduce claustrophobic conditions.

Referring toFIG. 2, illustrated is a diagram showing two resonant coils2001and2002with strong mutual inductance between them, in connection with various embodiments discussed herein. In various embodiments, the resonant frequencies of the two coils2001and2002can be very different from each other.FIG. 2shows the equivalent circuit diagram of the two resonant coils with mutual inductive coupling.

InFIG. 2, C1(2201), L1(2101) and R1(2301) are the equivalent capacitance, inductance and resistance of coil12001, and C2(2202), L2(2102) and R2(2302) are the equivalent capacitance, inductance and resistance of coil22002. M is the mutual inductance between the coil1and the coil2. For simplicity of calculation, M can be assumed to be positive. However, due to the sign of the net induced magnetic flux, the induced voltage of coils from each other can be positive or negative. For example, if the two coils have no overlap, then the induced voltage has a negative sign. If the two coils are facing each other and the distance between them is small, the sign is positive. If the two coils have overlap, the sign may be positive. The positive sign also applies for concentric configurations. In various examples and embodiments discussed herein, the coil configurations can be overlapping, facing each other, or concentric. In the following analysis, the positive sign is used. Thus, using Kirchhoff's Law, equations (2) can be obtained as follows:

If coil22002is going to be used as a shield, then

1ω⁢⁢C⁢⁢2=0.
At high field strength jωL2>>R2, thus R2can be thought of as 0, giving equation (3):

The resonant frequency of coil1with coil2as a shield is given by equation (4):

The resonant frequency f0of coil1increases due to the

-M2L⁢2⁢⁢term.
If L2is similar to L1and there is strong mutual inductance M between coil1and coil2, then the frequency increase is very large. To maintain the same resonant frequency, C1can increase by a large amount. The coil2shield can be concentric, or can overlap or face coil1at a very close distance as long as the net magnetic flux sign remains positive. Therefore, an open shield-like coil can increase coil resonant frequency significantly.

At 7T and higher magnetic fields traditional birdcage coils without close fitting cylindrical shielding only require very small capacitance to resonate because the frequency is high. For example, a head-sized 16-rung high-pass birdcage coil with a 26 cm diameter and a 26 cm length without an RF shield only needs about 4 pF to resonate based on analysis conducted via birdcage builder. This is very close to the parasitic capacitance between the two leads of the capacitor along the birdcage ring. As a result, the environment (e.g., parasitic capacitors) becomes a major part of the coil capacitor, that is, the coil resonates with the environment or it starts to radiate. Therefore, radiation loss is significant in coil loss, which results in very low coil Q, resulting in it becoming almost impossible to tune the birdcage coil without shielding at 7T and higher magnetic field. If a RF shield is used, then coil inductance decreases and the required resonant capacitance increases. As a result, the percentage of parasitic capacitance becomes less, and coil Q increases, resulting in the coil becoming resonant again and tunable. As can be seen, the RF shield reduces the coil efficiency. But the gain of coil Q can compensate for the coil efficiency loss. This is the primary reason that most existing birdcage transmitter coils at 7T and higher magnetic fields require close fitting cylindrical shields which is not claustrophobic-friendly, thereby limiting clinical utility.

To mitigate the claustrophobic situation created by existing approaches, various embodiments described herein can comprise an open and in-plane RF shield for a birdcage coil. Referring toFIG. 3, illustrated is a first example MRI RF coil apparatus300that can be employed in or as a RF antenna in a MRI apparatus (e.g., as a RF antenna150iin MRI apparatus100), according to various embodiments discussed herein. Coil apparatus300comprises an example 8-rung birdcage coil310with an open and in-plane RF shield320(e.g., which can also be a birdcage circuit (e.g., a circuit with a birdcage structure, such that it comprises two approximately parallel (e.g., within a threshold deviation) rings connected to one another by a plurality of (e.g., N) rungs that are approximately perpendicular (e.g., within a threshold deviation) to the two rings, and optionally comprises capacitors (e.g., on at least one of the ring(s) or rung(s), when capacitors are included)). InFIG. 3, the connections and capacitors indicated by310(and Ac and Bc) are for the 8-rung birdcage coil. The conductive connections indicated by320(and As and Bs) are for the open RF shield, and can comprise, for example, thin wire or small width copper.

The shield320can be electrically isolated from the birdcage coil310, and the shield320can be on the same plane of birdcage coil310. InFIG. 3(and similarly with other MRI RF coil apparatuses discussed herein), shield320and birdcage coil310are not electrically connected at the crossing points. In various embodiments, parasitic capacitance between shield320and birdcage coil310at the crossing points can be minimized by using narrower copper traces or jump wires. In various embodiments, the shield320can be arranged such that there is a very small distance between the plane of the birdcage coil310and the plane of the shield320, for example, due to mechanical reasons. As an illustrative example, the wires for the birdcage310and the shield320can have diameters of one mm, two mm, or another (e.g., larger or smaller) diameter, which can result in a difference between the plane of the birdcage coil310and the plane of the shield320. Other diameters can also be used in various embodiments. In various embodiments, the birdcage310and shield320can have substantially the same diameter, namely, they can have the same diameter within a margin that includes small differences in diameter arising from the size of wires or other circuit elements.

Although inFIG. 3the birdcage coil310and shield320are represented in a plane, in various embodiments, both the birdcage coil310and shield320can be arranged cylindrically around a common axis, with the birdcage coil310having the two points labeled Ac inFIG. 3representing the same point and the two points labeled Bc inFIG. 3representing the same point, and the shield320having the two points labeled As inFIG. 3representing the same point and the two points labeled Bs inFIG. 3representing the same point. Additionally, in birdcage coils (e.g., coil310, etc.) and shields (e.g., shield320, etc.) discussed herein, the “rings” or “end rings” are the two circular conductive paths between the pair of points inFIG. 3, etc., with the same two letter indicator (e.g., Ac and Ac, Bc and Bc, As and As, Bs and Bs, etc.), while the “rungs” are the conductive paths that connect the two rings of the birdcage coil or shield. Moreover, while example birdcage coils shown herein show only capacitors located on the rings of the birdcage coil, in various embodiments, capacitors can be located only on the rings (e.g., in high-pass embodiments), only on the rungs (e.g., in low-pass embodiments), or on both the rings and rungs (e.g., in band-pass embodiments) of birdcage coils.

Referring toFIG. 4, illustrated is a pair of diagrams showing two alternative example configurations400and450of an open shield (420or470) and birdcage coil (410or460) that can be employed in or as a RF antenna in a MRI apparatus (e.g., as a RF antenna150iin MRI apparatus100), according to various embodiments discussed herein. In the example configuration shown inFIG. 3, both rings of shield320are disposed outside of the rings of the birdcage coil310in the rung direction. As can be seen in configuration400and450ofFIG. 4, in various embodiments, the rings of shields (420or470) can be either inside or outside of the ring of the birdcage (410or460) in the rung direction. All of these configurations provide effective shielding.

The effectiveness of the open shields (e.g.,320,420, or470, etc.) of various embodiments can be explained via the image method. The birdcage coil (e.g.,310,410, or460, etc.) can be used to generate two perpendicular uniform B1field in quadrature mode. One of these B1fields can be used for the following analysis. The uniform B1field can be considered a virtual coil. This virtual coil can be described by its equivalent inductance L and equivalent capacitance C. As can be seen fromFIGS. 3-4, the open RF shield (e.g.,320,420, or470, etc.) can be rotated through a small angle azimuthally from the birdcage (e.g., rotated relative to a configuration wherein rungs of the birdcage coil and open RF shield are aligned) along the birdcage axis or B0direction. Referring toFIG. 5, illustrated is a diagram showing the induced B1sfield from an open shield using the image method relative to a B1field from a birdcage coil, according to various embodiments discussed herein. For a physical rotation between birdcage coil (e.g.,310,410, or460, etc.) and open shield (e.g.,320,420, or470, etc.) of 0.5α, the rotation angle between the imaged birdcage coil and the birdcage coil is α. The following discussion uses a for simplicity. The mutual inductance between the birdcage coil (e.g.,310,410, or460, etc.) and the open shield (e.g.,320,420, or470, etc.) can be estimated.

First, the distances between open shield rings and their birdcage direct neighboring rings can be assumed to be small and ignorable, or that the rung length is much greater than the distance between the shield rings and the direct neighboring rings. Second, the RF loss from the shield can be assumed to be small enough to be ignored. The induced voltage in the open shield can be written as in equation (5):

Vinduced=-d⁡(∅)dt=-M⁢dIbirdcagedt(4)
where ϕ is the uniform mode virtual coil magnetic flux going through the induced uniform mode virtual coil of the open shield, and Ibirdcageis the equivalent current going through the virtual coil of the uniform B1field of the birdcage coil. Since the open shield is rotated from the birdcage by a small angle 0.5α, which is much smaller than 2π/N, where N is the total number of rungs, the mutual inductance between the open shield and birdcage is close to L*cos(α) regarding the uniform mode virtual coil. Therefore, the new resonant frequency of the birdcage coil with open shield can be written, per equation (4), as equation (6):

Since α is small, the frequency increase can be very significant, involving an increase by

1(1-cos⁡(α)2)
times. Therefore, significantly larger value capacitors can be used to achieve the same resonant frequency. At 7T and higher magnetic field, this open shield reduces radiation loss significantly. As a result, various embodiments comprise or employ birdcage coils configured to operate at high B0field (e.g., 7T or higher) that have good coil Q and are tunable. Furthermore, shields according to various embodiments discussed herein can accommodate opening(s) for claustrophobia minimization.

In addition to the aforementioned applications for birdcage coils at 7T and higher B0field, open shield apparatuses and techniques discussed herein can also be employed in other contexts. One example application of such open shields is in dual (or multi-) tune RF coil(s). X-nuclei MRI can provide additional information not available via proton MRI. Thus, dual or multi-tune RF coil(s) can have a unique role in X-nuclei MRI. There are several ways to implement a dual tune coil.

Referring toFIG. 6, illustrated is an example circuit diagram of a first approach for implementing a dual tune coil, in connection with various aspects discussed herein. As shown inFIG. 6, a pole (e.g., via the circuit elements indicated in650) can be inserted into a LC resonant circuit600to implement a dual tune coil.

A second approach to implement a dual tune coil is to use a “nest” design. The nested design can employ two concentric birdcage coils, an outer coil with a larger diameter and an inner coil with a smaller diameter. There are two ways to arrange a nested configuration. In a first configuration, the inner coil can be the lower frequency coil and the outer coil can be the higher frequency coil. In the first configuration, the lower frequency coil employs trap circuits at the higher frequency so that the inner lower coil will not shield the higher frequency coil. In the second configuration, the inner coil is the higher frequency coil and the outer coil is the lower frequency coil. In the second configuration, the outer coil can simply serve as a RF shield because the lower frequency coil acts like an inductor at higher frequency, which is similar to pure inductive shield. Regardless of the configuration, in existing systems, the diameter of one of the coils must be larger than the other one. As a result, one of the coils loses coil efficiency because it is a little bit farther away from the scanned object.

In various embodiments, the open and in-plane shield (e.g.,320,420,470, etc.) can be configured as a lower frequency coil, which can provide a dual tune coil (e.g., apparatus300,400,450, etc.) without sacrificing any coil efficiency due to coil size. Referring toFIG. 7, illustrated is a diagram of an example dual-tune MRI RF coil apparatus700that can be employed in or as a RF antenna in a MRI apparatus (e.g., as a RF antenna150iin MRI apparatus100), according to various embodiments discussed herein. Coil apparatus700comprises a first example 8-rung birdcage coil710with an open and in-plane RF shield720, and can be similar to coil apparatus300, discussed above, with RF shield720configured as a second birdcage coil. In various embodiments, either birdcage coil710or RF shield720can be configured to operate at the lower frequency, with the other configured to operate at the higher frequency. Additionally, in various embodiments, this technique can be extended beyond dual-tune coils to any of a variety of multi-tune coils, such as a triple-tune coil, etc. For example, Referring toFIG. 8, illustrated is a diagram of an example dual-tune MRI RF coil apparatus800that can be employed in or as a RF antenna in a MRI apparatus (e.g., as a RF antenna150iin MRI apparatus100), according to various embodiments discussed herein. Furthermore, althoughFIGS. 7-8show RF shields extending beyond the inner birdcage coils in the rung direction on both sides, in various embodiments, other arrangements can be employed, as discussed in connection withFIG. 4, above.

Examples herein can include subject matter such as a method, means for performing acts or blocks of the method, at least one machine-readable medium including executable instructions that, when performed by a machine (e.g., MRI machine, for example as described herein, etc.) cause the machine to perform acts of the method or of an apparatus or system according to embodiments and examples described.

A first example embodiment comprises a magnetic resonance imaging (MRI) radio frequency (RF) birdcage coil apparatus comprising: a first circuit comprising N rungs, two end rings and breaking point capacitors located either at rungs (low-pass) or at rings (high-pass) or both (band-pass), wherein the first circuit forms a first birdcage coil; a second circuit comprising N rungs, two end rings and optional breaking point capacitors either at rungs (low-pass) or at rings (high-pass) or both (band-pass), wherein the second circuit forms a RF shield when the breaking point capacitors are omitted or a second birdcage coil when the breaking point capacitors are included; an optional third circuit comprising N rungs, two end rings and optional breaking point capacitors either at rungs (low-pass) or at rings (high-pass) or both (bandpass), wherein the optional third circuit forms a RF shield when the breaking point capacitors are omitted or a third birdcage coil when the breaking point capacitors are included; wherein the first circuit, the second circuit, and the optional third circuit have the same diameter or a trivial diameter difference (e.g., arising from thicknesses (e.g., <3 mm, etc.) of wires of the first, second, and/or optional third circuits); wherein the first, second, and third circuits are not electrically connected to each other (e.g., at any cross points where rungs and rings of different birdcages/RF shields cross, etc.); wherein the first, second, and optional third circuits share a common axis in the B0direction, but are rotated from each other at different azimuthal angles.

A second example embodiment comprises the first example embodiment, wherein the second circuit is non-resonant as a RF shield and comprises no capacitors.

A third example embodiment comprises the first example embodiment, wherein the first birdcage coil is tuned to a first frequency, the second circuit is non-resonant, and the third circuit is omitted.

A fourth example embodiment comprises the first example embodiment, wherein the first birdcage coil is tuned to a first frequency, the second circuit comprises the second birdcage coil and is tuned to a second frequency, and the third circuit is one of omitted or non-resonant.

A fifth example embodiment comprises the first example embodiment, wherein the first birdcage coil is tuned to a first frequency, the second circuit forms the second birdcage coil and is tuned to a second frequency, and the third circuit forms the third birdcage coil and is tuned to a third frequency.

The following examples are additional embodiments.

Example 1 is a magnetic resonance imaging (MRI) radio frequency (RF) coil array configured to operate in at least one of a transmit (Tx) mode or a receive (Rx) mode, the MRI RF coil array comprising: a first birdcage circuit comprising two rings of the first birdcage circuit, N rungs of the first birdcage circuit that electrically connect the two rings of the first birdcage circuit, and a plurality of capacitors of the first birdcage circuit, wherein the first birdcage circuit forms a first birdcage coil; and a second birdcage circuit comprising two rings of the second birdcage circuit, and N rungs of the second birdcage circuit that electrically connect the two rings of the second birdcage circuit, wherein a diameter of the first birdcage circuit is within a threshold distance of a diameter of the second birdcage, wherein the threshold is based on thicknesses of wires of the first birdcage circuit and the second birdcage circuit, wherein the first birdcage circuit is electrically isolated from the second birdcage circuit, and wherein the first birdcage circuit and the second birdcage circuit have a common cylindrical axis, and wherein the N rungs of the second birdcage circuit are azimuthally rotated through a first angle relative to the N rungs of the first birdcage circuit.

Example 2 comprises the subject matter of any variation of any of example(s) 1, wherein the second birdcage circuit is non-resonant and configured to operate as a RF shield for the first birdcage coil.

Example 3 comprises the subject matter of any variation of any of example(s) 1-2, wherein N is at least 8.

Example 4 comprises the subject matter of any variation of any of example(s) 1-3, wherein the plurality of capacitors of the first birdcage circuit are arranged on the two rings of the first birdcage circuit.

Example 5 comprises the subject matter of any variation of any of example(s) 1-3, wherein the plurality of capacitors of the first birdcage circuit are arranged on the N rungs of the first birdcage circuit.

Example 6 comprises the subject matter of any variation of any of example(s) 1-3, wherein the plurality of capacitors of the first birdcage circuit are arranged on the two rings and the N rungs of the first birdcage circuit.

Example 7 comprises the subject matter of any variation of any of example(s) 1-6, wherein the second birdcage circuit comprises a plurality of capacitors of the second birdcage circuit, and wherein the second birdcage circuit forms a second birdcage coil.

Example 8 comprises the subject matter of any variation of any of example(s) 7, wherein the first birdcage coil is configured to operate at a first frequency and the second birdcage coil is configured to operate at a second frequency, wherein the first frequency is the Larmor frequency of a first nucleus in a given B0 field aligned along the cylindrical axis, wherein the second frequency is the Larmor frequency of a second nucleus in the given B0 field aligned along the cylindrical axis, wherein the first frequency is different than the second frequency and the first nucleus is different than the second nucleus.

Example 9 comprises the subject matter of any variation of any of example(s) 1-8, wherein the MRI RF coil array is configured to operate in a B0 field with a field strength of 7T or higher.

Example 10 comprises the subject matter of any variation of any of example(s) 1-9, wherein the N rungs of the second birdcage circuit extend beyond one ring of the two rings of the first birdcage circuit in a direction parallel to the common cylindrical axis.

Example 11 comprises the subject matter of any variation of any of example(s) 1-9, wherein the N rungs of the second birdcage circuit extend beyond both rings of the two rings of the first birdcage circuit in a direction parallel to the common cylindrical axis.

Example 12 comprises the subject matter of any variation of any of example(s) 1-9, wherein the N rungs of the first birdcage circuit extend beyond both rings of the two rings of the second birdcage circuit in a direction parallel to the common cylindrical axis.

Example 13 is a magnetic resonance imaging (MRI) radio frequency (RF) coil array configured to operate in at least one of a transmit (Tx) mode or a receive (Rx) mode, the MRI RF coil array comprising: a first birdcage circuit comprising two rings of the first birdcage circuit, N rungs of the first birdcage circuit that electrically connect the two rings of the first birdcage circuit, and a plurality of capacitors of the first birdcage circuit, wherein the first birdcage circuit forms a first birdcage coil; a second birdcage circuit comprising two rings of the second birdcage circuit, N rungs of the second birdcage circuit that electrically connect the two rings of the second birdcage circuit, and a plurality of capacitors of the second birdcage circuit, wherein the second birdcage circuit forms a second birdcage coil; and a third birdcage circuit comprising two rings of the third birdcage circuit, and N rungs of the third birdcage circuit that electrically connect the two rings of the second birdcage circuit, wherein a diameters of the first birdcage circuit, the second birdcage circuit, and the third birdcage circuit are within a threshold distance of each other, wherein the threshold is based on thicknesses of wires of the first birdcage circuit, the second birdcage circuit, and the third birdcage circuit, wherein the first birdcage circuit, the second birdcage circuit, and the third birdcage circuit are electrically isolated from each other, and wherein the first birdcage circuit, the second birdcage circuit, and the third birdcage circuit have a common cylindrical axis, wherein the N rungs of the second birdcage circuit are azimuthally rotated through a first angle relative to the N rungs of the first birdcage circuit, and wherein the N rungs of the third birdcage circuit are azimuthally rotated through a second angle relative to the N rungs of the first birdcage circuit, wherein the second angle is different than the first angle.

Example 14 comprises the subject matter of any variation of any of example(s) 13, wherein the third birdcage circuit is non-resonant and configured to operate as a RF shield for at least one of the first birdcage coil or the second birdcage coil.

Example 15 comprises the subject matter of any variation of any of example(s) 13-14, wherein N is at least 8.

Example 16 comprises the subject matter of any variation of any of example(s) 13-15, wherein the third birdcage circuit comprises a plurality of capacitors of the third birdcage circuit, and wherein the third birdcage circuit forms a third birdcage coil.

Example 17 comprises the subject matter of any variation of any of example(s) 16, wherein the first birdcage coil is configured to operate at a first frequency, the second birdcage coil is configured to operate at a second frequency, and the third birdcage coil is configured to operate at a third frequency, wherein the first frequency is the Larmor frequency of a first nucleus in a given B0 field aligned along the cylindrical axis, wherein the second frequency is the Larmor frequency of a second nucleus in the given B0 field aligned along the cylindrical axis, wherein the third frequency is the Larmor frequency of a third nucleus in the given B0 field aligned along the cylindrical axis, wherein the first frequency, the second frequency, and the third frequency are different, and wherein the first nucleus, the second nucleus, and the third nucleus are different.

Example 18 comprises the subject matter of any variation of any of example(s) 13-17, wherein the MRI RF coil array is configured to operate in a B0 field with a field strength of 7T or higher.

Example 19 is an apparatus, comprising: a magnetic resonance imaging (MRI) radio frequency (RF) coil apparatus configured to operate in at least one of a transmit (Tx) mode or a receive (Rx) mode, the MRI RF coil array comprising: a first birdcage circuit comprising two rings of the first birdcage circuit, N rungs of the first birdcage circuit that electrically connect the two rings of the first birdcage circuit, and a plurality of capacitors of the first birdcage circuit, wherein the first birdcage circuit forms a first birdcage coil; and a second birdcage circuit comprising two rings of the second birdcage circuit, and N rungs of the second birdcage circuit that electrically connect the two rings of the second birdcage circuit, wherein a diameter of the first birdcage circuit is within a threshold distance of a diameter of the second birdcage, wherein the threshold is based on thicknesses of wires of the first birdcage circuit and the second birdcage circuit, wherein the first birdcage circuit is electrically isolated from the second birdcage circuit, and wherein the first birdcage circuit and the second birdcage circuit have a common cylindrical axis, and wherein the N rungs of the second birdcage circuit are azimuthally rotated through a first angle relative to the N rungs of the first birdcage circuit, and wherein the apparatus comprises at least one mechanical opening between one or more of the N rungs of the first birdcage circuit and one or more of the N rungs of the second birdcage circuit.

Example 20 comprises the subject matter of any variation of any of example(s) 19, wherein the MRI RF coil array is configured to operate in a B0 field with a field strength of 7T or higher.

Example 21 is a MRI apparatus comprising a MRI RF coil array according to any variation of any of example(s) 1-20.

Circuits, apparatus, elements, MRI RF coils, arrays, methods, and other embodiments described herein are described with reference to the drawings in which like reference numerals are used to refer to like elements throughout, and where the illustrated structures are not necessarily drawn to scale. Embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure and appended claims. In the figures, the thicknesses of lines, layers and/or regions may be exaggerated for clarity. Nothing in this detailed description (or drawings included herewith) is admitted as prior art.

Like numbers refer to like or similar elements throughout the description of the figures. When an element is referred to as being “connected” to another element, it can be directly connected to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.).

In the above description some components may be displayed in multiple figures carrying the same reference signs, but may not be described multiple times in detail. A detailed description of a component may then apply to that component for all its occurrences.

The following includes definitions of selected terms employed herein. The definitions include various examples or forms of components that fall within the scope of a term and that may be used for implementation. The examples are not intended to be limiting. Both singular and plural forms of terms may be within the definitions.

References to “one embodiment”, “an embodiment”, “various embodiments,” “one example”, “an example”, or “various examples” indicate that the embodiment(s) or example(s) so described may include a particular feature, structure, characteristic, property, element, or limitation, but that not every embodiment or example necessarily includes that particular feature, structure, characteristic, property, element or limitation. Furthermore, repeated use of the phrases “in one embodiment” or “in various embodiments” does not necessarily refer to the same embodiment(s), though it may.

“Circuit”, as used herein, includes but is not limited to hardware, firmware, or combinations of each to perform a function(s) or an action(s), or to cause a function or action from another circuit, logic, method, or system. Circuit can include a software controlled microprocessor, a discrete logic (e.g., ASIC), an analog circuit, a digital circuit, a programmed logic device, a memory device containing instructions, and other physical devices. A circuit can include one or more gates, combinations of gates, or other circuit components. Where multiple logical circuits are described, it may be possible to incorporate the multiple logical circuits into one physical circuit. Similarly, where a single logical circuit is described, it may be possible to distribute that single logical logic between multiple physical circuits.

“Computer-readable storage device”, as used herein, refers to a device that stores instructions or data. “Computer-readable storage device” does not refer to propagated signals. A computer-readable storage device can take forms, including, but not limited to, non-volatile media, and volatile media. Non-volatile media can include, for example, optical disks, magnetic disks, tapes, and other media. Volatile media can include, for example, semiconductor memories, dynamic memory, and other media. Common forms of a computer-readable storage device can include, but are not limited to, a floppy disk, a flexible disk, a hard disk, a magnetic tape, other magnetic medium, an application specific integrated circuit (ASIC), a compact disk (CD), other optical medium, a random access memory (RAM), a read only memory (ROM), a memory chip or card, a memory stick, and other media from which a computer, a processor or other electronic device can read.