The present invention relates to a radiofrequency (RF) multi-modal antenna for use in magnetic resonance applications, and in one particular example an Integrated Multi-modal Antenna with coupled Radiating Structures (I-MARS).
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgement or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.
Ultra-high-field (UHF) whole body Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) systems with a main magnetic field strength of 7 Tesla or higher, have seen significant development. Imaging and spectroscopy of certain regions of the human body, such as extremity and head, using UHF systems has demonstrated superior quality and sensitivity in comparison to using scanners of lower field strengths. However, the advantage of UHF systems for a large body section such as a hip joint or the abdomen, or deep anatomies such as prostate and heart, has not been well demonstrated mainly due to the lack of suitable radiofrequency (RF) coils.
For UHF body MRI/MRS applications, the RF transmit magnetic fields (B1+) may exhibit severe inhomogeneity. In a lower field MRI/MRS system, the RF transmission is often performed by a volume RF coil, typically of a cylindrical shape and located inside and adjacent to the inner wall of the scanner bore. However, such coils are associated with non-uniform B1+ fields at UHF. In MRI/MRS, B1+ is responsible for exciting the protons in the imaging region; after the B1+ is removed the excited protons go through a relaxation process while emitting the so-called “magnetic resonance (MR) signal”. It is based on this signal that images and spectra can be created for diagnosis and research purposes. Inhomogeneous B1+ is associated with spatially non-uniform excitation of protons, severely degrading the uniformity of the signal and therefore the clinical value of the MRI/MRS.
Parallel transmit systems (pTx) have been one of the most promising techniques developed to improve excitation profiles in UHF applications. pTx coils typically comprise an array of transmit elements distributed around the body section to be scanned, and RF amplifiers to independently drive individual coil elements. To achieve desired excitation profiles with the pTx system, numerical algorithms are then used to optimize the amplitude, phase and/or shape of the signal waveforms to drive the transmit elements. For example, constructive interferences of the individual B1+ fields can be used to provide sufficient excitation at a targeted scan region. In another example, the designed RF waveforms are combined with the MRI gradient systems to provide the so-called “spatially selective” pulses, with which the entire field of view or only a selective region can be excited with uniform intensity. As opposed to traditional coils, these techniques in combination pTx RF coils allow to control the coil efficiency and reduce the specific absorption rate (SAR), a measure of the RF energy absorbed by tissues.
Surface coil arrays that have both transmit and receive abilities, namely RF transmit-receive or transceive coils, are becoming popular because both RF transmission and reception systems are integrated to maximize the efficiency, instead of competing for space in close proximity to the region of interest. RF transceive coils offer improved power efficiency in transmit mode and better signal-to-noise ratio (SNR) in receive mode. Additional electronics are needed to allow the same RF coil elements to switch between the transmit and receive modes.
Conventionally, RF arrays use surface coil elements that are historically of loop shapes. In theory, a loop coil is equivalent to a magnetic dipole, ideally suited to produce magnetic fields perpendicular to the loop plane. In reception, changes in magnetic flux (produced by the excited nuclear magnetization in the imaged subjects) induce a current in the loop according to the Faraday's Law of induction. The resonance of a loop antenna is achieved by adjusting the inductance (size of the metallic loop) and capacitance (discrete or distributed form) of the circuit. At higher radio frequencies associated with UHF MRI/MRS, however, the transmit and receive magnetic field profiles of loop-shaped RF elements are less than ideal, encouraging the search for better RF elements.
Recently, dipole antennas as RF surface coil elements have become popular for UHF imaging applications. A dipole antenna typically consists of two identical conductive arms, symmetrically located with respect to the feeding/port. The resonance of a ½ wavelength dipole, as typically used in MRI applications, is achieved by creating standing waves of electrical currents oscillating between the two arms. It has been shown that the electric current pattern on a dipole antenna was more suited for UHF than their loop counterparts.
Regardless of the type of antenna, there are several important considerations when designing array elements for UHF applications. They include:
Rapid development and uptake of dipole antenna multi-element array coils has occurred in the pursuit of obtaining an ideal current pattern that yields high efficiency (criterion 1), high element SNR (criterion 1) and low element SAR (criterion 2) for use at 7T, as described for example in Lattanzi R, Sodickson DK. “Ideal current patterns yielding optimal SNR and SAR in magnetic resonance imaging: computational methods and physical insights”. Magnetic Resonance in Medicine 2012; 68(1):286-304.
Recent designs aimed to shorten their physical length for practical application from the theoretical half-wavelength (approximately 48 cm long in air for 7T applications). These designs include “fractionated dipole antenna” (Raaijmakers A J E, Italiaander M, Voogt I J, Luijten P R, Hoogduin J M, Klomp D W J, van den Berg CAT. “The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla”. Magnetic Resonance in Medicine 2016; 75(3):1366-1374.), a “single-side adapted dipole (SSAD)” (Raaijmakers AJE, Ipek O, Klomp D W J, Possanzini C, Harvey P R, Lagendijk J J W, van den Berg CAT. “Design of a Radiative Surface Coil Array Element at 7 T: The Single-Side Adapted Dipole Antenna”. Magnetic Resonance in Medicine 2011; 66(5):1488-1497) and hybrid loop-dipole (“loopole”) (Lakshmanan K, Cloos M, Lattanzi R, Sodickson D, Wiggins GC. “The loopole antenna: capturing magnetic and electric dipole fields with a single structure to improve transmit and receive performance”. 2014; Milan, Italy. p 397). These examples demonstrate considerable promises for 7T in vivo applications. However, these designs do not actively consider criteria 3 or 4. In fact, these existing element designs are sensitive to variations in loading conditions, and the decoupling between elements typically relies on having a large distance between elements, preventing high-density array designs and reducing imaging performance in certain Regions of Interest (ROI).
Although dipole current distribution may be suitable at 7T, conventional dipoles suffer from poor stability when the loading condition is varied (e.g., the position and/or electrical properties change among patients). The subsequent changes in the tuning and/or matching of the elements would significantly reduce their efficiency, degrade the image quality, and in extreme cases damage hardware. Similar issues are associated with conventional loop-shaped antennas. Alternative designs, such as shielded resonators or multi-layer resonators, have been proposed to achieve lower coupling (criterion 3) and higher loading stability compared to a conventional loop coil (criterion 4). Recently, a similar structure has been used to design a “self-isolated” loop coil. However, such technology has not been presented with RF dipole elements.
In one broad form the present invention seeks to provide a multi-modal antenna for use in magnetic resonance applications, the multi-modal antenna including: an elongate first conductive element; an elongate second conductive element at least partially aligned with and spaced from the first conductive element; and, a dielectric material at least partially separating the first and second conducting elements so that the first and second conductive elements are electromagnetically coupled and/or electrically connected, and wherein at least one of the first and second conducting elements are configured to be electromagnetically coupled and/or electrically connected to an RF system so that the multi-modal antenna can at least one of transmit and receive RF electromagnetic signals for performing magnetic resonance imaging or spectroscopy.
In one embodiment at least one of: the first and second conducting elements operate in one of: a transmission line mode; a dipole mode; and, a combination of a transmission line mode and a dipole mode; and, the dielectric layer and the first and second conductive elements form a transmission line.
In one embodiment the first conductive element is stimulated by the RF system and the second conductive element is stimulated by the first conductive element.
In one embodiment the first and second coupled conductive elements are stimulated by the MR signal from the subject.
In one embodiment the first and second conductive elements cooperate to define a closed-loop current including conductive currents passing along the first and second conductive elements and displacement currents passing through the dielectric material.
In one embodiment at least one of the conductive elements has a dipole configuration.
In one embodiment at least one of the conductive elements includes a slot or cut-out to define two arms, and wherein the RF system is electrically connected and/or electromagnetically coupled to each arm.
In one embodiment each conductive element at least one of: includes slots or cut-outs; has a length greater than a width; has a width greater than a thickness; is substantially laminar; is substantially planar; is at least partially flexible so that the multi-modal antenna can conform to a shape of a subject; is at least partially curved so that the multi-modal antenna can conform to a shape of a subject; includes an axial cross sectional shape that is at least one of: rectangular; circular; and, elliptical; has a paddle-shaped profile including one or more end portions wider or narrower than a mid-portion; has one or more meandering portions extending widthwise and lengthwise to increase an effective electrical length of the conductive element; includes multiple paddle stages; includes multiple paddle stages having different relative widths; and, includes multiple stages having different relative widths, and wherein a chamfer angle between stages can be adjusted.
In one embodiment the first and second conductive elements are interconnected via at least one of: lumped elements, additional conductive elements; and a direct connection.
In one embodiment the second conductive element at least one of: is smaller than the first conductive element; is shorter than the first conductive element; is narrower than the first conductive element; and, has a complementary profile to the first conductive element.
In one embodiment a spacing between the first and second conductive elements is at least one of: at least 0.1 mm; at least 1 mm; less than 10 mm; and, about 3 mm.
In one embodiment the first and second conductive elements are spaced at least one of: in a substantially parallel arrangement; and, asymmetrically.
In one embodiment the dielectric material is at least one of: is partially sandwiched between the first and second conductive elements; is provided in a layer; includes a number of layers of dielectric material; and, includes at least two different materials having different dielectric properties.
In one embodiment the multi-modal antenna includes: a dielectric layer; an outer conductive layer on at least one surface of the dielectric layer; and an inner conductive layer within the dielectric layer.
In one embodiment: the outer conductive layer includes the first conductive element; and, an inner conductive layer includes the second conductive element.
In one embodiment the dielectric material has a permittivity constant of at least one of: at least 1; less than 10; less than 35; less than 50; less than 100; less than 250; less than 500; less than 1000; and, about 3.5.
In one embodiment the antenna includes at least one further conductive element and/or at least one further dielectric structure.
In one embodiment the antenna includes at least one secondary element that modifies an electromagnetic response of the antenna.
In one embodiment the at least one secondary element includes at least one of: at least one secondary dielectric material; and, at least one secondary conductive element.
In one embodiment the at least one secondary element spans a cut-out in the first conductive element.
In one embodiment the multi-modal antenna is configured to minimise an electric field within the subject.
In one embodiment the multi-modal antenna includes a housing configured to maintain a desired spacing between the subject and the first and second conductive elements.
In one embodiment the housing includes a foam for engaging the subject, the foam having a defined thickness to maintain the desired spacing.
In one embodiment the RF system includes at least one of: a signal generator configured to generate RF signals that are applied to the antenna to generate the RF electromagnetic field; a detector that detects signals originating within the subject; and, a control system that causes the RF system to send control signals that can be used to control supporting electronics including at least one of: active detuning circuits; switching electronics; and, active switches.
In one embodiment active switching electronics are implemented into the multi-modal antenna to enable at least one of: active detuning to allow separate transmit and receive antenna operation modes; active on/off switching of different segments in conductive elements to allow control of current and field distributions; active changing of the resonant frequency; and, active changing of the effective electrical length of the multi-modal antenna.
In one broad form the present invention seeks to provide a multi-modal antenna array for use in magnetic resonance applications, the multi-modal antenna array including a plurality of RF antennas, each RF antenna including: an elongate first conductive element; an elongate second conductive element at least partially aligned with and spaced from the first conductive element; and, a dielectric material at least partially separating the first and second conducting elements, wherein the first and second conductive elements are electromagnetically coupled and/or electrically connected, and wherein at least one of the first and second conducting elements are configured to be electromagnetically coupled and/or electrically connected to a multi-modal system so that the RF antenna can at least one of transmit and receive RF electromagnetic signals for performing magnetic resonance imaging or spectroscopy.
In one embodiment the antenna array includes additional decoupling technique between the antennas in the array.
In one embodiment active detuning is implemented to allow separate transmit and receive antenna array configurations.
It will be appreciated that the broad forms of the invention and their respective features can be used in conjunction and/or independently, and reference to separate broad forms is not intended to be limiting. Furthermore, it will be appreciated that features of the method can be performed using the system or apparatus and that features of the system or apparatus can be implemented using the method.
Various examples and embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
An example of a multi-modal antenna for use in magnetic resonance applications, such as magnetic resonance imaging and/or spectroscopy will now be described.
In this example, the multi-modal antenna includes an elongate first conductive element and an elongate second conductive element at least partially aligned with and spaced from the first conductive element. A dielectric material is provided that at least partially separates the first and second conducting elements so that the first and second conductive elements are electromagnetically coupled and/or electrically connected. In use, one of the first or second conducting elements is configured to be electromagnetically coupled and/or electrically connected to an RF system so that the multi-modal antenna can at least one of transmit and receive RF electromagnetic signals for performing magnetic resonance imaging or spectroscopy.
In this configuration, one of the conductive elements is stimulated by the RF system, whilst the other conductive element is stimulated by electromagnetic fields generated by the stimulated conductive element. Thus, in one example, the first conductive element can be primarily stimulated by RF system, either directly via an electrical connection, or indirectly, for example via an inductive connection, and then the second conductive element is stimulated by the first conductive element, but it will be appreciated that reversed configurations could be implemented, in which the second conductive element is primarily stimulated. Additionally, and/or alternatively, when operating in receive mode, the first and second coupled conductive elements are stimulated by the MR signal originating within the subject.
In either case, the first and second conductive elements cooperate to define a closed-loop current including conductive currents passing along the first and second conductive elements and displacement currents passing through the dielectric material.
The above-described configurations result in a number of improved antenna characteristics. For example, the configuration minimises the external electric field that is generated, whilst maintaining a high external magnetic field, allowing the RF antenna to effectively stimulate the subject for magnetic resonance applications, whilst maintaining a high power efficiency, and low RF energy exposure. This further reduces coupling between different multi-modal antennas, whilst also providing high stability with regard to imaging subjects and body parts. Accordingly, it is apparent that the new antenna configurations can meet the criteria discussed above and represent a significant advancement over traditional arrangements.
A number of further features will now be described.
Typically the first and second conducting elements operate in a transmission line mode, a dipole mode, and more typically a combination thereof. In one specific example, the dielectric layer and the first and second conductive elements form a transmission line.
In one example, the first and/or second conductive element has a dipole configuration, and can include one or more slots or cut-outs. In one example, the slots or cut-outs define two arms, with the RF system being electrically connected to each arm, although it will be appreciated that slots or cut-outs can be provided in either of the first or second conductive elements, to adjust electrical properties as desired.
Each conductive element typically has a length greater than a width and a width greater than a thickness. The relative geometry (incl length, width and thickness) is adjusted to achieve optimal performance considering the wavelength of the applied signals. However, the length is typically in the region of 100 mm to 500 mm, 360 mm to 400 mm and more typically in the region of 376 mm to 380 mm, excluding shorter variations such as with meanders, paddle or lumped elements, as will be described in more detail below. The width is typically in the region of 10 mm to 25 mm and more typically about 18 mm, whilst the thickness is of the order of less than a few mm. Thus, it will be appreciated that the conductive elements are typically a thin substantially laminar body, and optionally, substantially planar, although the conductive elements may be curved and/or flexible so that the multi-modal antenna can more easily conform to a shape of a subject. For example, the multi-modal antenna could include flexible conductive elements embedded in a fluidic or otherwise deformable dielectric material. The conductive elements are typically made of copper or other similar materials, or a combination of multiple conductive materials.
In axial cross-section, the conductive elements typically have a rectangular shape, although this is not essential and other arrangements can be used, including, but not limited to circular, square and/or elliptical shapes.
In one example, the antenna has a paddle-shaped profile, including one or more end portions wider or narrower than a mid-portion or could include one or more meandering portions extending widthwise and lengthwise. The paddle-shape profiles could include multiple stages, which can have different relative widths, and may include chamfer regions where the stages join, with a chamfer angle being adjusted to obtain desired characteristics. These different arrangements, including the stages, paddle-shaped profile and meandering portions, act to more favourably redistribute electrical current density of the antenna structure for higher external magnetic fields generation and lower maximum local electrical energy in the subject, and/or increasing an effective electrical length of the antenna structure and reducing a physical length. These configurations can assist in making the antenna configuration more suitable for use in clinical or other environments, whilst maintaining the effectiveness of the antennas. A reduction in the physical length of the antenna can alternatively, or additionally, be achieved using lumped elements that interconnect the first and second conductive elements, which also provides the ability to adapt the distribution of electrical current. Current distribution could additionally and/or alternatively be achieved using additional conductive elements and/or dielectric elements and/or direct connections.
In one example, the second conductive element is smaller than the first conductive element, and could for example be shorter and/or narrower than the first conductive element, which can assist if the second conductive element is wholly embedded within the antenna, as will be described in more detail below. The second conductive element may or may not also have a similar profile to the first conductive element. In the former scenario, the conductive elements have substantially the same shape. In either case, the characteristics of the antenna are predominantly defined by the overlapping shape between the conductive elements, and the distribution of dielectric material between and/or around them, so the conductive elements could have significantly different shapes, with characteristics of the antenna being governed by the region of overlap of the conductive elements.
Typically a spacing between the first and second conductive elements is at least on 0.1 mm, at least 1 mm, typically less than 10 mm or more typically about 3 mm, although it will be appreciated that other spacings could be used depending on the preferred implementation, the intended use, the dimensions of the conductive elements, and the nature of the dielectric material. The conductive elements are typically in a substantially parallel arrangement, although this is not essential and other arrangements, such as asymmetrically spacing, relatively angling of the first and second conductive elements, or the like, could be used, depending on the characteristics of the antenna that are desired for the particular magnetic resonance application.
In one example, the dielectric material is partially sandwiched between the first and second conductive elements and may be provided in a layer, with conductive elements provided on one or more sides of, and optionally embedded within the layer. The dielectric material may also include two or more different materials having different dielectric constants, and in one example, can include two or more layers of dielectric material. The dielectric material has a permittivity constant of at least 1, less than 10, less than 35, less than 50, less than 100, less than 250, less than 500, less than 1000, or about 3.5, although different values could be used depending on the preferred implementation, the desired thickness of the dielectric layer, or the like.
In one configuration, the multi-modal antenna includes a dielectric layer, an outer conductive layer on at least one surface, and optionally extending paritally or completely around the exterior surfaces of the dielectric layer, with an inner conductive layer within the dielectric layer. In this example, the outer conductive layer can include the first (active) conductive element, whilst the inner conductive layer includes the second (passive) conductive element, although this is not essential and reversed arrangements could be used, with the internal conductive element being the active element.
In one embodiment the antenna includes at least one further conductive element and/or dielectric structure, and so for example, the antenna may include multiple second conductive elements spaced from the first conductive element, or could include third conductive elements spaced from the first and second conductive elements, thereby further helping ensure a desired distribution of currents within the antenna and/or fields within the subjects.
In one example, the antenna includes a secondary element that modifies an electromagnetic response of the antenna. This could include a secondary dielectric material and/or a secondary conductive element, and in one example spans a cut-out in the first or second conductive element, which can modify coupling between arms in the active dipole, and/or modify the magnetic and electric field distribution within the subject.
In general, the antenna is provided in a housing, optionally containing the first and second conductive elements, which is configured to maintain the desired spacing between the subject and the first and second conductive elements and may include a foam for engaging the subject, with the foam having a defined thickness to maintain the desired spacing.
As mentioned above, the multi-modal antenna is typically coupled to an RF system, which in one example can form part of a magnetic resonance apparatus configured to perform magnetic resonance imaging or spectroscopy. The RF system can include a signal generator configured to generate RF signals that are applied to the antenna to generate the RF electromagnetic field and may also include a detector that detects signals from the subject and/or a control system that causes the RF system to send control signals that can be used to control supporting electronics, such as active detuning circuits, switching electronics and/or active switches. Such active switching electronics can be implemented into the multi-modal antenna to enable at least one of: active detuning to allow separate transmit and receive antenna operation modes; active on/off switching of different segments in conductive elements to allow control of current and field distributions; active changing of the resonant frequency; and, active changing of the effective electrical length of the multi-modal antenna.
Whilst the antennas could be used separately, more typically a number of antennas are part of an antenna array. In this instance, properties of the antennas, in particular its multi-modal characteristics, can help reduce coupling between the individual antennas in the array. However, this can be further enhanced through the use of additional decoupling techniques, for example by connecting conductive elements in different antennas using inductive components.
Whilst the individual antennas and the antenna arrays can be used in transceive mode, active detuning circuits could be added to any of the individual antennas and antenna-elements in an array to enable additional transmit-only or receive-only modes. This is typically achieved by implementing electronically controlled switches, for example PIN diodes or other switching devices.
An example of a conventional dipole antenna is shown in
The dipole antenna is typically made of two arms 101 of conducting material, such as copper, with a slot 105 for RF signal feeding/receiving, which is typically achieved using a transmission line 111 connected to the arms 101, via connectors 112, although this could alternatively be achieved using indirect connections, such as via inductive coupling or the like. The transmission line 111 is typically connected to an RF system, such as a signal generator and/or sensor (not shown). Lumped elements may be used for tuning and matching purposes. In their simplest form, dipole antennas are designed to have an electrical length that approximates the half-wavelength of the transmitted or received signal. For the purpose of explanation, this dipole antenna will be referred to in the following study as a “Configuration A”, has a length of 380 mm and width of 22 mm.
Examples of multi-modal antenna configurations will now be described with reference to
The first example multi-modal antenna configuration shown in
A second example multi-modal antenna configuration is shown in
A third example multi-modal antenna configuration is shown in
To investigate the performance of the above designs, electromagnetic simulations were performed on these configurations in software Sim4Life (ZMT, Zurich, Switzerland), when loaded with the phantom shown in
Table 1 shows the Power and SAR10g efficiency of Configurations A-D with different relative permittivities and dielectric thicknesses. Table 1 shows the transmit B1 power efficiency as a measure of the peak B1+ and the B1+ at a depth of 5 cm, as well as the peak-spatial SAR10g (psSAR10g) and the B1+SAR efficiency (ratio between the B1+ and the square root of the SAR10g), for all configurations. For the configurations B-D, the dielectric thickness is varied (d=1, 3, 5 or 10 mm); so is the relative permittivity (εr=1, 3.5, 5, 10 or 35). An electrical conductivity of σ=0.0015 S/m was used. All results were normalized to 1W of accepted power.
As summarized in Table 1, all those configurations have different characteristics in terms of providing power efficiency (B1+/√Power) or SAR efficiency (B1+5cm/√SAR10g) (Criteria 1 and 2). In general, increasing d of Configurations B-D improved the power efficiency at 5 cm while maintaining the SAR efficiency in most cases. Additionally, in Configurations B-D εr=3.5-10 gave the best compromises between power and SAR efficiency.
Configuration B represents configuration, in which the conductive currents on the two dipoles have similar magnitude, albeit opposite phase. In Configuration C, the conductive currents mostly reside on the active dipole. Configuration D is a design that is symmetrical in radial direction. In this case, conductive currents mostly reside on the inner, passive dipole.
There exist multiple current modes within the structures of Configurations B-D. Within each configuration, the transmission-line mode currents on the pair of dipoles, which are out of phase from each other, together with the displacement currents within the dielectric substrates, form a closed current loop. In this loop-mode of resonance, the antenna structure operates in transmission line mode, whilst the dielectric substrate acts mainly as distributed capacitance. It is noted that besides induction, the closed-loop current is partially responsible for the excitation of the passive dipole in each of the Configurations B-D. The transmission-line mode co-exists with the dipole mode of resonance on the dipole pairs, while the dipole mode currents are in phase on the conductor pairs. The co-existence of the two resonance modes is described by the term “multi-modal”. “Integrated” in ‘I-MARS’ simply refers to the fact that the active, passive dipoles and dielectric substrate in each configuration are acting as a complete resonance structure. In fact, when the two resonance modes are considered as a whole, there exists an excess of electrical current on the active-passive dipoles pair, causing a net current. This net current is in a similar magnitude to that of the conventional dipole (Configuration A).
To further investigate the effects of the different design features of I-MARS (passive dipole placement, size, dielectric properties) on their sensitivity to load changes and inter-element coupling, additional simulations were conducted. A common baseline of all configurations was first established by tuning the antennas of Configurations A-D to resonate at 297 MHz with S11=−20 dB when the phantom was 10 mm from the front of each element. Simulations were repeated with the phantom 15 mm from the front of each element, without altering the matching and tuning circuits from the corresponding baseline simulations. The S11 at 297 MHz was recorded, as well as the shift in resonant frequency. In yet another set of simulations, by introducing another antenna of the same design to the corresponding baseline simulations, two elements of each design were simulated with a center-to-center distance of 55 mm. Their S12 was recorded in Table 2 for analysis, with Table 2 showing sensitivity to loading and inter-element coupling for different configurations.
According to Table 2, the εr=3.5, d=3 mm variants of Configurations B-D perform better overall than the εr=3.5, d=10 mm variants. The former with smaller dielectric thickness d had much smaller resonance frequency shift when the load changed, and had noticeably better Sit values between two like antennas. In fact, the resonance frequency shift of the Configurations B-D of the εr=3.5, d=3 mm variants were an order of magnitude smaller than that of the Configuration A (conventional dipole). These advantages of the smaller dielectric thickness d=3 mm also outweigh the SAR efficiency (B1+5cm/√SAR10g) provided by the larger d=10 mm, which is less than 3% as illustrated in Table 1. Among all the configurations and their variants, Configuration B with εr=3.5, d=3 mm had the best S11=−14.7 dB when the load was moved away; Configurations B and D with εr=3.5, d=3 mm had the smallest frequency shift of −2.5 MHz; and Configuration D with εr=3.5, d=3 mm had the best inter-element isolation of S12=−11.6 dB with a center-to-center distance of 55 mm.
Summarizing the investigations so far, the I-MARS coils satisfy all the design criteria listed in the background. Similar to conventional dipole designs, the conductive currents of the I-MARS elements have a “dipole mode” current on the conductive materials mostly in the longitudinal direction, as shown in
There are several practical aspects to consider making I-MARS coils more suitable for in vivo applications. The tuning of the I-MARS coils is accomplished by designing the cross-sectional profile (widths of the inner and outer conductors and their relative ratios), the electrical properties of the dielectric substrates and the physical length of the coil elements. The length of the presented I-MARS configurations is 380 mm, making it impractical to use in some applications.
The optimal length, besides other geometric parameters, of I-MARS is determined on a case-by-case basis, while considering a number of metrics, such as, B1 power efficiency, B1SAR efficiency and stability (to be explained later). If increasing the electrical length is desirable, meanders or lumped elements can be introduced to achieve the same overall electrical length with a shorter physical length. Using Configuration D as an example,
Assisted with numerical electromagnetic simulations, the performance of the proposed I-MARS elements were compared with state-of-the-art dipole coil elements for UHF MRI/MRS.
The fractionated antennas of
This comparison includes I-MARS coils of three variations, all of which are based on Configuration D. The first variation, I-MARS Straight, as shown in
I-MARS Meander and I-MARS Paddle variations shown in
The centers of all the conventional and I-MARS elements were aligned with the center of the torso-shaped phantom shown in
To investigate the stability of all coil elements against loading changes, the scattering parameters were calculated when the body-mimicking phantom was located at different distances to the coil. A baseline simulation was established for each coil element at the original phantom position (10 mm away from the coil). The matching network and tuning lumped elements were optimized to achieve S11=−20 dB at 297 MHz. The S11 parameters were calculated again when the phantom was positioned 5, 15 and 20 mm away from the coils with the tuning and matching networks determined for the baseline simulation.
As shown in
Among the three conventional dipole elements, the SSAD antenna had the best performance, which is however noticeably inferior to the I-MARS designs. It is worth noting that in addition to better S11, the I-MARS elements had a smaller bandwidth compared with existing dipole coil designs, potentially leading to improved signal-to-noise ratios.
Another aspect of loading variation is the change in load electrical properties, mimicking change in body composition between patients. This was investigated by simulating the coil elements at a distance of 10 mm from a phantom with relative permittivity of 38.5 and conductivity of 0.46 S/m. After achieving a S11=−20 dB, the simulations were repeated with a different set of electrical properties of the phantom (relative permittivity of 71.5 and conductivity of 0.86 S/m), while using identical matching and tuning circuits.
B1 Power and SAR10g Efficiency
As shown in
To characterize the coupling between individual elements of an array, the scattering parameters of two elements of the same type were modelled, when they were located at varied distances. Simulation setup was similar to that of the previous study concerning changing loading conditions. Here, a second element of the same type was brought to the vicinity of the first element with a center-to-center distance of 120 mm, 80 mm, 70 mm and 55 mm. In all individual simulations, the two elements were tuned at 297 MHz and matched at −20 dB.
Table 3 shows the transmission coefficient S12 when varying the inter-element distance between a pair of dipole elements of the same type. Among the conventional elements, the Fractionated2 element had the best decoupling performance. In comparison, all I-MARS based designed out-performed the conventional designs. The I-MARS Meander had the best isolation at larger distances (80 mm and 120 mm), with a S12 3 dB lower than that of the Fractionated2 dipole. The I-MARS Straight had the lowest coupling with short distances (55 mm and 70 mm), with S12˜2.5 dB lower than the Fractionated2 dipole. The I-MARS Paddle behaved as well the Fractionated2 dipole at all inter-element distances, while having a more practical element dimension. Overall, the results indicate that the I-MARS coils have intrinsically high isolation between like elements, when the inter-element distance is varied in loaded conditions.
Compatibility of I-MARS with Existing Decoupling Methods
Although the I-MARS elements possess intrinsically high self-isolation facilitating dense coil arrays, even higher levels of decoupling would be desirable. In particular, the use of pTx techniques and receive performance of RF coils greatly benefit from lower coupling, to increase the degrees of freedom in transmission and reduce noise correlation, respectively. At lower fields, loop elements are typically used in local surface array coils, which can be decoupled using a variety of techniques, including:
However, these techniques cannot be directly applied to dipole elements, as loop antennas couple magnetically whereas dipole antennas mostly couple electrically, which is more challenging to mitigate. The use of passive elements and new amplifier designs were investigated to decouple dipole antennas, but was shown to affect the field distribution, add bulk and minimal distance constraints between neighbor elements, and may limit bandwidth and power efficiency.
The unique design of I-MARS elements enables decoupling techniques between two I-MARS elements. In particular, it has been identified that inter-element isolation can be improved using inductive decoupling, by directly connecting multi-modal antennas 901 with inductors 909, as shown for example in
As a proof of concept, when two I-MARS Meander elements with a center-to-center distance of 95 mm were connected with an inductor of 220 nH, decoupling of S12=−35 dB was achieved at 297 MHz when loaded with a torso as shown in
The effect of the decoupling network on the B i-field was simulated using two I-MARS Meanders (with RF shields) with a center-to-center distance of 70 mm. The ‘Perfect decoupling’ result shown in
The calculated B1+ fields are shown in
To verify the performance predicted by numerical simulations, arrays of I-MARS elements were manufactured using the FR-4 process, with R04360G2 laminates (Rogers Corporation, Chandler, Ariz., USA). The low-loss substrate R04360G2 was adopted owing to its high relative permittivity of 6.15 and low electrical loss (Dissipation Factor 0.0038 at 10 GHz/23° C.).
As an example, manufactured I-MARS Paddle elements with their 3D printed PETG housings are shown in
The I-MARS antennas are robust to loading changes and have high isolation between neighboring elements. In the presented configurations, additional inductive decoupling was not necessary or implemented, thanks to sufficient decoupling provided by the required distance between elements. Since retuning and/or re-matching of RF antennas are uncommon for in vivo MR imaging, these features make possible imaging of different body sections without compromising transmit and receive performance. UHF imaging will also benefit from the high B1+ efficiency against RF power and SAR provided by the I-MARS. Here, the constructed 8-element I-MARS Meander coil array prototype was employed for imaging healthy volunteers of various body sections, including unilateral hip, unilateral shoulder, bilateral hip, prostate and lumbar spine. Across these five imaging scenarios, the geometric configurations of the I-MARS Meander array were readily adjusted to provide the best conformity, as illustrated in
For unilateral hip and shoulder imaging, as illustrated in
As shown in
Numerical electromagnetic simulations using a finite-difference time-domain (FDTD) method have been performed for each of the imaging scenarios. The simulations were assisted by software package Sim4Life (ZMT, Zurich, Switzerland) with digital human models, as shown in
Images were acquired on different healthy volunteers using the prototype I-MARS coil array:
Images were acquired on a prototype whole-body 7T MR research scanner (Siemens Healthcare, Erlangen, Germany). A custom 8-channel transmit/receive switch was employed to interface the I-MARS coil array to the MR scanner. The medical research ethics committee of the University of Queensland approved the current study, and informed written consent was obtained from all participants who had no history of significant musculoskeletal pathology.
Coronal unilateral and bilateral hip DESS images (0.56 and 0.7 mm isotropic resolution without interpolation) are shown in
Sagittal views of the lumbar spine we-DESS are shown with 0.67 mm isotropic resolution without interpolation on the same volunteer, when performed using four posterior elements only in
It will be appreciated from the above that a wide range of different configurations could be implemented that allow for the combined dipole and transmission-line modes of operation, which in turn lead to a number of the benefits previously outlined. A number of these variations are shown in
For example, the I-MARS antennas could include additional secondary elements, such as capacitive elements to adjust properties of the antenna, for example to perform tuning for specific applications. An example of this is shown in
In this example, the antenna 1300 includes an outer conductive element 1301 in the form of a dipole including a slot 1305 and an inner conductive element 1302 contained within a dielectric layer 1303. The secondary element additional layer includes an outer secondary conductive element 1321 and a dielectric layer 1323 extending across the slot in one side of the antenna, which can alter coupling between arms of the dipole.
In the example of an antenna element shown in
In these examples, the inner and outer conductive elements of the I-MARS Paddles 1401, 1402 are largely of the same shape as in the example of
It will be appreciated that the overlap between the two the conductive elements, together with the dielectric between them, determines the characteristics of the antenna, and so variations to the inner conductive element can be used to alter the characteristics of the resulting antenna, without altering the appearance of the antenna. However, conversely, the size and/or shape of the outer conductive element could be altered, whilst the inner conductive element remains unchanged to also alter the characteristics of the antenna.
The simulation results and acquired images presented in this work suggest that the proposed Integrated Multi-modal Antenna with coupled Radiating Structures (I-MARS) elements provide advantages for UHF MR imaging. Compared with the state-of-the-art dipole coil elements, the individual I-MARS has high efficiency in terms of producing transmit magnetic fields normalized to accepted power and normalized to peak SAR10g; demonstrating superior stability against loading changes; and presenting intrinsically higher isolation between neighboring elements when the distance between elements changes significantly. Furthermore, I-MARS elements are compatible with decoupling techniques that provide better than −25 dB isolation in the tested configurations. This combination of advantages is unique to I-MARS, making a multi-element I-MARS array uniquely suitable for multi-anatomy UHF imaging, where array elements can be rearranged to accommodate different body parts without the need for additional adjustments of tuning, matching and decoupling, and without sacrificing coil performance.
This work aims to provide an RF coil-element design addressing all four of the aforementioned design criteria, making it ideally suited for RF transmission and/or reception for ultra-high field MRI/MRS. The proposed coil-element has low sensitivity to loading changes; provides superior inter-element isolation (when part of a coil array), and a better efficiency regarding RF energy deposition. These benefits enable imaging versatility, allowing the proposed antenna or an array of such antennas to be re-arranged for imaging various body parts with optimal performance in all configurations. Namely, the elements can be arranged to conform to the body shapes and body parts, while varied inter-element distance and varied body composition will not introduce a notable loss of efficiency.
Throughout this specification and claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers. As used herein and unless otherwise stated, the term “approximately” means ±20%.
Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.
Number | Date | Country | Kind |
---|---|---|---|
2020902725 | Aug 2020 | AU | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/AU2021/050846 | 8/3/2021 | WO |