MRI transceiver comprising a birdcage coil and two or more dipole antennae

Abstract

The present disclosure relates to an apparatus for use with a Magnetic Resonance Imaging (MRI) system, in particular an RF coil 10 for use with a MRI system. 1. The RF coil 10 for a magnetic resonance (MR) imaging system, the RF coil 10 comprises a birdcage coil 104 comprising axially extending conductive rungs 102 circumferentially arranged between respective conductive end rings; and two or more dipole antennae 102 equiangularly positioned around the circumference of the birdcage coil 104 between at least a subset of the conductive rungs 102 of the birdcage coil 104, wherein the birdcage coil 104 and the two or more dipoles 102 are respectively tuned to operate at different RF frequencies, and together increase the homogeneity of a magnetic field across an imaging slice to be taken by the MR imaging device.

Description

[0001] P608525PC00

[0002] MRI TRANCEIVER

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to an apparatus for use with a Magnetic Resonance Imaging (MRI) system. In particular, the present disclosure relates to a radiofrequency (RF) coil for use with an MRI imaging system.

[0005] BACKGROUND

[0006] Magnetic Resonance Imaging (MRI) is a versatile technique used to image changes in brain anatomy and function. The clinical advantages of 7-Tesla (7T) MRI include high resolution and high contrast, which enhance lesion detection and broaden the applicability to various brain disorders (1,2).

[0007] However, as the magnetic field strength increases, the Larmor wavelength for protons in the human head decreases, and the RF wavelength in tissues becomes smaller than anatomical structures. At high magnetic fields, severe inhomogeneity in the applied transmit field (RF field) is observed. This inhomogeneity causes variations in the achieved pulse angle across different brain regions, resulting in a spatially varying signal-to-noise ratio (SNR) (3). Moreover, these effects are dependent on the imaging sequence used and may lead to deviations in contrast between different locations within the image. Such inhomogeneities are particularly pronounced in the temporal lobes and the cerebellum, making it difficult to assess anatomical structures and detect potential pathology in the peripheral areas of the brain. Furthermore, the inhomogeneous transmit field leads to additional Specific Absorption Rate (SAR) restrictions. The global SAR, or mean SAR over a given volume, increases approximately with the square of the applied magnetic field strength. However, due to the inhomogeneities in the transmit field, SAR is unevenly distributed across the brain, leading to larger differences between regions and higher SAR peaks in specific areas. This further imposes stringent limitations on the duty cycle of the applied sequence (4).

[0008] There are several known techniques to address this issue. The main examples known are discussed below.

[0009] 1. Additional hardware

[0010] Parallel excitation arrays: A potential solution involves the employment of parallel excitation arrays with multiple independent transmit coils. This approach aims to create a more uniform Bl field by modelling the RF waveform and the RF pulse sequences on each specific channel. However, the complexity of designing and controlling multiple P608525PC00 independent transmit coils so that they can operate simultaneously and in unison is a significant challenge. Each coil requires precise tuning and calibration, making the system complex and potentially less robust. Additionally, the computational power required to model and control these arrays in real-time can be substantial, potentially limiting the practicality of this solution in clinical settings (5).

[0011] Dielectric pads: The use of high-permittivity materials, specifically flexible "dielectric pads," which have been proven to be very useful in addressing RF inhomogeneities in high- field MRI systems (6-8). However, the main issue with dielectric pads is the difficulty in optimising their design for different head sizes and anatomical variations. Moreover, the effect of these pads is less pronounced in regions farther from the pads, such as deep brain structures. This spatial limitation reduces the overall effectiveness of the approach, and it can also be uncomfortable for patients due to the need for close physical contact with the pads during imaging.

[0012] 2. Pulse sequences

[0013] Adiabatic RF pulses, which modulate both the frequency and amplitude of the applied RF field above the adiabatic threshold, show relative insensitivity to Bl inhomogeneities. This method allows for uniform rotation of net magnetization with a constant flip angle, improving outer-volume suppression (9). The TR-FOCI pulse is an example that has been used in twice-refocused spin-echo diffusion pulse sequences to mitigate the impact of B1+ inhomogeneity on signal intensity across the brain at 7T (10). While adiabatic pulses can effectively reduce Bl inhomogeneities, they often require higher RF power, which can raise SAR issues. Additionally, the use of such pulses can lengthen the scan time, reducing patient throughput and potentially causing discomfort for patients who must remain still for extended periods.

[0014] 3. Post-imaging correction

[0015] Several algorithms have been proposed to correct for Bl inhomogeneity after image acquisition. For example, calibration data at coarsely sampled Bl values can be collected in conjunction with measured Bl maps. Different calibration curves are derived for grey matter and white matter instead of relying on simple linear scaling based on local Bl values (11,12). However, post-imaging correction adds a significant layer of complexity to the imaging workflow. It requires additional computational resources and time for processing, which can delay diagnosis and reduce the overall efficiency of the imaging process. Moreover, the accuracy of the correction depends on the quality of the Bl maps and the calibration data, which may not always be optimal. P608525PC00

[0016] Given the challenges associated with current solutions, there is a need for a further solution that does not require additional workflow or post-image processing.

[0017] SUMMARY OF THE DISCLOSURE

[0018] Embodiments of the present disclosure aim to provide a more integrated and efficient solution to address RF inhomogeneity and SAR limitations at ultra-high magnetic fields in MR imaging, thereby overcoming or at least improving upon the problems and / or results of the aforementioned known techniques.

[0019] A first aspect of the present invention provides an RF coil for a magnetic resonance (MR) imaging system, the RF coil comprising: a birdcage coil comprising axially extending conductive rungs circumferentially arranged between respective conductive end rings; and two or more dipole antennae equiangularly positioned around the circumference of the birdcage coil between at least a subset of the conductive rungs of the birdcage coil, wherein the birdcage coil and the two or more dipoles are respectively tuned to operate at different RF frequencies, and together increase the homogeneity of a magnetic field across an imaging slice to be taken by the MR imaging device.

[0020] A first aspect of the present invention provides an RF coil for a magnetic resonance (MR) imaging system, which offers several advantages. This RF coil includes a birdcage coil with axially extending conductive rungs arranged around conductive end rings, and two or more dipole antennae positioned equiangularly around the circumference of the birdcage coil. By tuning the birdcage coil and the dipole antennae to operate at different RF frequencies, the invention enhances the homogeneity of the magnetic field across the imaging slice. This improved field uniformity is beneficial for obtaining higher quality images, reducing artifacts, and ultimately leading to more accurate diagnoses in clinical settings.

[0021] In an embodiment, the birdcage coil and the two or more dipole antennae may be independently electrically driven with respective RF drive signals.

[0022] An advantage of having the birdcage coil and the dipole antennae independently driven by respective RF drive signals is that it allows for greater flexibility and precision in tuning the magnetic field. This independent control can result in improved field homogeneity and reduced interference, ultimately enhancing the quality of the MR imaging and leading to more accurate clinical diagnoses. P608525PC00

[0023] In an embodiment of the present invention, the two or more dipole antennae may be tuned to operate at the proton frequency.

[0024] An advantage associated with tuning the two or more dipole antennae to operate at the proton frequency is that it allows for enhanced imaging quality in proton-based MR imaging. This tuning ensures that the antennae are optimally aligned with the desired frequency, resulting in clearer and more accurate images, which are crucial for precise clinical diagnoses.

[0025] In an embodiment of the present invention, the two or more dipole antennae may comprise a tuning circuit, the tuning circuit may be arranged to match the impedance of the dipole antennae to the impedance of a transmitter, when in use.

[0026] An advantage associated with having the two or more dipole antennae comprising a tuning circuit that matches the impedance of the dipole antennae to that of the transmitter is that it ensures optimal power transfer. This impedance matching is crucial for maximizing the efficiency of the RF energy transmission, thereby improving the overall performance of the MR imaging system. Efficient energy transfer leads to clearer and more precise imaging, which is vital for accurate clinical diagnoses.

[0027] In an embodiment of the present invention, the tuning circuit may comprise an arrangement of a first series capacitor, a second series capacitor and a parallel capacitor, wherein the parallel capacitor may be disposed between the first and second series capacitors.

[0028] An advantage of having the tuning circuit comprise an arrangement of a first series capacitor, a second series capacitor, and a parallel capacitor, with the parallel capacitor disposed between the first and second series capacitors is that it allows for fine-tuning the resonance frequency of the dipole antennae. This precise tuning ensures optimal impedance matching and maximal power transfer, enhancing the efficiency and performance of the MR imaging system, which results in clearer and more accurate images.

[0029] In an embodiment of the present invention the tuning circuit may comprise a first dipole leg and a second dipole leg, the first dipole leg may be coupled to the second dipole leg via the matching circuit. P608525PC00

[0030] An advantage of having the tuning circuit comprise a first dipole leg and a second dipole leg, where the first dipole leg is coupled to the second dipole leg via the matching circuit, is that this configuration enhances the precision in impedance matching. This precise impedance matching is crucial for maximizing the efficiency of the RF energy transmission, thereby improving the overall performance of the MR imaging system. Efficient energy transfer leads to clearer and more precise imaging, which is vital for accurate clinical diagnoses.

[0031] In an embodiment of the present invention, the first dipole leg may be coupled to the tuning circuit via a first inductor and the second dipole leg may be coupled to the tuning circuit via a second inductor.

[0032] An advantage associated with having the first dipole leg coupled to the tuning circuit via a first inductor and the second dipole leg coupled to the tuning circuit via a second inductor is that this configuration allows for enhanced precision in the tuning process. By incorporating inductors, the system can achieve finer adjustments to the resonance frequency of the dipole antennae, thereby ensuring optimal impedance matching. This precise tuning capability is crucial for maximizing the efficiency of RF energy transmission, which ultimately leads to clearer and more accurate MR imaging. Such improvements in imaging quality are vital for precise clinical diagnoses, contributing to better patient outcomes.

[0033] In an embodiment of the present invention, the tuning circuit may be positioned in a different plane to the dipole antennae to reduce interference between the tuning circuit and the magnetic field generated by the birdcage coil.

[0034] An advantage associated with positioning the tuning circuit in a different plane to the dipole antennae is that it reduces interference between the tuning circuit and the magnetic field generated by the birdcage coil. This reduction in interference ensures that the system operates more efficiently, leading to improved imaging quality. By minimizing disruptions, the system can achieve clearer and more precise MR images, which are crucial for accurate clinical diagnoses.

[0035] In an embodiment of the present invention, the two or more dipole antennae may comprise a balun, the balun may be arranged on a coaxial cable of the two or more dipole antennae to reduce distortion of the two or more dipole antenna from the coaxial cable. P608525PC00

[0036] An advantage associated with having the balun arranged on a coaxial cable of the dipole antennae is that it reduces distortion of the dipole antennae from the coaxial cable, ensuring more accurate and reliable signal transmission. This reduction in distortion is critical for maintaining the integrity of the RF signals, which in turn enhances the overall performance of the MR imaging system, leading to clearer and more precise images essential for accurate clinical diagnoses.

[0037] In an embodiment of the present invention, the balun of the two or more dipole antennae may be tuned to the proton frequency.

[0038] An advantage associated with having the balun of the two or more dipole antennae tuned to the proton frequency is that it optimizes the performance of the MR imaging system, ensuring accurate and effective signal transmission. This precise tuning reduces signal loss and distortion, enhancing the quality of the MR images, which is crucial for accurate and reliable clinical diagnoses.

[0039] In an embodiment of the present invention, each of the two or more dipole antennae may be driven separately.

[0040] An advantage associated with driving each of the two or more dipole antennae separately is that this configuration allows for more precise control of the RF energy distribution. This separate driving mechanism enhances the flexibility and adaptability of the MR imaging system, enabling it to adjust to various imaging requirements and conditions. Consequently, this precision contributes to the production of higher quality MR images, which are essential for accurate and reliable clinical diagnoses.

[0041] In an embodiment of the present invention, the birdcage coil may be tuned to operate at the potassium frequency.

[0042] An advantage associated with tuning the birdcage coil to operate at the potassium frequency is that it enhances the specificity and accuracy of MR imaging for potassium ions. This precise tuning capability is particularly beneficial in medical diagnostics, as it allows for clearer and more detailed visualization of potassium distribution within biological tissues. Such improvements can lead to better diagnostic accuracy and more effective treatment planning, ultimately contributing to improved patient care and outcomes.

[0043] In an embodiment of the present invention, the birdcage coil may be arranged as a low- pass birdcage coil. P608525PC00

[0044] An advantage associated with arranging the birdcage coil as a low-pass birdcage coil is that it can effectively filter out higher frequency harmonics and noise, ensuring a cleaner and more accurate signal. This enhanced signal fidelity contributes to better image quality in MRI scans, which is critical for precise and reliable clinical diagnoses.

[0045] In an embodiment of the present invention, the birdcage coil may comprise a cable trap, the cable trap may be arranged to facilitate the wrapping of a coaxial cable thereabouts to block common-mode currents.

[0046] An advantage associated with having the cable trap arranged to facilitate the wrapping of a coaxial cable is that it effectively blocks common-mode currents. This blocking capability minimizes interference and enhances the quality of the RF signals, ultimately contributing to more accurate and reliable MR imaging. By ensuring a cleaner signal, the overall performance of the MR imaging system is improved, leading to clearer and more precise images that are essential for accurate clinical diagnoses.

[0047] In an embodiment of the present invention, the amount of conductive rungs of the birdcage coil may be a multiple of the amount of dipole antennae.

[0048] An advantage associated with having the number of conductive rungs of the birdcage coil be a multiple of the number of dipole antennae is that it enhances the uniformity and efficiency of the RF field. This configuration ensures optimal distribution of RF energy, leading to improved imaging quality and precision in MR imaging systems.

[0049] In an embodiment of the present invention, each of the conductive rungs may comprise a capacitor that bridges a gap between said first dipole leg and the second dipole leg.

[0050] An advantage associated with having each of the conductive rungs comprise a capacitor that bridges a gap between the first dipole leg and the second dipole leg is that it enhances the RF coil's ability to maintain consistent and uniform current distribution. This configuration effectively reduces signal distortion and increases the overall image quality in MRI scans, which is crucial for achieving precise and reliable clinical diagnoses. By ensuring a more stable and coherent RF field, the design improves the performance of the imaging system, ultimately leading to clearer and more detailed images.

[0051] In an embodiment of the present invention, the birdcage coil may comprise a first and second voltage source which may be coupled to conductive rungs of the birdcage coil, the P608525PC00 first voltage source and the second voltage source may be positioned on conductive rungs 90 degrees from each other to ensure a quadrature drive.

[0052] An advantage associated with having the birdcage coil comprise a first and second voltage source, which are coupled to conductive rungs and positioned 90 degrees from each other to ensure a quadrature drive, is that it enhances the efficiency and homogeneity of the magnetic field. This precise configuration minimizes signal loss and distortion, thereby significantly improving the quality and accuracy of MR imaging. Such enhancements lead to better diagnostic capabilities and more reliable clinical outcomes.

[0053] In an embodiment of the present invention, the birdcage coil may further comprise one or more end rung capacitors, the one or more end rung capacitors may be positioned in the conductive end rings of the birdcage coil and may be configured to prevent the formation of eddy currents in the birdcage coil.

[0054] An advantage associated with having the birdcage coil comprise one or more end rung capacitors positioned in the conductive end rings is that it prevents the formation of eddy currents within the birdcage coil. This prevention is crucial as it minimizes unwanted interference, which in turn enhances the overall performance and accuracy of MR imaging systems. By ensuring a more stable and coherent RF field, the design contributes to clearer and more detailed images, essential for precise and reliable clinical diagnoses.

[0055] In an embodiment of the present invention, the RF coil may comprise a birdcage coil comprising eight axially extending conductive rungs circumferentially arranged between respective conductive end rings and four dipole antennae equiangularly positioned around the circumference of the birdcage coil between at least a subset of the eight conductive rungs of the birdcage coil.

[0056] An advantage associated with having the RF coil comprise a birdcage coil with eight axially extending conductive rungs circumferentially arranged between conductive end rings and four dipole antennae equiangularly positioned around the circumference is that it enhances the uniformity and efficiency of the RF field. This precise arrangement ensures optimal distribution of RF energy, leading to improved imaging quality in MR systems. The configuration effectively reduces signal distortion and loss, thereby contributing to clearer and more accurate images, which are essential for reliable clinical diagnoses.

[0057] A second aspect of the present invention provides a kit of parts comprising the RF coil of the first aspect and an MRI imaging system. P608525PC00

[0058] Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner.

[0059] BRIEF DESCRIPTION OF THE FIGURES

[0060] One or more embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which like reference numerals refer to like parts, and wherein:

[0061] Figure 1 shows a front view and a side view of the structure of the RF coil, in accordance with an embodiment of the present disclosure;

[0062] Figure 2 shows a cross-section view of a birdcage coil, in accordance with an embodiment of the present disclosure;

[0063] Figure 3 shows a schematic of the birdcage coil of Figure 2, in accordance with an embodiment of the present disclosure;

[0064] Figure 4 shows a side view of the birdcage coil including the source and the cable trap, in accordance with an embodiment of the present disclosure;

[0065] Figure 5 shows a side view of a rung of the birdcage coil including a series capacitor, in accordance with an embodiment of the present disclosure;

[0066] Figure 6 shows a model of a dipole antenna, in accordance with an embodiment of the present disclosure;

[0067] Figure 7 shows a schematic of the dipole antenna of Figure 6, in accordance with an embodiment of the present disclosure; P608525PC00

[0068] Figure 8 shows an alternative view of the dipole antenna model of Figure 6 and a side view of the inductors of the dipole antenna, in accordance with an embodiment of the present disclosure;

[0069] Figure 9 shows a balun connected to the dipole antenna source, pre and post heat-shrink processing, in accordance with an embodiment of the present disclosure;

[0070] Figure 10 shows a series of example MRI scans which compare the homogeneity of the present disclosure compared to existing solutions;

[0071] Figure 11 shows a series of example MRI scans and graphs which compare the homogeneity of the present disclosure compared to an existing solution; and

[0072] Figure 12 shows a comparison of Bi+maps corresponding to an RF coil experiment and simulations relating to various levels of tuning and their associated Scaterring parameters matrix (S-parameters),

[0073] DETAILED DESCRIPTION

[0074] The present disclosure relates to an RF coil design that integrates a 1H dipole transceiver array with a 39K birdcage on a single layer, optimised for 7T MRI applications. While dipoles and birdcages may be well-established designs frequently used both in commercial and research settings, they are typically employed as individual elements acting as transceivers (13-18) or combined with additional transmit / receive (Tx / Rx) arrays on separate geometrical layers (19-22). The present disclosure, however, combines these elements into a single layer, significantly enhancing performance by leveraging the interaction between the two coil types to reduce image inhomogeneity at 7T.

[0075] The underlying concept of the present disclosure is therefore to integrate dipole antennae and a birdcage array together into a single layered unit that operates in use to increase the homogeneity of the RF field within the interior of the unit, thereby leading to improved (in terms of increased field homogeneity and therefore imaging consistency) MR imaging results across an MR scan slice. This is achieved by placement, typically although not exclusively equiangularly, of dipole coils around the birdcage element. As will be seen later, a birdcage coil typically has an RF field maximum running along its central axis of symmetry, with the field depleting towards the edge. Introducing a local dipole antenna at the edge of the birdcage introduces a local RF maximum around the dipole along its length. By careful placing of multiple (two or more) dipoles around the edge of the birdcage array then multiple local RF field maxima can be obtained, one around each dipole, which interact P608525PC00 with the RF field from the birdcage (with the axial maximum) to increase and homogenise (as in the homogeneity is increased) the resultant RF field within the whole arrangement. For example, one embodiment of the present disclosure consists of four 1H dipole transceivers and an eight-rung low-pass birdcage tuned to the potassium (39K) frequency, all constructed around the same acrylic cylinder. As shown in Figure 10, at 1002, the incorporation of the 39K birdcage, despite not being in resonance with 1H, significantly improves the performance of the 1H dipole array by reducing RF inhomogeneity, a critical challenge at the 7T field strength.

[0076] In isolation, the 39K birdcage coil is particularly useful in ultra-high field MRI applications, such as 7T MRI, where RF inhomogeneity can severely affect image quality. By integrating the 39K birdcage with a 1H dipole transceiver array, the combined design leverages the interaction between the two coil types to provide a more uniform RF field, improving the overall imaging performance.

[0077] The 39K birdcage coil has at least the advantage of ensuring a more uniform distribution of the RF field.

[0078] In some embodiments, the low-pass quadrature birdcage may be designed and constructed with conductive copper tape such as RS PRO 176-7498. The resonator may be composed of two end rings (diameter = 250mm, width = 25mm) and eight rungs (length = 250mm, width = 25mm) distributed equally around an acrylic tube (Simply plastics, outer diameter = 250mm, wall thickness = 3mm, length = 300mm). Series capacitors may be inserted into the middle of each rung as standard for a low-pass birdcage. Additionally, a lOnF capacitor may be inserted into each end ring in order to stop the eddy currents. Both sources may be located on the rungs 90 degrees from each other to ensure a quadrature drive. A matching network comprised of parallel and series capacitors may be inserted into the source to tune and match the birdcage to its operating frequency.

[0079] Further, in isolation, a 1H dipole is a type of radiofrequency (RF) coil used in magnetic resonance imaging (MRI) systems, specifically designed to operate at the proton (1H) frequency. Dipole antennas are known for their simplicity and effectiveness in generating and receiving RF signals, making them a popular choice for various MRI applications.

[0080] The 1H dipole coil typically consists of two conductive elements, often made of copper, arranged in a linear configuration. These elements are positioned parallel to each other and are separated by a small gap. The length of each element is approximately one-quarter P608525PC00 of the wavelength of the RF signal at the proton frequency, which is around 64 MHz for a

[0081] I.5T MRI system and 297 MHz for a 7T MRI system.

[0082] The dipole coil operates by generating an oscillating magnetic field when an RF current is applied to the conductive elements. This magnetic field interacts with the protons in the body, causing them to resonate and produce an MR signal. The dipole coil can be used for both transmitting and receiving RF signals, making it a versatile component in MRI systems

[0083] The 1H dipole coil is particularly useful in high-field MRI applications, such as 7T MRI, where its design helps to mitigate the challenges associated with RF inhomogeneity and specific absorption rate (SAR) limitations. Moreover, the 1H dipole can be used in parallel imaging techniques, where multiple coils are employed to improve image quality and reduce scan times. Further, the simplicity and effectiveness of the 1H dipole design make it a valuable tool for research and development in MRI technology, allowing for the exploration of new imaging techniques and coil configurations.

[0084] In some embodiments, the 1H dipole array may share characteristics of the previously developed 8Tx-dipole design(23) with each centre-shortened dipole (230 mm length, 15 mm width) etched from 35 pm copper on a 1.6 mm-thick FR-4 substrate. In that arrangement, the tuning / matching circuit consisted of two hand-wounded copper-wire series inductors, two series and one parallel capacitor placed on a printed-circuit board (same FR-4 substrate as the dipoles) elevated by 13 mm with respect to the dipole legs' level, and symmetrically positioned with respect to them.

[0085] Embodiments of the present disclosure will now be discussed in relation to Figures 1 to

[0086] II.

[0087] As previously mentioned, an embodiment of the present disclosure relates to an RF coil for use with a magnetic resonance (MR) imaging system, such as shown the RF coil 10 in Fig.l, The RF coil 10 of Fig.l comprises a birdcage coil 104, e.g. a 39K birdcage, and two or more dipole antennae 102a-d e.g. two or more 1H dipole antennae. The birdcage coil 104 comprises axially extending conductive rungs circumferentially arranged between respective conductive end rings of the birdcage coil, this is detailed in relation to Figs. 2 and 3. The two or more dipole antennae 102a-102d are equiangularly positioned around the circumference of the birdcage coil 104 between at least a subset of the conductive rungs of the birdcage coil 104, wherein the birdcage coil 104 and the two or more dipoles 102a-102d are respectively tuned to operate at different RF frequencies, and together P608525PC00 increase the homogeneity of a magnetic field across an imaging slice to be taken by the MR imaging device.

[0088] Figs. 2 and 3 detail the arrangement of the birdcage coil 20. The birdcage coil 20 is an example birdcage coil architecture that could be used as the birdcage coil 104 in Fig.l The birdcage coil 20 comprises a first voltage source 206a, a second voltage source 206b, a plurality of conductive rungs 202a-202f,204a-204b wherein the conductive rungs 202a- 202f,204a-204b comprise a series of capacitors, and end rung capacitors 208a-208b.

[0089] In this regard, the first voltage source 206a and the second voltage source 206b may be located on the rungs 90 degrees from each other to ensure a quadrature drive. A matching network (as can be seen in Fig. 3) comprised of parallel and series capacitors is inserted into the first and second source 206a-206b to tune and match the birdcage coil 20 to its operating frequency. The conductive rungs 202a-202f,204a-204b comprise a series of capacitors positioned in the middle of each conductive rung. These series capacitors seek to provide low-pass filter functionality. The end rung capacitors 208a-208b are provided in order to prevent eddy currents from forming within the birdcage coil 20. Moreover, the end rung capacitors 208a-208b play a large role in determining the resonant frequency of the coil. The provided capacitor values in Fig.2 are provided by way of example, the birdcage coil 20 is not limited to these values. In fact, the capacitor values can be varied in order to tune the birdcage coil 20 to desired frequencies.

[0090] Fig.3 shows a schematic view of the birdcage coil 20 as in Fig.2. Therefore, it can be seen that the conductive rungs 202a-202f 204a-204b are arranged in parallel extending from the end rings. Similarly, the end rings comprise the end rung capacitors 208a-208b. As previously mentioned, it can be seen that the first voltage source 206a and the second voltage source 206b comprise respective matching networks 207a-207b which comprise parallel and series capacitors to tune and match the birdcage coil 20 to its operating frequency.

[0091] Figure 4 shows a side view of the birdcage coil 20 including the source 206a and the cable trap 402. The cable trap 402 is inserted into each source 206a-206b of the birdcage coil 20. The cable trap 402 may be tuned to a specific frequency such as potassium frequency (13.9MHz). Fundamentally, the cable trap 402 is a component that enables the coaxial cable of the birdcage coil 20 to be wrapped around it. A cable trap 402 in a birdcage coil 20 are used to block unwanted currents on the outside of the coaxial cable. These unwanted currents, known as common-mode currents, can cause interference, radiation, and sensitivity to external factors, which can degrade the performance of the coil and P608525PC00 potentially cause safety issues. By using a cable trap 402, the birdcage coil 20 becomes less sensitive to changes in the cable's position and external electromagnetic interference, ensuring more stable and reliable operation

[0092] Figure 5 shows a side view of a conductive rung 202a-202f 204a-204b of the birdcage coil 20 including a series capacitor, in accordance with an embodiment of the present disclosure. In particular, Fig.5 shows that the conductive rung 202a-202f 204a-204b comprises a central gap. The gap portion is designed into the conductive rung 202a-202f 204a-204b such that it is bridged by a series capacitor, such as the capacitors 202a-202f 204a-204b in Figs.2 and 3.

[0093] In other words, the gap in the conductive rungs 202a-202f 204a-204b of a birdcage coil 20 are there to accommodate capacitors. These capacitors are crucial for tuning the birdcage coil 20 to the desired resonant frequency. By placing capacitors in these gaps, the birdcage coil 20 can achieve the necessary electrical characteristics to a homogeneous magnetic field, which is essential for high-quality MRI imaging. It can be said, that this arrangement enables precise control over the birdcage coil's 20 resonance and helps in creating a uniform magnetic field inside the birdcage coil 20, which may be vital for accurate imaging.

[0094] Birdcage coils are typically made from materials that ensure good electrical conductivity and mechanical stability such copper, aluminium, silver, copper plated steel etc.

[0095] Fig.6 shows a model of a dipole antenna 102, in accordance with an embodiment of the present disclosure. This dipole antenna 102 of Fig.6 is an example of the dipole antenna 102a-102d of Fig.l. The example dipole antenna 102 of Fig.6 is shown to comprise a first dipole leg or rung 1022a, a second dipole leg or rung 1022b, a first series capacitor 1024a, a second series capacitor 1024b, a parallel capacitor 1026, a first inductor 1028a and a second inductor 1028b. The dipole antenna 102 may be tuned to hydrogen or proton frequency e.g. around 64 MHz for a 1.5T MRI system and 297 MHz for a 7T MRI system.

[0096] As shown in the schematic diagram of Fig.7, it can be seen that the first dipole leg or rung 1022a is connected to the first inductor 1028a. The first inductor 1028a is in turn connected to a node between the first series capacitor 1024a and the parallel capacitor 1026. The alternative end of the parallel capacitor 1026 is connected to a node between the second series capacitor 1024b and the second inductor 1022b. It can be said that this arrangement of capacitors and inductors forms the tuning / matching circuits of the dipole antenna 102 in order to match the impedance of the dipole antenna 102 to the impedance P608525PC00 of a transmitter or receiver. This can be further seen in Fig.8, wherein the dipole antenna 102 is shown from a different angle.

[0097] Fig.8 more clearly shows the elevation of the first, second series capacitor and parallel capacitors above the first and second dipole legs or rungs 1022a-1022b. The elevated capacitors are connected to the first and second dipole legs or rungs 1022a-1022b via the first and second inductor 1028a-1028b. The tuning circuits in a birdcage coil 104 are elevated i.e. positioned in a different plane, from the first and second dipole legs 1022a- 1022b of the dipole antenna 102 primarily to minimize electromagnetic interference and to ensure optimal performance. Elevating the tuning circuits helps reduce unwanted coupling between the tuning components and the conductive elements of the birdcage coil 104. This ensures that the tuning circuits do not interfere with the magnetic field generated by the birdcage coil 104. Elevating the tuning circuits can also aid in better thermal management and improved ease of maintenance / adjustment.

[0098] Dipole antennas are typically made from materials that ensure good electrical conductivity and mechanical stability such copper, aluminium, silver, copper plated steel etc.

[0099] Fig.9 shows a balun 904 connected to the dipole antenna source 902, pre and post heatshrink processing, in accordance with an embodiment of the present disclosure. The balun 904 (short for balanced to unbalanced) is used in a dipole antenna, such as that of the dipole antenna 102 of previous figures, to transition between the balanced dipole antenna 102 and the unbalanced feed line, typically a coaxial cable. The balun 904 aids in preventing RF current on the coaxial cable. Without a balun 904, RF currents may flow on the on coaxial cable causing it to radiate and potentially distort the antenna's radiation pattern. Moreover, baluns 904 can help match the impedance of the dipole antenna 102 to the coaxial cable which ensures efficiency in power transfer and reduced signal loss. Finally, by reducing unwanted currents on the coaxial cable, the baluns 904 can help reduce interference with other electronic devices and improve overall signal quality.

[0100] Therefore, an embodiment of the present disclosure provides an RF coil for a magnetic resonance (MR) imaging system, the RF coil comprising: a birdcage coil (e.g. the birdcage coils shown in Figs.2-5) comprising axially extending conductive rungs circumferentially arranged between respective conductive end rings; and two or more dipole antennae (e.g. the dipole antenna shown in Figs.6-9) equiangularly positioned around the circumference of the birdcage coil between at least a subset of the conductive rungs of the birdcage coil, wherein the birdcage coil and the two or more dipoles are respectively tuned to operate P608525PC00 at different RF frequencies, and together increase the homogeneity of a magnetic field across an imaging slice to be taken by the MR imaging device.

[0101] However, it should be understood that various arrangements of RF coil are envisioned. This means that the RF coil may have a birdcage coil with differing amounts of conductive rungs and differing amounts of two or more dipole antennae. This is not limited to the aforementioned example that utilises eight conductive rungs and four dipole antennae. By way of examples, the RF coil may be formed of any non-lH nuclei birdcage coil and an independent number of dipoles. Therefore, as way of a further example, the RF coil may be formed of any non-lH nuclei birdcage coil with eight conductive rungs (this amount can differ) and up to seven dipole antennae (this amount can differ) interspersed between the conductive rungs. Fundamentally, the present invention relates to any single layer RF coil arrangement that incorporates any general birdcage design in X-nuclei combined with proton dipole antennae.

[0102] In an embodiment of the present disclosure, the birdcage coil and the dipole antennae may be independently electrically driven with different respective RF drive signals.

[0103] Figures 10 and 11 show experimental results 100 110 of embodiments of the present disclosure.

[0104] To validate the performance of the RF coil in accordance with an embodiment of the present disclosure, phantom experiments were conducted using a homogenous phantom (Siemens MR Biograph Phantom Spherical D170) that simulates realistic tissue properties (er= 81, o = 0.95 S / m). The transmit efficiency was characterised by acquiring B1+ maps on a 7T MRI scanner using the AFI sequence (TR = 200ms, FA = 60deg, 4.4x4.4x4.6mm3, 64 slices, 1 average, BW = 2.1xlO9Hz / Pixel, TA = 6min41s). The performance of the 4- channel dipole array with the 39K birdcage 1002 was compared against an 8-channel dipole array 1004, an 8Tx Nova coil 1006, and a Rapid lH / 23Na coil 1008. The B1+ map acquired on the central axial slice from all four coils are shown in Fig.10 as we see increased average B1+ and improved homogeneity (std / mean) in our proposed 4ch dipole array with 39K birdcage compared to the other existing designs.

[0105] Embodiments of the present disclosure offer several distinct advantages over existing RF coil configurations used in high-field MRI (quantitative comparison 110 is conducted between the proposed design 1102 1104 and the 8Tx Nova coil 1106 1108, which is the current market lead commercial coil for 7T proton imaging): P608525PC00

[0106] 1. Significant Improvement in Bl Homogeneity:

[0107] One of the most notable benefits of this innovation is the marked improvement in Bl homogeneity. Quantitatively, the B1+ mapping from the phantom experiments demonstrated a more uniform distribution of the transmit field when using the combined dipole-birdcage design compared to the other coils tested. Comparing the present invention to the currently available solutions e.g. IH-only coil (Nova 8Tx), the present invention achieves 30% lower std / mean whilst maintaining similar mean B1+ levels across the central axial (as seen in Fig.10). Specifically, the innovation showed a reduction in B1+ inhomogeneity by approximately 50% across the same profile on the central slice B1+ map. (Fig.11)

[0108] The proposed design offers a hardware solution through an integrated RF coil that significantly improves Bl homogeneity for 7T proton MRI. Unlike traditional approaches that rely on complex pulse sequences or require extensive post-processing, this innovation simplifies the clinical workflow. It provides a more efficient and practical solution to enhance image quality at 7T, making it easier to implement in routine MRI practice.

[0109] 2. Increased Efficiency:

[0110] In addition to improved homogeneity, 4ch dipole array with 39K birdcage exhibits similar average B1+ across the central slice, whilst reaching 40% higher maximum B1+ comparing to the 8Tx Nova coil. (Fig.10) The increased transmit efficiency suggests less power is needed to achieve the same B1+ levels, which can help in managing SAR constraints, reducing the risk of tissue heating, and making it safer for patients, especially during prolonged or high-intensity scans.

[0111] 3. Simplified Design:

[0112] Compared to the traditional approach to assemble multiple coil elements on multiple layers, we offer a simplified design by integrating the dipole array and the birdcage into a single layer. This makes more efficient use of the available space within the coil structure. This can be particularly advantageous in scenarios where space is limited, such as in specialised MRI systems or when accommodating additional hardware or patient-specific customisations.

[0113] Clinical Neuroimaging: The present innovation can be directly applied in clinical settings, particularly in neuroimaging where high-field MRI is essential for detecting small or subtle brain lesions, mapping brain function, and diagnosing complex neurological disorders. The improved Bl homogeneity and increased transmit efficiency offered by this coil design are P608525PC00 crucial for enhancing image quality and diagnostic accuracy at 7T, making it a valuable tool for hospitals and imaging centres specialising in neurological conditions.

[0114] Research Applications: In academic and research institutions, the present disclosure can be utilised for advanced MRI studies that require high-resolution imaging of the brain or other organs. Its improved performance at 7T makes it ideal for exploring new imaging techniques, investigating brain function and structure, and studying various pathologies at a more detailed level than currently possible.

[0115] Figure 12 provides further testing 120 of the RF coil in both simulated 1204 1206 1208 and experimental 1202 settings. Fig.12 illustrates Bi+maps corresponding to various levels of tuning and matching presented in the Scaterring parameters matrix (S- parameters), compared to the experimental Bi+map. In the Scattering parameters matrix, Tuning / Matching is represented with the diagonal terms of the matrix while the nondiagonal terms present the coupling of the coil elements.

[0116] Varying Bi+maps are shown for the experimental data 1202 of the RF coil using an arrangement of eight conductive rungs and four dipole antennas and the simulated data 1204 1206 1208. The experimental data 1202 shows that the RF coil in accordance with the present invention enhances the Bi field homogeneity. Therefore, based on the improved Bi field homogeneity, the RF coil, in accordance with the present invention, can obtain higher quality images, reducing artifacts, and ultimately leading to more accurate diagnoses in clinical settings

[0117] Among the simulations, Tuning / Matching 2 1206 demonstrates the greatest similarity to the experimental results, with an Sii-parameter (diagonal terms in the matrix) around -15 dB. Simulation results indicate that achieving optimal S-parameters aids in accurately capturing the coupling effects between the birdcage and the dipole array, thereby enhancing Bi field homogeneity.

[0118] Similarly, the optimal coupling values (non-diagonal terms of the scattering parameters matrix) among the 1H proton dipole elements would be playing roles to achieve the optimal transmit homogeneity and efficiency.

[0119] It will be appreciated that various changes and modifications can be made to the present disclosure without departing from the scope of the appended claims.

[0120] References P608525PC00 Okada T, Fujimoto K, Fushimi Y, Akasaka T, Thuy DHD, Shima A, et al. Neuroimaging at 7 Tesla: a pictorial narrative review. Quant Imaging Med Surg. 2022 Jun;12(6):3406-35. van der Kolk AG, Hendrikse J, Zwanenburg JJM, Visser F, Luijten PR. Clinical applications of 7 T MRI in the brain. Eur J Radiol. 2013 May l;82(5):708-18. Vaughan JT, Garwood M, Collins CM, Liu W, Delabarre L, Adriany G, et al. 7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images. Magn Reson Med. 2001;46(l):24-30. Collins CM, Liu W, Wang J, Gruetter R, Vaughan JT, Ugurbil K, et al. Temperature and SAR Calculations for a Human Head Within Volume and Surface Coils at 64 and 300 MHz. J Magn Reson Imaging. 2004;19(5):650-6. Zhu Y. Parallel excitation with an array of transmit coils. Magn Reson Med. 2004 Apr;51(4):775-84. Jacobs PS, Benyard B, Cember A, Nanga RPR, Cao Q, Tisdall MD, et al. Repeatability of B1+ Inhomogeneity Correction of Volumetric (3D) Glutamate CEST via High- Permittivity Dielectric Padding at 7T. Magn Reson Med. 2022 Dec;88(6):2475-84. Garcia M, Chaim K, Otaduy M, Rennings A, Erni D, Vatanchi M, et al. Investigating the influence of dielectric pads in 7T magnetic resonance imaging - simulated and experimental assessment. Curr Dir Biomed Eng. 2020 Nov 26;6:20203007. van Gemert J, Brink W, Webb A, Remis R. High-permittivity pad design tool for 7T neuroimaging and 3T body imaging. Magn Reson Med. 2019 May;81(5):3370-8. Tannus A, Garwood M. Adiabatic pulses. NMR Biomed. 1997 Dec; 10(8) :423-34. . Abbasi-Rad S, Cloos MA, Jin J, O'Brien K, Barth M. inhomogeneity mitigation for diffusion weighted MRI at 7T using TR-FOCI pulses. Magn Reson Med. 2024;91(6):2508-18. . Singh A, Cai K, Haris M, Hariharan H, Reddy R. On Bl Inhomogeneity Correction of In Vivo Human Brain Glutamate Chemical Exchange Saturation Transfer Contrast at 7T. Magn Reson Med Off J Soc Magn Reson Med Soc Magn Reson Med. 2013 Mar l;69(3):818-24. P608525PC00 . Windschuh J, Zaiss M, Meissner JE, Paech D, Radbruch A, Ladd ME, et al. Correction of Bl-inhomogeneities for relaxation-compensated CEST imaging at 7T. NMR Biomed. 2015 May;28(5):529-37. . Ahmad SF, Kim YC, Choi IC, Kim HD. Recent Progress in Birdcage RF Coil Technology for MRI System. Diagnostics. 2020 Nov 27;10(12): 1017. . Cheng T, Magill AW, Comment A, Gruetter R, Lei H. Ultra-high field birdcage coils: A comparison study at 14. IT. In: 2014 36th Annual International Conference of the IEEE Engineering in Medicine and Biology Society [Internet]. 2014 [cited 2024 Aug 12]. p. 2360-3. Available from: https: / / ieeexplore.ieee.org / abstract / document / 6944095 . Fu Z. A 9.4 Tesla Transmit and Receive Mri Quadrature Birdcage Coil Design and Fabrication for Mouse Imaging [Internet] [M.Sc.]. [United States — Alabama]: Auburn University; 2014 [cited 2024 Aug 12]. Available from: https: / / www.proquest.eom / docview / 2778642562 / abstract / 21CCC21340F74E48PQ / l. Peterson DM, Carruthers CE, Wolverton BL, Meister K, Werner M, Duensing GR, et al. Application of a birdcage coil at 3 tesla to imaging of the human knee using MRI. Magn Reson Med. 1999;42(2):215-21. . Rios NL, Pouliot P, Papoutsis K, Foias A, Stikov N, Lesage F, et al. Design and construction of an optimized transmit / receive hybrid birdcage resonator to improve full body images of medium-sized animals in 7T scanner. PLOS ONE. 2018 Feb l;13(2):e0192035. . Solis SE, Cuellar G, Wang RL, Tomasi D, Rodriguez AO. Transceiver 4-leg birdcage for high field MRI: knee imaging. Rev Mex Fisica. 2008 Jun;54(3):215-20. . Dregely I, Ruset IC, Wiggins G, Mareyam A, Mugler III JP, Aites TA, et al. 32-channel phased-array receive with asymmetric birdcage transmit coil for hyperpolarized xenon-129 lung imaging. Magn Reson Med. 2013;70(2):576-83. . Golestanirad L, Keil B, Angelone LM, Bonmassar G, Mareyam A, Wald LL. Feasibility of using linearly polarized rotating birdcage transmitters and close-fitting receive arrays in MRI to reduce SAR in the vicinity of deep brain simulation implants. Magn Reson Med. 2017;77(4): 1701-12. . Paska J, Cloos MA, Wiggins GC. A rigid, stand-off hybrid dipole, and birdcage coil array for 7 T body imaging. Magn Reson Med. 2018;80(2):822-32. P608525PC00 Paska J, Wang B, Chen AM, Madelin G, Brown R. Triple-tuned birdcage and singletuned dipole array for quadri-nuclear head MRI at 7 T. Magn Reson Med. 2024;91(5):2188-99. Clement J, Tomi-Tricot R, Malik SJ, Webb A, Hajnal JV, Ipek O. Towards an integrated neonatal brain and cardiac examination capability at 7 T: electromagnetic field simulations and early phantom experiments using an 8-channel dipole array. Magn Reson Mater Phys Biol Med. 2022 Oct l;35(5):765-78.

Claims

P608525PC00CLAIMS1. An RF coil for a magnetic resonance (MR) imaging system, the RF coil comprising: a birdcage coil comprising axially extending conductive rungs circumferentially arranged between respective conductive end rings; and two or more dipole antennae equiangularly positioned around the circumference of the birdcage coil between at least a subset of the conductive rungs of the birdcage coil, wherein the birdcage coil and the two or more dipoles are respectively tuned to operate at different RF frequencies, and together increase the homogeneity of a magnetic field across an imaging slice to be taken by the MR imaging device.

2. The RF coil of claim 1, wherein the birdcage coil and the two or more dipole antennae are independently electrically driven with respective RF drive signals.

3. The RF coil of any preceding claim, wherein the two or more dipole antennae are tuned to operate at the proton frequency.

4. The RF coil of any preceding claim, wherein the two or more dipole antennae comprise a tuning circuit, the tuning circuit being arranged to match the impedance of the dipole antennae to the impedance of a transmitter, when in use.

5. The RF coil of claim 4, wherein the tuning circuit comprises an arrangement of a first series capacitor, a second series capacitor and a parallel capacitor, wherein the parallel capacitor is disposed between the first and second series capacitors.

6. The RF coil of any of claims 4 and 5, wherein the tuning circuit comprises a first dipole leg and a second dipole leg, the first dipole leg being coupled to the second dipole leg via the matching circuit.

7. The RF coil of claim 6, wherein the first dipole leg is coupled to the tuning circuit via a first inductor and the second dipole leg is coupled to the tuning circuit via a second inductor.

8. The RF coil of any of claims 4 to 7, wherein the tuning circuit is positioned in a different plane to the dipole antennae to reduce interference between the tuning circuit and the magnetic field generated by the birdcage coil.P608525PC009. The RF capacitor of any preceding claim, wherein the two or more dipole antennae comprise a balun, the balun being arrange on a coaxial cable of the two or more dipole antennae to reduce distortion of the two or more dipole antenna from the coaxial cable.

10. The RF coil of claim 9, wherein the balun of the two or more dipole antennae is tuned to the proton frequency.

11. The RF coil of any preceding claim, wherein each of the two or more dipole antennae are driven separately.

12. The RF coil of any preceding claim, wherein the birdcage coil is tuned to operate at the potassium frequency.

13. The RF coil of any preceding claim, wherein the birdcage coil is arranged as a low- pass birdcage coil.

14. The RF coil of any preceding claim, wherein the birdcage coil comprises a cable trap, the cable trap being arranged to facilitate the wrapping of a coaxial cable thereabouts to block common-mode currents.

15. The RF coil of any preceding claim, wherein the amount of conductive rungs of the birdcage coil is a multiple of the amount of dipole antennae.

16. The RF coil of any preceding claim, wherein each of the conductive rungs comprises a capacitor that bridges a gap between said first dipole leg and the second dipole leg.

17. The RF coil of any preceding claim, wherein the birdcage coil comprises a first and second voltage source coupled to conductive rungs of the birdcage coil, the first voltage source and the second voltage source being positioned on conductive rungs 90 degrees from each other to ensure a quadrature drive.

18. The RF coil of any preceding claim, wherein the birdcage coil further comprises one or more end rung capacitors, the one or more end rung capacitors being positioned in the conductive end rings of the birdcage coil and configured to prevent the formation of eddy currents in the birdcage coil.P608525PC0019. The RF coil of any preceding claim, wherein the RF coil comprises a birdcage coil comprising eight axially extending conductive rungs circumferentially arranged between respective conductive end rings and four dipole antennae equiangularly positioned around the circumference of the birdcage coil between at least a subset of the eight conductive rungs of the birdcage coil.

20. A kit of parts comprising the RF coil of claim 1 and an MRI imaging system.

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