Radio frequency (RF) antenna element with (de)tuning system

Abstract

A radio frequency (RF) antenna element with a (de)tuning system, comprising an RF antenna element with a resonant conductive loop and a (de)tuning system, the (de)tuning system comprising a photosensitive switching element to (de)tune the resonant conductive loop. The (de)tuning system comprises an injection light source optically coupled to the photosensitive switching element.

Description

Technical Field

[0001] Magnetic resonance imaging (MRI) methods, which exploit the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images, are now widely used, in particular in the field of medical diagnostics, because they are superior in many respects to other imaging methods for imaging soft tissues, do not require ionizing radiation and are generally non-invasive.

[0002] Typically according to the MRI method, the body of the patient to be examined is arranged in a strong uniform magnetic field B0, the direction of which simultaneously defines the axis of the coordinate system related to the measurement results (usually the z-axis). The magnetic field B0 causes different energy levels for individual nuclear spins according to the magnetic field strength that can be excited (spin resonance) by applying an electromagnetic alternating field (RF field) of a defined frequency (the so-called Larmor frequency or MR frequency). From a macroscopic point of view, the distribution of individual nuclear spins produces an overall magnetization that can be deviated from the equilibrium state by applying an electromagnetic pulse (RF pulse) of appropriate frequency, while the corresponding magnetic field B1 of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession motion about the z-axis. The precession motion describes the surface of a cone, the aperture angle of which is called the flip angle. The amplitude of the flip angle depends on the intensity and duration of the applied electromagnetic pulse. In the example of the so-called 90° pulse, the magnetization is deflected from the z-axis to the transverse plane (flip angle 90°).

[0003] After termination of the RF pulse, the magnetization relaxes back to the original equilibrium state, wherein the magnetization in the z direction is reestablished with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z axis relaxes with a second and shorter time constant T2 (spin-spin or transverse relaxation time). The transverse magnetization and its changes can be detected by means of a receiving RF antenna (coil array), which is arranged and oriented in the examination volume of the magnetic resonance examination system in such a way that changes in the magnetization are measured in the direction perpendicular to the z axis. The decay of the transverse magnetization is accompanied by dephasing that occurs after the RF excitation caused by local magnetic field inhomogeneities, which promotes the transition from an ordered state with the same signal phase to a state in which all phase angles are uniformly distributed. The dephasing can be compensated by means of a refocusing RF pulse (e.g. a 180° pulse). This generates an echo signal (spin echo) in the receiving coil.

[0004] In order to achieve spatial resolution in an imaged object such as a patient to be examined, magnetic field gradients extending along three main axes are superimposed on a uniform magnetic field B0, resulting in a linear spatial correlation of the spin resonance frequency. The signal picked up in the receiving antenna (coil array) then contains components of different frequencies that can be associated with different locations in the body. The signal data obtained via the receiving coil corresponds to the spatial frequency domain of the wave vector of the magnetic resonance signal and is called k-space data. K-space data typically includes multiple lines of different phase encodings acquired. Each line is digitized by collecting multiple samples. A set of k-space data is converted into an MR image by means of Fourier transform.

[0005] The transverse magnetization also dephases in the presence of a magnetic field gradient. This process can be reversed by appropriate gradient reversals forming a so-called gradient echo, similar to the formation of an RF-induced (spin) echo. However, in the case of a gradient echo, in contrast to an RF-refocused (spin) echo, the effects of main magnetic field inhomogeneities, chemical shifts and other off-resonance effects are not refocused.

[0006] The invention relates to a radio frequency (RF) antenna element for a magnetic resonance examination system with a (de)tuning system. In particular, the (de)tuning system is arranged for optically detuning the RF antenna element. The radio frequency (RF) antenna element with the (de)tuning system is for use with a magnetic resonance examination system for picking up magnetic resonance signals. The (de)tuning system is for detuning the RF antenna element during transmission of an RF excitation field in the magnetic resonance examination system and for tuning the RF antenna element to resonance during pick-up of magnetic resonance signals. Background Art

[0007] The article "Optically detunable, inductively coupled coil for self-gating in small animal magnetic resonance imaging" by M. Korn et al. (in MRM65 (2011) 882-888) discloses a radio frequency (RF) antenna element with a (de)tuning system, which is arranged to optically detune the RF antenna element. The known RF antenna element is formed by a single loop coil of a small resonant surface coil that can be dynamically detuned by using a capacitor shunted by two pin diodes connected to a photodiode. Under illumination, the photocurrent of the photodiode switches the pin diode into a conductive state, thereby detuning the surface coil.

[0008] Document US2004 / 0019273A1 discloses an assembly comprising a standard magnetic resonance system, an optical interface assembly and a catheter assembly. The optical interface assembly converts the electrical command / gating signals of the magnetic resonance system into optical signals and transmits these optical signals through the catheter assembly.

[0009] The paper “Active optical-based decoupling circuit for receiver endoluminal coil” by Reina Ayde et al. (ISMRM-ESMRMB (No. 1343, 2014-04-28, p. 1343)) discloses an optical decoupling coil in which two oscillating inductors separate the photodiode from the rest of the coil.

[0010] Document US6144205 discloses a local RF antenna assembly for a magnetic resonance imaging (MRI) system comprising a conductive RF antenna structure. It discloses a photosensitive semiconductor switch that selectively switches a reactive electrical device to a resonant circuit in response to the impact of light, thereby nulling the response of the resonant circuit at the Larmor frequency. Summary of the invention

[0011] It is an object of the present invention to provide a radio frequency (RF) antenna element having a (de)tuning system with improved noise characteristics.

[0012] This object is achieved by a radio frequency (RF) antenna element having a (de)tuning system, wherein:

[0013] - the RF antenna element comprises a resonant conductive loop and a (de)tuning system comprising a photosensitive switching element to (de)tune the resonant conductive loop, and

[0014] - The (de)tuning system comprises an injection light source optically coupled to the light-sensitive switching element.

[0015] The (de)tuning system is used to tune the RF antenna element to resonate in the Larmor band or to detune the RF antenna element to be non-resonant in the Larmor band. The (de)tuning system includes a switch element that switches the (de)tuning system between tuning and detuning. That is, the switch element in its open or closed (non-conductive or conductive) corresponds to the detuning or tuning of the RF antenna element (or vice versa). In fact, when a strong RF field is applied (for example, to excite, reverse or refocus the spin), the RF antenna element is detuned for receiving magnetic resonance signals. In this way, the RF antenna element and its sensitive electronic devices are protected from the influence of such strong RF fields. When no strong RF field is applied, the RF antenna element is made to resonate in the Larmor band and is sensitive to receiving very weak magnetic resonance signals, for example due to earlier RF excitation. According to the present invention, the injection light source is optically coupled to the photosensitive switch element. The photosensitive switch element is closed when photons from the injection light source are incident, and is opened when no photons are incident. A simple and inexpensive implementation is to use a (pin) photodiode as the photosensitive switch element. By (direct) injection of photons, the photosensitive switch element changes from its open state to its closed state, and stopping the injection of photons changes the photosensitive switch element from its closed state back to its open state. By injecting photons from the injection light source into the photosensitive switch element, mobile charge carriers are generated in the photosensitive switch element, so that the photosensitive switch element becomes conductive. When no photons are incident on the photosensitive switch element, for example, when the injection light source is turned off, the switch element is opened. That is, by turning on / off the injection of photons from the injection light source into the photosensitive switch element, the photosensitive switch element is closed / opened, and thus the RF antenna element is switched between its (non-)resonant states. Accordingly, by turning on / off the injection light source, the RF antenna element is switched between its (non-)resonant states. The (direct) injection of photons into the photoswitching element does not require a separate photovoltaic element to generate charge carriers to render the photoswitching element conductive.

[0016] These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent claims.

[0017] In an embodiment of the present invention, the (de)tuning system comprises: a photodetector, which is connected in parallel with the photosensitive switching element; and a bias light source, which is optically coupled to the photodetector. The photodetector generates a negative bias voltage on the photosensitive switching element in response to incident radiation from the bias light source. When no photons from the injection light source are injected into the photosensitive switching element, then the photodetector is illuminated by radiation from the bias light source and a negative bias (photo) voltage is generated in the photodiode and applied to the photosensitive switching element. Therefore, the resistance of the switching element increases in the open state and the switching element electrically behaves like a capacitor rather than a resistive conductor. This causes (thermal) noise to be generated in the RF antenna element, so that the signal-to-noise ratio of the received magnetic resonance signal is increased. In the closed state of the switch, no radiation from the bias light source is incident on the photodetector and then no negative bias voltage is generated and the photosensitive switching element has a low resistance. This can be achieved, for example, by controlling the bias light source and the injection light source so that the bias light source is turned on when the injection light source is turned off, and the bias light source is turned off when the injection light source is turned on and the light-sensitive switch element is closed.

[0018] In another embodiment of the invention, the electrical interface circuit is provided with one or more inductors in series with the photosensitive switching element and a resistor in parallel with the photodetector for the bias light source. The inductor acts as a filter to attenuate or even block the radio frequency signal from the (de)tuning system. The resistor is added to reduce transient signals due to the switching of the photosensitive switching element. Thus, the electrical interface circuit provides low RF interference to the acquisition of the magnetic resonance signal and also reduces the dead time of the acquisition of the magnetic resonance signal. Inductors tend to be large in size, so it is often more convenient to place both of them at different locations, and sometimes a more symmetrical arrangement is also advantageous.

[0019] For example, the light-sensitive switching element is formed by back-to-back photodiodes. The back-to-back diodes implement that they cannot be tuned to conduct by the RF signal to be blocked, i.e. for which the RF antenna element is set to its non-resonant state. A single photodiode can be switched on during the positive half-cycle of the RF signal if this is greater than the required forward voltage of the single photodiode (added to the negative bias voltage).

[0020] In further embodiments, the injection light source and the bias light source may utilize a common light source arranged with an optical switch system to couple the common light source to a photodetector and the light-sensitive switch element.

[0021] These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawings therein. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 shows a schematic representation of an embodiment of a radio frequency (RF) antenna element having a (de)tuning system of the present invention;

[0023] Figure 2 A schematic representation of a further embodiment of a radio frequency (RF) antenna element with the inventive (de)tuning system is shown. DETAILED DESCRIPTION

[0024] Figure 1 A schematic representation of an embodiment of a radio frequency (RF) antenna element with a (de)tuning system of the present invention is shown. The radio frequency antenna element is formed as a conductive coil loop 11. The (de)tuning system 12 comprises a switchable resonant circuit 111, which is coupled to the coil loop 11. The switchable resonant circuit comprises a capacitor 15 connected in series with the coil loop 11, and an inductor 16 switchable by a photosensitive switching element connected in parallel with the capacitor 15 and in series with the inductor 16. By opening / closing the photosensitive switching element (i.e., changing its characteristics between capacitive and resistive), the conductive loop is switched between its non-resonant state and its resonant state. To close the photosensitive switching element, photons from an injection light source 13 are injected into the photosensitive switching element via an optical link 112. When no photons from the injection light source are incident on the photosensitive switching element, the photosensitive switching element is in its open state. A (de)tuning controller 17 is provided to control the injection light source 17. Switching of the injection light source causes the (de)tuning system to switch the coil loop between its resonant state and its non-resonant state.

[0025] The light sensitive switching element may be a simple pin (photo) diode, or alternatively a back to back circuit pin (photo) diode may be used.

[0026] Figure 2 A schematic representation of a further embodiment of a radio frequency (RF) antenna element having a (de)tuning system of the present invention is shown. Figure 2 In an embodiment of the invention, the photodetector 22 is connected in parallel with the photosensitive switch element 14 to form a circuit. The bias light source 21 is optically coupled 113 to the photodetector. The bias light source is turned on / off by a (de)tuning controller. When photons from the bias light source 21 are incident on the photodetector 22, then the photodetector 22 generates a negative bias voltage applied to the photosensitive switch element. The bias light source is enabled when the injection light source is turned off. When the photosensitive element is in its off state, the negative bias voltage is applied to the photosensitive element.

[0027] Instead of switching off the injection light source and the bias light source, the optical links 112, 113 may be interrupted.

[0028] The photodetector can be a GaN-based semiconductor laser or an InGaN blue LED, which can generate a negative bias voltage of up to 75V.

Claims

1. A radio frequency antenna element having a detuning / tuning system, wherein: The radio frequency antenna element comprises a resonant conductive loop (11) and a detuning / tuning system (111), wherein the detuning / tuning system comprises a photosensitive switch element (14), wherein the photosensitive switch element is configured to detune / tune the resonant conductive loop, wherein the detuning / tuning system comprises: - an injection light source (13) which is optically coupled to the photosensitive switch element; characterized in that the radio frequency antenna element comprises: - a photodetector (22) connected in parallel with the light-sensitive switching element; and - a bias light source (21) optically coupled to the photodetector, wherein the photodetector is configured to generate a negative bias voltage across the photosensitive switch element in response to incident radiation from the bias light source.

2. The radio frequency antenna element with a detuning / tuning system according to claim 1, comprising an electrical interface circuit having at least one inductor connected in series with the light-sensitive switching element and a resistor connected in parallel with the photodetector.

3. A radio frequency antenna element with a detuning / tuning system according to any one of the preceding claims, wherein: The light-sensitive switching element is formed by diodes in a back-to-back arrangement.

4. The radio frequency antenna element with a detuning / tuning system according to claim 1, wherein: The injection light source and the bias light source are formed by a common light source having an optical switch system configured to optically couple the common light source to the injection light source and the bias light source, respectively and independently.

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