Patent Description:
Magnetic resonance imaging (MRI) methods utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.

According to the MRI method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system to which the measurement is related. The magnetic field B0 causes different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the corresponding magnetic field B <NUM> of this RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession motion about the z-axis. The precession motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the example of a so-called <NUM>° pulse, the magnetization is deflected from the z axis to the transverse plane (flip angle <NUM>°).

After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z-direction relaxes with a second and shorter time constant T2 (spin-spin or transverse relaxation time). The transverse magnetization and its variation can be detected by means of receiving RF antennae (coil arrays) which are arranged and oriented within an examination volume of the magnetic resonance examination system in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied by dephasing taking place after RF excitation caused by local magnetic field inhomogeneities facilitating a 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 (for example a <NUM>° pulse). This produces an echo signal (spin echo) in the receiving coils.

In order to realize spatial resolution in the subject being imaged, such as a patient to be examined, magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B0, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennae (coil arrays) then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils correspond to the spatial frequency domain of the wave-vectors of the magnetic resonance signal and are called k-space data. The k-space data usually include multiple lines acquired of different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image by means of Fourier transformation.

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

The paper <NPL>, discloses a radio frequency (RF) antenna element with a detuning system arranged for optically detuning 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 a capacitor shunted with two pin-diodes connected to a photodiode. Under illumination, the photocurrent of the photodiodes switches the pin-diodes into the conducting state, detuning the surface coil.

Patent publication <CIT> discloses a surface coil system for single channel MRI reception. The paper "<NPL>) discloses an optically decoupled coil where two shock inductors separate photodiodes from the rest of the coil. Patent publication <CIT> discloses a system for testing a RF device. The system includes a light emitting source configured to transmit an optical test signal based on an RF signal.

An object of the invention is to provide an RF antenna element with a detuning system that can be controlled more accurately.

This object is achieved by an RF antenna element for MRI as defined in claim <NUM>. The RF antenna element comprises a detuning system and a resonant electrically conductive loop. The detuning system comprises an electroluminescent switching element configured to detune the resonant electrically conductive loop, wherein the electroluminescent switching element is coupled to the resonant electrically conductive loop, wherein the detuning system includes a photo-electric conversion element that is optically coupled to the electroluminescent switching element and configured to detect an electroluminescent signal from the electroluminescent switching element.

According to the invention an electroluminescent switching element is provided in the detuning system. The electroluminescent element coupled to the resonant electrically conductive loop generates an electroluminescence optical signal in response to a voltage applied to it by its detuning system. The electrically conductive loop forms the RF antenna. The electrically conductive loop may be a round loop, but may also be formed by another electrical conductor geometry that has sensitivity to pick-up magnetic flux. This electroluminescent optical signal is detected by a photo-electrical converter which in response produces an electronic feedback signal that is representative for the detuning state of the resonant electrically conductive loop. Thus, the electronic feedback signal makes available information on the detuning state of the resonant electrically conductive loop. On the basis of that information the detuning system can be more accurately controlled. Quite effective electroluminescence is found in GaInN-semiconductors which operate efficiently in a wavelength-band around <NUM>. For GaInN-based devices photovoltaic power generation efficient is about <NUM>%, electroluminescence efficiency is about <NUM>% and photo-induced electroluminescence has about <NUM>% efficiency. Alternatively Si-based semiconductor materials may be used, although these may have a lower efficiency.

The use of an electro-optical converter in a radio frequency (RF) antenna element with a resonant electrically conductive loop is known per se from the German patent application <CIT>. This known RF antenna element includes a local coil antenna that is coupled to a signal amplifier to amplify the magnetic resonance signal picked up by the local coil antenna. The amplified magnetic resonance signal is digitised and then applied to an electro-optical converter which derives an optical signal from the digital magnetic resonance signal. The optical signal is transmitted over an optical signal path to an optical receiver. The optical receiver turns the received optical signal into a digital receiver signal that is applied to a signal processor for reconstruction of the magnetic resonance image. This known RF antenna element does not feature an electro-optical converter in conjunction with a detuning system.

These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent Claims.

An example not part of the invention and presented for illustration purposes only concerns an RF antenna element with a detuning system of the invention, the electroluminescent element is formed by a photo detector circuited in series with the switch element. A control optical source is provided coupled to the photodetector. When optical radiation, e.g. light, (such as blue light with a wavelength around <NUM>), is incident on the photodetector, the photodetector generates a control voltage to the switching element to open or close the switching element so that the electrically conductive loop is switched between resonant and non-resonant. Depending on the voltage over the photodetector in the form of an electroluminescent element, an (optical) electroluminescent signal is generated by the photodetector (by way of electroluminescence and photo induced electroluminescence). This electroluminescent signal is converted into the electronic feedback signal that is representative for the detuning state of the resonant electrically conductive loop.

According to the invention, the switching element forms the electroluminescent element. In a preferred embodiment of the invention, an injection optical source is coupled to the switching element and when optical radiation, e.g. (blue) light, is incident on the switching element, the switching element becomes electrically conducting and is switched to its closed state. Thus, the switching element can be switched between its closed and open states by switching on or off of the injection optical source. Accordingly the electrically conductive loop is switched between its resonant and non-resonant state. The voltage, i.e. the induced radio frequency voltage, across the switching element causes, due its electroluminescent properties, to generate the electroluminescent signal. This electroluminescent signal is converted into the electronic feedback signal that is representative for the detuning state of the resonant electrically conductive loop.

The electronic feedback signal represents the switching element's (open or closed) state and accordingly provides information on whether the electrically conductive loop is in its resonant or its non-resonant state.

As the voltage over the electroluminescent element depends on the load of the electrically conductive loop, the electronic feedback signal depends on the load. Thus the electronic feedback signal may be employed to monitor the actual load onto the electrically conductive loop. There may be about an order of magnitude difference between the signal level of the electronic feedback signal for an open circuit condition and for a closed circuit condition.

In further embodiments of the RF antenna element comprising a detuning system according to the invention, parts of the optical paths overlap between the electroluminescent element and the control optical source or the injection optical source and between the photo-electrical conversion element and the electroluminescent element. In this way the number of optical connections is reduced. Also the optical efficiency is improved because only a single fibre may capture the emitted photons. This is achieved using an optical separator, such as a dichroic mirror. The dichroic mirror reflects the light from the electroluminescent element to the photo-electrical conversion element and transmits light from the injection optical source and from the control optical source. The dichroic mirror or dichroic beam splitter achieves near-complete separation of the transmitted and reflected optical signals. This further avoids crosstalk between optical signals. Alternatively, a conventional <NUM>/<NUM>-beamsplitter or a polarising beam splitter may be employed, which, however comes with a loss of optical signal in both transmission and reflection. Further alternatives for the optical separator may be to employ a diffraction grating or a Fresnel-like <NUM>-beamsplitter.

These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawings.

<FIG> shows a diagrammatic representation of an embodiment of the radio frequency (RF) antenna element comprising a detuning system of the invention wherein the electroluminescent element is formed by the switching element in the detuning circuit <NUM>.

The antenna element <NUM> is formed by the electrically conductive coil loop <NUM> in which a tuning capacitor <NUM> is circuited in series to render the electrically conductive loop resonant in the Larmor frequency band and sensitive to pick-up magnetic flux due to magnetic resonance. The detuning circuit is electrically coupled to the electrically conductive coil loop <NUM> and includes the switching element <NUM> with an inductance <NUM> in series with the switching element <NUM>. In order to close the switching element <NUM> optical radiation (e.g. light) from the injection optical source <NUM> is incident on the electroluminescent switching element <NUM>, e.g. a pin-diode via an optical link <NUM>. The switching operation is controlled by the control unit <NUM> that controls the injection optical source <NUM> to be switched on and off, or to control an interruption of the optical link <NUM>. When the electroluminescent switching element is made conductive (i.e. the switch is closed) the inductance <NUM> is in series with the tuning capacitance <NUM>, so that the resonance frequency of the electrically conductive loop <NUM> is shifted. Thus, by switching the injection optical source <NUM> on or off the electrically conductive loop's state is switched between resonant and non-resonant. Because the switching element is electroluminescent, it generates luminescent radiation (luminescence light) <NUM> which is detected by the photo-electrical conversion element <NUM> and converted into the electronic feedback signal. The feedback signal carries information on the state of the switching element. Variations in the feedback signal may relate to variations in the bias current due to temperature changes and may give insight into the thermal load. Additionally, the induced voltages of the RF transmit pulses will lead to voltage/ current variations at the diodes / switching elements. Depending on the actual implementation the variations will occur in different frequency ranges. There can e.g. appear a partial rectification of the RF signals by the PIN diode, which leads to currents in the low frequency (<<NUM>) range. The corresponding load variation of the photovoltaic power supply will thus cause a PEL signal. The forward resistance of PIN diodes also tends to vary with applied RF power, thus creating signals at higher harmonics of the applied frequency. These signals are as well indicative of the induced RF voltages. Altogether it might be advantages to measure PEL signals at the MR (Larmor) frequency or at multiples of the MR frequency in order to get insights into the induced RF signals. The induced RF signals themselves allow valuable conclusions about the actual RF transmit field strength at the location of the coil and/or the proper function of the RF coil (a too low signal might be caused by a problem in the transmit chain or by a broken receive coil). Since there are typically several coil channels a rough local characterization of the transmit field may be obtained.

<FIG> shows a diagrammatic representation of another embodiment of the radio frequency antenna element comprising a detuning system wherein the electroluminescent element is formed by the switching element <NUM> in the detuning circuit <NUM>. This embodiment is similar to that of <FIG>. In the embodiment of <FIG>, the optical paths <NUM>, <NUM> of the injection optical radiation and the electroluminescence from the electroluminescent element partially overlap. The injection optical light to the switching element and the electroluminescent light from the switching element are separated by an optical separator, e.g. a dichroic mirror <NUM>. Typically, for a single blue LED photodetector about <NUM>. 6V is sufficient to generate the electroluminescence. By employing several of these devices in series the total voltage may be increased. The generated luminescent radiation (luminescent light) <NUM> is detected by the photo-electrical conversion element <NUM> and converted into the electronic feedback signal.

Claim 1:
A radio frequency (RF) antenna element for magnetic resonance imaging (MRI),
- the RF antenna element comprising a detuning system (<NUM>) and a resonant electrically conductive loop (<NUM>) and
- the detuning system comprising an electroluminescent switching element (<NUM>) configured to detune the resonant electrically conductive loop, wherein
- the electroluminescent switching element is coupled to the resonant electrically conductive loop, wherein
- the detuning system includes a photo-electric conversion element (<NUM>) that is optically coupled to the electroluminescent switching element and configured to detect an electro-luminescent signal from the electroluminescent switching element.