Magnetic resonance examination system and magnetic resonance examination method

Abstract

Magnetic resonance examination system (1) comprising: a main body (2) comprising a transmitting coil system (3); an optical receiver (4) for receiving an optical freespace signal; a local coil device (6) comprising: a detection RF coil (7) for detecting an MR signal, a signal processing unit (8) configured to receive the MR signal, which is analog, and to transmit a corresponding signal, which comprises the MR signal in an analog form, to an optical emitter (9), the optical emitter (9), which is configured for transmitting the corresponding signal by an optical freespace link (5) to the optical receiver (4). Further a corresponding method is described.

Description

[0001] Magnetic resonance examination system and magnetic resonance examination method

[0002] The invention concerns a magnetic resonance examination system and a magnetic resonance examination method.

[0003] A detection radiofrequency (RF) coil as part of a local coil is an essential part of the magnetic resonance (MR) measurement system. It is also highly interactive, as it is handled by the operator and placed directly on the patient, and it plays a major role in defining the achievable image quality. Detection- only coils usually need to be detuned during the excitation pulse. Multi-channel RF coils combine the high SNR of small coils with the possibility to shorten the examination time using parallel imaging methods. Mechanically flexible coils that can be form-fitted to the patient's body provide additional means of SNR improvement .

[0004] Traditionally, the signal from the detection RF coil is transmitted to an MR scanner via coaxial cable.

[0005] The development of flexible multichannel detectors is increasingly hitting its limits in terms of scalability, handling and safety when using conventional coaxial cables for data transmission. Coaxial cables of RF coils are associated with a multitude of safety issues in MR examinations, and negatively impact patient comfort. They also limit the development of RF coils with a very high number of detection channels and of coils that exhibit very high mechanical flexibility, both of which can greatly increase the achievable MR image quality. The complexity of the cabling increases dramatically with the number of detector channels and the flexibility of the coils is severely limited by large cable strands.

[0006] Further, transmission of the signal detected by the RF coil via optical fiber is an option, such as shown in Demir, Taner, Ozgur Yilmaz, und Ergin Atalar, „OPTICAL TRANSMISSION SYSTEM for HIGH FIELD SYSTEMS", in Turk Manyetik Rezonans Dernegi 16, Yillik Top- lantisi; Istanbul, TUR, 2011; and in Memis, O.G., Eryaman, Y., Aytur, 0. and Atalar, E. (2008) , "Miniaturized fiber-optic transmission system for MRI signals", Magn. Reson. Med., 59: 165-173, https: / / doi.org / 10.1002 / mrm.21462. However, this still limits patient comfort and safety and necessitates cable management (e.g., plugging, routing) .

[0007] To solve these problems, wireless transmission of the MR signal has been proposed. The development of wireless MRI RF coils is technically challenging. An MRI image is calculated from a signal that is induced in an RF coil that is best-case located as close as possible to the object being examined. This signal has a high carrier frequency (e.g., 128 MHz for 3 T) and a high dynamic range (approx. 90 dB) . In addition, the radio-frequency coil is located in the MRI device, where high electromagnetic fields prevail (e.g., 3 T static magnetic field and gradient fields up to 80 mT / m switched at kHz rate) . As most standard components are not compatible with these conditions, dedicated components must be developed, manufactured and tested. In addition, care must be taken to ensure that the image quality is not impaired by electromagnetic interference .

[0008] Previous work on wireless RF coils (summarized in Nohava L, Ginefri J-C, Willoquet G, Laistler E and Frass-Kriegl R (2020) "Perspectives in Wireless Radio Frequency Coil Development for Magnetic Resonance Imaging", Front. Phys. 8:11. doi: 10.3389 / fphy .2020.00011) has focused primarily on radio and Wifi frequencies (GHz range) for data transmission. The signal is usually processed (i.e., decimated and digitized) directly at the local coil, i.e., before transmission to the signal processing unit of the MR scanner .

[0009] To overcome the problems that RF wireless signal transmission is prone to electromagnetic interference (EMI) in MR scanners causing imaging artifacts and that RF wireless signal transmission exhibits limitations regarding bandwidth and power consumption, an optical freespace approach was proposed, see e.g., US 7,173,426 Bl; DE 102007056223 Al; and Van Helvoort MJAM, Van Den Brink, JS, "Light data communication link device for use in magnetic resonance examination systems". In these documents, the coil signal is digitized prior to the optical wireless transfer.

[0010] Traditionally, the analog signal from the RF coil is transferred from the RF coil to the MR scanner via coaxial cables . Therefore , the MR scanner' s receiver electronics are designed to process analog signals . Therefore , digital signals from the wireless transmission have to be back-converted to the analog regime or processed by other means ("of fline" , i . e . , not directly by the conventional MR scanner ) . Further, digiti zation of the signal on the local coil requires more electronic components on the coil , which poses stringent requirements regarding MR compatibility, increases power requirements as well as si ze and weight of the on-coil circuitry .

[0011] It is an obj ective of the present invention to alleviate or overcome one or more of the problems of the prior art . In particular, it is an obj ective of the present invention to provide a magnetic resonance examination system and method with a local coil device with reduced power consumption, reduced interference with the MR electro-magnetic fields , reduced weight and / or reduced si ze . Preferably, no changes are required to the receive architecture of a classical cable-based MR scanner (" system transparency" ) .

[0012] This is solved by a magnetic resonance examination system comprising : a main body comprising a transmitting coil system; an optical receiver for receiving an optical freespace signal , wherein optionally the optical receiver is mounted on the main body or on an external holder ( in particular outside a bore of the main body) ; a local coil device comprising : a detection RF coil for detecting an MR signal , a signal processing unit configured to receive the MR signal , which is analog, and to transmit a corresponding signal , which comprises the MR signal in an analog form and which in particular is at least partially analog, to an optical emitter, the optical emitter, which is configured for transmitting the corresponding signal (which comprises the MR signal in an analog form and which in particular is at least partially analog) by an optical freespace link to the optical receiver .

[0013] This is further achieved by a magnetic resonance examination method comprising the steps : detecting an MR signal with a detection RF coil comprised by a local coil device; processing the MR signal, which is analog, and transmitting a corresponding signal, which comprises the MR signal in an analog form and which in particular is at least partially analog, to an optical emitter comprised by the local coil device; transmitting the corresponding signal (which comprises the MR signal in an analog form and which in particular is at least partially analog) by an optical freespace link to an optical receiver (e.g., mounted on a main body, wherein the main body comprises a transmitting coil system, or on an external holder) .

[0014] As conventionally, the transmitting coil system of the main body may produce excitation fields. The main body may be stationary. The detection RF coil is provided on the mobile / portable local coil device. The detection RF coil measures the response to the excitation fields. The resultant MR signal is analog. It is processed in the signal processing unit and the corresponding signal, which comprises the MR signal in an analog form, is transmitted to the optical emitter. The optical emitter transmits the corresponding signal (which comprises the MR signal in an analog form) by the optical freespace link to the optical receiver (e.g., provided mounted on the main body (i.e., in-bore) or mounted on the external holder (separate from the main body, i.e., outside the bore) ) . As mentioned, the optical receiver can be mounted outside the main body, which relaxes some requirements for the actuators for positioning the receivers. Subsequently, this corresponding signal may be further processed and received by a (in particular conventional) receiver of an MR scanner.

[0015] I.e., the cables between an MRI detector coil (RF coil) , which can be placed close to or directly on a patient, and the receive electronics of a (e.g., conventional) MR scanner are replaced by an optical wireless (at least partially) analog link. The local coil device may exploit the high sensitivity of (a) close-fitting, flexible coil (s) and potentially the high parallel imaging capabilities of multi-channel coils. By replacing coaxial cable connections by an analog, optical wireless link, the coil can be placed on the patient more freely and handling of the coil is simplified, as there is no need for cable management. Wireless signal transfer makes coaxial cables obsolete. Even if some control lines could still be required, they would have greatly relaxed requirements regarding signal quality . ( These control lines could be replaced by the wireless back-channel described below . ) The optical range of the electromagnetic spectrum ( THz ) provides practically unlimited bandwidth and immunity to electromagnetic interference (EMI ) . Optical links can be reali zed with low power consumption and low space requirements , and can be designed to obey eye safety limits . EMI could be further reduced by using an integrated optical receiver for the optical wireless link . At least partially analog signal transfer ensures " system transparency" , i . e . , it does not require any changes on the MR scanner' s receive architecture . Local coil devices with analog optical wireless signal transmission can be used on conventional MR scanners making use of their signal processing and image reconstruction pipelines . Further, the amount and power requirements of on-coil components can be greatly reduced by transmitting an ( at least partially) analog signal . This enhances MR compatibility and reduces the probability of image arti facts due to additional on-coil components .

[0016] The local coil device may be rigid or mechanically flexible . It may comprise single or multiple detection channels .

[0017] The optical receiver is in particular separate from the local coil device . Optionally, the optical receiver is connected to an MR scanner for processing the MR signal . I f the optical receiver is mounted on the external holder, it is preferable i f the optical receiver is inside the same MR scanner room as the main body . In comparison to mounting the optical receiver on the main body, a separate mount outside the MR scanner bore , on the one hand relaxes constraints in terms of MR compatibility and electromagnetic interference for both, the optical receiver and the alignment system mentioned below (with an extended choice of actuators ) . On the other hand, a longer optical freespace distance may imply a larger required movement range for the beam alignment system, and avoiding losses along the longer optical freespace path may become more di f ficult .

[0018] Under the corresponding signal compri sing the MR signal in an analog form it is in particular understood that the corresponding signal comprises the information of the detected MR signal in an analog form. The signal processing unit may for example amplify, compress and / or multiplex the MR signal to produce the corresponding signal. E.g., the analog MR signal may be connected to or modulated / multiplexed onto a digital carrier signal.

[0019] Optionally, the optical emitter may comprise a laser light source, in particular a modulatable laser. It is advantageous if the laser can be modulated fast enough. Thus, optionally, the modulatable laser light source has a modulation bandwidth of more than 10 MHz, preferably more than 100 MHz, more preferably more than 130 MHz, even more preferably more than 500 MHz. Optionally, the laser light source is a vertical cavity surface emitting laser (VCSEL) . Optionally, the laser light source is single-mode. Optionally, the optical emitter, in particular the laser light source, is configured for producing a linearly modulatable optical output power (e.g., by direct modulation of the driving current or by external modulators) . Optionally, the optical freespace link uses light waves in the infrared and / or visible spectrum. In particular the optical freespace link uses electromagnetic waves with a frequency of more than 1 THz. Optionally, the processing unit is linear. Optionally, the processing unit is sufficiently linear such that it (in particular its components) does not reach compression, in particular under the strongest possible input signals depending on the detection RE coil.

[0020] Optionally, the optical receiver has a (electrical) bandwidth of more than 10 MHz, preferably more than 100 MHz, more preferably more than 130 MHz, even more preferably more than 500 MHz. The optical receiver is in particular mounted on the main body in-bore or on an external holder. The optical receiver may in particular be mounted on a platform, which is mounted on the main body or on the external holder. Optionally, the optical receiver is an analog receiver. Optionally, the optical receiver is linear. Optionally, the optical receiver is sufficiently linear such that it (in particular its components) does not reach compression, in particular under the strongest possible input signals depending on the detection RE coil. Optionally, the optical receiver comprises an integrated photodetector, e.g., an integrated silicon photodetector. Optionally, the main body comprises a bore. Optionally, the optical receiver is mounted within a top section of the bore of the main body. Optionally, the optical receiver comprises: an optical detector and / or a linear transimpedance amplifier (TIA) for converting the photocurrent of the photodiode to a voltage and / or an output driver. The optical receiver may be implemented using discrete components or may be integrated in an application-specific integrated circuit (ASIC) (which minimizes EMI) . Optionally, the optical detector of the optical receiver (in particular the photodiode) is connected to a transimpedance amplifier (TIA) , which is preferably fast and / or linear and / or has a (electrical) bandwidth of more than 10 MHz, preferably more than 100 MHz, more preferably more than 130 MHz, even more preferably more than 500 MHz. Optionally, the TIA is sufficiently linear such that it (in particular its components) does not reach compression, in particular under the strongest possible input signals depending on the detection RE coil.

[0021] Optionally, the main body comprises a magnet, gradient (transmit) coils and / or an RE body (transmit) coil. Optionally, the main body comprises (in particular conventional) receive electronics connected to the optical receiver, which in particular restore signal properties .

[0022] Optionally, the local coil device comprises a non-magnetic battery for power supply. Also, technologies for wireless power supply (e.g., energy harvesting, inductive or optical power transfer) are possible. Advantageously, the present system can be implemented with low power requirements.

[0023] For a high signal-to-noise ratio (SNR) , it is preferable that a high optical power from the optical emitter is detected at the optical receiver. Thus, it is preferable if the optical emitter has an emission angle of less than 10°, preferably less than 5°, more preferably less than 2°, even more preferably less than 1° or less than 0.2°. Optionally, the (widened) beam is focused onto the optical detector so that a large fraction (in particular more than 50%, preferably more than 80%, in terms of radiant flux / power) of the emitted light is received. Broad beam optical wireless links and selecting one of multiple receivers, as used for digital data transfer, are not as suitable for analog data transfer. A high-quality signal from an analog optical wireless trans fer of MR signals is achieved with a maximum SNR and minimum distortion ( e . g . , compression) . For a high SNR, the maximum optical power from the optical emitter should be detected at the optical receiver . Increasing the si ze of the optical detector of the optical receiver is associated with an SNR and bandwidth penalty . Therefore , an alignment of the optical receiver with respect to the optical emitter is preferable . Advantageously, the optical receiver is movably mounted on the main body or on the external holder . E . g . , the optical receiver could be mounted on a platform movably mounted to the main body or on the external holder . Thus , the optical receiver can be moved and its optical detector centered on the beam from the optical emitter . This could for example be done manually . Static optical set-ups based on lenses are not as capable in maintaining the link performance under motion . Preferably, the optical receiver is movable in two or three dimensions and / or in a plane orthogonal to a beam from the optical emitter and / or in a direction substantially parallel to a beam from the optical emitter .

[0024] Optionally, the system comprises an actuator for moving the optical receiver relative to the main body or the external holder in at least one direction . Preferably, the actuator is configured for moving the optical receiver relative to the main body or the external holder in at least two non-parallel directions . In this way, an automatic alignment is possible . E . g . , the actuator could comprise a piezo-electric linear XY-stage . Preferably, it has a travel range of several centimeters and a high movement speed .

[0025] With analog optical wireless signal trans fer, the precise alignment of optical emitter and receiver is essential for preserving signal quality . Therefore , optionally, the system comprises a beam position detection system, which is preferably movably mounted on the main body or the external holder together with the optical receiver . The beam position detection system is configured to detect a position of a beam emitted by the optical emitter ( in particular relative to the optical receiver ) . This allows a precise , dynamic alignment of the optical receiver relative to the optical emitter . Optionally, the actuator is configured to move the optical receiver to center a beam from the optical freespace link on the optical detector of the optical receiver based upon the detection of the beam position by the beam position detection system . The beam position detection system also allows to adj ust for patient motion in in-vivo MRI . Optionally, there can be provided for a segmented photodetector, wherein a center photodiode serves as the optical detector for the receiver and the other photodiodes ( i . e . , the additional segments , which can be arranged as satellite diodes ) are part of the beam position detection system . It is also possible that there is provided for a segmented photodetector, wherein at least some of the photodetector segments that are used for beam alignment are also used as detectors for the optical receiver ( i . e . , for detecting the MR signal which is subsequently transmitted via the freespace optical link) .

[0026] By placing the optical receiver for the optical wireless link on a moveable platform driven by an MRI-compatible actuator with real-time feedback from beam position detectors and a control algorithm, stability of the analog optical wireless link is provided . As an alternative to moving the optical receiver, the beam can also be aligned by moving the optical emitter, e . g . , by using an MRI-compatible actuator such as a micro-electromechanical system (MEMS ) mirror . However, providing for an alignment on the side of the optical receiver allows to minimi ze the components necessary on the local coil device . Also , such an automatic alignment system greatly increases usability as it shortens the setup time before the patient measurement . Optionally, the beam position detection system is mounted on the moveable platform (which also holds the optical receiver ) .

[0027] Alternatively, or in addition to moving the optical receiver or the platform, the emission direction of the optical emitter can be steered to align with the optical receiver . In particular, there can be provided for an actuator for steering the beam emittable by the optical emitter to align with the optical receiver, wherein the actuator optionally comprises a micro-elec- tromechanical system (MEMS ) mirror and / or a piezo-electric actuator .

[0028] Optionally, the beam position detection system comprises at least two photodetectors arranged at least partially surrounding the optical detector of the optical receiver . Optionally, the at least two photodetectors each cover a di f ferent area outside of the optical detector of the optical receiver . By determining the signal strength (beam intensity) on these at least two photodetectors , the optical detector of the optical receiver can be centered with regard to the beam from the optical emitter . Optionally, the beam position detection system comprises four photodetectors , wherein each one of the four photodetectors is located respectively in one of four quadrants , with the optical detector of the optical receiver in the center of the quadrants . Optionally, at least one further photodetector is provided spaced further apart from the optical receiver than the at least two photodetectors . The at least one further photodetector can be used for initial alignment . Optionally, this photodetector is larger than the at least two photodetectors . As soon as the beam from the optical emitter hits the further photodetector, the beam or the receiver can be moved to roughly hit the at least two photodetectors , and their signal will be used for fine alignment . By adj usting the relative position of the beam position detection system together with the optical receiver, according to feedback from the at least two photodetectors in terms of their individual photocurrents , the beam can be centered in a controlled manner, in particular almost perfectly on the optical detector of the optical receiver ( in particular with a closed-loop control ) . A microcontroller may be used to detect and process the signals from the beam position detection system ( in particular from the at least two photodetectors ) and control the moveable platform to align the optical detector of the optical receiver with the beam spot .

[0029] Optionally, the system comprises a back-channel optical emitter for emitting an optical freespace control signal , which back- channel optical emitter is mounted on the main body or the external holder, wherein the local coil device further comprises a back-channel optical receiver for receiving the optical freespace control signal. The back-channel allows to transmit control signals from the MR scanner to the local coil device and / or allows synchronization. E.g., the local coil device, in particular the detection RE coil, can be controlled to switch between a transmit and receive mode (in particular by the MR scanner) . The MR scanner runs the acquisition sequence which includes alternating periods of signal excitation and signal reception. Obeying the stringent timing (tens of ps) requirements set by this MR sequence, the detection coil can be deactivated during excitation periods to maintain patient safety and protect sensitive receive electronics. Since the back-channel is also wireless, it does not inhibit the portability of the local coil device. Optionally, this back-channel can also be used to transfer reference signals to the local coil device, e.g., for on- coil processing steps of the MR signal. The requirements on signal quality are greatly relaxed for the back-channel; therefore, a broader beam angle can be used and no precise alignment system is needed. The back-channel optical emitter has an emitting angle of preferably more than 10°, even more preferably more than 20°, or even more preferably more than 30°. Optionally, the signal processing unit receives the control signal and is configured to switch the detection RE coil between an active state and a deactivated state based upon the control signal. The back- channel optical emitter and the optical receiver may be mounted on the same one or on different ones of: the main body and the external holder.

[0030] Optionally, the signal processing unit (of the local coil device) is further configured to transmit a predefined reference signal to an optical emitter and the optical emitter is further configured for transmitting the reference signal by the optical freespace link to the optical receiver. Optionally, the predefined reference signal is analog, e.g., a sinewave signal. When using an optical wireless link to transfer an analog signal from a moving patient to the receiver, the freespace distance of the signal link undergoes changes. Therefore, a reference signal that is preferably constantly transmitted with the corresponding signal is helpful for calibration purposes. The attenuation that the reference signal experiences on the transmission path via the freespace link can be measured and one or more of the received reference signal's parameters (e.g., frequency and / or amplitude) can be compared to the emitted (i.e., original) reference signal and used to correct other signals transmitted via the optical freespace link, e.g., the corresponding signal. The reference signal can be used to correct for distortion or compression of the corresponding signal transferred via the freespace optical link. Thus, signal quality is improved. Optionally, the reference signal has a predefined amplitude and / or frequency, e.g., a sinusoidal signal. Optionally, the reference signal is multiplexed on the corresponding signal by the signal processing unit. Optionally, the frequency of the reference signal is adjustable (e.g., by the signal processing unit) . Thus, also the frequency response of the transmission path can be corrected (equalizing) . Optionally, the reference signal is only transmitted once (e.g., initially at the start of a measurement with the detection RF coil) . Optionally, the reference signal is transmitted repeatedly, in particular continuously with the corresponding signal (i.e., continuously while the corresponding signal is also transmitted) . In this way, also fluctuations in the optical emitter's power, movements of the patient, temperature drifts and similar effects can be compensated. The correction of the data signals can either be implemented directly online in the optical receiver or by means of post-processing after the recording.

[0031] Optionally, a time-of-f light measurement of a signal transmitted via the freespace optical link (from the optical emitter to the optical receiver) can be conducted. Optionally, the system comprises a time-of-f light measurement unit for measuring a time- of-flight of a signal transmitted via the freespace optical link (from the optical emitter to the optical receiver) . In this way, a length of the transmission path can be determined. Thus, runtime changes and phase variations may be corrected for.

[0032] Alternatively, or in addition, a predefined reference signal (e.g., as described above) can also be transmitted from the back-channel optical emitter to the back-channel optical receiver . Optionally, the system is configured to transmit multiple coil channels . Thus , optionally, the local coil device comprises a detection RF coil array for detecting a plurality of MR signals , wherein the detection RF coil array includes the detection RF coil , wherein the signal processing unit is configured to receive the plurality of MR signals , which are analog, and to multiplex the plurality of MR signals to produce a multiplexed signal , which comprises the plurality of MR signals in an analog form and which in particular is at least partially analog, and to transmit the multiplexed signal to the optical emitter ; wherein the optical emitter is configured for transmitting the multiplexed signal (which comprises the plurality of MR signals in an analog form and which in particular is at least partially analog) by the optical freespace link to the optical receiver . In particular, the multiplexed signal comprises the corresponding signal or is equivalent to the corresponding signal . In particular, the detection RF coil array comprises a plurality of detection RF coils ( including the detection RF coil mentioned previously) .

[0033] Optionally, each of the plurality of MR signals is associated with a channel , and for each channel the local coil device comprises a tuning and / or matching and / or deactivation circuit and / or a low-noise preampli fier .

[0034] Returning to the method, it is preferable i f the method further comprises the steps : detecting a position of a beam of the optical freespace link over which the corresponding signal is transmitted; moving the optical receiver according to the detected position of the beam such that a signal strength of the corresponding signal transmitted over the optical freespace link increases , in particular such that the beam is centered on the optical detector of the optical receiver . In particular, the position of the beam is detected with the beam position detection system . In particular, the beam position detection system is moved together with the optical receiver ( e . g . , by being mounted on the movable platform, which is moved by the actuator ) . Optionally, the method further comprises the steps : emitting an optical freespace control signal with a back- channel optical emitter mounted on the main body or the external holder ; receiving the control signal by an optical back-channel receiver comprised by the local coil device ; switching the detection RF coil between an active state and a deactivated state based upon the control signal ( in particular, detuning circuits of the detection RF coil or coil array are switched, in particular during the MR excitation pulses ) . In particular, the detection RF coil is deactivated when the transmitting coil of the main body transmits an excitation signal . Further, a frequency reference can be provided via the back- channel , e . g . , for mixing and / or multiplexing . E . g . , two di f ferent wavelengths can be used for the detuning signal and the frequency reference signal . Further, via the back-channel , signals can be trans ferred to control modulation settings ( e . g . , carrier spacing and / or codes ) .

[0035] Optionally, the method further comprises the steps : transmitting a predefined reference signal by the optical emitter via the optical freespace link to the optical receiver ; correcting the corresponding signal received by the optical receiver based upon the predefined reference signal received by the optical receiver . Optionally, the predefined reference signal is transmitted via the optical freespace link multiplexed on the corresponding signal .

[0036] Optionally, the method further comprises the steps : detecting at least one further MR signal by a detection RF coil array comprised by the local coil device , wherein the detection RF coil array includes the detection RF coil ; wherein processing the MR signal and transmitting a corresponding signal comprises : processing the at least one further MR signal , wherein each one of the at least one further MR signal is analog, multiplexing the MR signal and the at least one further MR signal to produce a multiplexed signal , which comprises the MR signal and the at least one further MR signal in an analog form, transmitting the multiplexed signal to the optical emitter ; wherein transmitting the corresponding signal by the optical freespace link comprises : transmitting the multiplexed signal by the optical freespace link from the optical emitter to the optical receiver .

[0037] Optical signal transmission is ideally suited for various multiplexing schemes due to the high available optical and electrical bandwidth . The multiplexed signal is transmitted via a single optical wireless signal link . I f a reference signal is transmitted, the reference signal is optionally multiplexed together with the MR signal and the at least one further MR signal . The MR signal and the at least one further MR signal correspond to the plurality of MR signals mentioned above . Optionally, the multiplexed signal received by the optical receiver is demultiplexed .

[0038] Optionally, in multiplexing the MR signal and the at least one further MR signal to produce a multiplexed signal , the MR signal and the at least one further MR signal are multiplexed by frequency division multiple access , time division multiple access , wavelength division multiple access and / or code division multiple access , in particular by hybrid code division multiple access ( CDMA) . Hybrid CDMA ( i . e . , mixing the analog MR signals with digital codes ) provides advantages regarding the scalability to higher numbers of coil detection channels while facilitating a simple architecture with low component count .

[0039] Optionally, the corresponding signal received by the optical receiver, or the multiplexed signal received by the optical receiver after demultiplexing, is trans formed to match the signal properties ( e . g . , the center frequency) expected by the MR scanner' s receive architecture . For this purpose , e . g . , frequency mixing can be conducted . Alternatively, the receive architecture can be adapted to process the corresponding signal or demultiplexed multiplexed signal directly .

[0040] Optionally, the magnetic resonance examination method is used in a magnetic resonance imaging method . Optionally, the magnetic resonance examination system is configured to conduct the magnetic resonance examination method.

[0041] By way of example, the invention is further explained with respect to some selected embodiments shown in the drawings. However, these embodiments shall not be considered limiting for the invention .

[0042] Fig. 1 schematically shows a preferred embodiment of a magnetic resonance examination system.

[0043] Fig. 2 schematically shows some components of the embodiment of Fig. 1 in more detail.

[0044] Fig. 3a schematically shows an example of an optical receiver and a beam position detection system for the embodiment of Fig. 1.

[0045] Fig. 3b schematically shows another example of an optical receiver and a beam position detection system for the embodiment of Fig . 1.

[0046] Fig. 4 schematically shows an example of an alignment system for the embodiment of Fig. 1.

[0047] Fig. 5 schematically shows a first example of a freespace optical link with multiplexing for the embodiment of Fig. 1.

[0048] Fig. 5 schematically shows a second example of a freespace optical link with multiplexing for the embodiment of Fig. 1.

[0049] Fig. 1 schematically shows a preferred embodiment of a magnetic resonance examination system 1. The system 1 comprises a main body 2 with a transmitting coil system 3, a magnet 3a and gradient coils 3b. The transmitting coil system comprises an RF body coil 3c. The main body 2 further comprises a bore 16 and a patient bed or object holder 17 within the bore 16. However, the invention can equally be applied to open MR examination / imaging systems. A patient or object 18 to be imaged (e.g., a sam- ple / phantom) may be positioned on the patient bed or object holder 17 . An optical receiver 4 for receiving an optical freespace signal is mounted to the main body 2 . In an alternative embodiment not shown, the optical receiver 4 is mounted on an external holder . The optical receiver 4 is connected to an interface plug 19 , which in turn is connected to ( conventional ) MR receive electronics 20 , which can process detected MR signals .

[0050] The system 1 also comprises a local coil device 6 (which is in particular portable ) , which can be placed near or on the patient or obj ect 18 to be imaged to detect RF signals in response to MR excitation signals generated by the transmitting coil system 3 . The local coil device 6 comprises a detection RF coil 7 for detecting an MR (RF) signal , a signal processing unit 8 configured to receive the MR signal , which is analog, and to transmit a corresponding signal , which comprises the MR signal in an analog form and is at least partially analog, to an optical emitter 9 , and the optical emitter 9 , which is configured for transmitting the corresponding signal by an optical freespace link 5 to the optical receiver 4 .

[0051] As conventionally, the transmitting coil system 3 on the main body 2 may produce excitation fields . The detection RF coil 7 measures the response to the excitation fields . The resultant MR signal is analog . It is processed in the signal processing unit 8 and a corresponding signal , which comprises the MR signal in an analog form, is transmitted to the optical emitter 9 . The optical emitter 9 transmits the ( at least partially analog) corresponding signal by the optical freespace link 5 to the optical receiver 4 provided mounted on the main body 2 ( i . e . , in-bore ) . Subsequently, this corresponding signal may be forwarded via the interface plug 19 to the receiver 20 of the MR scanner, where it is further processed .

[0052] By using a wireless link, the local coil device 7 can be moved freely . The optical range of the electromagnetic spectrum ( THz ) provides practically unlimited bandwidth and immunity to electromagnetic interference (EMI ) . Optical links can be reali zed with low power consumption and low space requirements , and can obey eye safety limits . Further, the amount and power requirements of components on the local coil device 6 can be greatly reduced by transmitting the ( at least partly / partially ) analog corresponding signal . This enhances MR compatibility and reduces the probabi lity of image arti facts due to additional on-coil components .

[0053] Mounted on the main body 2 is further a back-channel optical emitter 13 for emitting an optical freespace control signal . The local coil device 6 further comprises a back-channel optical receiver 14 for receiving the optical freespace control signal . The control signal can for example be used for synchroni zation or detuning the detection RF coil 7 . To ensure a high SNR ratio , the optical emitter 5 emits a beam with a narrow emitting angle . On the other hand, requirements on the optical freespace control signal are much lower, such that the back-channel optical emitter 13 can emit with a broader emitting angle .

[0054] Fig . 2 schematically shows some details of the system 1 of Fig . 1 in more detail , in particular the components in the MR signal path from the detection RF coil 7 to the MR receive electronics 20 . The signal processing unit 8 comprises a matching / deactiva- tion circuit M / D, a low noise ampli fier LNA and a band-pass filter . The matching / deactivation circuit M / D is connected to the detection RF coil 7 , to receive the MR signal from the detection RF coil 7 and to match and detune the detection RF coil 7 when necessary . The MR signal is forwarded from the matching / deactivation circuit to the low noise ampli fier LNA, where it is ampli fied, and to the band-pass filter, where it is filtered . The resulting ( i . e . , corresponding) signal is forwarded to the optical emitter 9 , which transmits the corresponding signal via the optical freespace link 5 to the optical receiver 4 . In this embodiment , the corresponding signal is ( fully) analog .

[0055] As discussed earlier, the optical emitter 9 preferably emits with a narrow emitting angle . To center the beam of the optical freespace link 5 on the optical receiver 4 , the optical receiver 4 and a beam position detection system 11 are mounted on a platform, which is movable with respect to the main body 2 . There is provided for an actuator 10 for driving the platform in two dimensions . The beam position detection system 11 comprises four photodetectors , which are arranged surrounding the optical detector of the optical receiver 4 . Accordingly, the position of the platform and therewith the optical receiver 4 is adj usted according to feedback from the four photodetectors in terms of their individual photocurrents . Thus , the beam of the optical freespace link 5 can substantially be centered in a controlled manner on the optical detector of the optical receiver 4 . A microcontroller 21 is provided which provides closed-loop feedback . The corresponding signal is forwarded to the interface plug 19 and via a receive chain to the receive electronics 20 .

[0056] Further, as described above , the back-channel optical emitter 13 transmits an optical freespace control signal to the back-channel optical receiver 14 , which is forwarded to the matching and detuning unit M / D .

[0057] The local coil device 6 further comprises a non-magnetic battery 22 for power supply .

[0058] In an alternative embodiment , shown smaller in the background in Fig . 2 , the local coil comprises a detection RF coil array 15 (with a plurality of detection RF coils 7 including the one mentioned above ) for detecting a plurality of MR signals . The plurality of analog MR signals is ( in particular after the match- ing / detuning unit M / D and the low noise ampli fier LNA) forwarded to a multiplexing unit MUX comprised by the signal processing unit 8 and a multiplexed signal is produced (which is equivalent to the corresponding signal ) . In this embodiment , the multiplexed signal is at least partially analog . ( The multiplexed signal comprises the MR signals or signals corresponding to the plurality of MR signals in analog form, but could be mixed with digital code signals in case of hybrid CDMA. With other multiplexing methods ( e . g . , FDMA) , also the multiplexed signal is fully analog . ) The multiplexed signal is then transmitted by the optical emitter 9 via the optical freespace link 5 to the optical receiver 4 , and is subsequently demultiplexed at the demultiplexing unit DEMUX .

[0059] Fig . 3a shows an example of the beam position detection system 11 and the optical receiver 4 , consisting of its optical detector, a highspeed transimpedance ampli fier HS TIA and a ( e . g . , 50 Ohm) driver, in more detail . As can be seen, the beam position detection system comprises four photodetectors 12 which are arranged around the optical detector of the receiver 4. These are each connected via transimpedance amplifiers TIAs to the microcontroller 21 (see Fig. 2) for beam steering. The optical detector of the optical receiver 4 is connected via a highspeed transimpedance amplifier HS TIA and a (e.g., 50 Ohm) driver to the interface plug (see Fig. 2) .

[0060] Fig. 3b schematically shows another example of an optical receiver 4 and a beam position detection system 11 for the embodiment of Fig. 1. It works essentially the same as the embodiment of Fig. 3a. However, in this embodiment, the four photodetectors 12 also function as the optical detectors for the optical receiver 4. Thus, the same photodiodes are used for beam alignment as for data transmission. Each of the four photodetectors 4 is connected via highspeed transimpedance amplifiers HS TIA on the one hand via a respective buffer to the microcontroller 21 (see Fig. 2) for beam steering and on the other hand to the same summation amplifier, which in turn is connected via the (e.g., 50 Ohm) driver to the interface plug (see Fig. 2) .

[0061] Fig. 4 shows another example for the beam position detection system 11, on the left (under (a) ) in a perspective view and on the right (under (b) ) in a top view. In this embodiment, it comprises four additional (discrete) photodetectors 12b, which are provided spaced apart further from the optical detector of the optical receiver than the four photodetectors 12a. The four additional photodetectors 12b are used for initial alignment. As soon as the beam hits one of the four inner photodetectors 12a, these will be used for fine alignment. Also, the focusing lenses 23 are shown, which focus the beam of the optical freespace link 5. Further, on the right (under (b) ) a potential beam path 24 of the beam of the optical freespace link 5 on the platform as the platform is moved is shown.

[0062] Fig. 5 schematically shows a first example of the freespace optical link 5 with multiplexing, which could for example be used in the embodiment of Figs. 1 and 2. In this embodiment, the MR signals from the detection RF coil array 15 are multiplexed by analog FDMA. Therein, narrowband coil signals are orthogonalized in frequency and thus transmitted over di f ferent carriers .

[0063] Fig . 6 schematically shows a second example of the freespace optical link 5 with multiplexing, which could for example be used in the embodiment of Figs . 1 and 2 . In this embodiment , the MR signals from the detection RF coil array 15 are multiplexed by hybrid CDMA. Here , the detection RF coil signals are continuously multiplied, i . e . , modulated, by periodically repeated bipolar ( + 1 , - 1 ) code signals that are mutually orthogonal , before they are additively superimposed and sent over the optical freespace link 5 . At the synchroni zed optical receiver 4 , a matched filter bank, comprising of one matched filter for each local coil channel , recovers the individual local coil signals while suppressing the respectively undesired ones . It does so by multiplying the overall received signal with the respective code signals assigned to the coil channels and successively integrating and dumping over the period of the code signals . The method poses a scalable solution with respect to the number of coil channels that can be multiplexed while keeping the additional hardware requirements for multiplexing at the local coil device 6 low . More speci fically, multiplexing ever more local coil signals amounts to using a code set with greater cardinality . Furthermore , the modulation demands only a bipolar switching operation according to the code signal at the local coil device 6 side which is oftentimes already embodied in the design of modern active RF mixers .

Claims

22Claims1. Magnetic resonance examination system (1) comprising: a main body (2) comprising a transmitting coil system (3) ; an optical receiver (4) for receiving an optical freespace signal ; a local coil device (6) comprising: a detection RF coil (7) for detecting an MR signal, a signal processing unit (8) configured to receive the MR signal, which is analog, and to transmit a corresponding signal, which comprises the MR signal in an analog form, to an optical emitter (9) , the optical emitter (9) , which is configured for transmitting the corresponding signal by an optical freespace link (5) to the optical receiver (4) .

2. System (1) according to claim 1, wherein the optical emitter (9) has an emission angle of less than 10°, preferably less than 5°, more preferably less than 2°, even more preferably less than 1° or less than 0,2°.

3. System ( ) according to claim 1 or 2, wherein the optical receiver (4) is movably mounted on the main body (2) or on an external holder.

4. System (1) according to claim 3, comprising an actuator (10) for moving the optical receiver (4) relative to the main body (2) or to the external holder in at least one direction.

5. System (1) according to any one of claims 3 or 4, comprising a beam position detection system (11) , which is movably mounted on the main body (2) or on the external holder together with the optical receiver (4) .

6. System (1) according to claim 5, wherein the beam position detection system (11) comprises at least two photodetectors (12) arranged at least partially surrounding an optical detector of the optical receiver (4) .. System (1) according to any one of the previous claims, comprising a back-channel optical emitter (13) for emitting an optical freespace control signal, which back-channel optical emitter (13) is mounted on the main body (2) or on an external holder, wherein the local coil device (6) further comprises a back-channel optical receiver (14) for receiving the optical freespace control signal.

8. System (1) according to any one of the previous claims, wherein the signal processing unit (8) is further configured to transmit a predefined reference signal to the optical emitter (9) and the optical emitter (9) is further configured for transmitting the reference signal by the optical freespace link (5) to the optical receiver (4) .

9. System (1) according to any one of the previous claims, wherein the local coil device (6) comprises a detection RF coil array (15) for detecting a plurality of MR signals, wherein the detection RF coil array (15) includes the detection RF coil (7) , wherein the signal processing unit (8) is configured to receive the plurality of MR signals and to multiplex the plurality of MR signals to produce a multiplexed signal, which comprises the plurality of MR signals in an analog form, and to transmit the multiplexed signal to the optical emitter (9) ; wherein the optical emitter (9) is configured for transmitting the multiplexed signal by the optical freespace link (5) to the optical receiver (4) .

10. Magnetic resonance examination method comprising the steps: detecting an MR signal with a detection RF coil (7) comprised by a local coil device (6) ; processing the MR signal, which is analog, and transmitting a corresponding signal, which comprises the MR signal in an analog form, to an optical emitter (9) comprised by the local coil device ( 6 ) ; transmitting the corresponding signal by an optical freespace link (5) to an optical receiver (4) .

11. Method according to claim 10, comprising the steps:detecting a position of a beam of the optical freespace link (5) over which the corresponding signal is transmitted; moving the optical receiver (4) according to the detected position of the beam such that a signal strength of the corresponding signal transmitted over the optical freespace link (5) increases, in particular such that the beam is centered on an optical detector of the optical receiver (4) .

12. Method according to any one of claims 10 or 11, comprising the steps: emitting an optical freespace control signal with a back- channel optical emitter (13) ; receiving the control signal by an optical back-channel receiver (14) comprised by the local coil device (6) ; switching the detection RF coil (7) between an active state and a deactivated state based upon the control signal.

13. Method according to any one of claims 10 to 12, comprising the steps: transmitting a predefined reference signal by the optical emitter (9) via the optical freespace link (5) to the optical receiver ( 4 ) ; correcting the corresponding signal received by the optical receiver (4) based upon the predefined reference signal received by the optical receiver (4) .

14. Method according to any one of claims 10 to 13, comprising the steps: detecting at least one further MR signal by a detection RF coil array (15) comprised by the local coil device (6) , wherein the detection RF coil array (15) includes the detection RF coil (7) ; wherein processing the MR signal and transmitting a corresponding signal comprises: processing the at least one further MR signal, wherein each one of the at least one further MR signal is analog, multiplexing the MR signal and the at least one further MR signal to produce a multiplexed signal, which comprises the MR signal and the at least one further MR signal in an analog form,25 transmitting the multiplexed signal to the optical emitter ( 9 ) ; wherein transmitting the corresponding signal by the optical freespace link (5) comprises: transmitting the multiplexed signal by the optical freespace link (5) from the optical emitter (9) to the optical receiver (4) .

15. Method according to claim 14, wherein in multiplexing the MR signal and the at least one further MR signal to produce a multiplexed signal the MR signal and the at least one further MR signal are multiplexed by frequency division multiple access, time division multiple access, wavelength division multiple access and / or code division multiple access, in particular by hybrid code division multiple access.

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