RF preamplifier for amplifying a signal of an RF antenna of a magnetic resonance imaging system

Abstract

An RF preamplifier (10) for amplifying a signal of an RF antenna (11) of a magnetic resonance imaging system (1), wherein the RF preamplifier (10) is an integrated transconductance amplifier with at least one folded cascode.

Description

[0001] RF preamplifier for amplifying a signal from an RF antenna of a magnetic resonance imaging system

[0002] The invention relates to an RF preamplifier for amplifying a signal from an RF antenna of a magnetic resonance imaging system.

[0003] Furthermore, the invention relates to a circuit for detecting a signal from an RF antenna of a magnetic resonance imaging system.

[0004] The invention also relates to a local coil for a magnetic resonance imaging system.

[0005] The invention further relates to a magnetic resonance imaging system comprising:

[0006] - a sample table for holding an object to be examined;

[0007] - at least one magnet for generating a magnetic field;

[0008] - at least one transmitting coil for generating an excitation signal;

[0009] - a data processing system for processing detected RF signals.

[0010] Magnetic resonance imaging (MRI), along with other non-invasive techniques such as computed tomography or ultrasound imaging, enables the elucidation of complex structures and the temporal sequences of a wide variety of processes in many areas of science, research, and medicine. For example, MRI examinations can detect diseases early and with high accuracy. The possibilities of imaging techniques, however, extend far beyond the diagnostic field in medicine and are used in a wide variety of fields, such as materials research to elucidate the structure of crystals or drug research with regard to elucidating the structure of molecules.Magnetic resonance imaging (MRI) is currently the most important non-invasive imaging technique because it provides particularly clear insights into the samples being examined thanks to its three-dimensional representation. As described, for example, in DE 10 2019 205 114 B4, magnetic resonance imaging (MRI) uses strong external magnetic fields to align the nuclear spins of an object under investigation. These spins are then excited to precession around this alignment by alternating electromagnetic fields (generated by so-called RF transmit coils / transmitters). The precession, or return, from this higher-energy, excited state to a lower-energy state in turn emits an alternating electromagnetic field, which can be received, for example, via radio frequency (RF) antennas.The magnetic resonance signals to be received are typically very weak and close to the noise threshold. Magnetic gradient fields are generated for spatial encoding of the detected signals, which retroactively allow volume element assignment. Image reconstruction is performed by evaluating (e.g., using Fourier transformation) the signals received from the RF antennas.

[0011] Because of the small measurement signals, it is all the more crucial to record and process the signals without interference. In this context, the signal-to-noise ratio (SNR) is crucial, on which the quality of an MR scan depends to a large extent. The SNR in turn depends on factors such as the geometry, homogeneity and field strength of the magnetic fields, the geometry and orientation of the antennas, the length and orientation of electrical signal lines, the excitation of the object under examination, and also significantly on the electronic components used to record and process the measurement signals. Signals are typically recorded by local coils (English: local coils" or "coil arrays"). The local coils usually have several antennas in which a voltage signal is induced due to the alternating field emitted by the object under examination.This (usually very low) voltage signal can be amplified, for example, by means of a low-noise amplifier (LNA) before the signal is further processed.

[0012] Preamplifiers in MRI systems, which typically consist of discrete components, take up a lot of space, particularly when multi-channel local coil arrays are used. This high space requirement is particularly disadvantageous in the already densely built-up area around the MRI tube and near the local coil antennas. Signal transmission within these systems is generally analog. The transmitted signals can be disrupted, for example, by the strong magnetic fields of the MRI system or by the excitation signals from the object being examined, which can lead to a further reduction in signal quality.

[0013] A further complication is that individual components of an MRI system (such as receiving or .

[0014] Transmitting coils are typically only designed for a specific frequency or a narrowband frequency range, which can be determined, for example, by a main magnetic field or properties of the sample to be examined. This means that MRI systems often have transmitting and receiving coils and amplifiers that are specifically designed for an individual system and a concrete application. MRI systems are typically set up to record signals from hydrogen atoms. Alternatively or additionally, signals from other atoms or nuclides can also be measured, which can drastically expand the possible applications of MRI systems. Exposed to the same magnetic field, other nuclides have different resonance frequencies than hydrogen. Therefore, the entire signal chain must be set up accordingly in order to be able to process alternative or additional frequencies.This significantly increases the already high demands placed on amplifiers. This often results in limited flexibility and sometimes necessitates the installation of multiple components in parallel, differing only in their adaptation to a specific frequency of the measurement signal. Given the limited space within an MRI system, installing such (parallel) components is difficult or impossible.

[0015] To date, attempts have been made to reduce the size of discrete amplifier components in order to take up less space within MRI systems. Furthermore, preamplifiers can be located outside the MRI tube, although this requires that unamplified signals be transmitted over relatively long electrical cables. Longer cables are typically more susceptible to external interference such as alternating fields and thus lead to a deteriorated SNR (due to impaired common-mode rejection). Furthermore, MRI systems often have up to 64 such preamplifiers, which is why the arrangement and installation of the preamplifiers presents considerable technical challenges.

[0016] In summary, preamplifiers for amplifying signals from an MRI system should take up little space, ensure a high SNR despite significant external magnetic fields and alternating fields exciting the object under examination, and be flexible for use with signals of different frequencies.

[0017] One solution is to implement preamplifiers as integrated circuits (ICs). A significant disadvantage of this approach is that small circuits are particularly susceptible to interference due to the Hall effect. Furthermore, the Hall voltage increases linearly with the strength of the external magnetic field and is inversely proportional to the charge carrier density, i.e. the number of charge carriers present in relation to a volume. Semiconductors, which form the basis for integrated circuits, inherently have a relatively low charge carrier density. This means that particularly high Hall voltages occur in integrated circuits, which in turn lead to signal distortion and / or a deteriorated SNR. Furthermore, GaAs FETs, which have particularly good noise characteristics, are often used in discrete amplifiers.Integrated circuits, however, are based on silicon, which means that the advantages of GaAs-based components are lost. Due to these disadvantages alone, commercial MRI systems almost exclusively use discrete amplifiers, which are correspondingly less sensitive to the interference in an MRI system, although the relatively large size is accepted as a disadvantage. For example, a broadband CMOS amplifier for MRI is known from Horneff, Andreas, et al. "A New CMOS Broadband, High Impedance LNA for MRI Achieving an Input Referred Voltage Noise Spectral Density of 200pV / Hz." 2019 IEEE International Symposium on Circuits and Systems (ISCAS). IEEE, 2019. The amplifier is designed as a differential voltage amplifier and has feedback via a "DC servo loop". A differential voltage input is used to suppress magnetoresistive effects and the Hall effect.The amplifier features a folded cascode design. The disadvantages of this design are a narrow bandwidth (1.5 - 90 MHz), high power consumption, lower gain and a higher noise figure of 30 mA at 1.8 V (55 mW), as well as high current consumption (16 mA) for a pair of input amplifiers.

[0018] Another amplifier for MRI systems is known, for example, from Sun, Chenhao, et al., "Wideband receive-coil array design using high-impedance amplifiers for broadband decoupling." Magnetic Resonance in Medicine 90.5 (2023): 2198-2210. The disclosed circuit is based on conventional or discrete amplifiers. The LNA implemented here has a high input impedance to create broadband preamplifier decoupling from RF antennas or receive channels. This is achieved by means of a transformer between the RF antenna and the amplifier. With a gain of 15 dB over a bandwidth of 25 MHz, the amplifier is relatively narrowband and requires an additional amplifier stage to compensate for transmission losses to a reconstruction computer or digitization unit.

[0019] Sporrer, Benjamin, et al. "A fully integrated dual-channel on-coil CMOS receiver for array coils in 1.5-10.5 T MRI." IEEE transactions on biomedical circuits and systems 11.6 (2017): 1245-1255. Shows another CMOS amplifier for array coils. This publication demonstrates a non-analog transmission of amplified MR signals using digitization. The disclosed amplifier contains two amplifier channels and an ADC (analog-to-digital converter). The amplifier is designed to amplify frequencies between 63 MHz and 450 MHz. The noise figure (NF) is above 0.9 dB. The power consumption is 200-236 mW per channel. The ADC achieves a dynamic bandwidth of 81.9 dB at a bandwidth of 1 MHz. An optical link is used for signal transmission. The input of the amplifier includes a high-impedance (HZLNA) and low-impedance (LZLNA) input (both differential).Both LNAs of the respective amplifier channel are implemented as active-feedback amplifiers.

[0020] In this design, each LNA is followed by a mixer (downconversion) that mixes the received signal directly to an intermediate frequency (IE), with subsequent further amplification by a transconductance amplifier that is used as a low-pass and anti-aliasing filter.

[0021] Disadvantages include a limited output amplifier power of 36 dB in 6 dB steps and a high noise figure of 0.9 dB.

[0022] Cao, Xueming, et al. "Design of a 3T preamplifier whose stability is insensitive to coil loading." Journal of Magnetic Resonance 265 (2016): 215-223. The disclosed amplifier circuit exhibits improved amplifier stability, such as parasitic oscillations caused by changing load impedances at the amplifier input. The amplifier is built on a PCB with discrete SMD components. Disadvantages include a limited frequency bandwidth (123 MHz) and a relatively high noise figure of 1.02 dB.

[0023] Integrated amplifiers for MRI systems known to date from science and research typically exhibit high sensitivity to magnetic fields and / or low bandwidth and are not designed for varying loads. Furthermore, the integrated amplifier circuits proposed to date, without additional external protective measures (e.g., active detuning circuits using PIN diodes), are susceptible to current or voltage spikes that can result, for example, from signals stimulating the examination subject.

[0024] It is therefore an object of the present invention to eliminate or at least mitigate the disadvantages of the prior art. Preferably, the object of the invention is to provide an RE preamplifier for amplifying a signal from an RF antenna of a magnetic resonance imaging system, a circuit for detecting a signal from an RF antenna, a local coil for a magnetic resonance imaging system, and a magnetic resonance imaging system with which a high SNR over a wide frequency range can be achieved with a small size.

[0025] This task is solved by an RF preamplifier of the type mentioned above, wherein the RF preamplifier is an integrated transconductance amplifier with at least one folded cascode.

[0026] Furthermore, the object is achieved by a circuit for detecting a signal of an RF antenna of a magnetic resonance imaging system comprising:

[0027] - an RF preamplifier according to the invention with a first control signal input for a first control signal and a second control signal input for a second control signal for setting an amplification factor and / or an operating point of the RF preamplifier;

[0028] - a control unit which is configured to generate the first control signal and the second control signal in order to control, in particular to regulate, the amplification factor and / or the operating point of the RF preamplifier.

[0029] The object is also achieved by a local coil for a magnetic resonance imaging system with an RF preamplifier according to the invention or a circuit according to the invention, wherein the at least one RF antenna is configured to detect an RF signal, wherein the RF preamplifier is configured to amplify the RF signal detected by the RF antenna.

[0030] The object is also achieved by a magnetic resonance imaging system of the type mentioned at the outset with a local coil according to the invention for detecting the RF signals.

[0031] A magnetic resonance imaging system contains relatively strong magnetic fields which can be static and homogeneous, but can also have (locally) strong gradients and / or temporal dependencies. For this purpose, the magnetic resonance imaging system has at least one magnet, but typically a large number of magnets. For example, one or more superconducting magnets and / or permanent magnets can be used. Superconducting magnets are particularly frequently used in clinical applications because they can generate a particularly strong magnetic field. In addition, gradient coils can be provided, for example, which generate locally varying magnetic fields. An object to be examined can be located on a sample table in these magnetic fields. For example, the magnetic resonance imaging system can have a tube, wherein the sample table can be arranged inside the tube at least during a measurement.The object under investigation can, for example, be an organic object under investigation.

[0032] Furthermore, the magnetic resonance imaging system has at least one transmitting coil for generating an excitation signal. The excitation signal can, for example, excite hydrogen nuclei (protons) in the object under examination that are aligned along the external magnetic field. The excitation signal has an excitation frequency.

[0033] Due to this excitation, the object under investigation can emit an RF signal (also known as a high-frequency signal). The RF signal can, for example, have a frequency of at least 100 kHz up to 1 GHz.

[0034] The magnetic resonance imaging system further comprises a data processing system for processing acquired RF signals. The data processing system may, for example, comprise a computer. For example, the data processing system may be configured to create an image from the acquired RF signals.

[0035] To detect the RF signals, the magnetic resonance imaging system has at least one local coil. The local coil has at least one RF antenna. The RF antenna can, for example, be configured to pick up signals with the frequency of an expected RF signal. The RF antenna can have a resonant frequency. For example, the RF antenna can have an adjustable resonant frequency. In addition, the local coil has an RF preamplifier and / or a circuit for detecting a signal from an RF antenna of a magnetic resonance imaging system. The at least one RF antenna is configured to detect the RF signal, wherein the RF preamplifier is configured to amplify the RF signal detected by the RF antenna.

[0036] The circuit is preferably arranged within a local RF coil element of the local coil. The circuit can, for example, be present in a rigid housing of a local RF coil, for example a head coil, or in a flexible local RF coil, which can, for example, be designed in the form of a blanket or other flexible structure in order to be able to adapt to the shape of the examination object. The shape of the examination object can, for example, be a body contour of a person. An RF coil (also referred to as a head coil or "RF coil") typically has several RF antennas (also referred to as "coil loops").

[0037] Optionally, the circuit can be located on an outside of a local RF coil.

[0038] The (at least one) RF preamplifier is of central importance for the magnetic resonance imaging system, as the RF preamplifier amplifies the RF signal from which the image ultimately results. The RF signal picked up by the RF antenna is typically small and isolated, and therefore susceptible to external interference, as even small external interference can cause relatively significant distortion of the RF signal. It is therefore advantageous to amplify the RF signal as quickly as possible and close to its source, i.e., to keep the cable length between the RF antenna and the RF preamplifier as short as possible.

[0039] The RF preamplifier is an integrated transconductance amplifier with at least one folded cascode. In contrast to most other more widely used operational amplifier circuits, the transconductance amplifier has a current instead of a voltage output. The transconductance amplifier (also known as an operational transconductance amplifier (OTA)) has two voltage inputs in addition to the current output. The transconductance amplifier (OTA) converts a differential voltage between the two voltage inputs into a proportional output current. Transconductance amplifiers are generally known from the prior art, for example in Tietze U., Schenk C., and Gamm, E., Halbleiter-Schaltungstechnik, 16th edition, 2019, Springer-Verlag GmbH Germany: 595-600 or in Geiger, Randall L. , and Edgar Sanchez-Sinencio. "Active filter design using operational transconductance amplifiers: A tutorial." IEEE circuits and devices magazine 1 . 2 ( 1985 ) : 20-32 . Basic properties of transconductance amplifiers of fenbart . Transconductance amplifiers are typically used as filters because they can have variable frequency and / or bandwidth. For example, OTAs are widely used in audio electronics.

[0040] Surprisingly, an integrated transconductance amplifier is particularly well suited as an RF preamplifier for the RF signals of an RF coil of a magnetic resonance imaging system. The inherently differential design of the transconductance amplifier leads to reduced susceptibility to interference from external magnetic fields. Transconductance amplifiers also allow the amplification of signals within a particularly wide bandwidth. Preferably, the RF preamplifier can amplify RF signals in a frequency range from 500 kHz up to at least 100 MHz. Particularly preferably, the RF preamplifier can amplify RF signals in a frequency range from 100 kHz up to at least 600 MHz. Such broadband amplification is not achievable with conventional amplifier circuits.This broadband capability allows the RF preamplifier to be used to amplify various RF signals at different frequencies without requiring separate adaptation to an individual RF antenna. For example, an RF antenna can be narrowband, i.e., it can have a small bandwidth and selectively pick up only certain RF frequencies. The RF preamplifier can amplify these signals over a wide frequency range and, within this frequency range, therefore does not need to be adapted to the resonant frequency of the RF antenna. The (broadband) RF preamplifier can therefore be used particularly flexibly and easily to amplify signals from RF antennas.

[0041] Transconductance amplifiers exhibit very high common-mode rejection due to the wide common-mode input voltage range. Thus, any resulting non-linearities and magnetoresistive effects (such as the Hall effect) can be suppressed. Consequently, both static magnetic fields (B o Both temporally variable magnetic fields (gradient fields) and time-varying magnetic fields (gradient fields) have a minor influence on the RF preamplifier. This also makes the orientation of the RF amplifier within the external magnetic fields less critical.

[0042] The RF preamplifier is preferably implemented in CMOS technology. CMOS stands for Complementary Metal Oxide Semiconductor and is a term for semiconductor components in which both p-channel MOSFETs and n-channel MOSFETs are used on a common substrate. MOSFET stands for metal oxide semiconductor field effect transistor. The integrated form, particularly in CMOS technology, makes it possible to transfer the previously discrete and extremely noise-sensitive preamplifier into a robust and much smaller integrated component (IC) without losing any amplification power. CMOS integration also enables a significantly more energy-efficient implementation and a more electromagnetically compatible solution than previous discrete preamplifiers for magnetic resonance imaging.

[0043] The transconductance amplifier has a high input resistance or high input impedance. This high input resistance has proven to be extremely advantageous in the implementation of the RF preamplifier. The RF preamplifier thus decouples an (optional) RF antenna connected to the two voltage inputs from the current output or another signal chain. Energy induced in the RF antenna by the object under investigation therefore essentially remains in the RF antenna. The transconductance amplifier only measures a voltage without drawing any significant energy from the resonant circuit of the RF antenna. The RF preamplifier also has at least one folded cascode. This embodiment is generally known as a "folded cascode OTA".The transconductance amplifier can, for example, have at least four transistors, two of which form a differential input pair. The transconductance amplifier can, for example, have at least two, preferably at least four, in particular eight, transistors. The folded cascoding of series-connected transistors at the amplifier inputs can reduce or completely avoid the Miller effect, which is disadvantageous in other transconductance amplifier architectures. The folded cascode can be used to achieve an increased cutoff frequency or bandwidth of the RF preamplifier. Reducing or avoiding the Miller effect also has a positive effect on the noise figure and thus improves the SNR.In addition, the folded cascode can significantly increase the output voltage range of the RF preamplifier, enabling high amplification over a wide bandwidth using the RF preamplifier, with the RF preamplifier not saturating or only saturating with difficulty. The wide bandwidth of the integrated RF preamplifier is a significant advantage and enables the use of a single amplifier chip for different MRI systems and magnetic field strengths (e.g., less than 0.1 Tesla, 0.55 Tesla, 1.5 Tesla, 3 Tesla, 4 Tesla, 7 Tesla, 10.5 Tesla, 11.4 Tesla, 14 Tesla, etc.), thus eliminating the need to produce multiple analog or discrete amplifiers designed for the applicable field strength.A conventional implementation of an extremely low-noise multi-frequency amplifier capable of covering bandwidths up to or over 1 GHz and featuring a compact design would result in significant parasitic capacitances and impedances, making the desired properties unattainable. Such an implementation is therefore only possible in integrated form (e.g., in CMOS) in accordance with the RF preamplifier according to the invention.

[0044] The RF preamplifier preferably has at least one current mirror. The current mirror can be formed by one or more transistors. For example, the RF preamplifier can have at least two current mirrors, with at least one current mirror being provided for each voltage input. For example, a current mirror can be connected downstream of each differential input pair (formed by two transistors). The Miller effect can be avoided by the folded cascoding of these series-connected current mirrors at the amplifier inputs.

[0045] A transconductance amplifier with a folded cascode circuit and current mirrors is generally known, for example, from Daoud, Houda, et al. "Folded cascode OTA design for wide band applications." International Conference on Design and Test of Integrated Systems in Nanoscale Technology, 2006. DTIS 2006. IEEE, 2006.

[0046] By using a folded cascode circuit of series-connected current mirrors around a cascode node, the differential input transistor pair is separated from the current mirror cascode. This, in turn, enables a rail-to-rail design with lower supply voltages (e.g., 5 V or 3.3 V).

[0047] For example, the RF preamplifier can be supplied with a positive supply voltage and a negative supply voltage.

[0048] The invention is based on the surprising finding that the specific embodiment of the RF preamplifier as an integrated transconductance amplifier with at least one folded cascode makes it possible to manufacture the RF preamplifier in a particularly small design and yet still ensure a high SNR over a wide frequency range, since this circuit variant is particularly less sensitive to the external magnetic fields of a magnetic resonance imaging system compared to conventional preamplifiers.

[0049] The RF preamplifier is low-noise. A (RE) preamplifier is described as low-noise if it has a noise figure (NF) of less than 1.5 dB. However, a noise figure (NF) of less than 1 dB is preferred. The typical gain of a low-noise amplifier is between 10 and 20 dB for single-stage amplification or between 10 and 40 dB, depending on the number of amplifier stages. The RF preamplifier can have at least one or more amplifier stages. The RF preamplifier is preferably extremely low-noise with a noise figure of less than 1 dB, in particular less than or equal to 0.5 dB.

[0050] Miniaturization also offers advantages for the entire MRI system. The integrated design of the RF amplifier significantly reduces the space required compared to conventional amplifiers with discrete components. The relatively small RF preamplifier enables a higher RF antenna density within a local coil. The more receive channels (i.e. RF antennas) that can be used, the faster or higher the resolution MR scans that are possible. The current standard is 32 channels. 128 channels have been tested. The integrated RF preamplifier could, for example, enable 356 channels or an even higher number of channels, as the size can be drastically reduced, thus creating more space. The additional space can be used for other applications, such as magnetic field monitoring / influence, transcranial magnetic field stimulation (TMS), electroencephalography (EEG), etc.or for further improvements in the area of ​​methodology or diagnostics, or even to provide more space and comfort for the patients / test subjects in terms of design.

[0051] The RF preamplifier, for example, may have no feedback. The RF preamplifier without feedback can also be referred to as an open-loop RF preamplifier. The feedback-free design of the RF preamplifier largely prevents saturation of the RF preamplifier. Furthermore, a significantly higher bandwidth can be achieved without feedback than with feedback. The open-loop RF preamplifier also exhibits particularly high linearity between the two voltage inputs. The RF preamplifier without feedback is therefore even less sensitive to external interference such as external magnetic fields.

[0052] The RF preamplifier can have an amplifier circuit which is symmetrical with respect to a plane of symmetry. The symmetry can be, for example, a geometric symmetry, i.e. the RF preamplifier can be essentially mirror-symmetrical with respect to the plane of symmetry. The plane of symmetry can, for example, lie parallel to a normal vector to the main extension plane of the integrated circuit. The circuit can be constructed symmetrically so that the signals from the two voltage inputs are processed symmetrically. For example, corresponding line lengths within the RF amplifier can be of equal length. This means that interference (such as that due to the Hall effect) can have a symmetrical effect on signals from both voltage inputs. Since the RF amplifier amplifies a differential signal, symmetrical interference can cancel each other out and thus have hardly any influence on the output signal of the RF amplifier.

[0053] The RF preamplifier may have a first control signal input for a first control signal and a second control signal input for a second control signal for adjusting an amplification factor and / or an operating point of the RF preamplifier. For example, a resistance at the current output of the RF preamplifier can be adjusted using the first and / or second control signal. This allows adjustment of the output current. The first control signal may be a control current I ABC (Ampli fier Bias Current (ABC ) ). By the control current I ABC At the output of the RF preamplifier, an operating point of the RF preamplifier can be set. The second control signal can be a control current I x By means of the control current I xany non-linearities between the two voltage inputs can be reduced. The first and second control signals are particularly advantageous when the RF preamplifier is a feedback-free RF preamplifier. The control signals do not represent feedback because they are not applied to the two voltage inputs. By controlling the two control signals, it is possible to adjust the output gain of the RF preamplifier, to control the linearity and frequency response of the RF preamplifier, and to determine the amplification power depending on the situation. These properties, which are particularly advantageous in connection with the specific application of MRI, can only be achieved with a transconductance amplifier.

[0054] The symmetrical design of the RF preamplifier further simplifies and improves the adaptation and control of a gain-bandwidth product (GBW) and a gain slope across the bandwidth of the RF preamplifier. This enables a significant extension of the frequency range that can be stably amplified, particularly in low and high frequency ranges, for example in frequency ranges from 1 kHz to 100 kHz and 600 MHz to 1 GHz. The RF preamplifier can exhibit continuous and stable gain, particularly in a frequency range from 100 kHz to 600 MHz. These frequency ranges are particularly relevant in connection with ultra-high-field MR systems or extremely low-field or portable MR systems. The RF preamplifier can be used particularly flexibly due to its easy adaptation option, without any adaptation of the RF preamplifier hardware.

[0055] The circuit for detecting a signal from an RF antenna of a magnetic resonance imaging system has, in addition to the RF preamplifier according to the invention, a control unit which is designed to generate the first control signal and the second control signal in order to control, in particular to regulate, the amplification factor and / or the operating point of the RF preamplifier. The control unit can be controlled, for example, by a central MRI control device. The control unit can be, for example, a microcontroller. The control can be adapted, for example, to an expected frequency of the RF signal and / or an expected or actual output level of the output current of the RF preamplifier. The control unit can, for example, have or be an integrated circuit.

[0056] The integrated RF preamplifier can be programmed or adapted for various MR frequencies via optical and / or analog connections by the control unit. This allows use at various relevant MR field strengths and resonances (multi-core) with optimal performance (frequency modularity).

[0057] The circuit can optionally comprise a coaxial signal converter which is configured to receive an output signal from the RF preamplifier and to output it as an asymmetrical output voltage signal; wherein the control unit is configured to control, in particular to regulate, a third control signal for controlling a further amplification factor of the coaxial signal converter. The coaxial signal converter is connected to the current output of the RF preamplifier and receives the output signal from the RF preamplifier. The coaxial signal converter amplifies the output signal and converts it into a voltage signal in order to output it as an asymmetrical output voltage signal. The asymmetrical output voltage signal is output by the coaxial signal converter in a single-ended manner (i.e. asymmetrically and thus not differentially). The coaxial signal converter can be connected to a coaxial cable for outputting the asymmetrical output voltage signal.The digitization unit can be configured to receive and digitize the asymmetric output voltage signal of the coaxial signal converter.

[0058] The circuit can, for example, have a resistor array that is configured to receive an output signal from the RF preamplifier and output it as a differential output voltage signal; wherein the control unit is configured to control, in particular to regulate, a third control signal for controlling a further amplification factor of the resistor array. The resistor array is connected to the current output of the RF preamplifier and configured to generate a differential output voltage as a differential output voltage signal. The resistor array is controllable by a further third control signal. The resistor array can amplify the output signal. An output gain can be set by the third control signal. The digitization unit can be configured to receive and digitize the differential output voltage signal from the resistor array.Optionally, the resistor array can be arranged in parallel to the coaxial signal converter so that the circuit generates and outputs both a differential output voltage signal and an asymmetric (single-ended) output voltage signal.

[0059] The circuit can, for example, have a digitization unit that is configured to digitize an output signal of the RF preamplifier. The output signal of the RF preamplifier can be the output current of the RF preamplifier or the differential output voltage signal or the asymmetrical output voltage signal. The data processing device can, in particular, be configured to process digital data. The digitization unit can receive the (analog) output signal of the RF preamplifier or the differential output voltage signal or the asymmetrical output voltage signal and digitize it to thereby obtain digital data. The digitization unit can have an analog-to-digital converter (ADC).

[0060] The digitization unit can, for example, comprise an integrated circuit. The digitization unit can be an integrated circuit. To prevent external influences on the output signal of the RF preamplifier as much as possible, it is advantageous to keep the cable length between the RF preamplifier and the digitization unit as short as possible. The digitization unit and the RF preamplifier can be separate components, which can be arranged, for example, on a (common) circuit board or on separate circuit boards.

[0061] At least the RF preamplifier and the digitization unit can, for example, be arranged monolithically on a common semiconductor substrate. Monolithic in this context means that both the RF preamplifier and the digitization unit are manufactured on the same semiconductor wafer (in this case a silicon wafer). The main advantages of monolithic integration are miniaturization, high reliability, improved performance, and lower costs. By manufacturing all components on one chip, signal paths can be shortened and parasitic effects reduced. Optionally, the RF preamplifier and / or the digitization unit can be implemented as an AS IC (application-specific integrated circuit).

[0062] Optionally, the RF preamplifier and the resistor array and / or the coaxial signal converter can be arranged monolithically on a common semiconductor substrate.

[0063] Digitizing the MR signal (i.e., the output signal) as quickly as possible has the advantage of eliminating disruptive interactions between signal transfer lines (common-mode rejection), which can affect the stability of discrete amplifiers and even drastically degrade the signal-to-noise ratio. The system's modular design, based on a building block principle, allows the front end of MR transmit and receive coils to be adapted to the respective needs and conditions. Depending on the transmit or receive system, the output signal of one or more integrated RF preamplifiers can be sent to a common digitization unit (e.g., an ADC). The digitization unit can be located inside or outside the local MR transmit or receive coil (i.e., RF antenna).From there, the digitized MR signal can be forwarded, preferably optically, to an MR backend (e.g. to the data processing system).

[0064] Optionally, the circuit can have a mixing unit that is configured to convert a first frequency of the output signal of the RF preamplifier into a second frequency. For example, the second frequency can be a frequency matched to the digitization unit and thus simplify or improve the digitization of the output signal. The mixing unit can, for example, be connected in series between the output of the amplifier and an input of the digitization unit. The mixing unit can have an integrated circuit. The RF preamplifier, the mixing unit and the digitization unit can be monolithically integrated, i.e. arranged on a common semiconductor substrate.

[0065] An integrated module comprising the RF preamplifier and / or the digitization unit can optionally comprise additional components such as the mixing unit and / or one or more filters, which can be switched on and off, for example. The mixing unit and / or the filter(s) can be controllable; in particular, parameters of the mixers and filters can be adjusted based on an operating frequency of the RF preamplifier (or the input signal). Optionally, several RF preamplifiers can be combined in a group, i.e. an array, and designed monolithically. Therefore, an integrated module can comprise several RF preamplifiers that are connected in parallel to one another. The several RF preamplifiers can be essentially identical. The signal from one RF antenna, for example, can be applied to each of the several RF preamplifiers of the integrated module.The RF preamplifiers of the integrated module can be controlled jointly in one variant. The multiple RF preamplifiers of the integrated module can be controlled individually in another variant.

[0066] An RF preamplifier may be configured to amplify the signal from multiple RF antennas sequentially (i.e., one after the other in time).

[0067] It is preferred that the local coil has a plurality of RF antennas and a plurality of RF preamplifiers, wherein each RF preamplifier amplifies the signal of exactly one RF antenna.

[0068] The digitization unit may have a plurality of inputs and digitize the amplified signals (i.e., the output signals) of the RF preamplifiers.

[0069] It is preferred that the digitization unit output an optical digital signal. Optical signals are not affected by external magnetic fields and are therefore particularly insensitive to external interference that can be expected in an MRI system.

[0070] In another embodiment, the digitized signal from the digitization unit can be transmitted wirelessly. The digitization unit can have a transmitter module for this purpose.

[0071] Optionally, a programmable interface is provided in the circuit to switch additional integrated optional modules or features (such as filters, mixing units, etc.) on or off in order to enable the greatest possible flexibility / compatibility.

[0072] The combination of the integrated module's modular design and the broadband capability of the RF preamplifier allow existing MRI systems to be retrofitted or upgraded accordingly. This allows for cost-effective upgrading or conversion of older MRI systems and contributes to the more sustainable use of this cost-intensive technology.

[0073] Furthermore, a modular and optionally integrated design of the circuit makes it easier to add or switch on and off integrated optional features or elements in order to ensure the highest possible compatibility. For example, a feature or element may be required for at least one MRI system to be retrofitted and not required for at least one other MRI system. So that the circuit can be used on both MRI systems to be retrofitted, it contains the feature or element, whereby this feature or element is switchable and / or adjustable. Adjustability can be provided for features or elements that are required on at least two MRI systems to be retrofitted but are used with different parameters.

[0074] In the following, the invention is described in more detail with reference to figures, to which it is not intended, however, to be limited.

[0075] Fig. 1 shows schematically the structure of a magnetic resonance imaging (MRI) system;

[0076] Fig. 2 shows schematically an RF preamplifier;

[0077] Fig. 3 shows schematically a local coil with a circuit comprising the RF preamplifier from Fig. 2;

[0078] Fig. 4 shows schematically an amplification factor of the circuit of Fig. 3 as a function of a frequency of the input signals;

[0079] Fig. 1 schematically shows a magnetic resonance imaging system (MRI system) 1. The magnetic resonance imaging system 1 has a sample table 2 for receiving an examination object 3. In this exemplary embodiment, the magnetic resonance imaging system 1 is a clinical magnetic resonance imaging system, which is why the examination object 3 is schematically shown as a human in Fig. 1. In addition, the magnetic resonance imaging system 1 has a superconducting magnet 4 for generating a magnetic field, as well as several gradient coils 5 and shim coils 6. The magnetic resonance imaging system 1 has a transmitting coil 7 for generating an excitation signal. A data processing system 8 is provided for processing acquired RF signals. Corresponding magnetic resonance imaging systems 1 are known from the prior art. According to the invention, a local coil 9 is provided with an RF preamplifier 10 and an RF antenna 11 (see Fig.3) is provided for detecting the RF signals. The data processing system 8 is electrically connected to the RF preamplifier 10 of the local coil 9. The local coil 9, together with the RF preamplifier 10, is exposed to the magnetic fields within the magnetic resonance imaging system 1.

[0080] Figure 2 schematically shows an RF preamplifier 10 for amplifying a signal from an RF antenna of a magnetic resonance imaging system 1. The RF preamplifier 10 is an integrated transconductance amplifier with a folded cascode and two current mirrors. The RF preamplifier 10 has no feedback and is therefore feedback-free. The RF preamplifier 10 has a positive voltage input 12A, a negative voltage input 12B and a current output 13. The RF preamplifier 10 is configured to amplify a voltage difference between the two voltage inputs 12A and 12B and to output a correspondingly amplified output signal at the current output 13. The RF amplifier 10 is supplied with electrical energy by means of a positive supply voltage at a first positive supply input 14 and a negative supply voltage at a second negative supply input 15.The RF preamplifier 10 has an amplifier circuit which is symmetrical with respect to a symmetry plane 16, i.e. signals at the voltage inputs 12A and 12B are processed symmetrically to one another.

[0081] The RF preamplifier 10 has a first control signal input 17 for a first control signal and a second control signal input 18 for a second control signal for setting an amplification factor and an operating point of the RF preamplifier 10. The RF preamplifier 10 is configured to continuously amplify signals with frequencies between 100 kHz and 600 MHz (see Fig. 3).

[0082] Figure 3 shows a local coil 9 with an RF antenna 11 and the RF preamplifier 10 from Fig . 2 .

[0083] The RF preamplifier 10 is part of a circuit 24 for detecting a signal from the RF antenna 11 of the magnetic resonance imaging system 1. The circuit 24 includes the RF preamplifier 10 and a control unit 25 configured to generate the first control signal and the second control signal in order to control the gain factor and / or an operating point of the RF preamplifier 10. The control unit 25 is connected accordingly to the first control signal input 17 and the second control signal input 18. The circuit 24 has a first subcircuit 26 with at least one coaxial signal converter and optionally a mixing unit, and / or a filter unit, and / or a digitization unit, which is configured to output an output signal of the RF preamplifier 10 as a non-differential signal (single-ended), and optionally to mix, and / or filter, and / or digitize it.The first subcircuit 26 is correspondingly connected to the current output 13 of the RF preamplifier 10. The first subcircuit 26 is controlled by a third control signal 26A of the control unit 25. The first subcircuit 26 comprises an integrated circuit.

[0084] The circuit 24 has a further subcircuit 27 with a resistor array and optionally a mixing unit, and / or a filter unit, and / or a digitization unit, which are configured to directly output an output signal of the RF preamplifier 10 as a differential signal, and / or to mix it, and / or to filter it, and / or to digitize it. The second subcircuit 27 is correspondingly connected to the current output 13 of the RF preamplifier 10. The second subcircuit 27 is controlled by a further third control signal 27A of the control unit 25. The second subcircuit 27 has an integrated circuit.

[0085] The first and second subcircuits 26 and 27 of the circuit 24 may each further comprise a mixing unit configured to convert a first frequency of the output signal of the RF preamplifier 10 into a second frequency. The output signal 13 of the RF preamplifier 10 may optionally be amplified again in the first subcircuit 26, then optionally mixed, and / or filtered, and / or digitized.

[0086] The control unit 25 is connected to the first and second subcircuits 26 and 27 and the current output 13 of the RF preamplifier 10 and can control the RF preamplifier 10 according to the output signal (in analog or digital form).

[0087] The RF preamplifier 10, the control unit 25, the second subcircuit 27, with a resistor array, an optional mixing unit, an optional filter unit, and an optional digitization unit, and the first subcircuit 26, with a coaxial signal converter, an optional mixing unit, an optional filter unit, and an optional digitization unit, are arranged monolithically on a common semiconductor substrate. The circuit 24 is therefore a (single) integrated circuit comprising the RF preamplifier 10, the control unit 25, and the first and second subcircuits 26 and 27.

[0088] The RF antenna 11 is connected to the circuit 24, specifically to the positive voltage input 12A and the negative voltage input 12B, so that an RF signal induced in the RF antenna 11 by the object under investigation 3 is detected by the circuit 14 and amplified by the RF preamplifier 10.

[0089] The local coil 9 includes the RF antenna 11 and the circuit 24 including the RF preamplifier 10. Optionally, the local coil 9 can include additional RF antennas 11 and RF preamplifiers 10, cabling, and a housing.

[0090] Fig. 4 schematically shows an amplification factor 19 of the circuit 24 from Fig. 3 as a function of a frequency 20 of the input signals. The amplification factor 19 is constant over a wide frequency range. In the first edge region 21 and the second edge region 22, a frequency dependence can be seen. By means of the first control signal, the second control signal and the third control signal, different amplification curves 23A, 23B and 23C can be selected, for example in order to adapt the amplification behavior of the RF preamplifier 10 in the first edge region 21 and the second edge region 22 to an application or in order to adapt a further amplification by the coaxial signal converter. The amplification factor 19 can, for example, be adapted as a function of the output signal. A change in the first control signal leads to an adapted amplification curve 23A.A change in the second control signal leads to a changed gain curve 23B. In particular, a symmetrical design of the RF preamplifier 10 leads to a flexible adaptability of the gain behavior of the RF preamplifier 10, as is shown schematically in Figure 3. A change in the third control signal 26A leads to a changed gain curve 23C. The gain curves 23A, 23B and 23C differ in particular with regard to a level of the gain factor 19 and the behavior in the edge regions 21 and 22. For example, the adjusted gain curve 23C is particularly suitable for amplifying RF signals with frequencies in the first edge region 21, i.e. between approximately 1 kHz and 100 kHz. For example, the adjusted gain curve 23B is particularly suitable for amplifying RF signals with frequencies in the second edge region 22, i.e. suitable for frequencies between approximately 100 MHz and 1 GHz.

Claims

Claims:

1. RF preamplifier (10) for amplifying a signal of an RF antenna (11) of a magnetic resonance imaging system (1), characterized in that the RF preamplifier (10) is an integrated transconductance amplifier with at least one folded cascode.

2. RF preamplifier (10) according to claim 1, characterized in that the RF preamplifier (10) has no feedback.

3. RF preamplifier (10) according to claim 1 or claim 2, characterized by an amplifier circuit which is symmetrical with respect to a plane of symmetry (16).

4. RF preamplifier (10) according to one of the preceding claims, characterized by a first control signal input (17) for a first control signal and a second control signal input (18) for a second control signal for setting an amplification factor (19) and / or an operating point of the RF preamplifier (10).

5. Circuit (24) for detecting a signal from an RF antenna (11) of a magnetic resonance imaging system (1), comprising: - an RF preamplifier (10) according to claim 4; characterized by: - a control unit (25) which is configured to generate the first control signal and the second control signal in order to control, in particular to regulate, the amplification factor (19) and / or the operating point of the RF preamplifier (10).

6. Circuit (24) according to claim 5, characterized by: - a coaxial signal converter configured to receive an output signal from the RF preamplifier (10) and output it as an asymmetrical output voltage signal; wherein the control unit (25) is configured to control, in particular to regulate, a third control signal for controlling a further amplification factor of the coaxial signal converter.

7. Circuit (24) according to claim 5 or claim 6, characterized by: - a resistor array configured to receive the output signal of the RF preamplifier (10) and output it as a differential output voltage signal; wherein the control unit (25) is configured to control, in particular to regulate, a third control signal for controlling a further gain factor of the resistor array.

8. Circuit according to one of claims 5 to 7, characterized by - a digitization unit configured to digitize the output signal of the RF preamplifier (10) or the asymmetric output voltage signal or the differential output voltage signal.

9. Circuit according to claim 8, characterized in that the digitization unit comprises an integrated circuit.

10. Circuit according to claim 9, characterized in that at least the RF preamplifier (10) and the digitization unit are arranged monolithically on a common semiconductor substrate.

11. Circuit (24) according to one of claims 5 to 10, characterized by - a mixing unit configured to convert a first frequency of the output signal of the RF preamplifier (10) or the asymmetric output voltage signal or the differential output voltage signal into a second frequency.

12. Local coil (9) for a magnetic resonance imaging system (1), characterized by: at least one RF antenna (11) and an RF preamplifier (10) according to one of claims 1 to 4 or a circuit (24) according to one of claims 5 to 11, wherein the at least one RF antenna (11) is configured to detect an RF signal, wherein the RF preamplifier (10) is configured to convert the RF signal detected by the RF antenna (11) to strengthen.

13. Magnetic resonance imaging system (1) comprising: - a sample table (2) for holding an object to be examined (3); - at least one magnet (4) for generating a magnetic field; - at least one transmitting coil (7) for generating an excitation signal; - a data processing system (8) for processing detected RF signals; characterized by - at least one local coil (9) according to claim 12 for detecting the RF signals.

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