NMR apparatus with high efficiency current source

Abstract

An NMR device (10) having an NMR magnet system for generating an NMR magnetic field, the NMR magnet system including an NMR coil (11) for modifying the magnetic field, and the NMR device having a stable and low-noise current source supplying current to the NMR coil, the current source including a linear current regulator (12) electrically connected to a first end of the NMR coil, characterized in that the current source includes a digital voltage source (13) employing switching technology, the digital voltage source being electrically connected to a second end of the NMR coil such that the NMR coil is connected as an electrical load between the linear current regulator and the digital voltage source, and the current regulator (12) is connected to the digital voltage source (13) via a regulating line (14) for regulating voltage, and the linear current regulator regulates the digital voltage source such that the voltage on the linear portion side of the arrangement is within a preselected operating range, thereby consuming as little electrical power as possible in the linear portion of the arrangement. Thus, the NMR coil can be regulated with high electrical efficiency and supplied with current stably and with low noise, wherein the uniformity of the NMR coil remains stable and constant even under changing external conditions.

Description

Technical Field

[0001] This invention relates to an NMR device having a magnetic coil system for generating a uniform magnetic field, the magnetic coil system including at least one NMR coil for modifying the NMR magnetic field, and the NMR device having a stable current source configured to supply current to the NMR coil, the current source including a linear current regulator electrically connected to a first terminal of the NMR coil, wherein the current source includes a digital voltage source electrically connected to a second terminal of the NMR coil, such that the NMR coil is connected as an electrical load between the linear current regulator and the digital voltage source, the linear current regulator being connected to the digital voltage source via a regulating circuit for regulating voltage, wherein the current source is low-noise and the digital voltage source is implemented using switching technology, and the linear current regulator is configured to regulate the digital voltage source such that the voltage on the linear current regulator side is within a preselected operating range, thereby consuming as little electrical power as possible in the linear current regulator.

[0002] NMR devices with these characteristics are known from US6492817B2 [Reference 00].

[0003] A similar NMR device is described in an article by Nick Arango et al. (reference [0]), in which the power supply is not as low-noise, the digital voltage source is not implemented using switching technology, and the linear current regulator has a correspondingly different design.

[0004] NMR devices with magnetic coil systems for generating uniform magnetic fields have been in use worldwide since 1960. For example, an example is disclosed in DE10104365C1 (=Reference [1]).

[0005] A stable and low-noise current source configured to supply current to an NMR coil in such an NMR device, and the current source comprising a linear current regulator electrically connected to a first end of the NMR coil, is described in detail in the applicant’s technical manual entitled “BSMS System for AVANCE NEO” (Version 002, Chapter 10 “SCB20”, Subsections 10.1-10.4, pp. 83-90, = Reference [2]). Background Technology

[0006] This invention relates generally to the field of nuclear magnetic resonance (NMR), and more particularly to a cooled, typically superconducting NMR magnet system in operation, wherein the uniformity or intensity of the NMR magnetic field is further improved or altered by an NMR coil.

[0007] Nuclear magnetic resonance spectroscopy is a widely used and powerful method in instrumental analysis. It allows the study of the electronic environment of individual atoms in a substance (e.g., hydrocarbon molecules or biological inorganic complexes) and the interactions between individual atoms and their neighbors. For example, this method can elucidate the composition, structure, and kinetics of the substance, and also determine its concentration.

[0008] In NMR measurements, the material is placed in a strong, static, uniform magnetic field B0, which orients the nuclear spins within the material. A high-frequency electromagnetic pulse is then emitted into the material. The resulting electromagnetic field, also at a high frequency, is detected in an NMR spectrometer. Information about the properties of the material can then be obtained.

[0009] In both high-resolution magnetic resonance spectroscopy and imaging magnetic resonance, the requirements for magnetic field homogeneity are extremely high. To achieve homogeneity specifications, an electrically cryogenic shimming device (Kryoshim) is typically used. The coils of this Kryoshim generate the basic field distribution. By setting an appropriate current, the coils can improve the homogeneity of the NMR magnet at the sample location and thus increase the measurement resolution.

[0010] EP0780698B1 (=Reference [3]) discloses a high-resolution NMR spectrometer with a superconducting NMR magnet system, wherein the superconducting NMR magnet system is cooled to a low temperature by means of a pulse tube cooler and the superconducting NMR magnet system is arranged in a vacuum container in the cold zone of a cryostat.

[0011] Sometimes cold ferromagnetic materials (e.g., iron or steel alloys) are used as shimming elements to improve uniformity, as described in publication DE102015225731B3 (=reference [4]). Regardless of how the magnet is cold-shimmed, residual inhomogeneities will eventually exist, which must be corrected by another shimming system comprising shimming elements arranged outside the vacuum container (usually in the magnet bore). These shimming elements can be like cryogenic shimming devices, either comprising shimming coils powered by shimming current, or comprising ferromagnetic materials, or a combination of both.

[0012] Current shimming current sources used in NMR equipment are based on analog circuit technology. These shimming current sources are generally stable and low-noise, but they are very inefficient due to the high voltage and high current present in the current regulation loop, which generates a lot of waste heat.

[0013] The aforementioned reference [1] discloses an NMR device including a shimming system of an electrical load that serves as a current source. However, the aforementioned NMR device also suffers from the efficiency problem mentioned above because of the simultaneous presence of high voltage and high current in the current regulation circuit, which generates a relatively large amount of waste heat during NMR operation.

[0014] This is also clearly seen in the previously cited reference [2], which details, at least to date, the best stable and low-noise current source available for use in the NMR apparatus of reference [1]. Chapter 10 of reference [2] describes the currently used current source "SCB20" in more detail, starting on page 83. In particular, it can be seen from the block diagram on page 86 that all the power here is also provided by the linear portion of the arrangement.

[0015] This existing technology provides the required current from the feed voltage in a completely linear fashion. If the resistance of the load (including the power supply cable) is small, the power portion of the unwanted voltage is dissipated in the linear amplifier and converted into heat. On the one hand, this wastes current and loads the power supply more than actually needed. On the other hand, this necessitates specialized heat dissipation devices in NMR equipment, typically heat sinks, but these heat sinks cause the current source to become excessive. Furthermore, typical low-noise linear amplifiers capable of operating within the required voltage range are located in the audio range. However, these are increasingly being competed for by Class D amplifiers optimized for audio applications, so it can be expected that the low-noise linear amplifiers used here will sooner or later cease to be commercially available, or at least will not be as readily available as before in the near future. Audio Class D amplifiers also cannot meet the specific purpose of feeding NMR coils, as this requires continuous current. Summary of the Invention

[0016] In contrast, the objective of this invention is to modify, by means of readily available and readily accessible technical means and at the lowest possible cost, a current source having the previously defined characteristics for an NMR coil in an NMR device, such that the NMR coil is supplied with an adjustable, stable, and especially low-noise current with the highest possible electrical efficiency, particularly for applications specifically as a current source for shimming coils, but also for other applications in the field of NMR, such that the very high uniformity of the NMR magnetic field remains largely stable and constant even when external conditions change.

[0017] This relatively complex task is solved by the present invention in a surprisingly simple and effective manner by the linear current regulator comprising an electrical amplifier for feeding current into the NMR coil.

[0018] The electrical amplifier is primarily used to achieve the required low noise levels. By using a current source modified according to the present invention, it is now possible to power the NMR coils in an NMR device with exceptionally high electrical efficiency.

[0019] The solution according to the invention includes a high-efficiency, variable digital voltage source employing switching technology, an adjustable, stable, and low-noise linear current regulator employing hybrid analog and digital technologies, and an NMR coil fed as an electrical load of the current source. The arrangement according to the invention is characterized by a high inductance portion compared to the resistive portion of the load's impedance. The NMR coil, serving as the electrical load, is intentionally positioned between the voltage source and the current regulator in the circuitry. Due to the high inductance of the coil-type load, particularly high-frequency interference from the switching digital voltage source, it is kept away from the sensitive low-noise linear current regulator.

[0020] A key feature of the solution according to the invention is the efficient supply of voltage at the electrical connection of the NMR coil, which functions as a load. This voltage is regulated in such a way that power loss in the linear current regulator connected to the other electrical connection of the load is minimized, thereby minimizing waste heat. Therefore, along with efficient voltage generation, the efficiency of the entire system, consisting of the digital voltage source, the linear current regulator, and the NMR coil as an electrical load, is optimized, and thus power loss generated during NMR operation is minimized.

[0021] By placing an inductive load between the digital voltage source and the linear current regulator, interference generated by the necessary switching processes in the voltage source is filtered and kept away from the current regulator. This fundamentally allows for the high purity of the preset current flowing through the linear current regulator and, consequently, the high purity of the preset current flowing through the NMR coil as the load. The high purity of the preset current, and thus the high purity of the magnetic field generated in the NMR coil, is a crucial prerequisite for successful NMR experiments.

[0022] To better understand this invention, it should be noted that, compared to the closest prior art, the main advantages of the modifications proposed in this invention are the good commercial availability of the amplifier, the reduced space requirements of the layout, and its significantly improved electrical efficiency.

[0023] Ideally, current is supplied directly from the feed voltage via a switchable ground. However, for the low noise required in NMR applications, a linear regulator is still necessary, and stability is also desirable. However, this linear regulator should only produce a limited voltage drop, thus consuming a small portion of the electrical power. Furthermore, amplifiers with a maximum voltage variation range smaller than the feed voltage are more readily available on the market.

[0024] If the linear current regulator is directly connected after the switching voltage source, then the linear regulator must be floated.

[0025] If multiple high-efficiency current sources are used for an NMR spectrometer, such as those required to operate multiple shimming coils, then the linear regulator for each current source must be designed, isolated, and supplied with a separate feed voltage. Typically, each current source requires more than one switched feed, such as one for each feed connection of the linear regulator, and sometimes even more. This is associated with considerable cost, and the solution requires so much space that the size advantage gained through higher efficiency is no longer valid.

[0026] Therefore, the core idea of ​​this invention is to place the linear regulator on the ground side of the coil, while on the other side, a digital voltage source provides a voltage potential higher or lower than ground. Here, "ground" is understood to mean electrical ground or ground potential.

[0027] Especially when using multiple high-efficiency current sources, their linear regulators can share a common feed section. Control of the digital-to-analog converters, analog-to-digital converters, and operational amplifiers can be achieved through multi-channel, non-isolated components. These components are, for example, semiconductor components containing multiple analog-to-digital converters, digital-to-analog converters, and / or operational amplifiers, along with corresponding connection contacts. In this way, the functions of different linear current regulators can be integrated into a common component. This significantly saves space and cost. Furthermore, the high inductance of the NMR coil helps to perform additional filtering of the current before it reaches the sensitive linear regulator.

[0028] Linear current regulators can have a limited voltage range, more precisely, a much smaller voltage range than that of digital voltage sources. The voltage range of a linear current regulator must not be exceeded—unlike the closest prior art, in which the current regulator covered the entire voltage range.

[0029] In this situation, the digital voltage source must be adjusted to adhere to the stated voltage range at the desired current. This also applies to transient phases, during which the current flowing through the NMR coil is driven to the target current. This is achieved through slow changes in the current of the linear current regulator and digital readjustment of the voltage generated by the digital voltage source based on voltage measurements at the NMR coil's ground side.

[0030] When the target current is reached, the digital voltage source is configured within its digital range to minimize the voltage drop across the linear portion of the current source and to allow for a defined margin of error in the linear current regulator's adjustment. This means that for a "positive" current, the voltage is at the lower end of the linear current regulator's voltage range; for a "negative" current, the voltage is at the upper end of the linear current regulator's voltage range. Here, "positive" refers to a current flowing into the linear portion of the current source.

[0031] To further clarify this invention, some key terms are explained in detail below:

[0032] Stable and low-noise current source:

[0033] Under a resistive load of 1A and 10Ohm, the actual measured noise from 1Hz to 100Hz is less than 1μArms, and the actual measured noise from 1Hz to 200kHz is less than 20μArms. The requirement of this invention is to keep the noise at the lowest possible level with reasonable effort. To achieve this, the entire circuit needs to have sufficiently good suppression of interference across the entire frequency range. This will also be explained in detail below in the description of the pre-regulator.

[0034] Linear current regulator:

[0035] A current regulator with a linear output stage is a linear regulator. A linear current regulator can be implemented analog and / or digitally. In the case of this invention, both are used: a "fast" analog current regulator is controlled by a slower digital regulation loop.

[0036] For the stability of linear current regulators, the following limits should be pursued: gain drift <11ppm / ℃ and offset drift <±1μA / ℃.

[0037] Digital voltage source employing switching technology:

[0038] A digital voltage source employing switching technology is a DC-DC voltage converter that efficiently converts an input DC voltage into another DC voltage using switching elements and one or more energy storage devices. In the case of this invention, a buck converter is typically used, i.e., to achieve a reduction relative to the feed voltage.

[0039] Preferred embodiments and improvements of the present invention

[0040] In a particularly preferred embodiment of the NMR device according to the invention, the digital voltage source can operate in a pulsed manner.

[0041] The operating mode is that of a digital voltage source using switching technology. Pulsed voltage sources (also known as switching voltage sources) are very efficient and therefore generate less heat for the same output power.

[0042] In a preferred improvement of the type of embodiment, the pulse frequency of the digital voltage source is in the range of 10 kHz to 1 GHz, preferably in the range of 30 kHz to 200 kHz.

[0043] At frequencies too low (<10kHz), the filter element placed between the digital voltage source and the NMR coil becomes too large and slow. At frequencies too high (>1GHz), switching losses become too high, efficiency deteriorates, and excessive heat is generated. However, the latter will continue to improve with technological advancements. In principle, higher frequencies are preferred because the filter can be smaller and / or better.

[0044] In other advantageous improvements to the described type of embodiment, the digital voltage source can operate in a pulse width modulation (PWM) manner. Here, with the feed voltage of the digital voltage source constant, the duty cycle (the ratio of on-time to periodic time) is increased or decreased. The duty cycle thus determines the voltage applied to the NMR coil.

[0045] A defined output voltage is generated from an existing input voltage at a given duty cycle. Pulse width modulation (PWM) allows for simple implementation and good predictability of the digital voltage source's output voltage. Furthermore, the pulse frequency of the digital voltage source defines a well-defined fundamental frequency, which can be filtered using fixed components.

[0046] Other equally advantageous improvements feature a digital voltage source with a filter (typically an analog filter) comprising at least one inductive element (particularly one or more coils) and one or more capacitive elements (particularly capacitors) connected in parallel to ground, wherein the filter is configured to smooth the voltage of the pulsed digital voltage source. Preferably, the filter is arranged directly between the digital voltage source and the NMR coils.

[0047] The first stage of the filter may consist of a PWM storage choke and a capacitor for filtering out the DC component of the rectangular voltage from the digital voltage source. Additionally, a buffer may be provided for damping the filter.

[0048] The second stage includes, for example, a filter choke and capacitors to better filter the PWM frequency, as well as another damper for damping the filter. Here, the PWM frequency is the pulse frequency of the pulsed voltage source.

[0049] The first stage should include inductors and / or capacitors. The second stage can also be implemented differently from the LC stage, such as in a linear manner (but this may correspondingly mean more losses). Alternatively, the second LC stage can be integrated into the first stage, thus eliminating the need for the second stage. Accordingly, the first stage will be slightly larger. The advantage of splitting it into two parts is that the second stage can be spatially separated from the PWM stage (which, for example, serves multiple current sources simultaneously), and thus the individual voltages can be better shielded from interference caused by other current sources.

[0050] Another advantageous embodiment of the NMR device according to the invention is characterized in that the digital voltage source is configured to provide not only a negative voltage value but also a positive voltage value.

[0051] The direction of the current flowing through the NMR coil depends on the sign of the voltage value. This is advantageous because it allows the magnetic field of the NMR coil to be influenced and altered in both directions.

[0052] One particularly preferred embodiment of the invention is characterized in that the linear current regulator includes a shunt resistor and an analog-to-digital converter connected across the shunt resistor for measuring the current flowing through the NMR coil.

[0053] Since the current is set digitally, it must also be able to be measured digitally.

[0054] The common method for measuring current is using a shunt resistor. Alternative current measurement methods are not stable or accurate enough.

[0055] These improved embodiments are advantageous, wherein the analog-to-digital converter is connected to the digital conditioning unit.

[0056] By properly programming the digital adjustment unit, the analog current regulator can adjust the digital voltage source, ensuring that the voltage on the analog side of the arrangement structure is within a pre-selected operating range. Consequently, the power consumed in the analog section of the arrangement structure is extremely low.

[0057] In the digital realm, various adjustment techniques can be optimized and implemented as programs. Furthermore, subsequent adjustments are much simpler.

[0058] Digital voltage sources must operate digitally anyway, hence the advantage of digital regulators.

[0059] The improved version of the above embodiment of the present invention is particularly preferred, characterized in that the digital adjustment unit is connected to the electric amplifier through a digital-to-analog converter, and preferably connected to the digital voltage source through an adjustment line.

[0060] The electrical amplifier operates in analog mode; while the digital conditioning unit operates in digital mode. To control the electrical amplifier, a digital-to-analog converter is used to bridge the digital-to-analog boundary. This also allows for the achievement of the necessary stability and resolution.

[0061] In another particularly preferred embodiment of the NMR device according to the invention, the digital voltage source is controlled according to a predetermined time grid. If multiple current sources are arranged spatially adjacently, mutual interference can be reduced if the digital voltage sources are activated synchronously in time.

[0062] By controlling with a fixed time grid, interference between multiple high-efficiency current sources of the same type is avoided. Discrete voltage levels are generated using the fixed time grid. The discrete voltage levels of these digital voltage sources are compensated for by a linear current regulator.

[0063] In a particularly advantageous improvement of the embodiment, the integer divisor of the measurement interval of the analog-to-digital converter is selected as a time grid, or the time grid is selected such that the digital filtering element suppresses the frequency of the time grid as well as possible so that it does not interfere with the current flowing through the NMR coil.

[0064] Common analog-to-digital converters (ADCs) have integrated this digital filtering functionality. Therefore, the filters in an ADC can be raster-matched and / or vice versa.

[0065] An advantageous variant of the aforementioned improved scheme is characterized by the presence of at least one pre-regulator for the feed of the digital voltage source.

[0066] A pre-regulator (“power stabilizer”) typically integrated into a digital voltage source is used to further suppress interference in the mid-frequency range (10Hz-10kHz) from the voltage source’s power supply. On the other hand, a PWM filter only filters frequencies above 10kHz.

[0067] Each current source should be low-noise, generate as little interference as possible over a wide frequency range, and be very stable (i.e., with as little interference as possible at the lowest frequency).

[0068] In the mid-frequency range, the linear regulator is no longer effective enough, and the analog filter also fails to function properly. Therefore, the feed voltage of the digital voltage source is pre-regulated to keep interference from the digital voltage source in the mid-frequency range away from the load.

[0069] Other advantageous embodiments of the invention are characterized in that the current source is configured such that the current flowing through the NMR coil can vary in the range of -20A to +20A, particularly in the range of -1A to +1A.

[0070] The current flowing through the NMR coil, which is connected as an electrical load, can be set at high resolution and accuracy (e.g., up to 20 bits) (within a given range, meeting the requirements of the NMR system), and can be varied over time if necessary. Therefore, along with the NMR coil, minimal changes in the NMR magnetic field of the NMR device caused by external events can be corrected.

[0071] Typical key data for a stable and low-noise current source are as follows:

[0072] - Switching frequency: >10kHz (in pulsed operation)

[0073] - Current: <±1A per current source

[0074] - Feed voltage: ±24V

[0075] - Noise @ 1A, 10 ohm resistive load: 1Hz-100Hz: <1μArms, 1Hz-200kHz: <20μArms

[0076] - Load: 0-20Ω, 0-1mH

[0077] - Resolution: >20 bits

[0078] - Stability: Gain drift <11ppm / ℃ and offset drift <±1μA / ℃

[0079] In practice, embodiments of the present invention have proven particularly effective, wherein the NMR coil is configured as a coil for homogenizing the NMR device and / or for homogenizing the NMR magnetic field generated by the NMR magnet system, or as a coil for changing the NMR magnetic field, particularly with the aid of a flux pump.

[0080] A possible scenario is that, for example, when operating multiple shim coils, dozens of such current sources are needed, with their current and voltage requirements unknown beforehand. Only in the final application can it be determined which power source must drive which shim coil with which current.

[0081] For flux pumps, the voltage is relatively low, therefore the maximum current is sometimes higher. Flux pumps in NMR equipment are used to compensate for the attenuation of the main magnetic field of the NMR magnet system through inductive coupling.

[0082] Other advantages of the invention will become apparent from the description and drawings. Similarly, the features mentioned above and further elaborated can be used individually or in any combination according to the invention. The illustrated and described embodiments should not be construed as an exhaustive enumeration, but rather as exemplary features used to describe the invention. Attached Figure Description

[0083] The present invention is illustrated in the accompanying drawings and explained in more detail with the aid of embodiments.

[0084] Figure 1 A schematic block diagram of key parts of an NMR device modified according to the present invention is shown; and

[0085] Figure 2 A schematic block diagram of a preferred embodiment of the present invention is shown. Detailed Implementation

[0086] In general, the present invention relates to a modified NMR device 10 comprising an NMR magnet system (not shown separately in the figures) for generating a uniform magnetic field, the NMR magnet system including at least one NMR coil 11 for modifying the NMR magnetic field. Furthermore, a stable and low-noise current source is provided, configured to supply current to the NMR coil 11, and the current source includes a linear current regulator 12 electrically connected to a first terminal of the NMR coil 11.

[0087] like Figure 1 As shown, the NMR device 10 according to the present invention is characterized by, compared with the prior art, that the current source includes a clock-driven digital voltage source 13 employing switching technology, which is electrically connected to the second terminal of the NMR coil 11, such that the NMR coil 11 is connected as an electrical load between the linear current regulator 12 and the digital voltage source 13. The linear current regulator 12 is connected to the digital voltage source 13 via a regulating line 14 for regulating voltage. According to the present invention, the linear current regulator 12 is configured to regulate the digital voltage source 13 such that the voltage on the linear portion side of the arrangement structure is within a preselected operating range (e.g., 0 to 3.3V), thereby consuming as little electrical power as possible in the linear portion of the arrangement structure.

[0088] Typically, the digital voltage source 13 can operate in pulse mode, wherein the pulse frequency is in the range of 10 kHz to 1 GHz, preferably in the range of 30 kHz to 200 kHz. In one specific embodiment of the invention, a frequency of 104.1666 kHz is used.

[0089] In particular, the digital voltage source 13 can operate in pulse width modulation mode. The magnitude of the output voltage is determined by the pulse width modulation of the digital voltage source 13. The linear current regulator 12 determines the current flowing through the NMR coil 11, and in particular, the fine-tuning of this current. Typically, the current regulator 12 cannot adequately compensate for only high-frequency interference.

[0090] Figure 2 A block diagram of a particularly preferred embodiment of the NMR device 10 according to the present invention is shown.

[0091] Digital voltage source 13 is configured to provide both positive and negative voltage values, as can be seen in the accompanying drawings from the symbols V+ and V- at the two voltage input terminals. Typically, this involves voltages between +24V and -24V. Therefore, this voltage range is greater than the preselected voltage range of the linear portion of the circuit.

[0092] As in Figure 2 As clearly shown, in this embodiment, the digital voltage source 13 has a filter 15 (hereinafter also referred to as an "analog filter"), which includes at least one inductive element, particularly one or more coils 15', and one or more capacitive elements, particularly capacitors 15', connected in parallel to ground. The filter 15 is configured to smooth the voltage of the digital voltage source 13. Figure 2 In the diagram, filter 15 is shown spatially separate from digital voltage source 13; however, the filter can also be integrated into the digital voltage source. In any case, the filter is arranged between the voltage source and the NMR coil.

[0093] In the illustrated embodiment, the linear current regulator 12 includes a shunt resistor 16 and an analog-to-digital converter 17 connected across the shunt resistor for measuring the current flowing through the NMR coil 11. The analog-to-digital converter 17 is connected to a digital regulation unit 18.

[0094] In addition, the linear current regulator 12 includes an electrical amplifier 19 for feeding current into the NMR coil 11.

[0095] The digital adjustment unit 18 is connected to the electric amplifier 19 via the digital-to-analog converter 20, and preferably to the digital voltage source 13 via the adjustment line 14.

[0096] The digital voltage source 13 is controlled according to a specified time grid so that multiple simultaneously used current sources do not interfere with each other. The integer divisor of the measurement interval of the analog-to-digital converter 17 (hereinafter also referred to as "ADC") can be selected as the time grid. However, the time grid can be selected such that the digital filtering elements suppress the frequency of the time grid as well as possible. The measurement interval of the analog-to-digital converter corresponds to the ADC frequency in the frequency space.

[0097] If arbitrary PWM frequencies or the voltages generated thereby are allowed, then the voltage signal of a digital voltage source will contain all these arbitrary frequencies, as well as mixed frequencies with those of other current sources. These cannot be filtered out well enough by analog filters; at the very least, such filters either lack space in the layout or are too expensive or consume too much power. Uncontrolled mixed frequencies can directly cause interference in NMR measurements. However, especially for shimming applications, current stability is crucial.

[0098] Ideally, a Nyquist filter should exist before the analog-to-digital converter (ADC) to filter out frequencies above half the ADC frequency. However, due to the extremely precise measurements required in NMR operation, such a filter would introduce excessive errors, or the cost and complexity of continuous post-calibration would be too high / complex, if generally possible. Therefore, Nyquist filters are typically not used. Consequently, mixed frequencies that cause measurement errors are generated in the ADC. Common Sigma-Delta ADCs have integrated digital filters that suppress selected frequencies (and their multiples) very well.

[0099] The largest measurement error occurs at the point of greatest interference, specifically at the PWM frequency (and its multiples) of the digital voltage source, which cannot be perfectly filtered out by the analog filter. Therefore, the PWM frequency and the ADC filter are matched to prevent the ADC from incurring measurement errors due to PWM interference.

[0100] In the arrangement according to the present invention, there should be at least one pre-regulator for powering the digital voltage source 13. Figure 2 The embodiment shows two pre-regulators 21', 21', which are shown here as spatially integrated within the digital voltage source 13, but in other embodiments they may also be arranged separately from the digital voltage source 13, as shown in the appendix. Figure 2 Like filter 15 in the middle.

[0101] The current source of the NMR device 10 modified according to the present invention is generally configured such that the current flowing through the NMR coil 11 can vary in the range of -20A to +20A, particularly in the range of -1A to +1A.

[0102] In a particularly important application of the invention, the NMR coil 11 is configured as a coil for homogenizing the NMR device 10 and / or for homogenizing the NMR magnetic field generated by the NMR magnet system. However, for example, the NMR coil can also be used to change the NMR magnetic field. The device then functions as a flux pump.

[0103] The magnitude of the current is determined by a linear current regulator. This will continue as long as the power supply produces a voltage—a voltage that, taking into account the current and load, produces a voltage within its permissible range on the operational amplifier. Otherwise, the current would be indeterminate, unregulated, unstable, and noisy, and the current regulator could be damaged without protection circuitry. The current regulator operates under a voltage less than that the digital voltage source can produce.

[0104] Typically, an electrical amplifier is constructed as one or more interconnected operational amplifiers. Most operational amplifiers can be fed into or senken with current, meaning that current can also flow into the tip of the triangle representing the operational amplifier in a circuit diagram; in this invention, this is referred to as "positive" current.

[0105] At first glance, ground is not important to the circuit:

[0106] The positive current originates from the +V feed section, is converted to a lower voltage, flows through the NMR coil, and then flows into the negative feed section of the operational amplifier.

[0107] The negative current originates from the positive feed section of the operational amplifier, flows through the NMR coil, enters the digital voltage source, and exits through the -V feed section.

[0108] The fact that it is not mentioned here is also meaningful, because as long as the voltage difference is the same, the same current can be obtained on both sides of the NMR coil with different potentials / voltages.

[0109] But there are still boundary conditions, and here they come into play again:

[0110] The voltage generated by the pulsed digital voltage source must be filtered / stabilized relative to a certain potential. This potential is advantageously ground.

[0111] - In order to save a power supply section and to be able to control the operational amplifier with reference to ground, the linear output stage (i.e., the power amplifier) ​​is not symmetrically powered, that is, the negative power supply section of the power amplifier is ground here.

[0112] However, the operational amplifier symbol in the block diagram does not represent just a single power amplifier. Under normal circumstances, other operational amplifiers that do not carry all the current are also required. Nevertheless, these other operational amplifiers may require ± feed sections, but these feed sections do not need to provide as much current as the feed section of the power amplifier.

[0113] List of reference numerals in the attached diagram:

[0114] 10 NMR devices

[0115] 11 NMR coils

[0116] 12 Linear Current Regulator

[0117] 13 Digital voltage source employing switching technology

[0118] 14 Adjusting the circuit

[0119] 15 Filters

[0120] 15' Inductor

[0121] 15” capacitor element

[0122] 16 Shunt resistor

[0123] 17 Analog-to-Digital Converter

[0124] 18 digital control units

[0125] 19. Electrical Amplifier

[0126] 20 Digital-to-Analog Converters

[0127] 21', 21" pre-regulator

[0128] List of references:

[0129] Publications considered for determining patentability:

[0130]

[00] US6492817B2

[0131] [0]NICK ARRANGO et al., "Open-source, low-cost, flexible, current feedback-controlled driver circuit for local B0 shim coils and other applications", published February 7, 2022, source (URL: https: / / cds.ismrm.org / protected / 16MProceedings / PDFfiles / 1157.html) [retrieved on July 27, 2023]

[0132] [1]DE10104365C1≈GB2411238B≈US2005 / 0174118A1

[0133] [2] The applicant’s technical manual, titled “BSMS System for AVANCE NEO”, version 002, Chapter 10 “SCB20”, subsections 10.1-10.4, pages 83-90.

[0134] [3]EP0780698B1≈US5,744,959A

[0135] [4]DE102015225731B3≈EP3182147B≈US9,766,312B1≈CN106898452B≈JP6340403B

Claims

1. An NMR device (10) having an NMR magnet system for generating an NMR magnetic field, the NMR magnet system including at least one NMR coil (11) for modifying the NMR magnetic field, and the NMR device having a stable current source configured to supply current to the NMR coil (11), the current source including a linear current regulator (12) electrically connected to a first terminal of the NMR coil (11), wherein, The current source includes a digital voltage source (13) electrically connected to the second terminal of the NMR coil (11), such that the NMR coil (11) is connected as an electrical load between the linear current regulator (12) and the digital voltage source (13). The linear current regulator (12) is connected to the digital voltage source (13) via a regulating line (14) for regulating voltage. The current source is low-noise, and the digital voltage source (13) is implemented using switching technology. The linear current regulator (12) is configured to regulate the digital voltage source (13) such that the voltage on the linear current regulator (12) side is within a pre-selected operating range, thereby reducing power consumption in the linear current regulator (12). The linear current regulator (12) is characterized in that it includes an electrical amplifier (19) for feeding current into the NMR coil (11), the linear current regulator (12) includes a shunt resistor (16) and an AD converter (17) connected across the shunt resistor, the AD converter being used to measure the current flowing through the NMR coil (11), the AD converter (17) being connected to a digital adjustment unit (18), and the digital adjustment unit (18) being connected to the electrical amplifier (19) via a DA converter (20).

2. The NMR device according to claim 1, characterized in that, The digital voltage source (13) is capable of pulsed operation.

3. The NMR device according to claim 2, characterized in that, The pulse frequency of the digital voltage source (13) is in the range of 10 kHz to 1 GHz.

4. The NMR device according to claim 2 or 3, characterized in that, The digital voltage source (13) can operate in pulse width modulation mode.

5. The NMR device according to claim 2 or 3, characterized in that, The digital voltage source (13) has a filter (15) comprising at least one inductor and one or more capacitors grounded in parallel, wherein the filter (15) is configured to smooth the voltage of the digital voltage source (13).

6. The NMR apparatus according to any one of claims 1 to 3, characterized in that, The digital voltage source (13) is configured to provide not only negative voltage values ​​but also positive voltage values.

7. The NMR apparatus according to any one of claims 1 to 3, characterized in that, The control of the digital voltage source (13) is performed according to a specified time grid so that multiple current sources used at the same time will not interfere with each other.

8. The NMR device according to claim 7, characterized in that, The integer divisor of the measurement interval of the AD converter (17) is selected as a time grid, or the time grid is selected such that the digital filter element suppresses the frequency of the time grid.

9. The NMR device according to claim 8, characterized in that, There is at least one pre-regulator (21', 21'') for powering the digital voltage source (13).

10. The NMR apparatus according to any one of claims 1 to 3, characterized in that, The current source is configured such that the current flowing through the NMR coil (11) can vary in the range of -20A to +20A.

11. The NMR apparatus according to any one of claims 1 to 3, characterized in that, The NMR coil (11) is configured as a coil for homogenizing the NMR device (10) and / or for homogenizing the NMR magnetic field generated by the NMR magnet system, or as a coil for changing the NMR magnetic field.

12. The NMR device according to claim 1, characterized in that, The digital adjustment unit (18) is connected to the digital voltage source (13) through the adjustment line (14).

13. The NMR device according to claim 3, characterized in that, The pulse frequency of the digital voltage source (13) is in the range of 30kHz to 200kHz.

14. The NMR device according to claim 5, characterized in that, The filter includes one or more coils (15') and one or more capacitors (15'') connected in parallel to ground.

15. The NMR device according to claim 10, characterized in that, The current source is configured such that the current flowing through the NMR coil (11) can vary in the range of -1A to +1A.

16. The NMR device according to claim 11, characterized in that, The NMR coil (11) is configured as a coil for homogenizing the NMR device (10) with the aid of a flux pump and / or for homogenizing the NMR magnetic field generated by the NMR magnet system, or for changing the NMR magnetic field.

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