WO2022008548A1

WO2022008548A1 - Magnetic field probe, particularly for magnetic resonance applications, and tracking arrangement comprising the same

Info

Publication number: WO2022008548A1
Application number: PCT/EP2021/068725
Authority: WO
Original assignee: Eidgenössische Technische Hochschule (ETH); Universität Zürich
Priority date: 2020-07-07
Filing date: 2021-07-07
Publication date: 2022-01-13

Abstract

A magnetic field probe (2), particularly for magnetic resonance applications, comprises a sample (4) that exhibits nuclear magnetic resonance (NMR) at an operating frequency, and an electrically conductive structure (6) surrounding the sample and configured for pulsed NMR excitation of the sample and for receiving an NMR signal generated by the sample. In order to improve performance and compactness of the field probe, the sample is configured as a capsular sample comprising a shell (10) which contains a fluid core (12) of a sample substance that exhibits NMR at the operating frequency. An arrangement for tracking the position and orientation of an object or subject disposed in a main magnetic field region of a magnetic resonance (MR) apparatus during operation thereof, is provided with a carrier structure made of non-conducting material and rigidly attachable to the object or subject, and with a plurality of at least three magnetic field probes according to one of the preceding claims mounted to said carrier structure.

Description

Magnetic field probe, particularly for magnetic resonance applications, and track- ing arrangement comprising the same

Field of the Invention The present invention generally relates to a magnetic field probe, particularly for mag netic resonance (MR) applications, and to an arrangement for tracking the position and orientation of an object or subject.

Background of the Invention Magnetic field probes comprising a substance that exhibits nuclear magnetic resonance (NMR) are increasingly used to measure the spatiotemporal magnetic field evolution during MR procedures ( DeZanche , N., Barmet, C., Nordmeyer-Massner, J.A., Pruess- mann, K.P., 2008. NMR probes for measuring magnetic fields and field dynamics in MR systems. Magn. Reson. Med. 60, 176-186 ; and Barmet, C., De Zanche, N., Wilm, B.J., Pruessmann, K.P., 2009. A transmit/receive system for magnetic field monitoring of in vivo MRI. Magn. Reson. Med. 62, 269-276).

. Such "NMR field probes" are typically formed by an NMR-active liquid sample enclosed in a capillary surrounded by a solenoid coil for signal excitation and detection. The sen- sitive volume is shaped by the solenoid’s spatial transmit and receive characteristics. One key downside of this approach is that the resulting effective droplet is only softly de fined, leading to a sub-optimal tradeoff between signal yield and k-space range that can be covered without probe de-phasing. What is more, the inhomogeneous transmit field amplitude causes a flip angle gradient throughout the excitation volume, which reduces the signal to noise ration (SNR). For optimal performance, the sensitive volume should be spherical, sharply delineated, and fully excitable with uniform flip angle.

In order to reach these goals, it has been proposed to use physical droplets of sample liquid suspended in a gelled medium ( Nussbaum , J., Gross, S., Brunner, D.O., Barmet, C., Schmid, T., Dietrich, B. E., Weiger, M., and Pruessmann, K.P., 2016. Spherical drop let design and adiabatic excitation for enhanced performance and flip angle control of NMR field probes. Proceedings of the Annual Meeting of ISM RM, Singapore, 2016. Ab stract 2178). In this earlier work it has been shown that only a field probe with spherical sample performs equally well under gradient fields of any direction. Furthermore, a physically sharply confined sample enables increasing the probe SNR by optimization of coil and RF pulse design. In these respects, the probes based on gel-suspended sample droplets are not optimal. On the one hand, the samples are not perfectly spherical, which complicates susceptibility matching processes to achieve high field homogeneity within the sample. On the other hand, the sample are embedded in a capillary that needs to have a certain length to achieve high field homogeneity within the sample. This makes the overall probe head very long.

Summary of the Invention

In view of the above, it is the principal object of the present invention to overcome the limitations and disadvantages of currently known NMR field probes.

According to one aspect of this invention, there is provided a magnetic field probe, par ticularly for magnetic resonance applications, comprising: a sample that exhibits nuclear magnetic resonance (NMR) at an operating fre quency, and an electrically conductive structure surrounding the sample and configured for pulsed NMR excitation of the sample and for receiving an NMR signal generated by the sample.

The sample is configured as a capsular sample comprising a shell which contains a fluid core of a sample substance that exhibits nuclear magnetic resonance (NMR) at the op erating frequency.

Unless explicitly mentioned otherwise, the terms "conductive", "conductivity" etc. shall be understood in relation to electrical conductivity.

The terms "shell" and "core" of the capsular sample shall be understood in generally known manner. In other words, the "core" shall refer to an internal region of the sample which is completely surrounded by the "shell", where the latter is an external layer. Pref erably, the shell has a thickness that is substantially smaller than the core size. In order to contain the fluid core, the shell needs to have appropriate mechanical stability. In prin ciple, the shell could be a hard, solid layer or it could be in a glassy state. However, de pending on the method and materials used to from the capsular samples, the shell will typically have some resilience. In particular, the shell may be made of a gel or of a poly mer.

The inventors have found that magnetic field probe of the above type, i.e. an NMR field probe provided with a capsular sample can be produced and that they have many ad vantageous properties. In particular, such probes can be produced with a well-defined sensitive volume. Compared with conventional NMR field probes using a sample droplet trapped within a capillary tube, the magnetic field probes of the present invention are more compact and thus suitable for miniaturized setups. By using an appropriate tech- nique, the capsular samples can be produced without any gas entrapments. By virtue of their uniform and continuous shell, the capsular samples do not suffer from leakage problems as occasionally encountered with plugged capillary designs. Quite importantly, the capsular samples can be produced with a nearly perfect spherical shape, which is highly advantageous in many MR applications. However, the present invention is not limited to strictly spherical capsular samples. For example, in certain situations, it may be preferable to use capsular samples with an ellipsoidal shape.

Advantageous embodiments of the invention are defined in the dependent claims and are discussed in the description below.

According to one embodiment (claim 2), the magnetic field probe further comprises an electrically non-conductive jacket encasing the sample and the conductive structure. Preferably, the jacket has a magnetic susceptibility that is substantially identical to the magnetic susceptibility of the conductive structure. This will basically make the conduc- tive structure, e.g. a copper coil, magnetically invisible. For this purpose, the jacket can be cast from a two-component epoxy containing a suitable dopant and forming an ellip soid with susceptibility matched to the conductive structure. Examples for such jacket structures are described in WO 2007/118715 A1. According to an advantageous embodiment (claim 3), the shell is formed from an appro priate precursor liquid, i.e. from a liquid which undergoes a hardening or solidifying step by applying appropriate external conditions. The precursor liquid used for shell formation shall be immiscible with the core liquid and it should harden quickly. Examples of the hardening step are polymerization and gelation. Suitable examples of such shell liquids are gelatin, agar agar or alginate.

For certain applications, the shell preferably comprises an inner layer, e.g. a gel or poly mer layer, which is covered by an external layer, particularly a thin metallic layer (claim 4). Such a metallic layer may serve as a seal against degassing and/or contamination. It should nonetheless be sufficiently thin so as to allow the intended use of the magnetic field probe for MR applications.

Liquid filled capsules with diameters in the range of few hundreds of micrometers up to few millimeters are needed in many industries as, e.g., for pharmaceuticals, cosmetics, chemistry and food. Such capsules are often referred to as "microcapsules". Currently there exist various encapsulation methods. Among these, prilling by vibration is a method that allows encapsulating liquids with a very spherical, solid shell in a controlled manner and is therefore particularly suitable for the encapsulation of NMR-active liquids. Therefore, according to an advantageous embodiment (claim 5), the capsular sample is obtained by prilling. The basic working principle of prilling by vibration is as follows: two immiscible liquids, one for the core of the capsule and one that will form the shell, are pumped through two concentric nozzles. The nozzles vibrate such that the stream of liq uids is broken and droplets are formed. Due to the surface tension of the liquids, the droplets become more or less spherical with the core liquid trapped inside the shell liq uid. By applying a voltage at a nearby located electrode, the droplets are dispersed by electrostatic repulsion. Finally, they fall into a bath that causes hardening of the shell.

As already mentioned above, one advantageous embodiment is based on having a sub stantially spherical capsular sample (claim 6). In such cases it is possible and particu larly advantageous to provide the capsular sample with a jacket that is also substantially spherical.

As will be understood, it is also advantageous if the shell has a magnetic susceptibility that is substantially identical to the magnetic susceptibility of the sample substance (claim 7). This can be achieved by incorporating suitable paramagnetic dopants into the shell and/or into the sample substance. By appropriate selection of the dopant(s) added into the sample substance, one may also apply some selection to the relaxation time of the sample substance. In a further variant, the shell has a magnetic susceptibility that is substantially identical to the magnetic susceptibility of a surrounding jacket. This is par ticularly useful in situations where the shell has a comparatively smooth inner surface and a comparatively rough outer surface adjacent to the surrounding jacket.

Ideally, the sample substance should be a fluorine or hydrogen NMR-active liquid with a single peak, i.e. having a singlet signal in the relevant NMR spectrum (claim 7). Further more, the sample substance should preferably be dopable with paramagnetic ions or compounds. Good candidates fulfilling all these criteria for 1 H NMR are water, cyclohex ane, cyclooctane, methanol, isopropanol, acetone and crown ethers. For the case of 19F NMR, preferred sample substances are hexafluorobenzene, hexafluoroisopropanol, trifluoroethanol, hexafluoroacetylacetone, nonafluoro-tert-butanol, hexafluoro-2,3-bis(tri- fluoromethyl)-2,3-butanediol (‘perfluoropinacol’), and fluorinated crown ethers. In a fur ther advantageous embodiment, the sample substance is an ionic liquid, as described in Looser, A., Barmet, C., Fox, T., Blacque, O., Gross, S., Nussbaum, J., Pruessmann,

K.P., Alberto, R., 2018. Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic Liquids for NMR Field Probes. Inorg. Chem. 57, 2314-2319.

(https://doi.Org/10.1021/acs.inorgchem.7b03191 )

In many advantageous embodiments of the magnetic field probe, the capsular sample has a diameter of 0.5 to 2 mm, preferably of 0.8 to 1 .3 mm (claim 10). This has been found to yield a good probe signal with a remarkably compact device.

As will be understood, the electrically conductive structure can be configured in various manners basically known in the art. For the present invention, an advantageous embodi ment (claim 11 ) is based on a conductive structure comprising at least one loop portion, typically about 3 to 7 loops formed from a copper wire.

According to a particularly advantageous embodiment contributing to a novel tracking technology that is precise, versatile, safe and practical for broad use, the magnetic field probe further comprises means for preprocessing NMR signal generated by the sample into a digital preprocessed NMR signal, and means for wirelessly transmitting the digital preprocessed NMR signal to an external unit for further processing (claim 12). In one embodiment thereof (claim 13), the preprocessing means comprise: means for converting the raw NMR signal to a digital NMR signal; means for carrying out a phase extraction process to obtain a phase signal which constitutes said digital preprocessed NMR signal. Advantageously, the wireless transmitting means operate with a carrier frequency be tween 2.402 GHz and 2.480 GHz (claim 14).

The properties and performance of the above defined magnetic field probes are highly advantageous for applications requiring a compact assembly comprising a plurality of field probes. Therefore, a further aspect of the present invention relates to an arrange ment for tracking the position and orientation of an object or subject disposed in a main magnetic field region of a magnetic resonance (MR) apparatus during operation thereof, the arrangement comprising: a carrier structure made of non-conducting material and rigidly attachable to the ob- ject or subject; and a plurality of at least three magnetic field probes according to one of the preceding claims mounted to said carrier structure.

While it is generally understood that tracking of position and orientation may be achieved with an arrangement of three magnetic field probes, it is often preferable to have a larger number. One such arrangement corresponds to a non-degenerate tetrahe dron.

Brief description of the drawings The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better under stood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, which show the following: Fig. 1 an embodiment of a magnetic field probe according to the present in vention, as a schematic vertical section;

Fig. 2 an arrangement for producing capsular samples for a magnetic field probe, as a schematic representation; Fig. 3 various stages in the manufacturing of a magnetic field probe, as pho tographic representations; Fig. 4 signal of a magnetic field probe; top: T2 estimations by fitting the theoretical curve of a point sample

(green) and a spherical sample (red) under a gradient field to the real part of the demodulated probe signal; middle: phase of the demodulated probe signal; bottom: T1 estimation with the inversion recovery method.

Detailed description of the invention

As shown in Fig. 1 , a magnetic field probe 2 comprises a sample generally denoted as 4. The sample exhibits NMR at an operating frequency, and it is surrounded by an elec- trically conductive structure 6 which is configured for pulsed NMR excitation of the sam ple and for receiving an NMR signal generated by the sample. In the example shown here, the conductive structure 6 is configured as a solenoid with a plurality of loops. The sample 4 and the conductive structure 6 are encased within a jacket 8 formed of an electrically non-conductive material. In the embodiment shown here the jacket 8 is con- figured as an elongated cylinder. In other embodiments, the jacket can be formed as a sphere or a spheroidal body. As also shown in Fig. 1 , the sample 4 is configured as a capsular sample comprising an electrically non-conductive shell 10 which contains a fluid core 12 of a sample substance that exhibits NMR at the desired operating fre quency.

Example

Methods

There are several companies that offer commercial encapsulation solutions for industrial and lab-scale applications. For this example, the lab-scale encapsulation system "En- capsulator B-390" from BLJCHI (BLICHI Labortechnik AG, Flawil, Switzerland (www.bu- chi.com)) was chosen.

Core liquid For sufficient SNR of the final probe, the sample liquid has to be a single peak fluorine or hydrogen NMR-active liquid. Furthermore, it should be dopable with paramagnetic ions or compounds. Good options fulfilling all these criteria are water, cyclohexane, cy clooctane and hexafluorobenzene. The final goal is a fluorine containing capsule, which does not interact with the MR experiment. This makes hexafluorobenzene a desirable core liquid. Similar to hexafluorobenzene, cyclohexane and cyclooctane are hydrophobic and therefore immiscible with the liquid phase of water-based polymers such as gelatin, agar agar or alginate. Hence, for the experiments reported here, cyclohexane and hex afluorobenzene were chosen as core liquid. To enable optical evaluation of the cap sules, the core liquid was dyed blue with Solvent Blue 36 (1 ,4-Bis(isopropylamino)an- thraquinone).

Shell liquid

The shell liquid has to be immiscible with the core liquid and should harden fast. Well established practical options are gelatin, agar agar or alginate. In initial tests gelatin proved most suitable and was therefore chosen as shell material. To harden the gelatin shell, the capsules fell into a petroleum bath that was kept at 10^eC inside a water-cooled jacketed beaker. The cooling hardens the gelatin to a certain degree. To get a very hard shell, it was cross-linked by adding glutaraldehyde to the hardening bath.

Parameter relations for droplet formation

The formation of droplets is only possible if certain parameters match. For the formation of homogeneous micro-spheres (single nozzle and only one liquid) with prilling by vibra tion, there exists a formula for the optimal set of parameters. It shows the interplay be tween machine parameters like the nozzle diameter D, the liquid volumetric flow rate Q, the vibration frequency f and fluid properties like dynamic viscosity h, density p and sur face tension s [1]:

In the case of two concentric nozzles, the liquid properties in Eq. 3.1 may be approxi mated by an average value of the core and shell liquid properties. However, this is still problematic because the properties of the used liquids are not perfectly known, espe cially those of gelatin which strongly depend on temperature and concentration. Never theless, the knowledge of this formula provides a starting point to find matching parame ters.

The capsule diameter in liquid state is given by the flow rate and the frequency:

Therefore, capsule size can be increased by either reducing frequency or increasing the flow rate. However, this is only successful in accordance with the other parameters as described in Eq. 3.1.

Regarding the eventual capsule size, it is important to account for shrinkage due to hardening and drying.

Encapsulation procedure

A schematic view of an arrangement for carrying out the encapsulation procedure is shown in Fig. 2. The liquid stream disintegrates into droplets, which are dispersed by charging them. The capsules then fall into the hardening bath.

Starting from the afore mentioned theoretical parameter relations the following approach for producing cyclohexane filled capsules was developed empirically:

Nozzle parameters: diameter of the inner nozzle: 300 pm, diameter of the outer noz zle: 400 pm, temperature: 50^eC, vibration frequency: 50 Hz, electrostatic voltage: 1000 V

Core: blue-colored cyclohexane was pumped to the inner nozzle using a syringe pump at a flow rate of 300 ml/h

Shell: gelatin solution with 10 wt% gelatin, kept at 60^eC in a heating bath was pressed through the outer nozzle. The flow rate was regulated by air compression in the gelatin bottle (over-pressure of 400 - 600 mbar) and by a compressing screw at the liquid inlet of the nozzle. Hardening bath: petroleum, cooled to 10^eC and stirred magnetically in order to pre vent capsules from sticking together.

Cross-linking: when all capsules were in the bath, petroleum was extracted until the bath contained about 400 ml. Then, 5 ml toluene saturated with glutaraldehyde was added and left for 30 min to cross-link the shells.

Washing: after the cross-linking process, the capsules were extracted from the bath with a sieve, rinsed with acetone and finally rinsed with water. They were left for drying in a petri dish.

Selection: under the microscope the most spherical capsules were selected.

This process was also carried out with different experimental parameters and with hex- afluorobenzene or a mixture of cyclohexane and hexafluorobenzene.

Probe head Resulting cyclohexane capsules were each cast into a coil. For this purpose, a 5-turn solenoid coil was wound with an enameled copper wire (diameter = 138 pm) on a 1 mm capillary and then stripped off the capillary. Afterwards, the coil was wetted with epoxy of wire-matched susceptibility and the capsule was pushed inside. Due to the surface ten sion of the epoxy, the capsule is automatically drawn to the center of the coil. After hard- ening, the coil with the capsule was cast in epoxy of wire-matched susceptibility in shape of an long cylinder (diameter = 3 mm, length = 25 mm) or a sphere (d=9 mm) to achieve high field homogeneity inside the sample.

For signal excitation and acquisition the probe heads were connected to custom-de- signed RF chains and a stand-alone spectrometer, as described in [2]

Evaluation

The sphericity and size of the capsules were evaluated optically with a microscope. Sim ilarly, the position of the droplet inside the coil was assessed. The final probe perfor- mance was determined by measuring the free induction decay (FID) and comparing it to the theoretical behavior.

Results and discussion Images taken at various stages of the manufacturing process are shown in Fig. 3. The final capsules had a diameter of about 0.8 mm and a very thin shell which can be seen as brownish trace around the capsule when cast into epoxy, see Fig. 3 b-e) inside the coil. It can also be seen that some of the droplets were too small. Along the capillary, however, they were centered owing to the surface tension of the epoxy, see Fig. 3 d).

Signal evaluation

Fig. 4 shows the real part (top) and the phase (middle) of the demodulated signal of the long cylinder probe with a cyclohexane microcapsule. To characterize the probe, the curve of an ideal point sample (green) and the curve of a spherical sample under a con stant gradient (red), both decaying with T2, were fitted to the signal. The good match of the second fit supports the hypothesis that there are external field inhomogeneities that can be approximated to first order. Furthermore, the occurrence of the zero point in the FID is characteristic for a sharply defined NMR sample.

The residual phase of the signal (Fig. 4, middle) after subtraction of the linear modula tion and the constant starting phase should ideally be constantly zero. Flowever, such remaining phase components, that are not related to external field fluctuations, are fre quently observed in field probe signals [3, 4] The main reason for this behavior are in- homogeneities of the BO field that were neglected in the model. They can arise due to susceptibility variations inside the probe head that may occur, e.g., because of magnetic contamination (e.g., dirt) inside the sample. Moreover, the distribution of receive sensi tivity scales the impact of these field inhomogeneities on the residual phase. A possibil ity to tackle this issue is proposed in [5, 6] by calibrating the field probes.

The bottom image of Fig. 4 shows the result of the inversion recovery experiment and the found T 1. The T 1 of 4.8 s is very long and for applications it should be reduced by doping. The T2 of 26.4 ms is much shorter than expected for pure cyclohexane. This may be the result of the discrepancy of the model like additional field inhomogeneities that were neglected.

Hexafluorobenzene capsules While for cyclohexane varying combinations of parameters resulted in capsules, pure hexafluorobenzene could not be encapsulated with the present setup. However, encap sulation of hexafluorobenzene was achieved by using a mixture with cyclohexane. In the production, the flow rate of the core liquid was reduced to 250 ml/h and the nozzle tem- perature was lowered to 40^eC to keep evaporation of hexafluorobenzene as low as pos sible. Furthermore, omitting the washing prevented some capsule breakage. Presently, the encapsulation of hexafluorobenzene remains very challenging, so using another shell material or even a different production method should be considered. References

[1] C. Weber, “Zum Zerfall eines Flussigkeitsstrahles,” Zeitschrift fuer angewandte Mathematik und Mechanik, pp. 136-154, 1931. doi: 10.1002/zamm.19310110207

[2] F. Bloch, W.W. Hansen, and M. Packard, “Nuclear induction,” Physical Review, vol. 69, pp. 127-127, 1946. doi: 10.1103/PhysRev.69.127 [3] N. De Zanche, C. Barmet, J. a. Nordmeyer-Massner, and K. P. Pruessmann,

“NMR Probes for measuring magnetic fields and field dynamics in MR systems,” Magnetic Resonance in Medicine, vol. 60, no. 1 , pp. 176-186, 2008. doi: 10.1002/mrm.21624

[4] B. E. Dietrich, D. O. Brunner, B. J. Wilm, C. Barmet, and K. P. Pruessmann, “Con- tinuous magnetic field monitoring using rapid re-excitation of NMR probe sets,”

IEEE Transactions on Medical Imaging, vol. 35, no. 6, pp. 1452-1462, 2016.

[5] B. E. Dietrich, B. J. Wilm, D. O. Brunner, Y. Duerst, C. Barmet, and K. P. Pruess mann, “k-t-calibration improves continuous field monitoring for image reconstruc tion,” in Proc Int Soc Magn Reson Med Sci Meet Exhib, vol. 22, 2014, p. 1842. [6] B. E. Dietrich, “Field cameras for magnetic resonance systems,” Ph.D. disserta tion, ETH Zurich, Zurich, 2015.

Claims

1. A magnetic field probe (2), particularly for magnetic resonance applications, com prising: - a sample (4) that exhibits nuclear magnetic resonance (NMR) at an operating frequency, and an electrically conductive structure (6) surrounding the sample and configured for pulsed NMR excitation of the sample and for receiving an NMR signal gen erated by the sample; characterized in that the sample is configured as a capsular sample comprising a shell (10) which con tains a fluid core (12) of a sample substance that exhibits NMR at the operating frequency.

2. The magnetic field probe according to claim 1 , further comprising an electrically non-conductive jacket (8) encasing the sample and the conductive structure, wherein the jacket preferably has a magnetic susceptibility that is substantially identical to the magnetic susceptibility of the conductive structure.

3. The magnetic field probe according to claim 1 or 2, wherein the shell is formed by a gel or polymer, particularly from gelatin, agar agar or alginate.

4. The magnetic field probe according to one of claims 1 to 3, wherein the shell com prises an inner layer which is covered by an external layer, particularly a thin me- tallic layer.

5. The magnetic field probe according to one of claims 1 to 4, wherein the capsular sample is obtained by prilling.

6. The magnetic field probe according to one of claims 1 to 5, wherein the capsular sample is substantially spherical.

7. The magnetic field probe according to one of claims 1 to 6, wherein the shell has a magnetic susceptibility that is substantially identical to the magnetic susceptibility of the sample substance or is substantially identical to the magnetic susceptibility of a surrounding jacket.

8. The magnetic field probe according to one of claims 1 to 7, wherein the sample substance is a liquid compound exhibiting singlet 1 H NMR, particularly a liquid se lected from water, cyclohexane, cyclooctane, methanol, isopropanol, acetone and crown ethers.

9. The magnetic field probe according to one of claims 1 to 7, wherein the sample substance is liquid compound exhibiting singlet 19F NMR, particularly a liquid se lected from hexafluorobenzene, hexafluoroisopropanol, trifluoroethanol, hex- afluoroacetylacetone, nonafluoro-tert-butanol, hexafluoro-2,3-bis(trifluoromethyl)- 2,3-butanediol (‘perfluoropinacol’), fluorinated crown ethers and ionic liquids.

10. The magnetic field probe according to one of claims 1 to 9, wherein the capsular sample has a diameter of 0.5 to 2 mm, preferably of 0.8 to 1.3 mm.

11 . The magnetic field probe according to one of claims 1 to 10, wherein the electri cally conductive structure comprises at least one loop portion.

12. The magnetic field probe according to one of claims 1 to 11 , further comprising means for preprocessing NMR signal generated by the sample into a digital pre- processed NMR signal, and means for wirelessly transmitting the digital preprocessed NMR signal to an exter nal unit for further processing.

13. The magnetic field probe according to claim 12, wherein the preprocessing means comprise: means for converting the raw NMR signal to a digital NMR signal; means for carrying out a phase extraction process to obtain a phase signal which constitutes said digital preprocessed NMR signal.

14. The field probe according to claim 13, wherein the wireless transmitting means op erate with a carrier frequency between 2.402 GHz and 2.480 GHz.

15. An arrangement for tracking the position and orientation of an object or subject disposed in a main magnetic field region of a magnetic resonance (MR) apparatus during operation thereof, the arrangement comprising: a carrier structure made of non-conducting material and rigidly attachable to the object or subject; and a plurality of at least three magnetic field probes according to one of the pre ceding claims mounted to said carrier structure.