NMR-MAS probehead with an optimized MAS-DNP coil block for fast sample rotation

Abstract

An NMR-MAS probehead having an MAS stator (3) receiving an MAS rotor (5) that is surrounded by an RF coil (4) and that has a sample substance, and having a first microwave guide (1) supplying microwave radiation into a sample volume (0) through a coil block (2). The coil block is constructed from dielectric material, is inserted into the wall of the MAS stator so that it surrounds the RF coil and the MAS rotor, and has a first bore (4) that extends coaxially with the longitudinal axis of the elongate MAS rotor, the RF coil being fastened to the inner wall of said first bore, as well as a second bore (8) that extends coaxially with the longitudinal axis of the first microwave guide and has a hollow, elongate second microwave guide (8) supplying microwave radiation from the first microwave guide into the sample volume.

Claims

1. A Nuclear Magnetic Resonance Magic Angle Spinning (NMR-MAS) probehead comprising: an MAS stator configured to receive an elongate MAS rotor that is surrounded by an RF coil, wherein the MAS rotor is configured to hold a sample substance in a sample volume, and a hollow, elongate first microwave guide configured to supply microwave radiation into the sample volume through a coil block introduced in a wall of the MAS stator, wherein the coil block is constructed from dielectric material, wherein the coil block has a given geometric extent and is inserted into the wall of the MAS stator to at least partly surround the RF coil and the MAS rotor, and wherein the coil block comprises: a first bore that extends coaxially with a longitudinal axis of the elongate MAS rotor, wherein the RF coil is fastened to an inner wall of the first bore, and a second bore that extends coaxially with the longitudinal axis of the first hollow, elongate microwave guide and that comprises a hollow, elongate second microwave guide configured to supply microwave radiation from the first microwave guide into the sample volume.

2. The NMR-MAS probehead as claimed in claim 1, wherein the first bore of the coil block is dimensioned to receive the MAS rotor with a diameter of less than 2 mm.

3. The NMR-MAS probehead as claimed in claim 2, wherein the first bore of the coil block is dimensioned to receive the MAS rotor with a diameter of 0.4 mm to 1.9 mm.

4. The NMR-MAS probehead as claimed in claim 1, wherein the first microwave guide has a geometric structure that focuses the microwave radiation guided therethrough onto an opening, facing the microwave guide, of the second microwave guide in the coil block.

5. The NMR-MAS probehead as claimed in claim 4, wherein the first microwave guide, on a side facing the sample volume, has a reduction in cross section for focusing the microwave radiation.

6. The NMR-MAS probehead as claimed in claim 4, further comprising a microwave lens configured to focus supplied microwave radiation onto a lens-side opening of the second microwave guide and arranged between the first microwave guide and the coil block.

7. The NMR-MAS probehead as claimed in claim 6, wherein the microwave lens is arranged at a coil-block-side free end of the first microwave guide.

8. The NMR-MAS probehead as claimed in claim 1, further comprising a microwave mirror configured to reflect microwave radiation that emerges from the second microwave guide and passes through the sample volume and arranged on an inner side of the MAS stator that lies opposite the second microwave guide with respect to the MAS rotor.

9. The NMR-MAS probehead as claimed in claim 8, wherein the microwave mirror is attached to the coil block or is spatially integrated into the coil block.

10. The NMR-MAS probehead as claimed in claim 8, wherein the microwave mirror is cylindrical and concave in a direction of the sample volume, and wherein the microwave mirror is configured to focus microwave radiation from the sample volume that is incident on the microwave mirror onto the longitudinal axis of the MAS rotor.

11. The NMR-MAS probehead as claimed in claim 1, wherein the RF coil is a single-layer solenoid coil constructed from a plurality of spaced apart turns, and wherein a winding thickness d and a turn spacing D of the solenoid coil are optimized so that at least 80% of the microwave radiation is transmitted through the RF coil.

12. The NMR-MAS probehead as claimed in claim 11, wherein the RF coil has a winding thickness of 0.05 mm d 0.30 mm and a pitch of the turns of between 0.1 mm and 1.0 mm.

13. A coil block configured for a Nuclear Magnetic Resonance Magic Angle Spinning (NMR-MAS) probehead, wherein the NMR-MAS probehead comprises an MAS stator configured to receive an elongate MAS rotor configured to hold a sample substance in a sample volume and a hollow, elongate first microwave guide configured to supply microwave radiation into the sample volume, and wherein the MAS rotor is surrounded by an RF coil, wherein the coil block is constructed from an electrically nonconductive dielectric material, wherein the coil block is configured to be inserted into the wall of the MAS stator to at least partly surround the RF coil and the MAS rotor of the NMR-MAS probehead in an operational state of the NMR-MAS probehead, and wherein the coil block comprises: a first bore that extends coaxially with a longitudinal axis of the MAS rotor in the operational state of the NMR-MAS probehead, the RF coil being fastened to an inner wall of said the first bore, and a second bore that extends coaxially with the longitudinal axis of the first microwave guide in the operational state of the NMR-MAS probehead and comprises a hollow, elongate second microwave guide.

14. The coil block as claimed in claim 13, wherein an internal diameter of the second microwave guide is smaller than an internal diameter of the first microwave guide.

15. The coil block as claimed in claim 14, wherein the internal diameter of the second microwave guide is smaller than the internal diameter of the first microwave guide by at least 50%.

16. The coil block as claimed in claim 13, wherein a backside of the coil block that lies opposite the second microwave guide with respect to the RF coil has a metallic coating or has a metal film that the backside acts as a microwave mirror.

17. The coil block as claimed in claim 13, wherein the second bore has an inner wall metallic coating that acts as the second microwave guide integrated into the coil block.

18. The coil block as claimed in claim 13, wherein the inner wall of the first bore has a corrugation or groove structure, on which the RF coil is fastened in a force-fit and/or an interlocking fashion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated in the drawing and will be explained in more detail on the basis of exemplary embodiments.

(2) In the drawing:

(3) FIG. 1 shows a schematic cross-sectional illustration of a first embodiment of the NMR-MAS probehead according to the invention;

(4) FIG. 2A shows a schematic cross-sectional illustration of an NMR-MAS probehead according to the invention, with a coil block carrying the RF coil in a bore and a second microwave guide;

(5) FIG. 2B shows a schematic cross-sectional illustration of an NMR-MAS probehead according to the closest prior art as per reference [1] with a plano-convex microwave lens and a flat microwave mirror;

(6) FIG. 3 shows a schematic cross-sectional illustration of an NMR-MAS probehead according to the prior art as per reference [1] in comparison with the invention according to FIG. 1;

(7) FIG. 4 shows a schematic 3D CAD view of a coil block that has been modified according to the invention;

(8) FIG. 5 shows a size comparison between a 1.3 mm MAS stator (left) and a 3.2 mm MAS stator (right);

(9) FIG. 6 shows a central cut through the H-field amplitude distribution of a 3D EM simulation in the case of two arrangements according to the prior art (left) and an arrangement according to the invention (top right), wherein the scaling of the field values was chosen to be the same; and

(10) FIG. 7 shows a graphic with experimental results of the DNP gain by the arrangement according to the invention in comparison with arrangements according to the prior art.

DETAILED DESCRIPTION

(11) The invention relates to a novel configuration of a coil block for a MAS-DNP-NMR probehead, the main application of which being as a constituent part of a magnetic resonance apparatus. The embodiments of the probehead according to the invention, as illustrated in FIGS. 1 and 2A, for instance, each comprise an NMR-MAS probehead with an MAS stator 3 for receiving an elongate MAS rotor 5 that is surrounded by an RF coil 4, said MAS rotor having a sample substance in a sample volume 0, and having a hollow, elongate first microwave guide 1 for supplying microwave radiation into the sample volume 0 through a coil block 2 introduced in a wall of the MAS stator 3.

(12) The arrangement according to the invention is distinguished by virtue of the coil block 2 being constructed from dielectric material, the coil block 2 having such a geometric extent and being inserted into the wall of the MAS stator 3 in such a way that it at least partly surrounds the RF coil 4 and the MAS rotor 5, the coil block 2 having a first bore 4 that extends coaxially with the longitudinal axis of the elongate MAS rotor 5, the RF coil 4 being fastened to the inner wall of said first bore, and the coil block 2 having a second bore 8 that extends coaxially with the longitudinal axis of the first hollow, elongate microwave guide 1 and that comprises a hollow, elongate second microwave guide 8 for supplying microwave radiation from the first microwave guide 1 into the sample volume 0.

(13) The internal diameter of the second microwave guide 8 is smaller than the internal diameter of the first microwave guide 1, preferably by at least 50%.

(14) The second bore 8 can have a metallic coat on its inner wall and thereby act as a second microwave guide 8 that is integrated into the coil block 2.

(15) The inner wall of the first bore 4 can have corrugations or a groove structure on which the RF coil 4 is fastened in force-fit and/or interlocking fashion.

(16) Additionally, the end of the tubular second microwave guide 8 distant from the coil can have a widening 8, in particular a trumpet-like widening, as may be identified in FIG. 4.

(17) In embodiments of the invention, the first microwave guide 1, in particular in view of its diameter and its form, can have such a geometric structure that it focuses the microwave radiation guided therethrough onto the opening, facing it, of the second microwave guide 8 in the coil block 2.

(18) In the embodiments of the invention illustrated in FIGS. 1, 2A and 6, which are an alternative thereto, a microwave lens 6 for focusing the supplied microwave radiation onto the lens-side opening of the second microwave guide 8 is arranged between the first microwave guide 1 and the coil block 2 at the coil-block-side free end of the first microwave guide 1. This microwave lens 6 is arranged and geometrically embodied in respect of its focal length on the side facing the sample volume 0 in such a way that the lens-side opening of the second microwave guide 8 lies in the focus of the microwave lens 6.

(19) Moreover, in the embodiments of FIGS. 1, 2A and 6, it is possible to identify that a microwave mirror 7 for reflecting the microwave radiation that emerges from the second microwave guide 8 and passes through the sample volume 0 is present on the inner side of the MAS stator 3 that lies opposite the second microwave guide 8 in respect of the MAS rotor 5. This microwave mirror 7 can be attached to the coil block 2 or spatially integrated into the coil block 2. It can be constructed to be cylindrical and concave in the direction of the sample volume 0 and can have such a structural configuration that it focuses the microwave radiation that comes from the sample volume 0 and that is incident thereon onto the longitudinal axis of the MAS rotor 5.

(20) The backside of the coil block 2 that lies opposite the second microwave guide 8 in respect of the RF coil 4 can have such metallic coating or can be provided with a metal film in such a way that it acts as a microwave mirror 7.

(21) While FIG. 1 shows many details of the NMR-MAS probehead according to the invention and of the coil block 2 that was modified according to the invention in a schematic sectional illustration through a horizontal plane, FIG. 2A, in an even more schematic drawing of the principles, elucidates the arrangement according to the invention in a direct comparison with the arrangement, illustrated in FIG. 2B, according to the closest prior art as per reference [1]. The significant differences in the structure and configuration of the respective coil block 2 require no further comments. FIGS. 2A and 2B also show the observer that the principal applications of the present invention lie predominantly in systems with smaller external diameters of the MAS rotor 5 than in the conventional prior art.

(22) FIG. 3 illustratesas a counterpart to the illustration of the invention in FIG. 1an NMR-MAS probehead with a coil block according to the prior art, as described in reference [1]. This closest prior art likewise makes use of a lens and a mirror, however precisely not with the above-described modifications according to the invention, as becomes clear, in particular, from the significantly different structure and positioning of the coil block and, in particular, the complete lack of the second microwave guide integrated into the coil block.

(23) The notable modifications of the coil block 2 according to the invention can be identified in concentrated form in the partly transparent spatial illustration of FIG. 4:

(24) What is noteworthy to the present invention is a structure of the coil block 2naturally not displayable in the drawingmade of an electrically nonconductive dielectric material and its geometric arrangement within the MAS stator 3 in such a way that the coil block 2 receives the RF coil 4 in its first bore 4 and hence also surrounds the MAS rotor 5. Here, the RF coil 4 is fastened on the inner wall of the first bore 4 in interlocking and/or force-fit fashion. What is also significant to the invention is the hollow, elongate second microwave guide 8 that is arranged in the second bore 8 of the coil block 2.

(25) FIG. 5 contrasts a 1.3 mm MAS stator (left) with a conventional 3.2 mm MAS stator (right) in a direct size comparison. It is possible to clearly identify the difficultiesalready purely geometrical difficultiesthat occur in relation to the microwave access to the sample volume in the arrangement with the smaller MAS rotors. Until now, the latter have prevented a more efficient use of the radiated-in microwave power; this can now be decisively improved by the present invention.

(26) FIG. 6 illustrates a 3D EM simulation of the H-field amplitude in a horizontal sectional plane in an arrangement according to the closest prior art as per reference [1] in a 3.2 mm embodiment (top left), according to further prior art in a 1.3 mm embodiment (bottom left) and in an arrangement according to the invention (top right). Using this, the technical advance on account of the present invention is clearly elucidated:

(27) Previous Prior Art (Bottom Left):

(28) The Gaussian beam comes from the right and strikes coil and rotor; although the illumination is quite good, the beam is reflected and diffracted in uncontrolled fashion after the first strike, as a result of which a significant fraction is lost.

(29) Closest Prior Art (Top Left):

(30) The Gaussian beam comes from the right, strikes the cylindrical lens and is focused in a direction such that it strikes the rotor virtually perpendicularly. As a result, the rotor wall can be adapted in uniform manner since the rotor then acts not like a curved surface but like a plane dielectric. The coil transmits the beam in largely unimpeded fashion such that the latter is reflected at the cylindrical mirror and passes through the sample again. As a result of the two-fold passage, the power received by the sample increases.

(31) Invention (Top Right):

(32) On account of the second microwave guide 8 that, according to the invention, is arranged in the coil block 2 and on account of the positioning and dimensioning according to the invention of microwave lens 6 and microwave mirror 7, a once again significantly improved efficiency of the radiated-in microwave power is obtained, which, in particular, has a significant impact in the case of systems with MAS rotor diameters<<3.2 mm (here: 1.3 mm) in particular.

(33) Finally, the efficiency gain on account of the invention is illustrated graphically in FIG. 7 in exemplary fashion. Here, the DNP gain variable is plotted on the ordinate axis against the power on the probehead in watt, specified on the abscissa axis.

(34) What can be gathered from the experimental data combined in FIG. 7 is how the coil block that was modified according to the invention behaves in respect of the polarization gain (DNP gain) as a function of the radiated-in microwave power. Here, what can clearly be identified is that the arrangement according to reference [1] (illustrated top left in FIG. 6) already improves the DNP gain in a 3.2 mm system. The corresponding central curve (black triangular symbols) in FIG. 7 already lies quite significantly over the lower curve (open diamond symbols), which shows the measurement results using a conventional arrangement (shown bottom left in FIG. 6) according to the older prior art in the case of a 1.3 mm system, with the teaching of reference [1] reaching practical limits on account of the geometric restrictions.

(35) The arrangement according to reference [1] already leads to the same polarization effect already being obtained in the case of significantly lower microwave power than in an arrangement according to the older prior art. As a consequence, it is thus possible to resort to smaller and more cost-effective microwave sources. If the provided power is not restricted, a higher DNP gain may also be obtained therewith in saturation under certain circumstances.

(36) Of course, the measurement values in an arrangement according to the invention (uppermost curve with black square symbols) once again lie far above the DNP gain values that are obtainable using the arrangement according to reference [1]. Additionally, what is moreover true here is that a 1.3 mm system that was modified according to the invention was used when measuring this uppermost curve, while the central curve was recorded using a 3.2 mm system.

LIST OF REFERENCE SIGNS

(37) 0 Sample volume 1 First microwave guide 2 Coil block 3 MAS stator 4 RF coil 4 First bore 5 MAS rotor 6 Microwave lens 7 Microwave mirror 8 Second microwave guide 8 Second bore 8 Trumpet-like widening

CITATIONS

(38) Publications considered for assessing the patentability: [1] DE 10 2016 207 998 B3 [2] Cryogenic sample exchange NMR probe for magic angle spinning dynamic nuclear polarization, A. Barnes et al., Journal of Magnetic Resonance 198 (2009) 261-270 [3] WO 2015/175507 A1 [4] US 2016/0334476 A1 [5] EP 3 030 917 B1 [6] US 2017/0074952 A1