TRANSCEIVER COIL ARRANGEMENT FOR AN MAS NMR PROBE HEAD AND METHOD FOR DESIGNING A TRANSCEIVER COIL ARRANGEMENT

Abstract

A transceiver coil arrangement for an MAS NMR probe head has a first transceiver coil with a longitudinal axis Z for generating a first HF magnetic field B1, the first transceiver coil having at least one solenoid-shaped section with an electrical conductor having a path width W and N3 windings, wherein all windings run around the longitudinal axis Z of the transceiver coil 1. The electrical conductor has a slope S and each half-winding is tilted at a tilt T relative to the longitudinal axis Z, wherein T0 for at least a portion of the half-windings. According to the invention, at least two of the following variables change over the course t of the length of the electrical conductor: Tilt T=T(t), slope S=S(t), conductor path width W=W(t), allowing the transceiver coil to be optimized to improve the homogeneous region.

Claims

1. A transceiver coil arrangement for an MAS NMR probe head having a first transceiver coil with a longitudinal axis Z for generating a first HF magnetic field B1, the first transceiver coil having at least one solenoid-shaped section which has an electrical conductor with a conductor path width W and N3 windings, wherein all of said windings run around the longitudinal axis Z of the transceiver coil, and wherein the electrical conductor has a slope S and each of said windings has a half-winding tilted at a tilt T relative to the longitudinal axis Z, wherein T0 for at least a portion of the half-windings, the transceiver coil being configured such that at least two of the following parameters change over the course t of the length of the electrical conductor of the transceiver coil: Tilt T=T(t), Slope S=S(t), Conductor path width W=W(t).

2. The transceiver coil arrangement according to claim 1, wherein the electrical conductor of the first transceiver coil is a band-shaped conductor.

3. The transceiver coil arrangement according to claim 1, wherein the slope S changes over the course t of the length of the electrical conductor, and wherein the conductor path width W changes within each winding.

4. The transceiver coil arrangement according to claim 1, wherein the slope S and the tilt T of the electrical conductor of the first transceiver coil change along the course of the electrical conductor.

5. The transceiver coil arrangement according to claim 4, wherein the tilt T at axial ends of the first transceiver coil is smaller than at an axial center.

6. The transceiver coil arrangement according to claim 1, wherein the transceiver coil arrangement comprises at least one further transceiver coil for generating a second HF magnetic field B2 radially outside the first transceiver coil, and wherein the first transceiver coil and the further transceiver coil are arranged around the common longitudinal axis Z in such a way that HF magnetic fields B1, B2 generated by the first transceiver coil and the further transceiver coil are aligned perpendicular to each other.

7. The transceiver coil arrangement according to claim 1, wherein the electrical conductor of the first transceiver coil comprises a forward winding section and a return winding section, wherein the forward winding section comprises forward windings and, starting from a connection region, leads in a predetermined winding sense to an axial end of the transceiver coil, wherein the return winding section comprises return windings and, starting from the axial end of the first transceiver coil, leads to the connection region in the predetermined winding sense, wherein the windings of the return winding section have a slope S with sign opposite to those of the forward winding section, and wherein forward and return windings of the electrical conductor, with the exception of crossover regions in which the forward and return windings cross over each other, are arranged on a common cylindrical jacket surface around the longitudinal axis Z.

8. An MAS NMR probe head having a transceiver coil arrangement according to claim 1.

9. A method for producing a transceiver coil arrangement according to claim 1, the method comprising: performing an optimization of a target function, wherein said target function is either the signal-to-noise ratio of a predetermined NMR experiment or a function that comprises at least two variables that influence the signal-to-noise ratio SNR, and wherein said optimization uses at least two optimization parameters that vary over the course of the length of the electrical conductor and that are selected from the following parameters: Slope S, Tilt T, Conductor path width W, and constructing the transceiver coil arrangement in accordance with the optimized target function.

10. The method according to claim 9, wherein said optimization comprises: a) defining the number N of windings, where N3, b) determining in each case a starting value for the optimization parameters, c) determining the target function with the determined starting values for the optimization parameters, d) adjusting the optimization parameters, wherein for the at least two selected parameters a non-constant function is used as a function of a running parameter t running between 0 and winding number N of the transceiver coil arrangement, with t and 0tN, e) determining the target function with the adjusted optimization parameters, and f) repeating steps (d)-(e) until the target function is within a predetermined target interval.

11. The method according to claim 9, wherein one of the at least two variables of the target function influencing the signal-to-noise ratio SNR is a radial homogeneity of the HF magnetic field B1, which is produced by the transceiver coil during operation within the field of view, and the selected optimization parameters are the slope S and the tilt T of the windings.

12. The method according to claim 11, wherein the tilt of the windings is adapted over the course of the length of the electrical conductor such that the tilt T at axial ends of the first transceiver coil is smaller than at an axial center of the first transceiver coil.

13. The method according to claim 9, wherein one of the at least two variables of the target function influencing the signal-to-noise ratio SNR is an axial homogeneity of the HF magnetic field B1 generated by the transceiver coil.

14. The method according to claim 9, wherein one of the at least two variables of the target function influencing the signal-to-noise ratio SNR is a B1 amplitude/rating, and the selected optimization parameters are the slope S and the conductor path width W.

15. The method according to claim 9, wherein the tilt T of the windings in a center of the transceiver coil is selected such that a B1 amplitude/rating is maximized for a given ratio S/W of slope S to conductor path width W.

16. The method according to claim 9, wherein the transceiver coil arrangement comprises a further transceiver coil for generating a further HF magnetic field B2, and one of the at least two variables of the target function influencing the signal-to-noise ratio is a ratio B1/B2 of the amplitude/rating of the first HF magnetic field B1 and the further HF magnetic field B2.

17. The method according to claim 9, wherein the electrical conductor has a conductor thickness d and a rounding radius r, wherein at least one of the conductor thickness d and the rounding radius r of the electrical conductor is used as an additional optimization parameter, which varies over the course of the length of the electrical conductor.

18. The method according to claim 9, wherein the transceiver coil is produced from a metallic tube using milling, laser or water jet cutting, and makes use of a coated carrier, wherein the coating is produced by etching, milling or laser ablation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] FIG. 1 shows a transceiver coil according to the invention with a constant tilt in the center and reduced tilt of the outermost two windings, varying slope and periodically varying conductor path width with two maxima per winding for a single coil configuration.

[0081] FIG. 2 shows a transceiver coil with varying tilt, varying slope and periodically varying conductor path width with two maxima per winding for a cross-coil configuration.

[0082] FIG. 3a shows a plan view of a transceiver coil arrangement according to the invention in a cross-coil configuration with a first transceiver coil according to FIG. 2.

[0083] FIG. 3b shows a perspective view of the transceiver coil arrangement from FIG. 3a.

[0084] FIG. 4 shows a transceiver coil according to the invention with varying tilt, varying slope and periodically varying conductor path width with four maxima per winding for a cross-coil configuration.

[0085] FIG. 5a shows a plan view of a transceiver coil arrangement according to the invention in a cross-coil configuration with a zero-pitch transceiver coil with periodically varying slope and periodically varying conductor path width.

[0086] FIG. 5b shows a perspective view of the transceiver coil arrangement from FIG. 5a.

[0087] FIG. 6 shows a transceiver coil according to the invention for a single coil configuration, in which, in addition to the slope and the conductor path width, the tilt also varies, wherein the conductor path width varies periodically with a maximum in each revolution.

[0088] FIG. 7 shows a transceiver coil according to the invention with a slope varying from winding to winding, varying conductor path width and greatly varying tilt, wherein the conductor path width varies periodically.

[0089] FIG. 8 shows a transceiver coil according to the invention with a constant slope, varying conductor path width and varying tilt.

[0090] FIG. 9 shows an NMR probe head according to the invention.

[0091] FIG. 10a shows a detail of a solenoid-shaped coil section for illustrating the coil parameters in a coil with tilted windings.

[0092] FIG. 10b shows a detail of a solenoid-shaped coil portion for illustrating the coil parameters in a coil with non-inclined windings.

DETAILED DESCRIPTION

[0093] The transceiver coil according to the invention has coil parameters which vary along the course of the electrical conductor of the transceiver coil. FIG. 10a and FIG. 10b each show a section of a solenoid-shaped coil with a strip-shaped conductor 2 (conductor path), on the basis of which some of the coil parameters are first illustrated. The solenoid-shaped coils in FIG. 10a and FIG. 10b are arranged along a longitudinal axis Z (coil axis), wherein the longitudinal axis Z is perpendicular to an XY-plane (not shown). The solenoid-shaped coils are parameterized by the conductor path width W of the conductor 2, a gap width D of a gap 10, a local slope S (not shown) and/or a pitch P of the windings, a tilt T of the windings, and a radius R of the windings. In the embodiment shown here, a total of three windings are shown.

[0094] The conductor width W indicates the width of the conductor 2. The conductor path width W is the width of the conductor path perpendicular to the conductor center. In the coils shown in FIG. 10a and FIG. 10b, the conductor path width W is constant along the longitudinal axis Z (i.e., W=const).

[0095] The gap width D indicates the width of the intermediate space 10 between the strip-shaped conductor of adjacent windings of the conductor path 2.

[0096] The pitch P of the windings indicates the propulsion in the Z-direction of a complete winding and is determined via the central line of the conductor path 2. A constant pitch P does not exclude that the local slope S varies within one winding.

[0097] The inclination T of the windings indicates the inclination of the windings with respect to the longitudinal axis Z and corresponds to the amplitude of a sinusoidal modulation of the Z position of the conductor center plane over one winding. If the slope and tilt are constant over several windings, it can be easily determined from Max(Z(t)Z(t+1))S)/2, where t varies in the interval tn . . . tn+1.

[0098] The radius R of the windings indicates the radius on which the conductor 2 lies.

[0099] The solenoid coils shown in FIG. 10a and FIG. 10b each have a constant conductor path width W, a constant gap width D, a constant slope S and hence also a constant pitch P, with the coil shown in FIG. 10a being a tilted coil (T0), and the coil shown in FIG. 10b being a non-tilted coil (T=0).

[0100] In general, the central line of the conductor 2 is defined in Cartesian coordinates as

[00001]$\left(\begin{array}{c}R\left(t\right)*\sin \left(2t\right)\\ R\left(t\right)*\cos \left(2t\right)\\ P\left(t\right)*t+T\left(t\right)*\cos \left(2t+\right)\end{array}\right),$

with t{0 . . . N}, where : Orientation of the inclination of the windings.

[0101] The envelope of the conductor 2 in Cartesian coordinates is defined as

[00002]$\left(\begin{array}{c}R\left(t\right)*\sin \left(2t\right)\\ R\left(t\right)*\cos \left(2t\right)\\ S\left(t\right)*tW\left(t\right)/2+T\left(t\right)*\cos \left(2t+\right)\end{array}\right),\mathrm{with}t\left\{0.Math.N\right\}.$

[0102] The conductor path width is in particular W(t)=W.sub.0+W.sub.i (sin(2t+k)).sup.2i, the tilt T is constant over each half-winding. Normally, the tilt direction is =0 (tilt about the Y-axis) or /2 (tilt about the X-axis) and the radius R(t)=R.

[0103] In the following, various variants of the transceiver coil geometry according to the invention are described with which the performance of the NMR coil head according to the invention along the course of the electrical conductor 2 can be optimized by varying the coil parameters.

[0104] FIG. 1 and FIG. 2 show transceiver coils 1a, 1b according to the invention with a constant tilt T about the x-direction (i.e., direction of tilt=/2). The pitch P is constant in the central region of the transceiver coils 1a, 1b and decreases towards the two axial ends 4a, 4b of the transceiver coils 1a, 1b. The conductor path width W varies periodically and has two regions with maximum conductor path width per winding and two regions with minimum conductor path contact. Furthermore, W(k)=W(k+0.5)=Wmin and W(k+0.25)=W(k+0.75)=Wmax.

[0105] The parameters of the transceiver coils 1a, 1b varying according to the invention are the conductor path width W, as well as the pitch P and thus also the slope S. In these embodiments, the slope S is constant per winding. In general, however, it can also vary over the length of a winding.

[0106] The two transceiver coils 1a and 1b differ in terms of the arrangement of the minimum and maximum conductor path widths relative to the direction of tilt =/2 or =0 the transceiver coils 1a, 1.sub.b.

[0107] In principle, it is advantageous in the case of tilted windings (T0) if the minimum conductor path width on the sectional plane is orthogonal to the tilt axis. In the case of a tilt about the X-axis, as shown in FIG. 1, the minima of the conductor path width lie on the sectional plane of the coil with the YZ-plane. The tilt results in a field component being generated along a Y-axis, i.e., 90 to Z. This field component must penetrate the transceiver coil which is facilitated if a larger gap width D is present between the conductor sections of two windings. Shielding currents are formed on the conductors themselves, which increase resistive losses and weaken the field and thus lead to performance losses. The performance can be increased by positioning the minima of the conductor path width W or the maxima of the gap width D along the YZ-plane and the maxima of the conductor path width W or the minima of the gap width D in the direction of the XZ-plane, since the ratio of gap width and conductor path width can be optimized at every spatial position.

[0108] Due to their good performance, the transceiver coil 1a shown in FIG. 1 is particularly suitable for transceiver coil arrangements 100a, 100b with a single coil configuration.

[0109] If a further transceiver coil 11 (see FIG. 3a and FIG. 3b) generating an HF magnetic field B2, which is oriented in the X-direction (i.e., a saddle coil, a resonator, . . . ), surrounds or is surrounded by the transceiver coil 1b according to the invention, this field B2 must penetrate the first transceiver coil 1b. The conductor paths (windings) of the first transceiver coil 1b stand in the way and partially shield the field of the further transceiver coil 11. In this case, the performance of the further transceiver coil 11 can be optimized by reducing the conductor path width W of the first transceiver coil 1b in the direction of the X-axis, as shown in FIG. 2. This optimization of the further transceiver coil 11 is to the detriment of the performance of the first transceiver coil 1b, but offers better permeability in the X-direction. Therefore, the transceiver coil 1b shown in FIG. 2 is particularly suitable for transceiver coil arrangements 100b with a cross-coil configuration.

[0110] FIG. 3a and FIG. 3b show the transceiver coil arrangement 100b with such a transceiver coil 1b and a further transceiver coil 11 in a cross-coil configuration. The transceiver coil arrangement 100b comprises the first transceiver coil 1b shown in FIG. 2 for generating the first HF magnetic field B1 and the further transceiver coil 11 for generating the second HF magnetic field B2 for an NMR probe head 23 according to the invention (see FIG. 9). Here, the first transceiver coil 1b is arranged coaxially, radially inside the further transceiver coil 11, so that the second magnetic field B2 generated by the further transceiver coil 11 is substantially perpendicular to the first magnetic field B1 generated by the first transceiver coil 1b. The further transceiver coil 11 is formed here as an Alderman-Grant resonator consisting of two halves 5 and 5, and radially surrounds the first transceiver coil 1b, wherein the further transceiver coil 11 comprises two opposite openings 12 (windows). The first transceiver coil 1b and the further transceiver coil 11 are oriented relative to one another in such a way that the regions of the first transceiver coil 1b, in which the conductor path width W of the electrical conductor 2 of the first transceiver coil 1 has minimum values, lie within the windows 12 of the further transceiver coil 11 so that the second magnetic field B2 generated by the further transceiver coil 11 runs through the regions with minimum conductor path width W of the first transceiver coil 1a. As a result, the first transceiver coil 1b has a high transparency for the second HF magnetic field B2. As an alternative to the embodiment shown in FIG. 3a and FIG. 3b, the further transceiver coil 11 can also be arranged within the first transceiver coil 1b (not shown). Likewise, the further transceiver coil can also be designed as a saddle coil, in particular as a multi-winding saddle coil instead of as a resonator (not shown).

[0111] FIG. 4 shows a very specific embodiment of the transceiver coil 1g according to the invention. The conductor path width W changes periodically for the transceiver coil 1g and has four maxima and four minima per winding: two minima along the tilt axis (X-direction) in order toin analogy to the transceiver coil 1b from FIG. 2create transparency for a second HF magnetic field B2 of a cross-coil configuration, and two minima perpendicular to the tilt axis (along the Y-direction), in order toin analogy to the transceiver coil 1a from FIG. 1get less in their own way. These minima generally optimize the performance of the first transceiver coil 1g for transceiver coils with tilted windings, whereas the minima that increase transparency optimize the performance of the further transceiver coil (not shown in FIG. 4) at the expense of the first transceiver coil 1g.

[0112] The parameters of the transceiver coil 1g varying according to the invention are the conductor path width W, as well as the pitch P and thus also the slope S and the tilt T.

[0113] FIG. 5a and FIG. 5b show a further embodiment of a transceiver coil arrangement 100c in a cross-coil geometry with a first transceiver coil 1c. The windings of the transceiver coil 1c have a local slope S(t)=0 over a large part of their length. Such a winding forms a non-closed ring, i.e., S(t)=0 for t=t0 . . . t0+1- or t=t0+ . . . /2 . . . t0+1-/2, with >0, where E>0 prevents a short circuit; t=t0 is the beginning of the winding. Solenoid coils 1c designed in this way are known as zero-pitch coils, since a large part of the winding has a local pitch of 0. However, the pitch P of a complete winding has a value not equal to 0, which value is constant in this embodiment (|P|=const). Such a transceiver coil 1c can thus be designed as a combination of non-closed rings without (local) slope and sections of the electrical coil section with slope S(t)0. The ratio W/D of conductor path width W to gap width D can be kept constant in a simple manner over the transceiver coil 1c if the conductor path width is designed to be constant. As a result, the quality of the transceiver coil 1c can be maximized, and/or the electrical fields can be minimized, in a particularly simple manner. However, the transceiver coil 1c in FIGS. 5a and 5b has a periodically varying conductor path width W, as a result of which the gap width D between the windings also varies periodically. The optimization of the performance of the one transceiver coil 1c and its influence on the performance of a cross-coil arrangement is particularly easy to calculate in this configuration. Furthermore, the transceiver coil 1c has a constant tilt T.

[0114] The parameters of the transceiver coil 1c varying according to the invention are the conductor path width W, the slope S or pitch P and the tilt T.

[0115] The first transceiver coil 1c shown in FIG. 5a and FIG. 5b is preferably designed in a crisscross geometry with forward windings 14 and return windings 15 which are preferably arranged alternately.

[0116] FIG. 6 shows a transceiver coil 1d according to the invention in which both the local slope S and the conductor path width W as well as the tilt T vary. The conductor path width W has exactly one maximum and one minimum in each winding, wherein the conductor path width W averaged over a winding decreases towards the axial ends 4a, 4b. The transceiver coil 1d from FIG. 6 can be particularly well matched to multiple cores and is therefore suitable in particular for a 1-coil transceiver coil arrangement 100d.

[0117] The parameters of the transceiver coil 1d varying according to the invention are the conductor path width W, the tilt T and the pitch P and thus also the slope S. The tilt T at the axial ends of the coil is T=0. As a result, such a coil can be mounted particularly easily in a defined installation space, for example between the bearings of an MAS stator, and makes particular good use of the available volume.

[0118] FIG. 7 shows a transceiver coil 1e according to the invention in which both the local slope S and the conductor path width W as well as the tilt T vary. In this embodiment, the slope S of the windings changes discretely, i.e., S(t)=constant within one winding. Although this discretization does not provide the optimum homogeneity, it allows for achieving a sufficiently good homogeneity level. Both the conductor path width W and the tilt T and pitch P decrease towards the axial ends 4a, 4b.

[0119] The parameters of the transceiver coil 1e varying according to the invention are the conductor path width W, the tilt and the pitch P and thus also the slope S.

[0120] The transceiver coil 1e is suitable in particular for a 1-coil transceiver coil arrangement 100e.

[0121] The transceiver coil if shown in FIG. 8 has a constant pitch P. However, the tilt T between the windings and the conductor path width W varies.

[0122] The ratio W/D of conductor path width W to gap width D between the windings is constant here. In combination with tilted windings (T not equal to 0), this results in the maximum conductor path widths W being arranged at the bottom (Y-direction) in windings of the left half of the transceiver coil 1f, while the maximum conductor path widths W are arranged at the top (+Y-direction) in windings of the right half of the transceiver coil 1f. The transceiver coil 1f shown in FIG. 8 can be used particularly advantageously when the total length of the transceiver coil arrangement is limited, for example for a transceiver coil arrangement geometry, in which a transceiver coil has to be inserted between two bearings. The transceiver coil 1f can be used in particular for simple free induction decay (FID) experiments in which maximization of the axial and radial homogeneity is not necessary.

[0123] The parameters of the transceiver coil 1f varying according to the invention are the conductor path width W, the gap width D and the tilt T.

[0124] The transceiver coil 1f is suitable in particular for a 1-coil transceiver coil arrangement 100f.

[0125] FIG. 9 shows a schematic illustration of an NMR probe head 23 according to the invention. A static magnetic field for performing NMR measurements is aligned parallel to the Z-axis during operation in the example shown here. The NMR probe head 23 comprises a transceiver coil 1a-g or a transceiver coil arrangement 100a-f according to the invention, which is connected to the matching network 24 and further comprises a spectrometer connection 21. The NMR probe head 23 shown in FIG. 9 is a MAS (magic angle spinning) probe head in which the longitudinal axis Z of the transceiver coil 1 is tilted, preferably by the magic angle (=54.74), with respect to the Z axis along which the elongated extension of the NMR probe head 23 extends.

LIST OF REFERENCE SIGNS

[0126] 1a-g Transceiver coils [0127] 2 Electrical conductor [0128] 4a, 4b Axial ends [0129] 5, 5 Halves of the further transceiver coil [0130] 10 Intermediate space between windings [0131] 11 further transceiver coil [0132] 12 Openings/windows of the further transceiver coil [0133] 21 spectrometer connection [0134] 23 NMR probe head [0135] 24 matching network [0136] 100a-f Transceiver coil arrangements [0137] Z Longitudinal axis of the transceiver coil [0138] W Conductor path width [0139] D Gap width [0140] S Local slope [0141] P Pitch [0142] T Tilt [0143] R Radius