NMR probe head

Abstract

To provide an NMR probe head with a coil structure capable of reducing an electric field generated within a sample during operation.SOLUTION: At least one receiving / transmitting coil 1 for generating a high frequency B1 magnetic field has conductor portions 2a and 2b and a connection region 4. The conductor portions have a forward winding portion and a backward winding portion. The forward winding portion has forward windings 3a and 3b, and extends from the connection region 4 as a starting point to axial ends 5a and 5b of the receiving / transmitting coil 1 in a predetermined winding direction. The backward winding portion has backward windings 6a and 6b, and extends from the axial ends 5a and 5b of the receiving / transmitting coil 1 as starting points to the connection region 4 in the same winding direction. The windings of the backward winding portion have a pitch P of an opposite sign to the windings of the forward winding portion.SELECTED DRAWING: Figure 1a

Description

Technical Field

[0001] The present invention relates to an NMR probe head comprising a transmit-receive coil assembly having at least one transmit-receive coil for generating a high-frequency B1 (HF B1) magnetic field, the transmit-receive coil having at least one electrical coil part and a connection region, the electrical coil part having a forward winding part and a reverse winding part, the forward winding part having a forward winding and reaching an axial end of the transmit-receive coil in a predetermined winding direction starting from the connection region, the reverse winding part having a reverse winding and extending to the connection region in the same winding direction as the predetermined winding direction starting from the axial end of the transmit-receive coil, and the winding of the reverse winding part having a pitch of a sign opposite to that of the winding of the forward winding part.

Background Art

[0002] When measuring a lossy (especially conductive) measurement sample, when an electric field is generated in the measurement sample by the coil of the NMR probe head, various performance degradations occur, from a decrease in the excitation bandwidth and a decrease in the signal-to-noise ratio due to heating to the destruction of the measurement sample that is easily affected by temperature.

[0003] In order to shield the electric field generated by the probe head, for example, by a Faraday shield (see [Pel 2016], [Krahn 2008] at the end of this specification), or by a coil ([Stringer 2005], [Dillmann 2007]) in which only the electric field of one turn of the winding is visible to the sample, it is known to reduce the electric field in the sample.

[0004] As another measure for reducing the electric field in the NMR probe head, for example, it is common to reduce the inductance of the coil at the highest measurement frequency by using a cross-coil configuration ([Gorkov 2007], [Grant 2009]), replacing a solenoid coil with a plurality of inductively coupled single-turn resonators ([US5003265]), or connecting a plurality of solenoid coils in parallel ([US5180982]). Also, a form in which a solenoid coil with a plurality of windings connected in parallel is used is known ([JP4787033]). However, generally, only when measuring at the proton frequency can the loss be minimized by using a low-inductance coil or resonator. Losses in measurements at other frequencies (X nuclei) are usually not recognized as a problem.

[0005] It is known from [US5180982] and [EP1571459] to use a plurality of coils having windings with different winding directions, and from [US6751847] to use a plurality of coils wound with the same winding direction but opposite pitches.

[0006] For example, a coil assembly having a center tap is known from [US5180982] and [EP1571459]. In this case, starting from the center tap, a plurality of coil portions having windings extend in opposite directions with opposite winding directions and both have a positive gradient. The two coil portions in [US5180982] are wound so that the gradient does not become zero, while in [EP1571459], two superconducting disk-shaped windings are aligned in a direction perpendicular to the central axis and connected by a via (bridge element). The drawback here is that the potential difference across two halves of the coil is only about half of the potential difference of a solenoid having twice the number of windings over the entire length of the coil. Therefore, there is a limit to reducing the electrical loss. Furthermore, when the inductance of the transmit-receive coil decreases, the efficiency of the multi-core circuit decreases, leading to a performance degradation.

[0007] To reduce the electric field, [US6751847] proposes a coil having a forward winding on the outside of a cylindrical dielectric carrier and a reverse winding on the inside. Thus, the forward winding and the reverse winding are located on cylindrical surfaces having different radii, respectively. The forward winding and the reverse winding have the same winding direction but opposite gradients. For a measurement sample, such a coil acts like a coil with half the number of turns. The conductor material of the inner winding shields the potential of the outer winding. The electric field generated in the measurement sample by such a coil substantially coincides with the electric field generated by the inner winding of the coil. However, when the windings are separated inside and outside the dielectric carrier, capacitive coupling occurs between the inner winding and the outer winding. Since these windings have a high potential difference, especially in the region of the lead wires, this structure has a strong "capacitance" effect, especially when the carrier is made of a material having a high dielectric constant such as alumina-based ceramic / sapphire material or zirconia-based ceramic material. Therefore, the natural frequency is significantly reduced compared to a coil having windings only on one side of the carrier. To compensate for this, the number of turns, and thus the inductance of the coil, must be reduced, which results in various performance degradations when tuning multiple measurement frequencies with a single transmit-receive coil. Summary of the Invention Problems to be Solved by the Invention

[0008] An object of the present invention is to provide an NMR probe head having a coil structure that can reduce the electric field generated in a sample during operation while suppressing other performance degradations. Means for Solving the Problems

[0009] According to the present invention, this object is achieved by the NMR probe head according to claim 1.

[0010] In the NMR probe head according to the present invention, the forward winding and the reverse winding of the electric coil part are arranged on the same cylindrical surface centered on the longitudinal axis Z' except for the crossing region (cross-shaped structure) where the forward winding and the reverse winding cross each other.

[0011] Each coil part has a forward winding part and a reverse winding part arranged on the same cylindrical surface between the axial end of the transmitting and receiving coil and the connection region. That is, windings advancing in different directions are arranged on the same surface. This surface is preferably a cylindrical surface. In this case, the forward winding and the reverse winding are located at the same radial distance centered on the longitudinal axis. However, it is also conceivable that the forward winding and the reverse winding extend on the same cylindrical surface having a cross-section of a polygon, for example, a square. Whatever the cross-sectional shape, the conductor of the forward winding part extends from the connection region to the axial end of the transmitting and receiving coil in a predetermined winding direction, and then the conductor returns from there to the connection region in the same winding direction. Here, the winding of the reverse winding part has the opposite sign of the pitch to the winding of the forward winding part, and preferably the magnitude of the pitch is the same. The pitch of the winding is understood to be the height of one turn of the winding, that is, the amount of change in the Z' value of the center position of the conductor path after exactly one rotation around the longitudinal axis. The connection region is used to connect the electric coil part to the matching circuit, but can include a plurality of connection parts for a plurality of electric coil parts. The coil part extends between two connection parts of the connection region, and the applied voltage is obtained between the starting point of the forward winding and the ending point of the reverse winding of each coil part.

[0012] In order to arrange the forward winding and the reverse winding on the same cylindrical surface, the forward winding and the reverse winding must cross each other. The crossing occurs in the smallest possible region (crossing region) on the outer periphery of the cylindrical surface. Preferably, one conductor of the forward winding part and the reverse winding part remains on the cylindrical surface, and the other conductor having the form of a bridge element crosses over it. It is beneficial if the crossing region is less than 20% of the conductor length of the forward winding or the reverse winding, more preferably 10%, and particularly preferably less than 5%.

[0013] When several coil parts are provided, for example, when the connection area is arranged between two coil parts (i.e., not at the axial end of the transmitting and receiving coil), these coil parts can be formed as separate components that are electrically connected to each other within the connection area, or can be formed as part of a conductive structure having a connection area.

[0014] The NMR probe head is preferably tuned to at least two frequencies.

[0015] The potential along the conductor of the coil part is determined by a matching circuit to which the electrical coil part is connected via a connection area. In the prior art, during operation, it is common practice to design the matching circuit such that the potentials at the connection points are opposite and as equal as possible in magnitude (balanced circuit). When such a matching circuit is used for the probe head according to the present invention, there are potentials with opposite signs in the forward winding and the reverse winding. The winding at the transition from the forward winding part to the reverse winding part is called a "return winding". In this return winding, there is a point in the conductor where the potential becomes zero during operation. The return winding is at the axial end of the transmitting and receiving coil and occupies a special position. This is because in the area of the return winding, there are changes in the sign of the potential and pitch of the coil part. Depending on the number of turns of the winding of the coil part, the return winding may be assigned to the forward winding part or the reverse winding part, or part may be assigned to the forward winding part and part to the reverse winding part.

[0016] In a preferred embodiment, the coil part has a return winding having a point of zero potential, and preferably, the forward winding and the reverse winding of the coil part, excluding the return winding, are alternately arranged. In this embodiment, the windings of the forward winding part and the reverse winding part of the coil parts 2a, 2b are alternately arranged such that there is one turn of the reverse winding between two turns of the forward winding, and the potentials of adjacent windings can compensate each other as much as possible.

[0017] The structure of the winding and the connection region is selected such that the magnitudes of the operating potentials are the same or of the same order at comparable positions of adjacent windings (e.g., the start, center, or end of the winding). Here, when U1 is the voltage applied to the winding of the first turn, U1 is the voltage applied to the winding of the first turn, and UN is the voltage applied to the winding of N turns, if U1 / UN = (N / 2 - 1) / (N / 2), the potentials are considered to be of the same order.

[0018] Therefore, it is preferable that the windings of the forward winding portion and the reverse winding portion are alternately arranged axially so that a maximum potential difference (the sum of the potential differences of all adjacent winding pairs) occurs between adjacent windings during operation. For this purpose, in particular, the first turn of the forward winding portion and the last turn of the reverse winding portion (i.e., the first turn and the last turn of the winding of the electric coil portion) are arranged adjacent to each other. In this way, windings of opposite potentials are arranged adjacent to each other.

[0019] In a special embodiment of the probe head according to the present invention, the connection region is arranged at the first axial end of the transmitting and receiving coil, the forward winding portion extends from the connection region to the second axial end of the transmitting and receiving coil, and the reverse winding portion extends from the second axial end of the transmitting and receiving coil to the connection region. In principle, the electric coil portion forms two solenoid-like portions connected in series and arranged alternately axially, and these solenoid-like portions have the same winding direction, and the forward winding and the reverse winding have pitches of opposite signs. Therefore, the electric coil portion is particularly composed only of windings wound around the longitudinal axis Z' of the transmitting and receiving coil.

[0020] In another embodiment, the transmitting and receiving coil has at least two electrical coil parts, and the connection area is arranged preferably in the center between the two coil parts. In this case, the forward winding of the first electrical coil part starts from the connection area and reaches the first axial end of the transmitting and receiving coil, and the reverse winding of the first electrical coil part starts from the first axial end of the transmitting and receiving coil and reaches the connection area. The forward winding of the second electrical coil part starts from the connection area and reaches the second axial end of the transmitting and receiving coil, and the reverse winding of the second electrical coil part starts from the second axial end of the transmitting and receiving coil and reaches the connection area.

[0021] Therefore, this embodiment includes two coil parts, each having a forward winding and a reverse winding, and in each coil part, the forward winding and the reverse winding are arranged on the same cylindrical surface. It is preferable that all the windings of the two coil parts (except for the intersection area) are arranged on the same cylindrical surface. Each coil part forms two solenoid coils connected in series and arranged alternately in the axial direction, and these coil parts are connected in parallel. The connection area is arranged between the first axial end and the second axial end of the transmitting and receiving coil. These coil parts extend in opposite directions in the axial direction, but have the same winding direction.

[0022] Preferably, in the connection area (even if the forward winding and the reverse winding are alternately arranged within the coil part), the first turn (the connection turn of the first coil part) of the forward winding (or the reverse winding) of the first coil part is arranged adjacent to the first turn (the connection turn of the second coil part) of the forward winding (or the reverse winding) of the second coil part. This simplifies the design and technical implementation of the connection area. In particular, the two coil parts can be arranged symmetrically with respect to the connection area.

[0023] Alternatively, an alternating arrangement of the forward winding and the reverse winding (i.e., including the connection turns) can be provided across both coil parts. This can further reduce the electric field within the measurement sample.

[0024] The center line of the coil part is generally defined by the following formula.

Number

[0025] When S(t) = const. and T(t) = 0, it becomes a normal solenoid without a reverse winding part.

[0026] In the case of the coil according to the present invention, the following equation holds for each coil part.

Number

[0027] In a simple embodiment of the transmitting and receiving coil according to the present invention, both the forward winding part and the reverse winding part have a constant pitch P. Preferably, the forward winding part and the reverse winding part have the same magnitude of pitch P but in opposite directions, and the number of turns NH of the winding in the forward winding part is equal to the number of turns NR of the winding in the reverse winding part. This means that when the pitch P is constant and the forward winding part and the reverse winding part have windings with the same number of turns (NH = NR = N / 2), for t = 0...NH (with respect to the forward winding part), P(t) = P, and for t = NH...N (with respect to the reverse winding part), P(t) = -P. Particularly preferably, the transmitting and receiving coil has a solenoid-shaped forward winding part without inclination (i.e., T(t) = 0) and a solenoid-shaped reverse winding part. Such a coil can be easily made from, for example, a wire or a strip-shaped conductor wound around a carrier.

[0028] The strip-shaped conductor has a thickness that is small compared to the conductor path width (particularly at least one order of magnitude smaller) and has a substantially rectangular cross-section. The strip-shaped conductor preferably consists of a thin metallization, particularly a substrate coated with HTS.

[0029] Preferably, the conductor path thickness W of the conductor is at most 500 μm and / or at least twice the penetration depth of the high-frequency B1 magnetic field into the conductor.

[0030] In a particularly preferred embodiment, the forward winding and the reverse winding are arranged on a cylindrical surface (i.e., R(t) = const.). This can be made particularly easily because a cylindrical carrier can be used.

[0031] The electrical coil part is particularly preferably formed as a strip coil part with a conductor path width W. The conductor path width is the conductor width in a direction perpendicular to the conductor center. In the strip coil part, the conductor path width is greater than the thickness of the coil part. The strip width may be constant over the entire conductor (W = const.) or may vary (W = W(t)). If the conductor path width W(t) is unequal along the path t along which the conductor extends, the conductor path width may vary within one turn of the winding. The strip coil part may be made, for example, from a tubular blank by etching, milling, or cutting away the areas that are not required. Furthermore, a columnar conductor may be wound around a carrier and pressed flat on it, or a strip-shaped conductor may be wound around a carrier and pressed against its surface. Without pressing, the conductor will not be arranged flat on the carrier due to torsion during winding. Also, a round conductor may be formed into a strip-shaped conductor by pressing after being wound around a carrier.

[0032] In a specific embodiment, within the forward winding part and / or the reverse winding part, the conductor path width of the conductor and / or the gap width D between adjacent windings of the forward winding part and / or the reverse winding part vary in the direction of the length t of the conductor (W = W(t), where t = 0...N).

[0033] In particular, the conductor path width W of the electric coil part and / or the gap width D between adjacent windings of the transmitting and receiving coil may vary within each winding. Preferably, the conductor path width varies between a minimum value and a maximum value within each winding, and the conductor path width decreases and increases at least twice, preferably periodically, within each winding. Thus, each winding of the coil part has two regions with the minimum conductor path width and two regions with the maximum conductor path width. In such an embodiment, the efficiency of the transmitting and receiving coil with T(t)≠0 can be made higher than that of a coil with a constant conductor path width. Furthermore, this allows for obtaining higher permeability with respect to the magnetic field of the second coil in the cross-coil assembly without unduly degrading the quality. When a plurality of regions of the electric coil part with the minimum conductor path width are arranged with a 180° shift with respect to rotation about the longitudinal axis, the plurality of regions with the minimum width are located on mutually opposing cylindrical sections of the cylindrical surface, which means that a plurality of open spaces with the maximum gap width are located on mutually opposing cylindrical sections. This allows for forming more transmission regions that can be used to superimpose the second high-frequency magnetic field generated by the second coil of the MR probe head on the first high-frequency magnetic field. And the sample volume becomes "visible" for both high-frequency magnetic fields. At the same time, the transmitting and receiving coil can assume a conductor path width that optimizes the quality of the coil in the remaining regions. In this way, the probe head can be efficiently tuned to multiple frequencies.

[0034] In another particular embodiment, the gradient S of the winding, in particular the pitch P, varies in the direction of the length t of the electric coil part.

[0035] The change in the gradient S = S(t) can be achieved by changing the pitch P (the gradient of one winding), but it can also be achieved by locally changing the gradient S within one winding while keeping P constant.

[0036] Thus, the gradient S depends on the position along the conductor (S = S(t)). The pitch P is defined by the following formula as the integral of the gradient over one winding. [Number] That is, the gradient S is defined as the distance in the direction of the longitudinal axis that the conductor advances during exactly one revolution, and P = z’(tn) - z’(tn+1). Here, tn is the starting point of the nth winding. Since t is a dimensionless parameter, if the gradient S is constant, the pitch P is equal to the gradient S. That is, S(t) = S in the range of t = t0...t0+1.

[0037] A plurality of windings may be formed as a plurality of inductively or capacitively coupled independent windings. At this time, the pitch P represents the distance between two adjacent independent windings. The homogeneity along the z-axis (on-axis) can be improved by a variable gradient S. Even when the coil pitch P is constant, the local gradient S can be made variable within one winding, and the sign can also be reversed. The pitch P of one winding is expressed as positive when the Z’ coordinate of the center line at t0+1 is larger than the Z’ coordinate at t0. This also applies when the Z’ coordinate takes a value smaller than that at t0 in the interval between t0 and t0+1.

[0038] In a finite solenoid coil, the amplitude of the B1 magnetic field drops at both axial ends. By reducing the pitch P at both axial ends, the windings can be brought closer to each other. In this way, the shortage of current density caused by the coil being finite can be compensated. Therefore, in particular, when the pitch P at both axial ends of the transmitting and receiving coil is smaller than that at the axial center, the homogeneity along the longitudinal axis Z’ can be improved in a short transmitting and receiving coil operating far from the natural frequency.

[0039] When a "short" transmit-receive coil having a center tap operates at or near its resonant frequency, it is beneficial to reduce the pitch P at the center to reduce the magnetic field generated by the substantially sinusoidal current distribution along the length direction of the coil portion at the center, which can be compensated by "compressing" the windings within the coil portion. In such a coil, the highest current density occurs in the return winding(s), which generally compensates substantially for the lack of current density in a short solenoid coil.

[0040] To improve the homogeneity in the radial direction (off-axis), in a further embodiment of the MR probe head according to the present invention N the windings of the transmit-receive coil along the conductor are configured such that the inclination with respect to the longitudinal axis Z' becomes non-uniform so that T = T(t) holds.

[0041] The inclination T is defined as the amplitude of the sinusoidal variation of the Z' position of the center plane of the conductor over one turn of the winding. In principle, a coil with an inclination T≠0 can also be represented by a general gradient of S'(t)=S(t)+T(t)cos(2πt + φ). When writing S'(t) as a Fourier series, T represents the (k = 1) periodic part of the gradient S. For each turn of the winding, the gradient S can be written as follows.

Equation

Equation

[0042] The slope T may vary along the path t along which the conductor extends (T = T(t)). In this case, the slope is constant, at least partially, over at least one-half turn of the winding (half a revolution of the winding). Thereafter, the slope varies from one half-turn of the winding to the next. This means that each of the half-turn windings is inclined at a slope T with respect to the longitudinal axis Z', and T ≠ 0 in at least some of the plurality of one-half turn windings (half-turn windings). The slope T of the winding affects the B1 amplitude and the radial homogeneity. In the case of a coil with a slope T ≠ 0, the gradient S is different between the first half-turn winding and the next half-turn winding. Usually, the direction of the slope is φ = 0 (slope around the Y' axis) or φ = π / 2 (slope around the X' axis), and the radius R(t) = R.

[0043] In the case of a coil where the gradient S is an arbitrary function S(t), the effect of T(t) can also be represented via S(t), so the definition of the slope has only a limited meaning. T(t) reflects the periodically varying part of the non-uniform gradient S, which corresponds to the slope of the (elliptical) coil with respect to the Z' axis.

[0044] When the longitudinal axis of the transmit-receive coil is not arranged parallel to the static magnetic field B0 used for NMR measurement, such as in MAS (magic angle rotation) measurement, especially when the transmit-receive coil is formed as a solenoid coil or is composed of solenoid-like parts, the amplitude of the high-frequency B1 magnetic field generated by the transmit-receive coil, and thus the efficiency of the transmit-receive coil, can be increased. In MAS NMR measurement, the measurement sample is rotated about the longitudinal axis Z', and the longitudinal axis Z' is preferably inclined by the magic angle θ (θ = 54.74°) with respect to the Z axis defined by the static magnetic field B0, and the longitudinal extension of the NMR probe head during operation (i.e., when the NMR probe head is attached to the NMR apparatus) extends in this direction. By tilting the windings of the transmit-receive coil, the magnetic field component of the high-frequency B1 magnetic field parallel to the static magnetic field B0 can be minimized, or the magnetic field component of the high-frequency B1 magnetic field orthogonal to the static magnetic field B0 can be maximized. However, by tilting the windings, in contrast to a conventional (non-tilted) solenoid coil, the high-frequency B1 magnetic field within the measurement volume is no longer parallel to Z', and as a result, the conductors of the coil "get in the way of the magnetic fields they generate". By reducing the conductor path width of the tilted solenoid coil in these regions, the transmission efficiency and the achievable signal-to-noise ratio of the tilted solenoid coil according to the present invention can be improved.

[0045] In a particularly preferred embodiment, at least two of the tilt T, the gradient S, and the conductor path width W vary along the path along which the conductors of the transmit-receive coil extend, and in particular, the gradient S and one of the tilt T and the conductor path width W vary. In this way, a transmit-receive coil optimized with respect to the signal-to-noise ratio (SNR) can be realized.

[0046] In a further embodiment, at least one winding is configured such that the gradient S = 0 over substantially the entire circumference, in particular outside the intersection region (so-called "zero pitch" coil). Such a winding forms an open ring, i.e., S(t) = 0 holds at t = t0...t0 + 1 - ε or t = t0 + ε / 2...t0 + 1 - ε / 2. Here, ε > 0, and the fact that ε > 0 prevents short circuits, and t = t0 is the starting point of the winding. In the case of a non-gradient transmitting coil (i.e., when T = 0), the winding is arranged perpendicular to the longitudinal axis over the entire range where the gradient S = 0. Such a transmitting coil can be formed as a combination of an open "ring" without pitch and a portion of the electrical coil part with a gradient S > 0. As a result, the ratio of the conductor path width to the gap width can be kept constant over the transmitting coil. As a result, it becomes particularly easy to maximize the quality of the transmitting coil and / or minimize the electric field.

[0047] Further advantages of the present invention can be found in the description and the drawings. Similarly, the above-mentioned features and the features detailed below can be used individually or collectively in any combination according to the present invention. The illustrated and described embodiments should not be understood as an exhaustive list, but rather as having typical features for the description of the present invention.

Brief Description of the Drawings

[0048]

Figure 1a

Figure 1b

Figure 2a

Figure 2b

Figure 3

Figure 4a

Figure 4b

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0049] Figures 1a and 1b show a particularly preferred embodiment of the transmit-receive coil 1 for the NMR probe head 18 (see FIG. 10) according to the present invention in perspective view and developed view. A measurement sample 19 to be inspected is disposed in the transmit-receive coil 1. The transmit-receive coil 1 includes two coil portions 2a, 2b having a forward winding 3a, 3b extending from the connection region 4 to the axial end portions 5a, 5b of the transmit-receive coil 1 and a reverse winding 6a, 6b returning from each of the axial end portions 5a, 5b to the connection region 4. The windings having the connection region 4 as the end point / start point are called connection windings 16a, 16b. The forward winding 3a of the first coil portion 2a and the forward winding 3b of the second coil portion 2b have the same winding direction. The reverse windings 6a, 6b have a pitch P opposite to that of the forward windings 2a, 2b, but each has the same winding direction. The forward windings 3a, 3b and the reverse windings 6a, 6b are connected via return windings 15a, 15b, and the return windings 15a, 15b are respectively disposed at the end portions 5a, 5b on the side opposite to the connection portion 9 of the coil portions 2a, 2b, and invert the sign of the pitch P of the winding. The conductors of the return path (reverse windings) are wound on the same plane (in this case, a cylindrical surface) as the conductors of the forward path (forward windings). One turn of the reverse windings 6a, 6b is disposed in the space between two turns of the forward windings 3a, 3b. The necessary crossing 7 occurs in the smallest possible region (crossing region 8) of the outer periphery. A plurality of crossings 7 of the forward windings and the reverse windings 3a, 3b, 6a, 6b are realized by bridge elements. These bridge elements are connection elements that protrude from and return to a common peripheral surface. The two coil portions 2a, 2b are mirror-symmetrical to each other with respect to the connection region 4, and the forward windings and the reverse windings are alternately arranged within each conductor portion, but in the connection region 4, the first turn (i.e., the connection winding 16a of the first coil portion 2a) of the forward winding (or reverse winding) of the first coil portion 2a is disposed adjacent to the first turn (i.e., the connection winding 16b of the second coil portion 2b) of the forward winding (or reverse winding) of the second coil portion 2b.

[0050] Figures 2a and 2b show a perspective view and a developed view of another embodiment of the transmitting and receiving coil 1a, which includes only one coil part 2 having a forward winding 3 and a reverse winding 6. Here, the conductor starts from the axial end 5a of the transmitting and receiving coil 1a and reaches the other axial end 5b of the coil in a predetermined winding direction, and then returns from there to the first axial end 5a in the same winding direction but with the opposite pitch P. Therefore, in this embodiment, in contrast to the embodiment shown in FIG. 1, the connection region 4' is arranged at the first axial end 5a. Similar to the embodiment shown in FIG. 1, in the embodiment shown in FIG. 2, the reverse winding 6 is also located on the same plane as the forward winding 3, and one turn of the reverse winding 6 is arranged in the space between two turns of the forward winding 3. In this case, the forward winding 3 and the reverse winding 6 are connected via a return winding 15 arranged at the end 5b of the coil part 2 on the side opposite to the connection region 4', where the sign of the pitch P of the winding is inverted. The first turn of the forward winding and the last turn of the reverse winding (the windings of the connection region 4' respectively) are the connection windings 16.

[0051] In all embodiments of the coil structure according to the present invention, the coil parts 2, 2a, 2b according to the present invention have both forward and reverse windings, and the reverse windings 6a, 6b are on the same plane as the forward windings 3a, 3b and have opposite pitches P. In the special embodiments shown, one turn of the reverse windings 6a, 6b is arranged in the space between two turns of the forward windings 3a, 3b within each coil part 2a, 2b.

[0052] When there are two coil parts 2a, 2b (FIGS. 1, 5 to 9), the structure of the embodiment of the transmitting and receiving coil of the present invention shown is mirror-symmetrical, and at this time, the reverse windings or the forward windings of the two coil parts are arranged adjacent to each other in the connection region 4.

[0053] In all embodiments, there are connection portions 9 in the connection regions 4, 4', through which the connection winding lines 16a, 16b of the two conductor portions 3a, 3b are connected to the matching circuit 10 (see FIG. 10) to supply energy to the transmitting and receiving coils 1, 1a while transmitting high-frequency pulses or to detect the signal induced in the transmitting and receiving coils after the excitation of the measurement sample 19. The connection region can be provided at one end, one or more central regions, or both ends, and particularly in embodiments with a plurality of coil portions, it can be provided at both the central region and the end.

[0054] In the prior art, during operation, it is common practice to design the matching circuit 10 such that opposite potentials with absolute values as equal as possible exist at the connection portion 9. Thereby, usually, the electric field that can be generated in the measurement sample by one embodiment of the transmitting and receiving coil is minimized. In the case of the structure of the transmitting and receiving coil according to the present invention, windings with opposite potentials are both arranged at the same radial distance from the longitudinal axis of the transmitting and receiving coil, and at this time, the potentials of adjacent windings 3 to 6; 3a to 6a; 3b to 6b compensate each other, that is, their potentials are as equal as possible in magnitude but opposite in sign. A simulation of the change in the equipotential lines of the electric field around the conductor of the transmitting and receiving coil 1a similar to the coil in FIG. 2 is shown in FIG. 3. Since the windings with the largest potential difference are arranged adjacent to each other, the electric field is concentrated in the gap and decays very rapidly as the distance from the transmitting and receiving coil 1a increases. Therefore, the electric field only slightly reaches the inside of the transmitting and receiving coil 1a where the measurement sample 19 is located. When the NMR measurement head is loaded with a conductive measurement sample or a measurement sample with high dielectric loss, the electric field during high-frequency pulse transmission dissipates in the measurement sample, which may heat the measurement sample. During reception, noise caused by the electric field is picked up from the measurement sample. This is particularly inconvenient when the transmitting and receiving coil is cooled to cryogenic temperatures and has a temperature significantly lower than that of the measurement sample. By minimizing the electric field in the measurement sample 19 as possible with the structure according to the present invention, a good signal-to-noise ratio in the cryogenic-cooled NMR probe head is ensured even when operating with a lossy measurement sample 19.

[0055] Further optimization of the NMR probe head according to the present invention can be achieved by changing the coil parameters of the transmitting and receiving coils.

[0056] Figures 4a and 4b schematically show the details of a solenoid coil having a strip-shaped conductor (conductor path 11). First, the coil parameters will be described based on these. The solenoid coils in Figures 4a and 4b are arranged along the longitudinal axis Z' (coil axis), and the longitudinal axis Z' is perpendicular to the X'-Y' plane. The solenoid-shaped coil is parameterized by the conductor path width W of the conductor path 11, the gap width D of the space 12, the pitch P of the winding, the inclination T of the winding, and the radius R of the winding. In the illustrated embodiment, a total of three turns of winding are shown.

[0057] The conductor path width W indicates the width of the conductor path 11. The conductor path width W is determined by the outermost ends of the conductor path 11. In the embodiment shown here, the conductor path width W is kept constant over the entire length of the conductor (i.e., W(t) = const.).

[0058] The gap width D indicates the width of the space 12 between the windings of the conductor path 11. The gap width D is determined by the outermost ends of the region between adjacent windings of the conductor path 11.

[0059] When the gradient is constant over the length of one turn of the winding, S(t) = S from tn to tn+1, and the pitch P of the winding is defined by the following formula.

Equation

[0060] The slope T of the winding indicates the slope of the winding with respect to the longitudinal axis Z', and corresponds to the amplitude of the sinusoidal change in the Z' position of the conductor center plane over one turn of the winding. When the pitch P and the slope T are constant over a plurality of turns of the winding, it can be easily determined from Max(Z(t)-Z(t+1))-S) / 2. Here, t varies in the interval from t0...t0+1.

[0061] The radius R of the winding indicates the radius along which the conductor path 11 extends in the case of a cylindrical coil. In general, for a non-cylindrical coil as well, since it can be represented by R, let R = R(t).

[0062] The solenoid coils shown in FIGS. 4a and 4b each have a constant conductor path width W, a constant gap width D, and a constant gradient S (therefore, the pitch P is also constant). The coil shown in FIG. 4a is an inclined coil (T≠0), and the coil shown in FIG. 4a is a non-inclined coil (T = 0).

[0063] The embodiments of the transmitting and receiving coils 1, 1a of the present invention shown in FIGS. 1 and 2 each have a constant conductor path width W, a constant pitch P, and a slope T = 0.

[0064] Special modifications of the transmitting and receiving coil structure according to the present invention will be described below. By changing the coil parameters, the performance of the NMR coil head according to the present invention can be further improved.

[0065] FIG. 5 shows another embodiment of the transmitting and receiving coil 1b, where the conductor path width W and the gap width D along the conductor path are unequal (W = W(t)), and the conductor path width (excluding the intersection region 8) is constant within each turn of the winding (W = const.), but changes from one turn to the next. In the central connection region 4 (where the potential difference is large), the conductor path width W is maximum and the gap width D is minimum. As it progresses in the direction of the axial ends 5a, 5b (the direction in which the potential difference becomes smaller), the conductor path width W becomes smaller, and accordingly the gap width D becomes larger. At the axial ends 5a, 5b (where the minimum potential difference exists), the conductor path width W is minimum and the gap width D is maximum. Therefore, in this embodiment, the conductor path width W and the gap width D are functions of the potential difference.

[0066] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 5 includes two coil portions 2a, 2b having forward windings 3a, 3b and reverse windings 6a, 6b, and the conductor starts from the central connection region 4 and reaches each axial end 5a, 5b of the transmitting and receiving coil 1 at a predetermined pitch P, and then returns from there to the original central connection region 4 at the opposite pitch P. However, the unequal conductor path width W and / or gap width D can also be realized in an embodiment having only one coil portion. In this case, the gap width D increases and the conductor path width W decreases from the first axial end 5a toward the second axial end 5b.

[0067] With the transmitting and receiving coil 1b shown in FIG. 5, in particular, when the ratio W / D of the conductor path width to the gap width in the region of low potential difference is selected such that the electrical loss in the transmitting and receiving coil is minimized, on the one hand, it is possible to minimize the electric field within the measurement volume, and on the other hand, it is possible to improve the quality of the transmitting and receiving coil 1b. In the region where the potential difference is large, additional electrical losses within the transmitting and receiving coil are tolerated in order to reduce the losses caused by the electric field within the measurement sample (not shown in FIG. 5). If the gap width D becomes too small in the region where the potential difference is large, the risk of voltage drop during operation increases. Therefore, there is a lower limit for the gap width D, which is determined by the dielectric breakdown voltage. It is beneficial if D > 0.01 mm, and particularly beneficial if D > 0.1 mm. This makes it possible to significantly reduce the electric field in the measurement sample while providing reproducible manufacturability and sufficient dielectric breakdown voltage for a typical measurement sample diameter in the range of 0.5 mm to 5 mm.

[0068] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 5 also has non-tapered windings (T(t) = 0). The pitch P of the windings is a constant value (|P| = const.). This exception applies to the return windings 15a, 15b and one connecting winding, which have a pitch P that is half the value due to certain boundary conditions (inversion of the pitch P, position of the connection region). Nevertheless, such a coil can be regarded as a coil with a constant pitch P. The gradient S within the winding per turn is S(t) = 0 in all regions except the intersection region 8. Therefore, the constant pitch |P| = const. of the aforementioned windings is realized by the intersection region 8.

[0069] The solenoid coil formed in this way is known by the term "zero pitch" coil. Similarly, the coil according to the present invention for which S(t) = 0 for all t outside the intersection region shall also be referred to as a zero pitch coil.

[0070] In general, as long as adjacent conductor paths do not touch, conductor path widths W and / or gap widths D that are unequal can be used together with any gradient S and / or inclination T.

[0071] FIG. 6 shows another embodiment of the transmit-receive coil 1b, where the conductor path width W and the gap width D vary within each turn of the winding along the conductor in which they extend (W = W(t)). Preferably, as shown in FIG. 6, in the intersection region 8 and the region 13 that is radially opposed to the center of the intersection region 8 (when the transmit-receive coil 1c is in a wound state), the conductor path width W is minimized and the gap width D is maximized. In two further regions 14 of each turn of the winding, between the intersection region 8 and the region 13 that is radially opposed thereto, the conductor path width W is maximized and the gap width D is minimized. In the embodiment shown in FIG. 6, the minimum values and / or the maximum values are shifted by 180° with respect to rotation about the longitudinal axis. Thus, there are two regions (the intersection region 8 and the radially opposed region 13) where the conductor path width W is minimum and two regions (the further regions 14) where the conductor path width W is maximum. Here, the conductor path width W preferably varies periodically.

[0072] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 6 includes two coil portions 2a, 2b having forward windings 3a, 3b and reverse windings 6a, 6b, and the conductor starts from the central connection region 4 and reaches each axial end 5a, 5b of the transmit-receive coil 1 at a predetermined pitch P, and then returns from there to the original central connection region 4 at the opposite pitch P. However, the unequal conductor path width W and / or gap width D can also be realized in an embodiment having only one coil portion.

[0073] In the transmit-receive coil 1c shown in FIG. 6, by increasing the gap width D, a highly permeable space (“window”) can be provided from which another magnetic field can be radiated. As a result, for example, N a second high-frequency magnetic field generated by a second transmit coil (not shown) of the transmit-receive coil assembly of the MR probe head can be superimposed on the first high-frequency magnetic field of the transmit-receive coil 1. By providing the second transmit coil, other NMR active nuclei in addition to protons can be examined.

[0074] The embodiment shown in FIG. 6 is a zero pitch coil, similar to the embodiment shown in FIG. 5, where the winding is not inclined (T(t) = 0) and the pitch P is a constant value (|P| = const.).

[0075] However, unequal conductor path widths W and / or gap widths D can be used with any gradient S and / or inclination T. Further, the conductor path width W and / or the gap width D can be varied so that the conductor path width does not become minimum in the intersection region 8.

[0076] FIG. 7 shows yet another embodiment of the transmitting and receiving coil 1d, in which the pitch P is substantially constant (i.e., excluding the return windings at both ends of the conductor part, i.e., the axial end parts 5a, 5b, and the connecting windings in the central connection region 4 if necessary). Note that the gradient S varies within each winding per turn. This means that P(t) is constant for each part, and P(t) = P1, P2,... in the windings of the first turn, the second turn,.... Similar to the embodiments described above, the windings of the embodiment shown in FIG. 7 have no inclination (T = 0).

[0077] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 7 includes two coil parts 2a, 2b having forward windings 3a, 3b and reverse windings 6a, 6b. The conductor starts from the central connection region 4 and reaches the axial end parts 5a, 5b of the transmitting and receiving coil 1 in a predetermined winding direction and a predetermined pitch P, and then returns from there to the original central connection region 4 in the same winding direction but with the opposite pitch P. The winding direction of the conductor is different between the two coil parts, i.e., for example, it is in the positive direction in the forward winding part 2a and in the negative direction in the reverse winding part 2b. However, the coexistence of a non-uniform gradient S and a constant pitch P in the winding can also be realized in an embodiment having only one coil part.

[0078] FIG. 8 shows yet another embodiment of the transmitting and receiving coil 1e, where the pitch P of the winding, the conductor path width W, and the gap width D are unequal over the entire length t of the conductor, and the inclination T of the winding is 0 (P = P(t) and T = 0). In the embodiment shown in FIG. 8, the pitch P, the conductor path width W, and the gap width D are maximum in the central connection region 4 and decrease toward the axial end portions 5a, 5b. At the axial end portions 5a, 5b, the pitch P, the conductor path width W, and the gap width D are minimum (optionally excluding the connecting winding for one turn).

[0079] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 8 includes two coil portions 2a, 2b having forward windings 3a, 3b and reverse windings 6a, 6b, and the conductor starts from the central connection region 4 and always reaches the axial end portions 5a, 5b of the transmitting and receiving coil 1 in a predetermined winding direction and a predetermined pitch P, and then returns from there to the original central connection region 4 in the same winding direction but with the opposite pitch P. However, the unequal pitch P can also be realized in an embodiment having only one coil portion. In the case of an operation close to the natural frequency, the pitch P decreases in this case from the first axial end portion toward the second axial end portion. In the case of an operation sufficiently lower than the natural frequency, the pitch P decreases from the first axial end portion toward the center of the coil and then increases again toward the second axial end portion.

[0080] In this transmitting and receiving coil 1e, the axial uniformity can be improved, that is, the finite coil length can be compensated.

[0081] FIG. 9 shows yet another embodiment of the transmitting and receiving coil 1f, where the winding is inclined with respect to the longitudinal axis (S ≠ 0). In this case, the inclination is constant, that is, T = const. It can be clearly seen that the Z' value increases within a half turn of the winding (here, a half turn of the winding starting from the intersection region of the forward winding), and decreases within the remaining half turn of the winding (here, a half rotation of the forward winding ending at the intersection region), so that the Z' position changes in a sine wave shape.

[0082] The pitch P (excluding the return windings of the two axial ends 5a and 5b and the connecting winding for one turn) is constant throughout the entire transmitting and receiving coil 1f.

[0083] Such a coil is particularly useful when it is desired to generate a high-frequency magnetic field that is not parallel to the cylindrical axis (the longitudinal axis Z' of the transmitting and receiving coil). This is particularly useful for MAS-NMR measurement samples in which the cylindrical axis Z' and the direction of the static magnetic field are arranged at the magic angle. In such a configuration, by setting a constant inclination T(t)=const., the measurement efficiency of the measurement sample can be increased.

[0084] Similar to the embodiment shown in FIG. 1, the embodiment shown in FIG. 9 includes two coil portions 2a and 2b having forward windings 3a and 3b and reverse windings 6a and 6b. The conductor starts from the central connection region 4 and reaches the axial ends 5a and 5b of the transmitting and receiving coil 1 in a predetermined winding direction and pitch P, and then returns from there to the original central connection region 4 in the same winding direction but with the opposite pitch P. The inclination T can also be changed every one turn of the winding or every half rotation. However, a non-zero inclination T or an unequal inclination T can also be realized in an embodiment having only one coil portion.

[0085] FIG. 10 shows a schematic diagram of an NMR probe head 18 according to the present invention. The external magnetic field for performing NMR measurement is parallel to the Z axis during operation in the example shown here. The NMR probe head 18 has a transmitting and receiving coil 1 according to the present invention, and the transmitting and receiving coil 1 is connected to a matching circuit 10 and also has a spectrometer connection portion 17 for each measurement channel. The NMR probe head shown in FIG. 10 is a MAS (magic angle rotation) probe head in which the longitudinal axis Z' of the transmitting and receiving coil 1 is inclined with respect to the Z axis of the NMR probe head 18, preferably at the magic angle θ (θ = 54.74°).

[0086] List of cited references, etc. [Pel 2016] Pel et al. 1H,23Na and 35Cl Imaging in Cementitious Materials with NMR Appl Magn Reson(2016)47:265-276 [Krahn 2008] Krahn et al. Resonator with reduced sample heating and increased homogeneity for solid-state NMR J.Magn.Reson.191(2008)78-92 [Stringer 2005] Stringer et al. Reduction of RF-induced sample heating with a scroll coil resonator structure for solid-state NMR probes J.Magn.Reson.173(2005)40-48 [Dillmann 2007] Dillmann et al. A novel low-E field coil to minimize heating of biological samples in solid-state multinuclear NMR experiment J.Magn.Reson.187(2007)10-18 [Gorkov 2007] Gorkov et al. Using low-E resonators to reduce RF heating in biological samples for static solid-state NMR up to 900 MHz Journal of Magnetic Resonance 185(2007)77-93 [Grant 2009] Grant et al. A Modified Alderman-Grant Coil makes efficient cross-coil probe for high field solid-state NMR of lossy biological samples Journal of Magnetic Resonance 201(2009)87-92 [EP1571459]EP 1 571 459 B1 [JP4787033]JP 4787033 B2 [US5003265]US 5,003,265 [US5180982] US 5,180,982 [US6252403] US 6,252,403 B1 [US6751847]US 6,751,847 B1 [US6958608] US 6 958,608 B2 [Explanation of symbols]

[0087] 1, 1a to 1f Transmitting and receiving coils 2, 2a, 2b Coil section 3, 3a, 3b Coil section forward winding 4,4' Connection area 5a, 5b Axial ends of the receiving and transmitting coils 6,6a,6b Reverse winding of coil section 7 Conductor Crossing 8 Intersection Area 9 Connection of the connection areas 4, 4' 10 Matching circuit 11 Conductor conductor path 12 Space between the windings of the receiver / transmitter coils 13 Region of the coil where the conductor width W is minimum and the gap width D is maximum 14 Region of the coil part where the conductor path width W is maximum and the gap width D is minimum (further region) 15, 15a, 15b Return winding 16, 16a, 16b Connecting windings 17 Spectrometer connection 18 NMR probe head 19 Measurement sample

Claims

Claim 1 An NMR probe head comprising a transmit-receive coil assembly having at least one transmit-receive coil (1; 1a; 1b; 1c; 1d; 1e; 1f) for generating a high-frequency B1 magnetic field, wherein the transmit-receive coil (1; 1a; 1b; 1c; 1d; 1e; 1f) has at least one electrical coil part (2; 2a, 2b) and a connection region (4; 4'), the electrical coil part (2; 2a, 2b) has a forward winding part and a reverse winding part, the forward winding part has a forward winding (3; 3a, 3b) and, starting from the connection region (4; 4'), in a predetermined winding direction, reaches the axial ends (5a, 5b) of the transmit-receive coil (1; 1a; 1b; 1c; 1d; 1e; 1f), the reverse winding part has a reverse winding (6; 6a, 6b) and, starting from the axial ends (5a, 5b) of the transmit-receive coil (1; 1a; 1b; 1c; 1d; 1e; 1f), in the same winding direction as the predetermined winding direction, reaches the connection region (4; 4'), and the windings of the reverse winding part have a pitch P with a sign opposite to that of the windings of the forward winding part, the forward winding and the reverse winding (3, 6; 3a, 3b, 6a, 6b) of the electrical coil part (2; 2a, 2b) are arranged on the same cylindrical surface centered on the longitudinal axis Z' except in an intersection region (8) where the forward winding and the reverse winding (3, 6; 3a, 3b, 6a, 6b) intersect each other, the windings of the transmit-receive coil (1e; 1f) have an unequal inclination with respect to the longitudinal axis Z' along the length direction of the electrical coil part (2; 2a, 2b), NMR probe head. Claim 2 the electrical coil part (2; 2a, 2b) has a return winding (15) having a point that becomes zero potential during operation, the forward winding (3; 3a, 3b) and the reverse winding (6; 6a, 6b) of the electrical coil part (2; 2a, 2b) excluding the return winding (15) are alternately arranged, The NMR probe head according to claim 1, characterized in that. Claim 3 The connection region (4') is arranged at a first axial end portion (5a) of the transmission / reception coil (1a), the forward winding portion starts from the connection region (4') and reaches a second axial end portion (5b) of the transmission / reception coil (1a), and the reverse winding portion starts from the second axial end portion (5b) of the transmission / reception coil (1a) and reaches the connection region (4'), wherein the NMR probe head according to claim 1 or 2 is characterized in that.

4. The transmission / reception coil (1; 1b; 1c; 1d; 1e; 1f) has at least two of the electric coil portions (2a, 2b), The connection region (4) is arranged preferably at the center between the two electric coil portions (2a, 2b), The forward windings (3a, 3b) of the first electric coil portion (2a) start from the connection region (4) and reach a first axial end portion (5a) of the transmission / reception coil (1; 1b; 1c; 1d; 1e; 1f), and the reverse winding (6a) of the first electric coil portion (2a) starts from the first axial end portion (5a) of the transmission / reception coil (1; 1b; 1c; 1d; 1e; 1f) and reaches the connection region (4), The forward winding (3b) of the second electric coil portion (2b) starts from the connection region (4) and reaches a second axial end portion (5b) of the transmission / reception coil (1; 1b; 1c; 1d; 1e; 1f), and the reverse winding (6b) of the second electric coil portion (2b) starts from the second axial end portion (5b) of the transmission / reception coil (1; 1b; 1c; 1d; 1e; 1f) and reaches the connection region (4), wherein the NMR probe head according to claim 1 or 2 is characterized in that.

5. The NMR probe head according to claim 1, wherein the forward winding and the reverse winding (3, 6; 3a, 3b, 6a, 6b) are arranged on a cylindrical surface.

6. The NMR probe head according to claim 1, wherein the electric coil portion (2; 2a, 2b) is formed as a strip coil portion (2; 2a, 2b) with a conductor path width W.

7. The NMR probe head according to claim 6, wherein the conductor path width W of the electric coil portion (2a, 2b) and / or the gap width D between adjacent windings of the transmission / reception coil (1b; 1c) vary in the longitudinal direction of the electric coil portion (2a, 2b).

8. The NMR probe head according to claim 7, characterized in that the conductor path width W of the electric coil part (2; 2a, 2b) and / or the gap width D between adjacent windings of the transmitting / receiving coil (1c) varies within each winding per turn.

9. The NMR probe head according to claim 1, characterized in that the gradient S of the winding, particularly the pitch P, varies in the longitudinal direction of the electric coil part (2a, 2b).

10. The NMR probe head according to claim 9, characterized in that the pitch P at the axial end portions (5a, 5b) is smaller than that at the axial center of the transmitting / receiving coil (1e).

11. The NMR probe head according to claim 1, characterized in that for at least one turn of the winding of the transmitting / receiving coil (1; 1a; 1b; 1c; 1d; 1e; 1f), the gradient S is S = 0 outside the intersection region (8).

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