Abstract
A nuclear magnetic resonance coil configuration having at least one flat or cylindrical coil (18), through which current flows in operation, which coil generates a high-frequency magnetic B.sub.1 field at the location of a sample (16) which is oriented parallel to an x-axis, and which for the purpose of connection to a tuning network is connected to at least two electrical feed lines (11), through which in-phase currents flow in operation, and which generate a high-frequency magnetic B.sub.2 field in the sample (16), the orientation of which encloses an angle with the direction of the B.sub.1 field, is characterized in that the following applies for the angle : =180, where <90. In this way, a B.sub.1 field profile, which is as rectangular as possible and is particularly steep on both sides, can be generated.
Claims
1. A nuclear magnetic resonance coil configuration for examination of a sample, the coil configuration comprising: at least one flat or cylindrical coil through which a current flows in operation, said current flow in said coil generating a B.sub.1 high-frequency magnetic field at a location of the sample, wherein said B.sub.1 high-frequency magnetic field is oriented parallel to an x-axis; and at least two electrical feed lines, said electrical feed lines being disposed, structured and dimensioned to connect said coil to a tuning network, wherein at least a fraction of said current generating said B.sub.1 high-frequency magnetic field also flows through said electrical feed lines in operation, said current flow in said electrical feed lines thereby generating a B2 high-frequency magnetic field in the sample having a same phase as said B.sub.1 high-frequency magnetic field, said B.sub.2 high-frequency magnetic field being oriented at an angle with respect to a direction of said B.sub.1 high-frequency magnetic field, wherein =180, with <90.
2. The coil configuration of claim 1, wherein 45.
3. The coil configuration of claim 2, wherein 15.
4. The coil configuration of claim 1, wherein at least two of said electrical feed lines have a crossing point.
5. The coil configuration of claim 1, further comprising ring elements disposed in a bottom region of the coil configuration, wherein at least two of said electrical feed lines are electromagnetically coupled to said ring elements, said ring elements having a crossing point.
6. The coil configuration of claim 1, further comprising at least one HF screen, said screen being disposed such that a sum of said B.sub.1 and B.sub.2 fields is substantially zero in at least a sub-region of the sample.
7. The coil configuration of claim 1, wherein the coil configuration comprises at least four electrical feed lines forming at least two pairs of said at least two electrical feed lines, said at least four electrical feed lines generating B.sub.2.sup.i high-frequency fields in the sample during operation, wherein a sum of said B.sub.2.sup.i fields is substantially zero in at least in a sub-region of the sample.
8. The coil configuration of claim 7, wherein at least pairs of electrical feed lines are fitted on opposite sides of the coil configuration.
9. The coil configuration of claim 1, wherein the coil configuration comprises at least two coils which each generate a B.sub.1.sup.i high-frequency field in operation, said B.sub.1.sup.i fields being aligned to enclose an angle between them, wherein ||<10 or |180|<10.
10. The coil configuration of claim 9, further comprising at least one HF screen, wherein, when operating at at least one measuring frequency, a sum of said B.sub.1.sup.i and said B.sub.2 fields is substantially zero in at least a sub-region of the sample.
11. The coil configuration of claim 10, wherein at least one said HF screen has openings.
12. The coil configuration claim 11, wherein said openings are slots.
Description
BRIEF DESCRIPTION OF THE DRAWING
(1) FIG. 1a shows a coil configuration with single-turn saddle coils and four feed lines in a developed view according to the prior art;
(2) FIG. 1b shows the schematic course of the field lines of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for an configuration according to FIG. 1a;
(3) FIG. 1c shows the direction of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for a coil configuration according to FIG. 1a;
(4) FIG. 1d shows a coil configuration with two parallel-connected two-turn saddle coils and three feed lines in a developed view according to the prior art;
(5) FIG. 1e shows a birdcage resonator configuration with two feed lines and capacitive coupling in a developed view according to the prior art;
(6) FIG. 1f shows a coil configuration with two series-connected single-turn saddle coils and two feed lines in a developed view according to the prior art;
(7) FIG. 1g shows the schematic course of the field lines of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for an configuration according to FIG. 1f;
(8) FIG. 1h shows the direction of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for a coil configuration according to FIG. 1f;
(9) FIG. 2a shows a schematic diagram of the course of the magnetic field on a log-lin scale of the sum of the B.sub.1 and B.sub.2 fields for coil configurations according to the prior art;
(10) FIG. 2b shows a schematic diagram of the course of the magnetic field on a log-lin scale of the sum of the B.sub.1 and B.sub.2 fields for coil configurations according to the invention;
(11) FIG. 2c shows a schematic diagram of the course of the magnetic field on a log-lin scale of the sum of the B.sub.1 and B.sub.2 fields for coil configurations with optimally placed HF screens according to the prior art;
(12) FIG. 2d shows a schematic diagram of the course of the magnetic field on a log-lin scale of the sum of the B.sub.1 and B.sub.2 fields for coil configurations according to the invention with optimally placed HF screens;
(13) FIG. 3a shows a schematic diagram of a first embodiment of the coil configuration according to the invention with crossing points in the feed lines;
(14) FIG. 3b shows the schematic course of the field lines of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for an configuration according to FIG. 3a;
(15) FIG. 3c shows the direction of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for a coil configuration according to FIG. 3a;
(16) FIG. 4a shows a schematic diagram of a second embodiment of the coil configuration according to the invention with series-connected single-turn coils and two feed lines;
(17) FIG. 4b shows the schematic course of the field lines of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for an configuration according to FIG. 4a;
(18) FIG. 4c shows the direction of the B.sub.1 and the B.sub.2 field in the planes A-A and B-B respectively for a coil configuration according to FIG. 4a;
(19) FIG. 5 shows a developed diagram of a third embodiment of the coil configuration according to the invention with crossing points within the coil;
(20) FIG. 6a shows a schematic diagram of a fourth embodiment of the coil configuration according to the invention with two pairs of feed lines of which that in the direction of the x-axis (0) includes a crossing point and the second in the direction of the x-axis (180) is designed without crossing point;
(21) FIG. 6b shows a coil configuration as in FIG. 6a, in which however both pairs of feed lines are mounted in the direction of the x-axis; and
(22) FIG. 7 shows a coil configuration consisting of a resonator and a saddle coil, wherein the feed lines of the resonator are designed without crossing point, those of the saddle coil with crossing point, and the HF screen has an opening.
DESCRIPTION OF THE PREFERRED EMBODIMENT
(23) According to the invention, a B.sub.1 field profile which is steep on both sides is generated in that the feed lines of the coil or the resonator have at least one crossing point in the vicinity of the bottom ring element(s). As a result of this crossing point, the orientation of the magnetic field through the feed lines is reversed so that it is opposed to the orientation of the high-frequency magnetic field in the central region of the coil. FIG. 3a shows an embodiment according to the invention of a single-turn saddle coil with four feed lines, in which the two pairs of feed lines each have a crossing point directly at the ring element. In this case too, by definition, the orientation of the B.sub.1 field has an angle of 0 to the x-axis. However, in contrast to the embodiments according to the prior art, the B.sub.2 field has an angle of 180 to the x-axis as shown in FIG. 3b for the plane B-B. This is particularly clear in FIG. 3c, in which the B.sub.1 and B.sub.2 fields are shown schematically by arrows and have an opposing direction.
(24) When the high-frequency B.sub.2 field, which is generated between the feed lines, has an angle with 90<<270 to the B.sub.1 field in the central region of the coil or resonator, the vectorial sum of the B.sub.1 and B.sub.2 fields can exhibit a zero crossing, at least however a significant reduction of the magnitude of the B field in a sub-region of the edge region instead of an increase. This is particularly efficient at an angle which is as close as possible to 180 so that an angle with 170<<190 is preferred. The resulting high-frequency magnetic field profile is shown schematically in FIG. 2b. At the same time, the bottom flank of the field profile is even steeper than the top flank, as the B.sub.2 field in the edge region of the profile of the B.sub.1 field has already been subtracted therefrom. In contrast with this, a further sub-region of the edge region is produced, in which the B field is increased. If, additionally, screens are fitted, for a coil according to the invention, it is possible to generate a nearly rectangular B field profile both in the top region and in the bottom region, as the return flow of the B field into the volume outside the sample can be limited.
(25) A particularly advantageous embodiment, in which the two feed lines are only spaced apart radially, is shown in FIG. 4a. In the developed diagram, the two feed lines lie above one another. The feed lines are both fitted so that they are both positioned at approximately +90 or 90 to the x-axis (see FIGS. 4b and 4c bottom). The conductors are connected such that the B.sub.2 field in the sample in the plane B-B has an angle of approximately 180 to the x-axis, as shown as field lines in FIG. 3b and schematically in FIG. 3c. This form of feed line is particularly advantageous for series-connected single and multi-turn coils. Furthermore, the magnetic coupling to a further coil, the high-frequency magnetic B.sub.3 field of which is oriented along the y-axis, is reduced. Basically, any radial position can be used for fitting the feed lines when they are spaced apart in this way both radially and angularly such that the resulting B.sub.2 field has an angle of ideally approximately 180 to the B.sub.1 field in the central region of the coil. Particularly preferred, however, are the embodiments with only angular (FIG. 3a) or radial (FIG. 4a) spacing, as in both cases it is particularly easy to fit a further coil configuration orthogonally to the first and to electrically decouple it.
(26) A variant of the invention, in which the coil has a screen 52 in the top and bottom region, is shown in FIG. 5. The crossing points in the bottom region ensure a partial cancellation of the field in the bottom edge region so that the spacing of the screens at the bottom is usually different from the top. It depends on the detailed embodiment of the crossed feed lines, and in particular also the position of the crossing point, as to whether the spacing of the screens at the bottom is larger or smaller than the top. The associated B field profile (i.e. the sum of the B.sub.1 and B.sub.2 fields) for ideal positioning of the HF screens is shown schematically in FIG. 2d on a log-lin scale.
(27) FIGS. 6a and 6b show two variants of an embodiment which is particularly advantageous, particularly when using self-resonant structures such as birdcage or Alderman-Grant resonators, for example, but also with all coils which are resonantly tuned by means of integrated capacitors. Here, the one pair of feed lines to the coil is crossed; however, a second pair is not crossed. On average, the sum of the generated fields gives approximately zero over the relevant bottom region of the sample, so that the B field profile is approximately equal to the B.sub.1 field profile and is therefore independent of the current which is fed via the feed lines. A coil designed in this way can have the same B field profile over a very large tuning range. This is particularly of interest for resonators which, for example, are designed to be tunable from .sup.19F to .sup.1H, and also for broadband probe heads which, in part, are designed to be tunable over several octaves. In addition, with this embodiment, the coupling of the feed lines to a further coil configuration which is fitted inside or encompasses the first coil configuration is greatly reduced, as the inductive coupling with the two pairs of feed lines has the opposite sign and is therefore self-cancelling as long as both pairs of feed lines are positioned symmetrically and the further coil configuration generates an substantially symmetrical high-frequency magnetic field.
(28) With the variant shown in FIG. 6a, the crossed pair of feed lines is fixed on one side of the coil (0), whereas the uncrossed pair of feed lines is fitted on the opposite side (i.e. at 180). The fields of the two feed lines cancel one another at least on the axis of the cylindrical coil/resonator, otherwise they substantially cancel one another at least on average over a section through the sample orthogonal to the main axis. In alternative embodiments, the feed lines can be fitted at any angles to the x-axis.
(29) In contrast to this, in the variant according to FIG. 6b, both the uncrossed and the crossed pair of feed lines are fitted on the same side, i.e. at approximately 0. As a result, the fields of the two feed lines are locally superimposed and cancel one another almost completely in the sample. As the two feed lines of a pair of feed lines have a large potential difference in operation, the feed lines must be run with sufficient spacing in order to avoid electrical flashovers. These necessary spacings limit the cancellation of the fields to a certain extent.
(30) FIG. 7 shows an embodiment in which the coil configuration comprises two coils/resonators which are coupled inductively to one another in that the two generated B.sub.1.sup.1 and B.sub.1.sup.2 fields lie in the same (xz-) plane. Here, the first coil is fully encompassed by the second coil. If each of the two coils is tuned to one resonant frequency, then, as a result of the coupling, this results in two modes of different frequency, with which, in operation in each case, current flows in the two coils. In doing so, in the lower mode, the current of the two coils is in the same direction (that is to say the fields add), and in the mode of higher resonant frequency, the currents in the two coils flow in the opposite direction (that is to say the fields subtract). In general, an inner coil/resonator is tuned to a higher frequency and an outer coil/resonator to a lower frequency, as in the reverse case (or when tuned to the same frequency) the common resonance of the pair of coils at the high frequency is very inefficient. If a single or multi-turn coil is combined with a resonator and the feed lines to the coil are uncrossed in accordance with the prior art then a steep B.sub.1+B.sub.2 profile and in particular an efficient suppression of the residual fields in the edge region cannot be achieved by means of screening for either of the two resonant frequencies.
(31) This problem is solved surprisingly easily in that at least one pair of feed lines of one of the coils of the pair of coils is crossed. As a result, it is possible to adjust the sum of the B.sub.1.sup.i+B.sub.2.sup.i fields for at least one of the two resonant frequencies so that they cancel one another in the edge region of the coil and generate a steep high-frequency magnetic field profile both at the top and the bottom. If, in addition, an HF screen is fitted, a high-frequency B field profile with extensive cancellation of the fields in the edge region can be achieved for at least one frequency. In FIG. 7, the feed lines 72 of the inner coil 74 are uncrossed, whereas the feed lines of the outer coil 18 encompassing it include a crossing point 73.
(32) In order to be able to achieve an adequate magnitude of the B.sub.2.sup.i fields through the feed lines of one or more outer coil(s) in the region of the sample, it may be necessary to make one or more openings or slots in the screen.
(33) Positioning of the Crossing Point:
(34) The crossing points in the pairs of feed lines can be designed such that the two conductors cross above the bottom ring elements, on the ring elements or below the ring elements. In a preferred embodiment, the crossing points lie within the coil window. As a result, a reversal of the field direction of the B.sub.2 field generated by the feed lines is achieved as efficiently as possible.
(35) In a particularly easy to realize embodiment, the crossing points are situated directly below the ring elements. As a result, additional conductor lengths are minimized and therefore the efficiency of the coils is only slightly reduced.
(36) Comparison with Known Coils:
(37) A type of coil, in which crossing points which reverse the field direction exist in the top and bottom region of the coil, is disclosed in U.S. Pat. No. 5,929,639 (see, for example, FIG. 3a therein). The difference compared with the present invention is that, in this case, the coil is no longer to generate a dipolar field in the test volume and the regions of reversed field are to couple as equally strongly with a second coil as the central region; i.e. the integral of the field in the edge regions is equal to that of the central region. As a result, the field profile is not steeper, but the field has two additional zero crossings. As in the prior art, the feed lines to these coils are fed outwards and downwards uncrossed, so that these coils effectively have three zero crossings but not four zero crossings of the B.sub.1 field in the test volume.
