MR coil for intraoral MR measurements

Abstract

The invention relates to an MR coil (1) for intraoral MR measurements at a target section in the dental field, wherein the target section comprises at least part of a dental arch (10), wherein the MR coil (1) has coil conductors (4) which form at least one resonant circuit. The MR coil (1) has a central part (2) and two legs (3a, 3b) projecting from the central part, in which coil conductors (4) of the at least one resonant circuit are arranged. The MR coil (1) can be placed on the dental arch (10) in such a way that the legs (3a, 3b) encompass the dental arch and such that one leg (3b) is arranged on the inside and one leg (3a) on the outside of the dental arch. The MR coil (1) has a shape adapted to the shape of the target section.

Description

[0001] The invention relates to a magnetic resonance coil (MR coil) for intraoral MR measurements in the dental field and to an MR coil arrangement.

[0002] Regardless of the grammatical gender of a particular term, it includes persons with male, female or other gender identities.

[0003] There is diagnostic interest in further developing magnetic resonance imaging (MRI) of teeth, jaws, and gums in order to, for example, better depict inflammatory processes in the gums and tooth roots. MR measurements in the dental field place high demands on the local receiving coils due to the very fine structure of the teeth and tooth roots. In order to obtain an MR image with sufficiently high resolution, the signal-to-noise ratio (SNR) must be very high, especially at the tooth roots. Secondly, it is advantageous to be able to image not just a single tooth, but a larger section of the jaw or an entire half of the jaw, since it is not always possible to determine in advance which tooth or teeth are affected by the disease process. Furthermore, the area surrounding the affected tooth is also required for adequate diagnostics, and for this purpose, it is also advantageous to be able to image the jaw as a whole.Thus, by imaging a larger jaw section or half of the jaw, the measurement time can be significantly reduced, thereby increasing patient comfort and patient throughput.

[0004] The article “Intraoral Approach for Imaging Teeth Using the Transverse B1 Field Components of an Occlusally Oriented Loop Coil”, Magnetic Resonance in Medicine, 72:160-165 (2014) describes a flat MR coil that a patient can hold between the chewing surfaces in the mouth by clenching their teeth.

[0005] However, this coil has the disadvantage that, due to its contact with the chewing surfaces of the teeth, it is relatively far from the tooth roots. This limits the coil's penetration depth. Since the coil axis is aligned in the z-direction of the MRI scanner, only the stray field oriented in the x- and y-directions contributes to the MR signal. Furthermore, the coil is connected to the MRI scanner via a cable, which can negatively impact handling and patient comfort.

[0006] DE 10 2014 220 116 A1 describes an RF coil (radiofrequency coil) for MR measurements of a target volume in the oral region of a subject under examination. However, due to its design, this RF coil can only examine a single tooth or a very few teeth. A preferred diameter of 2-3 cm is stated.

[0007] The invention therefore aims to provide an MR coil for MR measurements in the dental field that overcomes the limitations of known MR coils. In particular, it is an object of the invention to provide an MR coil for intraoral MR measurements that allows the largest possible area to be examined with an improved signal-to-noise ratio.

[0008] The invention solves this problem by an MR coil according to claim 1, as well as an MR coil arrangement according to claim 15.

[0009] The invention provides an MR coil for intraoral MR measurements at a target section in the dental field, wherein the target section comprises at least part of a dental arch. The MR coil has coil conductors that form at least one resonant circuit. Furthermore, the MR coil has a central part and two legs projecting from the central part, in which coil conductors of the at least one resonant circuit are arranged. The MR coil is designed to be placed on the dental arch such that the legs encompass the dental arch and, if applicable, the jaw in the target section such that one leg is arranged on the inside and one leg on the outside of the dental arch, wherein the MR coil has a shape adapted to the shape of the target section.

[0010] The coil according to the invention is characterized, firstly, in that it encompasses the dental arch with its two legs in such a way that the coil conductors arranged in the legs are located very close to the tooth roots. This allows an MR measurement with a very high signal-to-noise ratio to be performed on the tooth roots, gums, and optionally in the jaw. Furthermore, the coil is preferably long enough that a region of several teeth can be captured by a single measurement. To achieve this, the shape of the MR coil is adapted to the target section, which comprises at least part of a dental arch. The MR coil therefore preferably has the shape of a dental splint.This makes it possible to capture a large area of the jaw, for example a single jaw quadrant, the entire lower jaw or upper jaw, or even the entire jaw (lower jaw and upper jaw) with the MR coil simultaneously in a single MR measurement, for example to create an MR image of the entire jaw quadrant, half of the jaw, or the entire jaw. At the same time, the MR coil is comfortable to wear thanks to its shape adapted to the target section. During an MR measurement, it can simply be inserted into the mouth and placed over the teeth. Because its shape is adapted to the dental arch, the patient does not have to hold it in place by biting. On the other hand, the patient can close their mouth, especially with an inductively coupled MR coil, and the MR coil is no more uncomfortable than a dental splint.

[0011] The MR coil is adapted to the shape of the target section, wherein the target section comprises at least part of a dental arch. Thus, the MR coil preferably has an arch shape adapted to the dental arch. However, the adapted shape can also be adapted to a straight section of the dental arch, e.g., in the region of the molars. For example, the limbs on the inside and outside of the dental arch can have different shapes. The MR coil preferably has a standardized shape, i.e., the shape is generally adapted to the shape of a standard dental arch, but not to a patient's individual dentition. However, it is possible to provide a set of several MR coils according to the invention in different sizes, for example to be able to cover different patient dentition sizes, e.g., one child size and 2-3 different sizes for adult patients.

[0012] The MR coil is particularly suitable for intraoral MR measurements on humans, preferably for diagnostic MR imaging in the dental field. The target section in the dental field comprises at least part of a dental arch. The terms "dental area" and "dental arch" are to be understood broadly here. "Dental arches" are understood in particular to mean the two horseshoe-shaped rows of teeth in the human being, which are located in the upper and lower jaws, respectively. In a healthy dentition, a dental arch consists of 16 teeth. The dental arches are supported by the jaw. Thus, the target section can also comprise the jaw section belonging to the part of the dental arch, the tooth roots, and / or the gums located in this section. If necessary, the target section can also comprise only a jaw section and, if applicable, the associated gums, if no teeth are present in this area. The target section can be part of a dental arch, e.g.¼ or 1 / 3 of a dental arch, a quadrant (i.e. half of a dental arch), an entire dental arch or both dental arches.

[0013] MR measurements are primarily imaging measurements. For example, the MR coil is suitable for acquiring high-resolution two-dimensional (2D) or three-dimensional (3D) MR images of the target segment. For example, imaging sequences with ultrashort echo time (UTE) can be used for this purpose.

[0014] The MR coil has one or more coil conductors, e.g., made of copper conductors. These, together with one or more capacitive elements (e.g., capacitors), form a resonant circuit. The resonant circuit is preferably resonant at the MR frequency. Depending on the field strength of the main magnetic field of the MRI device, the resonant frequency of the resonant circuit can be between 5 and 300 MHz. Typical frequencies are 23 MHz (0.5T), 63 MHz (1.5T), 123 MHz (3T), and 300 MHz (7T). Typically, an MR coil is resonant at a specific MR frequency. However, it is also possible to adapt the MR coil to different main magnetic field strengths using an adjustable capacitor, whose capacitance can be adjusted, for example, between 10 and 100 pF.

[0015] The MR coil has a central portion, which is preferably designed to be placed on the chewing surfaces of the teeth in the target section of the dental arch. The central portion is thus preferably flat and elongated along the row of teeth. It is preferably between 5 and 20 mm, more preferably 8-12 mm wide. The central portion preferably has the shape of a horseshoe or part of a horseshoe. The arched shape of the horseshoe adapts the MR coil to the shape of the target section. The central portion is preferably between 3 and 20 cm, more preferably between 5 and 15 cm long, for example, to cover a jaw quadrant or the entire lower or upper dental arch.

[0016] The two legs are connected to the two long sides of the central section and preferably protrude at an angle of 70°-130° from the plane of the central section, which runs approximately parallel to the occlusal plane during use. The two legs thus encompass the teeth and, if applicable, the associated jaw, with one leg positioned on the inside and one on the outside of the dental arch. The inside here refers to the side facing the oral cavity, and the outside to the side facing the cheek. The coil conductors of the resonant circuit arranged in the legs are therefore positioned very close to the tooth roots, enabling a very high signal-to-noise ratio for MR measurements, particularly imaging MR measurements, in this area. This allows the SNR of the entire tooth structure, including the tooth roots, gums, and jawbone, to be increased many times over.

[0017] The MR coil is preferably designed to be inductively coupled to an MR receive coil. This allows the reception field of the MR receive coil to be amplified, thereby increasing the SNR of the MR measurement. This has the advantage that the MR coil does not need to be connected to the MRI device via cables. The MR coil thus locally increases the SNR, making the region of interest in the target section more visible on the MR images. The MR receive coil is, for example, the body coil integrated into the MRI device. However, it is preferably a local coil, such as a head coil.

[0018] According to one embodiment, the target section comprises at least 4 teeth of a dental arch, preferably at least 5 teeth, more preferably at least the majority of one jaw quadrant, more preferably at least the majority of two jaw quadrants of a lower or upper jaw. In contrast to the RF coil of DE 10 2014 220 116 A1, the MR coil thus allows an MR measurement on a larger section of the dental arch or jaw. In particular, the MR coil according to this embodiment allows the imaging of an entire jaw quadrant, the lower jaw or upper jaw, or even the imaging of the entire jaw. In this case, the MR coil can be configured such that limbs that protrude upwards and downwards are connected to the longitudinal sides of the central part on the outer and inner sides. In this embodiment, the MR coil is designed like a dental splint for the lower and upper jaws together.

[0019] According to one embodiment, the coil conductors comprise at least one butterfly coil, each comprising at least two wings, which are arranged in the two limbs of the MR coil such that at least two wings are opposite one another on the inside and outside of the dental arch and are coupled to one another—in particular crosswise—in the region of the central part. A butterfly coil, in this case, is understood to mean an MR coil having two wings, which in this case are arranged in the two limbs of the MR coil. In a butterfly coil, the current paths preferably cross between the two wings. In this case, the coil conductors preferably cross in the region of the central part. In particular, the butterfly coil can have one or more turns in each of the two wings, which thus lie opposite one another on the inside and outside of the dental arch and are crosswise connected to one another in the region of the central part.The proposed design is advantageous because the main field of the resonant circuit is then parallel to the occlusal plane, thus aligning a normally lying patient in a typical MRI scanner in the xy plane. In a typical MRI scanner, the direction of the main magnetic field is horizontal, aligned longitudinally along the patient tunnel. It is therefore advantageous if MR coils for detecting the MR signal can capture a B1 field perpendicular to the Z direction particularly well. This is the case with the MR coil according to the invention in this embodiment. Advantageously, the windings running in the two limbs are coupled to one another in the middle section, for example, conductively connected, preferably crosswise. This ensures that the magnetic fields generated by the resonant circuit in the two wings / limbs mutually reinforce each other.

[0020] According to one embodiment, the target section of the MR coil comprises at least two jaw quadrants of a lower or upper jaw, with the coil conductors comprising a butterfly coil per jaw quadrant. This has the advantage that, on the one hand, an entire dental arch or an entire half of the jaw (lower jaw or upper jaw) lies in the sensitive area of the MR coil. The structure of two butterfly coils can advantageously amplify the magnetic field of the two butterfly coils, since the two butterfly coils are arranged on the right and left sides of the jaw during use. By appropriately connecting the two butterfly coils, they can operate in a constructive mode, generating an in-phase field in both jaw quadrants. In this mode, the B1 field of the MR coil is constructively amplified in both jaw quadrants. This enables a particularly high SNR while simultaneously maintaining a large sensitive area of the MR coil.Such an MR coil is also called a “double quadrant coil” here. In one embodiment, two double quadrant coils, one for the upper jaw and one for the lower jaw, can be combined to capture the entire jaw simultaneously.

[0021] According to one embodiment, the two butterfly coils are coupled to each other, in particular via a conductor section, in such a way that the B1 field generated by the two butterfly coils is structurally superimposed. This has the aforementioned advantage of a particularly high SNR throughout the entire lower or upper jaw. The coupling can be achieved, for example, by cross-connecting the two butterfly coils at a position that, during use, is in front of the front teeth. This creates a single coil with a correctly oriented B1 field.

[0022] According to an alternative embodiment, the two butterfly coils are independent, i.e., not connected via a conductor, but inductively coupled. This results in a split into two modes, with only one mode generating an in-phase field in both jaw quadrants. Preferably, the resonance is stimulated, where the B1 field in the jaw is constructively amplified.

[0023] According to one embodiment, the coil conductors comprise flat conductor tracks, in particular copper tracks, which are applied to an insulating substrate. This embodiment is advantageous because it allows the MR coil to be manufactured in any desired shape. The substrate can be made of any insulating material, in particular a non-conductive or electrically insulating material, for example, synthetic resin, e.g., epoxy resin, plastic, rubber, or Plexiglas. For example, the substrate can have the shape of a dental splint. The conductor tracks are then applied to this. This makes it possible to manufacture the MR coil in an advantageous shape adapted to the shape of the teeth. The substrate can be flexible to a certain extent, for example, by up to 10 degrees, in order to adapt to different denture shapes. The conductor tracks can, for example, have a width of 1 to 5 mm and a thickness of 5-100 µm.Preferably, all coil conductors are designed as flat conductor tracks.

[0024] The MR coil can advantageously be covered or coated with a tissue-friendly material (dental material), such as plastic, synthetic resin, or rubber. This allows the MR coil to be worn comfortably in the mouth like a dental splint. The material is advantageously suitable for sterilization, so that the MR coil can be used by different patients—after appropriate sterilization.

[0025] According to one embodiment, a capacitance of the at least one resonant circuit is formed in that two coil conductors of the at least one resonant circuit run essentially parallel to one another at least in sections, with a dielectric being arranged between the two coil conductors. The dielectric can, for example, be the insulating substrate described above. This embodiment has the advantage that no or fewer capacitors are required, which leads to cost savings. The MR coil preferably contains no additional components in the form of capacitors. This makes the MR coil more reliable because there are fewer solder joints or components that can be damaged. Two coil conductors preferably run essentially parallel to one another at least in sections by being applied to opposite sides of an insulating substrate.The uniform thickness of the substrate ensures an essentially constant distance between the conductor tracks.

[0026] “Substantially” can be understood as a deviation of up to ±15%, preferably up to ±10%.

[0027] According to one embodiment, the at least one resonant circuit is designed as a transmission line resonator (TLR). In a TLR configuration, the coil conductor is arranged such that it simultaneously represents the inductances and the capacitance to form a resonant circuit. This has the advantage that no discrete capacitors are required, which take up additional space in the mouth and may require complex plastic encapsulation. Furthermore, production is cheaper because no additional components are required. Finally, the configuration is more robust because fewer defect-prone solder joints are required. The resonant frequency is determined by the geometry, in particular the size of the TLR, the conductor width, the conductor length, the thickness of the dielectric between the conductors, etc. A TLR configuration is easier to implement for higher frequencies (≥123 MHz), but is also possible at 23 MHz (0.5 T).At low frequencies (especially ≤ 63.5 MHz), a multi-turn design may be preferable to achieve the resonant frequency. A multi-turn TLR is also called a multi-turn transmission-line resonator (MTLR).

[0028] According to an advantageous embodiment, the resonant circuit has at least one abrupt change in impedance, in particular an interruption. Such an abrupt change in impedance in a transmission line or a coil conductor causes a reflection of the transmitted signal, and two such reflectors create a standing wave between them. The resonant frequency is determined, among other things, by the distance between the two abrupt impedance changes and the effective dielectric constant of the transmission line. In this embodiment, the resonant circuit can also be designed as a TLR.

[0029] According to one embodiment, the at least one resonant circuit comprises at least two interrupted conductor rings that run substantially parallel to one another, with a dielectric arranged between the two conductor rings. This is an advantageous embodiment of a TLR. For example, coil conductors can be arranged on opposite sides of a flat dielectric, for example as conductor tracks on opposite sides of a flat substrate. This embodiment combines the above-described advantages of a resonant circuit with no or fewer additional components in the form of capacitors with the advantages of using coil conductors that are applied to a substrate, for example as flat conductor tracks, in particular made of copper. At the interruptions, the current is naturally equal to 0, so that a standing wave with a current node and a voltage antinode at the interruptions is created in the resonant circuit.The interruptions in the interrupted conductor rings are, in particular, sections where the coil conductors are not continuous, but have a gap. The gap can be, for example, 0.1-2 mm. Advantageously, the at least two interrupted conductor rings each have one or more interruptions, e.g., 1-5 interruptions. The interruptions can be directly opposite each other on the two sides of the dielectric, but they can also be offset from one another along the conductor ring.

[0030] According to one embodiment, the at least two interrupted conductor rings are each designed as a butterfly coil, wherein the conductor rings each have at least two turns, with at least one turn running in one leg of the MR coil. In this case, the TLR thus forms a butterfly coil. This is advantageously arranged in the legs of the MR coil, as described above, in such a way that an entire quadrant of the jaw can be viewed through one butterfly coil. A TLR butterfly coil therefore has two conductor rings, each of which has at least one turn in one leg and at least one turn in the other leg of the MR coil. In each leg / wing, at least two conductor rings thus run essentially parallel to one another, in particular on both sides of a dielectric substrate or carrier material.

[0031] According to one embodiment, in this embodiment the interruptions in the conductor rings are arranged in the region of the central part. In particular, this is in the region of the crossover. It has been shown that the formation of the voltage antinode at this point results in field amplification or the B1 field being particularly advantageous for the purpose of MR formation on teeth and jaws. However, depending on the resonance frequency, other embodiments are also possible in which the interruptions are arranged at other points on the conductor rings. There can also be 2 or 3 or more (e.g. up to 5) interruptions per conductor ring, which are distributed, for example, around the conductor ring.

[0032] According to one embodiment, the interrupted conductor rings have more than two turns, with the conductor rings in each leg of the MR coil in particular having two or more turns. The design with more than one turn increases the capacitance and inductance of the resonant circuit, thereby lowering the resonant frequency. Thus, a design with more than one turn allows the use of the MR coil according to the invention at lower magnetic field strengths, for example, at 23 MHz (0.5 T) and below.

[0033] According to one embodiment, the resonant frequency of the MR coil can be changed. Preferably, it is possible to decouple the MR coil from the respective transmit coil of the MRI device while the radio-frequency pulses (RF pulses) required for the MR measurement are transmitted. This is advantageous to avoid heating of the MR coil and transmission field distortions. This can advantageously be achieved by placing two antiparallel diodes in parallel with a capacitor of the resonant circuit. In the case of a TLR, such antiparallel diodes can also be connected across the interruption. The diodes act like a passive switch, which bridges the capacitor when a high induced voltage is present, as is to be expected during the RF transmit pulse. As a result, the resonant circuit is no longer resonant at the MR frequency at this moment.At low voltage, the capacitor is no longer bridged by the diodes, and the MR coil then has the set resonant frequency. As an alternative to the diodes in parallel with the capacitor, an additional resonant circuit could also be used as a "blocking circuit," similar to the one used for detuning MR receiver coils. However, this would require more space.

[0034] According to one embodiment, the MR coil comprises a covering surrounding the coil conductors, which is particularly flexible to adapt to different jaw geometries. The covering is preferably made of a material suitable and approved for use in the oral area (dental material), e.g., a suitable plastic, rubber, or caoutchouc. It allows a patient to place the MR coil in the mouth and, if necessary, bite on it to hold the MR coil in position. Furthermore, the covering advantageously leads to a comfortable wearing experience for the MR coil.

[0035] The invention is also directed to an MR coil arrangement comprising an MR coil according to one of the preceding claims and a pickup coil inductively coupled thereto. The pickup coil can be a "conventional" MR receive coil or MR transmit-receive coil of an MRI device, for example, a body coil, or advantageously a head coil or another local coil of an MRI device. The local coil can be, for example, a surface coil or a phased array coil. However, the pickup coil can also be a dedicated coil specifically manufactured for the purpose of receiving the signal from the intraoral coil according to the invention and, if necessary, transmitting it to the MRI device. As a result, the MR coil itself advantageously requires no electronics for the reception chain. Rather, the MR coil can contain only the conductor tracks and, if necessary, discrete passive components, as well as, as a rule, a carrier material / substrate and / or a covering.It also provides great patient comfort as the mouth can be closed again for the measurement.

[0036] Finally, the invention is also directed to an MRI device which has an MR coil or MR coil arrangement according to the invention.

[0037] Furthermore, the invention is directed to a use of an MR coil as described here for intraoral MR measurements at a target section in the dental field.

[0038] The invention is described in more detail below using exemplary embodiments with reference to the accompanying drawings. The drawings show: Fig. 1 is a perspective view of an MR coil according to an embodiment of the invention; Fig. 2 a perspective view of the coil conductors of an MR coil according to an embodiment of the invention, mounted on a dental arch; Fig. 3 a perspective top view of the coil conductors of an MR coil of the embodiment of the Fig. 2; Fig. 4 is a diagram showing the current path of a double quadrant coil according to a first embodiment; Fig. 5 is a diagram showing the current path of a double quadrant coil according to a second embodiment; Fig. 6 through a jaw with double quadrant coil and B1 field lines drawn; Fig. 7 a schematic representation of a simple transmission line resonator (TLR); Fig. 8 a perspective view of the coil conductors of a dental coil designed as a TLR butterfly coil; Fig. 9 a perspective view of the dental coil of the Fig. 8, placed on a row of numbers; Fig. 10 is a perspective view of the coil conductors of a dental coil designed as a TLR; Fig. 11 an enlarged section of the coil conductors of the Fig. 10 at the intersection point; Fig. 12. A perspective view of the coil conductors of a dental coil designed as a TLR butterfly coil with a common center bar.

[0039] In the figures, identical parts are designated by the same reference numerals.

[0040] Fig. Figure 1 shows the basic shape of an MR coil 1 according to an embodiment of the invention. In this embodiment, the target section essentially comprises an entire dental arch, optionally including the gums and the associated lower or upper jaw. The MR coil 1 has the basic shape of a dental splint. In particular, it has a flat, horseshoe-shaped central portion 2. From the central portion, a leg protrudes from the inside and outside. The leg on the outside is designated 3a, the leg on the inside 3b. The MR coil can be placed on the dental arch such that the two legs 3a, 3b encompass the dental arch and come to rest near the inner surfaces of the teeth or jaw (leg 3b) and the outer surfaces of the teeth or jaw (leg 3a). Fig. 1 the coil conductors are not shown, they run inside the MR coil, including the legs.

[0041] Out of Fig. Figure 2 shows the course of the coil conductors 4 according to an embodiment of the invention, namely the so-called "double quadrant coil." The MR coil 1 is attached to a lower jaw with a dental arch 10 and underlying gums 12 and jaw, as would be done for an MR measurement. The coil conductors 4 of the MR coil in this case comprise two butterfly coils 5a and 5b. Each of these butterfly coils consists of a turn 6a on the inside of the dental arch and a turn 6b on the outside of the dental arch. These turns run in the limbs 3a, 3b of the MR coil 1. On the chewing surface, i.e., in the region of the central part 2, the two turns 6a and 6b are coupled to each other, as shown in Fig. 3 is shown in more detail.

[0042] Advantageously, the two windings 6a and 6b of a butterfly coil 5a or 5b are connected crosswise at the crossing point 7. In the Fig. In the example shown in Figure 3, this is done by means of a typically soldered conductive connection and a capacitor 15. The capacitor 15 provides the necessary capacitance to form a resonant circuit from the coil conductors 4. The size of the capacitor 15 is selected such that the resonant circuit is resonant at the MR frequency. In this embodiment, two counter-directed diodes 14a and 14b are connected in parallel with the capacitor 15. These diodes close at high voltages, such as those that occur during the transmission of the RF pulses for the MR measurement. This "detunes" the resonant circuit so that it is no longer resonant at the MR frequency. This has the advantage that the MR coil is then decoupled from the transmit coil and the transmit field is neither amplified nor distorted. Both butterfly coils 5a, 5b are connected in the same way to capacitor 15 and diodes 14a, 14b at crossing point 7.

[0043] Fig. Figure 4 shows a first possibility of how the two butterfly coils 5a, 5b can be coupled. In the example shown, the two butterfly coils 5a, 5b are conductively connected crosswise in front of the teeth, creating a single resonant circuit with a correctly oriented B1 field. The current flow is determined from the B field lines of the Fig. 6. In particular, the Fig. 6, that the B1 field of the two butterfly coils is amplified in the area of jaw 20 when correctly connected. In this way, a significant increase in the SNR in the area of interest, the teeth and tooth roots, is possible.

[0044] Fig. Figure 5 shows a second way in which the two butterfly coils 5a, 5b can be coupled. In this case, they only couple inductively. In other words, two independent butterfly coils 5a, 5b are attached to the MR coil, with crossing points near the occlusal surfaces of the teeth, each connected to a quadrant of the jaw. Thus, the two coils inductively couple not only to a receiving coil, but also to each other. This leads to a splitting into two modes, with only one mode generating an in-phase field in both jaw quadrants. The resonance mode is selected, which constructively amplifies the B1 field in the jaw, as shown in Fig. 6 shown.

[0045] Fig. Figure 7 schematically shows a simple transmission line resonator (TLR) 22. This consists of two superimposed conductor rings 24, 25, between which a dielectric, such as an insulating substrate, is arranged. The opposing conductors 24, 25 act as a capacitance, thus eliminating the need for a discrete capacitor to form a resonant circuit. At 30, the two conductor rings 24, 25 each have an interruption.

[0046] Fig. Figure 8 schematically shows a butterfly TLR 23, which is suitable for being placed on a jaw quadrant and thus forming part of a coil conductor 4 of an MR coil 1 according to an embodiment of the invention. For better visibility of the wiring, the insulating material between the two layers of the TLR is hidden and only the coil conductors 4 are shown. The TLR 23 has the shape of a butterfly coil with two wings 6a and 6b, each formed by a single turn and which are arranged opposite each other on the inside and outside by a row of numbers 10. The butterfly coil comprises two interrupted conductor rings 24, 25, which run essentially parallel to each other, for example, on two sides of an insulating substrate or carrier material. Discrete capacitors are therefore no longer necessary. Interruptions 30 of the conductor rings 23, 24 are provided at the intersection point.By varying the width of the conductor tracks 4, the spacing between the conductor rings 24, 25, and the number of turns, it is possible to adjust the resonant frequency of the butterfly TLR 23 to achieve a desired MR frequency. The conductor tracks 4 are always shown straight here, but this is due to the schematic representation. Preferably, the conductor tracks, like the MR coil, are adapted to the shape of the sensitive area or target section, for example, slightly curved with the dental arch.

[0047] Fig. 9 shows the Butterfly-TLR 23 of the Fig. 8 placed on a jaw 12 with dental arch 10.

[0048] In an alternative embodiment (not shown), it is also possible to provide more than two interruptions 30. This acts like a series connection of several capacitors. This reduces the effective capacitance and thus increases the resonant frequency.

[0049] Fig. 10 shows a further embodiment of a butterfly TLR 26, which is constructed similarly to the butterfly TLR 23 of Fig. 8, but with two turns provided in each leg of the MR coil 1. The turns are concentric so that the conductor tracks can be arranged on a single substrate. Each of the two turns is in turn doubled with two parallel conductor rings 24, 25. In the area of the middle section, the turns of the butterfly coil 26 are connected to each other on the two sides of the toothed arch. This is also where the interruption 30 and 31 are located. This is shown in the Fig. 11 shown again enlarged.

[0050] Fig. Figure 12 shows yet another embodiment of a butterfly TLR 28. In this embodiment, the butterfly TLR has a common central web 29. In this embodiment, three interruptions 30 are necessary, one on the central web 29 and one each on the two wings 6a, 6b. Even more interruptions 30 are conceivable. The interruptions 30 on the central web 29 on the one hand and the two wings on the other hand are advantageously located in opposite conductor rings or layers of the TLR. In the illustration of the Fig. 12, the interruptions 30 are in the wings on the lower conductor ring, while the interruption 30 is arranged on the central web 29 on the upper conductor ring.

[0051] A prototype of an intraoral MR coil according to an embodiment of the Fig. 2 and Fig.3 was produced, and it was shown that it could be used to perform MR imaging with very high SNR on the dental arch. The inventive intraoral dental coil significantly improved the SNR compared to extraoral coils. It can cover an entire jaw quadrant, an entire jaw half, or even the entire jaw. The inventive MR coil has a great penetration depth, reaching into the jawbone and across the entire tooth roots. No cables are required, which makes the coils very easy to handle and significantly more comfortable for patients. Because the MR coil is realized entirely without active components, it is also cost-effective to implement. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2014 220 116 A1 [0006, 0018] Cited non-patent literature

[0000] Intraoral Approach for Imaging Teeth Using the Transverse B1 Field Components of an Occlusally Oriented Loop Coil", Magnetic Resonance in Medicine, 72:160-165 (2014

[0004]

Claims

[1] MR coil (1) for intraoral MR measurements on a target section in the dental field, wherein the target section comprises at least part of a dental arch (10), wherein the MR coil (1) has coil conductors (4) which form at least one resonant circuit, wherein the MR coil (1) has a central part (2) and two legs (3a, 3b) projecting from the central part, in which coil conductors (4) of the at least one resonant circuit are arranged, wherein the MR coil (1) is designed to be placed on the dental arch (10) in such a way that the legs (3a, 3b) encompass the dental arch and optionally the jaw in the target section in such a way that one leg (3b) is arranged on the inside and one leg (3a) on the outside of the dental arch, wherein the MR coil (1) has a shape adapted to the shape of the target section. [2] MR coil (1) according to claim 1, wherein the target section comprises at least 4 teeth of a dental arch, preferably at least the majority of one jaw quadrant, more preferably at least the majority of two jaw quadrants of a lower or upper jaw (20). [3] MR coil (1) according to claim 1 or 2, wherein the coil conductors (4) comprise at least one butterfly coil (5a, 5b) each comprising at least two wings (6a, 6b) which are arranged in the two legs (3a, 3b) of the MR coil (1) in such a way that at least two wings (6a, 6b) are opposite one another on the inside and outside of the dental arch and are coupled to one another in particular crosswise in the region (7) of the central part (2). [4] MR coil (1) according to claim 3, wherein the target section of the MR coil (1) comprises at least two jaw quadrants of a lower or upper jaw, and wherein the coil conductors (4) comprise one butterfly coil (5a, 5b) per jaw quadrant. [5] MR coil (1) according to claim 4, wherein the two butterfly coils (5a, 5b) are coupled to one another, in particular via a conductor section, in such a way that the B1 field generated by the two butterfly coils (5a, 5b) is constructively superimposed. [6] MR coil (1) according to one of the preceding claims, wherein the coil conductors (4) comprise flat conductor tracks, in particular copper tracks, which are applied to an insulating substrate. [7] MR coil (1) according to one of the preceding claims, wherein a capacitance of the at least one resonant circuit is formed in that two coil conductors (24, 25) of the at least one resonant circuit run at least in sections substantially parallel to one another, wherein a dielectric is arranged between the two coil conductors (24, 25). [8] MR coil (1) according to one of the preceding claims, wherein the at least one resonant circuit is designed as a transmission line resonator (23, 26, 28). [9] MR coil (1) according to claim 8, wherein the at least one resonant circuit comprises at least two interrupted conductor rings (24, 25) which run substantially parallel to one another, wherein a dielectric is arranged between the two conductor rings. [10] MR coil (1) according to claim 9, wherein the at least two interrupted conductor rings (24, 25) are each designed as butterfly coils (23, 26, 28), wherein the conductor rings each have at least two turns, of which at least one turn runs in a leg (3a, 3b) of the MR coil (1). [11] MR coil (1) according to claim 9 or 10, wherein the at least two interrupted conductor rings (24, 25) each have one or more interruptions (30). [12] MR coil (1) according to one of claims 9 to 11, wherein the conductor rings in each leg (3a, 3b) of the MR coil (1) have two or more turns. [13] MR coil (1) according to one of the preceding claims, which is designed to inductively couple to a pickup coil, in particular another MR receiver coil or MR transmit-receive coil. [14] MR coil (1) according to one of the preceding claims, which comprises a covering surrounding the coil conductors (4), which is in particular flexible in order to adapt to different jaw geometries. [15] MR coil arrangement comprising an MR coil (1) according to one of the preceding claims and a pickup coil inductively coupled thereto.

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