FIELD OF THE INVENTION

The invention relates to a magnetic resonance coil arrangement, as well as to a magnetic resonance imaging system. BACKGROUND OF THE INVENTION

Image-forming MR methods, which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, they do not require ionizing radiation, and they are usually not invasive.

According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field Bo whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based.

The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z- axis, so that the magnetization performs a precessional motion about the z-axis.

Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.

In order to realize spatial resolution in the body, magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body.

The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of samples of k-space data is converted to an MR image, e.g. by means of Fourier transformation.

Thus it can be summarized, that coils play an important role in magnetic resonance imaging.

An important parameter in coil design is the maximum root-mean-square

(rms) current the coil can handle. Increasing the maximum rms current permits to provide more current through the coil in less time, i.e. it permits the provision of more power.

Consequently, scan time can be reduced.

One aspect which limits the maximum rms current especially in coil systems is the heat dissipated from the coils during operation.

The axes of for example gradient coils in MR systems need to be electrically insulated from each other by means of an insulation layer. Typically in this layer, glass cloth is used as a spacer to put the axes into position and to reduce the electrical field in this area by creating sufficient distance between the axes. Typically, this structure will be impregnated with epoxy resin to fix all parts together. The glass cloth which has an open structure is meant to suck up the resin between the axes, bonding the structure together.

For high voltage gradient coils sufficient distance between coil axes to reduce the electrical field between the coil axes becomes more important to prevent partial discharges, noticeable as spikes in an MR system. However, by for example increasing the distance by increasing the thickness of the insulation, this also increases the thermal barrier for the heat flow towards a cooling infrastructure of for example a gradient coil, thereby reducing the maximum rms current of the gradient coil.

Several options exist to increase the maximum rms current. One option is to simply increase the operational temperature of the coil. However, this requires epoxy resins which are resistant to higher temperatures and the dissipated power will heat up the surrounding MR system, of which the temperature should also stay within certain limits. A second option to increase the maximum rms current is to reduce the thermal barrier to the cooling infrastructure. This can be achieved by using materials with higher thermal conductivity for these layers. To increase the heat conductivity of layers between gradient coil axes, often filled epoxy resin is used, wherein the filler is typically a ceramic powder mixed into the epoxy resin that improves the mechanical and thermal properties. However, in case openings between coil axes are narrow, epoxies with high viscosity, due to the mixed ceramic powder, are not able to penetrate uniformly the space between the coil axes which may lead to air bubbles in the epoxy filled areas. These bubbles in turn may lead for example to electric field concentrations which favor the appearance of partial discharges when operating the coils.

US 7,554,326 B2 does disclose a magnetic resonance imaging apparatus which includes a gradient magnetic coil having a plurality of line members found in a predetermined winding pattern, wherein a first resin material fills gaps between the plurality of coils, and the second resin material, which has higher thermal conductivity than that of the first resin material, fills gaps formed between the line members of the given coil.

SUMMARY OF THE INVENTION

From the foregoing it is readily appreciated that there is a need for an improved magnetic resonance coil arrangement with increased thermal performance, as well as an improved magnetic resonance imaging system.

In accordance with the invention, a magnetic resonance coil arrangement is provided which comprises electrical conductors for generating a magnetic field, wherein the electrical conductors are electrically isolated from each other by an electrical insulator, wherein the insulator comprises a nitride or aluminum oxide as electrical insulating material.

Embodiments of the invention have the advantage that a magnetic resonance coil arrangement can be provided with high heat conductivity such that a thermal barrier to a respective cooling infrastructure is reduced. Thus, overheating is avoided and the maximum rms current of the coil arrangement is increased.

In accordance with an embodiment of the invention, the electrical conductors are wound in a 'fingerprint' like coil structure (pattern), wherein the insulator is located between the individual coil windings of the fingerprint like coil structure and/or between the different coil axes.

Gradient coil construction often uses distributed 'fingerprint' like coils for x and y coil axes. In the manufacturing process these coils can be cut or etched from a copper sheet. In alternative cases the 'fingerprint' coil patterns can be made of round or rectangular conductors, hollow conductors, or wire(s). Usually the spiral like pattern is made in the flat to maintain the proper coil dimensions. In state of the art manufacturing processes, an insulating backing material of for example FR-4 prepreg laminate is bonded to the pattern to keep positions of the windings well defined in the further process of building the coil and providing a part of the insulator. The copper and backing material are then rolled to the proper form for assembly onto a gradient tube, where they can be stacked on top of other coil structures.

By using the thermal transfer properties of nitrides or aluminum oxide, the thermal transport between the various coil layers of the distributed fingerprint like coil structure is improved.

In accordance with an embodiment of the invention, the nitride or aluminum oxide is comprised as a filler material in the prepreg laminate that is part of the insulator.

Thus, a thermally conductive prepreg with the nitride or aluminum oxide being comprised as a filler material is used in the backing material of the coil patterns to enhance the thermal transport properties. In accordance with a further embodiment of the invention, the coil arrangement comprises a set of individual coils stacked on top of each other, wherein the individual coils are separated from each other by the insulator. This is especially relevant in case of gradient coils for x, y and z-directions stacked on top of each other.

In accordance with a further embodiment of the invention, the individual coils are separated from each other by an impregnated spacer. This spacer can be combined in a stack with one or more insulating layers to make the insulation, or one can simply use the spacer to create the insulation. For the spacer which is part of the insulation it is

advantageous, like for the other insulating layers in the stack, to use the nitride or aluminum oxide material. In case of the spacer, typically a cloth, the nitride material in the spacer material is giving the composite material, when impregnated with the epoxy resin, high thermal conductivity. Consequently, the problem is solved that a filler material mixed into the epoxy resin used for impregnation may clog the typically small space between individual coils stacked on top of each other. Thus, established well-working epoxy resins may still be used as an impregnation material while nevertheless the thermal barrier towards the cooling infrastructure is reduced. The thermally high conductivity material is thus not applied as a filler in the epoxy, but as a replacement of the typically used glass cloth by a better thermally conducting material.

In accordance with an embodiment of the invention, the cloth is impregnated with a resin, wherein the dielectric constant of the cloth matches the dielectric constant of the resin and/or wherein the nitride is boron nitride, silicon nitride or aluminum nitride. In case the dielectric constant of the cloth matches the dielectric constant of the resin, this permits to use it also in high voltage coil arrangements where besides the requirement of a high thermal conductivity the further requirement of similar dielectric constants to avoid electric field concentrations and thus partial discharges can be met.

Generally, it has to be noted that in low voltage coils such a dielectric constant matching may not be necessarily required, such that other types of nitrides may be employed to carry out the invention.

In accordance with a further embodiment of the invention, the coils are adjoiningly stacked on top of each other separated from each other by one or more impregnated spacers, possibly combined with an insulating layer, wherein the dielectric constant of said insulating layer matches the dielectric constant of the resin. Again, this also permits the usage of the coil arrangement for high voltage coils.

In accordance with a further embodiment of the invention, the nitride is boron nitride, silicon nitride or aluminum nitride. Especially boron nitride has several advantages: boron nitride is an excellent electrical insulator and has a high thermal conductivity, wherein additionally the dielectric constant of boron nitride matches the dielectric constant of typically used epoxy resins used as impregnation material, as well as the dielectric constant of the typically applied FR-4 material in the above mentioned prepreg or insulating layer. Thus, the usage of boron nitride is not just an arbitrary choice of a better heat conducting material for the cloth and the other insulating layers, but it is selected with care such that both requirements are met, namely high thermal conductivity and a matching of the dielectric constants of the insulator, the resin and the further insulating layers.

Even though the dielectric constant of silicon nitride and aluminum nitride do not exactly match the dielectric constants of the typically used epoxy resins and FR-4, they may still be an excellent choice for application in low voltage coils. The same holds with respect to aluminum oxide.

In accordance with a further embodiment of the invention, the coil

arrangement is an imaging gradient coil arrangement, wherein the coils are gradient coils. However, it has to be noted that the invention is not limited to just gradient coils but can be applied to any kind of coils used in MR systems, such as shimming coils or RF coils used for spin system excitation, which require electrical insulation for example between individual coil windings or between individual layers of coils stacked on top of each other.

In another aspect, the invention relates to a magnetic resonance imaging system comprising a coil arrangement as described above.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, preferred embodiments of the invention are described in greater detail by way of example only. Thus, the following drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

Fig. 1 shows a schematic of an MR system for implementation of the coil arrangement according to the invention,

Fig. 2 illustrates a schematic of a coil arrangement,

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Fig. 1, a schematic of an MR imaging system 1 is shown.

The system comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporarily constant main magnetic field BO is created along a z-axis through an examination volume.

A magnetic resonance generation manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially or otherwise encode the magnetic resonance, saturate spins and the like to perform

MR imaging.

More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole body gradient coils 4, 5 and 6 along x, y and z-axes of the

examination volume. An RF transmitter 7 transmits RF pulses or pulse packets, via a send/receive switch 8 to an RF antenna 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse sequences of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals may also be picked up by the RF antenna 9.

For generation of MR images of limited regions of the body or in general object 10, for example by means of parallel imaging, a set of local array RF coils 11, 12 and 13 are placed contiguous to the region selected for imaging. The array coils 11, 12 and 13 can be used to receive MR signals induced by RF transmissions effected via the RF antenna. However, it is also possible to use the array coils 11, 12 and 13 to transmit RF signals to the examination volume. The resultant MR signals are picked up by the RF antenna 9 and/or by the array of RF coils 11, 12 and 13 and are demodulated by a receiver 14 preferably including a pre-amp lifter (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via a send/receive switch 8.

A host computer 15 controls the gradient pulse amplifier 3 and the transmitter

7 to generate any of a plurality of imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging and the like.

For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in a rapid succession following each RF excitation pulse. A data acquisition system 16 performs analogue to digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume or the like. The image is then stored in an image memory where it may be accessed for converting slices or other portions of the image representation into appropriate formats for visualization, for example via a video monitor 18 which provides a man readable display of the resultant MR image.

Fig. 2 is a schematic illustrating a cross section of a gradient coil arrangement 202 according to the present invention. The coils already discussed with respect to Fig. 1 are stacked here on top of each other, separated by impregnated spacers and if needed an additional insulating layer 205/207. Coils 4, 5, 6 are the coils of the x, y, z gradient coil axes stacked together. In Figure 1, one of the coils consists of a hollow conductor, which carries also a cooling fluid, however also a separate cooling circuit (not carrying electrical current) in between the coils is an option. The coils typically consist of copper conductors, where the x and y coils 4 and 5 could be comprised of copper plates through which an electrical current can run in order to generate the gradient magnetic field.

For example, the coils 4 and 5 may comprise copper plates which are separated from each other by means of electrical insulation 204. As shown in Fig. 2, the adjoiningly stacked gradient coils 4 and 5 are separated from each other by one or two spacers 206, combined with one or two insulating layers 205, 207. Here layer 205 is the backing material that is bonded to the fingerprint pattern as was discussed previously.

In state of the art coil arrangements, the layer structure between the coils 4 and 5 (epoxy glass cloth / insulator) is a thermal barrier for the heat generated by the x and y coil 4 and 5. One can reduce this barrier by using better heat conductivity materials in this area.

The glass cloth layer 206 impregnated with an epoxy has a composite heat conductivity of about 0.5 W/m/K. The glass cloth (Si02) has a thermal heat conductivity of about 1 W/m/K and the epoxy resin of about 0.2-0.3 W/m/K. The additional Insulation layer 205, 207, typically an FR4 sheet material, also has a composite heat conductivity of about 0.5 W/m/K.

In particular for high voltage gradient coils having sufficient distance between coil axes is required to reduce the electrical field between the coil axes to prevent partial discharges, noticeable as spikes in an MR system. However, by an increased distance achieved by increasing the thickness of the glass cloth layer, this also increases the thermal barrier for the heat flow towards the cooling infrastructure of the gradient coil, thereby reducing the maximum rms current of the gradient coil.

The present invention solves this problem by replacing the standard glass cloth with a material having high heat conductivity, being a thermal insulator and having a dielectric constant that matches the dielectric constant of the epoxy resin. The preferred material is boron nitride, wherein the boron nitride is used as a cloth which may further be impregnated with a state of the art resin in order to provide electrical insulation and thermal conductance while avoiding electric field concentrations in regions with differing dielectric constants.

Referring back to Fig. 2, in the direction of increasing radius R, between coils 5 and 6 insulation 204 as provided between coils 4 and 5 is used.

In this case the z gradient coil 6 which comprises hollow loops through which a cooling liquid like water can flow is positioned on top of the last insulation layer 204. Thus, heat generated by the gradient coils 4 and 5 is transported through layers 204 and coil 5 towards the cooling infrastructure provided by coil 6, at which the generated heat is removed by means of a cooling liquid.

Only schematically depicted in Fig. 2 is a further shielding setup 212 in order to provide a gradient magnetic field shielding. Respective coils used in the setup 212 may also be designed in the manner as just discussed with respect to the coils 4 , 5, 6 including the insulation 204. As already discussed in detail, boron nitride is the preferred spacer material, as it is also for the other layers within 204, since boron nitride has a high heat conductivity of about 20 W/m/K compared to the silica glass (Si02) which has only a heat conductivity of 1 W/m/K and which is used in state of the art coil arrangements. Further, the usage of boron nitride has the advantage that its dielectric constant of about 4 matches the dielectric constant of the typically used epoxy resin and the typically used FR-4 material in the layers 204, 205, 206, 207 up to date.

Even though the discussion with respect to figure 2 related to cylindrical MR systems, the invention is also applicable to other kinds MR systems with the MR bore not having necessarily a cylindrical shape.