Patent Application
20100141260
1. Field of the Invention
The invention relates to magnetic resonance tomography and, in particular, to medical instruments for examining human and animal bodies. Magnetic resonance tomography (MRT) is also known as nuclear spin tomography.
2. Description of the Prior Art
MRT is an image-generating method based on the physical phenomenon of nuclear spin resonance. An object to be examined is subjected to a strong magnetic field. This causes an alignment of previously statistically distributed nuclear spins of the individual atoms. Excitement with high-frequency energy from outside causes measurable oscillations. The frequency is a function of the magnetic field strength. For spatial localization, magnetic fields which are inhomogeneous along the three spatial axes are generated using gradient coils. Transmitting coils are provided for emitting the high-frequency excitation energy. A reception of excited oscillations is effected with receiving coils. Transmitting coils and receiving coils are frequently combined with each other. In the following, these coils are also referred to as HF coils, because they serve for coupling-in or coupling-out high-frequency signals.
This non-invasive image-generating method makes it possible to obtain images of sections through a human or animal body along any desired axes.
Examples of transmitting and receiving coils are disclosed in U.S. Pat. No. 4,887,039. There pluralities of parallel conductors which are connected to each other via coupling capacitors are mounted on a cylindrical support. Feeding is effected by means of symmetrical conductors or coaxial cables. So-called phased-array arrangements are employed in order to achieve higher resolutions. For this, pluralities of independent coils having independent receiver inputs are connected for separate evaluation of the signals.
The construction of coils of this kind is very complex and the manufacturing costs are therefore relatively high. In coil arrangements of the future, an increasing number of coils will have to be provided, whilst the higher resolution will cause even greater demands to be made on the mechanical tolerances.
The problems outlined above may be in large part addressed by a high-frequency (HF) coil assembly having a simplified mechanical construction, whilst maintaining or improving electrical properties and mechanical stability. In addition, reduced mechanical tolerances and manufacturing costs are realized with such a coil assembly. The following are mere exemplary embodiments of a coil assembly and a magnetic resonance imaging device employing such characteristics and are not to be construed in any way to limit the subject matter of the claims.
An embodiment of a high-frequency (HF) coil assembly for magnetic resonance tomographs includes at least one flexible printed circuit board having inductors and at least one bar attached thereto for stiffening the flexible printed circuit board along one axis. The bar optionally has conductors for connecting electronic components on the printed circuit board with each other or with outside components of the magnetic resonance tomograph. An embodiment of a magnetic resonance imaging device includes at least one of such HF coil assemblies.
In the following, the invention will be described by way of example on embodiments with reference to the drawings without limitation of the general inventive concept.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
The coil assemblies described herein include a printed circuit board 10. In some cases, the printed circuit board 10 may be of a multi-layer construction, in which at least two layers are joined together, such as in the form of a laminate. At least one layer comprises a dielectric insulating material (insulator layer) having as low as possible dielectric losses in the operating frequency range of the coil assembly. Materials of this kind may comprise, for example, plastics such as PTFE (polytetrafluoroethylene), PE (polyethylene), or also ceramic materials. In order to increase the mechanical stability, fibers such as glass fibers or carbon fibers may be embedded.
A typical operating frequency range of the coil assemblies described herein is in the range of about 30 MHz to about several 100 MHz according to the prevailing outer magnetic field of the assembly. Firmly connected to the insulating material is at least one layer of conducting material (conductor layer), the shape of the coils having been formed in this layer. A connection between the layers, or the layers themselves, may be semi-flexible, so that internal mechanical tension cannot be caused, or can be reduced, when the assembly is subjected to bending. A conducting layer may be applied onto the first layer, optionally by chemical or electrochemical methods, in particular by electroplating or etching, or mechanically. Thus, it may be rolled on, for example in the form of a thin foil.
The flexible printed circuit board 10 carries besides the inductors at least one other electronic component, preferably a discrete component, which optionally comprises coils, resistors, and also semiconductors such as, for example, diodes. A discrete component is an electronic component which is not integrated into the printed circuit board and is self contained in its own housing. It can be soldered to the printed circuit board. Examples of discrete electronic components are SMD (surface mount devices) or wired capacitors, inductors, resistors and semiconductors. These components may be made by SMD technology in order to enable particularly space saving and efficient assembly.
Furthermore, at least one of bar 22 is attached to the printed circuit board. The bar may be laminated, glued, soldered or welded to flexible printed circuit board 10. In some cases, it is attached on a significant part of its length, and in further cases on its whole length. Alternatively, there may be gaps in the bar, for example at positions, where an electronic component is located under the bar. In some embodiments, the bar is stiff compared to the printed circuit board 10. Accordingly, the bar may include a thicker material. It may be arranged at right angle to the printed circuit board, but other angles may be employed. It stiffens the flexible printed circuit board in one axis parallel to the bar. The flexible printed circuit board may still be bent at right angle to the bar. The bar may be a printed circuit board which has conductors to connect components on the printed circuit board like inductors 20 and/or electronic components 21 with each other and/or with other parts of the magnetic resonance imaging device. The bar may serve for connecting parts on the printed circuit board to the outside. The connecting lines on the printed circuit bar of the coils have negative effects on the coils and, thus, the connecting lines are separated from the coils and have much less influence on the coils. Therefore, a low interaction with the coil can be achieved. In some cases, a plurality of bars is provided parallel at equal distances.
In further embodiments, the bar 22 comprises at least one strip line. Strip lines are electrical lines on printed circuit boards having specific characteristic impedance, such as 50 Ohms or 100 Ohms. Close to a strip line is at least one ground layer. Accordingly, the bar 22 may comprise at least one ground layer, and on top of this isolated by an insulating layer an electrical line. Alternatively, the electrical line may be sandwiched in between two ground layers which are connected together. All coils of the known prior art suffer from the problem, that it is difficult to transfer signals from the coils to other parts of the scanner.
In other embodiments, the bars are L-shaped or U-shaped. This results in a further increasing of stability. These bars form a triangle or a square in conjunction with the flexible printed circuit board 10.
In further embodiments, the printed circuit board is bent to a cylindrical form.
According to further embodiments second bars are provided, which are attached to the flexible printed circuit 10 at an angle to the first bars 22 to stiffen the flexible printed circuit board in a second axis, making it stable in two dimensions.
In other embodiments, reinforcement members are additionally provided on the flexible printed circuit board 10 to further increase the mechanical stability of the entire assembly. Particularly important are the locations on the assembly at which recesses have been provided, or at which various parts of printed circuit boards, or layers of insulating material, have been joined together. It is here that cracks or breaks predominantly form when mechanical stresses act on the entire assembly. The reinforcement members are provided additionally in order to avoid these.
It is of particular advantage for the reinforcement members to comprise a plastic material. Particularly expedient is the use of fiber-reinforced (glass fiber, carbon fiber, etc.) plastics.
Furthermore, it is of advantage for at least one of the reinforcement members to be incorporated into the assembly itself, preferably by pressing or casting.
It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide an HF coil assembly for magnetic resonance imaging. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.
