Nuclear spin examining apparatus with a radio frequency coil arrangement

p0001The invention relates to a magnetic resonance examination apparatus with an RF coil arrangement according to the preamble of claim 1. Such a radio frequency coil assembly is known eg from Radiology 160 (1986) S.695-699 known.

p0002It is known that the signal-to-noise ratio upon reception of magnetic resonance signals is unfavorable the more, the lower the magnetic flux of the stationary homogeneous magnetic field is, is exposed at a magnetic resonance examination of the examination region. In order to still achieve an acceptable signal to noise ratio even in nuclear magnetic resonance devices with low magnetic flux density, it is important that the RF coil system which receives the nuclear magnetic resonance signals having a high quality and a high sensitivity. It is known that a so-called solenoid coil with a sufficient number of turns has these properties. However, the number of windings can be chosen only so large that the length of the conductor, from which this coil is wound, is less than a quarter wavelength of the operating frequency, preferably small in comparison.

p0003Murton and Neale have therefore described a coil arrangement in which two coils are connected in parallel, each with three turns. Again, the self-resonant frequency is due to unavoidable coil capacity is still relatively low, so that wave propagation effects can be felt, leading to an increase in the dielectric losses and a decrease in quality.

p0004Object of the present invention is to provide a high-frequency coil system for a magnetic resonance apparatus, which has a favorable signal-to-noise ratio and a high quality even at a comparatively low magnetic resonance frequency. This object is achieved by the measures specified in the main claim.

p0005Thus, the invention uses more than two in parallel planes arranged on the circumference of a hollow cylindrical carrier body resonators, each formed by a conductor loop, which are tuned usually by means of an additional capacitor to the same frequency. The individual resonators are not galvanically connected to each other, but inductively coupled together. Only one of the resonators is connected to the high-frequency transmitter or to the high-frequency receiver in operation. However therefore Such RF coil system behaves with respect to the wave propagation effects and the associated dielectric losses as a high frequency coil with only one turn, in terms of sensitivity like a solenoid coil whose number of turns equal to the number of resonators.

p0006It should be mentioned at this point that from EP-A 281 787 a Oberflächenresonator for magnetic resonance examination apparatus is already known which consists of two arranged in approximately the same plane turns, which are electrically connected to one another and are inductively coupled to each other, the signal tap and the signals are input to the ends of one turn. It is also indicated that each one of these surface resonators above and arrange below the examination zone can.

p0007Further, from EP-B 142 760 an MR coil system known, which consists of an upper and a lower part, each with a resonator. The resonators are inductively coupled with each other, and only one of the resonators is fed energy. The two resonators are disposed in the upper and lower half of a cylindrical carrier body, wherein the field is perpendicular to the cylinder axis while it runs in the invention in the direction of the cylinder axis.

p0008In an RF coil system according to the invention there are so many vibrational modes as there are resonators. In a vibration mode which flows at all resonators flow with the same machining direction. This Schwindungsmodus is associated with the lowest resonant frequency which is below the frequency to which the individual resonators are tuned. In another mode of vibration, the currents have always the opposite sense of rotation in spatially adjacent resonators. In this vibrational mode, gives the highest resonant frequency, which is above the frequency to which the resonators are tuned. This vibration mode can be excited, characterized in that one of the resonators, a current is supplied with the respective frequency; by an external homogeneous RF field, it can however not be caused. When using a high-frequency coil assembly according to the invention for receiving nuclear magnetic resonance signals, these can thus be completely decoupled from a transmitting coil, which works on the same resonant frequency.

p0009In embodiment of the invention provides that in the operating state acts a vertically extending uniform, steady magnetic field in the area enclosed by the RF coil system examination region. In an MRI, a patient is examined lying generally. The RF coil system thus then has a horizontally extending longitudinal axis, and generates an extending direction in this high frequency magnetic field to which the vertical, homogeneous and stationary magnetic field is perpendicular - as it is required.

p0010The invention is explained hereinafter with reference to the drawing. Show it:<ul><li>Fig. 1 shows a magnetic resonance tomography apparatus in which the invention is applicable.</li><li>FIG. 2 is an electrical schematic diagram and </li><li>Fig. 3 shows the spatial layout of a high frequency coil according to the invention.</li><li>Fig. 4 shows a modification of this coil.</li></ul>

p0011The MRI scanner shown in FIG. 1 includes an RF coil system 1 with a hollow cylindrical cross-section, which generates in its interior a to the plane vertical, high frequency magnetic field that traverses the examination region in which the examination subject 2 is, for example, a patient whose longitudinal axis perpendicular to the plane runs. The nuclear spin tomograph has a main field magnet not shown in detail, generates a vertical direction extending in the steady, uniform magnetic field having a magnetic flux density of a few tenths of T or less. The stationary magnetic field and the RF field are therefore perpendicular to each other. There are also gradient - also not shown in detail - generate also extending in the vertical direction magnetic fields, but with a gradient in each of three mutually perpendicular directions.

p0012A control unit 3 controls the generation of the fields described above and processes the received from the examination zone nuclear magnetic resonance signals. In particular, they act on an electronic switch 4, by means of which the RF coil system 1 of the transmission mode - in which it is connected to an oscillator 5 - is switched to receiving mode in which it is connected to a high-frequency receiver 6, which amplifies the nuclear magnetic resonance signals, demodulated and is reacted in a sequence of digital data words that are processed in the unit of the third

p0013As is apparent from FIGS. 2 and 3, the RF coil system 1 comprises a number of resonators 11 ... 15. The resonators are arranged on a cylindrical support body 10 whose central axis 16 is perpendicular to the uniform, steady magnetic field B₀. Each resonator comprises a conductor loop which surrounds the annular support body and is only interrupted at a location which is capacitively bridged. The conductor loops are located in parallel to the longitudinal axis 16 of the carrier body 10 preferably perpendicular planes. The capacity of the open ends of the conductor loops 11 .. 15 bridging capacitors 110 ... 150 is selected so that all resonators are tuned (by itself) in the same resonant frequency. In the central conductor loop 13 that vote by means of two equally large, series connected capacitors 131 and 132, their connection point is grounded occurs. The facing away from this connection point of a connection of the two capacitors (132) is connected via a fitting and matching network 8 to the switch 4 (Fig. 1).

p0014Instead of each one point the conductor loops can be interrupted and bridged by a capacitor with suitable capacity and at a plurality of circumferentially each evenly staggered locations. As a result, while increasing the cost, in that it further reduces the dielectric losses occurring during operation or the quality even further increased.

p0015The resonators formed by a conductor loop and one or two capacitors are not electrically connected to each other. However, they are inductively coupled to each other, because the conductor loops are located in parallel planes. Because of the lack of galvanic connection between the individual conductor loops the propagation effects are determined only by the dimensions of which can be coupled with the RF transmitter 5 or the RF receiver 6 conductor loop. 13 With a diameter of the pipe 10 of 600 mm, the length of such a conductor loop (about 2 m) but still small compared to one-quarter wavelength (35 m) located at a magnetic flux density of the stationary homogeneous magnetic field B o of 0.2 T and the associated magnetic resonance frequency of approximately 8.5 MHz results. On the other hand results from the inductive coupling between the various resonators a concentration of the generated magnetic field, the 16 and extends in the direction of the center axis perpendicular to the resonators level, as indicated in Fig. 2, substantially in the space enclosed by the resonators room - similar to a solenoid coil with a corresponding number of turns. The quality and especially the coil sensitivity are therefore the better, the more resonators are present. In practice, one will therefore also use more than the shown in the figures five resonators.

p0016Apart from the strong field concentration, in particular in the center (in the region of the resonator 13), there is another advantage in excellent homogeneity of the RF field in the interior of the coil.

p0017In a high-frequency coil assembly comprising a plurality of resonators, the number of resonance frequencies of the number of the resonators corresponds; as the resonance frequency is that frequency referred to, wherein the ratio of the energy in the resonator to the energy of the resonator has supplied a maximum. The resonant frequencies are the farther apart, the stronger the inductive coupling between the individual resonators, ie the closer the conductor loops lie down together. Each resonant frequency is associated with a different mode of vibration. The lowest resonant frequency is obtained if all resonators a current with the same machining direction flows. This resonance frequency is lower than the frequency to which the individual resonators are tuned. The highest resonance frequency is obtained when the machining direction of flow of the resonator to resonator is inversely; this frequency is substantially higher than the frequency to which the resonators are tuned. The advantage of this vibration mode is that it can not be excited by a homogeneous external magnetic field. When operating such a coil arrangement as a receiving coil in the field of an (additional) transmitting coil, resulting in a significant decoupling between the coils, particularly when the receiving coil is made of an even number of resonators. It is advantageous that conventional transmission coils usually also lying on a cylinder jacket, but generate a perpendicular to the cylinder axis field. The receiver coil may thus be arranged concentrically to the transmitting coil in the interior.

p0018If examined in practice embodiment of an RF coil for magnetic resonance examinations in the head seven resonators were applied to a support body with a diameter of 320 mm. Each resonator consisted of 12 mm copper pipe, which was tuned by a capacitor of 320 pF at a frequency of about 11 MHz. The distance between the conductor loops was 20 mm. This arrangement was operated at the lowest resonance frequency was 6.8 MHz. Here there was an unloaded Q of 120 and a loaded Q of approximately 280. saddle coils have in such resonator frequencies although similarly high load and operating qualities, so that they are not inferior in the transmission mode. However, the sensitivity of a saddle coil is lower by around 40%, so that the RF coil system according to the invention has significant advantages in the receive mode. At low frequencies the quality of a high-frequency coil assembly takes to the invention far less than the from of a saddle coil, so that thus also result in yet depressants frequencies with respect to the transmission operation advantages.

p0019Above a high frequency coil has been described with a circular cross-section. The RF coil can also have a different cross section, for example, an elliptical or a square. The center points or centroids of the area enclosed by the individual conductor loops areas should be on a common straight line, and the parallel planes in which there are the conductor loops should intersect Just as vertical as possible.

p0020It is not necessary that all conductor loops are the same size. As indicated by dashed lines in Fig. 2, a further conductor loop 17 may be provided which is arranged concentrically to the central axis 16 and has a smaller diameter than this. This conductor loop must be tuned to the same frequency as the other resonators, including A capacitor 170 is required because of the reduced inductance of this loop with a larger capacity. The effect of this smaller resonator 17, which at one end of the RF coil system is located, seen in the fact that the magnetic flux in the axial direction towards even more drops to the outside so that the magnetic field is even more concentrated in the region enclosed by the coil space , Then, when the generator or the receiver is not in the middle resonator 13 but to the coil 17 adjacent resonator is connected 15, also follows the other side of the coil towards a greater decrease of the magnetic field. When using such a high-frequency coil as head coil inasmuch yield benefits when the then located left of the resonator 11 neck and shoulder area less RF energy is supplied and the coil quality therefore less stress this area.

p0021In Fig. 4 of the coil is arranged perpendicular to the coil axis conductive screen 18 of a 30 »m thick copper foil on the side facing away from the patient side inlet side. This increases the field in the center of the coil, thus causing an additional field concentration and hence a better signal-to-noise ratio.

p0022In addition, - when used as a head coil - on the side of the spool on which the patient is inserted with its head, another screen will be 19, which with an opening - indicated in Figure 4 with 20 phantom -. Is provided for the patient's head , This shields the shoulder region of the patient and stray fields, which generate an additional stress for the patient.