WO1988009926A1

WO1988009926A1 - Receiving aerial for nmr imaging apparatus

Info

Publication number: WO1988009926A1
Application number: PCT/FR1987/000197
Authority: WO
Inventors: Hervé Jacob; Jean Bossaert
Original assignee: Thomson-Cgr
Priority date: 1987-06-02
Filing date: 1987-06-02
Publication date: 1988-12-15

Abstract

The quality of the signal received is improved by making the aerial symmetrical. To this effect, the aerial has two branches (18-19) symmetrical in relation to the same plane (XOZ). These branches are connected by their ends to aerial tuning capacitors (C1-C2). The received signal is tapped (41) by a high frequency line (T) incorporated in one of the branches at the connection of one tuning capacitor (C1). The other end (31) of this high-frequency line arranged on the plane of symmetry is connected to an amplifying/impedance transforming circuit (25) of high input impedance.

Description

4,

RECEIVING ANTENNA FOR NUCLEAR MAGNETIC RESONANCE IMAGING APPARATUS

The present invention relates to a receiving antenna for nuclear magnetic resonance (NMR) imaging apparatus. It finds more particularly its application in the medical field where imaging by NMR proves to be valuable as an aid to diagnosis. It can nevertheless find its application in other fields.

An NMR imaging device essentially comprises means for subjecting a body to be examined to a constant and intense magnetic field BQ. Then, subject to this influence, the body receives a radio frequency excitation from a transmitting antenna to cause a resonance of the oscillation of the magnetic moments of its particles. As soon as this radio frequency excitation ceases, the resonance signal emitted by the body is measured when the magnetic moments of the particles tend to realign with the field B _Q. To receive the signal emitted, so-called surface antennas are sometimes used. The so-called surface antennas are antennas placed on the surface of the body. Their advantage compared to the antennas integral with the device is essentially that they can be placed in the immediate vicinity of the emitting particles. The transmitting antennas conventionally attached to the device are, for their part, much more distant. The use of these surface antennas results in a significant improvement in the signal-to-noise ratio of the detected signal. A surface antenna mainly consists of a conductor forming a current loop. The inductive impedance of this antenna circuit is tuned to the resonant frequency by means of a capacitor. The sensitivity of the antenna directly depends on the overvoltage coefficient of the resonant circuit thus produced. In the design of antenna circuits, we aim to minimize losses by acting on the one hand on the shapes and dimensions of the circuit and acting on the other hand on the choice of components, high quality, which compose them.

But other conditions must also be met in particular to minimize noise. For example, it is known to distribute the tuning capacities along the circuit: by doing so, the detection of electric fields is reduced. The impedances of the inductors and capacitors constituting the antenna are then reduced, which has the effect of reducing the voltages at their ends and the resulting electric field. In other words, we do not pick up the dielectric loss signal that appears in the body. Furthermore, to ensure the best reception of the resonance signal, this signal is led by a suitable line to a signal processing circuit. The adapted line is generally a coaxial line of characteristic impedance 50 Ohms. Another factor is directly involved in the quality of the signal received: it concerns the antenna synchronization. Figures la and lb show non-symmetrical and symmetrical antennas respectively. In the figure, the two radiating elements 1 and 2 are connected to the core 3 and to the braid of a coaxial cable 5. The latter is supplied by a signal source V. During transmission or reception , there is an imbalance between the radiating parts: a first part is limited to the antenna element 1, while the other part comprises the antenna element 2 and the braid> of the cable 5. In the figure 1b is a known symmetrization device. The supply of doublet 1-2 is carried out by means of a magnetic coupling 6, the middle point 7 of the secondary of which is connected to the braid of the cable 5. By thus balancing the supply, any sheath current in the cable is avoided coaxial. In this way the antenna becomes insensitive to external radioelectric disturbances. Unfortunately, this transformer symmetrization is not applicable to NMR imaging for which the presence of the magnetic coupling 6 is unacceptable. It follows that in NMR symmetrization is neglected.

The object of the present invention is to remedy this disadvantage by proposing balanced antennas. -For this symmetrization an original solution, without additional magnetic coupling, is proposed which at the same time performs the antenna impedance adaptation. In fact, coupling 6, which must now be dispensed with, has another characteristic: that of adapting the impedance of the antenna 1-2 to the characteristic impedance of the coaxial cable 5.

The subject of the invention is a reception antenna for nuclear magnetic resonance imaging apparatus comprising means for ensuring its symmetrization for the reception of the resonance signal and a circuit for ensuring the transformation of its impedance for the transmission of this resonance signal charac¬ terized in that the symmetrization means comprise an inductive loop tuned in frequency by a distributed capacitance comprising at least one capacitance arranged symmetrically in the loop, and a circuit for receiving the signal induced in the loop, connected to the terminals of one of the capacities, this reception circuit comprising a section of a high frequency line of which a mass conductor participates in the inductive loop and of which a core, which serves to conduct the received signal, is connected by a part to the rest of the loop and secondly to the impedance transformation circuit.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are given for information only and in no way limit the invention. In the figures, the same references designate the same elements. They show :

- Figure la and lb of the antenna elements of the prior art already described above;

- Figure 2: an NMR imaging device comprising an antenna according to the invention;

-Figure 3a and 3b a block diagram and a particular example of embodiment of an antenna according to the invention.

Figure 2 shows a nuclear magnetic resonance imaging device. This device essentially comprises first means 8 for submitting a body 9 supported by a plateau 10 at an intense magnetic field B _Q. In this situation the body 9 also receives a radio frequency excitation produced by generator means 11 and transmitted, for example, by a four-conductor antenna such as the conductor 12. As the radio frequency response of the signal is a volume response the ap¬ similar also includes coils 13, called gradient, also connected to the generator means 11 to particularize regions of the space where sections are to be imaged. For reception, perpendicular to these sections, is placed on the body 9 a surface antenna 14 provided with the improvement of the invention. This surface antenna is symmetrical and adapted. It is connected by a line 28 to processing circuits 15 connected to means 16 for viewing or memorizing for viewing or memorizing the images of the sections. The generator means 11 and the processing circuits 15 are controlled by a sequencer 17.

FIG. 3a schematically shows the antenna 1 provided with a symmetrization device. The antenna 14 comprises a current loop formed by two conductive strands 18 and 19 curved and symmetrical. These two strands are joined together, at their ends by two capacitors C, and C ₂ . Capacities C, and C ₂ are calculated to cause with the strands 18 and 19 a large overvoltage for a signal at the resonant frequency. The syme¬ trization is obtained here by connecting a reception circuit 20 to the terminals of one of the two capacitors. In a particular embodiment, FIG. 3 b, the assembly conforms to that of FIG. 3 a and the connection of a preferr reception circuit is shown in more detail. The antenna has symmetry with respect to a plane XOZ where Z is collinear with B _Q and where X is perpendicular to the plane of the antenna. This symmetry results in a partition of the loop into four half-strands denoted 21 to

24. The half-strands 22 and 23 on the one hand and 21 and 24 on the other hand are of equal length and have the same shape. The half-strands 22 and 23 are connected together at one of their ends, and at capacities C, and C _? respectively at their other ends. The half-strands 21 and 24 are also connected together at one of their ends and respectively at capacities C, and C ₂ at their other ends. The sections of the conductors of the half-strands are the same. They can for example be hollow conductors. In the invention, the half-strand 21 however forms with a conductor which is parallel to it, a section T of high frequency line. This section T is longer than a normal half-strand: the half-strand 24 is electrically connected to it at a certain location to determine the half-strand 21, and to constitute with it strand 18. The symmetrization is obtained by connecting the hot spot of the line at the end 40 of the half-strand 22 (connected to the capacitor C,) on the one hand, and to an adaptation-amplification circuit 25 on the other hand. In a preferred manner, the half-strands 22, 23 and 24 are formed from elements of coaxial cables, the useless cores of which are left in the air. The section T can be formed from an identical coaxial cable. In this example, the hot spot of the high frequency line is formed by the core of the section T. It is connected by a connection 41 to the half-strand 22.

The circuit 25 is placed in the immediate vicinity of the antenna (a few centimeters). It essentially comprises a field effect transistor (FET) 26. The transistor 26 is supplied by a supply 27, decoupled, and of known type, the potential (Vcc) of which is transported by the core of a coaxial connecting cable 28. The circuit 25 further comprises a bias resistor 29 and a decoupling capacity 30 mounted as a common transmitter. The circuit

25, which constitutes an amplifier with high input impedance, performs the transformation of the impedance of the antenna 14 to adapt it to that of the line 28. Indeed, the equality of the currents flowing in the conductive strands 18 and 19 and in the capacitors Ci and C ₂ causes a very high impedance of the resonant circuit presented at the input of the amplifier 25. While this circuit delivers in a low load via the line 28, by its high impedance input circuit 25 performs the impedance transformation. Line 28 is also used to transport the received signal to the processing means 15. In one example, the section T is made from a coaxial cable element with a characteristic impedance of 50 Ohms. At a resonance frequency of 20 MHz, the wavelength of this resonance signal is approximately 15 meters. The section T, the length of which can be of the order of 0.50 meters (much less than a quarter of the wavelength) and which is charged by an almost infinite impedance (that of circuit 25), behaves like a capacitive impedance of value C. The amplifier-adapter circuit 25 also has an equivalent alternative circuit composed of a resistor R "in parallel with a capacitor.

VS" . The frequency tuning of the current loop by capacitors C, and C ₂ is modified. Under these conditions we choose C, such that:

The end 31 of the section T connected to the circuit 25 is collinear with the axis Z. Also no common mode current can no longer flow in the core of the section T due to the electrical and mechanical symmetry of the device. In a particular embodiment the loop 14 is installed on an insulating sheet of a flexible and resistant material; the section of line T can be produced therein in the form of a microstrip. This facilitates placement on the body. The circuit 25 is placed directly downstream of the antenna, on the same sheet. The cable 28 connects this circuit, placed on the body in the tunnel of the NMR apparatus, to circuit 15.

Claims

1 - Receiving antenna (14) for a nuclear magnetic resonance imaging device comprising means (18-24) for ensuring its symmetrization (T) for receiving the resonance signal and a circuit for ensuring the transformation (25) of his impedance

- for the transmission of this resonance signal characterized in that the symmetrization means comprise a inductive loop (18-19) tuned in frequency by a distributed capacitor comprising at least one capacitor (C.-C) arranged symmetrically in the loop, and a circuit (25) for receiving the signal induced in the in-loop, connected (1) across one of the capacitors, this reception circuit comprising a section (T) of a high frequency line including a ground conductor (21) participates in the inductive loop and a core of which, which serves to conduct the received signal, is connected on the one hand to the rest (C.) Of the loop and on the other hand to the circuit (25) of

1 impedance transformation.

2 - Antenna according to claim 1 characterized in that the loop has a geometric symmetry with respect to a plane (XOZ), which cuts it substantially perpendicular to its own plane. o 3 - Antenna according to claim 2 characterized in that the end (31) of the section (T) of high frequency line connected to the impedance transformation circuit is contained in the plane of symmetry.

4 - Antenna according to any one of claims 1 to 3 characterized in that the section of the conductor which forms the rest of the loop is that of a coaxial cable identical to that of the section (T) of high frequency line.

5 - Antenna according to any one of claims 1 to 4 characterized in that the impedance transformation circuit is 0 a high input impedance circuit.

6 - Antenna according to claim 5 characterized in that the impedance transformation circuit is placed in the immediate vicinity of the antenna. 7 - Antenna according to claim 6 characterized in that the impedance transformation circuit comprises a transistor (26) with field effect mounted as a common emitter (29-30).

8 - Antenna according to claim 7 characterized in that the field effect transistor is supplied by a bias voltage (Vcc) decoupled (27) from the amplified signal and placed at a distance (28).

9 - Antenna according to claim 3 characterized in that the tuning capacities of the two symmetrical parts of the loop are equal.