EP3714281A1

EP3714281A1 - Tunable radio-frequency coil.

Info

Publication number: EP3714281A1
Application number: EP18825859.4A
Authority: EP
Inventors: Martin VIT; Daniel JIRAK
Original assignee: Technicka Univerzita v Liberci
Current assignee: Technicka Univerzita v Liberci
Priority date: 2017-11-24
Filing date: 2018-11-22
Publication date: 2020-09-30
Also published as: CZ307591B6; CZ2017753A3; WO2019101248A1

Abstract

A tunable radio-frequency coil comprises an outer hollow cylinder (1) and an inner hollow cylinder (2) which is at least partially insertable into the outer hollow cylinder (1), whereby each of the cylinders (1, 2) comprises a conductive loop (11, 21) at its outer edge (11, 21) and conductive strips (12, 22) which are mounted with one of its ends in the longitudinal direction on the conductive loop (11, 21), whereby the outer and the inner hollow cylinders (1, 2) are in the extended position oriented with the other free ends of the conductive strips (12, 22) towards each other.

Description

Tunable radio-frequency coil

Technical field

The invention relates to a tunable radio-frequency coil.

Background art

Radio-frequency coils are used for all transmitters and receivers in magnetic resonance imaging (MRI). The design of such coils consists of a system of conductors and capacitors resonating at a frequency which is needed to transfer a proton to another energy level.

For conducting medical nuclear magnetic resonance (NMR) spectroscopy, the device is generally configured to observe a single core, such as hydrogen protons ¹H, phosphorus-31 ³¹P, sodium ²³Na or carbon ¹³C. During combined treatment, when multiple cores having different resonance frequencies are to be observed, the coil needs to be replaced. However, during the replacement, the sample may be inappropriately handled.

A well-known solution to this problem is a multiple resonant peak coil, which, however, has a lower quality of the individual peaks for close frequencies and thus a lower quality output data. Another option is changing the coil parameters which have a direct influence on the resonance frequency. One of the basic parameters of the coil is inductance which indicates the magnitude of the magnetic induction flux at the unit electric current. Another important parameter is the capacitance indicating the amount of electrical charge around the conductor with a unit electric potential.

US4992737 discloses a radio-frequency coil which comprises a pair of opposed conductive loops which are connected to each other by conductive strips and a inset. The resonance frequency of the radio- frequency coil is preset in its design to operating frequency fO, whereby by interposing the inset into the radio-frequency coil design, the resonance frequency is reduced to a new operating frequency f1. The disadvantage of this arrangement is above all a limited number of tunable frequencies.

US 8680863 B1 describes a dual resonance MRI device comprising a pair of opposed outer end rings connected by straight segments and a pair of opposed inner end rings connected by straight arms, wherein there is at least one movable tuning ring arranged between them which is connected to at least one outer ring and reduces its inductance, thereby changing the operating frequency.

US 6396271 B1 describes a tunable radio frequency birdcage coil which comprises a pair of end ring conductors arranged in parallel planes and connected by a plurality of conductors and further comprises a pair of tuning rings and a non-conductive support cylinder provided with tuning leg conductors which serve to change the operating frequency.

The disadvantage of the devices described in the above-mentioned documents is especially a large number of capacitors used to reduce the voltage on the coil, which, however, decreases the range over which the radiofrequency coil can be tuned. As a rule, the capacitors are arranged such that they interrupt the end rings between the adjacent straight segments and, in addition, they interrupt the individual segments. The aim of the present invention is therefore to provide a radio-frequency coil which would enable to vary the resonance frequency and which would eliminate or significantly reduce the above-described drawbacks.

Principle of the invention

The aim of the present invention is achieved by a tunable radiofrequency coil, whose principle consists in that it comprises an outer hollow cylinder and an inner hollow cylinder which is at least partially insertable in the outer hollow cylinder, whereby each of the cylinders has at its outer edge a conductive loop and conductive strips which are mounted with one of their ends in the longitudinal direction on the conductive loop, whereby the outer and inner hollow cylinders are in the extended position oriented with the other free ends of the conductive strips towards each other. This arrangement is advantageous especially in the case of TEM and Birdcage coils, where changes can be made on hundreds of positions simultaneously by parallel connection given by the design. For surface radiofrequency coils, this design can also be used, however, in this case it is easier to use a capacitor with discrete parameters, i.e. time dependent and position independent. For more complex structures, however, degrees of freedom are disproportionately growing, and due to time and technical constraints, it is inappropriate to use elements with continuous parameters, i.e. time and position dependent. Another advantage is the difference compared to the structure with concentric parameters. Changing the properties is accomplished by changing the distributed parameters. Above all, this solution has a positive impact on improving the quality by limiting steep electric field gradients which do not arise in components with distributed parameters. Another benefit is that this design is cost-effective and material-efficient. For conventional radio-frequency coils, a plurality of capacitors is used, which is in this case replaced by a suitable geometric structure of the conductor.

Preferably, a layer enhancing dielectric properties is placed in the space between the conductive strips of the inner and outer hollow cylinders. In a preferred embodiment, each conductive strip, as well as the conductive loop, is embedded in a dielectric material. In another embodiment, a hollow cylinder made of a dielectric material is inserted between the outer and the inner hollow cylinders.

The outer and inner cylinders are provided with the same number of conductive strips and are mounted on a common longitudinal axis. These conductive strips are mounted in rings such that each conductive strip of the inner hollow cylinder is situated on an imaginary line connecting the longitudinal axis of the hollow cylinders and the conductive strip of the outer hollow cylinder.

At least one hollow cylinder is provided with means for its reciprocating displacement in the direction towards the second hollow cylinder and back. Description of drawings

The invention is schematically represented in the drawings, wherein Fig. 1 shows two conductive loops with conductive strips embedded in a dielectric material before being rolled into the desired shape, Fig. 2 shows two conductive loops with conductive strips in the dielectric material before being rolled into the desired shape constituting a parallel plate capacitor, Fig. 3 represents an axonometric view of an inner hollow cylinder partially inserted into an outer hollow cylinder of the tunable radio-frequency coil without a dielectric material, Fig. 4 shows an axonometric view of the inner hollow cylinder projecting from the outer hollow cylinder of the tunable radio-frequency coil without a dielectric material and Fig. 5 shows the tunable radio-frequency coil coupled with means for the reciprocating displacement of the hollow cylinders.

Examples of embodiment

The tunable radio-frequency coil comprises an outer hollow cylinder A and an inner hollow cylinder 2 coaxially arranged opposite each other, whereby the inner hollow cylinder 2 is at least partially insertable into the outer hollow cylinder . For an efficient and continuous increase in overall capacity, it is advantageous if the gap between the inner and outer hollow cylinders i, 2 is as small as possible and constant in size, whereby a non-contact design is easier in terms of mechanical structure and, in addition, it allows to achieve a smoother change in operating frequency.

The outer and the inner hollow cylinders 1_, 2 are coaxially mounted along a common longitudinal axis O, e.g. on a support structure 5, and are coupled to means 41 for their reciprocating displacement towards the second of the hollow cylinders A , 2, until the inner hollow cylinder 2 is completely inserted into the outer hollow cylinder A , and then back, until the inner hollow cylinder 2 is completely extended out of the outer hollow cylinder A . In another variant, only one of the hollow cylinders , 2 is movable and the second hollow cylinder , 2 is stationary. The hollow cylinders 1 , 2 are further coupled to AC power supply (not shown). The outer hollow cylinder 1 comprises a conductive loop 11., from which in the longitudinal direction of the hollow cylinder 1 , 2 project conductive strips 12, and the inner hollow cylinder 2 comprises a conductive loop 21 , from which in the longitudinal direction of the hollow cylinder !, 2 project conductive strips 22, whereby the free ends of the conductive strips 12, 22 of the outer and the inner hollow cylinders , 2 are in the extended position oriented towards each other. The individual conductive strips 12, 22 of the outer and the inner hollow cylinders , 2 are mounted in the same distance from each other, which means that when the inner hollow cylinder 2 is inserted into the outer hollow cylinder 1_ and at a proper angle, the conductive strips 12 of the outer hollow cylinder overlap the conductive strips 22 of the inner hollow cylinder 2 and the gaps between the conductive strips 12 of the outer hollow cylinder 1 correspond to the gaps between the conductive strips 22 of the inner hollow cylinder 2. Thus, in the portion of the inner hollow cylinder 2 which is inserted into the outer hollow cylinder l^ the conductive strips 22 of the inner hollow cylinder 2 are situated below the conductive strips 12 of the outer hollow cylinder 1, thereby constituting a parallel plate capacitor. Fig. 1 shows the structures of the outer and inner hollow cylinders 1, 2 before being rolled into the required shape, comprising the conductive strips H , 12 with the conductive strips 12, 22 embedded in the dielectric material 3. A parallel plate capacitor is formed by moving the structures of Fig. 1 on top of other, as is shown in Fig. 2, when the conductive strips 12, 22 of one of the cylinders , 2 at least partially overlap the conductive strips 12, 22 of the other of the cylinders , 2.

In order to form a non-conductive layer between the conductive strips 12, 22 of the inner and outer hollow cylinders 1, 2 ^ the conductive loop 11. , 21 with the conductive strips 12, 22 of each hollow cylinder 1 , 2 is embedded in a dielectric material 3, such as teflon, FR4, glass fiber, etc. In another uniilustrated embodiment, another hollow cylinder made of a dielectric material 3 is inserted between the hollow cylinders 1, 2. The change in the operating frequency of the device constructed according to the invention proceeds as follows.

By inserting the inner hollow cylinder 2 into the outer hollow cylinder 1., the overall length of the tunable radiofrequency coil is reduced, thereby reducing the inductance of the coil, and at the same time increasing the mutual area of the conductive strips 12, 22 embedded in the dielectric material 3, thereby increasing the capacity of the coil. During the insertion, the overall capacity of this coil increases far more rapidly than its inductance decreases, and consequently the operating frequency decreases.

Conversely, when the inner hollow cylinder 2 is pushed out of the outer hollow cylinder 1, the overall length of the tunable radio-frequency coil is extended, thereby increasing the inductance of the coil, while reducing the mutual area of the conductive strips 12, 22 embedded in the dielectric material 3, which reduces the capacity of the coil. The overall capacity of this coil decreases far more rapidly than its inductance increases, thereby increasing the operating frequency.

Industrial applicability

The invention is particularly applicable for magnetic resonance imaging where it is important to improve the quality of the resulting image by limiting sharp increases and decreases in the electric field gradients.

Claims

PATENT CLAIMS

1. A tunable radio-frequency coil, characterized in that comprises an outer hollow cylinder (1) and an inner hollow cylinder (2) which is at least partially insertable into the outer hollow cylinder (1), whereby each of the cylinders (1 , 2) comprises a conductive loop (11 ,21) at its outer edge (11 , 21) and conductive strips (12, 22) which are mounted with one of its ends in the longitudinal direction on the conductive loop (11 , 21), whereby the outer and the inner hollow cylinders (1 , 2) are in the extended position oriented with the other free ends of the conductive strips (12, 22) towards each other.

2. The tunable radio-frequency coil according to claim 1 , characterized in that at least one layer of a dielectric material is located between the conductive strips (22) of the inner hollow cylinder (2) and the conductive strips (12) of the outer hollow cylinder (1).

3. The tunable radio-frequency coil according to claim 1 or 2, characterized in that the outer and the inner hollow cylinders (1, 2) contain the same number of conductive strips (12, 22).

4. The tunable radio-frequency coil according to any of the preceding claims, characterized in that each conductive strip (22) of the inner hollow cylinder (2) is situated in the plane between the longitudinal rotary axis (O) and the conductive strip (12) of the outer conductive cylinder (1).

5. The tunable radio-frequency coil according to any of the preceding claims, characterized in that the inner and/or the outer hollow cylinder (1, 2) are provided with means (41) for their reciprocating displacement along their longitudinal rotary axis (O).