RU2574348C2 - Radio-frequency antenna for magnetic-resonance imaging with removable conductor - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to microwave engineering. A radio-frequency (RF) transmitting and/or receiving antenna is particularly in the form of a coil structure or coil or loop arrangement, having one or more removable conductors, especially for use in a magnetic resonance imaging (MRI) system or a magnetic resonance (MR) scanner, for transmitting RF excitation signals (Bi field) for exciting nuclear magnetic resonances (NMR), and/or for receiving NMR relaxation signals. The RF antenna is provided such that it can be easily adapted according to an application which either requires a large opening through the RF antenna or a parallel imaging capability.

EFFECT: high sensitivity of the RF antenna and image quality.

9 cl, 5 dwg

Description

Technical field

The present invention relates to a radio frequency (RF) transmitting and / or receiving antenna, in particular in the form of a coil structure or device in the form of a coil or loop having one or more removable conductors or conductor structures, in particular for use in a magnetic resonance imaging system (MRI) or magnetic resonance (MR) scanner to transmit RF excitation signals (field B ₁ ) to excite nuclear magnetic resonances (NMR) and / or to obtain NMR / RF relaxation signals. The present invention further relates to a magnetic resonance imaging (MRI) system or magnetic resonance (MR) scanner, also comprising an RF transmitting and / or receiving antenna.

State of the art

In an MRI or MR scanner system, the test subject, usually the patient, is exposed to a uniform main magnetic field (field B ₀ ), so that the magnetic moments of the nuclei inside the test subject tend to rotate around the axis under the influence of the applied field B ₀ (Larmor precession) with some the resulting magnetization of all nuclei parallel to the field B ₀ . The precession rate is called the Larmor precession frequency, which depends on the individual physical characteristics of the involved nuclei, namely their gyromagnetic ratio and the strength of the applied field B ₀ . The gyromagnetic ratio is the ratio between the magnetic moment and the spin of the nuclei.

By transmitting an RF excitation pulse (field B ₁ ), which is orthogonal to field B ₀ generated by the RF transmitting antenna, and adjusts the frequency of the Larmor precession of the nuclei of interest, the spins of the nuclei are excited and aligned in phase and receive a deviation of their resulting magnetization from the direction of field B ₀ , so that a transverse component is generated depending on the longitudinal component of the resulting magnetization.

After the end of the RF excitation pulse, relaxation processes of the longitudinal and transverse components of the resulting magnetization begin until the resulting magnetization returns to its equilibrium state. NMR relaxation signals that are emitted by transverse relaxation processes are recorded by the RF receiving antenna. The received NMR signals, which are temporary amplitude signals, undergo Fourier transform into the signals of the NMR frequency spectrum and are processed in a known manner to generate an MR image of the object being examined.

The RF transmitting and / or receiving antennas discussed above are also known in the form of the so-called MR coils of the body, which are fixedly fixed in the studied space of the MRI system to form an image of the entire object under examination and the so-called MR surface coils, which are located directly in the local zone or region should be examined, and which are created, for example, in the form of flexible pillows, or sleeves, or cells (a coil for the head or a coil of the “cell” type). For examining a local area or region, surgical devices like catheters or other invasive devices containing an RF transmitting and / or receiving antenna, in particular in the form of a small coil or loop for receiving MR relaxation signals, are also known.

Regarding the shape of the examination space, two types of MRI or MR scanners systems can be distinguished. The first system, the so-called open MRI system (vertical system), which contains an examination area, which is located between the ends of the vertical structure with a C-type frame. The second system is a horizontal MRI system, also called an axial MRI system, which contains a horizontally extending tubular or cylindrical examination space.

Patent 5457387 discloses an axial MRI device in which an RF coil assembly is provided having a pair of end rings that support the gradient coil assembly, with longitudinally extending longitudinally between the pair of end rings a plurality of coil elements that mechanically and selectively attach and disconnect to / from the gradient coil assembly. Such an RF coil assembly should prevent a decrease in the critical diameter inside the magnet gap of the MRI device and should provide a large aperture for the patient.

SUMMARY OF THE INVENTION

It has been found that disconnecting and moving one or more coil elements of an RF antenna can adversely affect the resonance properties of the RF antenna, so that the sensitivity of the RF antenna and image quality are degraded.

One objective underlying the present invention is to provide an RF transmitting and / or receiving antenna, in particular for use in MRI systems, said antenna comprising one or more circuit antenna elements, wherein the resonant properties of the RF antenna are not affected or only minimally when one of the at least one removable antenna element is removed.

Another objective underlying the present invention is to provide an RF transmitting and / or receiving antenna, in particular in the form of an MR surface coil, which can be adapted according to the requirements of certain NMR surveys with respect to either a large aperture through the antenna, or the ability to parallel imaging (visualization) without significantly affecting the resonant properties of the antenna.

These goals are achieved, according to paragraph 1 of the claims, a radio frequency transmitting and / or receiving antenna containing the structure of the first conductor, forming a first closed loop that is resonant at the desired RF frequency, and at least one structure of the second conductor, which can be connected with the structure of the first conductor so that the first closed loop is divided by at least one structure of the second conductor into at least a second and third closed loop, with at least one structure The cheers of the second conductor are tuned so that each of the second and third closed loops is resonant at the same or essentially the same desired frequency as the first closed loop.

Additional claims disclose preferred embodiments of the invention.

It should be understood that the features of the invention allow a combination of any combination, without departing from the scope of the invention, as defined by the attached claims.

Additional details, features and advantages of the invention result from the following description of preferred and exemplary embodiments of the invention with reference to the drawings.

Brief Description of the Drawings

FIG. 1 shows a schematic side view of a magnetic resonance imaging (MRI) system.

FIG. 2 shows an example of an RF antenna in the form of a single-turn coil or single loop.

FIG. 3 shows an example of an RF antenna comprising two resonant coils or two loops.

FIG. 4 shows an embodiment of an RF antenna according to the invention; and

FIG. 5 shows an equivalent circuit of the RF antenna according to FIG. four.

Detailed Description of Embodiments

Figure 1 shows the main components of an MRI system that can be provided by an RF transmitting and / or receiving antenna, according to the invention, for transmitting RF excitation pulses and / or for receiving MR / RF relaxation pulses. Figure 1 shows, for example, a vertical (open) MRI system having an examination area 10 between the upper and lower ends of an X-ray device with a C-type frame. However, the RF transmitting and / or receiving antenna according to the invention can also be used in a horizontal or axial MRI system, which comprises a horizontally extending tubular or cylindrical examination volume, as is well known.

The upper part and the lower part of the examination zone 10 are provided with corresponding magnetic systems 20, 30 for generating a substantially uniform main magnetic field (field B ₀ ) for ordering the orientation of the nuclear spins in the object being examined, and the magnetic flux density (magnetic induction) it can be an order of magnitude from some fractions of a tesla to several tesla. The main magnetic field penetrates substantially through the patient P in a direction perpendicular to the longitudinal axis of the patient P (i.e., in the X direction).

Typically, a planar or at least approximately planar RF transmit antenna 40 (in particular in the form of an RF surface resonator) serves to generate RF transmit excitation pulses (field B ₁ ) at the MR frequencies, said RF transmit antenna 40 being placed at or on at least one of the magnetic systems 20, 30. A planar or at least approximately planar MR / RF receive antenna (50) is used to receive successive NMR relaxation signals. This antenna can also be provided in the form of an RF surface resonator that is mounted in or on at least one of the magnetic systems 20, 30. At least one common RF / MR antenna, in particular an RF surface resonator, can also be used for RF transmission, and for RF reception, if it is suitably switched between transmission and reception, or two RF antennas 40, 50, which both can serve for alternating transmission of RF pulses and receiving RF signals together.

At least one of these RF transmitting and / or receiving antennas 40, 50 may be provided in the form of an RF transmitting and / or receiving antenna, respectively, according to the invention.

In addition, for the spatial selection or selection of the slice and spatial coding of the received RF relaxation signals emanating from the nuclei (localization of excited states), a plurality of gradient coils of the magnetic field 70, 80 are also provided, by means of which three gradient magnetic fields are generated that extend in the direction of the axis X. The first gradient magnetic field varies substantially linearly in the direction of the X axis, while the second gradient magnetic field changes substantially linearly in the direction o and Y, and the third gradient magnetic field varies essentially linearly in the direction of axis Z.

Finally, additional electrical devices or accessories are provided for the inspection. A device such as, for example, an RF receiving antenna in the form of an RF / MR surface coil 60, which is used additionally or, as an option, to a permanently integrated planar RF receiving antenna 50 (body coil, patch coil) and which is located directly on patient P or the area to be examined. Such an RF / MR surface coil 60 may be designed as a flexible lining, or sleeve, or cell, and may comprise or be provided in the form of an RF transmitting and / or receiving antenna according to the invention.

The above and the following principles of the invention and discussion are also applicable in the case of an axial or horizontal MRI system in which a patient or other object of examination is directed axially through a cylindrical or tubular examination space 10. The shapes and dimensions of the magnets and RF transmitting and / or receiving antennas are adapted to the shape of a cylindrical or tubular inspection space in a known manner.

Additionally, the RF antenna according to the invention is preferably used as an RF receiving antenna for receiving only MR relaxation signals. However, the principles of the invention are also applicable to the RF transmitting antenna for transmitting the RF excitation pulses and to the RF antenna, which is provided both for transmitting the RF excitation pulses and for receiving MR relaxation signals. All of these RF antennas can be used both in a vertical (open) RTM system and in a horizontal or axial RTM system. To cover all of these possible options, the following typically relates to an RF transmit and / or receive antenna.

Typically, the above-described RF transmitting and / or receiving antennas may each have one or more conductor structures (i.e., antenna elements), each of which is resonant for transmitting and / or receiving RF signals, independently of other such conductor structures or segments of the RF antenna , and each of which is provided with its own electronic RF transmitting and / or receiving unit, like RF signal generators, RF power amplifiers and / or RF receivers, as is well known, so that many independently operating RF transmitting and / or mnyh channels.

Such multichannel systems are usually used to improve the parallel visualization capabilities of the entire RF transmitting and / or receiving antenna and to some extent improve the signal-to-noise (s / w) ratio of the generated MR images, in particular, in the immediate vicinity of the conductors of the RF transmitting and / or receiving antenna.

However, especially in the case of an RF surface coil, the problem with such multi-element antennas may be that access through an RF antenna (which is sometimes required for some examinations) is limited or difficult. FIG. 2 and 3 show this problem for a typical very simple structure of an RF transmitting and / or receiving antenna in the form of a (surface) coil structure. FIG. 2 shows a single-element RF antenna having one loop or coil 1, access through which is relatively simple due to the large opening surrounded by coil 1. Compared to this antenna, the two-element RF antenna according to FIG. 3 has almost the same size W as the single-element RF antenna according to FIG. 2, however, it comprises a first coil 2 and a second coil 3 to allow parallel imaging, both coils having a common overlapping conductor structure 23, which greatly restricts access through the RF antenna.

The requirements for the RF transmitting and / or receiving antenna, with respect to both the large aperture and the ability of parallel imaging, are different for different types of examinations and / or different types of objects to be examined. Therefore, the appropriate RF transmitting and / or receiving antenna should be selected and possibly placed in the RTM system prior to such an examination. This, of course, is considered time-consuming, especially in the case when the requirements change during the examination of the same object. This may be the case, for example, when the first diagnostic imaging should be done, for which image quality and parallel imaging properties are more important than access to the object, and then surgical devices like a catheter, or biopsy needles, or another invasive device should be introduced at the site to examine a local area or area for which sufficiently large free access to the site is required, in particular if the RF transmitting and / or receiving antenna is a surface coil a neck, which is located directly on the area to be examined.

Another example of such changing requirements is when, after diagnostic MR imaging, high-performance focused ultrasound (HIFU) therapy should be performed, and the ultrasound should be transmitted through the RF surface coil to the object. In order to avoid any disturbance of the ultrasonic field, no conductor structures can continue through this region and, therefore, through the opening of the RF antenna.

According to the invention, this problem is overcome by providing an RF transmitting and / or receiving antenna, which, on the one hand, has the ability of parallel imaging and, on the other hand, can be easily provided with an opening that provides easy access through the RF antenna, without any significant changes in the resonant properties of the RF antenna.

Alternatively, according to the invention, a single-channel RF transmitting and / or receiving antenna having a large opening for easy access through the RF antenna is provided so that the parallel imaging ability can be easily obtained without reducing the region of interest of the entire RF antenna and without significant change resonant properties of an RF antenna compared to a single-element RF antenna.

One of the main ideas of the invention is to provide an RF transmitting and / or receiving antenna containing at least one structure of the first conductor, in particular, arranged in the form of one or more resonant coils or circuits, where the structure of the first conductor forms a first closed loop (i.e. includes an opening), which has specially defined dimensions such as to provide direct access to the object of examination, or areas of interest of such an object of interest, at least one structure being provided cheers of the second conductor, which can be electromagnetically connected or connected to the structure of the first conductor so that it divides the first closed loop into at least a second and third closed loop (i.e., it passes through the opening of the structure of the first conductor), and the structure of the second conductor is tuned so that each of the second and third closed loops is resonant at the same or essentially the same desired frequency as the first closed loop (i.e. without the structure of the second conductor), thus providing the ability to parallel visualize the RF transmitting and / or receiving antenna.

Such an RF transmit and / or receive antenna is schematically shown in the exemplary embodiment of FIG. 4. It contains the structure of the first conductor in the form of two first conductors 4, 5, which together form a first closed loop in the form of a loop or antenna coil, which is essentially circular or, as shown in FIG. 4, substantially rectangular in shape surrounding the aforementioned opening. Additionally, the RF antenna contains the structure of the second conductor in the form of one second conductor 6 passing through the opening and electromagnetically coupled or connected to the first two conductors 4, 5, thereby forming a second and third closed loop inside the first closed loop. This connection is provided so that the second conductor 6 can be removed from the first conductors 4, 5 and reconnected to these conductors 4, 5 in a simple manner. Additionally, each of the first conductors 4, 5 is provided with a first series-connected capacitor CI, and the second conductor 6 is provided with a second series-connected capacitor C2.

FIG. 5 shows an equivalent circuit of the RF antenna according to FIG. 4. Each of the two first conductors 4, 5 is provided with a series connection of the first capacitor CI and the first inductance LI, and the second conductor 6 is provided with a series connection of the second capacitor C2 and the second inductance L2. Additionally, this drawing shows the mutual induction M between the two first conductors 4, 5 and the mutual induction Ml2 between each first conductor 4, 5 and the second conductor 6. Additionally, it should be assumed that the electromagnetic energy that is received and / or transmitted via an RF antenna is transmitted from and / or to the RF antenna by means of a preamplifier or generator, respectively, in parallel (or as part) with both first capacitors CI.

Through this RF antenna, the first single current mode I0 can be generated by the first two conductors 4, 5, forming the first closed loop, if the second conductor 6 is removed. Additionally, if the second conductor 6 is connected to the first conductors 4, 5, a second current mode I1 appears in the second closed loop, and a third current mode I2 appears in the third closed loop, the second and third current modes I1, I2 being equal in this example.

The value of the second capacitor C2 in the second conductor 6 is selected for decoupling the two second current modes I1, I2:

where the frequency ω of these current modes I1, I2 is equal to

Frequency ω is the required resonant frequency of the RF antenna and is selected to use the RF antenna in an MRI system equal to the MR frequency by appropriate selection of the values of the first capacitors CI.

In this case, if the second conductor 6 is removed, the aforementioned first single current mode I0 has the same resonant frequency as the second and third current modes I1, I2, which occur if the second conductor 6 is connected to the first conductors 4, 5.

Therefore, the RF antenna can be used as a two-element or two-channel antenna (for example, for parallel imaging) or as a single-element or single-channel antenna by removing the second conductor 6 (if you need a large opening, for example, to gain access to the object of examination), both on the same and the same resonant frequency.

If the second conductor 6 is removed, both preamplifiers or generators parallel to each of the first capacitors CI observe the same resonant structure of conductor 4, 5. In the case of receiving signals, the information of one of the two preamplifiers can either be canceled or stored depending on the needs and requirements signal recovery, given the correlation and, in particular, the noise of the received signals.

As mentioned above, the two-element coil shown in FIGS. 4 and 5 is only an example. Of course, the RF transmitting and / or receiving antenna may be provided with a first conductor structure having more than two elements, i.e. having more than two first conductors 4, 5, and / or more than one removable structure of the second conductor (or one structure of the second conductor, which contains more than one removable conductor), which can be removed without significantly affecting the resonance properties of the RF antenna.

Therefore, when linking the structure of the second conductor with the structure of the first conductor, more than the second and third closed loops can be provided, for example three or five, etc. closed loops to get great parallel visualization capabilities.

Additionally, the dimensions or dimensions of the closed loops that are provided by linking the structure of the second conductor with the structure of the first conductor may all be the same or different from each other.

Additionally, if more than one second conductor 6 or second conductor structure is provided, they can be configured such that, according to requirements, one or more or all of these second conductors or conductor structures can be removed without significantly affecting the resonance properties of the RF antenna.

Additionally, the second conductor 6 may have a different shape than a straight shape, for example a curved shape.

According to another embodiment of the invention, at least one of the first conductors 4, 5 or structures of the conductor can be made customizable in order to compensate for possible frequency deviation when the removable (second) conductor 6 is removed.

In another embodiment, the detachable conductor structure may be combined with a decoupling amplifier to increase coupling with or without the detachable conductor structure, as described in "The NMR Phased Array", PB Roemer, WA Edelstein, C. E. Hayes, SP Souza , O. M. Mueller, in Magnetic Resonance in Medicine, Vol. 16, Is. 2, p. 192-225 (1990).

Finally, the removable structure of the second conductor can be made for single use. In particular, if the structure of the second conductor needs to be sterilized, it is preferable to sell it in a sterile bag as a disposable unit. Such use may have the advantage of performing surgical MRI procedures. The user can attach or remove the sterile disposable structure of the second conductor anywhere in the intervention.

Finally, the above principles of the RF transmitting and / or receiving antenna according to the invention can be applied to a planar coil of the whole body and a tubular coil of the whole body, which both can be fixedly mounted in the vertical or axial system of the MRI, respectively, and can be applied to the surface coil , like a flexible lining that fits directly onto the area to be examined, or “cell” coils, etc.

Although the invention is illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary, and not restrictive, while the invention is not limited to the disclosed embodiments. Changes to the above-described embodiments of the invention are possible, for example, in relation to other shapes and dimensions of the first conductors 4, 5 and the second conductor 6, as well as in relation to their number and location relative to each other, without departing from the basic principle of the invention as defined by the claims.

Modifications of the disclosed embodiments may be understood and practiced by those skilled in the art in practicing the claimed invention based on an analysis of the drawings, the present disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the use of the singular does not exclude the plural. A single processor or other unit may fulfill the functions of several elements set forth in the claims. The fact that some features are set forth in mutually different dependent claims does not mean that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims (9)

1. A radio frequency transmitting and / or receiving antenna comprising a structure (4, 5) of a first conductor, forming a first closed loop that is resonant at a desired RF frequency, and at least one structure (6) of a second conductor, which may be associated with the structure (4, 5) of the first conductor, so that the first closed loop is divided by at least one structure (6) of the second conductor into at least a second and third closed loop, and at least one structure (6) of the second conductor is configured so that every and h of the second and third closed loops is resonant at the same or essentially the same desired frequency as the first closed loop, and in which at least one structure (6) of the second conductor can be connected with the possibility of removal with the structure (4, 5) first conductor. 2. The radio frequency transmitting and / or receiving antenna according to claim 1, wherein at least one structure (6) of the second conductor is tuned by means of a capacitance (C2), which is connected in series to the structure (6) of the second conductor. 3. The radio frequency transmitting and / or receiving antenna according to claim 1, wherein the structure of the second conductor is provided in the form of a single conductor (6). 4. The radio frequency transmitting and / or receiving antenna according to claim 1, wherein the structure (4, 5) of the first conductor provides a first closed loop in the form of at least one loop or coil. 5. The radio frequency transmitting and / or receiving antenna according to claim 4, wherein one second conductor of structure (6) is provided, which divides the first closed loop into second and third closed loops having substantially the same dimensions. 6. The radio frequency transmitting and / or receiving antenna according to claim 1, which is provided in the form of a surface coil, or coil type cells, or chest coil for use in magnetic resonance imaging (MRI). 7. The radio frequency transmitting and / or receiving antenna according to claim 1, which is provided in the form of a radio frequency or magnetoresistive (RF / MR) surface coil for use in therapy with highly efficient focused ultrasound. 8. The radio frequency transmitting and / or receiving antenna according to claim 1, which is provided in the form of a coil for the whole body for use in a vertical or axial RTM system. 9. The device magnetic resonance imaging (MRI), containing a coil for the whole body according to p. 8.

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