MRI RF surface coil with reduced sensitivity in proximity of conductors

The invention relates to a radio-frequent (RF) coil system (17, 17′) for use in a magnetic resonance imaging (MRI) system. The RF coil system comprises at least one main coil (35) for transmitting an RF magnetic field (B1) into and/or receiving an RF magnetic field (B1′) from an examination volume (3) of the MRI system. The main coil has a main coil axis (37), which is or is to be oriented parallel to a main magnetic field (B0) in the examination volume, and at least one electrical conductor (39, 41) which extends mainly parallel to the main coil axis. According to the invention, the RF coil system comprises at least two electrical auxiliary coils (51, 53, 55, 57) which are assigned to said conductor of the main coil. The auxiliary coils are arranged on opposite sides of said conductor of the main coil. Each auxiliary coil has a coil axis (59, 61, 63, 65) which extends substantially parallel to the main coil axis at a distance from the conductor of the main coil to which the respective auxiliary coil is assigned, said distance being small relative to a main dimension (L) of the main coil. The auxiliary coils constitute passive electrical coils in which electrical currents are generated under the influence of an RF magnetic field (B11, B11′) present at the location of the auxiliary coils. The RF magnetic field (B12, B34) generated by the auxiliary coils as a result of said currents in the auxiliary coils suppresses said RF magnetic field present at the location of the auxiliary coils. Thus, the auxiliary coils provide a sensitivity reducing effect of the RF coil system in local regions (47, 49) which are at relatively small distances from the conductor of the main coil. For regions at a distance from the conductor of the main coil comparable to the main dimension of the main coil, said sensitivity reducing effect is negligible.

The invention relates to a radio-frequent (RF) coil system for use in a magnetic resonance imaging (MRI) system, comprising at least one electrical main coil for transmitting an RF magnetic field into an examination volume of the MRI system and/or receiving an RF magnetic field from the examination volume, said main coil having a main coil axis which is to be oriented substantially parallel to a main magnetic field of the MRI system in the examination volume, said main coil having at least one electrical conductor extending mainly parallel to the main coil axis.

The invention further relates to a magnetic resonance imaging (MRI) system comprising an examination volume, a main magnet system for generating a main magnetic field in the examination volume, a gradient magnet system for generating gradients of the main magnetic field, and an RF coil system for transmitting an RF magnetic field into the examination volume and/or receiving an RF magnetic field from the examination volume.

A radio-frequent (RF) coil system and a magnetic resonance imaging (MRI) system of the kinds mentioned in the opening paragraphs are generally known. The known MRI system is used to make images of the entrails of a patient's body by means of a nuclear spin resonance method. The main magnet system of the MRI system comprises a number of superconducting electrical coils for generating a relatively strong and uniform main magnetic field within a central region of the examination volume. The gradient magnet system comprises a number of electrical coils for generating gradients of the main magnetic field in three orthogonal directions. The known MRI system further comprises a first RF coil system for transmitting an RF magnetic field into the examination volume and a second RF coil system for receiving an RF magnetic field from the examination volume. It is also known to use a single RF coil system for transmission and receiving purposes. An image of the patient's body is constructed by observing nuclear spin resonance effects in a large number of positions in the patient's body, which are selected by altering said gradients. For each selected position, the first RF coil system transmits an RF magnetic field into the examination volume in order to generate nuclear spin resonance effects in the selected position. Subsequently, the second RF coil system receives an RF magnetic field which is generated in the selected position as a result of the nuclear spin resonance effects.

A disadvantage of the known RF coil system and of the known MRI system comprising the known RF coil system is that the RF coil system has a relatively high local sensitivity for regions of the patient's body which are relatively close to the individual electrical conductors of the RF coil system oriented mainly parallel to the main magnetic field, i.e. for local regions at a distance from said conductors which is small relative to a main dimension of the main coil In the case of a transmitting RF coil system, said high local sensitivity is caused by the fact that, in positions relatively close to the conductors, the magnetic field generated by the RF coil system is relatively strong. In the case of a receiving RF coil system, said high local sensitivity is caused by the fact that, at the location of the conductors, the magnetic field generated by the nuclei in positions relatively close to the conductors is relatively strong. In particular, said high local sensitivity is present in the case of a so-called surface coil system, i.e. an RF coil system which is placed directly on the patient's body to achieve an improved signal-to-noise ratio. As a result of said high local sensitivity, known MRI systems using surface coil systems are highly susceptible to movement artifacts in the generated image, which are caused by relatively strong signals generated at the surface of the patient, for example in the subcutaneous fat layers on the patient's chest or belly.

An object of the invention is to provide a radio-frequent (RF) coil system of the kind mentioned in the opening paragraphs and a magnetic resonance imaging (MRI) system of the kind mentioned in the opening paragraphs, in which said local high sensitivity of the RF coil system is reduced without substantially affecting the sensitivity of the RF coil system for positions which are not relatively close to the conductors of the RF coil system.

In order to achieve said object, an RF coil system in accordance with the invention is characterized in that the RF coil system comprises at least two electrical auxiliary coils assigned to said conductor of the main coil and arranged on opposite sides of said conductor of the main coil, each auxiliary coil having a coil axis extending substantially parallel to the main coil axis at a distance from said conductor of the main coil, wherein said distance is small relative to a main dimension of the main coil.

In order to achieve said object, an MRI system in accordance with the invention is characterized in that the RF coil system used therein is an RF coil system in accordance with the invention.

The auxiliary coils assigned to the conductor of the main coil are passive electrical coils in which electrical currents are induced under the influence of an alternating magnetic field present at the location of the auxiliary coils. In a transmitting mode of the RF coil system according to the invention, electrical currents are induced in the auxiliary coils predominantly under the influence of the RF magnetic field generated by the conductor of the main coil to which the auxiliary coils are assigned. In this mode, the electrical currents in the auxiliary coils cause an RF magnetic field of the auxiliary coils which opposes the RF magnetic field generated by said conductor of the main coil. Since the auxiliary coils are present on opposite sides of said conductor of the main coil and the coil axes of the auxiliary coils are at relatively small distances from said conductor, the RF magnetic field of the auxiliary coils is relatively small in positions at distances from said conductor of the main coil comparable to the main dimension of the main coil. As a result, said opposing effect of the RF magnetic field of the auxiliary coils is predominantly present in a local region at a distance from said conductor of the main coil which is small relative to the main dimension of the main coil, i.e. in a local region where the main coil would have a relatively high local sensitivity without the presence of the auxiliary coils. In positions at distances from said conductor of the main coil comparable to the main dimension of the main coil, the RF magnetic field of said conductor of the main coil is hardly affected by the RF magnetic field of the auxiliary coils.

In a receiving mode of the RF coil system according to the invention, electrical currents are induced in the auxiliary coils under the influence of the RF magnetic field generated by spinning nuclei. In this mode, the electrical currents in the auxiliary coils cause an RF magnetic field which opposes the RF magnetic field generated by the spinning nuclei. Since the auxiliary coils are present on opposite sides of the conductor of the main coil, to which the auxiliary coils are assigned, and the coil axes of the auxiliary coils are at relatively small distances from said conductor, the auxiliary coils are predominantly sensitive to the RF magnetic field which is generated by spinning nuclei present in positions at relatively small distances from said conductor of the main coil. As a result, said opposing effect of the RF magnetic field of the auxiliary coils is predominantly present in a local region at a distance from said conductor of the main coil which is small relative to the main dimension of the main coil, i.e. in a local region where the main coil would have a relatively high local sensitivity without the presence of the auxiliary coils. The spinning nuclei in positions at distances from said conductor of the main coil comparable to the main dimension of the main coil hardly generate electrical currents in the auxiliary coils, so that the sensitivity of said conductor of the main coil for these positions is hardly affected by the auxiliary coils.

A particular embodiment of an RF coil system according to the invention is characterized in that the main coil has a loop comprising a first and a second electrical conductor extending mainly parallel to the main coil axis, the RF coil system having a first and a second auxiliary coil assigned to said first conductor and arranged on opposite sides of said first conductor and a third and a fourth auxiliary coil assigned to said second conductor and arranged on opposite sides of said second conductor, each auxiliary coil having a coil axis extending substantially parallel to the main coil axis at a distance from the respective conductor of the main coil to which the respective auxiliary coil is assigned, wherein said distance is small relative to said main dimension. In this particular embodiment, the main coil is for example a square or rectangular coil. The local high sensitivity of the main coil for the region, which is relatively close to said first conductor of the main coil, is effectively reduced by means of said first and said second auxiliary coil, while the local high sensitivity of the main coil for the region, which is relatively close to said second conductor of the main coil, is effectively reduced by means of said third and said fourth auxiliary coil.

A particular embodiment of an RF coil system according to the invention is characterized in that the two auxiliary coils are connected in series in an electrical anti-phase mode. In this particular embodiment, the electrical anti-phase mode of the two auxiliary coils is for example achieved by means of a so-called butterfly-loop connection or symbol-8 shaped loop connection between the two auxiliary coils. As a result of the fact that the two auxiliary coils are connected in series, the electrical currents in the two auxiliary coils are equal. An advantage of this embodiment is that, in a design phase of the RF coil system, the magnetic field of the two auxiliary coils can be optimized by optimizing the dimensions and the surface areas of the two auxiliary coils.

A particular embodiment of an RF coil system according to the invention is characterized in that the conductor of the main coil extends substantially parallel to the main coil axis, and each auxiliary coil comprises two conductors extending substantially parallel to the main coil axis, wherein the distance between the coil axis of each auxiliary coil and the conductor of the main coil is larger than 0,5*B and smaller than 1,5*B, B being a distance between the two conductors of the respective auxiliary coil. In this embodiment, said distance between the coil axis of each auxiliary coil and the conductor of the main coil, to which the auxiliary coils are assigned, is sufficiently small to achieve the desired local sensitivity reducing effect of the auxiliary coils.

A further embodiment of an RF coil system according to the invention is characterized in that the first and the second conductor of the main coil extend substantially parallel to the main coil axis, and each auxiliary coil comprises two conductors extending substantially parallel to the main coil axis, wherein a distance B between the two conductors of each auxiliary coil is larger than 0,06*L and smaller than 0,25*L, L being a distance between the first and the second conductor of the main coil. In this embodiment, said distance B between the two conductors of each auxiliary coil is sufficiently small relative to a main dimension of the main coil, i.e. relative to the distance L between the first and the second conductor of the main coil, to achieve the desired local sensitivity reducing effect of the auxiliary coils.

A further embodiment of an RF coil system according to the invention is characterized in that the RF coil system comprises a skin contact surface, the first and the second conductor of the main coil extending in an imaginary plane at a distance D from said skin contact surface, and each auxiliary coil extending in an imaginary plane at a distance H from said skin contact surface, wherein 0<H<3*D. In this embodiment, a distance H-D is present between the imaginary plane, in which the auxiliary coils extend, and the imaginary plane in which the main coil extends. With the condition 0<H<3*D, the distance between the conductors of the auxiliary coils and the conductors of the main coil is sufficiently small relative to a main dimension of the main coil to achieve the desired local sensitivity reducing effect of the auxiliary coils.

A yet further embodiment of an RF coil system according to the invention is characterized in that D<H<1,5*D. In this embodiment, an optimum dimension is achieved of the regions, relatively close to the first and the second conductor of the main coil, for which the local high sensitivity of the main coil is effectively reduced by means of the auxiliary coils.

The MRI system in accordance with the invention shown inFIG. 1is an MRI system of the so-called closed cylindrical type comprising a mainly cylindrical housing, which is not shown inFIG. 1and extends in a Z-direction indicated inFIG. 1. The MRI system comprises a main magnet system1, which is enclosed by said housing and which surrounds an examination volume3in which a patient to be examined can be positioned. The main magnet system1comprises a number of superconducting electrical coils5for generating a relatively strong and substantially uniform main magnetic field B0parallel to the Z-direction in a central portion of the examination volume3. The MRI system comprises an electrical power supply7for the superconducting coils5and a cryogenic cooling device9with cooling channels11for cooling the superconducting coils5. The MRI system further comprises a gradient magnet system13, which is arranged between the main magnet system1and the examination volume3and comprises a number of electrical coils15for generating gradients of the main magnetic field B0in three orthogonal directions X, Y, and Z. The MRI system further comprises a radio-frequent (RF) coil system17,17′ in accordance with the invention, which is arranged between the gradient magnet system13and the examination volume3for transmitting an RF magnetic field into the examination volume3and for receiving an RF magnetic field from the examination volume3.

The MRI system is used to make images of the entrails of a patient's body by means of a nuclear spin resonance method. An image of the patient's body is constructed by successively observing nuclear spin resonance effects in a large number of positions in a portion of the patient's body which is present in the uniform main magnetic field B0. The positions in the patient's body are successively selected by altering the gradients of the main magnetic field B0by means of suitable currents in the electrical coils15of the gradient magnet system13. For this purpose, the MRI system comprises a control unit19, which controls the currents in the coils15of the gradient magnet system13in accordance with a predetermined sequence, and a power amplifier21for amplifying the control signals supplied by the control unit19to the coils15of the gradient magnet system13. For each selected position in the patient's body, the RF coil system17,17′ transmits an RF magnetic field with a predetermined frequency and pulse time into the examination volume3in order to generate nuclear spin resonance effects in the selected position. Subsequently, the RF coil system17,17′ is used to receive an RF magnetic field which is generated by the spinning nuclei in the selected position in connection with the nuclear spin resonance effects. For this purpose, the control unit19also controls the RF coil system17,17′ in accordance with a predetermined sequence which is linked up with the sequence according to which the gradient magnet system13is controlled. The control unit19supplies a control signal to an RF transmitting and receiving device23, which feeds the RF coil system17,17′ with a suitable current in order to generate the required RF magnetic field in the examination volume3. The RF transmitting and receiving device23also receives a current from the RF coil system17,17′ generated in the RF coil system17,17′ by the RF magnetic field from the examination volume3. The RF transmitting and receiving device23supplies a signal to a processor25of the MRI system, which is suitable for converting the signals received from the RF transmitting and receiving device23into an image. For this purpose, the processor25comprises a signal amplifier27, a demodulator29, an image reconstruction device31, and a display33.

The nuclear spin resonance effects to be generated by means of the RF coil system17,17′ comprise a precession of the spin axes of the spinning nuclei in a predetermined direction of precession along an imaginary conical surface having a central axis parallel to the Z-direction of the main magnetic field B0. Consequently, the component B1, of the RF magnetic field generated by the RF coil system17,17′, which is directed perpendicularly to the Z-direction (hereinafter called the RF magnetic field B1), is predominantly effective in generating the nuclear spin resonance effects. The component B1, is predominantly generated by conducting portions of the RF coil system17,17′ which extend parallel to the Z-direction. The RF magnetic field B1′, which is generated by the spinning nuclei in connection with the nuclear spin resonance effects and which is to be received by the RF coil system17,17′, is also directed perpendicularly to the Z-direction. Consequently, the conducting portions of the RF coil system17,17′, which extend parallel to the Z-direction, are predominantly sensitive to the RF magnetic field B1′.FIGS. 2aand2bdiagrammatically show a portion17of the RF coil system17,17′ comprising an electrical main coil35. The main coil35has a coil axis37which is or is to be oriented substantially parallel to the Z-direction of the main magnetic field B0. The main coil35has a loop comprising a first electrical conductor39and a second electrical conductor41, which extend substantially parallel to the coil axis37and accordingly form the predominantly effective and sensitive portions of the main coil35. The loop further comprises further electrical conductors43and45, which extend substantially perpendicularly to the coil axis37and accordingly form portions of the main coil35having only a minor effectiveness and sensitivity. In the transmitting mode, an alternating electrical current in said loop results in an RF magnetic field B1, as shown inFIG. 2b, in a selected position P in the patient's body at a distance DPfrom the loop. In the receiving mode, an RF magnetic field B1′, shown inFIG. 2band generated at the location of the loop in connection with nuclear magnetic resonance effects in said selected position P, results in an alternating electrical current induced in said loop.

In the transmitting mode, the strength of the RF magnetic field B1, generated by the first and the second conductor39,41in the selected position P strongly depends on the distance between the selected position P and the conductors39,41. In local regions47,49in the patient's body present at a distance from the conductors39and41which is small relative to a main dimension of the main coil35, e.g. the distance L between the two conductors39,41, the generated RF magnetic field B1, is relatively strong, so that the main coil35has a local high effectiveness for said local regions47,49. Similarly, in the receiving mode, a relatively strong current is induced in the first and the second conductor39,41under the influence of an RF magnetic field B1′ generated in connection with nuclear magnetic resonance effects in positions which are within said local regions47,49. As a result, in the receiving mode, the main coil35has a local high sensitivity for said local regions47,49. In particular, said local high sensitivity becomes manifest in embodiments wherein the RF coil system17,17′ is used as a so-called surface coil In such embodiments, the RF coil system17,17′ is placed directly on the patient's body to achieve an improved effectiveness and sensitivity and an improved signal-to-noise ratio for regions deep in the patient's body. However, it appeared that in such embodiments the local high sensitivity for the local regions47,49present at relatively small distances from the conductors39,41causes the generated images to be highly susceptible to movement artifacts, which are caused by relatively high signals generated at the surface of the patient's body, for example in the subcutaneous fat layers on the patient's chest or belly.

In order to reduce the above described local high sensitivity of the main coil35for the local regions47,49, present at relatively small distances from the first and the second conductor39,41, without affecting the effectiveness and the sensitivity of the main coil35for selected positions outside said local regions47,49, i.e. for positions at a distance from said conductors39,41in the order of the main dimension L of the main coil35, the portion17of the RF coil system17,17′ in accordance with the invention comprises a first electrical auxiliary coil51and a second electrical auxiliary coil53, which are assigned to and associated with the first conductor39of the main coil35, and a third electrical auxiliary coil55and a fourth electrical auxiliary coil57, which are assigned to and associated with the second conductor41of the main coil35, as shown inFIG. 2a. The first and the second auxiliary coil51,53are arranged on opposite sides of the first conductor39of the main coil35and each have a coil axis59,61extending parallel to the main coil axis37at a distance E from said first conductor39, said distance E being small relative to the main dimension L of the main coil35. The third and the fourth auxiliary coil55,57are arranged on opposite sides of the second conductor41of the main coil35and each have a coil axis63,65extending substantially parallel to the main coil axis37at said distance E from said second conductor41. Each auxiliary coil51,53,55,57has two electrical conductors67,69, which extend parallel to the main coil axis37over substantially the complete length of the respective conductor39,41to which the respective auxiliary coil51,53,55,57is assigned, wherein a distance B is present between said two conductors67,69of each auxiliary coil51,53,55,57which is small relative to the main dimension L of the main coil35. In the embodiment shown inFIGS. 2aand2b, the first and the second auxiliary coil51,53are electrically connected in series and the third and the fourth auxiliary coil55,57are also electrically connected in series. In this embodiment it is necessary that the first and the second auxiliary coil51,53are electrically connected in anti-phase mode, which is achieved by means of a so-called butterfly-loop connection or symbol-8 shaped loop connection71between the first and the second auxiliary coils51,53, as shown inFIG. 2a. As a result, the electrical currents in the first and the second auxiliary coils51,53have mutually opposite directions. Likewise, a butterfly-loop connection73is present between the third and the fourth auxiliary coil55,57.

In the following, the technical effect of the presence of the auxiliary coils51,53,55,57will be explained. The auxiliary coils51,53,55,57constitute passive electrical coils, in which electrical currents are induced as a result of a variation of the magnetic field present at the location of the auxiliary coils51,53,55,57. For the transmitting mode of the RF coil system17,17′, the technical effect will be explained with reference to the first and the second auxiliary coil51,53, and the skilled person will understand that the technical effect of the third and the fourth auxiliary coil55,57will be similar. In the transmitting mode of the RF coil system17,17′, the electrical currents in the first and the second auxiliary coil51,53are predominantly induced under the influence of variations of the RF magnetic field B11generated by the first conductor39of the main coil35at the location of the first and the second auxiliary coil51,53. As schematically shown inFIG. 2b, said RF magnetic field B11has mainly opposite directions at the location of the first and the second auxiliary coil51,53, so that the electrical currents induced in the first and the second auxiliary coil51,53also have opposite directions, which are allowed as a result of the butterfly-loop connection71. The electrical currents induced in the first and the second auxiliary coil51,53will generate an RF magnetic field B12, schematically shown inFIG. 2b, which is opposite to the RF magnetic field B11and consequently suppresses the RF magnetic field B11. Since the coil axes59,61and the conductors67,69of the first and the second auxiliary coil51,53are arranged at distances from the first conductor39of the main coil35which are small relative to the main dimension L of the main coil35, the RF magnetic field B12is relatively small in positions at distances from said first conductor39which are comparable to said main dimension L. Consequently, the first and the second auxiliary coil51,53predominantly oppose the RF magnetic field B11of the first conductor39in positions at distances from the first conductor39which are small relative to the main dimension L, such as the local region47in the patient's body close to the first conductor39for which the main coil35would have a relatively high local sensitivity without the presence of the first and the second auxiliary coil51,53. In positions at distances from the first conductor39comparable to the main dimension L, the RF magnetic field B1of the main coil35and the sensitivity of the main coil35are hardly affected by the presence of the first and the second auxiliary coil51,53.

For the receiving mode of the RF coil system17,17′, the technical effect of the auxiliary coils51,53,55,57will be explained with reference to the third and the fourth auxiliary coil55,57, and the skilled person will understand that the technical effect of the first and the second auxiliary coil51,53will be similar. In the receiving mode of the RF coil system17,17′, electrical currents in the third and the fourth auxiliary coil55,57are induced under the influence of variations of the RF magnetic field generated by the spinning nuclei.FIG. 2bschematically shows the RF magnetic field B11′ generated at the location of the third, and the fourth auxiliary coil55,57by the spinning nuclei in a position P′ in the local region49. Said RF magnetic field B11′ has mainly opposite directions at the location of the third and the fourth auxiliary coil55,57, so that the electrical currents induced in the third and the fourth auxiliary coil55,57also have opposite directions, which are allowed as a result of the butterfly-loop connection73. The electrical currents induced in the third and the fourth auxiliary coil55,57will generate an RF magnetic field B34, schematically shown inFIG. 2b, which is opposite to the RF magnetic field B11′ and consequently suppresses the RF magnetic field B11′. Since the coil axes63,65and the conductors67,69of the third and the fourth auxiliary coil55,57are arranged at distances from the second conductor41of the main coil35which are small relative to the main dimension L of the main coil35, the spinning nuclei in positions at distances from the second conductor41, which are comparable to the main dimension L, will generate mainly equally directed RF magnetic fields at the locations of the third and the fourth auxiliary coil55,57. As a result of the butterfly-loop connection73, the electrical currents in the third and the fourth auxiliary coil55,57induced by the latter RF magnetic fields will be relatively small or even zero, so that the latter RF magnetic fields are hardly or not suppressed by the presence of the third and the fourth auxiliary coil55,57. Consequently, the third and the fourth auxiliary coil55,57predominantly suppress the RF magnetic field generated by the spinning nuclei in positions at distances from the second conductor41which are small relative to the main dimension L, in particular by the spinning nuclei in the local region49for which the main coil35would have a relatively high local sensitivity without the presence of the third and the fourth auxiliary coil55,57. For positions at distances from the second conductor41comparable to the main dimension L, the sensitivity of the main coil35is hardly affected by the presence of the third and the fourth auxiliary coil55,57.

In the foregoing it is disclosed that, in order to obtain a reduction of the sensitivity of the main coil35which is limited to local regions at a distance from the first and the second conductor39,41which is small relative to the main dimension L of the main coil35, the auxiliary coils51and53should be arranged on opposite sides of the first conductor39, and the auxiliary coils55and57should be arranged on opposite sides of the second conductor41, while the coil axes59,61,63,65of the auxiliary coils51,53,55,57should be at a distance E from the respective conductor39,41of the main coil35which is small relative to said main dimension L. Preferably the distance B between the conductors67,69of each auxiliary coil51,53,55,57is also small relative to said main dimension L. It is noted that, in this respect, the expression “on opposite sides of” intends to indicate that the respective two auxiliary coils51,53and55,57must be arranged on opposite sides of an imaginary plane, which extends through the respective conductor39,41of the main coil35and is oriented perpendicularly to the imaginary plane in which the main coil35extends. It was found that, in the embodiment shown inFIGS. 2aand2b, a sufficient local sensitivity reducing effect of the auxiliary coils51,53,55,57is obtained if said distance E is smaller than 1,5*B and larger than 0,5*B. In order to obtain a sufficient local sensitivity reducing effect of the auxiliary coils51,53,55,57, the distance B between the conductors67,69of the auxiliary coils51,53,55,57should be smaller than 0,25*L and larger than 0,06*L. In an embodiment wherein the RF coil system17,17′ is used as a surface coil, an optimum range can also be defined for the distance between the auxiliary coils51,53,55,57and a skin contact surface of the RF coil system17,17′.FIG. 2bschematically shows the position of a skin contact surface75of the RF coil system17,17′, i.e. a surface via which the RF coil system17,17′ is in contact with the patient's skin during operation. As shown inFIG. 2b, a distance D is present between the skin contact surface75and an imaginary plane77in which the first and the second conductors39,41of the main coil35extend, and a distance H is present between the skin contact surface75and an imaginary plane79in which the auxiliary coils51,53,55,57extend. It was found that, if said distance H is within the range between 0 and 3*D, the distance between the conductors67,69of the auxiliary coils51,53,55,57and the conductors39,41of the main coil35is sufficiently small to achieve a sufficient local sensitivity reducing effect of the auxiliary coils51,53,55,57. Preferably, said distance H is larger than the distance D, i.e. the auxiliary coils51,53,55,57are arranged at a side of the main coil35which is remote from the skin contact surface75. An optimum dimension of the local regions47,49, for which the desired sensitivity reducing effect is achieved, is obtained if said distance H is between D and 1,5*D. It is noted that it is not essential that the respective two auxiliary coils51,53and55,57extend in a common imaginary plane, as is the case in the embodiment shown inFIGS. 2aand2b. The invention also encloses embodiments in which the two auxiliary coils assigned to a conductor of the main coil are arranged on opposite sides of said conductor and in different imaginary planes which each enclose an acute angle with the skin contact surface75.

In the embodiment shown inFIGS. 2aand2b, the auxiliary coils51,53and the auxiliary coils55,57are symmetrically arranged relative to the respective conductor39,41of the main coil35. It is noted, however, that the invention also encloses embodiments in which the auxiliary coils51,53and the auxiliary coils55,57are asymmetrically arranged relative to the respective conductor39,41. The invention encloses embodiments in which, for example, the coil axes59,61,63,65of the auxiliary coils51,53,55,57are arranged at mutually different distances from the respective conductor39,41, and embodiments in which the auxiliary coils51,53,55,57have mutually different shapes and/or dimensions. In embodiments wherein the auxiliary coils51,53,55,57have mutually different shapes and/or dimensions, the auxiliary coils preferably have substantially equal surface area's, but this is not essential for a proper function of the auxiliary coils51,53,55,57.

In the embodiment shown inFIGS. 2aand2b, the auxiliary coils51,53and the auxiliary coils55,57are connected in series in an electrical anti-phase mode by means of the butterfly-loop connections71,73. It is noted that said connection between the auxiliary coils51,53and between the auxiliary coils55,57is not an essential feature of the invention, and that the invention accordingly also encloses embodiments in which the two auxiliary coils51,53and the two auxiliary coils55,57each constitute a separate passive electrical loop. An advantage of connecting the two auxiliary coils51,53and the two auxiliary coils55,57in series and in anti-phase mode is that the electrical currents in the two auxiliary coils51,53and in the two auxiliary coils55,57are equal, so that, in a design phase of the RF coil system17,17′, the magnetic field of the auxiliary coils51,53,55,57can be optimized by optimizing the dimensions and the shape of the auxiliary coils51,53,55,57. The invention also encloses embodiments in which the auxiliary coils51,53,55,57are connected in series and in anti-phase mode by means of connections which are different from the butterfly-loop connections71,73. Referring toFIG. 2a, the invention for example encloses an embodiment in which the conductors69of the first and the second auxiliary coil51,53together constitute a single conductor which is common to the first and the second auxiliary coil51,53, and in which the conductors69of the third and the fourth auxiliary coil55,57together constitute a single conductor which is common to the third and the fourth auxiliary coil51,53.

It is noted that the invention also encloses RF coil systems which have a coil configuration which is different from the coil configuration of the RF coil system17,17′ shown inFIGS. 2aand2b. For example, the invention also encloses RF coil systems having more than one main coil. Particularly, an RF coil system in accordance with the invention may be provided with a number of main coils which are arranged in mutually transversely oriented planes so as to improve the efficiency of generating and/or the sensitivity of receiving a rotating RF magnetic field in the examination volume. In such an embodiment, the conductors of each main coil oriented parallel to the main magnetic field B0may be provided with separate auxiliary coils. It is further noted that, in order to be effective, an RF coil system in accordance with the invention should be provided with at least one conductor which is or is to be oriented mainly parallel to the main magnetic field B0. However, such a conductor does not need to be a straight conductor extending parallel to the main magnetic field B0, like the first and the second conductor39,41in the embodiment shown inFIGS. 2aand2b. The conductor may, for example, also be a curved conductor having a main or predominant orientation parallel to the main magnetic field B0. Accordingly, the expression “said main coil having at least one electrical conductor extending mainly parallel to the main coil axis” in the claims is to be interpreted in accordance herewith Accordingly, also the expression “the main coil has a loop comprising a first and a second electrical conductor extending mainly parallel to the main coil axis” in the claims is to be interpreted in the same manner.

The MRI system in accordance with the invention described before is an MRI system of the so-called closed cylindrical type. It is noted that the invention also encloses other types of MRI systems. An example is an MRI system of the so-called open type, in which the main magnet system and the gradient magnet system have been arranged in two separate housing portions at a distance from each other and in which the examination volume is an open volume present between said two housing portions.

The RF coil system17,17′ in accordance with the invention described before has both a transmitting and a receiving function. It is noted that the invention also encloses RF coil systems which only have a receiving function and RF coil systems which only have a transmitting function. In particular, RF coil systems in accordance with the invention, which are used as a surface coil, may only have a receiving function so as to co-operate with a further RF coil system, which is in a fixed position relative to the main magnet system of the MRI system and which has only a transmitting function or a combined transmitting and receiving function.