WO2014027261A1

WO2014027261A1 - Detuning circuit for mri local rf coils comprising ptc resistor

Info

Publication number: WO2014027261A1
Application number: PCT/IB2013/055821
Authority: WO
Inventors: Adrianus Petrus Maria VAN DEN BERG; Leo Johan Marie PIJPERS
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-08-13
Filing date: 2013-07-15
Publication date: 2014-02-20

Abstract

A de-tuning circuit (38) for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil (24) of an MR imaging system (10) having consecutive RF transmit phases and RF receive phases, the de-tuning circuit (38) comprising: an RF resonance circuit (40); at least one RF solid state switching member (44); a bias current generating circuit (42) provided for generating a DC bias current (I_b) through the at least one RF solid state switching member (44); at least one positive temperature coefficient (PTC) resistor (46) coupled in series with the at least one RF solid state switching member (44); and thermal bridging means that are arranged between the at least one RF solid state switching member (44) and the PTC resistor (46) and are specifically designed for transferring thermal heat from the at least one RF solid state switching member (44) to the PTC resistor (46); a method for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil (24) of an MR imaging system (10) comprising the steps of generating a DC bias current (I_b) through the RF solid state switching member (44) by the bias current generating circuit (42); adjusting the DC bias current (I_b) for application during a transmit phase to a value at which the resistance of the PTC resistor (46) exceeds a threshold value at a preselected switch-off temperature (T_SW); and transferring any heat that is dissipated by the RF solid state switching member (44) to the PTC resistor (46); and a magnetic resonance (MR) imaging system (10), comprising at least one of the disclosed embodiments of a de-tuning circuit (38) or combinations thereof for de-tuning the RF receive coil (24).

Description

DETUNING CIRCUIT FOR MRI LOCAL RF COILS

COMPRISING PTC RESISTOR

FIELD OF THE INVENTION

The invention pertains to a de-tuning circuit for de-tuning radio frequency (RF) receive coils of a magnetic resonance (MR) imaging system, a method for de-tuning RF receive coils of an MR imaging system, and an MR imaging system with a de-tuning circuit for de-tuning RF receive coils of the MR imaging system.

BACKGROUND OF THE INVENTION

In the art of magnetic resonance (MR) imaging systems comprising a main magnet for generating a substantially static magnetic field, a magnetic gradient coil system for generating gradient magnetic fields superimposed to the static magnetic field, an examination region provided to position a subject of interest within, at least one RF transmit coil that is provided for applying an RF magnetic field at the examination region during an RF transmit phase to excite nuclei within the subject of interest, and at least one RF receive coil that is provided for acquiring MR signals from the excited nuclei within the subject of interest during an RF receive phase, it is known to de-tune the RF receive coil during the RF transmit phase by a de-tuning circuit. This is done for protection of the RF receive coil, as the transmitted RF power exceeds the power of the received MR signals by several orders of magnitude. Examples of de-tuning circuits for application in MR imaging systems are described in documents US 8,013,609 B2 and US 6,747,452 Bl . Therein, a PIN diode is switched between a highly conductive state during a transmit phase for activating, and a state of low conductivity during a receive phase for de-activating, respectively, a circuit comprising inductors and/or capacitors that de-tunes an RF receive coil if activated.

In some positions in the examination region, however, the RF receive coil might couple to the RF transmit coil, for instance a body coil, which could result in excessive heating of the de-tuning circuit. For RF receive coil types that could be placed anywhere in the examination region, extra care has to be spent by an operator to avoid this. As a safety procedure conducted by an operator, this manual check is susceptible to human error, which might be a difficulty in an attempt to meet safety standards such as the IEC60601 safety requirements for temperature if applicable. It is therefore desirable to provide a de-tuning circuit with an improved safety that solves the above-mentioned problem of potential overheating.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a de-tuning circuit with an improved safety that solves the problem of potential overheating.

This object is achieved by a de-tuning circuit for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil of an MR imaging system having consecutive RF transmit phases and RF receive phases. The de-tuning circuit comprises - an RF resonance circuit including at least one de-tuning capacitor and at least one de-tuning inductor,

at least one RF solid state switching member coupled in series to one out of the at least one de-tuning capacitor and the at least one de-tuning inductor, and together with the one out of the at least one de-tuning capacitor and the at least one de-tuning inductor, coupled in parallel communication to the other out of the at least one de-tuning capacitor and the at least one de-tuning inductor for enabling or disabling the RF resonance circuit,

a bias current generating circuit provided for generating a DC bias current through the at least one RF solid state switching member, wherein an RF impedance of the at least one RF solid state switching member is controllable between a substantially electrically non-conducting state and a substantially electrically conducting state by variation of the bias current,

at least one positive temperature coefficient (PTC) resistor, wherein the at least one PTC resistor is coupled in series with the at least one RF solid state switching member, and

- thermal bridging means that are arranged between the at least one RF solid state switching member and the PTC resistor and that are specifically designed for transferring thermal heat from the at least one RF solid state switching member to the PTC resistor.

The phrase "solid state switching member", as used in this application, shall be understood particularly as an electronic switching device based on semiconductor technology. Examples of solid state switching members are PIN diodes or transistors.

Preferably, the thermal bridging means may include a thermal conductive paste arranged between the RF solid state switching member and the PTC resistor.

Alternatively or in addition, a common housing may be provided that encompasses the RF solid state switching member and the PTC resistor and provides thermal insulation with respect to the environment. The housing may preferably be made from any plastic material, and may further include a thermal insulation material arranged between the housing and the PTC resistor and the RF solid state switching member for a further improved thermal insulation.

The phrase "coupled in series" as applied to two electric or electronic components, as used in this application, shall be understood particularly as the components being connected along a single path, so that the same current flows through all of the components. The phrase shall in particular encompass an arrangement in that the two components are indirectly connected in series via a third component and in that the same current flows through all three components.

In a suitable implementation, a potential raise in temperature of the RF solid state switching member can be exploited in a combination with a temperature characteristics of a resistance of the PTC resistor to de-activate the RF receive coil, thereby protecting the de-tuning circuit, the RF receive coil as well as a subject under examination, usually a patient.

In a preferred embodiment, the at least one RF solid state switching member includes a PIN diode. At radio frequencies under consideration (42.6 MHz/Tesla for hydrogen 'Η), a PIN diode functions as an electric component whose RF impedance is an inverse function of a forward current flowing through it. Therefore, the RF impedance can be readily controlled between a substantially electrically non-conducting state and a

substantially electrically conducting state by variation of the bias current, and a cost-effective solution for the RF solid state switching member can be provided.

In another preferred embodiment, a resistance characteristic of the PTC resistor has a non-linear dependence from temperature with at least one inflection point in the temperature range between 0°C and 100°C. In case of heat being transferred from the RF solid state switching member to the PTC resistor, causing a rise of temperature of the PTC resistor, the non-linear temperature dependence with at least one inflection point in the temperature range between 0°C and 100°C can be used to achieve a strong effect of the rise of temperature on the resistance of the PTC resistor, which in turn can be easily exploited by way of electronic measurement techniques.

In a further aspect of the invention, the at least one PTC resistor is designed as a resettable polymeric PTC device. Such polymeric PTC devices are commercially available as a product and combine several advantages, like a specified trip current which characterizes a transition between a status of lower electrical resistance and a status of higher electrical resistance, a large ratio of the higher electrical resistance and the lower electrical resistance which can easily and robustly be exploited by way of electronic measurement techniques, and a reversible behavior between the status of lower electrical resistance and the status of higher electrical resistance induced by changes in temperature of the PTC resistor.

In another aspect of the invention, the bias current generating circuit comprises at least one RF current-limiting inductor. In case of RF power transmitted in the transmit phase coupling to the RF receive coil and the de-tuning circuit, the RF current- limiting inductor can protect the de-tuning circuit from over-currents.

It is another object of the invention to provide a method for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil of an MR imaging system having a main magnet for generating a substantially static main magnetic field, an examination region provided in the substantially static main magnetic field to position a subject of interest , usually a patient, within, further having consecutive RF transmit phases and RF receive phases, at least one RF transmit coil that is provided for applying an RF magnetic field at the examination region during the RF transmit phases to excite nuclei within the subject of interest, and at least one RF receive coil provided for acquiring MR RF signals from the excited nuclei of the subject of interest during the receive phases, and at least one of the disclosed embodiments of a de-tuning circuit or combinations thereof.

The method comprises the steps of

(b) generating a DC bias current through the RF solid state switching member by the bias current generating circuit during the transmit phase that is sufficient to bring the RF impedance of the RF solid state switching member into the substantially electrically conducting state,

(c) adjusting the DC bias current for application during the transmit phase to a value at which the resistance of the PTC resistor exceeds a threshold value at a preselected switch-off temperature, and

(d) transferring any heat that is dissipated by the RF solid state switching member to the PTC resistor via the thermal bridging means.

With this method, by choosing a suitable resistance of the PTC resistor for the applied bias current, a de-tune circuit can be implemented which will trip at a preselected temperature of the PIN diode. A rise in temperature of the RF solid state switching member due to an undesired and unintended coupling of the RF receive coil with the RF transmit coil can be employed to cause an abort of an RF receive coil scan procedure to prevent overheating, to ensure an RF receive coil integrity, and to save the subject under examination from bodily harm by burn.

It is yet another object of the invention to make available a magnetic resonance (MR) imaging system, comprising a main magnet provided for generating a substantially static magnetic field, a magnetic gradient coil system provided for generating gradient magnetic fields superimposed to the static magnetic field, an examination region provided to position a subject of interest within, at least one RF transmit coil that is provided for applying an RF magnetic field at the examination region during an RF transmit phase to excite nuclei within the subject of interest, at least one RF receive coil that is provided for acquiring MR signals from the excited nuclei within the subject of interest during an RF receive phase, and at least one of the disclosed embodiments of a de-tuning circuit or combinations thereof for de-tuning the RF receive coil.

By that, an MR imaging system can be provided whose RF receive coil integrity is ensured for any position of the RF receive coil within the examination region. Further, the subject under examination can be saved from any bodily harm from burn by an overheated de-tuning circuit, and the integrity of the receive coil can be ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

In the drawings:

Fig. 1 is a schematic illustration of a part of an embodiment of an MR imaging system including a de-tuning circuit in accordance with the invention,

Fig. 2 illustrates a schematic view of an embodiment of the de-tuning circuit pursuant to Fig. 1, and

Fig. 3 shows a trip current temperature characteristic of a PTC resistor of the de-tuning circuit pursuant to Fig. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance (MR) imaging system 10 comprising an MR scanner 12. The MR imaging system 10 includes a main magnet 14 provided for generating a substantially static magnetic field. The main magnet 14 has a central bore that provides an examination region 18 for a subject of interest 20, usually a patient, to be positioned within. In general, the invention is also applicable to any other type of MR imaging system providing an examination region.

Further, the MR imaging system 10 comprises a magnetic gradient coil system 16 provided for generating gradient magnetic fields superimposed to the static magnetic field. The magnetic gradient coil system 16 is concentrically arranged within the bore of the main magnet 14, as is well known in the art.

Further, the MR imaging system 10 includes an RF transmit coil 22 designed as a body coil that is provided for applying an RF magnetic field at the examination region 18 during an RF transmit phase to excite nuclei of the subject of interest 20, and an RF receive coil 24 designed as a local RF coil that is provided for acquiring MR signals from the excited nuclei within the subject of interest 20 during an RF receive phase. In a state of operation of the MR imaging system 10, RF transmit phases and RF receive phases are taking place in a consecutive manner. The RF transmit coil 22 is arranged concentrically within the bore of the main magnet 14. The RF receive coil 24 may be arranged at a region of interest in proximity to or even at the subject of interest 20, which in principle could be anywhere within the examination region 18.

Moreover, the MR imaging system 10 comprises an MR image reconstruction unit 28 provided for reconstructing MR images from the acquired MR signals and an MR imaging system control unit 26 with a monitoring unit provided to control functions of the MR scanner 12, as is commonly known in the art. Control lines 32 are installed between the MR imaging system control unit 26 and an RF transmitter 30 that is provided to feed RF power of an MR radio frequency to the RF transmit coil 22 via an RF switching unit 34. The RF switching unit 34 in turn is also controlled by the MR imaging system control unit 26, and another control line 36 is installed between the MR imaging system control unit 26 and the RF switching unit 34 to serve that purpose.

The RF switching unit 34 is provided to feed the RF power from the RF transmitter 30 to the RF transmit coil 22 during the RF transmit phases and to direct MR RF signals received by the RF received coil 24 to the MR image reconstruction unit 28 of the MR imaging system 10 during the RF receive phases.

In order to protect the RF receive coil 24 from taking up RF power transmitted by the RF transmitter 30, the RF switching unit 34 comprises a de-tuning circuit 38 for detuning the RF receive coil 24. The de-tuning circuit 38 that is shown in more detail in Fig. 2 is connected in series to two adjacent RF receive coil elements at RF receive coil element contacts 50, 52. The de-tuning circuit 38 comprises an RF resonance circuit 40 including a de-tuning capacitor Ci and a de-tuning inductor L. The de-tuning inductor L consists of two inductor parts L_{l s} L₂ that are wound in a bifilar manner for minimizing leakage inductance.

Further, the de-tuning circuit 38 includes an RF solid state switching member 44 designed as a PIN diode that is coupled in series to one out of the de-tuning capacitor Ci and the de-tuning inductor L, namely the de-tuning inductor L. Together with the de-tuning inductor L, the RF solid state switching member 44 is coupled in parallel communication to the other out of the de-tuning capacitor Ci and the de-tuning inductor L, namely the de-tuning capacitor Ci, for enabling or disabling the RF resonance circuit 40 in the de-tuning circuit 38. For a magnetic field strength of the main magnet of 1.5 T, the MR frequency for exciting hydrogen nuclei ('Η) within the subject of interest 20 would be about 64 MHz. In this frequency range, the PIN diode functions as an electric component whose RF impedance is an inverse function of a forward current flowing through it.

The de-tuning circuit 38 is furnished with a bias current generating circuit 42 provided for generating a DC bias current ¾ through the RF solid state switching member 44. The RF impedance of the PIN diode is controllable between a substantially electrically nonconducting state and a substantially electrically conducting state by variation of the bias current During the RF transmit phases, the RF switching unit 34 is controlled by the MR system control unit 26 in such a manner that the bias current generating circuit 42 of the de- tuning circuit 38 generates a DC bias current ¾ through the RF solid state switching member 44 that is sufficient to bring the RF impedance of the RF solid state switching member 44 down to the substantially electrically conducting state. By that, the RF resonance circuit 40 is transferred into an active state, de-tuning a resonance frequency of the RF receive coil 24 in order to protect it from picking up RF power transmitted by the RF transmit coil 22 at the MR frequency. In order to prevent an unintended change of the bias current ¾ from induction in the bias current generating circuit 42, the de-tuning circuit 38 comprises an RF current- limiting inductor L₃ that blocks any picked-up RF power transmitted by the RF transmit coil 22 due to its high impedance at the frequencies under consideration.

During the RF receive phases, the RF switching unit 34 is controlled by the MR system control unit 26 in such a manner that the bias current generating circuit 42 of the de-tuning circuit 38 generates a DC bias current ¾ through the RF solid state switching member 44 that is sufficiently low to bring the RF impedance of the RF solid state switching member 44 into the substantially electrically non-conducting state. By that, the RF resonance circuit 40 is transferred into a non-active state and is disabled from the de-tuning circuit 38. At the radio frequencies under consideration, the de-tuning capacitor Ci of the RF resonance circuit 40 has a very low impedance, so that the RF receive coil 24 is tuned to the resonance frequency required to receive RF MR signals from the excited nuclei within the subject of interest 20.

Moreover, the de-tuning circuit 38 comprises a positive temperature coefficient (PTC) resistor 46, which is coupled in series with the RF solid state switching member 44. Thermal bridging means are arranged between the RF solid state switching member 44 and the PTC resistor 46. The thermal means include a common hollow housing 48 made from plastic material that encloses both the RF solid state switching member 44 and the PTC resistor 46, thereby thermally insulating them from their surroundings, and a thermal conductive paste that the hollow housing 48 is filled up with. The thermal means are specifically designed for transferring thermal heat from the RF solid state switching member 44 to the PTC resistor 46. Thereby, any heat that is dissipated by the RF solid state switching member 44 is transferred to the PTC resistor 46 via the thermal bridging means, so that a temperature of the PTC resistor 46 follows a temperature of the RF solid state switching member 44. This is in particular true in a condition in which the RF receive coil 24 is arranged within the examination region 18 at a location in which it electromagnetically couples to the RF transmit coil 22, and RF power is dissipated in the RF solid state switching member 44.

The PTC resistor 46 is designed as a resettable polymeric PTC device. This type of PTC resistor 46 is commonly known for use as a self-resetting fuse for over-current protection, exploiting the effect of a very sharp increase in electrical resistance within a narrow temperature range due to a phase change of a conductive polymer within the PTC resistor 46. The phase change results from a rapid increase in the temperature of the PTC device, caused by a generation of heat within the PTC device by self- heating. A resistance characteristic of the PTC resistor 46 has a non-linear temperature dependency in an S-shaped fashion with an inflection point within a temperature range between 0°C and 100°C. A variable that characterizes the phase change to start is given by a current level through the PTC resistor 46, named the "trip current".

Because of the self-heating mechanism causing the phase change, the trip current of an individual resettable polymeric PTC device depends on a temperature level of the PTC device, and in fact decreases with increasing temperature. This is illustrated by the graph shown in Fig. 3. Therefore, in a situation in that the PIN diode of the de-tuning circuit 38 heats up due to coupling of the RF receive coil 24 with the body coil, also the temperature of the PTC resistor 46 will rise, resulting in a decrease of the trip current level according to Fig. 3.

This dependency is exploited in the embodiment in accordance with the invention shown in Fig. 2 by adjusting the DC bias current ¾ for application during the transmit phase to a current value at which the resistance of the PTC resistor 46 exceeds a threshold value at a preselected switch-off temperature T_Sw- Dependent on the type of PTC resistor 46, the trip current level will be lower than the bias current ¾ at the preselected switch-off temperature Tsw- In this situation the resistance of the PTC resistor 46 increases and the bias current ¾ decreases to a very low value, resulting in an abort of a current RF MR signal receive procedure of the MR imaging system. In this way the RF receive coil 24 will always stay under safe conditions.

By selecting a value for the resistance of the PTC resistor 46 that is appropriate for the used bias current a de-tuning circuit 38 can be implemented which will trip at the preselected switch-off temperature Tsw of the RF solid state switching member 44. As an example taken from the characteristic of Fig. 3, a bias current ¾ that is adjusted to 60% of a current level rated for the polymeric PTC device at a temperature of 25°C will cause the phase change and the sudden increase in resistance with it at a rise in temperature to the preselected switch-off temperature T_Sw of 75°C.

As the PTC resistor 46 is designed as a polymeric PTC device, it will automatically reset if the temperature has fallen below the preselected switch-off temperature Tsw at which the phase change will be reversed. After that, the RF receive coil 24 is ready for use again.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. REFERENCE SYMBOL LIST

10 MR imaging system

12 MR scanner

14 main magnet

16 magnetic gradient coil system

18 examination region

20 subject of interest

22 RF transmit coil

24 RF receive coil

26 MR imaging system control unit

28 MR image reconstruction unit

30 RF transmitter

32 control lines

34 RF switching unit

36 control lines

38 de-tuning circuit

40 RF resonance circuit

42 bias current generating circuit

44 RF solid state switching member

46 PTC resistor

48 Housing

50 receive coil element connection

52 receive coil element connection

Ci de-tuning capacitor

lb bias current

L de-tuning inductor

Li de-tuning inductor part

L₂ de-tuning inductor part

L₃ RF current- limiting inductor

Tsw switch-off temperature

Claims

CLAIMS:

1. A de-tuning circuit (38) for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil (24) of an MR imaging system (10) having consecutive RF transmit phases and RF receive phases, the de-tuning circuit (38) comprising:

an RF resonance circuit (40) including at least one de-tuning capacitor (Ci) and at least one de-tuning inductor (L);

at least one RF solid state switching member (44) coupled in series to one out of the at least one de-tuning capacitor (Ci) and the at least one de-tuning inductor (L), and together with the one out of the at least one de-tuning capacitor (Ci) and the at least one detuning inductor (L), coupled in parallel communication to the other out of the at least one de- tuning capacitor (Ci) and the at least one de-tuning inductor (L) for enabling or disabling the RF resonance circuit (40);

a bias current generating circuit (42) provided for generating a DC bias current (lb) through the at least one RF solid state switching member (44), wherein an RF impedance of the at least one RF solid state switching member (44) is controllable between a

substantially electrically non-conducting state and a substantially electrically conducting state by variation of the bias current (lb);

at least one positive temperature coefficient (PTC) resistor (46), wherein the at least one PTC resistor (46) is coupled in series with the at least one RF solid state switching member (44); and

thermal bridging means that are arranged between the at least one RF solid state switching member (44) and the PTC resistor (46) and that are specifically designed for transferring thermal heat from the at least one RF solid state switching member (44) to the PTC resistor (46).

2. The de-tuning circuit (38) as claimed in claim 1, wherein the at least one RF solid state switching member (44) includes a PIN diode.

3. The de-tuning circuit (38) as claimed in claim 1 or 2, wherein a resistance characteristic of the PTC resistor (46) has a non-linear temperature dependency with at least one inflection point in the temperature range between 0°C and 100°C.

4. The de-tuning circuit (38) as claimed in one of the preceding claims, wherein the at least one PTC resistor (46) is designed as a resettable polymeric PTC device.

5. The de-tuning circuit (38) as claimed in one of the preceding claims, wherein the bias current generating circuit (42) comprises at least one RF current- limiting inductor (L₃).

6. A method for de-tuning a magnetic resonance (MR) radio frequency (RF) receive coil (24) of an MR imaging system (10) having a main magnet (14) for generating a substantially static main magnetic field, an examination region (18) provided in the substantially static main magnetic field to position a subject of interest (20), usually a patient, within, further having consecutive RF transmit phases and RF receive phases, at least one RF transmit coil (22) that is provided for applying an RF magnetic field at the examination region (18) during the RF transmit phases to excite nuclei within the subject of interest (20), and at least one RF receive coil (24) provided for acquiring MR RF signals from the excited nuclei of the subject of interest (20) during the receive phases, and at least one de-tuning circuit (38) as claimed in any of the claims 1 to 5,

the method comprising the following steps:

(a) during the transmit phase, generating a DC bias current (¾) through the RF solid state switching member (44) by the bias current generating circuit (42) that is sufficient to bring the RF impedance of the RF solid state switching member (44) into the substantially electrically conducting state;

(b) adjusting the DC bias current (¾) for application during the transmit phase to a value at which the resistance of the PTC resistor (46) exceeds a threshold value at a preselected switch-off temperature (T_Sw);

(c) transferring any heat that is dissipated by the RF solid state switching member

(44) to the PTC resistor (46) via the thermal bridging means.

7. A magnetic resonance (MR) imaging system (10), comprising: a main magnet (14) provided for generating a substantially static magnetic field;

a magnetic gradient coil system (16) provided for generating gradient magnetic fields superimposed to the static magnetic field;

an examination region (18) provided to position a subject of interest (20) within;

at least one RF transmit coil (22) that is provided for applying an RF magnetic field at the examination region (18) during an RF transmit phase to excite nuclei within the subject of interest (20);

at least one RF receive coil (24) that is provided for acquiring MR signals from the excited nuclei within the subject of interest (20) during an RF receive phase;

at least one de-tuning circuit (38) for de-tuning the RF receive coil (24) as claimed in one of the claims 1 to 5.