WO2009104116A1

WO2009104116A1 - Magnetic resonance imaging apparatus having a switched-mode power supply

Info

Publication number: WO2009104116A1
Application number: PCT/IB2009/050594
Authority: WO
Inventors: Alexander C. De Rijck; Hendrik Huisman; Bjoern Weissler
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2008-02-19
Filing date: 2009-02-13
Publication date: 2009-08-27
Anticipated expiration: 2010-08-19

Abstract

A magnetic resonance imaging (MRI) apparatus is disclosed. The apparatus has also a switched-mode power supply which is operable at a switching frequency, wherein the switching frequency is adjustable within a given switching frequency interval. The apparatus further comprises a control unit which is adapted to determine a frequency value within the given switching frequency interval so that higher harmonics of the frequency value are outside of a frequency band used for the detection of MR signals. The control unit is further adapted to set the switching frequency of the switched-mode power supply to the determined frequency value, wherein the control unit employs for the determination of the frequency value information about the frequency band of the MR signals.

Description

MAGNETIC RESONANCE IMAGING APPARATUS HAVING A SWITCHED-MODE POWER SUPPLY

FIELD OF THE INVENTION

The invention relates to a magnetic resonance imaging apparatus and to a method of operating a switched-mode power supply of a magnetic resonance imaging apparatus.

BACKGROUND

In magnetic resonance imaging (MRI) pulse sequences consisting of switched radio frequency (RF) fields (Bi) and magnetic field gradient pulses are applied to an object, e.g. the body of a patient, placed in a stationary and substantially homogeneous main magnetic field (Bo) to generate phase encoded magnetic resonance signals, so called MR signals, which are then detected by a MR scanner and further processed by electronic equipment to obtain information from the object and to reconstruct images thereof. The application of these pulse sequences is also denoted as MR scan. Since its initial development, the number of relevant fields of application of magnetic resonance imaging has grown enormously. Magnetic resonance imaging can be applied to almost every part of the body, and can be used to obtain information about a number of important functions of the human body. The pulse sequence, which is applied during an MR scan, determines completely the characteristics of the reconstructed images, such as location and orientation in the object, dimensions, resolution, signal to noise ratio, contrast, sensitivity for movements, etc. An operator of a magnetic resonance imaging device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.

Usually, MR imaging devices comprise a superconducting main magnetic, for the generation of the static magnetic field Bo in the examination zone, gradient coils, for the generation of switched magnetic field gradients during the imaging sequence, and a radio frequency coil assembly. The RF coil assembly of known magnetic resonance imaging apparatuses includes a transmit coil to generate the above mentioned Bi-field for citation of nuclear spins, and one or more receive antennas used in conjunction with the transmit coil to detect and receive the magnetic resonance signals from the examined object. The receive antennas may be typically connected to a receive chain of the MR apparatus. The receive chain comprises radio frequency amplifiers, attenuators, modulators, filters and digitizers in order to allow for a narrow-band, low-noise detection of the magnetic resonance signals and for converting the detected magnetic resonance signals into digital signal samples. These digital signal samples are finally processed by means of a computer and converted into digital images of the object.

In known parallel MRI techniques, multiple receiving antennas with different spatial sensitivity profiles are employed, for example to enhance local sensitivity or to reduce the scan time for a diagnostic image. The latter can for example be achieved in accordance with the known SENSE technique by acquiring a smaller set of phase encoded magnetic resonance signals than would actually be necessary to completely cover the predetermined field of view in accordance with Nyquist's theorem.

The RF coil assembly including the transmit coil and the one or more receive antennas may be components of the above mentioned MR scanner that is in particular used to detect the above mentioned generated phase encoded magnetic resonance signals. The MR scanner is a sensitive detection system that even allows detecting weak magnetic resonance signals from the human body.

A circuit which performs switching operations at a given switching frequency generates inherently higher harmonics at frequencies that are multiples of the basic switching frequency. Such higher harmonics might be at frequencies that are inside the detection range of the MR scanner and are thus detectable in form of peaks by the MR scanner. The peaks might be falsely interpreted as MR signals by the subsequent signal processing unit and might finally lead to faulty reconstructed images.

The document US 5,546,001 discloses a magnetic resonance imaging system having a predetermined frequency band and comprising a switching device operating in response to a switching signal of a switching frequency. A switching signal generator supplies the switching signal to the switching device, wherein the switching signal generator comprises an element for supplying the switching signal to the switching device. The switching frequency is able to be changed in response to a control signal and an element for adjusting the switching frequency supplies the control signal to the switching signal supplying element so that a frequency which is a product of the switching frequency and an integer falls out of the predetermined frequency band.

It is an object of the invention to provide an improved magnetic resonance imaging apparatus. SUMMARY OF THE INVENTION

According to a first aspect of the invention, a magnetic resonance imaging apparatus is disclosed. In accordance with an embodiment of the invention, the MRI apparatus comprises an MR scanner which is adapted to detect MR signals if the frequencies of the MR signals are inside a frequency band of the MR scanner. The MRI apparatus also comprises a switched-mode power supply which is operable at a switching frequency that is adjustable within a given switching frequency interval. The apparatus further comprises a control unit which is adapted to determine a frequency value within the given switching frequency interval so that higher harmonics of the frequency value are outside of the frequency bands, wherein the control unit employs information about the frequency band of the MR scanner for the determination of the frequency value. The control unit is further adapted to set the switching frequency of the switched-mode power supply to the determined frequency value.

The MR scanner is thus adapted to detect MR signals, e.g. phase encoded MR signals, with frequencies that lie inside a detection range of the MR scanner that is defined by the frequency band. The control unit is adapted to set the switching frequency at which the switched-mode power supply is operated to the determined frequency value so that higher harmonic frequencies of the switching frequency do not fall into the detection range defined by the frequency band of the MR scanner. The switching frequency of the switched-mode power supply will thus not be a source of noise or of an error signal. The MRI apparatus according to the above mentioned US 5,546,001 uses an input switching signal having a first switching frequency with a harmonic frequency and generates an output switching signal in response thereto, wherein the output switching signal has a second switching frequency with another harmonic frequency, wherein the harmonic frequency of the input switching signal is within the image frequency band and wherein the other harmonic frequency of the output switching signal lies outside of the image frequency band. Since the control unit according to the apparatus in accordance with the invention employs in particular direct the information about the frequency band of the MR scanner, the determination of the switching frequency can be performed in a much simpler way and is less error-prone. Further, the control unit can be constructed in a more compact design as it only requires information about the frequency band of the MR scanner and does not require another input switching frequency which is employed for the determination of the actual switching frequency of the power supply.

In accordance with an embodiment of the invention, the control unit is adapted to determine the actual frequency band of the MR scanner. This is particularly advantageous when the detection range and thus the frequency band of the MR scanner can be varied within some bounds as it ensures that the control unit employs the actual frequency band of the MR scanner for the determination of the actual switching frequency of the power supply. In accordance with an embodiment of the invention, the control unit comprises a stable clock. The control unit is further adapted to lock the switching frequency of the switched-mode power supply to the stable clock when the switching frequency is set to the frequency value. The frequency value is thus locked to the stable clock of the control unit. In particular, locking of the frequency value to the stable clock may be applied during a MR scan in order to prevent a false detection of a MR signal due to a higher harmonic of the power supply's switching frequency that is inside of the detection range of the MR scanner. An MR scan hereby relates to the application of one or a sequence of RF pulses to the body to be examined.

In accordance with an embodiment of the invention, the switched-mode power supply comprises an internal clock. The switched-mode power supply is adapted to lock the switching frequency to the internal clock if the switching frequency is unlocked from the stable clock. The internal clock may be less stable than the stable clock of the control unit. In particular, the switching frequency may be locked to the internal clock in order to ensure that it is kept at least approximately at the set frequency value when no MR scan is performed. This allows locking the switching frequency to the stable clock in a relatively short time as the actual switching frequency is closely kept to the determined frequency value so that the lock with respect to the stable clock can tune the switching frequency rapidly to the determined frequency value.

In accordance with an embodiment of the invention, the control unit is adapted to set the frequency value of the switching frequency to a new frequency value in response to a change of the frequency band of the MR scanner to a new frequency band, wherein the control unit is adapted to determine the new frequency value by use of the new frequency band so that it is outside of the new frequency band. The MR scanner may comprise an RF coil assembly with one or more receive antennas that are used in conjunction with a transmit coil to detect and receive the MR signal from the examined body. Further the receive antennas may be connected to a receive chain included in the MR scanner so that due to modifications in the receive chain the detection bandwidth of the MR scanner can be changed and set to a new frequency band. The use of the control unit in accordance with the invention provides the advantage that since the control unit uses in essence information about the actual frequency band of the MR scanner, the new frequency value can be determined in an easy and quick way.

In accordance with an embodiment of the invention, the control unit is adapted to vary the frequency value of the switching frequency so that the actual switching frequency is always at the outside of the actual frequency band of the MR scanner. The control unit can for example vary the frequency value so that the frequency value oscillates harmonically between a highest and a lowest frequency value, wherein higher harmonics of the frequencies in the frequency range between the highest and lowest frequency values are at the outside of the scanner's actual frequency band. In accordance with an embodiment of the invention, the MR scanner comprises roll-off frequency intervals below and above the frequency band. The MR signals with frequencies in the roll-off frequency intervals are detectable by the MR scanner and the control unit is adapted to vary the frequency value of the switching frequency if a higher harmonic of the switching frequency is at the inside of one of the roll-off frequency intervals. The variation of the frequency value might in particular be necessary if the detection bandwidth of the MR scanner is relatively large so that it is not possible to determined a frequency value within the operational range of the power supply so that higher harmonics of the corresponding switching frequency lie at the outside of the roll-off frequency intervals.

In accordance with an embodiment of the invention, the frequency value of the switching frequency is variable according to a spread spectrum technique. The frequency value is spread within a broader bandwidth, e.g. the size of a roll-off frequency interval, in order to smear out a possible detection of higher harmonics in the roll-off frequency intervals. The detection of a peak in the roll-off interval caused by a higher harmonic of the switching frequency might lead to a misinterpretation of this peak as a true MR signal and may result in a wrong image. By varying the frequency value, the noise level detected in the roll-off intervals may be increased which results in a drop of the signal to noise ration within the roll-off intervals. The variation of the frequency value according to the spread spectrum technique prevents however the detection of a false peak caused by the higher harmonic that false within a roll-off interval and thus prevents the generation of a wrong image. In accordance with an embodiment of the invention, the spread spectrum technique relates to the application of frequency hopping within a predefined frequency interval.

In accordance with an embodiment of the invention, the frequency width of the spectrum relates in essence to the magnitude of the roll-off frequency interval. In accordance with an embodiment of the invention, the frequency width of the spread spectrum relates is determined by the quotient between the magnitude of the roll- off frequency interval and the order number of the higher harmonic that is inside the roll-off frequency. In accordance with an embodiment of the invention, the MRI apparatus comprises a super-conducting main magnet for the generation of a static magnetic field in the examination zone. The MRI apparatus further comprises gradient coils for the generation of switched magnetic field gradients during an MR scan. The MR scanner comprises a transmit coil to generate switched RF magnetic fields for performing the MR scan and for excitation of nuclear spins in the body. The MR scanner further comprises one or more receiving antennas that are used in conjunction with the transmit coil to detect and receive the MR signals from the examined body. The receive antennas are connected to a receive chain of the MRI apparatus. The receive chain comprises RF amplifiers, attenuators, modulators, filters and digitizers in order to allow for a narrow-band, low-noise detection of the MR signals and for converting the detected MR signals into digital signal samples. The switched-mode power supply of the magnetic resonance imaging apparatus may be adapted to provide electric power at a certain voltage to one or more of the electronic components of the receive chain of the MR scanner and may therefore be place in the vicinity of the MR scanner so that the higher harmonics of the power supply may be detected by the MR scanner if the higher harmonics falls inside of the detection range of the MR scanner.

In accordance with an embodiment of the invention, the MR scanner comprises MR surface coils, wherein the detected MR signals are amplified, digitized and pre-processed. The switched-mode power supply is adapted to provide electric power to a component of the MR surface coils, for example to a digitizer or pre-processor. Further, the magnetic resonance imaging apparatus in accordance with the invention might comprise a plurality of switched-mode power supplies, wherein each of the plurality of switched-mode power supplies is adapted to provide electric power to a particular component of the MR surface coils.

In accordance with an embodiment of the invention, the switched-mode power supply receives a single input voltage from a voltage supply that is a component of the magnetic resonance imaging apparatus. The switched-mode power supply is adapted to produce a pre-specified output voltage when it is set to a switching frequency within its operational range that is given by the switching frequency interval. In particular, a switched- mode power supply is adapted to adjust a duty cycle according to the actual switching frequency in order to produce the pre-specifϊed and wanted output voltage at an actual load. The switched-mode power supply may for example by a DC/DC converter which receives a DC-input voltage and outputs a DC-output voltage.

According to a second aspect of the invention, there is provided a method of operating a switched-mode power supply of a magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus comprises according to an embodiment of the invention a magnetic resonance scanner which is adapted to detect magnetic resonance signals that lie within a frequency band that defines the detection range of the MR scanner. The switched-mode power supply is operable at a switching frequency that is adjustable within a given switching frequency interval that defines the operating range of the switched- mode power supply. The method in accordance with the invention comprises the step of determining a frequency value within the given switching frequency interval so that higher harmonics of the frequency value are at the inside of the frequency band of the MR scanner, wherein for the determination of the frequency interval the frequency band of the MR scanner is employed. The method further comprises the step of setting the switching frequency of the switched-mode power supply to the frequency value in order to ensure that higher harmonics of the actual frequency value do not fall inside the frequency band of the MR scanner.

According to a third aspect of the invention, there is provided a computer program product with computer executable instructions that are adapted to cause when executed on a computer the computer to perform steps of the method in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the following preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which:

Fig. 1 shows a block diagram of a magnetic resonance imaging apparatus in accordance with the invention,

Fig. 2 shows a block diagram of an MR scanner, Fig. 3 shows a graph displaying a switching frequency and its higher harmonics,

Fig. 4 shows an illustration of the detection range of an MR scanner, and Fig. 5 shows a flow diagram illustrating steps of a method in accordance with the invention. DETAILED DESCRIPTION

In fig. 1 a magnetic resonance imaging apparatus 1 in accordance with the present invention is shown as a block diagram. The apparatus 1 comprises a set of main magnetic coils 2 for generating a stationary and homogeneous magnetic field and three sets of gradient coils 3, 4 and 5 for superimposing additional magnetic fields with controllable strength and having a gradient in a selected direction. Conventionally, the direction of the main magnetic field is labeled the z-direction, the two directions perpendiculars thereto are denoted as x- and y-directions, respectively. The gradient coils 3, 4 and 5 are energized via a power supply 9. The apparatus 1 further comprises a radiation emitter 6, an antenna or coil, for emitting radio frequency (RF) pulses to a body 7 placed in the examination zone of the apparatus 1. The radiation emitter 6 is coupled to a modulator 8 for generating and modulating the RF pulses. The apparatus 1 also comprises a magnetic resonance (MR) scanner 10. The MR scanner 10 comprises receiving antennas 11 for receiving MR signals from the body 7. The receiving antennas 11 may form a coil array for the purpose of parallel imaging and can for example be separate surface coils with different spatial sensitivity profiles. The MR scanner 10 further comprises a switched-mode power supply and an electronic device 13. The electronic device 13 provides the electronic circuitry that is required for digital or analogue (pre-) processing of the MR signals received by the receiving antennas 12. The electronic device 13 can further comprise sampling means for sampling the signals received by the receiving antennas 11 into digital signals. The electronic device 13 can furthermore be equipped with signal transmission antennas for wireless radio transmission of the digital signals in a multiplexed fashion to a corresponding data processing unit 17, for example a computer equipped with a radio antenna, for transformation of the received digital magnetic resonance signals into an image. This image can be made visible on a visual display unit 15. The modulator 8, the emitter 6 and the power supply 9 for the gradient coils 3, 4 and 5 are controlled by a control system 16 to generate the actual imaging sequence for parallel imaging.

The switched-mode power supply 12 is further connected with a power supply 18 that provides a DC voltage signal as input signal to the switched-mode power supply 12. Fig. 2 shows a block diagram of the MR scanner 10 of which the switched-mode power supply 12 is a component. The switched-mode power supply 12 is adapted to generate a specified output voltage from the input voltage provided by the power supply 18. The output voltage is employed to supply the electronic device 13 with electric power. Typically, the magnitude of the output voltage is lower than the magnitude of the input voltage but the magnitude of the output current is higher than the magnitude of the input current. The switch- mode power supply 12 does therefore not or to a negligible extend dissipate power within the MR scanner 10. The switched-mode power supply is therefore the preferred choice as voltage converter within the bore of a MRI apparatus as a linear voltage regulator dissipates far more power due to the possibly high voltage drop occurring within such a voltage regulator when transforming an input voltage to a lower output voltage. An embodiment of a switched-mode power supply rapidly switches at an adjustable switching frequency a power transistor between saturation and cutoff with a variable duty cycle whose average is the desired output voltage. The switching frequency for the switched-mode power supply 12 is provided by the control unit 14.

The receiving antennas 11 and the subsequent electronic device 13 allow for the detection of MR signals that lie within a detection range of the MR scanner 10 that is defined by a frequency band. Unfortunately, one or more higher harmonics of the switching frequency of the switched-mode power supply 12 may lie within this frequency band. The control unit 14 comprises a microprocessor 22 and storage 23 on which a computer program 24 is stored. The computer program 24 comprises instructions that are executable by the microprocessor 22. In operation, the microprocessor 22 executes the instructions and uses the information about the frequency band of the MR scanner 10 which might for example also be stored on the storage 22 to determine a frequency value for the switching frequency of the switched-mode power supply 12 so that higher harmonics of the switching frequency are at the outside of the frequency band. The control unit 14 sets then the switching frequency of the switched-mode power supply 12 to the determined frequency value. The higher harmonics are therefore not detectable by the MR scanner 10. In particular, the switching frequency is set to the determined frequency value before a MR scan is performed and kept at this value during the MR scan. In order to keep the switching frequency at the determined frequency value, the control unit comprises a stable clock 21 to which the switching frequency is locked, e.g. by use of a phase-locked- loop. The stable clock 21 may for example be a stable quartz oscillator. The switched-mode power supply 12 itself may further comprise an internal clock 12. The switching frequency may be locked to the internal clock 12 in case it is not locked to the stable clock 21. The internal clock 12 may be less accurate than the stable clock 21. The internal clock 12 provides the advantage that the switching frequency of the power supply 12 can be kept to at least within some bounds at the desired and determined frequency value. The lock which uses the stable clock 21 can then quickly set the actual switching frequency to the frequency value determined by the control unit 14.

Fig. 3 shows a graph displaying a switching frequency and its higher harmonics. The abscissa relates to the frequency axis and the ordinate of the graph relates to the signal strength. The graph is only to be regarded as illustration. In particular, the signal strengths do not reflect any real situations. The switching frequency at which the switched- mode power supply is operated is denoted as f_sw. Higher harmonics of the switching frequency are at frequencies f_n = n * f_sw, wherein n is a natural number. The MRI apparatus in accordance with the invention is adapted to determine the switching frequency of the switched-mode power supply so that the harmonic frequency of order n, nf_sw, is at a value below the detection range given by the frequency band (FB) of the MR scanner and so that the harmonic frequency of order n+1, (n+l)f_sw, is at a value above the detection range given by the frequency band (FB) of the MR scanner. Thus, higher harmonics of the switching frequency will therefore not be detectable by the MR scanner so that this source of false detection is eliminated.

Fig. 4 shows an illustration of the detection range 40 of an MR scanner. The abscissa relates to the frequency axis and the ordinate relates to the signal strength. The detection range 40 of the MR scanner is within the frequency range between a first frequency fi and a second frequency f₂ with a center frequency fc. In this range, a MR signal is for example detectable with a gain of at least -6 dB. Other definitions of the detection range might relate to the frequency interval in which a MR signal is detectable with a gain of at least - 3 dB. A first roll-off frequency interval is further present in the range between the first frequency fi and a third frequency f₃ and a second roll-off interval is present in the range between the second frequency f₂ and a fourth frequency fit. MR signal with frequencies that lie in the first or second roll-off frequency intervals are detectable according to the example presented here with a gain of at most -6 dB. It might not always be possible due to the limited operational range of the switched-mode power supply defined by the switching frequency interval to determine a frequency value for the switching frequency so that higher harmonics are not situated in the first and/or second roll-off frequency intervals. The control unit is therefore adapted, in case a higher harmonic of the switching frequency lies in the first and/or second roll-off frequency interval to vary the switching frequency. In particular, the control unit is adapted to vary the switching frequency so that the higher harmonics will vary over the full span of the roll-off frequency interval into which it falls. Thus, the higher harmonics will not be detected in form of a peak but will be smeared out over the full range of the corresponding roll-off frequency interval. The higher harmonics therefore contributes to an increase of the noise detected in the corresponding roll-off frequency interval and decreases therefore the signal to noise ratio in the corresponding interval but is not misinterpreted as a MR signal and does therefore not cause the generation of a faulty image. Fig. 5 shows a flow diagram illustrating steps of a method in accordance with the invention for operating a switched-mode power supply of a MRI apparatus. The MRI apparatus comprising a MR scanner which is adapted to detect MR signals that are inside a frequency band of the MR scanner. The switched-mode power supply is operatable at a switching frequency which is adjustable within a given switching frequency interval. According to step 50 of the method a frequency value is determined within the given switching frequency interval so that higher harmonics of the frequency value are outside of the frequency band, wherein the frequency band of the MR scanner is employed for the determination of the frequency value. According to step 51, the switching frequency of the switched-mode power supply is then set to the determined frequency value.

LIST OF REFERENCE NUMERALS

1 MRI apparatus

2 main magnet coils 3 gradient coil

4 gradient coil

5 gradient coil

6 radiation emitter

7 body 8 modulator

9 power supply

10 MR scanner

11 receiving antenna

12 switched-mode power supply 13 electronic device

14 control unit

15 visual display unit

16 control system

17 data processing unit 18 power supply

20 internal clock

21 stable clock

22 processor

23 storage 24 computer program

40 detection range of MR scanner

Claims

CLAIMS:

1. A magnetic resonance imaging apparatus (1) comprising: a MR scanner (10) being adapted to detect MR signals if the frequencies of the MR signals are inside a frequency band of the MR scanner, a switched-mode power supply (12) being operatable at a switching frequency, the switching frequency being adjustable within a given switching frequency interval, a control unit (14) being adapted to determine a frequency value within the given switching frequency interval so that higher harmonics of the frequency value are outside of the frequency band, the control unit being further adapted to set the switching frequency of the switched-mode power supply to the determined frequency value, wherein the control unit employs for the determination of the frequency value the frequency band of the MR scanner.

2. The MRI apparatus according to claim 1, wherein the control unit comprises a stable clock (21), wherein the control unit is adapted to lock the switching frequency of the switched-mode power supply to the stable clock when the switching frequency is set to the frequency value.

3. The MRI apparatus according to claim 2, wherein the switched-mode power supply comprises an internal clock (20), wherein the switched-mode power supply is adapted to lock the switching frequency to the internal clock if the switching frequency is unlocked from the stable clock.

4. The MRI apparatus according to claim 1, wherein the control unit is adapted to set the frequency value of the switching frequency to a new frequency value in response to a change of the frequency band of the MR scanner to a new frequency band, wherein the control unit is adapted to determine the new frequency value so that it is outside of the new frequency band.

5. The MRI apparatus according to claim 1, wherein the control unit is adapted to vary the frequency value so that higher harmonics of the actual frequency value are outside of the frequency band.

6. The MRI apparatus according to claim 1, wherein the MR scanner comprises roll-off frequency intervals (f3 ... fϊ, f₂... ft) below and above the frequency band, wherein MR signals with frequencies in the roll-off frequency intervals are detectable by the MR scanner, wherein the control unit is adapted to vary the frequency value of the switching frequency according to a spread spectrum technique if a higher harmonic is inside one of the roll-off frequency intervals.

7. The MRI apparatus according to claim 6, wherein the size of the spread spectrum is in essence equal to the magnitude of the roll-off frequency interval or wherein the size of the spread spectrum is in essence equal to a frequency span, wherein the frequency span is determined by the quotient between the magnitude of the roll-off frequency interval and the order number of the higher harmonics that is inside the roll-off frequency.

8. A method of operating a switched-mode power supply (12) of a MRI apparatus (1), the MRI apparatus comprising a MR scanner (10) being adapted to detect MR signals that are inside a frequency band of the MR scanner, the switched-mode power supply being operatable at a switching frequency, the switching frequency being adjustable within a given switching frequency interval, the method being performed by a control unit (14), the method comprising: determining a frequency value within the given switching frequency interval so that higher harmonics of the frequency value are outside of the frequency band, wherein the control unit employs the frequency band of the MR scanner for the determination of the frequency value, setting the switching frequency of the switched-mode power supply to the frequency value.

9. The method of claim 8, wherein the control unit comprises a stable clock, wherein the method comprises locking the switching frequency of the switched-mode power supply to a stable clock when the switching frequency is set to the frequency value.

10. The method of claim 8, wherein the MR scanner comprises roll-off frequency intervals below and above the frequency band, wherein MR signals with frequencies in the roll-off frequency intervals are detectable by the MR scanner, wherein the method comprises varying the frequency value of the switching frequency according to a spread spectrum technique if a higher harmonic of the switching frequency is inside of one of the roll-off frequency intervals.

11. A computer program product comprising a computer useable medium including a computer readable program, wherein the computer readable program causes when executed on the computer to perform steps of the method according to any one of the preceding claims 8 to 10.