Patent Description:
Current RF amplifiers in magnetic resonance imaging applications utitlize a large capacitor bank to store the pulse energy required to be generated. These RF amplifiers typically comprise a mains power supply, a capacitor bank for energy storage and a RF amplifier which uses the stored energy. The capacitor bank is usually recharged between pulses from the mains power supply. In general, the RF transmission happens in very short pulses with significant time in between where gradient fields are manipulated and or RF signals are being received.

The mains power supply that charges the capacitor banks in RF amplifiers often uses switched mode power supply techniques. The switched mode power supply starts recharging the capacitor bank when the capacitor bank voltage is below a threshold voltage. The charging current is limited in the power supply and some control algorithm may be added to limit the charging current in cases where the capacitor bank voltage is only slightly below the threshold voltage. The recharging therefore starts with some delay with respect to the initiation of the discharge by the RF amplifier. Hence, the full power supply current is not available during short high power RF pulses.

<CIT> describes that a magnetic resonance imaging apparatus includes an amplifier, a capacitor bank, and processing circuitry. The amplifier supplies, based on an imaging sequence, an RF pulse to an RF coil which generates a radio frequency magnetic field. The capacitor bank supplies an electric power to the amplifier. The processing circuitry judges whether an imaging by the imaging sequence is able to be executed, based on a condition of the RF pulse in the imaging sequence and an output efficiency of the amplifier. The European patent application <CIT> discloses magnetic resonance imaging equipment in which the control of (dis)charging a rechargeable battery is based on consumed power in real time.

It is an object of the invention to make the generation of RF pulses for MRI applications more efficient.

According to the invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are described in the sub claims.

Therefore, according to the invention, an MRI system is provided as defined in claim <NUM>.

Hence, the present invention addresses the power supply control for a MRI RF transmitter by using extra information. This information may be used in such a way that the recharging current to the capacitor bank may be activated even though the voltage drop is still very small. This would allow for a smaller capacitor bank value at the same performance level. According to the invention, there are different possibilities for determining when the capacitor bank recharging current should start and how big it should be.

In this respect, according to a preferred embodiment of the invention, a capacitor bank current sensor is provided which is coupled to the capacitor bank and which is adapted for measuring the actual current drawn from the capacitor bank and for generating the indication signal on the basis of this current. In this way, the control of the charging current fed from the mains power supply to the capacitor bank is directly influenced by the actual current drawn from the capacitor bank.

In addition or as an alternative, according to a preferred embodiment of the invention, a RF amplifier power sensor is provided which is coupled to the RF amplifier and which is adapted for measuring the actual RF power generated in the RF amplifier which can form the basis for calculating the current draw.

In general, it is possible that the RF amplifier is only coupled to the capacitor bank for feeding a current to the RF amplifier. However, according to a preferred embodiment of the invention, the RF amplifier is also directly coupled to the mains power supply for directly drawing a current from the mains power supply. It is desirable that the current from the mains power supply is available during the largest part of the RF pulse. Therefore, according to this preferred embodiment of the invention, not only the current from the capacitor bank but also the current directly obtained from the mains power supply may be used to generate RF power in the RF amplifier. This would also allow for a smaller capacitor bank value at the same performance level.

For the mains power supply, the invention allows for different types of power supplies. However, according to a preferred embodiment of the invention, the mains power supply is a switched mode power supply.

According to the invention, the MRI system further comprises an RF transmit coil and an information unit coupled to the power supply control unit and being adapted for generating the indication signal on the basis of the upcoming current drawn from the capacitor bank on the basis of the upcoming RF power demand for the transmit coil of the MRI system. Hence, information on the RF power demand may also be provided by the MRI system via separate communication to the power supply control unit and, thus, to the RF amplifier, informing it when and how much current will be drawn from the capacitor bank, based on and derived from the RF power demand.

The invention also relates to a method for generating RF pulses for a MRI system with a RF transmitter as defined in claim <NUM>.

Preferred embodiments of the above described method relate to the preferred embodiments of the RF transmitter described further above. According to the invention, the method also comprises the following method step: controlling the generation of the charging current for the capacitor bank based on the estimated upcoming current drawn from the capacitor bank. In such a way, a charging current can be fed to the capacitor bank directly before a RF pulse is generated and forwarded to the RF transmit coil.

In addition, according to a preferred embodiment of the invention, the method further comprises the method step of estimating the actual current drawn from the capacitor bank by measuring the actual current drawn from the capacitor bank.

As an alternative, the method step of measuring the actual current draw from the capacitor bank can be avoided if the actual current draw from the RF amplifier can be estimated based on different information, such as: time-varying current draw by (part of) the RF amplifier or time-varying RF amplifier output power. RF amplifier output power has a close relationship to current draw, but it also depends on other component values, such as temperature, actual bias currents in different parts of the RF amplifier, transistor efficiency and capacitor bank voltage.

According to a preferred embodiment of the invention the RF amplifier is also directly fed by a charging current from the mains power supply. As set out in detail further above, according to this preferred embodiment of the invention, not only the current from the capacitor bank but also the current directly obtained from the mains power supply may be used to generate RF power in the RF amplifier which allows for a smaller capacitor bank value at the same performance level.

Further, according to the invention the method comprises the method step of estimating the upcoming current drawn from the capacitor bank on the basis of the upcoming RF power demand for the transmit coil of the MRI system.

It is a further preferred option of the invention to track and store the efficiency of the RF amplifier over time and to control the generation of the charging current for the capacitor bank also based on the efficiency of the RF amplifier. In this respect, extra information about the actual RF amplifier efficiency may be made use of and the RF amplifier efficiency may be tracked over time, e.g. during variation of circumstances, such as temperature, ageing and, hence, using more complete information about RF sequences from the MRI system.

The invention also relates to a non-transitory computer-readable medium as defined in claim <NUM>.

This invention can also be used for any application where a large energy is buffered and then drawn on a very short time scale. In such situations the energy replenishing mechnism may kick in too late. This invention combines the contiuous time control algorithms - as known from power supplies - with discrete time information of other parts of the application and with the fact that the output power from the energy storage can be measured in time and value. Hence the control system can learn to precisely estimate the power draw in the system and use that for filling the energy buffer.

Further, this invention can also work in case multiple components draw power from the same energy storage as long as all power draws can be estimated reasonably accurately. For example in case of a single capacitor bank with two RF amplifier channels the power draw of each channel can be determined individually. The total power draw can be used to determine the recharging current to the capacitor bank.

Such an 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 any case, the scope of the invention is defined by the claims.

In <FIG> a MRI system according to a first example which is outside the scope of the invention is shown. This MRI system <NUM> is used for MRI examination of a patient <NUM> in an examination area <NUM> within the bore of a superconducting magnet <NUM>, which is used for generating a high static magnetic field. For positioning the patient <NUM> in the examination area <NUM>, the patient <NUM> is positioned on a patient support <NUM>, which may be driven into and out of the examination area <NUM> within the bore of the superconducting magnet <NUM>.

Here, the MRI system <NUM> is only shown with its most relevant components, i.e. components which are of certain relevance for the present invention. In this respect, the MRI system <NUM> according to the first example comprises a gradient coil <NUM> within the bore of the superconducting magnet <NUM> as well as RF transmit coils <NUM> and a RF receiver coil <NUM>. The RF transmit coils <NUM> emit RF pulses, which are supplied from a RF transmitter <NUM>, and generate a radio frequency magnetic field within the bore of the superconducting magnet <NUM>.

As is well known by the man skilled in the art, by transmitting RF pulses which have an orthogonal polarization to the magnetic field generated by the superconducting magnet <NUM> and matching the Larmor frequency of the nucleons of interest, the spins of the nucleons can be excited and brought into phase, and a deflection of their net magnetization from the direction of the field of the superconducting magnet <NUM> is obtained so that a transversal component in relation to the longitudinal component of the net magnetization is generated. After termination of the RF pulse the relaxation process of the longitudinal and reversal components of the net magnetization begin until the net magnetization has returned to its equilibrium state. Magnetic resonance signals, which are generated by the precessing magnetization are detected by means of the RF receiver coil <NUM>. The received magnetic resonance signals are time-based amplitude signals, which are further Fourier transformed to frequency-based magnetic resonance spectrum signals and further processed for generating a magnetic resonance image of the nucleons of interest.

According to the first example described here, a RF transmitter <NUM> is provided, which is schematically depicted in <FIG> in more detail. The RF transmitter <NUM> comprises a RF amplifier <NUM> for generating RF pulses and for forwarding these RF pulses to the RF transmit coil <NUM> of the MRI system <NUM>. Further, the RF transmitter <NUM> comprises a capacitor bank <NUM>, which is coupled to the RF amplifier <NUM>, for storing electric energy and for providing the RF amplifier <NUM> with a current for generating the RF pulses. A mains power supply <NUM> is coupled to the capacitor bank <NUM>, for generating a charging current for charging the capacitor bank <NUM> with electric energy.

It is an essential aspect of the first example that a power supply control unit <NUM> is coupled to the mains power supply <NUM>, for controlling the generation of the charging current for the capacitor bank <NUM>. The generation of the charging current for the capacitor bank <NUM> is controlled in such a way that the power supply control unit <NUM> receives an indication signal indicating the actual current drawn from the capacitor bank <NUM> and for controlling the generation of the charging current for the capacitor bank <NUM> on the basis of this indication signal. This indication signal is generated by a capacitor bank current sensor <NUM>, which is coupled to the capacitor bank <NUM> and which measures the actual current drawn from the capacitor bank <NUM> and, on the basis of the measured current, generates the indication signal, which is forwarded to the power supply control unit <NUM>. Hence, the control of the charging current fed from the mains power supply <NUM> to the capacitor bank <NUM> is directly governed by the actual current drawn from the capacitor bank <NUM>. Therefore, the actual current drawn from the capacitor bank <NUM> directly triggers a respective recharging of the capacitor bank <NUM> by the mains power supply <NUM>.

According to an alternative second example which is outside the scope of the invention, instead of measuring the current drawn from the capacitor bank <NUM>, a RF amplifier current sensor <NUM> is provided, which is coupled to the RF amplifier <NUM>. The RF amplifier current sensor <NUM> measures the actual current used in the RF amplifier <NUM> for generating the RF pulses. On the basis of this current, the RF amplifier current sensor <NUM> generates the indication signal, which is sent to the power supply control unit <NUM> for controlling the generation of electric energy in the mains power supply <NUM>. Hence, according to the second example, the actual current used in the RF amplifier <NUM> triggers the recharging of the capacitor bank <NUM>.

It is to be noted that though the second example here is described as an alternative to the first example described before, the options according to the first and second examples, respectively, may also be combined. This means that the RF transmitter <NUM> may be provided with a capacitor bank current sensor <NUM> as well as a RF amplifier current sensor <NUM> for generating signals indicative of the required current in the RF transmitter <NUM> and, hence, for a respective recharging of the capacitor bank <NUM> by the mains power supply <NUM>, which is controlled by the power supply control unit <NUM>.

<FIG> schematically depicts a RF transmitter <NUM> according to a third example which is outside the scope of the invention. This third example resembles the second example described before wherein in addition to the charging of the RF amplifier <NUM> by the capacitor bank <NUM> a direct charging of the RF amplifier <NUM> by the mains power supply <NUM> is possible due to a direct current line <NUM> between the mains power supply <NUM> and the RF amplifier <NUM>. Hence, not only the current from the capacitor bank <NUM> but also the current, which is directly obtained from the mains power supply <NUM> may be used to generate RF power, which allows for a smaller capacitor bank <NUM> at the same performance level.

In <FIG> a MR system <NUM> according to a preferred embodiment is shown As commonly known for MRI systems, also the present MRI system <NUM> according to the preferred embodiment of the invention is controlled by a MRI control unit <NUM>, which is omitted in <FIG> but explicitly depicted in <FIG>. This MRI control unit <NUM> is coupled to an information unit <NUM>, which is further coupled to the RF transmitter <NUM> according to the preferred embodiment of the invention. This RF transmitter <NUM> is depicted in more detail in <FIG>. From <FIG> it can be seen that the information unit <NUM> is coupled to the power supply control unit <NUM>. Actually, the information unit <NUM> receives information from MRI control unit <NUM> about the MRI process performed by the MRI system <NUM>. In this way, the information unit <NUM> generates an indication signal on the basis of the upcoming current drawn from the capacitor bank <NUM> on the basis of the upcoming RF power demand for the transmit coil <NUM> of the MRI system, which is fed to the information unit <NUM> by the MRI control unit <NUM>. In this way, it is even possible to feed a charging current to the capacitor bank <NUM> directly before a RF pulse is generated and forwarded to the RF transmit coil <NUM>. Thus, this embodiment makes it possible to prevent conceivable drops in available RF power on the basis of the planned MRI process, which is executed by the MRI system <NUM>.

Altogether, the present invention addresses the power supply control for the RF amplifier <NUM> of the RF transmitter <NUM> of a MRI system <NUM> by using extra information on required charging current for the capacitor bank <NUM>. This information is used to activate the recharging of the capacitor bank <NUM> in time and in some cases even before a RF energy drop has occurred. In this way, the generation of RF pulses for MRI applications becomes more efficient making it possible to use smaller capacitor bank values at the same performance level of the system.

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 defined by the claims and not limited to the disclosed embodiments.

Further, for the sake of clearness, not all elements in the drawings may have been supplied with reference signs.

Claim 1:
MRI system with an RF transmit coil (<NUM>) and an RF transmitter (<NUM>) comprising:
- an RF amplifier (<NUM>) for generating RF pulses and for forwarding these RF pulses to the RF transmit coil (<NUM>) of the MRI system (<NUM>),
- a capacitor bank (<NUM>) coupled to the RF amplifier (<NUM>), for storing electric energy and for providing the RF amplifier (<NUM>) with a current for generating the RF pulses,
- a mains power supply (<NUM>) coupled to the capacitor bank (<NUM>), for generating a charging current for charging the capacitor bank (<NUM>) with electric energy, and
- a power supply control unit (<NUM>) coupled to the mains power supply (<NUM>), for controlling the generation of the charging current for the capacitor bank (<NUM>), characterised in that the MRI system further comprises
- an information unit (<NUM>) coupled to the power supply control unit (<NUM>) and being adapted for generating an indication signal on the basis of an upcoming current drawn from the capacitor bank (<NUM>) on the basis of an upcoming RF power demand for the transmit coil (<NUM>) of the MRI system (<NUM>), wherein
the power supply control unit (<NUM>) is adapted for receiving the indication signal indicating the upcoming current drawn from the capacitor bank (<NUM>) and for controlling the generation of the charging current for the capacitor bank (<NUM>) on the basis of the indication signal.