MRI gradient amplifier operable at different slew rates

Abstract

The present invention relates to a method for supplying current to a gradient coil (107,207) of a magnetic resonance imaging system (100) by a gradient amplifier (222), the method comprising: supplying, by an electrical power supply (241), a voltage at first level to the gradient amplifier output to generate a gradient current in the gradient coil (107,207) to produce a magnetic gradient field at an slew rate, wherein the slew rate is set to a first value, resetting the slew rate to a second value, comparing the second value to the first value, and adjusting the voltage to a second level if the second value is different from the first value.

Claims

1. A magnetic resonance imaging system comprising: a gradient coil, a gradient amplifier for supplying current to the gradient coil, the gradient amplifier output being connected to the gradient coil, the gradient amplifier comprising: a control module with a digital controller and a modulator to provide pulse width modulation (PWM) signals a power chain with a power electronic stack and a filter to supply a filtered voltage across the gradient coil wherein the power electronic stack is provided with an electrical power supply and a capacitor arranged in parallel with a bridge switching power stage, wherein the electrical power supply is configured to supply a voltage at a first level over the bridge switching power stage to apply the filtered output voltage across the gradient coil to generate a gradient current in the gradient coil to produce a magnetic gradient field at a slew rate, wherein the slew rate is set to a first value; and a controller coupled to the electrical power supply, wherein the digital controller is adapted for resetting the slew rate to a second value including determining electrical power dissipated by the gradient coil currently producing the magnetic gradient field by multiplying the square of the current in the gradient coil by the resistance of the winding of the gradient coil and selecting the second value of the slew rate from one or more predetermined slew rate values if the dissipated electrical power exceeds a predetermined maximum allowed dissipation value, comparing the second value to the first value, and controlling the electrical power supply to adjust the voltage over bridge switching power stage to a second level if the second value is different from the first value.

2. The magnetic resonance imaging system of claim 1, wherein the resetting happens before data acquisition by the magnetic resonance imaging system.

3. The magnetic resonance imaging system of claim 1, wherein the resetting happens during data acquisition by the magnetic resonance imaging system.

4. The magnetic resonance imaging system of claim 1, wherein the first value is a maximum slew rate value allowed for the MRI system.

5. The magnetic resonance imaging system of claim 1, wherein the voltage at second level V.sub.2 is determined from the voltage at first level V.sub.1 using the equation:
V.sub.2=SV.sub.1, where S=max(SR/SR.sub.max,0.5) where SR is the second value of the slew rate and SR.sub.max is a maximum slew rate value allowed for the MRI system.

6. A computer program product comprising computer executable instructions, wherein execution of instructions effects to perform the method steps of a control module as set forth in claim 1.

7. The magnetic resonance imaging system of claim 1, wherein the second value is below the first value and the voltage at second level is smaller than the voltage at first level.

8. A gradient amplifier for supplying current to a gradient coil of a magnetic resonance imaging system, the gradient amplifier having an output operable for being connected to the gradient coil, the gradient amplifier comprising: a control module with a digital controller and a modulator to provide pulse width modulation (PWM) signals a power chain with a power electronic stack and a alter to supply a filtered voltage across the gradient coil wherein the power electronic stack is provided with an electrical power supply and a capacitor arranged in parallel with a bridge switching power stage, wherein the electrical power supply is configured to supply a voltage at a first level over the bridge switching power stage to apply the filtered output voltage across the gradient coil to generate a gradient current in the gradient coil to produce a magnetic gradient field at a slew rate, wherein the slew rate is set to a first value; a controller coupled to the electrical power supply being adapted for resetting the slew rate to a second value including determining electrical power dissipated by the gradient coil currently producing the magnetic gradient field by multiplying the square of the current in the gradient coil by the resistance of the winding of the gradient coil and selecting the second value of the slew rate from one or more predetermined slew rate values if the dissipated electrical power exceeds a predetermined maximum allowed dissipation value, comparing the second value to the first value, and controlling the electrical power supply to adjust the voltage over the bridge switching power stage to a second level if the second value is different from the first value.

9. The gradient amplifier of claim 8, wherein the second value is below the first value and the voltage at second level is smaller than the voltage at first level.

10. The gradient amplifier of claim 8, wherein the first value is a maximum slew rate allowed for the MM system.

11. The gradient amplifier of claim 8, wherein the voltage at second level V.sub.2 is determined from the voltage at first level V.sub.1 using the equation:
V.sub.2=SV.sub.1, where S=max(SR/SR.sub.max,0.5) where SR is the second value of the slew rate and SR.sub.max is a maximum slew rate allowed for the MRI system.

12. A method for supplying current to a gradient coil of a magnetic resonance imaging system by a gradient amplifier comprising a bridge switching power stage, the method comprising: supplying, by an electrical power supply, a voltage over the bridge switching power stage at first level to the gradient amplifier output to generate a gradient current in the gradient coil to produce a magnetic gradient field at a slew rate, wherein the slew rate is set to a first value, resetting the slew rate to a second value by operations including: determining electrical power dissipated by the gradient coil currently producing the magnetic gradient field by multiplying a square of the current in the gradient coil by a resistance of a winding of the gradient coil; and selecting the second value of the slew rate from one or more predetermined slew rate values, if the dissipated electrical power exceeds a predetermined maximum allowed dissipation value; comparing the second value to the first value, and adjusting the voltage over the bridge switching power stage to a second level if the second value is different from the first value.

13. The method of claim 12, wherein the second value is below the first value and the voltage at second level is smaller than the voltage at first level.

14. The method of claim 13, wherein the voltage at second level V.sub.2 is determined from the voltage at first level V.sub.1 using the equation:
V.sub.2=SV.sub.1, where S=max(SR/SR.sub.max,0.5) where SR is the second value of the slew rate and SR.sub.max is a maximum slew rate value allowed for the MRI system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 illustrates a magnetic resonance imaging system,

(3) FIG. 2 shows a schematic diagram for a gradient amplifier,

(4) FIG. 3 shows a typical trapezoidal current through the gradient coil, and

(5) FIG. 4 shows a flowchart of a method for supplying current to a gradient coil of a magnetic resonance imaging system by a gradient amplifier.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) In the following, like numbered elements in these figures are either similar elements or perform an equivalent function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

(7) FIG. 1 illustrates an exemplary magnetic resonance imaging (MRI) system 100 for generating images of a patient 101. MRI system 100 comprises magnetic assembly 103 to generate magnetic fields that will be applied to patient 101. Magnetic assembly 103 comprises magnet coils 105 adapted to produce a static magnetic field required to perform magnetic resonance imaging and gradient coils 107. The gradient coils 107 are made up of an X-axis gradient coil, Y-axis gradient coil, and Z-axis gradient coil. This enables to image different regions of the patient 101.

(8) MRI system 100 further comprises a gradient amplifier unit 109, and a control module 111. The gradient amplifier unit 109 includes an X-axis gradient amplifier Gx, Y-axis gradient amplifier Gy, and Z-axis gradient amplifier Gz. The gradient coil 107 is connected with the gradient amplifier 109. The X-axis gradient coil, Y-axis gradient coil, and Z-axis gradient coil of the gradient coil 107 are connected, respectively, with the Gx amplifier, Gy amplifier and Gz amplifier of the gradient amplifier 109.

(9) A gradient magnetic field in an X-axis direction, gradient magnetic field in a Y-axis direction, and gradient magnetic field in a Z-axis direction are formed, respectively, by electric currents supplied to the X-axis gradient coil, Y-axis gradient coil, and Z-axis gradient coil, respectively, from the Gx amplifier, Gy amplifier and Gz amplifier of the gradient amplifier. Control module 111 is connected with the gradient amplifier 109.

(10) Control module 111 generates control signals for controlling the gradient amplifier. In particular, control module 111 may generate control signals that induce gradient amplifier unit 109 to energize gradient coils 107.

(11) Controller 111 controls the electrical power supply 115. The electrical power supply 115 supplies a voltage for the gradient amplifier 109 to generate a gradient current in the gradient coil 107 to produce a magnetic gradient field.

(12) The controller 111 is shown as being connected to a hardware interface 154 of a computer system 152. The computer system 152 uses a processor 156 to control the magnetic resonance imaging system 100.

(13) The computer system 152 shown in FIG. 1 is illustrative. Multiple processors and computer systems may be used to represent the functionality illustrated by this single computer system 152. The computer system 152 comprises the hardware interface 154 which allows the processor 156 to send and receive messages to components of the MRI system 100. The processor 156 is also connected to a user interface 158, computer storage 160, and computer memory 162.

(14) The computer storage 160 is shown as containing MRI scan parameters 168. One of the scan parameters is the slew rate. The slew rate value may be stored by the controller 113.

(15) The computer storage 160 is further shown as containing a pulse sequence 170.

(16) The pulse sequence 170 either contains instructions or it contains a timeline which may be used in accordance with the scan parameters for constructing instructions which enable the magnetic resonance imaging system 100 to acquire magnetic resonance data 172.

(17) The computer storage 160 is shown as storing magnetic resonance data 172 acquired by the magnetic resonance imaging system 100.

(18) The computer memory 162 is shown as containing a module 174. The module 174 contains computer-executable code which enables the processor 156 to control the operation and function of the MRI system 100. For example the module 174 may use the pulse sequence 170 to acquire the magnetic resonance data 172.

(19) FIG. 2 shows a simplified architecture of an electronic system 200 comprising a gradient amplifier 222 and a gradient coil 207. Gradient amplifier 222 comprises a control module 201, optional, and a power chain 203. The control module 201 generates control signals for the power chain 203 in such a way that a setpoint received digitally from a source, not shown, such as a data acquisition system controller is accurately reproduced at the output of the power chain 203. The power chain 203 converts the main power to high voltage and high current that drive the gradient coil 207.

(20) The control module 201 comprises a controller 243 and a modulator 211. The controller 243 continuously dictates to the modulator 211 the required modulation setpoint in terms of output voltage based on the setpoint, actual and past measured output current and boundary conditions like voltages, damping the output filter, etc.

(21) The modulator 211 converts the modulation setpoint from the digital controller 209 into suitable Pulse Width Modulation (PWM) signals for all individual gate driver units of the power chain 203. These PWM signals are optimized for high voltage bandwidth and high ripple frequency under the condition that the first voltage is within defined limits. The controller 243 is coupled to an electrical power supply 241. The controller 243 may control the electrical power supply 241 to adjust the additional voltage V.sub.1 in accordance with a pre-determined slew rate of the gradient current through the gradient coil. For example, the controller 243 may control the electrical power supply 241 to reduce the voltage V.sub.1 if the pre-determined slew rate is below the slew rate corresponding to V.sub.1. The controller 243 comprises a first unit 245 for resetting the slew rate to a second value, in response to a determination by as second unit 247 that the first slew rate needs to be changed, a third unit 249 for comparing the second slew rate to a predetermined threshold value, and a forth unit 251 for controlling the electrical power supply 241 to adjust the voltage to a second level if the second value is below the first value.

(22) MRI scans that do not benefit from the maximum amplitude/slew rate are limited in maximum level of amplitude/slew rate during the waveform generation on the scan control computer. Those scans can be characterized beforehand, e.g. based on scan technique, echo time, echo spacing, resolution, etc. For those scans the controller 243 selects a lower value as maximum of the system. There are also system related reasons not to use the maximum amplitude and/or slew rate. This may for instance be related to the dissipation in the gradient coil at high frequencies. The usage of high amplitude/slew rate may also increase the probability to induce peripheral nerve stimulation (PNS). The controller 243 has a unit (not shown) to predict PNS and is can limit the maximum amplitude/slew rate to stay within certain PNS limits. The user can also modify the recipe on the host computer to limit the scan. This can be indirectly, e.g. by limiting the system to a lower noise level, using a lower maximum slew rate (SofTone) or directly setting the maximum slew rate in this recipe. In other words, there are multiple opportunities to avoid the maximum slew rate. The maximum slew rate is not very often used.

(23) The power chain 203 consists of a number of blocks that convert the main power to suitable high voltage and high current that drive the gradient coil 207. The power chain 203 comprises a power stack 213. The power stack 213 comprises an electrical power supply 241 that provides the main power. FIG. 3 shows a typical trapezoidal current 300 through the gradient coil 207. During the flat top portion of the trapezoid 301 the gradient amplifier needs to balance ohmic losses in the gradient coil in order to maintain coil current. Those losses require relatively low voltage V.sub.0 from the electrical power supply. To generate fast current gradients through the gradient coil 207 during the ramped portions 303 of the trapezoid, a high voltage across its terminals is required. That is, the inductance of the gradient coil 207 requires an additional voltage V.sub.1, i.e., the voltage at the first level, proportional to the rate of current change. The slew rate may be defined as the ratio of i.sub.c 305 and t 307.

(24) The main power is further filtered, rectified and stabilized to a nominal voltage. The power chain 203 comprises a power electronic stack 213, a filter 215 and a current sensor 217. The power electronic stack 213 comprises a capacitor 219 which is connected in parallel with a bridge 221 switching power stage. The bridge 221 may be for example a metal-oxide-semiconductor field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT) bridge. Switches 223 and 225 constitute a first half-bridge, 227 and 229 the second half-bridge. The half-bridges are separately driven by pulse width modulators of the control module 201.

(25) The control module 201 is connected with the four switches 223, 225, 227 and 229 via four respective lines 231. The power stack 213 generates a precise and controlled output stage voltage 233 from the main voltage by pulse-width modulation. A residual ripple is filtered out by the filter 215, and the filtered voltage 235 is across the gradient coil 207 as an output voltage. The filter may be for example a low pass filter.

(26) The sensor 217 may produce a feedback signal to the digital controller 209 indicative of the magnetic gradient field produced for the gradient coil.

(27) For a magnetic resonance imaging system, typically there will be one gradient power supply such the one described in FIG. 2 for each of the three different orthogonal directions.

(28) FIG. 4 is a flowchart of a method for supplying current to a gradient coil of a magnetic resonance imaging system by a gradient amplifier.

(29) In step 401, the electrical power supply supplies a voltage at first level to the gradient amplifier output to generate a gradient current in the gradient coil to produce a magnetic gradient field at a slew rate. The slew rate is set to a first value.

(30) In step 403, the controller 243 resets the slew rate to a second value, in response to the determination that the first slew rate needs to be changed. The resetting happens before or during data acquisition by the magnetic resonance imaging system. The change of the slew rate value may be driven by dissipation in the gradient coil caused and/or the power stacks. The determination that the first slew rate needs to be changed comprises determining electrical power dissipated by the gradient coil currently producing the magnetic gradient field; and selecting the second slew rate from one or more predetermined slew rates, if the dissipated electrical power exceeds a predetermined maximum allowed dissipation value.

(31) In another example, the determination that the first slew rate needs to be changed comprises receiving a request to reduce the first slew rate from a user of the magnetic resonance imaging system, the request being indicative of the second slew rate.

(32) In step 405, the second value is compared to the first value. The value is a maximum allowable slew rate. The voltage at second level V.sub.2 may be determined from the voltage at first level V.sub.1 using the equation:
V.sub.2=SV.sub.1, where S=max(SR/SRmax,0.5)
where SR is the second value of the slew rate and SRmax is a maximum allowable slew rate.

(33) In step 409, the voltage is adjusted to a second level if the second value is different from the first value.

LIST OF REFERENCE NUMERALS

(34) 100 MRI system 101 patient 103 magnetic assembly 105 magnet coil 107 gradient coil 109 gradient amplifier 111 control module 113 controller 115 electrical power supply 152 computer system 154 hardware interface 156 processor 158 user interface 160 computer storage 162 computer memory 168 scan parameters 170 pulse sequence 172 magnetic resonance data 174 module 200 electronic system 222 gradient amplifier 201 control module 203 power chain 207 gradient coil 209 digital controller 211 modulator 213 power stack 215 filter 217 sensor 219 capacitor 223-229 switches 231 line 233 output voltage 235 filtered voltage 241 electrical power supply 243 controller 300 current 301 trapezoid 303 ramped portions 305 i.sub.c 307 t