Query: Magnetic Resonance Imaging Power: High-Performance MVA Gradient Drivers

Candidate papers:

abstract: In high performance magnetic resonance imaging systems, the gradient driver is required to supply the gradient coil with a large current >600 A) and a high voltage >2000 V) to achieve a strong gradient field and a fast slew rate. In addition, extremely high fidelity for reproducing the current command from the central system is very critical to imaging quality. This paper presents the solutions for the different elements of the driver: 1) high bandwidth (BW) power inverter; 2) ripple cancelation filter; 3) multioutput power supply (PS) and 4) digital control. A high BW power inverter requires a stacked bridges structure to achieve a high output ripple frequency with the existing commercial power semiconductor modules. The high voltage and the high frequency for large power modules can be obtained easier and with lower loss using the new silicon carbide devices. The control needs a digital implementation and a very fast processor. Digital control provides compensation and feedforward to improve the response. A capability improvement is obtained by reducing the switching frequency when large currents with a very low frequency variation are needed. The control can handle it very well, but the filter has to be designed to eliminate more than one ripple frequency. Finally, many PS solutions have been used for multiple isolated outputs, but digital control compensation permits the use of much simpler unregulated PS and keeps the performance. A 2 MVA, 900 A/1200 V, platform has been built and fully tested. The experimental results proved the validity of the proposed structure and the modulation technique.
author_names: J A Sabate

Ruxi Rudy Wang

Fengfeng Tao

Song Chi
title: Magnetic Resonance Imaging Power: High Performance MVA Gradient Drivers
venue: IEEE Journal of Emerging and Selected Topics in Power Electronics
year: 2016

abstract: A high power, high efficiency switching amplifier capable of generating arbitrary gradient coil waveforms for a magnetic resonance imaging system is described. A high frequency "partial voltage" buck regulator feeds a "full voltage" full bridge inverter to relieve the full bridge of the necessity of switching when high voltage is needed by the load. When high voltage is needed, the partial voltage buck circuit that employs faster lower voltage IGBT devices assumes the switching duty. When low voltage is needed, the high voltage bridge inverter, operating from a low voltage DC bus, assumes the switching duty. Since switching losses are approximately proportional to bridge input voltage, the switching losses incurred by the inverter are minimized.
author_names: Robert Steigerwald

William Frederick Wirth
title: High power, high performance switching amplifier for driving magnetic resonance imaging gradient coils
venue: 2000 IEEE 31st Annual Power Electronics Specialists Conference. Conference Proceedings (Cat. No.00CH37018)
year: 2000

abstract: Access to scanners for magnetic resonance imaging (MRI) is typically limited by cost and by infrastructure requirements. Here, we report the design and testing of a portable prototype scanner for brain MRI that uses a compact and lightweight permanent rare earth magnet with a built in readout field gradient. The 122 kg low field (80 mT) magnet has a Halbach cylinder design that results in a minimal stray field and requires neither cryogenics nor external power. The built in magnetic field gradient reduces the reliance on high power gradient drivers, lowering the overall requirements for power and cooling, and reducing acoustic noise. Imperfections in the encoding fields are mitigated with a generalized iterative image reconstruction technique that leverages previous characterization of the field patterns. In healthy adult volunteers, the scanner can generate T1 weighted, T2 weighted and proton density weighted brain images with a spatial resolution of 2.2 x 1.3 x 6.8 mm3. Future versions of the scanner could improve the accessibility of brain MRI at the point of care, particularly for critically ill patients.
author_names: Clarissa Zimmerman Cooley

Patrick C McDaniel

Jason P Stockmann

Sai Abitha Srinivas

Stephen F Cauley

Monika Sliwiak

Charlotte R Sappo

Christopher F Vaughn

Bastien Guerin

Matthew S Rosen

Michael H Lev

Lawrence L Wald
title: A portable scanner for magnetic resonance imaging of the brain.
venue: Nature biomedical engineering
year: 2020

abstract: As radiologists demand increased power, speed, and duty cycle from their magnetic resonance imaging (MRI) systems, thermal management of the gradient driver sub system becomes more challenging. MRI systems contain three IGBT based gradient drivers that apply current waveforms to an inductive load located inside a cylindrical magnet. The effectiveness of the gradient driver cooling system determines the power, speed, and duty cycle of the MRI system. MRI gradient drivers are unique in the world of power electronics in that they must be able to generate arbitrary current waveforms with extremely high fidelity. This high fidelity requirement means that the IGBTs must switch at frequencies greater than >10 kHz, resulting in die level heat fluxes approaching 100W/cm2, dictating the use of liquid cooling. The gradient driver cooling system design is further challenged by a continuing loss of die level thermal time constant as IGBT manufacturers incorporate thinner die with each new generation of IGBTs. The loss of die level thermal time constant results in higher IGBT junction temperature excursions, placing more demands on cold plate performance. Numerous experiments were performed to characterize IGBT switching losses and transient thermal behavior. The data was used to create an electrical/thermal model to predict heat generation, junction temperature rise, and reliability. Monte Carlo simulations were performed on the electrical/thermal model to assess the impact of variations in IGBT manufacturing, cooling system performance, and scan sequences in order to derive requirements for a cold plate design. Cold plate performance was experimentally verified and the design was deemed robust, or insensitive to sources of variation
author_names: G K Morris

J A Galie
title: Design of a robust cold plate for high power gradient drivers in magnetic resonance imaging systems
venue: Thermal and Thermomechanical Proceedings 10th Intersociety Conference on Phenomena in Electronics Systems, 2006. ITHERM 2006.
year: 2006

abstract: In high performance magnetic resonance imaging (MRI) systems, the gradient driver is required to supply the gradient coil with large amplitude >600A) and high slew rate >2A/us) current waveform. In addition, extremely high fidelity for reproducing the current command from the central system is very critical for imaging quality. This paper presents an improved gradient driver structure and a new modulation scheme, with which the switching frequency will vary according to the slew rate of the current command. The corresponding output filter design is also introduced. With the proposed technique the switching loss can be reduced and evenly distributed in all the bridges, therefore the utilization ratio of the semiconductor devices is improved and the number of devices in parallel can be reduced. A 2 MVA platform has been built and fully tested. The experimental results proved the validity of the proposed structure and modulation technique.
author_names: Rixin Lai

J A Sabate

Song Chi

Wesley M Skeffington
title: High performance gradient driver for magnetic resonance imaging system
venue: 2011 IEEE Energy Conversion Congress and Exposition
year: 2011

abstract: This paper presents a magnetic resonance imaging gradient amplifier power supply with a series resonant converter (SRC) using an innovative optimal phase shift control. This provides very fast response by eliminating the dynamics associated with the resonant tank. One phase leg has zero voltage switching; the other leg has both zero voltage and zero current switching. This ensures very low switching losses. The switched gradient amplifier load presents specific challenges: multiple outputs with good cross regulation, and very fast load transient response. All load currents vary independently from no load to full load, and output voltages must be tightly regulated for all conditions. The solution presented uses post regulators for half of the output voltages. Several post regulator options are compared based on performance and reliability. The proposed approach has high power density and excellent performance.
author_names: J A Sabate

Michael Joseph Schutten

Qiming Li

William Frederick Wirth
title: Resonant power supply for magnetic resonance imaging gradient drivers
venue: IEEE 34th Annual Conference on Power Electronics Specialist, 2003. PESC '03.
year: 2003

abstract: The magnetic resonance imaging (MRI) gradient driver is a high performance inverter that provides large currents, >1000A, for the gradient magnetic fields used for imaging. It is challenging to achieve the required performance with the fast transitions on current pulses driven with high voltages, >2000V, for fast imaging. The currents provided by the driver are defined by the imaging procedure and are always changing from zero to maximum current, resulting on pulsating load from zero load to 10 times the rated power. The accuracy for the currents supplied to the gradient coils requires regulated voltages provided to the inverter not to sag below 10% of rated value. To avoid voltage sag and to limit peak power draw from the facility power utilities, the system has capacitors for energy storage at the inverter supply voltages and at the input of the power supplies for the inverter. The paper presents the power architecture for power supplies, inverter (gradient amplifier) and energy storage, and how the energy storage is optimized to avoid excessive demands on facility power feed that have to be designed for peak power. The architecture and energy storage selections reduced the input current and minimized the voltage sagging at the inverter input. The solution was implemented, and the results are presented here when operating standard imaging sequences.
author_names: Yash Veer Singh

J A Sabate

Ruxi Rudy Wang
title: Capacitive Energy Storage for Magnetic Resonant Imaging Gradient Driver
venue: 2019 IEEE Applied Power Electronics Conference and Exposition (APEC)
year: 2019

abstract: Gradient coils for magnetic resonance imaging (MRI) require large currents 500 A) for the gradient field strength, as well as high voltage 1600 V) for fast slew rates. Additionally, extremely high fidelity, reproducing the command signal, is critical for image quality. A new driver topology recently proposed can provide the high power and operate at high switching frequency allowing high bandwidth control. The paper presents additional improvements to the new driver architecture, and more importantly, describes the digital control design and implementation, crucial to achieve the required performance level. The power stage and control have been build and tested with the experimental results showing that the performance achieved with the new digital control capability, more than fulfills the system requirements
author_names: J A Sabate

Luis Garces

Paul Michael Szczesny

Qiming Li

William Frederick Wirth
title: High bandwidth high power gradient driver for magnetic resonance imaging with digital control
venue: Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
year: 2005

abstract: A switch mode gradient amplifier based on dual parallel H bridge topology was presented.Dynamic dead time control method was proposed to reduce the reverse recovery current of power MOSFET.The output filter was designed and optimized for both common and differential mode with good damping and small losses.Further a high performance current tracking controller,combing with state feedback,feed forward and nonlinear PI regulations,was proposed based on the decoupling of common and differential mode of the gradient amplifier model.To ease the requirements of sampling system,a simple PI controller was applied instead of the tracking controller during the steady period.Finally,a dual DSP control system was designed to implement the control algorithm,and a 200 A/150 V gradient amplifier was built in the laboratory.Experimental results show that only 1% overshoot occurs when tracking a trapezium current with 200 A amplitude and 200 ms rising time;the low frequency noise is in the order of mA when outputing DC current.
author_names: Jiang Xiaohua
title: Digital Controlled Gradient Amplifier for Magnetic Resonance Imaging
year: 2010

abstract: In Magnetic Resonance Imaging system, the most powerful device is gradient driver, which is also one of the key components to realize fast imaging with high quality. To meet the requirements of high end MRI products, the power rating of the gradient driver increase from several hundred kVA to 1MVA and even higher in the future. Commonly the gradient driver applies full bridge cascade technology and needs a lot isolated DC source with high power capability. To provide solution to this requirement, several technology platforms have been developed. This paper presents one of the most attractive options. In the proposed power supply structure, an AC pre regulator and a multi winding transformer are combined together with diode rectifiers to provide multiple isolated DC outputs. The AC pre regulator compensates the utility voltage variation by injecting voltage to the input line through a series transformer, and that make the multi winding transformer get a stable input. With this solution, the DC outputs voltage is only determined by the transformer leakage inductance and load current. Compared with high frequency DC DC conversion methods, this scheme does not need high frequency magnetic devices and capacitors, which are difficult to build and high cost for high power application. The switching devices (IGBTs) in this option can also operate at a much lower frequency (several kHz) to reduce the switching loss, and lower switching frequency also benefit the device selection. Beside that the rectifiers applied at the secondaries of the multi winding transformer operate at line frequency and that benefits the device selection and cost too. Here in this paper, the control algorithm of the power supply is presented and discussed. Both its self performance and co operation with the amplifier are evaluated with Saber simulation. The results indicate the attractiveness of the proposed scheme.
author_names: Pengcheng Zhu

Yunfeng Liu

J A Sabate
title: Multi output power supply with series voltage compensation capability for Magnetic Resonance Imaging system
venue: 2009 IEEE 6th International Power Electronics and Motion Control Conference
year: 2009