Paul Chang

Paul Chang

Alumni of the Research Group MR Spectroscopy

Main Focus

Background

Mechatronics engineer specialising in control systems and digital electronics. Projects undertaken thus far include:

  • image processing in magnetic resonance imaging (MRI);
  • controller design with applications in fractional and integer proportional-integrator-differentiator (PID) controllers;
  • software development for pH chemical sensing platforms.

Motivation

The magnetic eld strengths of magnetic resonance imaging (MRI) systems have been increasing since the MRI was first invented. The reason for thisupward trend in eld strength is the expected increases in:

  • signal-to-noise ratio (SNR);
  • blood oxygen level-dependent (BOLD) contrast; and
  • spectral resolution.

Despite the benefits of higher field strengths, there are also a number of drawbacks. One of these drawbacks is the decrease in the homogeneity of the B0 (main static) field. This degrades all of the potential benefits that can be gained at higher field strengths.

This project aims to homogenise the B0 field in real-time using a feedback system with field monitoring.

2015-02-09

Real-time Feedback B0 Shim System for Ultra-high Field MRI

Overview

The aim of this project is to improve current dynamic shim updating (DSU) methods which will be done by using a feedback system to control the shims based on field measurements.

By using a feedback system the cross-coupling between the shim coils, eddy currents induced by shim coils and disturbances in the static field due to physiological artifacts will be be compensated. The control system needs to switch shim coil currents quickly and efficiently and be robust against noise and disturbances.

Dynamic Shimming

Shim coils generate a set of 3D basis functions (typically spherical harmonic functions) to produce the required field.

Static shimming sets the shim terms before the measurement and these remain the same during the measurement. Dynamic shimming is the process of shimming while using different shim terms for each slice (or voxel) that is being measured at the time.

This requires developing a robust algorithm that calculates the spherical harmonic function. This software application needs to provide a user-friendly interface for defining regions of interest (ROIs) and calculating shim terms.

Dynamic shimming requires amplifiers that can be dynamically changed and for this purpose the Resonance Research Inc. MXW 28ch shim amplifiers are used. These include spherical harmonic functions up to the 4th order and partial 5th and 6th order functions.

Field Monitoring

To implement a feedback system we require a hardware to monitor the B0 field. This is done using a custom built field camera which consists of an array of field probes. This setup is used to characterise the shim system and to monitor the actual field during experiments.

The field probes produces free induction decay (FID) signals which have frequencies related to the strength of the field at that position. Hence, these probes can be used to measure the magnetic fields. The measurements are sensitive to the positions of the probes and therefore the positions need to be accurately determined for reliable measurements.

Controller

The controller used for the real-time feedback system is implemented on an embedded system for fast digital control. This was preferred over an analogue implementation because digital controllers are more flexible.

The field monitoring system data is input into the controller and compared with the set-points (which are the desired shim terms calculated from the B0 shim algorithm). The data are processed and the controller outputs are then sent to the gradient and shim amplifiers.

The controller is implemented on a system-on-chip Zynq 7020 board. This provides FPGA functionality to interface with custom Siemens gradient system communications, as well as a dual-processing system which provides access to common communication protocols and easier software design. An asymmetric multiprocessor (AMP) is implemented running Linux on one core and a standalone program on the other core.

Curriculum Vitae

Current

Max Planck Institute of Biological Cybernetics

2013-Current

Control/MR Engineer (PhD Student)

High-field Magnetic-Resonance Group

PhD in Neural and Behavioural Science

Graduate Schoolof Neural and Behavioural Science

Project: Real-time Feedback B0 Shim System for Ultra-high Field MRI

Education

1) Imperial College London

2011-2012

MSc in Control Systems

Department of Electrical Engineering

Thesis: Development of Versatile Software for Chemical Sensing Applications

2) University of Cape Town

2007-2010

BSc (Eng) in Mechatronics

Department of Electrical Engineering

Other Majors: Mathematics and Economics

Thesis: Combination of functional and structural MRI with rapid prototyping as a neurosurgical tool

3) Westville Boys' High School

2002-2006

Publications/Conference Papers

1) "The dual role of Parylene C in chemical sensing: Acting as an encapsulant and as a sensing membrane for pH monitoring applications" in Sensors and Actuators B: Chemical 2013

2) "Sensing H+ with conventional neural probes" in Applied Physics Letters 2013

3) "Free-standing Parylene C films as flexible pH sensing membranes" in IEEE Sensors Conference 2013

4) "Preoperative Three-Dimensional Model Creation of Magnetic Resonance Brain Images as a Tool to Assist Neurosurgical Planning" in Stereotactic and Functional Neurosurg 2013

5) "Comparison of PI and FPI Controllers on a Hydraulic Canal using Pareto Fronts" in CISSE 2012

6) "Combination of Functional and Structural MRI with Rapid Prototyping as a Neurosurgical Tool" in ISMRM 2011

7) "Programming Styles for an Open Source Epanet Project" in WDSA 2010

Work Experience

1) University of Cape Town

2011

Academic Course Tutor

Description: Tutored undergraduate courses. This involved academic and lab assistance and marking coursework.

2) Open Box Software

2010 (2 months)

Intern Software Developer

Description: Vacation work as an informations system software developer. Involved website development and writing software for the back-end database.

3) Stone Three Venture Technology

2009 (2 months)

Intern Engineer

Description: Vacation work that involved building hardware to froth analyzers. Writing software applications for in-house use.

3) Altech UEC

2007 (2 months)

Intern Engineer

Description: Vacation work that involved writing in-house software applications as well as testing hardware devices.

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