1. Abstract Motivation Background Experimental Methods/Results Future Work References Acknowledgements Neal Haas Peter Kleinschmidt Anne Loevinger Luisa Meyer Client: Orhan Unal, PhD 1 and Krishna Kurpad, PhD Advisor: Walter Block, PhD 2 1 Department of Medical Physics/Radiology – School of Medicine and Public Health University of Wisconsin – Madison 2 Department of Biomedical Engineering – University of Wisconsin Madison Dr. Orhan Unal—Department of Medical Physics/Radiology Dr. Krishna Kurpad—Department of Medical Physics/Radiology Dr. Susan Hagness—Department of Electrical and Computer Engineering Dr. Walter Block—Department of Biomedical Engineering The clinical MRI apparatus is not always the most convenient tool for use, therefore a smaller system with more rapid imaging is desired. The new, low-cost desktop unit would incorporate different gradient coil windings and a more efficient and easily modifiable computer operating system that would allow for the faster imaging. This modular system would be used for development and research prototyping. Goal: to develop a set of gradient coils in the z-direction (see figures) for the desktop MRI system. Discussion Hall Probe As part of a broader goal to develop novel applications of Magnetic Resonance Imaging (MRI), a low-cost and modular MRI system is currently being developed. One component of this project is the design, construction and testing of gradient coils to function within the system. Several common gradient coil designs serve as a basis for development. A simulation script was developed to approximate magnetic field strengths produced by a given coil design. In order to validate the simulation, a testing environment was developed using a Hall Effect Probe to measure the magnetic fields created by a coil of wires. A set of coils was constructed based on a Golay pair concept with the aim of generating a linearly varying x-gradient. Though the measured data showed some variability, it corresponded fairly well to the simulated fields. The simulation showed a high degree of linearity within a confined area of the magnetic field. The measured field also showed some linearity, though less reliably. Simulation Biot-Savart Law for magnetic field, B MATLAB script used to simulate magnetic field using the Biot-Savart Law Actual Field Simulated Field Validated shape of magnetic field created against simulated field – proved validity of simulations Many variables may be tainting results of findings o Key variable is noise in signal from hall probe Current circuit design is very economical. Commercially developed probes can be quite expensive, but will provide the needed accuracy to measure the fields created. Limitations Hall probe sensitivity Current available Hall probe baseline inconsistencies. Environment not free from other magnetic disturbances. Example: Power Supply Coil Design The coils designed in this project represent a proof of concept for generating the desirable fields. Refine coil design to optimize linearity. o Larger Image area can be obtained by separating coils more Increased strength of the field generated can be achieved by increasing current This requires more consideration of resistivity and heat tolerances of the wire Experimental Design The signal from the hall probe was quite noisy, and showed strong evidence of interference from other electromagnetic fields nearby in the lab. o This can be achieved by isolating the coils/probe. Improve Hall Probe accuracy by filtering noise/outside disturbances. Use more precise calibration of hall probe. Obtain data at a more fine scale o Mechanically automate data acquisition for faster validation o Acquire data for all horizontal planes of the coils rather than just about the central horizontal plane. Source: http://www.magnet.fsu.edu/education/tutorials/magnetaca demy/mri/page5.html Magnetic Resonance Imaging (MRI) is a non-invasive medical imaging technique which creates cross-sectional images of the body. MRI systems include: A main magnetic field - aligns nuclear spins of protons in a specimen; A radio frequency (RF) amplifier – pushes the protons out of alignment with the main field; A data acquisition unit - picks up radio waves emitted as poles of the proton realign; Gradient coils – in x-, y-, and z- directions create linearly varied magnetic fields to spatially code the data, o Gradient strength - measures change of field strength over a distance (mT/m) and is directly proportional to current supplied to the coil o Slew rate - how quickly the gradient coils can be turned on or off and depends on the inductance of the coils and the quality of the voltage amplifier. Setup: Current, Voltmeter, and Magnetic field perpendicular to each other. Magnetic Field causes force on moving charges in orthogonal directions. Displaced charges create a potential difference, or voltage. Measured voltage is proportional to magnetic field. Our probe has three appendages: 5V, Vout, Ground Block, W.F., et al. (2006). Magnetic Resonance Imaging. In Encyclopedia of Medical Devices and Instrumentation (2nd ed.). John Wiley & Sons, Inc. Haacke, E. Mark et al. Magnetic Resonance Imaging: Physical Principles and Sequence Design. Hardcover: June 15, 1999. Sanchez, H. et al. “A Simple Relationship for High Efficiency-Gradient Uniformity Tradeoff in Multilayer Asymmetric Gradient.” IEEE Transaction on Magnetics. Vol. 43, No. 2, February 2007. Stang, P. et al. “Experiments in Real-Time MRI with RT-Hawk and Medusa.” Intl. Soc. Mag. Reson. Med. 16 (2008). Pg. 348. Amplification Circuit manipulates Hall Probe signal to be read by DMM. Amplifies signal by 100. Stand to hold probe has graduated heights for measurement. Moved freely across grid Coils wound on forms constructed with acrylic o Fastened by aferrous metal screws Hall Probe calibrated using a circular wire on a 3” form. o A calculated field strength for this form was used to generate a calibration constant OBSERVATIONS The simulated field shows a significant degree of linearity at the center of the coils. The actual fields measured show similar trends to the simulated fields The field at the center of the coils yields a linear regression of 0.92, but behaves nonlinearly at the center of the coils There appeared to be numerous disturbances to the Hall probe including amplification noise and environmental EM artifacts