Transcript Slide 1

Neal Haas
Peter Kleinschmidt
Anne Loevinger
Luisa Meyer
1 and
Client: Orhan Unal, PhD
Krishna Kurpad, PhD
2
Advisor: Walter Block, PhD
Background
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 zdirections 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.
Source:
http://www.magnet.fsu.edu/education/tutorials/magnetaca
demy/mri/page5.html
Motivation
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.
1 Department
of Medical Physics/Radiology – School of Medicine and Public Health
University of Wisconsin – Madison
2Department of Biomedical Engineering – University of Wisconsin Madison
Abstract
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.
Experimental Methods/Results
• 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
Simulation
• Biot-Savart Law for magnetic field, B
Goal: to develop a set of gradient coils in the z-direction
(see figures) for the desktop MRI system.
• Amplification Circuit manipulates Hall Probe
signal to be read by DMM. Amplifies signal
by 100.
Simulated Field
Hall Probe
• 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
• MATLAB script used to simulate magnetic field using the
Biot-Savart Law
Actual Field
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
Discussion
• 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
Future Work
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.
oThis 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
oMechanically automate data acquisition for faster validation
oAcquire data for all horizontal planes of the coils rather than
just about the central horizontal plane.
References
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.
Acknowledgements
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