Download MRI Resonator Coil

MRI Resonator Coil
Concept Generation and Selection
The Resonators: Alan Nelson, Brady Quist, Danny Park
09
Table of Contents
Introduction ................................................................................................................................................3
Process Description ................................................................................................................................4
Body of Facts .............................................................................................................................................5
Critical Assumptions ..................................................................................................................................5
Critical Design Areas .................................................................................................................................5
Type of Resonator Coil ..........................................................................................................................6
Considered Resonators.......................................................................................................................6
Definition of Criteria..........................................................................................................................7
Chosen Resonator Coil: Low-Pass Bird Cage ...................................................................................8
Type of Outer Casing .............................................................................................................................8
Considered Outer Casings..................................................................................................................8
Definition of Criteria..........................................................................................................................9
Chosen Casing: We Design & Manufacturing Students Build ..........................................................9
Non-Critical Design Areas .......................................................................................................................10
Introduction
The Resonators group was created in order to design and fabricate a resonator coil for,
and with the help of, Dr. Neal Bangerter of the BYU Electrical Engineering Department.
Dr. Bangerter desires a coil for use in his MRI (Magnetic Resonance Imaging) research.
This coil will be different from most MRI resonator coils in that it will be designed to
excite precessions at the Larmor frequency (determined by the type of atom and the
strength of the main magnetic field) in sodium atoms rather than hydrogen.
MRI's depend upon particular atoms having ½ integer spins for imaging operations. Due
to the magnetic moments that this spin number creates, these atoms can be aligned using
a strong superconducting magnet. In order to get a reading as to the relative density of
this atom in a given sample—as in differentiating tissues within the human body through
a medical MRI—a resonating coil that creates an RF field must be powered up near the
sample. The RF field excites a precession in these atoms. The coil can be powered down
and switched into receive mode to detect the insuing radiation from the now-precessing
atoms. Through manipulation of magnetic fields (and therefore precession frequencies)
and RF pulses, spatial information can be mapped out into a usable image for diagnosing
medical problems.
Almost all MRI resonating coils today are tuned to the frequency required to excite
precessions in Hydrogen atoms. One of the greatest advantages of Hydrogen over other
elements is that it has the best SNR due to its great abundance in the human body.
Recently, attention has been turned to applying MRI to imaging other elements. Of
particular promise is the element sodium. Although sodium is about 10,000 times less
abundant than hydrogen (resulting in low SNR), there is valuable information available
from sodium density imaging of the human body that cannot be gained through hydrogen
imaging. Possible applications include (but are not limited to) the following:
1) Cartilage breakdown resulting deformation is the cause of osteoarthritis, and cannot be
reversed after those deformations have occurred. Hydrogen MRI images show no signs of
this breakdown until it has already caused these deformations. Research shows, however,
that the sodium content in the cartilage is reduced in the early stages of this breakdown.
Drug companies such as GlaxoKleinSmith have shown interest in developing drugs for
treating this breakdown before it is too late to stop the deformation. But, without the
proper diagnostic equipment, testing such drugs would be extremely difficult. Sodium
MRI shows promise in enabling the development and implementation of such treatments.
2) Functional MRI—monitoring body function in action—is becoming increasingly
important in a wide variety of fields—perhaps psychology most of all. In this field,
research participant may be exposed to various stimuli while the brain is being imaged to
see what areas are activated. Hydrogen MRI, however, show only a delayed response that
is visible as oxygen begins flowing to activated portions of the brain. Sodium, however,
is more instantaneous, and the development of Sodium MRI could be very useful in
enhancing the capabilities of functional MRIs.
3) Tumors affect the sodium content of the surrounding cell matrix, and sodium imaging
could potentially have application in better tumor detection.
Dr. Bangerter is interested in exploring these application of sodium MRI, but because
sodium is rarely imaged, sodium coils are not readily available, and custom coils cost
thousands of dollars. Our purpose is to design and fabricate a working resonator coil
tuned to the Larmor frequency of Sodium that can interface with the 3 Tesla MRI
machine at the University of Utah for use in his research.
Figure 1 is a very simplified block diagram of the MRI machine we will be designing a
resonator for. The resonator coil is an important part in the MRI process but is only one
part of a complex system.
Siemens 7T MRI Machine
Transmit/Receive
Switch
Current In
Computer
Controller
Pre-amplifier
Resonator Coil
(part to be designed)
Current
Out
Signal
Out
Figure 1: System Level Diagram illustrating how resonator fits into MRI
Process Description
MRI and resonator design is new to all in our group. As a result, many of our decisions
have seemed arbitrary. Along the way we have had many discussions about what our
design should be. However, our lack of knowledge seems to hinder us. To solve that
problem Dr. Bangerter suggested, and we agreed, to build a simple surface coil to learn
the principles before endeavoring to build a more complicated coil.
This document started by applying methods learned in class to justify decisions that have
already been made. While going through the motions we discovered a discrepancy with
the coil design we have chosen and the needs we are trying to meet. This led to more
research and discussion to solve the issue. As we talked with Dr. Bangerter we
discovered the shortcomings of the design that appeared to have more merit than the one
we had chosen.
To complete the document we made decisions together and assigned individual sections
to different members of the group to complete.
Body of Facts
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Need to design an MRI Resonator for Sodium(23) in a 7 Tesla Field
Resources - Dr. Bangerter, People at University of Utah, books/articles/online,
MRI machines at the University of Utah
Simulate Design in Matlab
Tune design once done
Design casing for resonator
Build Resonator (and acquire materials)
Tune Resonator
Analyze SNR of MR Images obtained
We can make a head, breast, or knee coil
Critical Assumptions
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There is enough time to design/tune/build a resonator and get an image
Interfacing resonator to MRI machine will be relatively trivial compared to
resonator design
Having components machined can be done in the time alotted
We will be able to get scan time on the U's MRI machines to test our equipment
The people who have offered to help will have enough time to give us help
Critical Design Areas
We chose two critical design areas to evaluate. First, the type of resonator coil to use.
The type of resonator coil to use defines the arrangement of the inductive and capacitive
components as seen in figure 2. Also, we choose to evaluate the outer casing to use for
the resonator (see figure 2). The outer casing comes in contact with the patient and also
creates the finishing touches on the coil, leaving a good or bad impression to those who
see it. The outer casing is a critical design area because of the safety from burns and
discomfort it gives to the patient.
Figure 2: Resonator Coil Block Diagram
Type of Resonator Coil
Considered Resonators
Surface Coil- This is a very simple resonator. It is simply a circle of a conductive
material that lies on one plane. Current goes through the inductor creating a field
perpendicular to the plane.
High/Low-Pass Bird Cage- These are slightly different implementations of the same
basic structure. This structure has two parallel conducting circles, connected by
conductive bars that run perpendicular to the circle’s plane. It has advantages in
homogeneity, plus it allows more breathing room to the imaging area.
Single Turn Solenoid- This is very similar to the surface coil, except it is exists in three
dimensions. It is a sheet of conductive material that is formed into a circle. This creates
a homogenous field. One detriment is that the solenoid is often very long and must be
perpendicular to the bore of the MRI machine.
Definition of Criteria
Design Time- The amount of time needed to design and manufacture a basic layout that
should resonate. This does not including tuning resonance, just the time needed to design
and manufacture the basic structure.
Cost of Design- The cost of design is the cost that will be incurred to actually produce
the working prototype of the resonator. A higher score represents a cheaper
implementation.
Homogeneity of Field- In magnetic resonance imaging a homogenous field significantly
improves image quality. This is true for both the homogeneity of the magnetic field, as
well as the resonating RF circuit.
Meets Customers Needs- The market for sodium magnetic resonators is very slim. It is
generally dominated by researchers who want to run new tests. Often they need a
resonator that works at the right frequency, but they want a resonator for a specific
anatomy. They would accept one that meets their needs, but we want to improve
customer satisfaction by giving them something that they want.
Ease of Tuning- This refers to the final tuning once the basic implementation has been
constructed. This is an iterative process, with a difficulty that varies based on the
complexity of the style of resonator.
Adequate Imaging Volume- When imaging any part of the anatomy, it is obviously
important that the resonator image the entirety of the desired anatomy. But, it is also
desirable for image quality if the entire anatomy can be imaged, without getting noise
from outside anatomies.
Expected SNR- It goes without saying that certain designs are better for the SNR than
others. As with any signal, a higher the SNR is better. This criteria evaluates this
important piece.
Versatile Anatomy- While not required, it is nice to build a resonator that can be used on
multiple parts of the anatomy. This will help increase customer satisfaction, but is not as
important as meeting the criteria for the desired anatomy.
Criteria
HighPass
Bird
Cage
LowPass
Bird
Cage
Weightin
g Factor
Surface
Coil
Single turn
Solonoid
Design and
Manufacturing
Time
Cost of Design
5%
10%
5
5
2
2
2
2
3
6
Homogeneity of
Field
25%
5
9
9
8
Meets Customers
Desires
Ease of Tuning
25%
5%
5
5
9
3
10
3
7
6
Adequate
Imaging Volume
Expected SNR
Versatile Anatomy
Total:
15%
10%
5%
100%
5
5
5
500
2
8
3
620
2
8
3
645
2
8
1
595
Chosen Resonator Coil: Low-Pass Bird Cage
As seen from the table above, the Low-Pass Bird Cage resonator is our best choice.
Although, the High-Pass Bird Cage is still a close second. The Bird Cage coils are better
than the surface coil mainly for the homogeneity of the field they produce, which means
the image will be more clear. The weakness in the single turn solenoid is the comfort of
the patient in imaging. With a single turn solenoid the patient would have to contort
himself into strange positions to get the any image. Overall, the Bird Cage coil fits our
customer's needs the best.
Type of Outer Casing
Considered Outer Casings
We have come up with three options for creating a casing (shielding) around our
coil such that it will be more aesthetically appealing and also provide insulation between
the coil and the test subject. We chose the Improvised Shielding as our reference
concept, and compared the other two options to that standard.
Improvised Shielding (at Imaging Time)- Simply involves Dr. Bangerter (who will be
the test resonator) will put some kind of pad/towel around his knee such that the coil will
not burn him as it gets hot from high amounts of current going through it.
We Design & We Build- Design shielding that will not interfere with operation and that
can be made by us, and then we make it.
We Design & MFG students build- Design the shielding, perhaps using CAD software,
and then recruiting the MFG students to fabricate the design. Dr. Bangerter suggested
this as a possibility, and has offered to cover any costs that this extra step would incur.
Definition of Criteria
Design Time- The amount of time needed to design and manufacture the shielding.
Cost of Design- The higher the score in this criteria, the less the materials cost.
Meets Customer's Desires- Rates how the concept meets what the customer wants in the
end product.
Safety From Burns- The reliability of the product in preventing accidental contact
between resonating coil and skin.
Ease of Use- How easy it is to set up and use the product in the MRI chamber.
Weighting
Factor
Criteria
Design and
Manufacturing
Time
Cost of
Design
Meets
Customer's
Desires
Safety from
burns
Ease of Use
Total:
Improvised
Shielding (at
Imaging Time)
We Design &
we Build
We Design &
MFG students
build
15%
5
3
1
15%
5
3
2
30%
5
7
9
20%
20%
100%
5
5
500
7
7
580
8
8
635
Chosen Casing: We Design & Manufacturing Students
Build
Applying the above criteria, and the weightings that seemed appropriate, we came to the
conclusion that, in order to best meet the customer's desires, and to create the best
product possible, we will attempt to create a design that the MFG students will be able to
help us manufacture. As can been seen in the above table, this is the best choice for our
current prioritites. This may prove to be more difficult than anticipated, but this is the
direction we are going to go in to begin in. If it does turn out to be too difficult, we will
fall back on designing and creating a covering on our own.
Non-Critical Design Areas
Magnetic shielding – The idea behind the magnetic shielding is that the coil will be
inside the huge MRI magnet when functioning. This will cause the resonator to act
different than when testing outside of the magnet. To imitate the magnet a large metal
cylinder can be used. This will just be sheet metal applied to a cylinder structure of
plastic about 3 feet in diameter and 4 feet long.
Physical structure – The physical structure of the resonator is just used to support the
inductive and capacitive components. We will use a cylindrical structure of plastic to
support our bird cage resonator.