Synopsis
This session explains the source of dielectric effects in MRI. It is furthermore explained how the dielectric effects can be used to improve image and spectra quality.HIGHLIGHTS
·
dielectric materials
can improve magnetic resonance imaging
·
high field mri coils
can be build from water with very little effort
·
modes of dielectric
materials for cylinders can easily be estimated
TARGET AUDIENCE
researchers and engineers working in the field of high
and ultra high field MRI.
OUTCOME
This talk will inform you about dielectrics, their
effects on MRI and how to use them to improve imaging. After this talk you
should have a general understanding about dielectrics. You will be able to
build a simple microscopy coil for your ultrahigh field mri just using
destilled water, a bucket and a pickup probe made from ordinary coaxial wire.
You will understand how to use dielectric bags to homogeneize your RF field and
thus improving data quality on 3.0, 7.0 Tesla and even higher field strength experiments.
PURPOSE
Why do we need Dielectric Materials and Resonators?
Since the introduction of 3.0
Tesla mri wavelength effects in the human body can no longer be neglected as
the high field strength also means a higher frequency is needed for the B1
field (1). However this
leads to decreased homogeneity in the body trunk at higher field strength due
to the shorter wavelength. Already in standart clinical applications on 3.0 T
this effect leads to areas of local signal voids and thus uneven image
brightness for example in breast and abdominal imaging (2). Dielectric
resonators are interesting alternatives to lumped element mri volume coils
especially at higher field strengths (3) and can also be used as surface coils
(4). We can summarize
the main challenges in high field MRI as:
·
Inhomogeneous B1+
·
Increased SAR
·
Shading artifacts
·
Areas of poor contrast
This leads to poor image quality (figure 1) already at 3T in the abdomen
and in the head also visible in the head.
RESULTS
Due to the decreased wavelength of the B1 radio wave at higher
field strength the wave becomes so short that we get standing wave patterns
inside the human body. This effect can be clearly shown with an electromagnetic
field simulation in the human head for different field strength (figure 2).
However the wavelength is not the only variable
leading to this undesired artifacts. One can also clearly show with
electromagnetic simulation that the dielectric constant εr of the object in which the wave is
traveling has a major impact.
We can combine this knowledge to produce a secondary B1
field by using a bag filled with a high dielectric that „fills up“ the lateral
voids in the head if positioned right.
It is then possible to design pads with the right
shape and dielectric constant to improve the B1 field in a selected region in
our sample with the help of electromagnetic simulations (figure 5) for example
targeted for imaging of the inner ear.
METHODS
How to make and use
high dielectric bags to improve imaging.
The production of high dielectric bags is very simple: the bag that
usually contains a slurry made from a high dielectrics ceramics power such as
barium titanat (BaTiO3) gets mixed with water, preferably this should be
deuterized if you do not want to see your bag later on the image, and then put
into a sealable plastic bag. The application is also simple: for the head at 7
Tesla for example one places the bags just around the backside of the head
approximatley from ear to ear (7). On 3 Tesla cardiac and
abdominal imaging, one simply puts the bag suspine on the patients chest
beneath the coil (see figure 1).
Once one realises the effect of dielectrics on mri,
especially the central brightning in the head, one can assume that it also be
possible to use this standing wave effects in dielectric materials to ones
advantage and build a resonator based on this principle. Indeed this is
possible and has been first shown by Wen et al. (8). The feasibilty of these resonators has been shown also at ultra high
fields by Neuberger et al. (9) Further designs have been presented in recent years by dutch groups (3,4,10–13).
CONCLUSION
Relevance to clinical practice
Understanding dielectric materials, their properties
and how they interact with modern MR systems is an important part of ultra high
field mri. The usage of high dielectric bags helps easily to improve image and
data quality while reducing SAR. It is now already used on a routine basis at 3
Tesla (14) and for research studies at 7 Tesla field strengh.The principle of how
these dielectric bags work has been understood and concerns about negative
influence on SAR have been neglected in previous studies (15).
Relevance to future research
Dielectric materials will continue to be an
interesting topic in the future of MRI. The basic concept has been addressed (5,16,17) and one can anticipate ongoing improvement of dielectric pad
composition and geometry for example by using a target field simulation
approach. The integration of dielectric materials in selected coils for ultra
high field MRI seems to be the next logical step. Different pads for different
body geometries might be required to get optimal results. Future work will show
if there is a „one size fits all“ solution or those pads should be more
personalized for maximum benefits in contrast-to-noise ratio, homogeneity of B1
and the reduction of the specific absorption rate.
Dielectric
materials can be used to build novel RF coils (8,12,13) for MRI. The intuitive way to build a simple
dielectric resonator from destilled water (8,10) for example for microscopy experiments makes them a
usefull skill for researchers working on high Field MRI systems. The high radio
frequency nature of ultra high field MRI systems allows for a multitude of new
concepts for novel resonators designs for volume and surface coils and has also
found its way into traveling wave MRI (11).
Acknowledgements
No acknowledgement found.References
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