Field Simulation Software to Improve Magnetic Resonance ... · SAR distribution of the body coil at 1.5 Tesla whole body MRI-system The Billie biological model of the Virtual Family

Darmstadt April 2010

Field Simulation Software to Improve

Magnetic Resonance Imaging

a joint project with the NRI in South Korea

CST Usergroup Meeting 2010

Darmstadt

OvG University Magdeburg

Institute for Biometry and Medicine Informatics

J. Mallow, T.Herrmann


J. Mallow, T. Herrmann

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Institute for Biometry and Medical Informatics

Prof. J. Bernarding, J. Mallow, T.Herrmann

Chair of Biomedical Magnetic Resonance

Prof. O. Speck, M. Kladeck

Chair of Microwave and Communication Engineering

Prof. A. Omar, I. Ali Elabyad

Neuroscience Research Institute, Gachon University, Incheon, South

Korea

Prof. Z-H. Cho

Leibniz Institute for Neurobiology, Magdeburg, Germany

Dr. J. Stadler

Working group



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MRI RF-coil development with

CST Microwave Studio 2009-2010

Goals:

Simulation based development of 1T, 1.5T, 3T and 7T MRI RF-coils

Designing of RF-coils for special fMRI-experiments

Designing of surface coils and phased array coils for 3T and 7T

Optimizing the B1-Field homogeneity for better image quality

Optimizing the matching for better Q-factor (S-parameter)

Verifying of RF-coils by calculating SAR with “biological models”

Investigation of the travelling wave concept to allow whole

body MRI excitation at ultra-high field

Surface-coils for different applications

Phased- Array 7T- Monkey head coil

CST Microwave Studio in MRI at OvG

University Magdeburg



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8-Channel receive head coil for 3T MRI Helmholtz Tx-Rx head coil for 7T

Doty - CP 7T extremity RF-coilB1-field homogeneity 8-Ch. head coil for 3T MRI





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Simulation and Construction of an 8-Channel

Receive Head Coil for 3T MRI

human head transversal slice imageB1-field homogeneity 8-Ch. head coil for 3T MRI



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Finding the optimized loop-distance for phased-array surface coils with CST





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One or more RF-coils are necessary to acquire MR images

A close distance between RF-coil and subject as well as and a good filling factor is recommended to get highest SNR

Standard procedure for clinical MRI-systems ( 1.5T and 3T):

use a body coil for excitation Tx combined with one or more phased array coils for receive Rx

To have the possibility to use flexible and close distance Rx RF-coils a body coil for excitation is very important to get homogenous B1+ distribution

In this case all SAR calculation have to be done one time just for the body coil

Whole Body MRI excitation



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Simulation model of 1.5 Tesla (63MHz) MRI whole body

system. Model designed with CST MWS 2010

B1+ field distribution of the body coil at 1.5 Tesla whole body MRI-

system. Calculations performed with CST MWS 2010

Siemens Sonata 1.5T and Trio 3T

whole body MRI

body coil for excitation Tx of B1+ and

receive with Rx phased array coils

Body Coil in Standard MRI-Systems



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Combination of Biological Model

and Field Simulation for SAR calculation

SAR distribution of the body coil at 1.5 Tesla whole

body MRI-system

The Billie biological model of the Virtual Family The Gustav biological model of the

CST Family



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Known problems:

At UHF conditions there is no body coil resonator available such as for 1.5T and 3T whole body MRI-systems

Reasons: SAR too high at 7T (5x SAR in compare to 3T) for standard

body coil architecture

Instable field distribution in loaded condition

Present state:

Every UHF RF-coil needs separate excitation and SAR calculation to work in UHF MRI-system

New solutions required

Body coil at Ultra High Field (UHF) MRI



12OvG University Magdeburg


Travelling Wave for Transmit (Tx) in

7T MRI Whole Body System

Reference: Brunner DO, De Zanche N, Fröhlich J, Paska J, Pruessmann KP; Travelling-

wave nuclear magnetic resonance; Nature, 2009, 457(7232):994-8

The useable B1+ field in RF-coils is restricted to dimensions

and geometry of the RF-coils itself

The useable B1+ field in travelling wave concept is restricted

to dimensions of the waveguide (RF-shield) only

The condition of travelling wave related to near field and far

field The physical near field condition is described from 0-10λthe far field begins at 10λ The behavior of the EM-wave is different in near field and far field This condition is depend on load in side of RF-shield



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Field simulation model of the 7T MRI system with turnstile dipole and scheme of

the travelling wave concept

Travelling Wave Near Field (NF-Tx) for Tx





RF-Shield & Gradient Coil: length = 158cm

RF-Shield diameter d= 64,1cm

RF-Shield

The cut-off frequency for propagating TE11 (H11) mode is 275 MHz for an RF-shield diameter d=64,1cm

The RF-shield of the bore is used as circular waveguide

Only Rx coils with passive or active detuning could be used

Turnstile Dipole and Patch Antenna are used for transmit Tx




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basic principle using an turnstile dipole to create a circular polarized B1-field

Port 1 Port 2

Port 1 + Port 2 (90° phased-shift)




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With tuning and matching capacitors for port 1 & 2

to allow optimal adjustments inside the bore

patch antenna prototype

optimization using software model implemented with CST MWS 2009

Patch Antennas for Near Field Tx (NF-Tx)



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inside bore for free space

Reflection parameter of the designed patch antenna

Patch antenna is matched and tuned for bore condition

The reflection parameters depend of position inside of the bore




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Depiction of antenna positioning for image acquisition

Schematic scenario to acquire an image of H2O

phantom with matching at the end of the waveguide






Sequence: MPRAGE TR: 2000ms, TE: 4.52ms, matrix: 256x256, FoV: 90x90mm, slice thickness: 1mm, slice distance: 1mm α = 25°

Tx: Monkey Dual Helmholtz Tx Coil

Rx: Monkey 12Ch. Rx Coil

Tx: Turnstile Dipole NF-Tx

Rx: Monkey 12Ch. Rx Coilthe Dual Helmholtz Tx Coil was disabled

orange as load




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By using phased array RF-coil for Rx to increase SNR

The patch antenna have to be place at the back side of the bore

because the patient table with Rx coil




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Tx: Monkey Dual Helmholtz Tx Coil

Rx: Monkey 12Ch. Rx Coil

Tx: Patch Antenna NF-Tx

Rx: Monkey 12Ch. Rx Coil- the Dual Helmholtz Tx Coil was disabled

Grapefruit as Load

Sequence: GRE TR: 100ms, TE: 10ms, matrix: 256x256, FoV: 155x155mm, slice thickness: 2.5mm, slice distance: 1mm, α = 25°






Summary and Conclusion

The travelling wave concept has the potential to work as a

whole body coil replacement for the excitation

The use of turnstile dipole and patch antenna for Rx leads to

a weaker SNR due to the distance to the load and the filling

factor

The use of field simulation software saved time and material

to find the optimal antenna design for excitation

Extension of field simulation software to incorporate hardware

architecture and biological models is a significant step

towards more realistic field simulation

The dynamic thermal solver could give the opportunity to

calculate a realistic SAR for RF-coils at Ultra-High-Field MRI

Systems

Field Simulation Software to Improve Magnetic Resonance ... · SAR distribution of the body coil at 1.5 Tesla whole body MRI-system The Billie biological model of the Virtual Family

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