Field Simulation Software to Improve Magnetic Resonance Imaging

Transcription

1 Field Simulation Software to Improve Magnetic Resonance Imaging a joint project with the NRI in South Korea CST Usergroup Meeting 2010 Darmstadt Institute for Biometry and Medicine Informatics J. Mallow, T.Herrmann Darmstadt April 2010

2 Working group 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 2

3 MRI RF-coil development with CST Microwave Studio 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 3

4 CST Microwave Studio in MRI at OvG University Magdeburg Surface-coils for different applications Phased- Array 7T- Monkey head coil

5 CST Microwave Studio in MRI at OvG University Magdeburg 8-Channel receive head coil for 3T MRI Helmholtz Tx-Rx head coil for 7T B1-field homogeneity 8-Ch. head coil for 3T MRI Doty - CP 7T extremity RF-coil 5

6 Simulation and Construction of an 8-Channel Receive Head Coil for 3T MRI B1-field homogeneity 8-Ch. head coil for 3T MRI human head transversal slice image 6

7 CST Microwave Studio in MRI at OvG University Magdeburg Finding the optimized loop-distance for phased-array surface coils with CST 7

8 Whole Body MRI excitation 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 8

9 CST Microwave Studio in MRI at OvG University Magdeburg Body Coil in Standard MRI-Systems Siemens Sonata 1.5T and Trio 3T whole body MRI body coil for excitation Tx of B1+ and receive with Rx phased array coils 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 MRIsystem. Calculations performed with CST MWS

10 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 10

11 Body coil at Ultra High Field (UHF) MRI 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 11

12 Travelling Wave for Transmit (Tx) in 7T MRI Whole Body System 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 0-10λ the 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 Reference: Brunner DO, De Zanche N, Fröhlich J, Paska J, Pruessmann KP; Travellingwave nuclear magnetic resonance; Nature, 2009, 457(7232):

13 Travelling Wave Near Field (NF-Tx) for Tx Field simulation model of the 7T MRI system with turnstile dipole and scheme of the travelling wave concept 13

14 Travelling Wave Near Field (NF-Tx) for Tx 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 RF-Shield RF-Shield & Gradient Coil: length = 158cm RF-Shield diameter d= 64,1cm 14

15 Travelling Wave Near Field (NF-Tx) for Tx basic principle using an turnstile dipole to create a circular polarized B1-field Port 1 Port 2 Port 1 + Port 2 (90 phased-shift) 15

16 Patch Antennas for Near Field Tx (NF-Tx) 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

17 Patch Antennas for Near Field Tx (NF-Tx) Reflection parameter of the designed patch antenna for free space inside bore Patch antenna is matched and tuned for bore condition The reflection parameters depend of position inside of the bore 17

18 Patch Antennas for Near Field Tx (NF-Tx) Schematic scenario to acquire an image of H 2 O phantom with matching at the end of the waveguide Depiction of antenna positioning for image acquisition 18

19 Travelling Wave Near Field (NF-Tx) for Tx orange as load Tx: Monkey Dual Helmholtz Tx Coil Tx: Turnstile Dipole NF-Tx Rx: Monkey 12Ch. Rx Coil Rx: Monkey 12Ch. Rx Coil the Dual Helmholtz Tx Coil was disabled Sequence: MPRAGE TR: 2000ms, TE: 4.52ms, matrix: 256x256, FoV: 90x90mm, slice thickness: 1mm, slice distance: 1mm α = 25 19

20 Patch Antennas for Near Field Tx (NF-Tx) 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 20

21 Travelling Wave Near Field (NF-Tx) for Tx Tx: Monkey Dual Helmholtz Tx Coil Rx: Monkey 12Ch. Rx Coil Grapefruit as Load Tx: Patch Antenna NF-Tx Rx: Monkey 12Ch. Rx Coil - the Dual Helmholtz Tx Coil was disabled Sequence: GRE TR: 100ms, TE: 10ms, matrix: 256x256, FoV: 155x155mm, slice thickness: 2.5mm, slice distance: 1mm, α = 25 21

22 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 22