Project Leader

Shajan Gunamony
Phone: +49 7071 601-734
Fax: +49 7071 601-702
shajan.gunamony[at]tuebingen.mpg.de

Current and former Lab members

     Shajan Gunamony (Research Scientist)
     Dr. Irena Zivkovic (PostDoc)
     Jens Hoffmann (PhD Student)
     Martin Eschelbach (PhD Student)
 
 

 

MRI RF Hardware

The focus in the RF lab is to develop transmit arrays that provides control over the B1+ field for RF shimming and dynamic parallel transmission methods. The transmit arrays are combined with tight fitting receive arrays for increased receive sensitivity. Precise numerical simulation and safety evaluation are carried out to ensure safe in-vivo use. Travelling wave imaging and prospective motion correction using NMR field probes are being explored.
 
The promise of increase in signal to noise ratio (SNR) proportional to the main magnetic field strength motivated a few academic laboratories to pursue magnetic resonance imaging (MRI) research at ultra-high field (UHF, ≥ 7 T), much higher than the field strength approved for clinical use. The magnetic resonance center of our institute is equipped with a 9.4 T scanner, the strongest magnet ever used for human MRI till date.
 
Radio frequency (RF) coils and the front-end RF hardware play a significant role in realizing the benefits offered by this unique scanner. In addition to conventional 1H imaging, multi nuclei imaging / spectroscopy is expected to gain from the increased field strength. UHF MRI operates in a regime where the shorter wavelength in tissue due to the high Larmor frequency leads to constructive and destructive interference of the transmit (B1+) field inside the human body. Hence, it is not only challenging but also opens up opportunities to develop novel methods for excitation.
 
The focus in the RF lab is to develop RF transmit arrays that provides control over the B1+ field for RF shimming and dynamic parallel transmission (pTx) methods together with precise numerical simulation to accurately estimate specific absorption rate (SAR). Anatomy specific, form-fitting receive arrays are combined with transmit arrays to increase the image SNR. Alternative approaches for spin excitation utilizing the waveguide properties of the scanner bore and prospective motion correction using NMR field probes are being explored.
 
Projects:

Transmit arrays and receive arrays

Volume resonators are an integral part of any clinical MR scanner. However, this design approach cannot be readily adapted at 9.4 T because the wavelength in tissue at the 1H Larmor frequency is already less than the size of a human head. By controlling the amplitude and phase of the current to the individual coil elements of a transmit array using static/dynamic RF shimming approaches, the strong in-homogeneities in the transmit field can be corrected. A novel RF setup with 16 transmit elements in two rows for 3D RF shimming was developed and combined with a 31-channel tight fitting receive array for optimum SNR.
 
B0 shimming capability, high SNR and efficient transmit excitation are the primary requirements of a whole-brain 23Na MR imaging setup. A novel three-layered coil (4-channel 1H dipole, 4-channel  23Na transmit and 27-channel  23Na receive array) was developed to meet these demanding requirements. Using this hardware together with optimized sequences, high quality in-vivo 23Na images were acquired with level of details not previously achieved.

EM Simulation and safety evaluation

The prediction of the SAR created in the subject’s body is vital for every self-developed coil. Hence, precise numerical simulations are an indispensable tool for accurate estimation. EM Simulations are performed using state-of-the-art software (CST Studio Suite including circuit co-simulation) with graphics processing unit (GPU) acceleration. To this end, the SAR in realistic, high-resolution human voxel models is simulated and this information is compressed using the Virtual Observation Points method to supervise and restrict the applied RF power for safe in vivo operation during pTx experiments.
 
Self-built RF coil must strictly meet official requirements for safe application on humans. However, a standardized method to test ultra-high field coils for operation in compliance with these requirements is missing. Our department took a leading role and developed procedures for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures including regular quality assurance testing. Self-built RF coil has to pass these tests before clearance for in vivo used is granted.

Travelling wave imaging

The elevated Larmor frequency at 9.4 T enables an alternative method for spin excitation and signal reception, termed traveling-wave imaging. It differs from conventional setups as it deliberately uses the scanner’s bore as a waveguide to convey the RF field from a remotely placed antenna towards the subject. These antennas are simple to construct, provide free space and are versatile as they can be used to image large a field-of-view of subjects in different positions.
Our research focuses on improvement of imaging performance by exploring innovative antenna designs and by increasing the number of independent RF channels to enable pTx and parallel reception. For example, a novel box antenna was designed that will enable excitation of two different modes by incorporating an RF MEMS switches. Further, a recently developed 3-channel transceiver antenna for simultaneous control over three independent waveguide modes provides void-free images of the entire human brain at 9.4 T using simple pTx techniques.

NMR field probes

NMR field probes consist of a liquid NMR active sample (1H, 19F or D2O) inside a sealed glass capillary which is excited by a solenoid RF coil wrapped around it. A promising application is reading out the position of the field probes by applying specified linear gradient fields while exciting the probes. This applies also to the position changes – e.g. motion of the field probes. Attached to the head of a subject this data can be used to track or correct for head motion, thus avoiding motion artifacts in the acquired images and improving the image quality.
Last updated: Saturday, 07.02.2015