Multi-purpose FLEXible coil ARray for high resolution 7T MRI – FLEXAR7

This project is structured in 3 main tasks.
Task 1 will consist in studying and optimizing by numerical simulations the radiofrequency detection system envisioned in this project and will by conducted jointly by IR4M and MRCE. Therefore, we will develop a mixt simulation method dedicated to arrays of transmission line resonator that will allow both accounting for the electromagnetic environment of the MRI experiment and including the electronic components of the acquisition chain of the radiofrequency signal.
Task 2 will be dedicated to the development of the electronic circuitry for connecting the array to the scanner and conditioning the NRM signal on one hand and to the development of dedicated imaging protocols on the other hand. This task will be conducted jointly by IR4M and MRCE. We will provide original solutions for mutual decoupling of adjacent coils, decoupling during emission and signal transmission, with robust performances regarding the flexibility of the array.
Task 3 will concern the evaluation of the performances of the detection systems developed throughout this project and more particularly of the final coil array. This systems will be studied on the bench using electromagnetic characterization tool specifically developed by IR4M in order to evaluate the achieved sensitivity gain and the improvement of the accessible field of view. We will then proceed to the validation by MRI of performances of the developed array. This will be ensured by MRCE and will be held in Vienna.

In the first one and half year, we have designed, using analytical modelling and 3D EM simulations, the single element that will compose the array. Following this, we have performed preliminary 3D EM simulations and bench measurement for evaluating the performance of the single element coil.
The successful completion of this first step has allowed us to investigate the design of a 12 element array. We have investigated a shielding ring-based technique, which can be optimized analytically for any coil geometry. Several simulation studies were done in order to evaluate the performance and limitations of this decoupling technique
Using 3D EM simulation, we have successfully demonstrated that shielding rings can be used to decouple TLR arrays for 7 T without degrading the detection sensitivity. Based on the preliminary results of the two simulations described above, a 12-channel phased array coil was simulated as well in order to evaluate the shielding ring decoupling performance in the array by calculating B1+ field and S-parameter matrix.
Based on the simulation results, a flexible printed circuit board of the 12-element and single element TLRs with and without shielding ring were fabricated. Furthermore the interfaces of the elements of the array containing, T/R switches, pre-amplifiers, power splitters and cabling were designed and fabricated. The effect of the shielding ring was then validated on the bench and the whole array was assembled and it’s performances were evaluated on the bench.
In parallel, we have investigated and proposed a new design for monolithic RF coil: the multi-turn Multi-gap transmission line. This design offers an additional degree of freedom in tuning self-resonant TLRs, as their resonance frequency is fully determined by the coil geometry (e.g. diameter, number of turns, conductor width, etc.). Thus it widens the application range of TLRs in MRI.

In the second half of the project, we will finalize the fabrication of the array and optimize it’s performances. Then, dedicated phantom mimicking the properties of various anatomical region will be developed. They will be used for evaluation the performances of the array in loading condition on the bench. Dedicated acquisition sequences will be developed and the performances of the array will be finally evaluated by MRI.
In parallel, we will start the writing of a full article presenting the obtained results and we will present the intermediate results at several scientific conferences.

Li, Z., Kriegl, R., Hosseinnezhadian, S., Poirier-Quinot, M., Laistler, E., Darrasse, L. Ginefri J-C. Investigation on shielding-ring based decoupling technique for small monolithic RF coils. in 32nd European Society for Magnetic Resonance in Medicine and Biology (ESMRMB). Edimbourg, (Ecosse), Octobre 2015
Kriegl, R., Ginefri, J-C., Poirier-Quinot, M., Darrasse, L., Moser, E., Laistler, E., Multi-turn Multi-gap Transmission Line Resonators - First Tests at 7 T. in 23rd Scientific Meeting of the International Society for Magnetic Resonance in Medicine (ISMRM 2015). Toronto (Canada) Mai 2015
Li, Z., Kriegl, R., Laistler, E., Poirier-Quinot, M., Darrasse, L. Ginefri J-C., Preliminary investigation on shielding-ring based decoupling technique for small monolithic RF coils. in 23rd Scientific Meeting of the International Society for Magnetic Resonance in Medicine (ISMRM 2015). Toronto (Canada) Mai 2015

Magnetic resonance imaging has become one of the major tools in non-invasive medical diagnostics, providing a multitude of quantitative and functional information with ever increasing performance. For applications demanding very high spatial or temporal resolution, sensitivity and, thus, image quality frequently becomes the limiting factor. To improve sensitivity, MR scanners with a higher static magnetic field may be employed, the performance of the radio frequency detection systems can be enhanced or more efficient image acquisition and reconstruction methods can be developed.
The scientific objective of this multi-disciplinary project is the combination of all these strategies to outperform currently available technologies for ultra-high resolution (<100 µm) MRI in humans. The biomedical application concerns imaging of joint injuries, nerve damage, and early detection of bony erosions in chronic polyarthritis.
The technological objective represents a major instrumental innovation in the form of a flexible, highly sensitive RF coil array comprising 32 miniature coil elements with diameters of 2-3 cm for imaging in a 7T whole-body scanner. By its flexibility, the array can be form-fitted to the target organ’s shape, rendering it a highly sensitive multi-purpose instrument for various anatomical sites.
The detection system to be developed will be based on the monolithic transmission line resonator principle. It will be designed by numerical simulation and fabricated on flexible Teflon substrate. Concurrently, electronic circuitry and imaging protocols will be developed. Electromagnetic characterization on the bench will be carried out and imaging performances of will be determined to evaluate the added diagnostic value.
The final output of the project is an innovative, high-sensitivity detection system and optimized acquisition protocols for 7T MRI allowing 3D-visualization of joint and nerve structures in vivo, at scales undetectable up to now.
The project is thoroughly structured in four tasks to ensure efficient parallel advances in instrumental and methodological work. Back-up solutions for critical tasks guarantee the timely accomplishment of the project aims.
We put in synergy the multi-disciplinary and complementary competences of the laboratory of Imagerie par Résonance Magnétique Médicale et Multi-Modalités (IR4M, Université Paris-Sud, Orsay, France) and of the Magnetic Resonance Center of Excellence (MRCE, Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Austria). This consortium has been specifically assembled to address all essential questions arising in the context. The combination of the strong expertise of IR4M in methodology and instrumentation for MRI and the internationally outstanding experience of MRCE in ultra-high field MR micro-imaging unique regroups scientific and technological knowledge perfectly matching this highly ambitious project.
The sensitivity improvement achieved in this project is essential for the diagnostics of joint and nerve pathologies and will also benefit other biomedical applications of MRI for demanding high detection sensitivity, such as investigations in cardiology, dermatology and neurosciences.