Ultra-High Field Magnetic Resonance for human applications. A novel splittable design of degenerate birdcage with integrated Tx/Rx switches and Butler matrix.
In UHFMRI the use of high static magnetic fields (e.g. 7T), can provide a substantial increase in the SNR and a consequent reduction in scan time or improved spatial resolution, due to the increased polarization of the sample. However, with increasing intensity of the B0 field the frequency of the B1 RF field increases proportionally, reducing the RF wavelength to dimensions comparable to those of the sample (~0.1 m). This shortening of the wavelength in tissue leads to a number of challenges, especially for relatively large coils necessary to image the head or the body, such as inhomogeneities of B1 transmit field and flip angle, decreased power efficiency, as well as increased SAR. In general, these problems are mitigated using multi-channel RF coils, by means of careful RF coil design on the hardware side, and software-related techniques such as RF shimming and pTx on the software side. In both cases, the use of a multi-channel transmit coil is required.
The aim of this thesis is the development of a multi-channel RF coil for UHFMRI applications on the human limbs. Several critical issues to be overcome are shown. The chosen design is a splittable, 8-channel degenerate birdcage with dedicated Tx/Rx switches fed by a Butler matrix. All this hardware components are developed in the framework of this thesis. The performance of this coil is demonstrated by simulation, workbench measurements and scanner measurements. An UHFMRI scanner operating at 7 Tesla is currently available at the IMAGO7 Foundation (Pisa) which promotes the research in this field and allows for in-magnet coil tests and measurements.
After a brief introduction about the physics of MRI and its application to imaging, the hardware of the system will be discussed, with main focus on RF coils. Next, the design of the three main components of this coil is discussed together with workbench and scanner results. Finally, the measurements carried out with the fully assembled prototype are presented, proving its good performance in terms of efficiency, B1+ homogeneity and SNR. The first in vivo images of the knee are also shown
