Optimized Human head array design for 10.5T MRI

Abstract

Ultrahigh magnetic fields (UHF) are increasingly being used in in biomedical research with humans because of the gains in signal-to-noise ratio (SNR) and, in some cases, contrast-to-noise ratios (CNR). This has led to the development of human MRI systems that are capable of imaging in the ultrahigh frequency domain (300 MHz and higher). However, the short wavelength of such high frequencies in the human body, caused by the average εr ~50, leads to a traveling wave behavior as the dimensions of the object to be imaged become larger than the wavelength; this causes significant field non-uniformities. Consequently, radiofrequency (RF) coil designs at the UHF leaving the strict near field domain and far field concepts have become more important. Radiative type antennas, particularly dipoles, have been proposed as building blocks for transmit arrays and have recently shown promising performance in the UHF range. It has been proposed and demonstrated that arrays consisting of transmit antennas indeed have the ability to mitigate B1 non-uniformity through an optimal combination of phase/amplitude and RF excitation waveforms. At the UHF, such array systems are essentially mandatory in terms of achieving a uniform B1+ field with sufficient transmit efficiency for brain imaging with a minimized specific absorption rate (SAR). Both dipole and a combination of loops with dipoles have been successfully used for applications to the human body and the desired high penetration at the UHF has been achieved. Compared to other coil types such as loop or microstrip type antennas, dipoles have the advantage that they can support symmetric B1+ field patterns, have favorable pointing vectors, and can achieve a greater depth of penetration. However, radiative antennas face greater challenges in minimizing mutual coupling between neighboring elements. This is particularly difficult for human head imaging due to the non-uniform gap between the subject and the antenna array. In addition, the decoupling of dipole antennas are made more difficult by the interaction of the dipole with the coaxial feed cable which typically needs to be routed typically in parallel with one leg of the dipole antenna. Combined with the mutual coupling among the coaxial cables in an array, this can change field patterns and degrade antenna performance. This problem has previously been addressed using balanced to unbalanced (balun) matching and cable traps or a combination of matching networks and cable traps. Here, we describe novel UHF coils which are optimized for applications to the human head at 10.5 T. We simulated, built and validated all of antenna arrays described in this study. We then demonstrated their use via electromagnetic (EM) simulations and MR experiments for B1+ efficiency, 10 g SAR and SAR efficiency within a uniform phantom before human applications.