Helmholtz pair for MR imaging and spectroscopy at 11 . 7

Introduction. For non-invasive imaging of targeted tissues, cells and biological tissue substitutes in living species the application of fluorinated agents are becoming more popular as F is a relative sensitive nucleus and images can be obtained without disturbing background signals as seen in H MRI. However, compared to H nuclei the number of available F nuclei is usually limited and thus optimal signal to noise detection is required. Apart from designing contrast agents with a large number of F nuclei, this can be aimed for by using the highest possible magnetic field, optimal RF coils and the shortest possible echo time and repetition time in acquisition. Furthermore, for anatomical matching a background H MR image is needed. The aim of this study was to realize these MR conditions to enable H and F MRI of rat leg bone and bone substitutes containing F contrast, by: 1. designing and building a Helmholtz pair RF probe that is capable of generating a homogeneous RF-field for H and F at 11.7T magnetic field 2. performing zero echo time (ZTE) H and F imaging to capture short T2 species. Materials and Methods MR was performed on a 11.7T MR-system (Bruker Biospin, Germany). The RF-coil consists of a Helmholtz pair with two separate, slightly bended, elements with a size of 25 x 35mm. The average distance between the elements is 25mm. Both elements are separately tuned and matched (balanced) and are combined with a home-built lumped element Wilkinson power splitter/divider [1]. Since the elements couple strongly a split in the resonant curve is observed. The elements must be tuned to the lowest frequency [2]. For best performance both nuclei have a separate power/divider. The circuit diagram of the complete coil can be found in figure 1. The circuit in the red box with the solid line is the RF-coil with the 2 similar elements. In the red box with the dashed line the circuit for the power splitter/ divider can be found. This circuit splits the Tx signal into two outputs with half the power and the same phase. During receive the splitter/ divider combines the received signals. Note the mirrored cable connections at the elements to achieve similar phases for both elements. Otherwise the transmit and receive RF-field will partly cancel out due to the 180° phase difference between the elements.