At frequencies above 150 MHz birdcage resonators, which perform well at lower frequencies start to display performance limitations. The main reason for this is that coil dimensions approach the RF wavelength and therefore the current distribution is no longer uniform. Additionally there are increased radiation losses. However it is still possible to build a highpass birdcage head coil for 170 MHz with reasonable homogeneity and performance under certain conditions. With a cavity resonator (1), which replaces the lumped elements with distributed circuits, one can expect a better performance than a birdcage resonator due to the reduced ohmic, conservative electric field and radiative losses. The cavity also stores the return field and acts as a self shielding structure. We compared both types of head volume coils under the same conditions on the bench and in a 4 Tesla imaging experiment. We built an eight element shielded highpass birdcage and an eight element cavity resonator with the same dimensions. The coils were 21cm long and the inner diameter was 27cm. We also evaluated that the optimal This is in agreement with recently reported data (2). For the bench measurements we used a head sized phantom containing 2.5 l of 50% normal saline solution. For the imaging experiment we positioned the same volunteer in both coils in the same way. For the S/N measurement we obtained MDEFT(3) images with a TR of 1.2ms, a 256x210 matrix, 3mm slices and a FOV of 22x20 cm. The isolation between the two quadrature channels for both coils was better than -25dB and the gain difference between the quadrature channels was less than 1dB for either of the coils. The Qo/Ql was a high 11.9 for the cavity resonator compared to 5.05 for the birdcage. Both coils were homogeneous in the +/-10% range over 12cm along the z-axis. However, the fall off in sensitivity towards the end-rings for the birdcage was significantly faster than for the cavity resonator.
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