Improving SNR and B1 transmit field for an endorectal coil in 7 T MRI and MRS of prostate cancer

Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B1+/- and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B1+/- fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4-fold decrease in local specific absorption rate is observed when normalized to the B1 values in the center of the prostate. Because of the 4-fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B1 value (20 mu T at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B1 duty cycle can be increased by a factor 4. Consequently, when using the two-element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for 1H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo 1H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B1+ of 20 mu T shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg. Magn Reson Med, 2012. (C) 2011 Wiley Periodicals, Inc.