Development of High Field MR Imaging and Spectroscopy Techniques of the Prostate

Abstract

This thesis summarizes the work of the development of new techniques for obtaining magnetic resonance imaging (MRI) and spectroscopy (MRS) of the prostate at the ultra high field of 7 tesla (T). The 7 T field strength presents various challenges such as the shortening of the wavelength and the lower power penetration in the human body as compared to lower field strengths. A dedicated endorectal surface coil (ERC) with two elements was developed for obtaining MRI and MRS prostate. The duality of the ERC coil improved the field of view coverage and power penetration, compared with a one element ERC. In addition, the ERC decreased the total specific absorption rate (SAR), a safety quantity, thus making possible the use of power intense adiabatic MR sequences while keeping SAR within safety limits. Further coil exploration was done by combining six external radiative antennas with the ERC. With this coil combination MRI of the whole pelvic region could be obtained with a signal to noise (SNR) increase when transmitting with the external antennas and receiving with the external and internal elements. In addition, the power available for the ERC was extended to the anterior side of the prostate with the external antennas. Therefore, MRS imaging (MRSI) of larger prostates could be obtained by transmitting with all elements and receiving with the ERC. The possible RF coupling between the internal and external elements was addressed by doing active RF decoupling. With the ERC available, improvements on the MRS techniques were explored. Two fully adiabatic sequences were developed: the cLASER, sequence, which allowed 3D MRS imaging (MRSI) acquisition by making the excitation pulse composite and the nsLASER sequence, which had a non-selective excitation pulse, thus making this one suitable for 2D MRSI acquisitions. Both, the cLASER and nsLASER were compared with an existing semi-adiabatic sequence that has shown good performance for acquiring MRSI in the prostate. The cLASER and nsLASER outperformed the semi-adiabatic sequence in areas were the available power deviated from the nominal power. No signal voids were seen with the fully adiabatic sequences. However, the SAR deposited by the developed sequences was elevated compared to the semi-adiabatic one, which was kept under the safety limits by increasing their total scan time. More adiabatic sequences were explored for the detection of lactate (i.e. a relevant metabolite in prostate cancer for knowing tissue oxygenation). A semi-LASER sequence with MEGA pulses for editing (MEGA-sLASER) and frequency offset corrected inversion (FOCI) pulses for refocusing was developed and tested in the brain. Two versions of the MEGA-sLASER with FOCI sequence were constructed for either lactate or GABA detection using different interpulse timings. The MEGA-sLASER with FOCI pulses showed a detection improvement of 20% and 75% for GABA and lactate respectively. At last, the variations arising from zero order field variations were investigated with field probes located in the housing of the ERC. Retrospective corrections showed SNR improvement and up to 40% linewidth reduction in MRSI data sets obtained in the human prostate.