Sequence design for magnetic resonance spectroscopic imaging of prostate cancer at 3 T

Magnetic resonance spectroscopic imaging (MRSI) has proven to be a powerful tool for the metabolic characterization of prostate cancer in patients before and following therapy. The metabolites that are of particular interest are citrate and choline because an increased choline-to-citrate ratio can be used as a marker for cancer. High-field systems offer the advantage of improved spectral resolution as well as increased magnetization. Initial attempts at extending MRSI methods to 3 T have been confounded by the J-modulation of the citrate resonances. A new pulse sequence is presented that controls the J-modulation of citrate at 3 T such that citrate is upright, with high amplitude, at a practical echo time. The design of short (14 ms) spectral-spatial refocusing pulses and trains of nonselective refocusing pulses are described. Phantom studies and simulations showed that upright citrate with negligible sidebands is observed at an echo time of 85 ms. Studies in a human subject verified that this behavior is reproduced in vivo and demonstrated that the water and lipid suppression of the new pulse sequence are sufficient for application in prostate cancer patients.