Off-resonance spin locking for MR imaging.

Off-resonance spin locking is investigated as a low power method for achieving low field spin-lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin-lattice relaxation times and equilibrium magnetizations in the off-resonance rotating frame (T1 rho(off), beta) were measured for tissue-mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin-locking pulse (f1/delta). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off-resonance spin-locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasing f1/delta, the two gels demonstrated a strong variation which provided improved discrimination compared to T1-weighted imaging. Off-resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patient volunteer is presented.