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 (T  1ρoff β) 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/Δ). 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/Δ, 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.

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