Exchange-mediated contrast agents for spin-lock imaging.

Measurements of relaxation rates in the rotating frame with spin-locking techniques are sensitive to substances with exchanging protons with appropriate chemical shifts. The authors develop a novel approach to exchange-rate selective imaging based on measured T(1ρ) dispersion with applied locking field strength, and demonstrate the method on samples containing the X-ray contrast agent Iohexol with and without cross-linked bovine serum albumin. T(1ρ) dispersion of water in the phantoms was measured with a Varian 9.4-T magnet by an on-resonance spin-locking pulse with fast spin-echo readout, and the results used to estimate exchange rates. The Iohexol phantom alone gave a fitted exchange rate of ~1 kHz, bovine serum albumin alone was ~11 kHz, and in combination gave rates in between. By using these estimated rates, we demonstrate how a novel spin-locking imaging method may be used to enhance contrast due to the presence of a contrast agent whose protons have specific exchange rates.