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. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.