NMRD investigation of DyDTPA- and GdDTPA-labeled starch particles - Selection of a suitable suspension medium and influence of the starch matrix on relaxivity

RATIONALE AND OBJECTIVES. The primary aim was to investigate the influence of the starch matrix on the T1 relaxivities of starch particles labeled wit h gadolinium and dysprosium diethylenetriamine pentaacetic acid (GdDTPA-SP and DyDTPA-SP), Achieving this required the selection of a medium that was suitable for suspending the particles and that had field-independent T1 rel axation rates, thereby eliminating errors in relaxivity determinations resu lting from a field-dependent background. METHODS. GdDTPA-SP with low and high gadolinium content, DyDTPA-SP, and emp ty DTPA-SP were suspended in an aqueous medium containing 5% (w/w) of a pol yethylene glycol-based block copolymer, 1/T1 NMRD profiles were obtained in the temperature range of 5 degrees to 35 degrees C. RESULTS, Using the block copolymer, particles did not settle, and samples c ould be prepared at a low temperature to avoid particle degradation. the in trinsic T1 relaxation rate of the suspension medium was field-independent a nd identical to that of water from 25 degrees to 35 degrees C, The T1 relax ivities of DyDTPA-SP were higher than those of dysprosium diethylenetriamin e pentaacetate-bis(methylamide) (DyDTPA-BMA) and decreased with increasing magnetic field strength,The T1 relaxivity of GdDTPA-SP was higher than that of GdDTPA at all fields, and decreased with decreasing temperature and inc reasing gadolinium content. CONCLUSIONS. The GdDTPA-SP results showed that the particulate starch matri x served a dual role, with opposing influences on relaxivity, It provided a means for increasing the rotational correlation time (tau(R)), which resul ted in higher relaxivities, However, it also retarded radial diffusion of w ater molecules within the particle interior, which significantly counteract ed the enhancing effect of tau(R). For DyDTPA-SP, the starch matrix provide d an additional diamagnetic contribution, resulting in relaxivities higher than those of DyDTPA-BMA, The block copolymer was suitable as a suspension medium for DyDTPA-SP and GdDTPA-SP and should also be applicable for other particulates.