MECHANISMS THAT CONTRIBUTE TO THE IN-VITRO RELAXATION AND SIGNAL INTENSITY OF WATER IN BARIUM-SULFATE SUSPENSIONS USED AS MRI CONTRAST AGENTS

The individual components of two commercially available barium sulfate (BaSO4) suspensions, Liquid HD and E-Z-paque (E-Z-EM Inc., Westbury, NY), were investigated to determine their contribution to relaxation. Longitudinal and transverse relaxation times of suspensions and soluti ons of the different BaSO4 particles and the vehicle used to keep them in suspension were measured separately at 2.0 T. A multiple echo Carr -Purcell-Meiboom-Gill (CPMG) pulse sequence was used for T2 determinat ions with different values of the echo spacing 2tau. Longer values of 2tau resulted in significant shortening of the calculated T2 relaxatio n times, indicating that the major mechanism leading to signal loss in BaSO4 suspensions is the diffusion of water molecules through suscept ibility gradients in the vicinity of suspended particles. At higher Ba SO4 concentrations, decreased water proton density also produces signi ficant signal loss. Viscosity has little effect on the relaxation. A c ombination of larger and smaller BaSO4 particle sizes was found to be more effective than smaller sizes only in enhancing signal decay.