Mechanisms that contribute to the in vitro relaxation and signal intensity of water in barium sulfate suspensions used as MRI contrast agents

Abstract

The individual components of two commercially available barium sulfate (BaSO 4) 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 solutions of the different BaSO 4 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 T 2 determinations with different values of the echo spacing 2τ. Longer values of 2τ resulted in significant shortening of the calculated T 2 relaxation times, indicating that the major mechanism leading to signal loss in BaSO 4 suspensions is the diffusion of water molecules through susceptibility gradients in the vicinity of suspended particles. At higher BaSO 4 concentrations, decreased water proton density also produces significant signal loss. Viscosity has little effect on the relaxation. A combination of larger and smaller BaSO 4 particle sizes was found to be more effective than smaller sizes only in enhancing signal decay.