LOW-MOLECULAR-WEIGHT LANTHANIDE CONTRAST AGENTS - EVALUATION OF SUSCEPTIBILITY AND DIPOLAR EFFECTS IN RED-BLOOD-CELL SUSPENSIONS

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW ) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two -compartment system for the evaluation of the magnetic dipolar and sus ceptibility contributions to the transverse (T-2) relaxation of solven t mater protons, The influence of RBC geometry and degree of metal che late compartmentalization on T-2 was investigated by variation of the osmolality and hematocrit (HC), respectively, The T-2-relaxation abili ty of Dy-chelates was markedly improved in RBC suspensions, in compari son to aqueous solutions, due to the presence of susceptibility effect s that more than compensated for the low dipolar relaxation efficacy, Despite a smaller susceptibility effect, the Gd-chelates were still th e most efficacious in shortening T-2 due to their comparatively larger dipolar relaxation contribution, The results obtained with the Dy-che lates allowed the evaluation of the relative contributions of suscepti bility and dipolar mediated relaxation for the Gd-chelates. The RBC ge ometry and degree of compartmentalization influenced strongly the T-2 relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hem olysis eliminated the susceptibility effect, essentially removing the T-2 relaxation ability of Dy-chelates. The T-2 relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipola r relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the differen t contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents. (C) 1997 Elsevier Science Inc.