MRI enhancement of perfused tissues using chromium labeled red blood cells as an intravascular contrast agent

It has been demonstrated that chromium (Cr) labeling significantly decreases the relaxation times of packed red blood cells (RBCs). In this study, the spin-lattice relaxation time (T₁) of human red cells was shortened from 836 ms to 29 ms and the spin-spin relaxation time (T₂) shortened from 134 ms to 18 ms, when the cells were labeled at a Cr incubation concentration of 50 mM. Labeling of canine cells at 50 mM resulted in a T₁ of 36 ms and a T₂ of 26 ms. A labeling concentration of 10 mM produced similar relaxation enhancement, with uptake of 47% of the available Cr, and was determined to be optimal. The enhancement of longitud-inal and transverse relaxation rates (1/T₁, 1/T₂) per amount of hemoglobin-bound Cr are 6.9 s^-1 mM^-1 and 9.8 s^-1 mM^-1 respectively, different from those of a pure Cr⁺³ solution. Labeling cells at 10 mM decreased the survival half-time in vivo from 16.6 days to 4.7 days in dogs. No difference in red cell survival was found with the use of hetero-transfusion versus auto-transfusion of labeled RBCs. Significant shortening of the T₁ (912 ms to 266 ms, P =.03) and T₂ (90 ms to 70 ms, P =.006) of spleen and the T, (764 ms to 282 ms, P =.005) and the T2 (128 ms to 86 ms, P =.005) of liver occurred when 10% of the RBC mass of dogs was exchanged with Cr labeled cells. Liver and spleen spin density changes (, P > 0.23) and muscle spin density and relaxation changes (P > 0.4) were insignificant. The in vivo T₁ of a canine spleen which had been infarcted did not change following transfusion with labeled cells, where the T₁ of liver did shorten. We believe this preliminary study suggests that Cr labeled red cells may have the potential to become an intravascular magnetic resonance imaging contrast agent.