Assessment of Blood Hemodynamics by USPIO-Induced R(1) Changes in MRI of Murine Colon Carcinoma.

The objective of this study is to assess whether ultrasmall superparamagnetic iron oxide (USPIO)-induced changes of the water proton longitudinal relaxation rate (R 1) provide a means to assess blood hemodynamics of tumors. Two types of murine colon tumors (C26a and C38) were investigated prior to and following administration of USPIO blood-pool contrast agent with fast R 1 measurements. In a subpopulation of mice, R 1 was measured following administration of hydralazine, a well-known blood hemodynamic modifier. USPIO-induced R 1 increase in C38 tumors (ΔR 1 = 0.072 ± 0.0081 s−1) was significantly larger than in C26a tumors (ΔR 1 = 0.032 ± 0.0018 s−1, N = 9, t test, P < 0.001). This was in agreement with the immunohistochemical data that showed higher values of relative vascular area (RVA) in C38 tumors than in C26a tumors (RVA = 0.059 ± 0.015 vs. 0.020 ± 0.011; P < 0.05). Following administration of hydralazine, a decrease in R 1 value was observed. This was consistent with the vasoconstriction induced by the steal effect mechanism. In conclusion, R 1 changes induced by USPIO are sensitive to tumor vascular morphology and to blood hemodynamics. Thus, R 1 measurements following USPIO administration can give novel insight into the effects of blood hemodynamic modifiers, non-invasively and with a high temporal resolution.