MR Imaging-Based In Vivo Macrophage Imaging to Monitor Immune Response after Radiofrequency Ablation of the Liver.

Purpose: To establish molecular magnetic resonance (MR) imaging instruments for in vivo characterization of the immune response to hepatic radiofrequency (RF) ablation using cell-specific immunoprobes.

Materials And Methods: Seventy-two C57BL/6 wild-type mice underwent standardized hepatic RF ablation (70 °C for 5 minutes) to generate a coagulation area measuring 6-7 mm in diameter. CD68 macrophage periablational infiltration was characterized with immunohistochemistry 24 hours, 72 hours, 7 days, and 14 days after ablation (n = 24). Twenty-one mice were subjected to a dose-escalation study with either 10, 15, 30, or 60 mg/kg of rhodamine-labeled superparamagnetic iron oxide nanoparticles (SPIONs) or 2.4, 1.2, or 0.6 mg/kg of gadolinium-160 (Gd)-labeled CD68 antibody for assessment of the optimal in vivo dose of contrast agent. MR imaging experiments included 9 mice, each receiving 10-mg/kg SPIONs to visualize phagocytes using T2-weighted imaging in a horizontal-bore 9.4-T MR imaging scanner, Gd-CD68 for T1-weighted MR imaging of macrophages, or 0.1-mmol/kg intravenous gadoterate (control group). Radiological-pathological correlation included Prussian blue staining, rhodamine immunofluorescence, imaging mass cytometry, and immunohistochemistry.

Results: RF ablation-induced periablational infiltration (206.92 μm ± 12.2) of CD68 macrophages peaked at 7 days after ablation (P < .01) compared with the untreated lobe. T2-weighted MR imaging with SPION contrast demonstrated curvilinear T2 signal in the transitional zone (TZ) (186 μm ± 16.9), corresponsing to Iron Prussian blue staining. T1-weighted MR imaging with Gd-CD68 antibody showed curvilinear signal in the TZ (164 μm ± 3.6) corresponding to imaging mass cytometry.

Conclusions: Both SPION-enhanced T2-weighted and Gd-enhanced T1-weighted MR imaging allow for in vivo monitoring of macrophages after RF ablation, demonstrating the feasibility of this model to investigate local immune responses.