Feasibility of Interstitial CT Lymphography Using Optimized Iodized Oil Emulsion in Rats

Objectives: To formulate an iodine-based contrast agent with an oil-in-water emulsion and to evaluate the feasibility of the agent for use as an interstitial computed tomographic (CT) lymphographic agent in a normal rat model. Materials and Methods: The effect of iodized oil (lipiodol) content and the type of surfactant/cosurfactant on the resultant emulsion size and polydispersity was investigated to obtain an optimized lipiodol emulsion for CT lymphography. Optimized emulsions (144 mg/mL) were injected in the hind paws of 6 rats, using 0.5 mL per paw. As control groups, iopamidol solution and lipiodol diluted with squalene to adjust the injection volume with iodine concentration equivalent to the emulsions were used. Precontrast and post-contrast CT images up to 1 week after contrast agent injection were obtained. Time-enhancement curves of the popliteal lymph nodes were obtained. Analysis of variance and post hoc analysis with the Dunn procedure were used for comparing mean peak enhancement, time to peak enhancement, and sustained duration of contrast enhancement. Results: Optimized emulsion formulations composed of 30% lipiodol and 282 mg/mL of 9: 1 surfactant mixture (Tween 80: TPGS [alpha-tocopheryl polyethylene glycol succinate], Tween 80: Kollidon 12 PF, or Tween 80: Span 85) exhibited mean particle size less than 120 nm, and they were stable without significant particle size change up to 1 month. Targeted lymph nodes in all emulsion groups showed continuously increasing enhancement until 4 or 8 hours after injection, followed by continuous washout. Peak enhancement (time to peak enhancement) was 172.4 +/- 54.5 HU (Hounsfield unit) (384.0 +/- 131.5 minutes) for Tween 80: TPGS; 172.8 +/- 28.0 HU (432.0 +/- 107.3 minutes) for Tween 80: Kollidon 12 PF, and 177.2 +/- 68.9 HU (294.0 +/- 190.2 minutes) for Tween 80: Span 85. For iopamidol, peak enhancement of 153.0 +/- 46.1 HU (0.5 +/- 0.5 minutes) occurred early with rapid washout. For lipiodol as a reference agent, contrast enhancement continuously increased even 1 week after injection without washout (peak enhancement, 486.0 +/- 97.4 HU). Peak enhancement among the emulsion groups and the iopamidol group was not statistically different (P = 0.95). All emulsion groups showed more prolonged enhancement than the iopamidol group; enhancement duration for the emulsion groups was 534.0 +/- 481.1 minutes for Tween 80: TPGS; 957.0 +/- 524.8 minutes for Tween 80: Kollidon 12 PF; and 750.0 +/- 566.0 minutes for Tween 80: Span 85, and enhancement duration for iopamidol was 8.2 +/- 12.3 minutes (all P < 0.05 in multiple comparisons). However, there was no significant difference in enhancement duration among the 3 emulsion groups (P > 0.05). Conclusions: Iodized oil emulsion made with a surfactant mixture (Tween 80 as the main surfactant and TPGS, Kollidon 12 PF, or Span 85 as the cosurfactant) provided sufficient and sustained contrast enhancement on CT of targeted lymph nodes with washout on delayed phase.