A liver-mimicking MRI phantom for thermal ablation experiments

Purpose: To develop a liver-mimicking MRI gel phantom for use in the development of temperature mapping and coagulation progress visualization tools needed for the thermal tumor ablation methods, including laser-induced interstitial thermotherapy (LITT) and radiofrequency ablation (RFA).Methods: A base solution with an acrylamide concentration of 30 vol. % was prepared. Different components were added to the solution; among them are bovine hemoglobin and MR signal-enhancing contrast agents (Magnevist as T1 and Lumirem as T2 contrast agent) for adjustment of the optical absorption and MR relaxation times, respectively. The absorption was measured in samples with various hemoglobin concentrations (0%–7.5%) at different temperatures (25–80 °C) using the near-infrared spectroscopy, measuring the transmitted radiation through the sample. The relaxation times were measured in samples with various concentrations of T1 (0.025%–0.325%) and T2 (0.4%–1.6%) contrast agents at different temperatures (25–75 °C), through the MRI technique, acquiring images with specific sequences. The concentrations of the hemoglobin and contrast agents of the gel were adjusted so that its absorption coefficient and relaxation times are equivalent to those of liver. To this end, the absorption and relaxation times of the gel samples were compared to reference values, measured in an ex vivo porcine liver at different temperatures through the same methods used for the gel. For validation of the constructed phantom, the absorption and relaxation times were measured in samples containing the determined amounts of the hemoglobin and contrast agents and compared with the corresponding liver values. To qualitatively test the heat resistance of the phantom, it was heated with the LITT method up to ∼120 °C and then was cut to find out if it has been melted.Results: In contrast to liver, where the absorption change with temperature showed a sigmoidal form with a jump at T ≈ 45 °C, the absorption of the gel varied slightly over the whole temperature range. However, the gel absorption presented a linear increase from ∼1.8 to ∼2.2 mm-1 with the rising hemoglobin concentration. The gel relaxation times showed a linear decrease with the rising concentrations of the respective contrast agents. Conversely, with the rising temperature, both T1 and T2 increased linearly and showed almost the same trends as in liver. The concentrations of hemoglobin and T1 and T2 contrast agents were determined as 3.92 ± 0.