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 gelabsorption presented a linear increase from ∼1.8 to ∼2.2 mm-1 with the rising hemoglobin concentration. The gelrelaxation 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 ± 0.42 vol. %, 0.098 ± 0.023 vol. %, and 2.980 ± 0.067 vol. %, respectively. The measured ex vivoliver T1 value increased from ∼300 to ∼530 ms and T2 value from ∼45 to ∼52 ms over the temperature range. The phantom validation experiments resulted in absorption coefficients of 2.0–2.1 mm−1 with variations of 1.5%–2.95% compared to liver below 50 °C, T1 of 246.6–597.2 ms and T2 of 40.8–67.1 ms over the temperature range of 25–75 °C. Using the Bland–Altman analysis, a difference mean of −6.1/1.9 ms was obtained for T1/T2 between the relaxation times of the phantom and liver. After heating the phantom with LITT, no evidence of melting was observed.Conclusions: The constructed phantom is heat-resistant and MR-compatible and can be used as an alternative to livertissue in the MR-guided thermal ablation experiments with laser to develop clinical tools for real-time monitoring and controlling the thermal ablation progress in liver.
This research was supported by a grant from the German Research Foundation (DFG) for the project “In-vitro Temperaturbestimmung und Computersimulation der Temperaturverteilung zur optimalen Planung und Steuerung der laserinduzierten Thermotherapie (LITT),” with the reference numbers VO 479/10-1 and SI 1289/1-1, to the institutes IDIR of the J.W. Goethe University of Frankfurt and Fraunhofer ITWM.
II.A. Measurement of liver properties
II.A.1. Optical absorption
II.A.2. MR relaxation
II.B. Adjustment of phantom properties
II.B.1. Phantom material preparation
II.B.2. Optical absorption adjustment
II.B.3. MR relaxation adjustment
II.C. Validation of the phantom
II.C.1. Optical absorption
II.C.2. MR relaxation
II.C.3. Heat resistance
III.A. Liver properties
III.A.1. Optical absorption
III.A.2. MR relaxation
III.B. Adjustment of phantom properties
III.B.1. Optical absorption adjustment
III.B.2. MR relaxation adjustment
III.C. Validation of the phantom
III.C.1. Optical absorption
III.C.2. MR relaxation
III.C.3. Heat resistance
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