Purpose: To measure the complex permittivity and thermal properties of a gel-saline phantom designed for thermometric studies in a 3T MRI scanner.
Methods: A gel-saline phantom was designed according to the ASTM International standard for measurement of radiofrequency (RF) induced heating on or near passive implants during magnetic resonance imaging. The dielectric constant and electrical conductivity of the phantom was obtained using an open-ended coaxial probe permittivity measurement method that relies on changes in the complex reflection coefficient of an optimized probe due to the material under test. A simple implementation of this method was achieved with a basic RF electronic circuit consisting of a signal generator, power splitter, circulator, and oscilloscope, as opposed to the widely used Vector Network Analyzers (VNA). The non-ideal behavior of the coaxial probe due to spurious reflections and ohmic losses were accounted for by a scattering matrix that relates the measured reflection coefficient to the reflection coefficient at the probe tip. The probe was calibrated with three standard terminations including a short circuit, an open circuit, and a saline solution with a similar permittivity to that of the phantom. Assessment of the thermal conductivity and specific heat capacity of the phantom is underway and will be discussed.
Results: A dielectric constant of 82.1 ± 1.4 and electrical conductivity of 0.47 ± 0.04 S/m was measured at a frequency of 127.8 MHz. This agrees with the recommended values of the ASTM standard of 80 ± 20 and 0.47 ± 10% S/m, respectively, for the phantom.
Conclusion: A novel equivalent circuit for open-ended probe permittivity measurement has been developed and verified in the electrical characterization of a tissue-equivalent phantom. The complex permittivity of the phantom agrees with recommended values and indicates its usability for safety assessment in MRI.