A heterogeneous human tissue mimicking phantom for RF heating and MRI thermal monitoring verification

Abstract

This paper describes a heterogeneous phantom that mimics a human thigh with a deep seated tumor, for the purpose of studying the performance of radiofrequency (RF) heating equipment and non-invasive temperature monitoring with magnetic resonance imaging (MRI). The heterogeneous cylindrical phantom was constructed with an outer fat layer surrounding an inner core of phantom material mimicking muscle, tumor and marrow-filled bone. The component materials were formulated to have dielectric and thermal properties similar to human tissues. The dielectric properties of the tissue-mimicking phantom materials were measured with a microwave vector network analyzer and impedance probe over the frequency range of 80 – 500 MHz and at temperatures of 24°C, 37°C, and 45°C. The specific heat values of the component materials were measured using a differential scanning calorimeter over the temperature range of 15 – 55°C. The thermal conductivity value was obtained from fitting the curves obtained from one-dimensional heat transfer measurement. The phantom was used to verify the operation of a cylindrical 4-antenna annular phased array extremity applicator (140 MHz), by examining the proton resonance frequency shift (PRFS) thermal imaging patterns for various magnitude/phase settings (including settings to focus heating in tumor). For muscle and tumor materials, MR imaging was also used to measure T1/T2* values (1.5 Tesla) and to obtain the slope of the PRFS phase change vs. temperature change curve. The dielectric and thermal properties of the phantom materials were in close agreement to well-accepted published results for human tissues. The phantom was able to successfully demonstrate satisfactory operation of the tested heating equipment. The MRI-measured thermal distributions matched the expected patterns for various magnitude/phase settings of the applicator, allowing the phantom to be used as a quality assurance tool. Importantly, the material formulations for the various tissue types may be used to construct customized phantoms that are tailored for different anatomical sites.

Intravenous magnetic nanoparticle cancer hyperthermia

“…88,89 For clinical use, it may be envisioned that dose planning will be undertaken similarly to that for radiation. The iron concentrations can be mapped by MRI, computed tomography, or magnetorelaxometry 51,90,91 and, knowing the precise SLP of the particles and field strength, the heating topography can be predicted, as has been done in human magnetic nanoparticle brain tumor hyperthermia treatments. 76 Subjection to a tissue/ blood-flow modeling program can approximate the heating profile without the need for multiple invasive thermocouples.…”

Tailor-Made Tissue Phantoms Based on Acetonitrile Solutions for Microwave Applications up to 18 GHz

“…These phantoms are supposed to exhibit the same dielectric properties, i.e., relative permittivity, as those of human tissues. There are both commercial and reported formulas for self-preparation phantoms [13], just as liquids [14], gel/semisolids [15], [16] or solids [17]. In the vast majority of cases, phantoms are made of ordinary ingredients such as water, sugar [18], salt, gelatinin-oil [19] or flour [20] with preservatives or combined with other compounds.…”

Focusing microwave hyperthermia in realistic environment for breast cancer treatment

“…Generally, to obtain the required properties, several samples of aqueous gelatin solution with different concentration of gelatin, water, oil and/or salt are tested. The procedure of making samples is similar to the one reported in [128]. Moreover, the challenging issue in constructing phantom is in avoiding air bubbles trapped into samples with high percentages of oil (more than 50% of sample volume).…”

“…The specific heat capacity is measured during the dynamic time of the heating process and it is determined using The other important thermal property, the thermal conductivity K ( / ℃), is obtained from the transient temperature measurements [128]. The thermal conductivity is determined by fitting the transient temperatures at different positions along the axis of a cylindrical shaped sample to a onedimensional heat transfer equation where thermal conductivity is treated as unknown variable and optimised to best fit the measurements.…”

“…However the used tissue-mimicking materials were not tested to confirm their thermal properties as the phantom was aimed at imaging studies, not hyperthermia. In microwave hyperthermia of other human organs, a thigh phantom with dielectric and thermal properties was developed [128]. Yet the reported phantom has a simple structure, which is made from cylindrical shaped fat, muscle and tumour.…”