Characterization of a dielectric phantom for high‐field magnetic resonance imaging applications

Abstract

Purpose: In this work, a generic recipe for an inexpensive and nontoxic phantom was developed within a range of biologically relevant dielectric properties from 150 MHz to 4.5 GHz. Methods: The recipe includes deionized water as the solvent, NaCl to primarily control conductivity, sucrose to primarily control permittivity, agar-agar to gel the solution and reduce heat diffusivity, and benzoic acid to preserve the gel. Two hundred and seventeen samples were prepared to cover the feasible range of NaCl and sucrose concentrations. Their dielectric properties were measured using a commercial dielectric probe and were fitted to a 3D polynomial to generate a recipe describing the properties as a function of NaCl concentration, sucrose concentration, and frequency. Results: Results indicated that the intuitive linear and independent relationships between NaCl and conductivity and between sucrose and permittivity are not valid. A generic polynomial recipe was developed to characterize the complex relationship between the solutes and the resulting dielectric values and has been made publicly available as a web application. In representative mixtures developed to mimic brain and muscle tissue, less than 2% difference was observed between the predicted and measured conductivity and permittivity values. Conclusions: It is expected that the recipe will be useful for generating dielectric phantoms for general magnetic resonance imaging (MRI) coil development at high magnetic field strength, including coil safety evaluation as well as pulse sequence evaluation (including B + 1 mapping, B + 1 shimming, and selective excitation pulse design), and other non-MRI applications which require biologically equivalent dielectric properties. C

Synthesized tissue‐equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions

“…Although human tissues have dielectric properties that cover a wide range, most have an electrical conductivity greater than that of water and a relative permittivity less than that of water at frequencies above 100 MHz . Ionic compounds, such as salt, are commonly used to increase electrical conductivity in tissue‐mimicking materials, whereas various substances, such as alcohol, sucrose, or polymers, have been used to reduce permittivity to the desired value . Alcohol and sucrose can be unfavorable because they introduce extraneous spectral components that complicate the MR signal evolution and can reduce the relaxation times.…”

Size‐adaptable “Trellis” structure for tailored MRI coil arrays

“…To determine coil variability in a range of subject sizes, we measured the scattering matrix in the above phantom and 2 others (134 and 195 mm in diameter, representing 50th and 99th percentile knee size, respectively). All phantoms were filled with salt and sucrose to approximate dielectric properties of the human knee at 3T (38.7 g of salt and 1341.5 g of sucrose per 1 L of water to generate ε r = 56.6 and σ = 0.37 S/m) …”

In vivo imaging of electrical properties of an animal tumor model with an 8‐channel transceiver array at 7 T using electrical properties tomography

“…The outer diameter of the phantom was 5.5 cm, and its length was 10 cm, similar to the overall size of utilized animals. The main body (compartment 1) of the phantom was filled with a gel solution including Agar, NaCl, CuSO 4 ·5H 2 O, and deionized water with a mass ratio of 1.2:0.453:0.1:100, of which EP, measured with an Agilent 85070E dielectric probe, and E5061B network analyzer (Agilent Technologies, Santa Clara, CA, USA) at 298 MHz, were σ = 0.94 ± 0.01 S/m and ɛ = 78.6 ± 0.1 ɛ 0 . A small balloon with a diameter of about 2.5 cm (compartment 2), positioned inside the main body, as shown in Figure b, was filled with a gel solution including Agar, NaCl, CuSO 4 ·5H 2 O, sucrose (S8501‐5KG, Sigma‐Aldrich), and deionized water with a mass ratio of 1.2:2.36:0.01:50:100; and the measured EPs were σ = 1.16 ± 0.03 S/m and ɛ = 67.4 ± 0.5 ɛ 0 .…”