Characterization and evaluation of tissue-mimicking gelatin phantoms for use with MRgFUS

Abstract

Background: A tissue-mimicking phantom that accurately represents human-tissue properties is important for safety testing and for validating new imaging techniques. To achieve a variety of desired human-tissue properties, we have fabricated and tested several variations of gelatin phantoms. These phantoms are simple to manufacture and have properties in the same order of magnitude as those of soft tissues. This is important for quality-assurance verification as well as validation of magnetic resonance-guided focused ultrasound (MRgFUS) treatment techniques.

Methods: The phantoms presented in this work were constructed from gelatin powders with three different bloom values (125, 175, and 250), each one allowing for a different mechanical stiffness of the phantom. Evaporated milk was used to replace half of the water in the recipe for the gelatin phantoms in order to achieve attenuation and speed of sound values in soft tissue ranges. These acoustic properties, along with MR (T1 and T2*), mechanical (density and Young's modulus), and thermal properties (thermal diffusivity and specific heat capacity), were obtained through independent measurements for all three bloom types to characterize the gelatin phantoms. Thermal repeatability of the phantoms was also assessed using MRgFUS and MR thermometry.

Results: All the measured values fell within the literature-reported ranges of soft tissues. In heating tests using low-power (6.6 W) sonications, interleaved with high-power (up to 22.0 W) sonications, each of the three different bloom phantoms demonstrated repeatable temperature increases (10.4 ± 0.3 °C for 125-bloom, 10.2 ± 0.3 °C for 175-bloom, and 10.8 ± 0.2 °C for 250-bloom for all 6.6-W sonications) for heating durations of 18.1 s.

Conclusion: These evaporated milk-modified gelatin phantoms should serve as reliable, general soft tissue-mimicking MRgFUS phantoms.

Keywords: Gelatin; MRgFUS; Phantoms; Tissue-mimicking.

Fig. 1

Typical gelatin phantom housed in our custom-built phantom holder. The holder is made of acrylic tubing a with a height of 15 cm and b with an inner diameter of 10 cm. A 0.1-mm film of clear PVC is adhered with silicone on both ends of the holder, creating an ultrasound transparent barrier

Fig. 2

Setup for performing the through-transmission measurements used in calculating the speed of sound and attenuation of the samples

Fig. 3

Thermal repeatability for the 125-bloom phantom, demonstrating the degree of consistency of achieving the same measured peak temperature for a given acoustic power. The order of applied power is shown in the legend

Fig. 4

Thermal repeatability for the 175-bloom phantom, demonstrating the degree of consistency of achieving the same measured peak temperature for a given acoustic power. The order of applied power is shown in the legend

Fig. 5

Thermal repeatability for the 250-bloom phantom, demonstrating the degree of consistency of achieving the same measured peak temperature for a given acoustic power. For clarity in this figure, only some representative runs are included. The order of applied power was 6.6, 6.6, 6.6, 10.1, 6.6, 13.2, 6.6, 13.2, 6.6, 15.4, 6.6, 17.6, 6.6, 20.7, 20.7, and 6.6 W (italicized values reflect the runs shown in the figure and listed in the legend)