Viscoelastic Characterization of Soft Tissue-Mimicking Gelatin Phantoms using Indentation and Magnetic Resonance Elastography

Summary

This article presents a demonstration and summary of protocols of making gelatin phantoms that mimic soft tissues, and the corresponding viscoelastic characterization using indentation and magnetic resonance elastography.

Abstract

Characterization of biomechanical properties of soft biological tissues is important to understand the tissue mechanics and explore the biomechanics-related mechanisms of disease, injury, and development. The mechanical testing method is the most straightforward way for tissue characterization and is considered as verification for in vivo measurement. Among the many ex vivo mechanical testing techniques, the indentation test provides a reliable way, especially for samples that are small, hard to fix, and viscoelastic such as brain tissue. Magnetic resonance elastography (MRE) is a clinically used method to measure the biomechanical properties of soft tissues. Based on shear wave propagation in soft tissues recorded using MRE, viscoelastic properties of soft tissues can be estimated in vivo based on wave equation. Here, the viscoelastic properties of gelatin phantoms with two different concentrations were measured by MRE and indentation. The protocols of phantom fabrication, testing, and modulus estimation have been presented.

Introduction

Most of the soft biological tissues appear to have viscoelastic properties that are important to understand their injury and development^1,2. In addition, viscoelastic properties are important biomarkers in the diagnosis of a variety of diseases such as fibrosis and cancer^3,4,5,6. Therefore, the characterization of viscoelastic properties of soft tissues is crucial. Among the many characterization techniques used, ex vivo mechanical testing of tissue samples and in vivo elastography using biomedical imaging are the two widely used methods.

Although various mechanical testing techniques have been used for soft tissue characterization, the requirements for sample size and testing conditions are not easy to be satisfied. For example, shear testing needs to have samples fixed firmly between the shear plates⁷. Biaxial testing is more suitable for membrane tissue and has specific clamping requirements^8,9. A compression test is commonly used for tissue testing, but cannot characterize specific positions within one sample¹⁰. The indentation test does not have additional requirements to fix the tissue sample and can be used to measure many biological tissue samples such as the brain and liver. In addition, with a small indenter head, regional properties within a sample could be tested. Therefore, indentation tests have been adopted to test a variety of soft tissues^1,3,11.

Characterizing the biomechanical properties of soft tissues in vivo is important for translational studies and clinical applications of biomechanics. Biomedical imaging modalities such as ultrasound (US) and magnetic resonance (MR) imaging are the most used techniques. Although US imaging is relatively cheap and easy to carry out, it suffers from low contrast and is hard to measure organs such as the brain. Capable of imaging deep structures, MR Elastography (MRE) could measure a variety of soft tissues^6,12, especially the brain^13,14. With applied external vibration, MRE could measure the viscoelastic properties of soft tissues at a specific frequency.

Studies have shown that at 50-60 Hz, the shear modulus of the normal brain is ~1.5-2.5kPa^5,6,13,14,15 and ~2-2.5 kPa for normal liver¹⁶. Therefore, gelatin phantoms that have similar biomechanical properties have been widely used for mimicking soft tissues for testing and validation^17,18,19. In this protocol, gelatin phantoms with two different concentrations were prepared and tested. Viscoelastic properties of the gelatin phantoms were characterized using a custom-built electromagnetic MRE device¹⁴ and an indentation device^1,3. The testing protocols could be used for testing many soft tissues such as the brain or liver.

Protocol

1. Gelatin phantom preparation Weigh gelatin, glycerol, and water according to Table 1. Mix the gelatin powder with water to obtain the gelatin solution. NOTE: The concentrations of the individual components for preparing the two phantoms are shown in Table 1. The higher the concentration of gelatin, the stiffer the phantom. Heat the gelatin solution to 60 °C in a water bath. Add glycerol to the gelatin solution while maintaining the tem…

Representative Results

Following the MRE protocol, a clear shear wave propagation in the gelatin phantoms at 40 and 50 Hz were observed (Figure 3). The viscoelastic properties measured from MRE, and indentation tests are shown in Figure 4. The estimated G' and G" values at each testing for each phantom are summarized in Table 2. Following the indentation protocol, the viscoelastic properties of each phantom at each test point are summarized in Table 3<…

Discussion

Gelatin phantoms are commonly used as tissue-mimicking materials for testing and validation of algorithms and devices^{17,19,22,23,24,25,26,27}. One of the pioneering studies using the gelatin phantom to compare MRE and dynamic shear testing was presented by O…

Materials

24-channel head & Neck coil	United Imaging Healthcare	100120	Equipment
3T MR Scanner	United Imaging Healthcare	uMR 790	Equipment
Acquisition board	Advantech Co	PCI-1706U	Equipment
Computer-Windows	HP	790-07	Equipment
Electromagnetic actuator	Shanghai Jiao Tong University		Equipment
Function generator	RIGOL	DG1022Z	Equipment
Gelatin	CARTE D’OR		Reagent
Glycerol	Vance Bioenergy Sdn.Bhd		Reagent
Indenter control program	custom-designed		Software; accessed via: https://github.com/aaronfeng369/FengLab_indentation_code.
Laser sensor	Panasonic	HG-C1050	Equipment
Load cell	Transducer Technique	GSO-10	Equipment
MATLAB	Mathworks		Software
Power amplifier	Yamaha	A-S201	Equipment
Voice coil electric motor	SMAC Corporation	DB2583	Equipment