Comparison of quantitative shear wave MR‐elastography with mechanical compression tests

Abstract

The mechanical properties of in vivo soft tissue are generally determined by palpation, ultrasound measurements (US), and magnetic resonance elastography (MRE). While it has been shown that US and MRE are capable of quantitatively measuring soft tissue elasticity, there is still some uncertainty about the reliability of quantitative MRE measurements. Mechanical properties of tissues such as Young's modulus, shear modulus, and bulk modulus are of special interest in tissue characterization. By palpation, the stiffness of the tissue, in particular its resistance to pressure and shear forces, is inspected by the physician's hand. Often, cancerous tissue can be detected since it appears as a hard lesion which is a result of increased stromal density (1). Within the last 10 years various ultrasound (US) and MR methods to quantitatively determine the elasticity of soft tissue have been established. Noninvasive US techniques that measure the elastic properties of soft tissue have been described by Ophir et al. (1) (5) showed that in many cases breast lesions cause changes of the elastic modulus as assessed by US measurements. MR elastography is a more recently proposed technique to measure tissue elastic moduli noninvasively. There are two principally different methods of MRE: static or quasi static (11-16) and dynamic (17-26). Static MRE uses two different static compressional states of the investigated material to determine its corresponding distortions. Dynamic MRE is based on the excitation of mechanical waves in soft tissue. In order to evaluate the quantitative precision of dynamic shear wave MRE, we compared quantitative shear wave MRE results with those from mechanical compression tests. MATERIALS AND METHODS Tissue-Mimicking Phantoms and Compression Test SpecimensFor the MRE measurements and compression tests a series of tissue phantoms and compression test specimens were produced. Ideally, the material of the phantoms and the specimens should mimic human soft tissue. Agar-agar gel was used as a test material because it shows mechanical properties similar to human soft tissues. To produce the phantoms, different amounts of agar-agar powder (Agar Agar Kobe I pulv., Roth, Karlsruhe, Germany) were stirred in distilled water and boiled for about 2 min. Then the liquid agar-agar was poured into cylindrical heat-resistant plastic molds having both a diameter and height of about 16 cm and allowed to cool to room temperature. At about 40°C a chemical cross-linking occurs and the agar-agar changes from a fluid to a solid state. As agar-agar gel is a biological material, water diluted formalin was applied to the surface of the phantoms to inhibit the growth of fungi; this allowed for a lifespan of the phantoms of several months. For the compression tests, cylindrical specimens of 4 cm height and 5 cm diameter were cast. To ensure that the corresponding phantoms and specimens had the same elastic properties, they were made from dilutions with identical agar-agar concentrations.The concentration of the agar-agar powder wa...

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