No abstract

MR elastography (MRE) enables the noninvasive determination of the viscoelastic behavior of human internal organs based on their response to oscillatory shear stress. An experiment was developed that combines multifrequency shear wave actuation with broad-band motion sensitization to extend the dynamic range of a single MRE examination. With this strategy, multiple wave images corresponding to different driving frequencies are simultaneously received and can be analyzed by evaluating the dispersion of the complex modulus over frequency. The technique was applied on the brain and liver of five healthy volunteers. Its repeatability was tested by four follow-up studies in each volunteer. Five standard rheological models (Maxwell, Voigt, Zener, Jeffreys and fractional Zener model) were assessed for their ability to reproduce the observed dispersion curves. The three-parameter Zener model was found to yield the most consistent results with two shear moduli mu(1) = 0.84 +/- 0.22 (1.36 +/- 0.31) kPa, mu(2) = 2.03 +/- 0.19 (1.86 +/- 0.34) kPa and one shear viscosity of eta = 6.7 +/- 1.3 (5.5 +/- 1.6) Pa s (interindividual mean +/- SD) in brain (liver) experiments. Significant differences between the rheological parameters of brain and liver were found for mu(1) and eta (P < 0.05), indicating that human brain is softer and possesses a higher viscosity than liver.

The purpose of this work was to develop magnetic resonance elastography (MRE) for the fast and reproducible measurement of spatially averaged viscoelastic constants of living human brain. The technique was based on a phase-sensitive echo planar imaging acquisition. Motion encoding was orthogonal to the image plane and synchronized to intracranial shear vibrations at driving frequencies of 25 and 50 Hz induced by a head-rocker actuator. Ten time-resolved phase-difference wave images were recorded within 60 s and analyzed for shear stiffness and shear viscosity. Six healthy volunteers (six men; mean age 34.5 years; age range 25-44 years) underwent 23-39 follow-up MRE studies over a period of 6 months. Interindividual mean +/- SD shear moduli and shear viscosities were found to be 1.17 +/- 0.03 kPa and 3.1 +/- 0.4 Pas for 25 Hz and 1.56 +/- 0.07 kPa and 3.4 +/- 0.2 Pas for 50 Hz, respectively (P < or = 0.01). The intraindividual range of shear modulus data was 1.01-1.31 kPa (25 Hz) and 1.33-1.77 kPa (50 Hz). The observed modulus dispersion indicates a limited applicability of Voigt's model to explain viscoelastic behavior of brain parenchyma within the applied frequency range. The narrow distribution of data within small confidence intervals demonstrates excellent reproducibility of the experimental protocol. The results are necessary as reference data for future comparisons between healthy and pathological human brain viscoelastic data.

No abstract

MR elastography (MRE) allows the noninvasive assessment of the viscoelastic properties of human organs based on the organ response to oscillatory shear stress. Shear waves of a given frequency are mechanically introduced and the propagation is imaged by applying motion-sensitive gradients. An experiment was set up that introduces multifrequency shear waves combined with broadband motion sensitization to extend the dynamic range of MRE from one given frequency to, in this study, four different frequencies. With this approach, multiple wave images corresponding to the four driving frequencies are simultaneously acquired and can be evaluated with regard to the dispersion of the complex modulus over the respective frequency. A viscoelastic model based on two shear moduli and one viscosity parameter was used to reproduce the experimental wave speed and wave damping dispersion. The technique was applied in eight healthy volunteers and eight patients with biopsy-proven high-grade liver fibrosis (grade 3-4).

Magnetic resonance elastography (MRE) is an increasingly used noninvasive modality for diagnosing diseases using the response of soft tissue to harmonic shear waves. We present a study on the algebraic Helmholtz inversion (AHI) applied to planar MRE, demonstrating that the deduced phase speed of shear waves depends strongly on the relative orientations of actuator polarization, motion encoding direction and image plane as well as on the actuator plate size, signal-to-noise ratio and discretization of the wave image. Results from the numerical calculation of harmonic elastic waves due to different excitation directions and simulated plate sizes are compared to experiments on a gel phantom. The results suggest that correct phase speed can be obtained despite these largely uncontrollable influences, if AHI is based on out-of-plane displacements and the actuator is driven at an optimal frequency yielding an optimal pixel per wavelength resolution in the wave image. Assuming plane waves, the required number of pixels per wavelength depends only on the degree of noise.

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...

No abstract