The measurement of anisotropic elasticity in skeletal muscle using MR Elastography

T. Oida, Y. Kang, T. Azuma, J. Okamoto, A. Amano, L. Axel, O. Takizawa, S. Tsutsumi, T. Matsuda Graduate School of Informatics, Kyoto University, Kyoto, Kyoto, Japan, Institute for Frontier Medical Sciences, Kyoto University, Kyoto, Kyoto, Japan, Siem ens-Asahi Medical Technologies Ltd., Shinagawa-ku, Tokyo, Japan, Department of Radiology, New York University School of Medicine, New York, New York, United States Introduction Magnetic resonance elastography (MRE) is a non-invasive method for measuring the elasticity of human tissue proposed by Muthupillai et al. [1]. Several studies on measuring the elasticity of skeletal muscle fiber have been reported [2, 3]. Among five anisotropic elasticity properties in the transverse isotropic material such as skeletal muscle tissue which has directional structure, only one of them has been measured in these studies. In this study, two of the transverse isotropic shear moduli are measured with different shear wave propagation direction. These anisotropic elasticity properties are potentially important for accurate assessment of human tissue character in physiological and pathological conditions. Methods In this study, two different shear waves were imaged by MRE. One was a wave which propagated or vibrated parallel to the muscle fiber orientation (parallel wave) and the other was one which propagated and vibrated orthogonal to the muscle fiber orientation (orthogonal wave). Two types of actuators were utilized for generating shear wave (Fig.1 and Fig.2). Two of the independent elastic properties (S44=2Gyz, S66=2Gxy in the stiffness matrix) were measured by estimating the wavelength of corresponding MRE images. Two experiments were performed. In the first experiment, a foot of pig was examined ex vivo to measure two transverse isotropic shear moduli Gxy and Gyz. Mechanical oscillations were applied as shown in Fig.1. All the MRE images were acquired with Magnetom Sonata (Siemens AG, Erlangen, Germany) with 100Hz mechanical oscillation. To acquire MRE images, a modified gradient echo sequence was applied including a sinusoidal MSG of 25mT/m whose direction and frequency were identical to those of mechanical oscillation. Images with ten phase offsets were obtained to estimate the wavelength accurately. For extracting shear waves, a spatio-temporal directional filter [4] was applied to these images in order to suppress reflection waves. Finally, the shear moduli were calculated from the wavelength of the extracted waves with an estimation of tissue density as 1 g/cm. In the second experiment, a forearm of a healthy young volunteer was examined in vivo using actuators as shown in Fig.2. While oscillation conditions, MRE sequences and the calculation method of shear moduli were same as those in the first experiment, images with only four phase offsets were acquired in this in vivo experiment to reduce total acquisition time. Results and Discussion Acquired MRE images are shown in Fig.3. Shear moduli in the directions parallel (Gyz) and orthogonal (Gxy) to the muscle fiber were averaged in the square area shown in each image. The averaged shear moduli were 21.4 (SD: 9.6) kPa and 12.9 (SD: 3.7) kPa in the pig ex vivo study, and 14.3 (SD: 8.1) KPa and 6.6 (SD: 16.0) KPa in human in vivo study, respectively. The shear moduli of the same pig specimen were measured by a viscoelastic analyzer (Rheogel-E4000, UBM, Kyoto, Japan). The resulting shear moduli of 21.1 (SD: 2.7) kPa (Gyz) and 14.8 (SD: 4.6) kPa (Gyz) corresponded well with the measurements of MRE. The shear modulus parallel to the muscle fiber direction (Gyz) in human in vivo study was comparable to that of previous studies [2, 3]. Conclusion In this study, two of the transverse isotropic shear moduli were measured with different shear wave propagation directions. Experimental results showed that the transverse isotropic material such as skeletal muscle tissue had different shear moduli parallel to and orthogonal to the fiber direction. Reference 1. Muthupillai R et al, ``Magnetic resonance elastography by direct visualization of propagating acoustic strain waves,’’ SCIENCE, vol.269, pp.1854-1857, 1995. 2. Thomas RJ et al, ``Noninvasive muscle tension measurement using novel technique of magnetic resonance elastography (MRE),’’ J. Biomechanics 36, pp.1917-1921, 2003 3. Uffman K et al, ``In Vivo Examination of Skeletal Muscle with MR Elastography,’’ Proc. Intl. Soc. Mag. Reson. Med. 11 (2004), p.1774, 2004 4. Manduca A et al, ``Spatio-temporal directional filtering for improved inversion of MR Elastography images,’’ Medical Image Analysis, 7, pp.465-473, 2003 Fig.1 : Oscillation system for measuring a foot of pig.