Noninvasive muscle tension measurement using the novel technique of magnetic resonance elastography (MRE)

A novel method for direct measurement of the state of skeletal muscle contraction is introduced called magnetic resonance elastography (MRE). Such a technique is useful for avoiding the indeterminacy inherent in most inverse dynamic models of the musculoskeletal system. Within a standard MRI scanner, mechanical vibration is applied to muscle via the skin, creating shear waves that penetrate the tissue and propagate along muscle fibers. A gradient echo sequence is used with cyclic motion-encoding to image the propagating shear waves using phase contrast. Individual muscles of interest are identified and the shear wavelength in each is measured. Shear wavelength increases with increasing tissue stiffness and increasing tissue tension. Several ankle muscles were tested simultaneously in normal subjects. Applied ankle moment was isometrically resisted at several different foot positions. Shear wavelengths in relaxed muscle in neutral foot position was 2.34±0.47cm for tibialis anterior (TA) and 3.13±0.24cm for lateral gastrocnemius (LG). Wavelength increased in relaxed muscle when stretched (to 3.80±0.28cm for TA in 45° plantar-flexion and to 3.95±0.43cm for LG in 20° dorsi-flexion). Wavelength increased more significantly with contraction (to 7.71±0.97cm in TA for 16Nm dorsi-flexion effort and to 7.90±0. 34cm in LG for 48Nm plantar-flexion effort). MRE has been shown to be sensitive to both passive and active tension within skeletal muscle making it a promising, noninvasive tool for biomechanical analysis. Since it is based on MRI technology, any muscle, however deep, can be interrogated using equipment commonly available in most health care facilities.