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 indeterminancy 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 +/- 0.47 cm for tibialis anterior (TA) and 3.13 +/- 0.24 cm for lateral gastrocnemius (LG). Wavelength increased in relaxed muscle when stretched (to 3.80 +/- 0.28 cm for TA in 45 degrees plantar-flexion and to 3.95 +/- 0.43 cm for LG in 20 degrees dorsi-flexion). Wavelength increased more significantly with contraction (to 7.71 +/- 0.97 cm in TA for 16 N m dorsi-flexion effort and to 7.90 +/- 0.34 cm in LG for 48 N m 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.