Introduction: Magnetic resonance elastography (MRE) is a non-invasive method to quantify biomechanical properties of human tissues. It has potential in diagnosis and monitoring of kidney disease, if established in clinical practice. The interplay of flow and volume changes in renal vessels, tubule, urinary collection system and interstitium is complex, but physiological ranges of in vivo viscoelastic properties during fasting and hydration have never been investigated in all gross anatomical segments simultaneously.
Method: Ten healthy volunteers underwent two imaging sessions, one following a 12-hour fasting period and the second after a drinking challenge of >10 mL per kg body weight (60–75 min before the second examination). High-resolution renal MRE was performed using a novel driver with rotating eccentric mass placed at the posterior-lateral wall to couple waves (50 Hz) to the kidney. The biomechanical parameters, shear wave speed (cs in m/s), storage modulus (Gd in kPa), loss modulus (Gl in kPa), phase angle and attenuation (α in 1/mm) were derived. Accurate separation of gross anatomical segments was applied in post-processing (whole kidney, cortex, medulla, sinus, vessel).
Results: High-quality shear waves coupled into all gross anatomical segments of the kidney (mean shear wave displacement: 163 ± 47 μm, mean contamination of second upper harmonics <23%, curl/divergence: 4.3 ± 0.8). Regardless of the hydration state, median Gd of the cortex and medulla (0.68 ± 0.11 kPa) was significantly higher than that of the sinus and vessels (0.48 ± 0.06 kPa), and consistently, significant differences were found in cs, , and Gl (all p < 0.001). The viscoelastic parameters of cortex and medulla were not significantly different. After hydration sinus exhibited a small but significant reduction in median Gd by −0.02 ± 0.04 kPa (p = 0.01), and, consequently, the cortico-sinusoidal-difference in Gd increased by 0.04 ± 0.07 kPa (p = 0.05). Only upon hydration, the attenuation in vessels became lower (0.084 ± 0.013 1/mm) and differed significantly from the whole kidney (0.095 ± 0.007 1/mm, p = 0.01).
Conclusion: High-resolution renal MRE with an innovative driver and well-defined 3D segmentation can resolve all renal segments, especially when including the sinus in the analysis. Even after a prolonged hydration period the approach is sensitive to small hydration-related changes in the sinus and in the cortico-sinusoidal-difference.
Introduction: In recent years, elastography has become a widely accepted methodology to assess the longitudinal shear elastic modulus of skeletal muscle. Ultrasound shear wave elastography is the gold standard used for such a purpose. However, its low sample rate (1–2 Hz) and the impossibility of being used in several muscles simultaneously limit potential biomechanical applications. In this work, we overcome such limitations by using a surface wave elastography method (NU-SWE).
Methods: The NU-SWE comprises a wearable device suitable for measuring several muscles simultaneously. Elasticity can be measured at high-frequency rates (∼15 Hz), by propagating several pulse trains of low-frequency (∼100 Hz) superficial waves separated by a short time interval. These pulses propagate along the medium surface and are recorded by a linear array of vibration sensors placed on the skin of each measured muscle. In this context, this work carried out a proof of concept, showing how NU-SWE enables performing experimental protocols previously impracticable with ultrasound elastography. Thus, we measured the longitudinal shear elasticity of the biceps brachii and brachioradialis muscles simultaneously at 15 Hz during isometric elbow flexions exerted at different torque development rates. Furthermore, for comparison, we measured the electromyographic activity of both muscles.
Results: Our results show that the maximum elasticity reached by the brachioradialis increases with contraction rate, while the biceps brachii behaves inversely.
Discussion: This study provides new insights concerning muscle synergism, evidenced by changes in muscle elasticity during torque production. More generally, it shows that NU-SWE could help widen elastography-driven research in biomechanics.