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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.

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.

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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).

In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency f v as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/f v . This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR ≤ 1/f v which we call "fractional" MRE, i.e., only a fraction of one vibration cycle per TR, can be used for motion sensitization. The benefit of fractional MRE is twofold: 1) acquisition times in seconds can be achieved for a single-phase difference wave image, and 2) materials that combine low elasticity, high viscosity, and short T* 2 relaxation times show an increased phase-to-noise ratio (PNR Manual palpation is a sensitive means of detecting pathologically altered tissue near the surface of the body. The sensitivity of this method is related to the resistance of soft tissue to shear forces, which varies by orders of magnitude in the human body (1,2). Accordingly, elastography techniques have been developed to quantify the shear elasticity of living human tissue based on soft-tissue imaging techniques, such as ultrasound or MRI, using either static mechanical compression or acoustic strain waves (3-9). Since the acoustic approach in MR elastography (MRE) has made rapid progress in the last few years, it is now possible to examine the elasticity of tissue that is not palpable from the body surface. Dynamic MRE can map spatial and temporal shear wave fields that depend on heterogeneity, anisotropy, and nonlinearity of the elasticity (10 -17).Although pilot studies demonstrated the potential of MRE, it has remained a relatively slow technique compared to the rapid acquisition schemes of other flow-or motion-quantifying MRI methods (18 -21). The extended time consumption of conventional MRE scans is due to the duration of bipolar gradients used to sensitize the sequence to slow mechanical vibration cycles (usually Ͼ5 ms, corresponding to vibration frequencies of f v Ͻ 200 Hz). This frequency range is compelled by the high viscosity of most soft tissues that results in a rapid damping of the shear wave amplitude with the penetration distance (22). Current MRE methods encode the motion by oscillating gradients with a minimum duration of one vibration cycle. Thus, the repetition time (TR) of an MRE sequence is always greater than 1/f v .In this work we propose the use of low-frequency shear vibrations with vibration cycles longer than the TR of the MRI sequence (TR Յ 1/fv). As a trade-off, only a fraction of one motion cycle can be magnetically encoded, and thus the phase difference signal is smaller. However, it will be shown that for soft and viscous materials with short transverse relaxation times this loss is more than compensated for by an increased signal resulting from reduced transverse relaxation. The short echo times (TEs) that are achievable with fractiona...

The diagnostic performance of multifrequency MR elastography in determining the degree of hepatic fibrosis increases with stage of fibrosis. Metrics obtained at the higher frequencies provide better diagnostic performance compared with the lower frequencies. Results of the AUROC analysis demonstrate the high accuracy of frequency-independent cutoff values for staging higher grades of hepatic fibrosis.

Nearly half a century after the first report of normal pressure hydrocephalus (NPH), the pathophysiological cause of the disease still remains unclear. Several theories about the cause and development of NPH emphasize disease-related alterations of the mechanical properties of the brain. MR elastography (MRE) uniquely allows the measurement of viscoelastic constants of the living brain without intervention. In this study, 20 patients (mean age, 69.1 years; nine men, 11 women) with idiopathic (n = 15) and secondary (n = 5) NPH were examined by cerebral multifrequency MRE and compared with 25 healthy volunteers (mean age, 62.1 years; 10 men, 15 women). Viscoelastic constants related to the stiffness (µ) and micromechanical connectivity (α) of brain tissue were derived from the dynamics of storage and loss moduli within the experimentally achieved frequency range of 25-62.5 Hz. In patients with NPH, both storage and loss moduli decreased, corresponding to a softening of brain tissue of about 20% compared with healthy volunteers (p < 0.001). This loss of rigidity was accompanied by a decreasing α parameter (9%, p < 0.001), indicating an alteration in the microstructural connectivity of brain tissue during NPH. This disease-related decrease in viscoelastic constants was even more pronounced in the periventricular region of the brain. The results demonstrate distinct tissue degradation associated with NPH. Further studies are required to investigate the source of mechanical tissue damage as a potential cause of NPH-related ventricular expansions and clinical symptoms.