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...

Many cardiovascular diseases and disorders are associated with hemodynamic dysfunction. The heart's ability to contract and pump blood through the vascular system primarily depends on the elasticity of the myocardium. This article introduces a magnetic resonance elastography (MRE) technique that allows noninvasive and time-resolved measurement of changes in myocardial elasticity over the cardiac cycle. To this end, lowfrequency shear vibrations of 24.3 Hz were induced in the human heart via the anterior chest wall. An electrocardiograph (ECG)-triggered, steady-state MRE sequence was used to capture shear oscillations with a frame rate of eight images per vibration cycle. The time evolution of 2D-shear wave fields was observed in two imaging planes through the short axis of the heart in six healthy volunteers. Correlation analysis revealed that wave amplitudes were modulated in synchrony to the heartbeat with up to 2.45 ؎ 0.18 higher amplitudes during diastole than during systole (interindividual mean ؎ SD). The reduction of wave amplitudes started at 75 ؎ 9 ms prior to changes in left ventricular diameter occurring at the beginning of systole. Analysis of this wave amplitude alteration using a linear elastic constitutive model revealed a maximum change in the myocardial wall stiffness of a factor of 37.7 ؎ 10.6 during the cardiac cycle. Magn Reson Med 61:668 -677, 2009.

In vivo quantification of the anisotropic shear elasticity of soft tissue is an appealing objective of elastography techniques because elastic anisotropy can potentially provide specific information about structural alterations in diseased tissue. Here a method is introduced and applied to MR elastography (MRE) of skeletal muscle. With this method one can elucidate anisotropy by means of two shear moduli (one parallel and one perpendicular to the muscle fiber direction). The technique is based on group velocity inversion applied to bulk shear waves, which is achieved by an automatic analysis of wave-phase gradients on a spatiotemporal scale. The shear moduli are then accessed by analyzing the directional dependence of the shear wave speed using analytic expressions of group velocities in k-space, which are numerically mapped to real space. The method is demonstrated by MRE experiments on the biceps muscle of five volunteers, resulting in 5.5 ؎ 0.9 kPa and 29.3 ؎ 6.2 kPa (P < 0.05) for the medians of the perpendicular and parallel shear moduli, respectively. The proposed technique combines fast steadystate free precession (SSFP) MRE experiments and fully automated processing of anisotropic wave data, and is thus an interesting MRI modality for aiding clinical diagnosis. Magn Reson Med 56:489 -497, 2006.

bSSFP combined with fractional MRE enables rapid measurement of liver stiffness in vivo. The used actuation principle supports a 2-dimensional analysis of the strain wave field captured by axial wave images. The measured data indicate individual variations of hepatic stiffness in healthy volunteers.

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