iii. Controlled static force In VCTE™ the static force applied by the operator is monitored 1) Magnetic resonance elastography with a force sensor. The force must be sufficient to ensure a Magnetic resonance elastography (MRE) was introduced in 1995 proper transmission of the vibration from the subcutaneous by Muthupillai [5] from the Mayo clinic (Rochester, USA). A few tissues to the liver parenchyma. The force should not be groups reported liver stiffness measurements using MRE [6] excessive to prevent vibration distortion and modification [7] [8-10]. Liver shear stiffness is often chosen as the output of the tissues characteristics. Furthermore the stiffness of stiffness parameter. The shear stiffness µ can be deduced soft tissue may vary when stress is applied on it. Controlling from Young’s modulus E using the simple relationship: µ=E/3. the static force is crucial in quantitative elastography. 2) Radiation force iV. Controlled algorithmMore recently, several techniques [11-14] based on radiationforce [15, 16] such as ARFI (Acoustic Radiation Force Impulse) The stiffness computation algorithm is the heart of VCTE™. have been proposed for liver stiffness measurement. These Highly sophisticated algorithms have been developed for tissue techniques use high intensity ultrasound beams to induce stiffness assessment. Data and image processing technologies displacements and tissue heating inside the liver remotely. are used to measure tissue stiffness in kilopascals and to Other techniques such as those developed at the Mayo clinic automatically reject invalid measurements.In practice, shear [17] or at the Institut Langevin (Paris, France) [18] are currently wave propagation can be very complex in some conditions investigated. or some organs such as lungs or intestines. These may yield poor strain rate images. These images are automatically invalidated by a specificreproducibleand standardized algorithm toand highquality results.Vi. StandardiZation ensure consistent, The standardization of elastography procedures is a challenging aspect of the emergence of new elastographic technologies in the V. elaStoGraPHiC medical field. Preliminary studies performed on tissue-mimicking materials showed a good correlation between stiffness measured techniques using MRE and VCTE™[19] [20]. Both techniques are based onthe use of external mechanical vibrators. Thus they are using a controlled 50 Hz frequency which is favourable to standardization. Elastography techniques may be separated into two groups: qualitative elastography and quantitative elastography. In radiation force-based ARFI technique, shear waves are induced remotely using a high intensity and focused ultrasound beam. A. QuAlitAtivE ElAstogrAphy While shear waves frequency is controlled in MRE and VCTE™, ARFI shear wave frequency can not be controlled and depends on Qualitative elastography as a modality is now offered in some elements such as the etiology-dependent mechanical properties ultrasound scanners. Based on the work of Ophir’s group [4], these of the organ and on the depth of the region of interest, etc. techniques give a color image of the relative stiffness within tissues. These techniques are qualitative because the mechanical A direct conversion of liver stiffness obtained using VCTE™ and impulse is not controlled. Indeed they do not use shear waves. ARFI is tempting. Indeed, theoretically Young’s modulus as provided by VCTE™ could be deduced from the shear velocity b. QuAntitAtivE ElAstogrAphy as measured by ARFI. However, this relationship is only valid at a given frequency, f: E(f)=3Vs(f)². Shear wave frequencies in Shortly after the VCTE-based Fibroscan® was commercialized VCTE™ and ARFI are 50 Hz and several hundreds hertz respectively. in 2004, other elastographic techniques were proposed to assess liver stiffness (LS). These techniques (see Table 2) rely Furthermore, in ARFI the frequency of the shear wave is wide ranging on the use of shear waves. and depends on the mechanical properties of the organ. The stiffer the tissues are, the higher the frequency is. As a consequence, Table 2. Quantitative elastography techniques. the frequency of the shear wave in a cirrhotic liver can be much IMAgIng MO- VIBRATIOn FREQUEnCY higher than the frequency of the shear wave in a healthy liver. DALITY MODE As ARFI and VCTE™ (or MRE) frequencies are significantly ACOUSTIC RADIATIOn Ultrasound Transient Wideband different, the classical relationship E=3Vs ² is invalid. A direct FORCE IMPULSE Radiation conversion of ARFI shear velocity to VCTE™ Young’s modulus is force not possible. VIBRATIOn-COn- Ultrasound Transient 50 Hz TROLLED TRAnSIEnT Mechanical ELASTOgRAPHY actuator MAgnETIC RESO- Magnetic Continuous 50 – 60 Hz nAnCE ELASTOgRA- resonance Mechanical PHY imaging actuator