i. tranSient elastography Transient Elastography [1-3] relies on the estimation of shear → Third, the shape of the vibration is servo controlled to wave velocity as a means to assess tissue stiffness. Stiffness ensure a correct shear wave generation whatever the or Young’s modulus is directly related to shear velocity by the examination conditions (fat thickness, soft and stiff following expression, E=3pVs², where p is the density, Vs the livers, abdominal wall distortion, etc). shear velocity and E the Young’s modulus or stiffness expressed in kilopascals. →Fourth, the electro-mechanical system used to induce the vibration is calibrated to ensure standardized and A very important feature of transient elastography is that the optimized vibration vibration be transient to avoid reflections and interferences With a constant shape of the vibration, a reference frequency occurring within the tissues. The transient shear wave travels of the shear wave generated by a calibrated mechanical through the tissues within tens of milliseconds which implies system (vibrator), VCTE™ allows comparison of shear that the ultrasound-based imaging modality be ultrafast to wave propagation parameters independently of the organ follow its propagation. Within these constraints stiffness can conditions, etiology, patient and operator. All quantitative be deduced from the strain rate images of the shear wave results given by a VCTE™-based device are comparable. propagation. b. ControllEd EnErgy ii. Vibration-Controlled→ VCTE™ uses low acoustic energy to follow the transient elastography propagation of the shear wave inside the organ. The sensitivity of the single ultrasound transducer is controlled to ensure that the amount of acoustic Echosens® adapted transient elastography to medical application energy sent into the patient is below the official levels by introducing a variety of new features. The improved technique for fetal imaging, abdominal, intraoperative, pediatric, is called Vibration-Controlled Transient Elastography (VCTE™)[1-3]. small organ, etc. (see Table 1). → The proper use of shear wave velocity analysis for Table 1. Fibroscan acoustic output exposure levels comply with clinical diagnosis requires the control of various preamendments from FDA (source: guidance for industry and FDA physical parameters to ensure an accurate, reliable, staff,CDRH). ISPTA.3 = derated Spatial-Peak Temporal-Average and reproducible assessment of tissue stiffness. These Intensity,ISPPA.3 = derated Spatial-Peak Pulse-Average Intensity include the control of vibration frequency, energy intensity, applied force and a standardized algorithm. ISPTA < 94 mW/cm² .3 These important controls ensure that the shear < 190 mW/cm² wave, which is the ultimate source of stiffness ISPTA.3 information, is properly induced in the medium. A. ControllEd vibrAtion One of the most important components of VCTE™ is the → VCTE™ is based on the transmission of mechanical control of the vibration. To ensure proper assessment of energy through the patient. The mechanical vibration tissue stiffness the vibration is controlled in shape, frequency can be compared to a flick; the energy amount is very and amplitude. The vibration needs to satisfy several very small compared to shear waves caused by organs such important criteria. as the heart or respiratory motions. The vibration is → First of all, as the stiffness value depends on the controlled and monitored during the examination. The frequency of the shear wave, the frequency of electro-mechanical system is calibrated periodically to the vibration is controlled to obtain a consistent ensure that these parameters key to performance and measurement that can be used for diagnostic purposes safety do not drift over time. whatever the mechanical properties of the organ under investigation, whatever the etiology of the VCTE™ is a non-invasive method which quantifies patient. tissue stiffness with only small mechanical and acoustic energies with no tissue heating. These amounts of energy only depend on the probe model; → Second, the amplitude of the vibration is adapted to they do not vary between stiff or soft livers. the morphology of the patients: small amplitude in children, large amplitude in obese patients to increase the penetration depth of the shear wave.