VCTE™

One of the most important components of VCTE™ is the control of the vibration. To ensure proper assessment of tissue stiffness the vibration is controlled in shape, frequency and amplitude.

Vibration is controlled in frequency

First of all, as the stiffness value depends on the frequency of the shear wave, the frequency of the vibration is controlled to obtain a consistent measurement that can be used for diagnostic purposes whatever the mechanical properties of the organ under investigation, whatever the etiology of the patient.

Vibration is controlled in amplitude

Second, the amplitude of the vibration is adapted to the morphology of the patients: small amplitude in children, large amplitude in obese patients to increase the penetration depth of the shear wave.

Vibration is controlled in shape

Third, the shape of the vibration is servo controlled to ensure a correct shear wave generation whatever the examination conditions (fat thickness, soft and stiff livers, abdominal wall distortion, etc).

Fourth, the electro-mechanical system used to induce the vibration is calibrated to ensure standardized and optimized vibration.

With a constant shape of the vibration, a reference frequency of the shear wave generated by a calibrated mechanical system (vibrator), VCTE™ allows comparison of shear wave propagation parameters independently of the organ conditions, etiology, patient and operator. All quantitative results given by a VCTE™-based device are comparable.

VCTE™ uses low acoustic energy.

VCTE™ uses low acoustic energy to follow the propagation of the shear wave inside the organ. The sensitivity of the single ultrasound transducer is controlled to ensure that the amount of acoustic energy sent into the patient is below the official levels for fetal imaging, abdominal, intraoperative, pediatric, small organ, etc...

VCTE™ is based on the transmission of mechanical energy through the patient. The mechanical vibration can be compared to a flick; the energy amount is very small compared to shear waves caused by organs such as the heart or respiratory motions. The vibration is controlled and monitored during the examination. The electro-mechanical system is calibrated periodically to ensure that these parameters key to performance and safety do not drift over time.

VCTE™ is a non-invasive method which quantifies tissue stiffness with only small mechanical and acoustic energies. These amounts of energy only depend on the probe model; they do not vary between stiff or soft livers.

Crucial in quantitative elastography.

In VCTE™ the static force applied by the operator is monitored with a force sensor. The force must be sufficient to ensure a proper transmission of the vibration from the subcutaneous tissues to the liver parenchyma. Yet the force should not be excessive to prevent vibration distortion and modification of the tissues characteristics. Furthermore the stiffness of soft tissue may vary when stress is applied on it. Controlling the static force is crucial in quantitative elastography.

Controlled algorithm

The heart of VCTE™.

The stiffness computation algorithm is the heart of VCTE™. Highly sophisticated algorithms have been developed for tissue stiffness assessment. Data and image processing technologies are used to measure tissue stiffness and to automatically reject invalid measurements. In practice, shear wave propagation can be very complex in some conditions or some organs such as lungs or intestines. These may yield poor strain rate images. These images are automatically invalidated by a specific and standardized algorithm to ensure consistent, reproducible and high quality results.

Fibroscan®: pionner in clinical Elastography

Fibroscan®, the only device based on VCTE™, was introduced by Echosens® in December 2003. It is the first device clinically validated for the assessment of liver fibrosis. Nowadays, more than 800 Fibroscan® devices are used worldwide in research and routine clinical practice. Market clearance of the Fibroscan® has been granted in Europe (CE marked), Canada and China and is pending in the USA and Japan.

VCTE™ applied to liver

VCTE™ applied to liver Fibroscan® consists in a dedicated acquisition platform that includes a single channel analog front end to emit and receive ultrasound and a servo motor control for the low frequency vibration generation. The probe contains an electro-dynamic transducer that is used to generate a transient vibration. As shown in Figure PROBE, a single element ultrasound transducer is mounted on the axis of the vibrator. The vibration consists in a period of sinusoid with a frequency of 50 Hz and a 2 mm peak-to-peak amplitude. The vibration therefore lasts only 20 ms. The choice of 50 Hz frequency is a consistent compromise between the depth of penetration of the shear wave and the accuracy of the measurement.

Liver is protected by the rib cage and is thus not subject to direct static force application. However Fibroscan® monitors the static force applied by the operator during the examination. The force must be sufficient to ensure a proper transmission of the vibration through the subcutaneous tissues. The force should not be excessive to prevent vibration distortion and modification of the tissues characteristics. In standard examination, measurements are possible when the static force is between 4 N and 8 N.

Fibroscan® standard probe is equipped with a 3.5 MHz center frequency and 9-mm external diameter ultrasound transducer that is typical of abdominal investigation. This transducer is used in conventional pulse echo acquisition to provide A-mode and M-mode images to the operator in real time for liver localization. When measurement is triggered, ultrafast pulse echosequence is performed (pulse repetition frequency of 6 kHz) during the propagation of the controlled shear wave. The acquisition lasts only 80 ms and strains induced in the liver by the propagation of the shear wave is measured using standard autocorrelation approach between successive ultrasound lines.