NOTA:
Ultrasound based-elastographic techniques
are classified in: strain techniques and shear wave elastography techniques.
Three types of elastographic techniques are included in the last category:
Transient Elastography (TE), point Shear Wave Elastography (pSWE) and shear wave elastography
(SWE) imaging (including 2D-SWE and 3D-SWE).
In
the pSWE category two techniques are included: Acoustic Radiation Force Impulse
(ARFI) elastography and ElastPQ which look very similar, but there are some
differences regarding their physical principles.
Regarding ARFI elastography
technique, the ultrasound probe produces an acoustic “push” pulse that
generates shear-waves which propagate into the tissue. Their speed, measured in
meters/second (m/s), is displayed on the screen and reflects the underlying
tissue stiffness (influenced mainly by liver fibrosis), the propagation speed increasing
with tissue stiffness. Using image-based localization and a proprietary
implementation of ARFI technology, shear wave speed may be quantified, in a
precise anatomical region, focused on a region of interest, with a predefined
size, provided by the system.
Very few information are available regarding the physical principles of ElastPQ
technique. According to the data provided by the manufacturer in the application for approval submitted
to the US Food and Drug Administration (FDA), ElastPQ system is relatively
similar with Aixplorer system® (SuperSonic Imagine S.A., Aix-enProvence,
France), which is a 2D-SWE. ElastPQ system generates an electronic voltage
pulse, which is transmitted to the transducer. In the transducer, a piezo
electric array converts the electronic pulse into an ultrasonic pressure wave.
When coupled to the body, the pressure wave transmits through body tissues. The
Doppler functions of the system process the Doppler shift frequencies from the echoes
of moving targets, such as blood, to detect and graphically display the Doppler
shift of these tissues as flow. The Doppler mode creates waves in soft tissues
and estimates the tissue stiffness by determining the speed at which these
shear waves travel.
The usefulness of ARFI elastography for non-invasive assessment of liver fibrosis was demonstrated in the last 2-3 years in international multicenter studies [5] and meta-analyses [6-8], but ElastPQ is a newly developed technique and few data are available.
The usefulness of ARFI elastography for non-invasive assessment of liver fibrosis was demonstrated in the last 2-3 years in international multicenter studies [5] and meta-analyses [6-8], but ElastPQ is a newly developed technique and few data are available.
2D –Shear Wave Elastography (2D-SWE):
The evaluation of liver stiffness [LS ] by 2D–SWE was
performed with an Aixplorer® ultrasound system (SuperSonic Imagine S.A.,
Aix-en-Provence, France), using a SC6-1 convex probe. By this technique a
quantitative elasticity map of the medium was obtained. This map is required to
image the propagation of the shear-wave and to measure its velocity. Because
the shear waves generated into the tissue by the acoustic pulse propagate at a
few meters per second, a frame rate of several kilohertz is needed to image
them. This is not possible using conventional ultrasound scanners (they usually
reach a frame rate of approximately 50 Hz). For this reason, an ultrafast,
ultrasonic scanner is required, able to remotely generate the mechanical shear
wave, by focusing ultrasound at a given location, and image the medium during
the wave propagation at a very high-frame rate (up to 6000 images/s). 2D-SWE
technique allows the acquisition of echographic images at a pulse repetition
that can reach 6000 Hz. The results of LS measurement may be displayed in units
of shear wave velocity (meters/second) or converted into units of Young’s modulus
(kPa), similar with TE.
