The aim of elastography is to assess tissue stiffness based on 3 steps:
- Excitation: transmission of stress in a tissue (mechanical,
vibrational,
shear)
- Acquisition: recording the signal induced by the tissue deformation due to the stress (RF or B-mode data)
- Analysis/post-treatment: analysis of tissue strain induced by the propagation of the stress.
Human body has a mechanical behavior similar to a soft homogeneous and isotropic linear elastic material.
The stiffness of tissue is assessed by the Young elastic modulus in kilopascal kPa.
A stress S induces a strain e,
dependent on the tissue elasticity.
The Young modulus E is defined by the ratio between the stress and the strain (Fig. 1).
Young Modulus E = S / e.
A stress links to 2 types of mechanical waves in the tissue:
- Compression wave that compresses tissue little by little,
inducing a displacement parallel to the propagation direction.
- Shear wave: responsible of a slip of different tissue layers,
relative to each other,
inducing a displacement perpendicular to the wave propagation direction;
The ultrasound elastography quantitative techniques do not directly measure the Young's modulus but the speed V of shear wave propagation.
The velocity V of the shear wave is related to shear modulus μ (shear):
μ = r V2 with r = tissue density
The shear modulus μ is itself connected to the elastic modulus:
E = 3 μ
The measurement of the shear wave propagation velocity V (in m/s) allows to assess the elastic modulus E according to the formula:
E = 3ρV2
For computations,
the tissue density is assumed to be constant and equal to 1000 kg/mm3.
There are several methods of ultrasound elastography (Fig. 2).
1.
Strain elastography
The quasi-static elastography is based on the deformation of a tissue due to a stress.
A stress is applied to the tissue (through the ultrasonic probe itself) and is observed by ultrasound imaging.
The strain induced by the propagation of the stress is analyzed:
- By comparing data before and after stress,
we estimate the displacement or strain tensor e.
- The stress s is not measurable: the resulting map does not give the Young's modulus.
The system provides parametric maps,
that differentiate rigid and soft tissues.
These maps are in color and grey-scale depending on the constructor (Fig. 3,
Fig. 4).
Several constructors offer this technic:
- Hitachi: real time tissue elastography,
eMode
- Siemens: mode eSie Touch elastography imaging
- General electrics: Ultrasound elastography imaging
- Philips: strain based elastography
- Toshiba: Real-time elastography
- Ultrasonix
- Esaote: ElaXto
2.
Transient elastography
This is the technique used by FibroScanTM (Echosens).
It uses a probe (3.5 MHz) containing a vibrator and an ultrasound transducer.
This is not an imaging guided system and the probe is positioned randomly in the skin surface.
The measures are done at a depth between 25 and 65 mm.
The exam takes about 5-10 minutes and consists of 10 measures at the same location.
The system immediately computes a median of the 10 measures.
This system is based on a mechanical pulse induced at the surface of the skin by an external vibrator which generates a transient shear wave (pulse) that propagates longitudinally.
Through the ultrasound transducer,
the velocity and the amplitude of the shear wave are measured in a region of interest.
The velocity V is converted into kPa,
and reflects the tissue stiffness (Fig. 5).
The system can also compute the wave attenuation in decibels per meter dB / m.
3.
ARFI = Acoustic Radiation Force Imaging
This technique of elastography uses a focused ultrasound pulse.
It provides an estimate of the stiffness of deep tissues,
non-accessible by external compression.
This system was initially developed by Siemens (ARFI Virtual Touch ™).
After identification of the area of interest by ultrasound imaging,
a focused ultrasonic wave is applied.
This wave leads to tissue displacement and a shear wave.
The velocity of the shear wave is measured and expressed in m / s (Virtual Touch quantification) (Fig. 6).
Until now this technique provided only a mean value of the velocity of the shear wave in a small region of interest.
Recently it became possible to obtain parametric map by multiplying the number of measurement in multiple ROIs.
But because the creation of the Shear wave is only obtained in one focused point this technique remains a non real time technique.
4.
Ultra-fast shear wave elastography
This is the technique developed by Supersonic Imaging also based on transient elastography ultrasound pulse.
However the possibility offered by the ultrafast techniques allows the generation a multiple shear wave along a same longitudinal axis (through a compression wave) leading to the propagation of a plane shear wave.
The ultrafast technique allows also the measurement of the velocity of this plane in each point of the image in real time providing a 2D quantitative measure of elasticity E in real time,
usually expressed through a parametric map in kPa (Fig. 7).
5.
Comparison of the elastography methods
Tables 1 and 2 compare the different methods (Table 1 Table 2) .