Method and device for measuring velocity of shear waves in biological tissue
A method for measuring a mean visco-elasticity value for a soft material uses a single probe carrying at least one transducer. At least one burst of mechanical vibrations is induced in a constraint zone in order to generate internal shear waves in the tissue propagating from the constraint zone into the tissue. The transient tissue displacements are measured with a transducer in at least one first measurement zone in the tissue, the first measurement zone being located away from the constraint zone. A mean visco-elasticity of the region of the tissue situated between the constraint zone and the first measurement zone is estimated from the measured transient tissue displacements of the tissue in the first measurement zone.
1. A method for measuring velocity of shear waves in a biological tissue, said method comprising the steps of:
a) energizing a transducer to generate an ultrasound beam;
b) directing the ultrasound beam towards the tissue;
c) generating mechanical vibrations in said tissue in a constraint zone where said tissue is exposed to said ultrasound beam;
d) generating a shear wave in said tissue from said mechanical vibrations in a first measurement zone which is outside of where said tissue is exposed to said ultrasound beam;
e) sending multiple pulses along at least two ultrasound lines spaced apart from each other and into said tissue where said shear wave is generated in said first measurement zone which is outside of where said tissue is exposed to said ultrasound beam;
f) imaging tissue displacements in said first measurement zone in the tissue and along the two ultrasound lines,
g) determining velocity of the shear wave between the two ultrasound lines based on said tissue displacements; and
h) displaying said velocity of the shear wave.
2. A method according to claim 1 , wherein said mechanical vibration is induced by a biological source of mechanical displacement, or by an external vibrator.
3. A method according to claim 1 , wherein said mechanical vibration is induced by ultrasonic radiation pressure in the tissue, said constraint zone being a constraint line.
4. A method according to claim 1 further comprising a detection step for detecting the shear wave propagation in said at least one first measurement zone, said detection step automatically triggering the inducing step a).
5. A method according to claim 1 , wherein step c) is performed by a computer program.
6. A method according to claim 1 , wherein a visco-elasticity of a region of the tissue situated between the constraint zone and the at least one first measurement zone is estimated based on determining the velocity of the shear wave, wherein the region of the tissue where a visco-elasticity is estimated further includes said constraint zone.
7. A method according to claim 6 , wherein the region of the tissue where a visco-elasticity is estimated further includes said at least one first measurement zone.
8. A method according to claim 1 , wherein said measuring step implements ultrasonic waves, said at least one first measurement zone being a measurement line.
9. A method according to claim 8 wherein said constraint line is parallel to said measurement line.
10. A method according to claim 9 , wherein said measurement line is located laterally away from said constraint line or from the other measurement line of a distance R.λ, R being a real greater or equal to 2 and λ being the wavelength of waves transmitted by the transducer and used to measure the tissue displacements.
11. A method according to claim 1 wherein said imaging transducer is included in a transducer array, further comprising a step of selecting said at least one imaging transducer of said transducer array in order to realize the measurement step b) by measuring the transient tissue displacements in the at least one first measurement zone.
12. A method according to claim 11 , wherein the transducer array is an ultrasound transducer array.
13. A method according to claim 1 wherein said imaging transducer is included in a transducer array, further comprising a step of selecting said at least one imaging transducer of said transducer array in order to induce said inducing step a) by generating said burst.
14. A method according to claim 13 wherein the transducer array is an ultrasound transducer array.
15. A method for measuring velocity of shear waves in a biological tissue, said method comprising the steps of:
a) energizing a transducer to generate an ultrasound beam;
b) directing the ultrasound beam towards the tissue;
c) generating mechanical vibrations in said tissue in a constraint zone where said tissue is exposed to said ultrasound beam;
d) generating a shear wave in said tissue from said mechanical vibrations in a first measurement zone which is outside of where said tissue is exposed to said ultrasound beam;
e) sending multiple pulses along at least two ultrasound lines spaced apart from each other and into said tissue where said shear wave is generated in said first measurement zone which is outside of where said tissue is exposed to said ultrasound beam;
f1) imaging tissue displacements in said first measurement zone in the tissue and along the two ultrasound lines;
f2) imaging tissue displacements in a second measurement zone in the tissue and along the two ultrasound lines;
g) determining velocity of the shear wave between the two ultrasound lines in said tissue displacements of the tissue in the first and second measurement zones; and
h) displaying said velocity of the shear wave.
16. A method according to claim 15 , wherein a visco-elasticity of a region of the tissue situated between the first and second measurement zones is estimated based on determining the velocity of the shear wave, wherein the estimation step implements a temporal comparison of the transient tissue displacements measured in said first and second measurement zones using any conventional signal processing techniques for motion and distortion estimation.
17. A method according to claim 16 , wherein the transient tissue displacements are known from at least a mechanical parameter included in the group formed by: shear wave group velocity, shear wave phase velocity, shear wave attenuation, shear viscosity and shear elastic modulus.