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Between 1998 and 2005, we have developed a new elasticity imaging technique of soft tissues. This technique, so called transient elastography, is based on low frequency wave propagation at the surface of the body coupled with ultrasound ultrafast imaging able to reach until 5000 images/s. This technique allows getting the local Young's modulus of soft tissues with a resolution of the order of the millimeter. The Young's modulus is a very important parameter, because it describes the rigidity of the tissues and characterizes their pathological state. Some studies have showed that a carcinoma could be 30 times harder than health tissues around. Moreover it is the parameter detected intuitively by physicians during palpation.
Fig. 1: Transient elastography imaging system. a,b) A medical ultrasound array is fixed on a low frequency mini-shaker. c) B-mode image obtain on patient with a carcinoma of about 1 cm diameter. d) Mapping of the shear modulus in the zone defined by the yellow dashed line. The scale changes between 0 and 80 kPa.
Our goal was to develop a "quantitative palpation" system able to map in 2D and soon in 3D the elasticity of tissues. The first imaging system was composed of a piezoelectric transducer array fixed on a low frequency mini-shaker, Fig. 1a. With the mini-shaker, the front face of the array gives a little low frequency transient vibration (~50 Hz) at the surface of the body. This little push is quasi-imperceptible and generates the propagation of low frequency shear waves inside the tissues. The wave speed of about few m/s (slower than the ultrasound) is directly related to the Young's modulus, i.e. to the tissues rigidity. Like this, one can have access with this "seismic" shear waves to a local measurement of the Young's modulus of the tissues, Fig. 1d. Tissues displacements generate by this low frequency wave are assessed with a high frame rate ultrasound imaging of the organs. By comparison two by two of the successive ultrasound images, using classical cross-correlation techniques, the local displacements of the tissues are calculated. The total time of the 2D data acquisition is about of 20 ms, which is enough faster to avoid problems from organs or patient movements.
After some years of research, this project lead to some first clinical tests in vivo for breast cancer detection in collaboration with the "Institut Curie" (June 2001). The results obtained were very promising [1].
Remote palpation with acoustic radiation force
During the Jérémy Bercoff Ph. D., we have modified the technique in an innovative way which allows precise assessments, easy utilizations and gives access to the relaxation time of soft tissues. Actually, instead of used some external mini-shaker at the surface of the body, it is possible to generate inside the tissues, some shear wave sources by using a focused ultrasound beam [2]. During insonifications for some hundreds of microseconds [3,4], the medium is pushed at the focal point of about some micrometers. This principle is related to the ultrasound radiation pressure and can be generated by the same array of transducers that the one used for ultrafast imaging. Like this with the same array, one can gives at the pratitioner the possibility to "virtually" palpate exactly where he wants just by moving the ultrasonic probe. Generating many shear sources simultaneously at different place, one can shown that it is possible to create a supersonic shear source, Fig. 2. This supersonic shear source induces two plane shear waves propagating inside the organs. This phenomenon is closely comparable to this one obtain in the sky with a supersonic plane [2].
Fig. 2: Generation of a supersonic shear source and ultrafast imaging of the wave. In gray level, displacements induce in a soft tissue homogeneous phantom by using some ultrasound beam focalized a different depth during some hundreds of microseconds. The gray level scale is varying from 0 to 10 µm and the size of each image is 40x40 mm².
The economic advantages of such a method using both ultrafast imaging and ultrasonic remote palpation, so called Supersonic Shear Imaging (S.S.I.) [2], are extremely important :
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With all the capacity of a classical ultrasonic scanner, the system gives access to two important biomechanical parameters of the human body: the Young's modulus and shear relaxation time of tissues. This technique does not set the physicians to modify their clinical protocol. Nevertheless, it needs some specific ultrasonic scanners dedicated for this method.
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This invention touches many application domains. This ultrafast ultrasonic scanner could be used also for imaging the mechanical properties of breast, liver, brain, ... and like this bring a tidy help in hospital, clinic or radiologist consulting room for pathological diagnosis (cancer, C-hepatitis, cirrhosis, …). For breast cancer detection, this new scanner could complete mammography. For liver pathologies, it could give a quantitative indication on the level of liver fibrosis.
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This scanner based on ultrasound technology is of low cost compared for example to MRI elasticity imaging [5].
A start-up, named Supersonic Imagine, has been launched in May 2005 to increase the value of this imaging technique for breast cancer detection.
2 J. Bercoff, M. Tanter and M. Fink, “Supersonic shear imaging: a new technique for soft tissue elasticity mapping”. IEEE Ultras. Ferro. Freq. contr., Vol. 51(4), pp. 396-409, 2004.
3 A.P. Sarvazyan, O.V. Rudenko, S.D. Swanson, J.B. Fowlkes, and S.Y. Emelianov, "Shear wave elasticity imaging - a new ultrasonic technology of medical diagnostic," Ultrasound in Medicine and Biology, Vol. 20, pp. 1419-1436, 1998.
4 K. Nightingale, M. Scott Soo, R. Nightingale and G. Trahey, “Acoustic radiation force impulse imaging: in vivo demonstration of clinical feasibility”, Ultrasound in Medicine and Biology, Vol. 28, pp. 227-235, 2002.
5 R. Sinkus, M. Tanter, S. Catheline, J. Lorenzen, C. Kuhl, E. Sondermann, M. Fink. “Imaging anisotropic and viscous properties of breast tissue by Magnetic Resonance-Elastography”, Magnetic Resonance in Medicine, Vol. 53, pp. 372-387, 2005.