February 2010

Virtual Touch… : Gnanakumar Vivekanandan, Senior Manager – Ultrasound, Siemens Healthcare

Tissue Strain Analytics could represent the most important development in ultrasound technology since the advent of Doppler imaging.


Tissue strain analytic applications that enable qualitative visual or quantitative value measurements of the mechanical stiffness (elasticity) properties of tissue are the new dimension of diagnostic information. These applications are not available using conventional sonographic imaging and represent the most important development  in ultrasound technology for non-invasive study of liver characterization since the advent of Doppler imaging. Tissue stiffness information is complementary and independent from the acoustic impedance information provided by B-mode (grayscale) imaging as well as vascular flow information provided by doppler imaging (Figure 1). Thus, tissue strain analytics provide information that is complementary to other ultrasound derived information in approaching diagnostic challenges. Siemens is first in India to provide a comprehensive suite of tissue strain applications that provides a new dimension of diagnostic information through either qualitative assessment or quantitative measurement of the mechanical stiffness of tissue.

Further, Accoustic Radiation Force Impulse (ARFI) imaging is a new tissue strain imaging technology that utilizes sound waves to interrogate the mechanical stiffness properties of tissue. Virtual Touch tissue imaging and Virtual Touch tissue quantification are the first and only commercially available software applications implementing the tissue  strain imaging technology.

Unlike conventional B-mode sonography, which provides anatomical detail based on differences in acoustic impedance, Virtual Touch imaging describes relative physical tissue stiffness properties. In this sense, Virtual Touch imaging is more similar to a physical palpation exam of tissue than conventional sonographic uation. In complement, Virtual Touch tissue quantification provides accurate numerical measurements related to tissue stiffness at user-defined anatomical locations. For example, a given lesion or structure can be qualitatively visualised for its overall stiffness relative to surrounding tissue as well as the relative stiffness of its internal structure. Subsequently, numerical measurements of the lesion can be made. Overall, Virtual Touch software is an advanced form of sonographic imaging and provides complementary information to a conventional ultrasound scan, while benefiting from anatomical localization.

Virtual Touch Tissue Imaging

A Virtual Touch software image is a qualitative grayscale map of relative tissue stiffness (elastogram) for a user defined ROI (Figure 2). This information is computed by examining the relative displacements of tissue elements due to an acoustic push pulse. For a given elastogram image, bright regions depict tissue that is more elastic (less stiff) than dark regions. While a Virtual Touch software image may be displayed side-by-side with a corresponding conventional ultrasound B-mode image, apparent tissue boundaries may differ between the images as they rely on different tissue contrast mechanisms. The Virtual Touch tissue imaging application forms an image by combining independently acquired multiple axial lines of tissue displacement information. Starting with the left most axial line within the ROI, a baseline conventional ultrasound signal description of the tissue is obtained. Next, a push pulse is applied along this line. Conventional tracking beams are applied along the same line to obtain the displaced tissue signal.


“Virtual Touch tissue imaging and Virtual Touch tissue quantification are the first and only commercially available software applications implementing the tissue  strain imaging technology. “


 

The baseline and post-push signals are compared using a cross-correlation algorithm. This allows computation of differences in tissue position, at each point along the axial line, between the relaxed and compressed states. The computed differences are related to the maximum displacement experienced at a given spatial tissue location due to the elastic properties of the tissue at that location. The more elastic a given tissue element, the more displacement it experiences. The above process is repeated for each axial line within the ROI as with a conventional B-mode scan. Finally, all computed displacements across the entire ROI are converted to an elastogram image depicting relative tissue stiffness (Figure 3).

Virtual Touch Tissue Quantification

In addition to qualitative imaging, ARFI technology may be utilized to measure a numerical value of shear wave speed3 as implemented by Virtual Touch tissue quantification (Figure 4). In general, the more stiff a region of tissue, the greater a shear wave’s speed as it travels through this region. Thus, the measured shear wave speed is an intrinsic and reproducible property of tissue. Shear waves are generated and travel  perpendicular to an accoustic push pulse induced displacement of tissue much like ripples resulting from a stone dropped into a pond. Thus, in contrast to conventional axially oriented ultrasound waves, shear waves do not directly interact with the transducer.

In addition, unlike conventional ultrasound waves, shear waves are attenuated approximately 10,000 times more rapidly, and thus require greater sensitivity to measure. However, as the shear wavefront travels through tissue, the generated displacements are detectable using ultrasound tracking beams. By observing the shear wavefront at several locations, and correlating these measurements with the elapsed time, the shear wave speed is quantified.

For Virtual Touch tissue quantification, an anatomical location for measurement is first identified using a ROI placed on a conventional ultrasound image. An acoustic push pulse is applied just lateral to this location, inducing a shear wave that travels through the ROI. Tracking beams, sensitive to greater than 1/100 the wavelength of sound, are applied adjacent to the push pulse path. These beams are continuously transmitted until the passing shear wavefront is detected. The time between generation of the shear wave and detection of the peak is utilized to compute the shear wave velocity. Multiple measurements are made for a given spatial location before a value is reported in order to ensure measurement quality (Figure 5).

The Indian Scenario

It is estimated that liver diseases are among the top ten killer diseases in India, causing lakhs of deaths every year. Besides, there are those who suffer from chronic liver problems, needing recurrent hospitalization and prolonged medical attention, which leaves them physically, mentally, emotionally and financially devastated. On the other hand, there are millions of cases of hepatic diseases, which go unreported or are reported when the matters have gone out of hand. Utter poverty coupled with lack of education and awareness prevents people from seeking medical advice until it is too late. In addition, high cost of treatment pose as major obstacle in convincing people about taking treatment.


“It is estimated that liver diseases are among the top ten killer diseases in India, causing lakhs of deaths every year.”


 Studies have indicated that some liver related diseases like Hepatitis B and Hepatitis C virus could burgeon into an epidemic much larger in scale than the dreaded HIV. However, these diseases being silent killers with long gestation periods do not attract the attention of the Government or the other influential bodies. The needs of patients with liver- related disease have been grossly underestimated and largely ignored.

Conclusions

Virtual Touch tissue imaging and quantification are the first and only commercially available implementations of acoustic radiation force impulse imaging. Through this modality, previously difficult or impossible elastographic examinations are made accurate and practical. Most importantly, Virtual Touch software technology enables a new dimension of tissue information to be applied for screening, diagnostic and therapeutic clinical applications and Virtual Touch applications may offer a way to reduce unnecessary biopsies and other invasive procedures otherwise needed to give an accurate diagnosis through easy uation of pathology.

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