Virtual Touch™ Software—Applications in Liver Disease
known as Virtual Touch tissue imaging and Virtual Touch tissue quantification and are the first commercially available applications that use ARFI technology for qualitative and quantitative assessment, respectively. Clearly, the acquisition of either qualitative or quantitative information is dependent upon the clinical requirement.6
Virtual Touch Tissue Imaging (Qualitative Imaging) This method is useful for characterizing and visualizing focal lesions (such as tumors) within normal healthy tissue, commonly liver. The principle is to apply smaller instantaneous energies throughout a region of excitation (ROE). Push pulses are applied throughout the ROE and are co-located with tracking beams. Data is repeatedly obtained throughout the ROE. All the displacement responses within the ROE are computed to create the final image, which is a qualitative grayscale map of relative tissue stiffness. The resultant image box includes the lesion of interest in its entirety and some surrounding healthy liver. Stiffer pathological tissues show less displacement than surrounding tissue, which is typically more compliant. In the generated image, bright regions depict more compliant tissue and the dark regions depict relatively stiffer tissue. The images are speckle free and usually have greater contrast than conventional B-mode images, which are viewed side by side (see Figure 1).
Virtual Touch Tissue Quantification
This method generates a numerical value and measures the velocity of propagated shear waves, which correlates to the stiffness of the tissue being assessed. ARFI excitations are applied and the resulting displacements are tracked both spatially and temporally. The principle of tissue quantification is that the stiffer the tissue, the faster the velocity of the shear wave in a region of interest (ROI). The shear wave velocity measurements are usually obtained in meters per second. A conventional B mode ultrasound image is obtained and a ROI is placed within the tissue or lesion of interest. A push pulse is applied causing a shear wave to travel through the ROI. Tracking beams are applied, and a numerical value for the shear velocity is calculated (see Figure 2). It is important to recognize that shear wave reconstruction methods can be inaccurate if structures reflect incident shear waves limiting their propagation into the structure. This can create shear wave interference patterns inside and outside the structures. 3,4
Safety Considerations
The acoustic energy that is used to displace tissue by a few micrometers can also cause tissue heating. A single ARFI excitation is typically associated with a temperature increase of 0.05 to 0.20 °C. Transducer heating can also occur as diagnostic transducers are used for excitation.3,5
It is important to highlight that the acoustic radiation force waves operate within the acoustic energy guidelines and are regulated in a similar way to conventional ultrasound energy. Multiple parameters are controlled to ensure that total tissue temperature increases are maintained within the diagnostic range. Excessive transducer heating in commercial systems is automatically prevented by limiting the frequency and magnitude of the push pulses.6–8
On the ACUSON S2000,
there is an additional method to limit transducer heating, which is a short, programmed 'cool down' period after the image is acquired where live imaging is prevented (for Virtual Touch Tissue Imaging only).
US RADIOLOGY Figure 1: Virtual Touch Imaging
Stiff lesion in patient with history of colorectal carcinoma. Source: Courtesy of Dirk Andre Clevert, Munich.
Figure 2: Virtual Touch Quantification
21 fps 6 cm
A region of interest is placed on the area where the stiffness quantification is to be measured. The shear wave velocity measurement in this fibrotic liver was 2.44 m/sec, which is displayed to the right of the image.
Virtual Touch Applications in Liver Disease The superficial location of the liver and its easy accessibility with ultrasound, along with the wide range of pathology that affects this organ, makes it a good target for interrogation with ARFI. The current major application of ARFI technology is in the evaluation of diffuse liver disease, in particular, liver fibrosis. The assessment of focal liver lesions and the use of Virtual Touch imaging in radio frequency ablation in the liver are under evaluation and show future potential as discussed below.
Applications in Diffuse Liver Disease— Liver Fibrosis
Virtual Touch quantification and the measurement of shear wave velocity have so far been shown to be most useful and reliable in the
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