Tissue internal pressure is closely related to its pathological status. For many diseases, their occurrence and development can cause tissue internal pressure change. One example is interstitial fluid pressure. For most solid tumors, their growth can cause increased interstitial fluid pressure, which has been identified as a barrier for the uptake of therapeutic agents and consequently a major cause of reduced treatment effect for cancers. Therefore, it is of great diagnostic value to measure and/or monitor tissue internal pressure of targeted tissues. In current medical practices, the methods for evaluating tissue internal pressure are mostly invasive and either needles or catheters are used. To reduce the harm of the invasive diagnosis, a noninvasive ultrasound method based on a comparative analysis of two ultrasound-based elasticity imaging is proposed and validated in this project.
The two ultrasound-based elasticity imaging methods are shear wave elasticity imaging (SWEI) and strain imaging. SWEI is a quantitative method that can provide the true elasticity of tissue (shear modulus). While strain imaging is a qualitative method, and it has been clinically implemented based on the assumption that the difference in strain magnitude is an indication of relative differences in elasticity between target tissue and surrounding tissue. However, externally and/or internally generated pressure may also lead to strain contrast although the shear modulus of the tissue under pressure remains the same. To visualize the effects of tissue internal pressure, we proposed a simultaneous analysis of shear modulus images and strain images.
The results of a study performed on a tissue-mimicking phantom are shown in Figure 1. As the inclusion pressure increases, there is no visible change in B-mode images and shear modulus maps, but the color density of the inclusion decreases in strain maps. Utilizing shear modulus ratio and strain ratio between the inclusion and the background of the phantom, the effects of pressure accumulated in the inclusion can be clearly visualized. These results show that the proposed method can noninvasively assessing tissue internal pressure in a qualitative way. To more effectively assessing tissue internal pressure, a quantity characterizing the difference between shear modulus ratio and strain ratio is expected to be defined and theoretically validated in future development. Moreover, preclinical study on a cancer model or other pressure related diseases such as compartment syndrome is expected.
- M. T. Islam, J. N. Reddy, and R. Righetti, “A model-based approach to investigate the effect of elevated interstitial fluid pressure on strain elastography,” Physics in Medicine and Biology, vol. 63, no. 21, 2018.
- A. Chaudhry, N. Kim, G. Unnikrishnan et al., “Effect of Interstitial Fluid Pressure on Ultrasound Axial Strain and Axial Shear Strain Elastography,” Ultrasonic Imaging, vol. 39, no. 2, pp. 137-146, 2017.