Objective
Acoustic radiation force (ARF)-based approaches to measure tissue elasticity require transmission of a focused high-energy acoustic pulse from a stationary ultrasound probe and ultrasound-based tracking of the resulting tissue displacements to obtain stiffness images or shear wave speed estimates. The method has established benefits in biomedical applications such as tumor detection and tissue fibrosis staging. One limitation, however, is the dependence on applied probe pressure, which is difficult to control manually and prohibits standardization of quantitative measurements. To overcome this limitation, we built a robot prototype that controls probe contact forces for shear wave speed quantification.
Methods
The robot was evaluated with controlled force increments applied to a tissue-mimicking phantom and in vivo abdominal tissue from three human volunteers.
Results
The root-mean-square error between the desired and measured forces was 0.07 N in the phantom and higher for the fatty layer of in vivo abdominal tissue. The mean shear wave speeds increased from 3.7 to 4.5 m/s in the phantom and 1.0 to 3.0 m/s in the in vivo fat for compressive forces ranging from 2.5 to 30 N. The standard deviation of shear wave speeds obtained with the robotic approach were low in most cases (< 0.2 m/s) and comparable to that obtained with a semiquantitative landmark-based method.
Conclusion
Results are promising for the introduction of robotic systems to control the applied probe pressure for ARF-based measurements of tissue elasticity.
Significance
This approach has potential benefits in longitudinal studies of disease progression, comparative studies between patients, and large-scale multidimensional elasticity imaging.
Radiofrequency ablation (RFA) is used to locally disrupt electrical propagation in myocardium and treat arrhythmias, and direct visualization of ablation lesions by acoustic radiation force methods may benefit RFA procedures. This work compares four imaging modalities, B-Mode, ARFI, STL-SWEI and MTL-SWEI, in their ability to resolve RFA lesions in four ex vivo experiments. Ablation lesions are shown to be marked by at least a local halving of ARFI displacements and doubling of shear wave speeds. In a controlled ablation of ex vivo porcine and canine cardiac tissue, STL-SWEI and ARFI are shown to have similar CNR, better than MTL-SWEI and B-Mode. The SWEI modalities are demonstrated to have improved imaging of distal lesion boundaries. Gaps smaller than 5 mm are visualized in ablation lines made of discretely-spaced ablations, and complex structures are reconstructed through depth in an “x” ablation experiment. Scans of suspended atria show increased noise, but successfully visualize ablations in ARFI, MTL-SWEI and STL-SWEI.
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