ARFI imaging is a relatively new technique that our laboratory has demonstrated can image relative differences in the mechanical properties of tissues in vivo and in vitro.
Acoustic radiation force is applied to absorbing or reflecting targets in the propagation path of an acoustic wave. The absorption of focused, high intensity acoustic beams generates acoustic radiation forces that in turn create distributions of stress within tissues. The deformations resulting from the stress are indicative of the elasticity and geometric structure of the material.
The displacement and recovery of tissues takes place on a 1-4 msec time scale and can be monitored to a 0.1-0.2 µm accuracy using cross-correlation techniques applied to echoes from conventional B-mode pulses emitted before and after the “pushing pulse”. We have observed differences in both the magnitude of displacement and recovery rates of different tissues to transient radiation force excitation.
While ARFI displacement images do not directly provide quantitative measurements of tissue mechanical properties, relative differences between tissues are evident, and there is good agreement between structures in matched conventional B-mode and ARFI images. ARFI images are speckle free, with resolution comparable to conventional ultrasound imaging. Figure 4 shows an example of a matched B-mode and ARFI image in a tissue phantom.
ARFI imaging has been shown to be capable of imaging a developing RFA generated lesion in an open chest subject with the imaging probe placed directly on the heart and from a 64 element ICE catheter inside the heart. We propose to further develop intracardiac ARFI imaging with a custom 128 channel Siemens AcuNav intra-cardiac echo catheter to non-invasively monitor cardiac ablations in vivo.
Additional Uses for ICE based ARFI imaging
The primary aim of this proposal is to develop the system and the techniques for visualizing ablated atrial myocardium for guiding atrial flutter and atrial fibrillation ablations. ARFI imaging has the potential to differentiate other tissues in the heart that could be of benefit guiding these and other cardiac ablation procedures.
It has recently been shown that placing ablation lesions near the myocardial venous junction in the pulmonary veins can be effective in ablation of AF. Traditional ICE cannot easily differentiate this border but it is likely that ARFI imaging will be able to provide the necessary contrast to demarcate this boundary .
Ventricular tachycardia originating from infracted myocardium are frequently ablated by placing lesions on the border of or in the spared layer of tissue below the infarct scar. ARFI imaging will likely be able to provide the soft tissue contrast necessary to differentiate normal myocardium from infarct scar and thus provide real time guidance for lesion placement during these procedures.
Cardiac ARFI also has the potential for characterizing myocardial stiffness and contraction and relaxation defects. These functions could aid in the characterization of cardiac function for a number of clinical conditions including coronary artery disease and transplant rejection.