Acoustic radiation force impulse (ARFI) imaging has been demonstrated to provide insight into the mechanical properties of tissue. The quality of ARFI images is dependent on the amount of acoustic energy from the radiation force pulse reaching the focus. Intra-cardiac probes provide an advantage for ARFI imaging of cardiac tissue, as the probe can be positioned close to the region of interest. The resulting ARFI images display local variations in tissue stiffnesses and show promise for monitoring and assessing the progress of cardiac ablations. The Siemens AcuNav intra-cardiac probe was used to image a tissue-mimicking phantom having 3 mm diameter spherical inclusions with an elastic modulus eight times greater than the surrounding tissue. The ARFI sequences formed high contrast, high resolution images of these inclusions up to depths of approximately 1.5 cm. The ARFI pulse sequences resulted in 0.8°C temperature increase on the transducer face, and the time constant associated with the return to equilibrium temperature was approximately 300 ms. The probe was used to examine an excised segment of an ovine right ventricle with a surface lesion created from radiofrequency ablations (RFA). In areas of healthy tissue, the ARFI images did not show any stiffer regions that would indicate the presence of a lesion. Although the lesion was not visible in conventional B-mode images, the ARFI images were able to show the boundaries between the lesion and the surrounding tissue.
When performing radiofrequency ablation (RFA) procedures, physicians
currently have little or no feedback concerning the success of the
treatment until follow-up assessments are made days to weeks later. To
be successful, RFA must induce a thermal lesion of sufficient volume
to completely destroy a target tumor or completely isolate an aberrant
cardiac pathway. Although ultrasound, computed tomography (CT), and
CT-based fluoroscopy have found use in guiding RFA treatments, they
are deficient in giving accurate assessments of lesion size or
boundaries during procedures. As induced thermal lesion size can vary
considerably from patient to patient, the current lack of real-time
feedback during RFA procedures is troublesome. We have developed a
technique for real-time monitoring of thermal lesion size during RFA procedures utilizing acoustic radiation force impulse (ARFI) imaging. In both ex vivo and in vivo tissues, ARFI imaging provided better thermal lesion contrast and better overall appreciation for lesion size and boundaries relative to conventional sonography. The thermal safety of ARFI imaging for use at clinically realistic depths was also verified through the use of finite element method models. As ARFI imaging is implemented entirely on a diagnostic ultrasound scanner, it is a convenient, inexpensive, and promising modality for monitoring RFA procedures in vivo.