In Vivo ARFI Images of Cardiac Ablation
We performed two important preliminary studies in animals. In the first study32 (Appendix 2), we demonstrated ARFI imaging of ablation from the epicardium with a hand held probe. In the second33 (Appendix 1), we demonstrated ARFI imaging of ablation using the AcuNav™ ICE catheter. Both studies confirm that ablation lesions can be imaged using this technique.
In Vivo ARFI Imaging of Ablation with a Hand Held Linear Array
In vivo ARFI images of a canine heart were acquired with a hand held Siemens probe at a frame rate of 10 Hz for three seconds. The chest was opened so that the heart could be imaged at a closer depth. The transducer imaged the heart through an ultrasonically transparent 0.5 cm standoff pad. Other details of the procedure are available in Appendix 2 (ref Fahey).
We observed that when matched with their corresponding ECG, the sequence of ARFI images reflected changes in myocardial stiffness as the muscle contracted and relaxed thoughout the cardiac cycle. The ARFI images acquired during diastole indicate that the myocardium is relatively homogeneous and compliant with radiation force-induced tissue displacements consistently above 10 µm. During systole, myocardial contraction is evident from the wall thickening within the B-mode images and radiation force-induced tissue displacements in the ARFI images have dramatically decreased reflecting the increase of myocardial stiffness as the muscle contracts.
A lesion was created via radiofrequency ablation on epicardial surface of the free wall of the left ventricle. As a result, the lesion moved with the heart, but remained functionally inactive through the cardiac cycle and therefore did not change in mechanical stiffness. B-mode and ARFI images were acquired with the transducer centered over the lesion. The resulting displacement images were examined to determine lesion contrast at various points of the cardiac cycle.
Matched B-mode and ARFI images at four points in the cardiac cycle were examined and are shown in Figure 5 a-d. The relative time of acquisition of these images with respect to the electrocardiogram is shown in panel e. The ARFI images show the ablation lesion as a hemispherical region with small displacements and little variation through the cardiac cycle. The surrounding untreated tissue had large tissue displacements during diastole and a sharp decrease in tissue displacements during systole.
Multi-beat synthesis was used to create tissue displacement plots through a cardiac cycle by sorting 30 frames by their relative times between QRS complexes. Tissue displacement plots at two points within the myocardium, one in the center of the lesion the other in untreated tissue, are shown in Panel f . The two selected regions of interest are marked by their respective shapes in the three B-mode images. From these plots, the cyclic variation in tissue stiffness within healthy cardiac tissue becomes more apparent as there is a 5 µm (or 100%) change between systolic and diastolic displacements. Conversely, the measured displacements inside the lesion are low and remain low throughout the entire cardiac cycle.
These results demonstrated that realtime ARFI imaging of cardiac tissue and ablation lesions is possible and can be used to characterize both spatial variations in elastic properties within individual ARFI images and temporal changes in stiffness across images. This finding could lead to additional criteria for differentiation between ablated and normal tissue.