Milestone 2: Correlation of and ARFI Image Parameters and Histopathology in Coagulated Myocardium
Applying RF ablation energy with a catheter tip produces a hemispherical gradation of temperature within the myocardium immediately adjacent to the metal electrode. The hottest tissue during the ablation is usually one to two millimeters beneath the catheter tip (figure 10). The therapeutic effect on the tissue is a function of the tissue temperature and duration. Thus, a continuum of tissue changes exists in the ablation region that correlates with the thermal gradient established during the energy application. A gradient of thermal effects are created in all ablation lesions and by imaging the lesion over its extent, we can visualize this gradient and correlate thermal tissue changes with changes in tissue compliance.
ARFI imaging produces an image where color encodes the measured displacement of the tissue. The correlation between measured displacement changes during ablation and the state of the myocardium has not been studied. Thus, the reduction in tissue compliance that differentiates normal from coagulated myocardium is unknown. It is hypothesized the gradient of displacement change will be relatively steep as the protein denaturation that leads to stiffening is a highly non-linear function of temperature. The steep gradient of coagulation necrosis should produce a steep gradient of compliance changes causing a well defined image boundary correlating with the border of tissue necrosis. This hypothesis is supported by the images in figure 10.
Fresh samples of bovine myocardium will be cut from the heart. 4 cm x 4 cm samples of the left and right ventricle and the left and right atria will be cut from the heart. The tissue samples will be pinned into place in a plexiglass chamber and superfused with 37° saline at a flow of 65 ml/ min (Figure 11). The bottom of the chamber will be lined with an ultrasound absorbing material. An ablation catheter with temperature sensing will be inserted vertically into the chamber to contact the tissue. An AcuNav imaging catheter will be inserted into the chamber parallel to the tissue surface and rotated such that the ablation catheter tip lies just outside the -12dB Tx-Rx system response. At this point, the imaging plane will pass diagonally through the ablation region just below the catheter tip. The tissue temperature will be monitored with a needle thermistor (YSI-) and the tissue and chamber allowed time to stabilize at a temperature of 37°C.
An ablation lesion will be created by applying energy in the temperature controlled mode for 60 sec. The exact temperature setting will be determined experimentally to create a lesion approximately 1 cm deep and 2 cm in diameter. ARFI images will be made of the tissue before, during, and after the ablation. ARFI images will be formed at 2 frames/sec with concurrent B-mode at 10 frames/sec.
After the lesion is created the chamber roof will be removed and small clips attached to the tissue surfaces at the intersections of the imaging plane. This will be done by guiding the clip attachment using the B-mode image. The tissue will be remove from the chamber and sectioned along the imaging plane using the clips for alignment of the blade. The sectioned tissue will be photographed, fixed in formalin, stained with Gomori’s Trichrome and photographed again. The photographs will be processed by pixilating the image and assigning a category of coagulation necrosis based on the stained tissue’s color. The ARFI images will be processed and a map of tissue displacement made across the imaging plane. The photograph and ARFI images will be aligned using the top and bottom edges of the imaging plane and each pixel assigned a coagulation zone and local displacement value. A histogram of displacement values will be created for each zone of necrosis and a discriminate analysis performed to determine the displacement value which best describes each zone.
The expected outcome of this experiment will be a mean value and confidence interval for tissue displacement in each zone of coagulation necrosis of the lesion.