MEDICAL LASER REPORT: Tracking images laser coagulation

Laser coagulation is a thermal technique that has found use in prostate, GI, and eye surgeries. Ideally the surgeon would have some sort of real-time imaging system that would provide information on how deeply and how intensely the tissue has been heated. A collaboration between three researchers at different institutions developed a technique that uses the spatial profile of the ultrasound waveform change to represent the spatial profile of coagulating tissue damage during heating. The technique could provide convenient and inexpensive real-time feedback for controlling coagulation damage.

Laser light absorbed into the tissue can seal bleeding blood vessels or selectively kill other types of tissue (such as for treating prostate cancer). When the light is absorbed, the energy turns into heat in the tissue. Heat moves through and alters tissue in a complex interaction involving conduction, convection, radiation, metabolism, evaporation, and physical phase change.

Coagulation occurs when tissue is heated to about 55 degrees C at which point proteins begin to denature. (In general, coagulation can be seen with the naked eye as whitening of the tissue. One of the most familiar examples of thermal coagulation and denaturing is when an egg white is cooked: it turns from a clear wet mass into an opaque solid one.) The treatment parameters (such as heating rate and duration) dramatically influence the overall tissue response. But because the process kills the cells involved, researchers want techniques that allow them to monitor the temperature inside the tissue and avoid damaging surrounding tissue.

Noninvasive assessment of the extent of thermal coagulation in tissue has been investigated, but most of the work has either been qualitative or has required off-line and expert image analysis. Also none of the techniques provided a spatial profile of the coagulation damage during the procedure.

Zhigang Sun, with the Industrial Materials Institute of the National Research Council Canada (Boucherville, Quebec), Hao Ying at Wayne State University (Detroit, MI), and Jialiang Lu at Hallibunon Corporation (Houston, TX) developed a novel cross-correlation technique for noninvasively detecting the spatial profile of the degree of coagulation damage along a diagnostic ultrasound beam axis in the tissue being heated.¹ The system works in real time, automatically.

The basic idea underlying the technique is that the more the tissue structure in a region has been altered by the heating (in other words, tissue denaturation), the more the ultrasound signal scattered from that region should change. The degree of signal waveform change infers the degree of tissue structure change.

The researchers coagulated 23 fresh canine liver samples using an Nd:YAG laser operating at 1064 nm, with light intensities ranging from 62 to 105 W/cm² and exposure times from 20 to 350 s. They used a 13-mm-diameter 10-MHz broadband single-element spherical focused ultrasound transducer. They obtained a reference waveform at the beginning of the procedure then repeatedly sent the same acoustic signal to the tissue being heated. They tracked the echo signals scattered from many small tissue regions. With this data, they quantified the changes in the waveform with respect to the reference signal.

The researchers developed an automatic procedure to compute the coagulation depth using the spatial profiles of the waveform change. The rms difference between the calculated depth and the depth measured by visual inspection was less than 1 cm.

The researchers want to further develop the theoretical models they used to describe waveform changes and to describe the tissue changes based on the cross-correlation coefficient. Also the setup they used had the laser-coupled fiber on one side of the tissue sample and the ultrasound transducer on the other. Further development will ideally place the laser source and ultrasound transducer on the same side of the tissue, perhaps even with the fiber sticking through a hole in the middle of the transducer.

Reference:

1. Z. Sun, H. Ying, J. Lu, IEEE Trans. On Biomed. Eng. 48 (2), 223 (Feb. 2001).