Aorta during moderate power argon irradiation

The thermal response of tissue during laser irradiation is highly dependent upon the optical properties of the tissue. Many laser treatments produce temperatures substantially above normal tissue temperatures (50 $^\circ$ -300 $^\circ$ C). At such temperatures, dehydration, protein denaturation, coagulation, charring, pyrolysis or ablation may occur. Any of these processes changes the appearance of the tissue and hence the optical properties of the tissue. Heretofore, there has been no attempt to measure the optical properties of tissue during irradiation.

The experimental apparatus in Figure 6.8 was used to measure the scattered transmission, the scattered reflectance, and the collimated transmission during irradiation of human aorta by an argon laser. Reflectance and transmission were measured during irradiation, digitized and saved on a computer for later analysis. The laser used was a 20W argon laser (Coherent CR-18) operating in the multi-line mode. Two integrating spheres (Labsphere) were used to measure scattered light. The larger sphere (24cm in diameter) was used to measure scattered transmission and the smaller (12cm) was used to measure scattered reflectance. Collimated transmission was measured with a 4mm diameter photodiode located 170cm from the exit port of the larger integrating sphere. The detectors used to measure light in the integrating sphere were also photodiodes (RCA SK2031). To ensure uniform heating of the sample, the central flat portion of the beam was used. This was obtained by expanding the laser beam with a 5 x microscope objective (f26.4mm) and collimating it with an f126mm convex lens, resulting in a net magnification of 4.8. The edges of the beam were blocked with a circular aperture 8mm in diameter, thereby allowing only the central `flat' portion of the Gaussian beam profile to reach the sample. The spot size was 8mm in diameter.

**Figure 6.8:** Experimental apparatus to measure optical properties of tissue during heating with an argon laser. The laser beam was expanded and passed through a diaphragm to obtain a flat beam profile. The sample was located between two integrating spheres to detect diffuse reflection and transmission. The photodiode was located about one meter from the sample to ensure that most of the light collected was collimated transmitted light.
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Aorta was obtained from the morgue the morning the experiments were done. The aorta was kept in chilled saline until used. The adventitia was removed leaving samples with full thickness media and the intima intact. Typical sample thicknesses were about 1.5mm. The aorta was not sandwiched between glass slides which allowed the tissue to change shape during irradiation. Typically the samples became thinner during irradiation due to dehydration. Both experiments described here use aorta samples from the same subject.

During irradiation the tissue passed through a series of phases. The first phase, coagulation, was marked by whitening of the tissue. This was followed by dehydration, by boiling, and finally by charring. Before each experiment, measurements of 0% and 100% scattered transmission, primary transmission and scattered reflectance were made to allow scaling of detector voltages into fractions of the total possible reflectance or transmission. These values are plotted in Figure 6.9 for two experiments with differing irradiances: 130 and 90W/mm². As expected, reflectance and transmission change more quickly for the higher irradiance. The tissue response for irradiances between 130 and 90mW/mm² was similar and, for clarity, is not shown.

The difference in magnitude illustrated by the scattered and primary transmission plots at the right is caused by differing sample thicknesses. (1.65mm for the higher irradiance and 1.70mm for the other.) Since one mean free path (mfp) is about 0.1mm, this is a significant thickness change. This difference is not as evident in the reflectance graph because the samples are so thick ( $\sim$ 15 mfp) that thickness variations do not substantially affect the net reflectance.

**Figure 6.9:** Reflection and transmission during argon laser irradiation of normal human aorta. Irradiances of 90 and 130W/mm² are shown. Dashed lines indicate explosions and onset of charring (see text) and correspond to the same lines in Figure 6.10.
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Both the primary and the scattered transmission drop significantly in the first 50-75 seconds. Presumably this is due to surface coagulation and dehydration. The initial drop in transmission is followed by an increase in transmission caused by subsurface vapor production. Once the vapor pressure exceeds the yielding point of the tissue the bubble of vapor explodes, announced by an audible ``pop.'' This time is indicated by the dashed lines in Figures 6.9 and 6.10. In Figure 6.10 the lower irradiance also has an anomalous early ``pop'' indicated. This may have been due to a small plaque deposit. The onset of charring is marked by a sudden decrease in the reflectance. This is also indicated by a labelled dashed line. Not surprisingly, as the tissue blackens during charring, the transmission also drops.

Halldórsson et al. have made similar types of measurements using a Nd-YAG laser on canine stomach wall [23]. They used much higher power densities, smaller spot sizes and shorter irradiation times. They found that the reflectance initially increased and the transmission decreased, followed by a rapid increase in reflectance and continued decrease in transmission during the evaporation or dehydration phase. Finally, during the carbonization phase they reported that the reflectance decreased and the transmission increased. Clearly, these results differ from those reported here. This discrepancy is most likely caused by the greater penetration depth of Nd-YAG (1060nm) light in tissue. This difference causes changes in the light distribution, and consequently, in the thermal profile and the damage pattern for the tissue.

Dimensionless optical properties were calculated for the measured reflections and transmissions. The use of dimensionless optical properties allowed optical properties to be obtained without knowing the thickness of the sample during irradiation. The thickness was not monitored during the experiment and so it was not possible to extract values for the absorption and scattering coefficients from the dimensionless optical parameters. The dimensionless parameters shown in Figure 6.10 are the transport albedo (a'), the optical depth $(\tau')$ , and the delta-Eddington anisotropy (g'). The index of refraction was assumed to be constant, despite some evidence that the index of refraction of tissue varies with water content [2]. This assumption was not too extreme since the estimated variation in the index of refraction (1.38 to 1.45) only changed the measured optical properties by 10%.

The most surprising finding is that there is very little change in the transport albedo until after the tissue explodes. At this time there is a slight decrease in the albedo corresponding to the onset of tissue boiling. After the onset of carbonization, the transport albedo drops sharply, indicating an increase in tissue absorption.

The initial drop in the anisotropy corresponds to tissue coagulation or blanching. This decrease indicates that the tissue scatters light more isotropically. Subsequent to the initial fall, the anisotropy increases linearly, corresponding to tissue dehydration and thinning. Thinning reduces the distance between scattering centers thereby increasing the effective size of the scatterers and, as in Mie scattering, the anisotropy as well.

The optical depth depends on the physical thickness of the sample. This thickness changes during irradiation due to tissue dehydration: if the absorption and scattering were constant, the optical depth would still change. The optical depth is quite sensitive to tissue coagulation and substantial changes occur in the first 50-75 seconds.

**Figure 6.10:** Optical properties during argon laser irradiation of normal human aorta. Irradiances of 90 and 130W/mm² are shown. Dashed lines indicate explosions and onset of charring. The anomalous ``pop'' was probably caused by the presence of plaque in the tissue.
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As expected the most drastic changes in optical properties are associated with charring: the albedo decreased and the delta-Eddington anisotropy g' increased. Unexpectedly, the effective albedo a' was relatively constant during coagulation and dehydration. These preliminary experiments illustrate a combined experimental and theoretical technique for measuring the optical properties of tissue during laser irradiation. Although the parameters presented are independent of tissue thickness, evaluation of absorption and scattering coefficients will require measurement of tissue thickness during irradiation.

Light Transport in Tissue

Aorta during moderate power argon irradiation