Plasma and Cavitation Dynamics during Pulsed Laser Microsurgery in vivo

We compare the plasma and cavitation dynamics underlying pulsed laser microsurgery in water and in fruit fly embryos (in vivo)—specifically for nanosecond pulses at 355 and 532 nm. We find two key differences. First, the plasma-formation thresholds are lower in vivo —especially at 355 nm—due to the presence of endogenous chromophores that serve as additional sources for plasma seed electrons. Second, the biological matrix constrains the growth of laser-induced cavitation bubbles. Both effects reduce the disrupted region in vivo when compared to extrapolations from measurements in water.

Figure 1

Assessment of cavitation damage in vivo. Three sequential confocal images are shown of fruit fly embryos before and after ablation at (a) $\lambda =355\mathrm{nm}$ and $E_P=1.22\mu \mathrm{J}$, i.e., $5\times$ threshold, or (b) $\lambda =532\mathrm{nm}$ and $E_P=8.26\mu \mathrm{J}$, i.e., $1\times$ threshold. Ablation occurred between the first two images. Each white circle is centered on the ablation site with a radius equal to the calculated $R_{\max }=12.9\mu \mathrm{m}$ in (a) and $65.6\mu \mathrm{m}$ in (b).

Figure 2

Cavitation bubble parameters versus laser pulse energy in water and in vivo. To facilitate comparison, we have included data previously reported for ablation in water at 532 and 1064 nm [10].

Figure 3

Cavitation bubble parameters from 355 nm ablation in water, in vivo, and in a 38 mM NADH solution.

Figure 4

Direct measurements of the growth and collapse of ablation-induced cavitation bubbles via confocal laser backscatter: (a) in water at 532 nm; and (b) in vivo at 532 nm. The average pulse energy, $E_P$, and calculated $R_{\max }$ for each data set are given. Filled symbols represent passage of the bubble front during expansion; open symbols represent the passage during bubble collapse. The gray lines represent distances at which we saw no evidence of bubble passage. For in vivo experiments, the separation between the ablation site and the imaging laser is parallel to the long axis (▪) or short axis (▴) of the embryo.

Figure 5

Fraction of the laser pulse energy represented in (a) the shock wave at 1.5 mm from the ablation site and (b) maximal expansion of the cavitation bubble against hydrostatic pressure. To facilitate comparison, we have included data previously reported for ablation in water at 532 and 1064 nm [10].