6.5.6 Photodisruption
Similar to plasma-induced photoablation, inphotodisruptionvery high irradiance
levels result in plasma formation. However, the amount of energy absorbed during
photodisruption generally is at least two orders of magnitude higher than during
plasma-induced photoablation. This condition produces a greater free electron
density and also a higher plasma temperature than in the case of plasma-induced
photoablation. When such a plasma has been newly formed, a condition called
plasma shieldingarises. In this effect the plasma absorbs and scatters further
incident light, thereby protecting (shielding) the underlying tissue structure from the
plasma effects. In ophthalmology, plasma shielding prevents potentially damaging
light from reaching the retina when performing procedures such as lens capsulo-
tomy. This is a treatment that involves breaking a secondary cataract, which is a
membrane that developed at the back of a lens following cataract surgery.
The photodisruption laser-tissue interaction mode uses pulses of nanosecond or
shorter duration and irradiances of 10^9 – 1014 W/cm^2. Because the molecules in the
target tissue undergo rapid ionization from a high-intensity laser beam, the
plasma-creation process in photodisruption creates localized mechanical effects
such as shock waves, jetting of material, and bubble formation followed by cavi-
tation. As a result of the high kinetic energy of free electrons, the temperature of the
plasma rises rapidly and the plasma electrons diffuse into the surrounding tissue
medium. The rapidly growing plasma creates a pressure wave or shock wave that
travels outward and soon separates from the plasma boundary. About 1–5 % of the
incident pulse energy is converted to shock wave energy, with short pulses on the
order of picoseconds resulting in a weaker shock wave. High-energy pulses on the
order of nanoseconds produce stronger shock waves. Such pulses are not desirable
for ophthalmology applications, because they can cause damage at points that are
remote from the focal region of the laser. However, pulses on the order of
nanoseconds can be used in lithotripsy for breaking up kidney stones or gallstones.
Cavitationoccurs when a vapor bubble that forms around the plasma grows to a
critical size and then collapses violently, thereby sending out another shock wave.
Jettingcan occur when the cavitation bubble is formed close to a solid surface (for
example, on a tooth surface). In this case a high-speed liquid jet is directed toward
the wall of the surface as the bubble collapses.
In addition to its use in lithotripsy, photodisruption is widely used for minimally
invasive surgery and in ophthalmology for drilling holes in the cornea or lenses and
for cataract surgery.
6.5 Light-Tissue Interaction Mechanisms 187