Threshold and ablation efficiency studies of microsecond ablation of gelatin under water

Laser thrombolysis is the selective ablation of thrombus occluding vessels by microsecond pulsed laser irradiation. To achieve efficient ablation of thrombus, the optimal wavelength, spot size, and pulse energy need to be determined.

A gelatin-based thrombus model confined in 3 mm inner diameter tubes was ablated under water using a 1 microsecond pulsed dye laser. Wavelength studies were conducted by varying the absorption of the gelatin between 10-2000 cm-1 corresponding to the waveband between 400-600 nm on the absorption spectrum of thrombus. A unique spectrophotometric method was developed to measure the ablated mass. An acoustic method was used to measure ablation thresholds under water as a function of absorption.

The mass removed per pulse per unit energy was nearly equal over an absorption range of 100-1000 cm-1 at pulse energies above threshold. Mass removal increased linearly with pulse energy but did not have a direct relationship with radiant exposure. Ablation thresholds indicate that the gelatin needed to be heated only to 100 degrees C for ablation to commence. Ablation masses measured were an order of magnitude higher than those predicted by thermal ablation models.

The results suggest that any wavelength between 410-590 nm can be used for effective thrombolysis. The ablation efficiency depends on the total energy delivered rather than the radiant exposure. The high ablation efficiencies suggest a dominance of the mechanical action of vapor bubbles over thermal ablation in the ablation process.