The phenomena at the onset of and during tissue ablation using continuous wave lasers were studied. Aortic and myocardial bovine tissues were exposed in air and in water to Nd-YAG (10 to 60 W) and argon (3.5 W) laser light. The transmitted light was measured, the surface of the tissue was filmed and the tissue was processed for histology. Three distinct phases were observed. Phase A was tissue denaturation. Phase B started with explosive vaporisation ('popcorn') in conjunction with a drop of about 50% in light transmission due to enhanced reflection and scattering. The 'popcorn' is possibly associated with a layered structure of the tissue. Phase C started with carbonisation of the tissue beginning in the centre and expanding in a cyclic fashion while the tissue was vaporised leaving a crater. Forward light transmission did not decrease. Thin layers of carbon (20 microns) and vacuoles (30 microns) suggested a large temperature gradient along the tissue ablation front. The ablation velocity was constant (r greater than 0.92) and tissue dependent. Reported models on laser tissue ablation need to be extended to include explosive vaporisation ('popcorn') at the onset and cyclic carbonisation during steady-state crater formation.

http://iopscience.iop.org/article/10.1088/0031-9155/35/8/009/pdf

Explosive onset of continuous wave laser tissue ablation.

Background and Objective

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.



Study Design/Materials and Methods

A gelatin-based thrombus model confined in 3 mm inner diameter tubes was ablated under water using a 1 μs 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.



Results

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°C for ablation to commence. Ablation masses measured were an order of magnitude higher than those predicted by thermal ablation models.



Conclusion

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. © 1996 Wiley-Liss, Inc.

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

Pros and Cons in Pta and Auxiliary Methods

Excimer laser ablation in the treatment of total chronic obstructions in critical limb ischaemia in diabetic patients. Sustained efficacy of plaque recanalisation in mid-term results.

Applications and limitations of laser-assisted angioplasty.

Using continuous wave (CW) argon ion laser light, a total of 253 laser exposures of varying power (1.5, 3, 5, 8 or 10 W) and duration (20-1,333 ms) were delivered to four segments of human atheromatous aorta obtained at autopsy. Exposure conditions were controlled by using an optically shielded laser catheter that provided a 500 micron spot of light of known power. Two thresholds for consistently reproducible ablation could be defined-an intensity threshold at 25.5 W/mm2 and a fluence threshold at 3.2 J/mm2. Above threshold, a fluence of 5.1 J/mm2 was found to produce the most efficient ablation, ie, removed the greatest volume (mm3) per energy delivered (J) compared to other fluence levels employed (p less than 0.0001). Between aortic segments, however, considerable variability in efficiency (mm3/J) was observed, possibly owing to different optical properties and/or plaque composition. Low-intensity laser radiation produced inconsistent ablation and extensive coagulation effects to surrounding tissue. When a fluence of 5.1 J/mm2 was constructed with a high-intensity laser beam and a short exposure time, consistent and efficient tissue removal resulted without histologic evidence of coagulation necrosis.

Ben Costello

Papers

Effects of varying argon ion laser intensity and exposure time on the ablation of atherosclerotic plaque.