A. Charlton

Erbium-YAG and holmium-YAG laser ablation of bone

Results are presented for the latent heat of ablation of bone using an erbium-YAG laser operating at 2.9Μm, and a holmium-YAG laser operating at 2.1Μm. The values are 8.2±1.0 kJ cm−3 and 18±2.0 kJ cm−3, respectively. Secondary damage to surrounding tissue is found to extend approximately 5Μm with the erbium laser and is greatly increased to 80Μm with significant charring in the case of holmium. These secondary damage zones are much smaller than those produced by the CO2 laser.

An in vitro comparison of the Erbium: YAG laser and the carbon dioxide laser in laryngeal surgery

This study compares the relative thermal damage caused by a surgical CO2 laser and the Erbium:YAG laser when used to incise the human vocal fold in vitro. Results show that charring is completely eliminated when using the Erbium:YAG laser. The depth of coagulative necrosis adjacent to an incision is reduced from 510 microns (+/- 75) using the CO2 laser to 23 microns (+/- 12) using the Ebrium:YAG laser and at the base is reduced from 125 microns (+/- 45) using the CO2 laser to 12 microns (+/- 8) using the Erbium:YAG laser. The potential advantages regarding post-operative healing after laryngeal surgery are discussed.

High Repetition Rate, High Average Power Er:YAG Laser at 2·94 μm

Abstract A simple kinetic model is presented which describes the temporal behaviour of an Er:YAG laser pulse and predicts c.w. operation. The use of transfer dye to selectively pump the erbium ions, and suitable optimisation of laser cavity parameters has allowed operation up to 40Hz with 4W average power. 10W at 100Hz is predicted.

Q-switching the Erbium-YAG Laser

Abstract An erbium-YAG laser has been Q-switched by two methods; a mechanical technique employing a rotating output coupler and electro-optically utilizing a LiNbO3 crystal. The performances of each technique are assessed and compared. Pulse durations of 150 to 850 ns have been produced, with pulse energies of 1 to 16 mJ depending on the technique used. A computer model based on the rate equations describes the temporal profile of the laser pulses, showing good agreement with experiment.

Studies of Er-YAG laser interactions with soft tissue

The ablation efficiency and depth of secondary thermal damage have been determined for a range of cadaveric soft tissues on exposure to radiation from a pulsed Er-YAG laser operating at 2.94μm. The tissues investigated included brain, small intestine, stomach, liver, heart, spleen, lung, aorta, cornea, kidney, skin and uterus. The results obtained are compared to those predicted by a simple one-dimensional model of the interaction. The amount of tissue damage varied between tissues. In cellular tissues it was approximately 20μm in extent on either side of the slot and at its base. In acellular tissues (aorta, cornea, etc.) the alteration in protein structure was more variable and was dependent upon the nature of the connective tissue fibres. Corneal collagen showed changes in protein structure up to 30μm from the edge of the slot, whereas aortic elastic fibres were little affected by the laser energy, apparently melting to form a coagulum that lined the slot.

A Tunable Visible Solid State Laser

Abstract Laser operation and radiative properties of organically doped porous sol-gel silica have been studied using a range of cavities. Measurements of diffusion, distribution and long term photostability of dopant dye molecules are described. Laser oscillation at wavelengths of between 360 and 630 nm are reported. This represents a substantial increase over the range previously reported for organic dye doped silica lasers. In a Littrow configuration a tuning range of 60 nm and a conversion efficiency of 16% have been observed. A linewidth of <0·05 nm has been obtained using a grazing incidence cavity.

Interaction of erbium laser radiation with corneal tissue

The potential of an erbium-YAG laser for corneal surgery has been assessed under a range of operating parameters. The ablation threshold has been measured at approximately 0.6 Jcm-2 and the depth of thermal damage evaluated for different pulse durations and energy densities. The minimum damage of

Holmium:YAG and erbium:YAG laser interaction with hard and soft tissue

The holmium YAG and erbium YAG lasers operating at 2.1 micrometers and 2.9 micrometers respectively, are the subject of great interest for various medical applications. The interaction of both these pulsed lasers with biological tissue involves absorption of the radiation by water leading to rapid heating and ablation, however the different absorption coefficients at these two wavelengths give rise to different ablation efficiencies and haemostatic properties for the two lasers. It is this cut/seal ratio that determines for which medical applications each of these lasers is most suited. The lasers were used to produce incisions in various tissues by translating the tissue at fixed speed beneath a focused laser beam. The laser energy density was varied between 100 and 500 J/cm2 and the lasers were operated at 2 Hz. After irradiation the tissues were fixed in formalin, processed routinely into paraffin wax, sectioned at 5 micrometers and stained with haemotoxylin and eosin. This allowed the dimensions of the incisions to be measured, as well as the depth of coagulative denatured tissue surrounding each incision. In this way the cut/seal ratio was determined for both the holmium YAG and erbium YAG laser in a range of hard and soft tissues. Results show that the latent heat of ablation for the holmium YAG laser interacting with soft tissue varies between 20-50 kJ/cm3, almost an order of magnitude larger than with the erbium YAG laser. Furthermore, the depth of coagulative necrosis with holmium YAG extends 100-400 micrometers , compared with 10-30 micrometers for erbium YAG. The two interactions clearly lead to vastly different results suggesting that the holmium YAG laser is suitable for producing lesions in highly vascular tissue where haemostasis is important, whereas the erbium YAG laser is better suited to avascular tissue requiring large depths of incision.

Comparison of cooling criteria with a cryogen spray and water/air spray

Skin cooling using a cryogen spray (tetrafluoroethane) has been shown to dramatically reduce the skin surface temperature whilst predictions show that the underlying dermal tissue is unaffected. This technique is repeated with a chilled water spray, along with a continuous airflow to enhance evaporation. Radiometric skin surface temperature measurements are recorded during trials utilizing this technique and the results are compared with theoretical predictions in order to determine the mechanism by which the heat is removed from the skin. The optimum spray conditions are achieved when the water is chilled to around 2 degrees Celsius with a continuous airflow of 50 liters/minute. Under these conditions skin surface temperature reduction is about 8 degrees Celsius - 10 degrees Celsius. The measured radiometric skin surface temperature change indicates that the mechanism by which this process removes heat from the skin is predominantly evaporation. Predictions of skin temperature change with varying skin depth indicate that the optimum spray time is around 100 ms.

Post-operative healing of erbium YAG laser incisions

A pilot study was made on the post-operative healing of incisions in the tails of anaesthetized rats made by an erbium YAG laser. The incisions are compared with those made by a scalpel. Histological sections taken at intervals between 3 h and 14 days show little difference between the two methods of incision.