Picosecond Laser-Induced Photothermal Skin Damage Evaluation by Computational Clinical Trial

Abstract

Background and objectives: Computational clinical trial (CCT) in the field of laser medicine promotes clinical application of novel laser devices, because this trial carried out based on numerical modeling of laser-tissue interactions and simulation of a series of treatment process. To confirm the feasibility of the computational clinical trial of skin treatment with a novel picosecond laser, this paper presents an evaluation method of the safety.

Study design/materials and methods: In this method, the light propagation and thermal diffusion process after ultrashort light pulse irradiation to a numerical skin model is calculated and the safety based on the photothermal damage is evaluated by computational modeling and simulation. As an example, the safety of a novel picosecond laser device was examined by comparing with several laser devices approved for clinical use.

Results: The ratio of the maximum thermal damage induced by picosecond laser irradiation was 1.2 × 10^-2 % at the epidermis, while that caused by approved laser irradiation was 99 % at the capillary vessels. The numerical simulation demonstrated that less thermal damage was observed compared with the approved devices. The results show the safety simulated by photothermal damage calculation was consistent with the reported clinical trials.

Conclusions: This computational clinical trial shows the feasibility of applying computational clinical trials for the safety evaluation of novel medical laser devices. In contrast to preclinical and clinical tests, the proposed computational method offers regulatory science for appropriately and quickly predicting and evaluating the safety of a novel laser device.

Keywords: Medical device evaluation; computational clinical trial; photothermal damage; picosecond laser; regulatory science; safety evaluation.

Fig. 1:

Schematic diagram of the computational modeling and simulation of light and heat transfer. After creating a numerical skin geometry model, light propagation in the skin model is calculated with optical properties to produce spatial distribution of energy deposition S(x,y,z). By using the absorbed energy distribution as the initial value of a heat source, thermal diffusion is analyzed to produce temperature distribution T(x,y,z,t). From the calculated spatiotemporal temperature change, a spatial distribution of thermal damage rate in the skin model is derived numerically.

Fig. 2:

(a) Three-dimensional structure of skin model and (b) its zx plane cut, consisting of epidermis, dermis, subcutaneous fat, and three kinds of blood vessels (capillary vessels, upper dermis vessels, and deep dermis vessels).

Fig. 3:

Spatial distributions of (a) light fluence and (b) energy deposition in the skin model on the zx plane after laser pulse irradiation with (i) Device A (755 nm, 2 mm □ 6.37 J/cm²), (ii) Device B (755 nm, 2 mm φ, 18 J/cm²), (iii) Device C (1064 nm, 2 mm φ, 10 J/cm²), and (iv) Device C (532 nm, 2 mm φ, 2.5 J/cm²) under severe conditions.

Fig. 4:

Optical penetration depth after laser pulse irradiation under severe conditions.

Fig. 5:

Energy deposition profiles in the skin model on the z axis with x and y equal to 0 mm after laser pulse irradiation under severe conditions.

Fig. 6:

Time variations of temperature profiles in the skin model for 100 ms after laser pulse irradiation with (a) Device A (755 nm, 2 mm □, 6.37 J/cm²), (b) Device B (755 nm, 2 mm φ, 18 J/cm²), (c) Device C (1064 nm, 2 mm φ, 10 J/cm²), and (d) Device C (532 nm, 2 mm φ, 2.5 J/cm²) under severe conditions.

Fig. 7:

Maximum temperature after laser pulse irradiation in (a) epidermis, (b) dermis, (c) capillary vessels, (d) upper dermis vessels, (e) deep dermis vessels, and (f) subcutaneous fat.