Introduction: Mechanical index (MI) is used for quantifying acoustic cavitation and the relationship between acoustic pressure and the frequency. In this study, modeling of the MI was applied to provide treatment protocol and to understand the effective physical processes on reproducibility and gene expression of fibroblast cells. Skin damage can occur of burn, cuts, abrasions and ulcers to varying degrees of severity. Fibroblasts are the major cell type in connective tissue and are critical for tissue restoration and remodeling after injury. Therapeutic ultrasound is commonly used to enhance the repair of tissue injuries and reduce associated pain. It was noted that these effects required specific exposure conditions
. So in this study, MI of threshold 0.2 in water and heigher (0.4) and lower (0.1) than it were investigatrd on the proliferation and collagene type 1 expression of fibroblast.
 
Materials and Methods: The acoustic pressure and MI equations are modeled and solved to estimate optimal MI for 1MHz and 40 kHz frequencies. Radial and axial acoustic pressure distribution was extracted. mechanical index of 0.10, 0.20, 0.40 that were compared with control and sham groups in continuous mode. The mature guniea pig skin was cut into small pieces (1mm2) and explanted in Medium. fibroblast cells isolated and growed of the skin explant after 25 days. The proliferation was calculated as a ratio of optical density (OD) of cells in MTT assay and type1 collagene expression was quantified with real time PCR utilizing SYBR.
 
Results: The optical density(OD) of experimental groups was normalized to that of corresponding 80000 groups at the day of 5th. Increasing reproducibility in the group of 0.21 mechanical index had the maximum reproducible (0.62±0.05) compared to control group (0.38±0.03). Fold change of expreesion level of fibroblast cells in 0.2 mechanical index has been achieved 4.35 ±0.03 compared to the control group.
 
Conclusion: Low intensity ultrasound with 0.20 MI is more effective on the proliferation rate and type1 collagene of Fibroblast cells. This model provides proper treatment planning in vitro and in vivo by estimating the cavitation phenomenon. Finally, the effect of low intensity ultrasound based on MI in the best limit obtained and suggested to used in skin wound healing, cell therapy and transplant.