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Fig. 5. The interpulse interval along a scan path lies in the sub-microsecond range, whereas inter-track intervals occur on the millisecond scale. A) Symmetric spacing increases the likelihood of bubble interference, as it does not exploit the spatial and temporal separation domains that limit pulse – bubble interactions. B) Asymmetric spacing minimizes consecutive spot overlap, reducing bubble interference from residual cavitation activity. The longer inter-track intervals allow cavitation bubbles to fully expand and dissipate before pulses from adjacent lines arrive. Here, individual cavitation events( bubbles 1, 2, 3,..., n) are illustrated using distinct colors to indicate their sequential generation.
p ¼ L ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
½ðp = 4Þð1 � UÞŠ, whereU is the areal fill fraction, and
L is the characteristic bubble projection diameter. This definition is calibrated such that square packing at kissing contact( U = p / 4 0.79) corresponds to‘/ L 0.414, consistent with the diagonal extent of the central diamond-shaped void. Although‘ does not represent the diameter of an equal-area circle, it provides a convenient and physically meaningful scaling for the linear size of residual gaps across different packing geometries. Values‘/ L [ 0.3 – 0.5 correspond to fill fractions between approximately 75 % and 90 %, consistent with empirically observed conditions for effective tissue separation and the optimum window of the overlap fractions defined in this work.
It should be emphasized that overlap factors F1 andF2 can be regarded as universal( or generic) in a geometric and dimensional sense, not in the sense of being independent of biological or optical tissue properties. The parameters are dimensionless geometric overlap frequencies, defined as the ratio between the projected cavitation bubble diameter and the laser spot spacings along and across the scan pathway, respectively; equivalently, their reciprocals may be interpreted as normalized spacings. Owing to their dimensionless nature, these factors are applicable to any bubble size, independent of the absolute spatial scale of a given laser system, and allow direct comparison across different devices, pulse energies, and clinical platforms. While tissue biomechanics and optical properties influence the absolute bubble size, and thus the effective numerical values of F1 and F2, the underlying overlap physics is governed by these universal geometric ratios.
Regarding the effectiveness of asymmetric cutting and the optimized result, it would be ideal to design experiments
to directly validate it, since experimental proof will always significantly increase the impact of the work. The novel task in this work is to help formally determine( from objective means) both dose and asymmetry( within optimum or at least adequate levels). The fact that all three aspects:( lower) pulse energy,( lower) dose, and( higher) asymmetry( positively) affect outcomes is evident and apparently universal, from previous literature.
Arba-Mosquera et al. [ 27 ] reported, aligned with other groups, that consistently in a large cohort consisting of three different countries, lowering the energy was one of the key factors to improve outcomes.
Arba-Mosquera et al. [ 26 ] mathematically modelled and demonstrated that too high energies are not consistent with minimum treatment fluence( due to the non-linear increase of bubble size with energy, i. e., above an energy range, an increase in pulse energy results in a fractional increase in bubble size, and under-proportional reduction in the required number of pulses; whereas below an energy range, a decrease in pulse energy results in a dramatic decrease in bubble size, with over-proportional increase in the required number of pulses). Further, this work interestingly opened the rigorous venue for asymmetric spacings. Using asymmetric settings, the optimum window shifts towards lower energies in a very relevant manner( without increasing the total treatment fluence). So that working with asymmetric spacings allows to effectively and safely work closer to the LIOB threshold.
Pradhan and Arba Mosquera [ 28 ] compared two ranges for pulse energies( ~ 115 nJ vs. ~ 90 nJ, with symmetric and asymmetric settings, respectively) for similar treatment fluence( ~ 750 mJ / cm 2) and demonstrated that higher energies using symmetric settings provided not as good outcomes