Changes in ultrasonic elastometry parameters of the liver parenchyma during its radiofrequency ablation (experimental study)

Abstract

Objective. To determine in the experiment the changes in the elasticity of the liver parenchyma during its radiofrequency ablation at different distances from the electrode and their correspondence to the zones of irreversible thermal damage of the tissue. Materials and methods. The elasticity of the parenchyma of six samples of isolated porcine liver during radiofrequency ablation in automatic mode for 12 min with an initial applicator power of 50 W and its subsequent automatic increase by 10 W/min until critical impedance values were reached was evaluated by ultrasonic elastometry with the determination of the Young's modulus. The elasticity of the liver in kilopascals was determined before the start of radiofrequency ablation, during its implementation every minute for 12 minutes and 15, 30 and 60 minutes after the procedure in three zones located at a distance of 1.0, 1.8 and 3.0 cm from the applicator. Results. Before radiofrequency ablation, the elasticity of the liver parenchyma ranged from 4.1 to 9.3 kPa and averaged (6.64 ± 1.55) kPa. At the maximum power of the applicator – (109.67 ± 4.97) W – the transverse size of the hyperechogenic “cloud” at the 12th minute of the procedure was (18.0 ± 1.41) mm. The value of Young's modulus in the first zone of elastometry statistically significantly increased from the 1st minute of radiofrequency ablation and by the 11th minute reached the level of (46.38 ± 5.43) kPa and did not change significantly thereafter. In the second zone, a statistically significant increase in the value of Young's modulus to (44.22 ± 6.55) kPa was observed throughout the procedure, and after its termination it changed statistically insignificantly. In the third zone, changes in the value of Young's modulus occurred 3 minutes after the start of the procedure and continued until its completion, but its maximum value – (15.63 ± 1.57) kPa – exceeded the baseline level only about 2 times, and an hour after the completion of radiofrequency ablation, the value of Young's modulus decreased statistically significantly. Conclusions. The stiffness of the parenchyma of isolated porcine liver increases significantly during radiofrequency ablation under conditions of its sufficient duration, and depending on the distance to the electrode, these changes have different phase character. In loci corresponding to the zone of irreversible tissue necrosis, the initial slow approximately twofold increase in Young's modulus during the first 3 to 4 minutes is followed by a rapid exponential increase in the next 5 to 6 minutes and the formation of a plateau with 6 to 8 times the initial level, after which the index does not change significantly. To determine the edge of liver parenchyma ablation by elastometry, in addition to the absolute value of Young's modulus at the end of radiofrequency ablation and the multiplicity of its increase relative to the baseline value, such criteria as the three–phase nature of the increase in this indicator and the absence of its decrease within an hour after the procedure are equally important.