To investigate the ultrasonic features of rabbit skeletal muscle injury and regeneration induced by microwave thermal ablation, and to observe histological findings and laboratory tests respectively.

Experimental rabbit models of skeletal muscle injured by microwave ablation (2450 MHz) with different power were established. 40 rabbits were randomly divided into two groups with 20 rabbits in each group, group 30 W and group 50 W. At different time points, before injury and one hour, one day, two days, 7 days, 28 days after injury, the corresponding 2D sonograms, color Doppler flow imaging (CDFI), contrast-enhanced ultrasound (CEUS) features and microscopic findings from the region of interest (ROI) taken from either model were analyzed and compared statistically, laboratory tests of serum markers CK, LDH and TnT level were also be detected dynamically.

(1) After different power microwave ablation, 2D sonograms showed there were distinct changes of the textures in both injured regions compared to the normal status, characterized in disappearance of their regular arrangement and directions. The echo level in ablation regions of two groups increased one hour after injury, but decreased gradually later, especially at one day after injury. Compared with regions of one hour after injury, the maximum diameters became larger significantly in two groups at one, two hour after injury [the maximum diameter of group 30 W, one day vs one hour: (23.1±5.8 vs 10.0±3.1) mm; the maximum diameter of group 50 W, one day vs one hour: (32.4±4.6 vs 16.0±4.1) mm, t=-7.72, -8.31, P<0.01; the maximum diameter of group 30 W, 2 day vs one day: (25.7±1.8 vs 23.1±5.8) mm, t=-2.35, P<0.05], while the maximum diameters became smaller significantly in two groups at 7 days, 28 days after injury respectively [the maximum diameter of group 30W, 7 days vs 2 days: (20.8±3.4 vs 25.7±1.8) mm, t=-2.23, P<0.05; the maximum diameter of group 50 W, 28 days vs 7 days: (23.6±2.6 vs 28.5±4.6) mm, t=-4.32, P<0.01]. At different time points after injury, the maximum diameter in group 50 W was significantly larger than in group 30 W [(16.0±4.1 vs 10.0±3.1) mm, (32.4±4.6 vs 23.1±5.8) mm, (31.0±5.0 vs 25.7±1.8) mm, (28.5±4.6 vs 20.8±3.4) mm, (23.6±2.6 vs 18.2±1.5) mm, t=-3.23, -3.59, -2.99, -6.53, -2.17, all P<0.05]. There were no blood signal and contrast agents displayed on CDFI or CEUS images after injury, and group 50 W images often showed larger filling-defect areas compared with group 30W. However, 7~28 days after injury, with the regeneration of muscular fibers, injured regions of the two groups were repaired gradually. The injured muscle texture appeared clearly and peripheral blood signals could be detected as grade Ⅱ-Ⅲ on CDFI images. No contrast agents area on CEUS images became smaller. (2) The corresponding histopathology revealed identifiable alterations such as muscular fibers degeneration, necrosis, regeneration, fibrosis or nourishing blood vessels variation, which contribute to the differentiation to normal status and between the two groups on ultrasonograms. (3) Compared with normal status, one hour after injury, serum CK level increased significantly in two groups [group 30 W: (1.58±0.67 vs 0.64±0.31) kU/L; group 50 W: (1.84±1.19 vs 0.64±0.31) kU/L, t=-4.623, -4.768, all P<0.01], while serum LDH level in group 50 W was also obviously higher than in group 30 W [(7049.60±3328.60 vs 4155.80±1745.80) U/L, t=-2.594, P<0.05]; one day after injury, serum LDH and TnT level in both two groups increased significantly [LDH in group 30 W: (8486.82±2438.90 vs 4155.8±1745.8) U/L; LDH in group 50 W: (9091.86±1068.50 vs 4155.8±1745.8) U/L, t=-4.762, -6.515, P<0.05; TnT in group 30 W: (21.67±8.80 vs 12.37±4.34) μg/L, TnT in group 50 W: (25.64±4.19 vs 12.37±4.34) μg/L, t=-3.306, -5.194, all P<0.01]; two days after injury, serum LDH level in both groups increased significantly [group 30 W: (7116.2±1887.2 vs 4155.8±1745.8) U/L, group 50 W: (8494.57.86±1199.30 vs 4155.8±1745.8) U/L, t=-3.289, -5.910, all P<0.01].

Different power of Microwave thermal ablation can cause different histopathologic changes and laboratory tests results in correspondence with various features on multimode ultrasonograms. It is visible and useful for ultrasonography to demonstrate the texture characteristics of skeletal muscle injured by thermal factors, and it will be helpful to guide the explorative ultrasound diagnosis and evaluation, and to provide complementary information of skeletal muscles injury and repair characterization.