Acoustic energy absorbed by tissue and thereby converted to heat induces coagulation necrosis within the focus. The morphological characteristics of the lesion change with the applied energy and the time of follow-up.
Immediately after ultrasound exposure with low energy levels, the lesion sometimes cannot be detected macroscopically. Even microscopy only shows an area that is less strongly stained by periodic acid-Schiff, without changes in the cellular structure [38,39]. Ultrastructural examination of the kidney has revealed damage to organelles within the first couple of hours. The initial healing process indicated the presence of these discrete lesions. Medium energy levels induced a sharp lesion that was macroscopically detectable 1 h after HIFU treatment and that was demarcated within the next few days . Focusing on the renal corti-comedullary border resulted in pronounced streaky bleeding of the medulla. Macroscopic and microscopic lesions appeared to be less extensive in the cortex. Nevertheless, focusing directly on the cortex also induced a distinct defect in this area. Histologically, acute changes involved epithelial displacement and epithelial destruction of the affected tubuli. Subsequently, the stroma collapsed, revealing empty medullar tubuli and ducts with slight fibroblastic activity at the margins .
Kohrmann et al.  applied HIFU to healthy kidney tissue of 24 patients immediately before nephrectomy. In 19 of the 24 cases, hemorrhage or necrosis was detected macroscopically. Histologically, interstitial hemorrhages and fiber rupture, as well as collagen fiber shrinkage with eosinophilia, were detected in the focal area. Chapelon et al.  studied the effects of HIFU on rat and canine kidneys (no tumor treatment) and demonstrated lesions consistent with coagu-lative necrosis or cavitation, depending on the duration and intensity of ultrasound. The lesion size also varied, depending on the acoustic intensity and the number of firings. Adams et al.  noted that, histologically, affected cells demonstrated pale eosinophilic cytoplasm and separation from one another. At the periphery of the lesions, areas of hemorrhage were noted in close proximity to normal-appearing tissue. Susani et al.  treated healthy and tumorous tissue in two patients with renal tumors before performing radical nephrectomy. Two renal-cell tumors were excluded from the study because the great amount of tumor necrosis did not allow the target zone to be identified. Clearly demarcated necrosis became hemorrhagic and was later replaced by granulation tissue. The size and location of the lesions corresponded exactly to the previously determined target areas.
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