Extracorporeal ultrasound-guided high intensity focused ultrasound therapy: Present limitations

Extracorporeal ultrasound-guided high intensity focused ultrasound (USgFU) ablates solid lesions non-invasively. Limitations manifested in preclinical and clinical trials are discussed in brief. The ablation efficiency (necrosis rate) is low. Tissue responses are monitored in real time during USgFU treatment, based upon the appearance of hyperecho within the insonated volume. However, hyperecho does not necessarily indicate tissue necrosis, with a low specificity and negative predictive value. A preoperative treatment plan cannot predict the ablation outcome satisfactorily. A reflection interface in the travel path of therapeutic ultrasound obstructs the propagation, refocuses beams and shifts the focus or burns adjacent tissues. A diagnostic transducer is located in the center of a therapeutic one in a USgFU device; hence there is a blind field of the diagnostic transducer; tissues within this area cannot be constantly observed during HIFU insonation. The motion of an organ may shift a target lesion leading to untoward tissue damages. The drastic variance between tissue types/individuals is the greatest challenge for the standardization of USgFU treatment. The quantitatively clinical data of limitations are still unavailable, and USgFU therapy is with a low level of evidence from the perspective of evidence based medicine. Some potential solutions are introduced briefly. Key words: High intensity focused ultrasound, ultrasound imaging, ablation efficiency, real-time monitoring, treatment plan.