Focused ultrasound treatment for central nervous system disease: neurosurgeon's perspectives

The concept of focused ultrasound (FUS) and its application in the field of medicine have been suggested since the mid-20th century. However, the clinical applications of this technique in central nervous system (CNS) diseases have been extremely limited because the skull inhibits efficient energy transmission. Therefore, early application of FUS treatment was only performed in patients who had already undergone invasive procedures including craniectomy and burr hole trephination. In the 1990s, the phased array technique was developed and this enabled the focus of ultrasonic energy through the skull, and in conjunction with another technique, magnetic resonance thermal monitoring, the possibility of applying FUS in the CNS was further strengthened. The first clinical trial using FUS treatment for CNS diseases was performed in the early 21^st century in patients with glioblastoma, which consists of highly malignant primary brain tumors. However, this trial resulted in a failure to make lesions in the tumors. Various causes were suggested for this outcome including different acoustic impedances across heterogeneous intracranial tissue (not only brain tissue, but also fibrous or tumor tissue). To avoid the influence of this factor, the targets for FUS treatment were shifted to functional diseases such as essential tremor, Parkinson's disease, and psychiatric disease, which usually occur in normal brain structures. The first trial for functional diseases was started in 2010, and the results were successful as accurate lesions were made in the target area. Nowadays, the indication of FUS treatment for functional CNS diseases is gradually widening, and many trials using the FUS technique are reporting good results. In addition to the lesioning technique using high intensity FUS treatment, the possibility of clinical application of low intensity FUS to CNS disease treatment has been investigated at a pre-clinical level, and it is expected that FUS treatment will become one of the most important novel techniques for the treatment of CNS diseases in the near future.