A study to evaluate the effectiveness and safety of ExAblate transcranial MRgFUS Thalamotomy treatment of medication refractory tremor-related diseases subjects with low skull density ratio (SDR) value. Essential tremor (ET) and Parkinson disease (PD) are the most prevalent tremor disorders. ET, considered as a pure tremor disease, is characterized by upper limb intention or postural tremor, while PD is characterized by a variety of motor and non-motor symptoms, among them rest tremor. A number of studies have demonstrated that Magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy is a minimally invasive and effective procedure suitable for medication-refractory tremor in patients with ET and patients with PD. However, the skull is the main barrier to MRgFUS thalamotomy therapy and patients are screened by calculating SDR value before treatment. The US FDA recommended SDR value \>0.45±0.05 as the inclusion criterion for screening patients with tremor treated by MRgFUS system. However, about 20%-50% of patients with SDR value are lower than this standard, which makes this part of patients excluded and unable to accept this treatment with many advantages such as non-invasive, no need for general anesthesia, and no need for hardware implantation. Therefore, this study intended to evaluate the safety and effectiveness of MRgFUS thalamotomy in the treatment of tremor-related patients with low SDR value, so as to provide clinical basis for more patients with tremor to benefit from this treatment.