## 1 Introduction  
Hyperthermia therapy (HT) is used to treat diseases through maintaining high-temperature heating of the whole body or a part for a long period, which is usually used alone or in conjunction with some other medical therapeutics such as chemotherapy and radiotherapy [1]. Many successful clinical trials have applied the therapy to treat different types of cancer tumors, such as recurrent breast cancer, liver cancer, uterine fibroids and bladder cancer [2; 3; 4; 5]. HT is mainly divided into three categories: local HT, regional HT and whole-body HT [6]. The purpose of local HT is to increase the temperature of the tumor without affecting the surrounding normal tissue. Generally, local HT is applied to treat tumors below the skin or in body cavities near the surface of the body through using external or interstitial heating modalities [7]. In contrast, regional HT is more complicated than local HT because its targets are deep-seated tumors in body cavities, especially locally advanced tumors. Non-invasive methods to realize regional HT include heating the blood, irrigating the body cavities and radiofrequency (RF). Whole-body HT is usually applied to treat metastatic cancer by using surface heating, radiation induction or extracorporeal induction. During the entire hyperthermia treatment period, the temperature of the tumor and surrounding tissues needs to be strictly controlled at the expected temperature. Among many techniques such as microwave, infrared and RF,high-intensity focused ultrasound (HIFU) is increasingly being developed as a promising modality to induce hyperthermia for clinical tumor treatment combined with diagnostic ultrasound guidance [8; 9]. For many kinds of benign and malignant tumors, HIFU plays an auxiliary role in the current standard treatments such as surgery, gene therapy and radiation. It was first reported by [PERSON] et al. in the early 1940s that HIFU can pass through tissue without harm and cause tissue ablation in a focal area. In the 1950s and 1960s, significant early work including high-intensity ultrasound and ultrasound visualization was completed. By the late 1990s, commercially available ultrasound-guided HIFU devices had been developed [10]. Extensive advances in phased-array applicator technology have enabled HIFU to perform better in focusing [11]. With the advances in imaging, the accurate focus position makes clinical application possible [12]. Magnetic resonance imaging (MRI) and ultrasound imaging are two guiding modalities with complementary guiding features [13; 14; 15] which have few side effects in humans. HIFU uses the characteristic that ultrasound can penetrate the human body without damage and focus on tumors in vivo causing a thermal effect (main effect), cavitation effect and mechanical effect. Since the high temperature generated by thermal effect on the focus can destroy the cell structure and damage the protein, it will lead to the coagulation necrosis of the tumor target [16; 17; 18] instead of burning the surrounding normal tissue. The sensor emits high-intensity ultrasound signals to the tissue target, and then the tissue absorbs ultrasound energy and converts energy into thermal energy during therapy. This heat deposition can cause the rapid temperature rise of tissue. Due to the focusing of the ultrasound beam, thermal energy is mainly concentrated on the tumor target, improving the temperature to the desired range (e.g., 5 \({}^{\circ}\)C-6 \({}^{\circ}\)C higher than the human body temperature). Therefore, there is almost no obvious heat deposition on the surrounding tissue, which avoids the unpredictable growth of healthy tissue caused by excessive temperature [19]. There are three typical types of HIFU equipment systems that have been put into clinical practice for treatment of tumors: HIFU systems guided by MRI (MRgHIFU), HIFU systems guided by ultrasound (USgHIFU) and HIFU systems guided by both MRI and ultrasound.