High Intensity Focused Ultrasound (HIFU)
The hyperthermia therapy technique using high intensity focused ultrasound (HIFU) was developed in the 1940s as an imaging tool which uses temperature to treat a variety of diseases. HIFU has been successfully applied in the treatment of cancer to destroy solid tumours of the bone, brain, breast, liver, pancreas, rectum, kidney and testes. In addition, it can be used to treat uterine fibroids, benign thyroid nodules, hypertrophic parathyroid gland ablation and for breast fibroadenoma ablation.
Currently, there is an emerging technique called transcranial magnetic resonance guided focused ultrasound (tcMRgFUS surgery): this is a non-invasive treatment for a variety of brain disorders. In 2015, the FDA gave approval for HIFU in the ablation of prostate tissue. This procedure uses a transrectal probe capable of transferring the heat from the focused ultrasound waves to destroy the cancerous prostate cells.
HIFU has advantages over conventional modalities such as surgery, radiotherapy and chemotherapy because this procedure treats tumours non-invasively is non-radioactive and has fewer complications after treatment. To date, 100,000+ cases have been treated in clinics with HIFU.
The theory behind HIFU ablation involves the important property of thermal necrosis due to the absorption of ultrasound energy. During transmission, the sound energy intensifies at the tissue site and induces cavitation damage. The focused ultrasound energy applies local heat and destroys the diseased tissue through ablation. This elevation of temperature to 90 degrees means no tissue can survive.
The FDA gave approval to the HIFU system called Sonablate 450 for the treatment of prostate cancer. HIFU is currently being used to treat uterine fibroids and to relieve pain from bone metastases. Clinical HIFU procedures are typically image-guided to permit treatment planning and targeting before applying a therapeutic or ablative level of ultrasound energy.
Magnetic Resonance-guided Focused Ultrasound (MRgFUS)
The aim of magnetic resonance-guided focused ultrasound (MRgFUS) is to deliver focused high-energy ultrasound waves into the tissue to cause thermal coagulation of the targeted tissue. The ultrasound transducer contains a piezoelectric plate which is capable of producing high-energy sound waves. During this process, the ultrasound field and vibration frequency are dependent on the shape and size of the organ(s) and/or tissue within the human body.
The ultrasound waves can be focussed using a combination of lens and/or reflectors. In some cases, the transducer can automatically adjust the sound wave according to the medical scenario. situation. The usage of an ExAblate machine allows for automatic focusing to be performed using phased arrays. These phased arrays permit the generation of a larger focal spot – in addition to controlling the location of the focus by the phase and amplitude of the radio-frequency (RF) signals – driving each component.
The phased array components can focus the ultrasound waves into a beam and can also produce multiple beams which can cause intensification at the target. These sound waves can pass through the skin and focus directly on a particular target (e.g., tumour) to deliver the energy. The ExAblate system contains a phased-array transducer with 208 array elements that are individually controlled. Also, there is a computer-controlled positioning system, a multichannel RF amplifier system and a user interface. All of these modules integrate with the MRI (1.5 T to 3 T) system to give guidance to the ultrasound therapy at the disease site.