High-intensity focused ultrasound (HIFU) is a non-invasive therapeutic technique that uses non-ionizing ultrasonic waves to heat or ablate tissue. HIFU can be used to increase the flow of blood or lymph or to destroy tissue, such as tumors, via thermal and mechanical mechanisms. Given the prevalence and relatively low cost of ultrasound generation mechanisms, The premise of HIFU is that it is expect a non-invasive and low-cost therapy that can at minimum outperform operating room care.
The technology is different from that used in ultrasonic imaging, though lower frequencies and continuous, rather than pulsed, waves are used to achieve the necessary thermal doses. However, pulsed waves may also be used if mechanical rather than thermal damage is desired. Acoustic lenses are often used to achieve the necessary intensity at the target tissue without damaging the surrounding tissue. The ideal pattern diagram is the beam-focusing of a magnifying glass of sunlight; only the focal point of the magnifying glass has high temperature.
HIFU is combined with other imaging techniques such as medical ultrasound or MRI to enable guidance of the treatment and monitoring.
History
Studies on localized prostate cancer showed that, after treatment, progression-free survival rates were high for low- and intermediate- risk patients with recurrent prostate cancer. The InsighTec ExAblate 2000 was the first MRgFUS system to obtain FDA market approval.
Medical uses
There is no clear consensus on the boundaries between HIFU and other forms of therapeutic ultrasound. In particular literature, HIFU refers to the high levels of energy required to destroy tissue through ablation or cavitation, although it is also sometimes used to describe lower intensity applications such as occupational therapy and physical therapy.
Either way, HIFU is used to non-invasively heat tissue deep in the body without the need for an incision. The main applications are the destruction of tissue caused by hypertharmia, increasing perfusion and physical therapy. The use of ultrasound in the treatment of musculoskeletal conditions is another use in the physiotherapy setting.
Neurological disorders
One of the first applications of HIFU was the treatment of Parkinson's disease in the 1940s. Although ineffective at the time, HIFU has the capacity to lesion pathology. A focused ultrasound system is approved in Israel, Canada, Italy, Korea and Russia to treat essential tremor, neuropathic pain, and Parkinsonian tremor. This approach enables treatment of the brain without an incision or radiation. In 2016, the US Food and Drug Administration (FDA) approved Insightec's Exablate system to treat essential tremor. Treatment for other thalamocortical dysrhythmias and psychiatric conditions are under investigation.
Cancers
Prostate
HIFU may be effective for detecting prostate cancer.
Liver
HIFU is studied in liver cancer and in many studies report a high response rate and positive outcome. During the treatment of metastasized liver cancer with HIFU, immune responses have been observed in locations that are distant from the focal region.
Prostate enlargement
Treatment of prostate enlargement (benign prostatic hyperplasia) by HIFU from inside the intestine (transrectal) has turned out to be unsuccessful.
In some countries, not in USA, HIFU has also been offered from the inside of the prostate, that is, via a catheter in the prostatic urethra. Evidence as of 2019 is lacking.
In England the National Institute for Health and Care Excellence (NICE) in 2018 classified the method as "not recommended".
Mechanism
HIFU beams are precisely focused on a small region of diseased tissue to locally deposit high levels of energy.
Focused ultrasound may be used to generate highly localized heating to treat cysts and tumors (benign or malignant). This is known as Magnetic Resonance guided Focused Ultrasound (MRgFUS) or High Intensity Focused Ultrasound (HIFU). These procedures generally use lower frequencies than medical diagnostic ultrasound (from 0.7 to 2 MHz), but higher the frequency means lower the focusing energy. HIFU treatment is often guided by MRI. Focused ultrasound may be used to dissolve kidney stones by lithotripsy. Ultrasound may be used for cataract treatment by phacoemulsification.