High intensity focused ultrasound

Overview
HIFU (high intensity focused ultrasound) (sometimes FUS or HIFUS) is a highly precise medical procedure using high-intensity focused ultrasound to heat and destroy pathogenic tissue rapidly. It is one modality of therapeutic ultrasound, and although it induces hyperthermia it should not be confused with this technique which heats much less rapidly and to much lower therapeutic temperatures (generally < 45°C).

This is typically under computerized MRI guidance, when it is sometimes called Magnetic Resonance guided Focused Ultrasound, often shortened to MRgFUS. Magnetic resonance imaging (MRI) is used to identify tumors or fibroids in the body, before they are destroyed by the ultrasound. MRgFUS is currently used in the US, Canada, Israel, Europe, and Asia to treat uterine fibroids. Current clinical trials are underway, examining the possible use of the technique in the treatment of cancers of the brain, breast, liver, bone, and prostate.

Therapeutic ultrasound is a minimally invasive or non-invasive method to deposit acoustic energy into tissue. Applications include tissue ablation (HIFU) (for tumor treatments, for example), hyperthermia treatments (low-level heating combined with radiation or chemotherapy), or the activation or enhanced delivery of drugs.

Aiming
The ultrasound beam can be focused in these ways:-
 * Geometrically, for example with a lens or with a spherically curved transducer.
 * Electronically, by adjusting the relative phases of elements in an array of transducers (a "phased array"). By dynamically adjusting the electronic signals to the elements of a phased array, the beam can be steered to different locations, and aberrations due to tissue structures can be corrected.

How HIFUS works
As an acoustic wave propagates through the tissue, part of it is absorbed and converted to heat. With focused beams, a very small focus can be achieved deep in tissues. When hot enough, the tissue is thermally coagulated. By focusing at more than one place or by scanning the focus, a volume can be thermally ablated. At high enough acoustic intensities, cavitation (microbubbles forming and interacting with the ultrasound field) can occur. Microbubbles produced in the field oscillate and grow (due to factors including rectified diffusion), and eventually implode (inertial or transient cavitation). During inertial cavitation, very high temperatures inside the bubbles occur, and the collapse is associated with a shock wave and jets that can mechanically damage tissue. Because the onset of cavitation and the resulting tissue damage can be unpredictable, it has generally avoided in clinical applications. However, cavitation is currently being investigated as a means to enhance HIFU ablation and for other applications.

Method of use
In HIFU therapy, ultrasound beams are focused on diseased tissue, and due to the significant energy deposition at the focus, temperature within the tissue rises to 65° to 85°C, destroying the diseased tissue by coagulation necrosis. Each "sonication" of the beams treats a precisely defined portion of the targeted tissue. The entire therapeutic target is treated by moving the applicator on its robotic arm in order to juxtapose multiple shots, according to a protocol designed by the physician. This technology can achieve precise "ablation" of diseased tissue, therefore being called HIFU surgery. Because it destroys the diseased tissue non-invasively, it is also known as "Non-invasive HIFU surgery". Anesthesia is not required. The treatment can be combined with radiotherapy or chemotherapy.

Uterine fibroids
Development of this therapy significantly broadened the range of treatment options for patients suffering from uterine fibroids. HIFU treatment for uterine fibroids was approved by the Food and Drug Administration (FDA) in October 2004.

Cancer
HIFU has been successfully applied in treatment of cancer to destroy solid tumors of the bone, brain, breast, liver, pancreas, rectum, kidney, testes, prostate. A lot of the initial studies have been performed by F. Wu and coworkers at the Chongqing Medical University, Chongqing, China. At this stage cancer treatments are still in the investigatory phases as there is a need to find more about their effectiveness.

The earliest widespread use of HIFU ablation was as a treatment for prostate cancer. Developed and refined by two companies in Europe and the United States, this treatment is administered through a trans-rectal probe and relies on heat developed by focusing ultrasound waves into the prostate to kill the tumor. Promising results approaching those of surgery have been reported in large series of prostate cancer patients. These treatments are performed under ultrasound imaging guidance, which allows for treatment planning and some minimal indication of the energy deposition.

In addition, several thousand patients with different types of tumors have been treated in China with HIFU using ultrasound imaging-guided devices built by several different companies. Currently, one of these devices has completed successful clinical trials for liver and kidney cancers at the Churchill Hospital, Oxford, UK, and now has CE Marking so can be used clinically in Europe and other countries where the CE Mark is accepted.

In International HIFU Centers in North & South American physicians use HIFU to ablate the entire prostate gland using a transrectal probe. It is an out patient procedure that usually last 1-3 hours. Results show it greatly reduces side effects common with other treatments for prostate cancer.

Delivering drugs to brain
In current research, HIFU is being used to temporarily break up the blood-brain barrier, allowing an influx of drugs into the brain. It is most effective when used in combination with an inhibitor like verapamil.

Treatment of Atrial Fibrillation
HIFU has been used to treat the most common heart arrhythmia, atrial fibrillation (AF). A minimally invasive catheter based system designed to ablate heart tissue responsible for propagating AF has been approved for use in Europe and is undergoing an FDA approved phase III pivotal efficacy trial in the U.S.

History
The first investigations of HIFU for non-invasive ablation were reported by Lynn et al in the early 1940s. Extensive important early work was performed in the 1950s and 1960s by William Fry and Francis Fry at the University of Illinois and Carl Townsend, Howard White and George Gardner at the Interscience Research Institute of Champagne Ill., culminating in clinical treatments of neurological disorders.In particular High Intensity ultrasound and ultrasound visualization was accomplished stereotaxically with a Cincinnati precision milling machine to perform accurate ablation of brain tumors. Until recently, clinical trials of HIFU for ablation were few (although significant work in hyperthermia was performed with ultrasonic heating), perhaps due to the complexity of the treatments and the difficulty of targeting the beam noninvasively. With recent advances in medical imaging and ultrasound technology, interest in HIFU ablation of tumors has increased.

The first commercial HIFU machine, called the Ablatherm, was developed by the French company EDAP-TMS (NASDAQ: EDAP) and launched in Europe in 2001 after receiving CE approval, bringing a first medical validation of the technology for localized prostate cancer. Comprehensive studies by practitioners at more than one site using the device have demonstrated clinical efficacy at more than 8 years with limited occurrence of side effects. Studies by Murat and colleagues at the Eduard Herriot Hospital in Lyon in 2006 showed that after treatment with Ablatherm, progression-free survival rates are very high for low- and intermediate- risk patients with recurrent prostate cancer (70% and 50% respectively) HIFU treatment of prostate cancer is currently an approved therapy in Europe, Canada, South Korea, Australia, and elsewhere. Clinical trials in the United States are expected to begin in 2006.

Advantages over other techniques
High Intensity Focused Ultrasound is often considered a promising technology within the non-invasive or minimally invasive therapy segments of medical technology. HIFU’s capacity to generate in-depth precise tissue necrosis using an external applicator, with no effect on the surrounding structures, is unique. The history of using therapeutic ultrasound dates back to early in the 20th century. Technology has continually improved and additional clinical applications, both diagnostic and therapeutic, have become an integral part of medicine today.

An important difference between HIFU and many other forms of focused energy, such as radiation therapy or radio surgery, is that the passage of ultrasound energy through intervening tissue has no apparent cumulative effect on that tissue.

Discoveries during use
Currently, the only proven imaging method to accurately quantify the heating produced during HIFU in vivo is Magnetic Resonance Imaging (MRI). MRI also has superior soft tissue contrast and can image in any orientation, making it the state of the art for guiding HIFU treatments. Clinically, MRI-guided HIFU treatments have been tested for uterine fibroids, breast fibroadenomas, breast cancer, bone metastases, and liver tumors. The largest number of patients treated with MRI-guided HIFU have been with uterine fibroids.

Ultrasound-guided HIFU treatments have been approved in Europe and Asia. MRI-guided treatments of uterine fibroids have been approved in Europe and Asia, and were granted FDA approval in the US in 2004.

Organizations
The International Society for Therapeutic Ultrasound, founded in 2001, aims to promote clinical, academic and industrial advancement in Therapeutic Ultrasound. Its primary activity is the annual International Symposium on Therapeutic Ultrasound, which has attracted experts in HIFU from throughout the world.

The Foundation for Focused Ultrasound Research is an unincorporated association promoting research into medical uses of high intensity focused ultrasound, including HIFU.

The Focused Ultrasound Surgery Foundation (FUSF) is working to shorten the time from technology development to patient treatment, develop new applications and accelerate the worldwide adoption of MR-guided focused ultrasound surgery.