Generally, a disturbance of mechanical energy that propagates through a medium in the forms of waves is defined as sounds. Meanwhile, ultrasound (US) is a form of sound that has a greater frequency which is larger than 20 000 Hz or 20 kHz . In other words, the mechanical vibrations above the detection of human hearing is also called ultrasound . Actually, sound can carry energy from its source to another part of area as long as the existence of the medium. Frequently, Ultrasound is used by all clinicians for diagnostic imaging which using the range frequency of 1- 10 MHz  and it is more likely caused no detrimental effect on the human body within the diagnostic ranges used. HIFU transducer delivers ultrasound with intensities of 10-1000 W/ at the focal spot and with peak compression pressures up to 30 MPa and peak rarefaction pressures up to 10 MPa
Usually, HIFU therapy takes the advantages of US waves as carriers of energy which can be propagated through human tissue or the specific target area. HIFU has used 2 methods to propagate the energy to the target area; firstly, US waves is generated from much number of sources and secondly, it will focus the energy into small spot.
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The important parameter for ultrasound is sonic intensity and the SI unit for sonic intensity is W/. It can be defined as a time average rate of sonic energy flow through a unit area. The sonic intensity is proportional to sonic pressure square and has positive correlation of sound . This means the higher the sonic intensity, the greater the energy accumulation at the target area. The relationship for sonic intensity and other parameter is summarized up at page 3.
Sonic intensity varies with space and time, and it usually expressed as peak or average intensity, and both quantities can refer to either spatial or temporal dimension . The higher intensity of US refers to average intensity higher than 5 W/. This higher energy can caused the coagulation necrosis of tissue and is used for ultrasonic surgery. However, Low average intensity of ultrasound usually ranges between 0.125-3 W/ causes non-destructive heating and eventually will stimulates physiological responses to an injury. The lower range of intensity usually used for physiotherapy.
The figure 3 shows the basic concepts for HIFU. When the ultrasound waves are focused into small tiny spot, the acoustic pressure is rapidly increases near focus where the tissues temperature also elevated to the sufficient level for effects of coagulation necrosis. The temperature for tissue increases as the more focusing US waves towards the target. Therefore, the tissue near the ultrasound source will has a low energy or acoustic pressure and it will spare the normal tissue near the source instead of damaging the inner target tissue. So, the HIFU is practically used for the invasive therapy for tumor ablation.
Mechanism of therapeutic action
The mechanism of tissue damage caused by ultrasound is consists of two predominant mechanisms. Firstly is by the conversion of mechanical energy into heat (thermal effects) and the second is through cavitation (mechanical effects) .
Heat generation due to absorption of the acoustic energy becomes the major effect of high acoustic intensity in tissue. The rapidly increase of temperature to or higher in tissue causing coagulation necrosis within a few seconds. The focusing causes the higher intensity at specific target area and over small volume. This focusing process minimizes the potential of thermal damage of tissue located between transducer and focal point because the intensities are much higher only the focal region.
This effect includes cavitation, microstreaming and radiation forces. Usually the mechanical effects are associated at high intensity but are not present at lower intensities. Cavitation is the creation or motion of gas cavity in acoustic field . Cavitation happens when alternating compression and expansion of tissue as an ultrasound field propagates through it. When the tissue expands or rarefaction pressure is of sufficient magnitude, gas can be extracted in the tissue causing bubble formation. The bubble literally further interact with ultrasound field. Cavitation also has two things to be considered; first is stable cavitation which bubble is exposed low pressure of acoustic field  resulting the stable oscillation of the bubble size. The second one is inertial cavitation in which the violent collapse and destruction of the bubble during exposure of the bubble or rarefaction phase. When inertial cavitation occurs near cell membrane, someone might anticipate mechanical, rather than thermal damage .
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Microstreaming is the phenomenon resulting from stable cavitation which is rapid movement of fluid near the bublle due to oscillating properties. It can produse high shear of forces that can destroy cell membranes. Meanwhile, radiation forces is developed when a wave is absorber or reflected.
Biological effects of HIFU usually are caused by thermal and cavitation and thermal mechanisms. The results of HIFU will lead to tissue heating, necrosis, apotosis and cell lysis. Coagulation necrosis always happens in tissue exposed to high intensity ultrasound when the temperature of tissue is elevated to a certain level for a certain time or in simple words the coagulation is time-dependent. The shock elevated of tissue temperature to more than C for 1 second will lead to instantaneous cell death (necrosis). This type of cell death is the primary mechanism for tumor cell destruction fot HIFU therapy.
The surgery of HIFU is initiated by Lynn et. al for destruction of central nervous system and this method is expanded or continued on by others for other clinical trials for example to treat benign and malignant tumors for prostate cancer, bowel bladder an also kidney . The HIFU technique is established by the Frank and William Fry and also for work on ablation for brain tissue. The development of HIFU projects lead to the progress of transducer of HIFU and also combination of powerful diagnostic tools such as ultrasonography and magnetic resonance imaging (MRI).
Piezoceramic transducer is a source for HIFU which has property to changing its thickness response to applied voltage. The voltage will create acoustic ultrasound wave frequency according to the level of voltage applied. Usually the frequency used in HIFU is 0.5 ââ‚¬" 10 MHz. typically, intensity should be applied above temperature threshold but yet below the cavitation threshold to avoid damage to surrounding structures . Depending on the frequency, this ranges between 750-4500 W/. Modern piezoceramics can operate with long term output consist and stable with focal therapy device requirements .
The HIFU beam is achieved either by placing lens in front of the transducer or made up the transducer the spherical geometry. The tissue of interest and the transducer coupling should be free of air and usually low loss media is applied such as degassed water. Shape of focal zone can change depending on exposure time, focal length.
Transducer is used for focusing the wave to the target tissue and various method focusing have been discussed. The simplest method is by spherically curved US transducer which has a beam focus fixed at the position from geometrical properties of transducer. After that, flat transducer was used for enable variation in focal length and focal geometry. Currently, phased array transducer is used to enable beam steering and focusing while focal spot can move for any direction within physically allowed ranges.
Although there is a lot of number of devices are used for prototype models, but there are lack devices for current clinical use. The two main types of device are extracorporeal and transrectal. The difference between both types is extracorporeal devices have been used to target many organs. The extracorporeal require a longer wavelength than transrectal sources on principle. The extracorporeal tend to employ transducer of larger dimensions and operate at lower frequencies and higher intensities than translectal devices. Transrectal device is designed to ablate prostate cancer tissue and incorporating small imaging and therapy transducer on a single probe.
Prototype device for extracorporeal types that be applied for commercial use is built by ter Haar et al which employs a spherical (PZT) ceramic transducer with 10 cm diameter and 15 cm focal length, driven at a frequency 1.7 MHz and operating at free field spatial intensities between 1000 W/ and 4660 W/.
Then, technology of HIFU has been developed and designed by Chongqing HAIFU Technology Company, P.R. China where they used lead zirconate titanate (PZT) transducer with focal length 10-16 cm and driven at a frequency 0.8 MHz or 1.6 Mhz and the intensity can reach up to 20 000 W/. It has also built in 3.5 MHz diagnostic scanner. The advanced HIFU also can operate with MRI compatible 10 cm diameter of focus transducer with an 8 cm of curvature operating at 1.5 MHz. This HIFU-MRI is developed by GE Medical Systems Milwaukee, WI.
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Generally, extracorporeal HIFU is used for transcutaneous in tissue depth ablation for organ accessible for ultrasound .
Meanwhile, Sonablate, Focal Surfery, Milpitas,CA use a 4 MHz PZT transducer for both imaging and also treatment. The intensity at the focus is 1680-2000 W/ and depending on its focal length which usually can be 3.0 cm, 3.5 cm or 4.0 cm. the focal length is dependant on the crystal used . For obtaining an image update as well as moving the transducer electronically to the next treatment location, 4 s interval of therapy which is power is switched on and then followed by 12 s power off.
Other device uses a 2 separate transducer which has develop by Ablatherm, Technomed International, Lyon, France. First transducer is rectangular transducer that has focal length of 4 cm and driven at 2.20 MHz that can produce intensity up to 1000 W/. Second transducer is for imaging part which has 7.5 MHz of frequency.