Ultrasounds

With echographic devices, it is possible to construct an image which leads to the most common medical application of ultrasound: imaging. Ultrasound waves deposit energy as they travel through tissue but the amount deposited during ultrasound imaging is completely insignificant. Ultrasound imaging is harmless and is so safe that it is used to image unborn babies. The premise behind HIFU is the destruction of tissue by depositing huge amounts of energy into it. This is accomplished by doing two things: increasing the intensity of the waves (similar to turning up the volume) and focusing the waves on a single point (like a magnifying lens). If done in the right conditions it will raise the temperature of tissue to a level sufficient to cause irreversible tissue damage (ablation).

HIFU: High Intensity Focused Ultrasound

HIFU technology uses a high-intensity convergent ultrasound beam generated by high power transducers to produce intense and immediate heat. HIFU is intended to allow the surgeon to necrose prostatic tissue without damaging intervening and surrounding tissue, thus eliminating the need for incisions, transfusions, general anesthesia and their resulting complications or radiations.

History of HIFU

The development of ultrasound transducers started with the work of P. Langevin, who used, in the 20th century, the piezoelectric properties of quartz crystal to build the first submarine sonar.

In the 1950s, the Fry brothers - Francis and William - imagined the first medical application of ultrasonic waves.

Their first works were related to the extra-corporeal treatment of neurologic disorders - Parkinson disease. Using a set of ultrasound transducers focused on the area to be treated, they could realize tiny biological lesions located deep inside the cerebral cortex. But the lack of an imaging device with enough performances and accuracy stopped the development of this type of therapies.

In the 80s, Lizzi, with a more advanced technology, set up a device for the treatment of glaucoma and intra-ocular tumors. This approach has been rapidly replaced by laser.

At the end of the 80s, the INSERM - French National Institute for Medical Research - Lyon Hospitals (France) and EDAP TMS, engaged in a research program on the interaction of High Intensity Focused Ultrasound (HIFU) on prostatic tissues. The main purpose of this work was to develop applications to treat malignant tumors: the prototype of the Ablatherm® was born and in 1993 a clinical trial with the first treatment on human was performed.

What Are the Physical Principles?

Ultrasound wave emission is based on transducer vibration. This results in dilatation and contraction modification of acoustical pressure. The acoustical pressure creates tissue movement (dilatation and contraction) which amplitude is directly related to the pressure level. As the tissue response is not perfectly elastic, energy is lost and converted into heat.

Focused Ultrasound Waves

By using a spherical shaped transducer, the ultrasound beam is concentrated on the transducer focus point, resulting in a maximum of pressure concentrated at this point. As tissue heating is directly related to pressure maximum, the necrotic lesion is formed at the transducer focus.

How Does HIFU Create a Lesion?

Tissue temperature increases in the focal area resulting in a necrotic lesion. The lesion extension is about 3/4 in front of the transducer focus and 1/4 beyond.

The lesion dimension is related to the firing duration: the lesion starts at the transducer focus and progresses toward the transducer during the firing sequence. With the software integrated algorithm controlling the firing sequence based on experimental and mathematical models, the Ablatherm® device is able to perform lesions up to 24 mm in length.