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Focused ultrasound is a very effective way to raise the temperature in a tumor in a completely non invasive way.
The principle is very simple: an ultrasounds emitter called a transducer is placed outside of the patient body. By its shape (usually a portion of a sphere) or by means of phased array operation, the transducer concentrates the ultrasound beams into a very small focal spot (the size of a rice grain). At the focal spot, the temperature rises rapidly while the temperature elsewhere remain the same. As an analogy, consider the sun rays focused with a lens: heating is only felt at the focal spot. To ablate a tumor, the focal spot is swept over the target volume. There are basically two strategies for sweeping the tumor with the focal spot:
The major advantages of focused ultrasound compared to other therapeutic techniques like radiation therapy are:
- The sequential aproach (point by point or line by line) in which a small portion of the tumor (a point or a line) is completely destroyed before moving to the next point or line.
- The volumetric approach in which the focal spot is swept rapidly and repeatedly over the complete tumor volume until the target volume is completely destroyed .
- damage to the tissue happens only at the focal spot. There is no effect of the ultrasound along the propagation path to the tumor.
- The technique is both non invasive and non ionizing (no irradiation).
- The physical mean of tissue destruction, heat, is deterministic. When tissues are heated above a threshold temperature, cellular death occurs within seconds with much certainty (this is not the case with radiation therapy).
Focused ultrasounds ablation: heating at the focal spot through focusing of ultrasounds
Focused ultrasound capacity to destroy a target precisely and non invasively, has been studied since the early 1950s [1-24]. What has essentially changed in the last few years to bring this technique into clinical practice is the rapid development of image guidance for the procedures. Real time image guidance during the procedure is indeed the key to safe, effective and reliable treatments.
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