In modern medicine, many forms of energy are used for their therapeutic effect. Radio-frequency electric current, microwave energy and cryogenically chilled probes are all used to ablate diseased tissue, such as cancer lesions in the body. These devices have significantly improved the treatment results over using scalpel and sutures alone.
Another form of energy that has come under consideration for use as a therapeutic agent is ultrasound waves or ultrasonic acoustic energy. Ultrasound has long been used for diagnostic procedures where the waves are generated and collected much the same way that sonar wave are. These ultrasound waves are of an energy level that has no therapeutic effects, meaning that the ultrasound waves do not change the tissue structure in any way. The frequency of such waves is generally in the 5 to 30 MHz range.
Another ultrasound device that has been employed for decades is the ultrasonic diathermy instrument. This instrument uses an ultrasonic transducer to generate ultrasound waves at a higher intensity level than the diagnostic units. The acoustic waves are transmitted into the body where they are attenuated and absorbed. This absorption of energy causes the tissue temperature to rise, thereby causing deep heating of tissue. This heating is not sufficient to cause cell death, but promotes healing by increasing blood flow to the region, the same as if heating compresses were used.
The acoustic waves in both the diagnostic and diathermy machines are collimated or non-focused. In the new field of ultrasonic therapy, acoustic waves are focused to a point within a patient's tissues. By concentrating this energy at a specific location, the energy density increases to the point that ablation or necrosis of the tissue occurs. Such therapy has been given the designation “High Intensity Focused Ultrasound” or “HIFU.”
Many different forms of HIFU generators or transducers have been proposed over the years. All of these devices take the energy output of an entire crystal face and through either curvature of the crystal itself or by focusing acoustic lenses concentrate the energy at a single point. An ordinary magnifying glass is a simple analogy where light energy is focused to a point in space. The concentrated energy can then easily raise the temperature of paper to greater than the flammability point.
One device is currently being marketed in Europe and Asia primarily for treatment of prostate disease but has also been proven to be effective in ablating lesions within the liver and kidneys as well. In this design, the circumference of the crystal is shaped as part of a concave surface. Since the acoustic waves propagate in a direction perpendicular to the face of the vibrating crystal, the waves will propagate in a manner such that they will converge at the focal point of the concave surface, FIG. 1, assuming that the crystal is in contact with a medium which will allow acoustic wave transmission, such as water or body tissues. As the waves propagate to the focal point, the acoustic energy density, watts/cm2, increases to a point at which the temperature of the tissue rises above the ablation point. In addition to thermal effects, cavitation and micro streaming of the liquids surrounding the cells has been identified as mechanisms of cell destruction. In any event, the cells are rendered unviable. The body will then remove the necrotic tissue with its normal cleansing mechanism.
In the embodiment described above, the focal zone is theoretically a point in space, but in actuality it will be a small three-dimensional volume. It has been described as having the shape of a rice kernel. In order to treat a larger volume, the transducer head must be manipulated by the physician to treat another location, move it again and so on until the operation is completed.
The several HIFU devices that have been used clinically share at least one attribute: the focal point is quite small in volume. In some cases, this is desirable since only a discrete volume of tissue needs to be treated or the treatment area is close to or contiguous with healthy or important structures such as bile ducts. However, in some cases, a much larger volume of tissue must be ablated, such an entire kidney lobe. When such a large volume needs to be ablated, the only possibility has been to treat a volume, then physically move transducer head to a different location and treat that volume, move the head and so on. This has increased the operation time and operator fatigue factor considerably. Conversely, if a transducer were constructed that provided a very large focal zone, the transducer would not be able to be used for smaller lesions or for discrete points around viable tissue or structures.