Читать книгу Intracranial Gliomas Part III - Innovative Treatment Modalities - Группа авторов - Страница 66

Clinical ultrasound treatments require the delivery of energy to a precise target volume of tissue so as to prevent injury to healthy brain. Higher frequencies yield a smaller focus, but at the expense of increased attenuation and, therefore, decreased depth of penetration. Focusing allows a greater intensity at depth and thus has significant advantages. There are a number of ways in which focused transducers can be constructed. A single-element spherically curved piezoelectric transducer is the simplest design but has a number of limitations. It has a fixed focus and therefore treatment over larger or multiple regions requires physical repositioning. Furthermore, the focal spot may be severely distorted if the ultrasound beam must pass through bone, as is required in transcranial applications. Acoustic lenses are constructed from a material with similar attenuation but different speed of sound than the coupling medium, so that the ultrasound beam exiting the lens converges to a focal spot similar to that generated from a spherical source. The focal depth may be adjusted in some lens designs, but phase correction and steering are not possible. The most flexible but complex design is a multi-element array where the elements are each driven independently, which allows both aberration correction when focusing through non-uniform tissues such as bone, to maintain a tight focus, and electronic steering, which allows multiple or large volumes to be covered more quickly.

Fig. 1. The focused ultrasound helmet. The patient’s head is placed inside the helmet after being attached to a BRW stereotactic head frame. This device contains over 1,000 ultrasound transducers that can be individually moved to focus on a specific target. A rubber dam is placed over the head of the patient and allows the skull to be cooled with degassed 17°C water. The helmet is compatible with clinical MRI scanners.

Typical clinical treatment configurations involve a transducer coupled to the patient via a coupling medium such as degassed water (Fig. 1). In treatments through the skull, the coupling medium may be circulated and cooled to avoid temperature elevations of the skin and scalp. Hairs on the beam path are normally removed, either mechanically or chemically, as it causes attenuation, but may be less of a concern with lower frequencies and intensities. Rigid fixation of the patient to the treatment assembly and transducer may be required, as is done in various intracranial therapies. Single-element fixed-focus transducers may be held by a computer-controlled 3-axis positioning system, which allows positioning throughout the treatment volume. In order to generate ultrasound, the transducer is excited with a radiofrequency (RF) signal, generated by a signal generator or oscillator and amplified with an RF amplifier. Finally, the input signal to the transducer passes through a matching circuit to couple the impedances of the amplifier and the transducer. The output acoustic power from the transducer can then be adjusted by either adjusting the duty cycle, the amplitude of the input signal to the amplifier, or the amplifier itself. For multi-element arrays, a similar driving system is typically required for each separate element.