User:Rakruger

Themoacoustic Imaging In Medicine

Fig. 2: First 3D thermoacoustic image of breast cancer. Form left to right, images depict axial, coronal and saggital views of the cancer (arrows).

Fig. 1. First 3D thermoaoustic images of a lamb kidney (C,D)in comparison to MRI images of the same kidney (A,B).

Thermoacoustic Imaging was originally proposed by Theodore Bowen, et al. in 1981 as a strategy for studying the absorption properties of human tissue using virtually any kind of electromagnetic radiation. But Alexander Graham Bell first reported the physical principle upon which thermoacoustic imaging is based a century earlier. He observed that audible sound could be created by illuminating an intermittent beam of sunlight onto a rubber sheet. Not until 1994, however, did researchers use an infrared laser to produce the first thermoacoustic image, albeit two dimensional and in a tissue-mimicking phantom. By 1998 researchers at Indiana University Medical Center[1] employed pulsed microwaves to produce the first fully 3D thermoacoustic images of biologic tissue - an excised lamb kidney (Fig. 1). The following year they created the first fully 3D thermoaocustic images of cancer in the human breast, again using pulsed microwaves (Fig. 2).

Thermoacoustic Wave Production

Sound can be induced in virtually any material, including biologic tissue, whenever time-varying electromagnetic energy is absorbed. The stimulating radiation that induces these thermal-acoustic waves may lie anywhere in the electromagnetic spectrum, from high-energy ionizing particles to low energy radio waves. The term “photoacoustic”[2] applies to this phenomenon when the stimulating radiation is of optical nature, while “thermoacoustic” refers to all radiating sources, including optical.

The process by which thermoacoustic waves are generated is depicted in the Figure 3. It can be understood as a four-step process:

Fig. 3. Schematic illustration of thermoacoustic imaging.

1. Biologic tissue is irradiated by an energy source that is absorbed by the body. The source of energy is non-specific, but typically consists of visible light, near infrared, radio waves or microwaves.
2. The absorbed energy is converted to heat, which raises the temperature of the tissue, typically by less than .001 degrees Centigrade.
3. The increase in the temperature of the tissue causes the tissue to expand in volume, however slightly.
4. This mechanical expansion produces an acoustic wave that propagates outward in all directions from the sight of energy absorption at the velocity of sound in biologic tissue, approximately 1.5 mm per microsecond.

When the tissue is irradiated with a pulse, the acoustic frequencies that characterize the acoustic wave span a range of frequencies from zero to 1/(pulse width). E.g., a 1 microsecond pulse produces acoustic frequencies from zero to approximately 1 megaHertz (MHz). Shorter pulses produce a wider range of acoustic frequencies. These frequencies are referred to as ultrasonic, and are also associated with medical ultrasound applications.

Image Formation Principles

Fig. 4: Generic thermoacoustic imaging instrumentation

Any thermoacoustic imaging device needs a source of electromagnetic radiation, be it a laser or a microwave antenna, to deliver energy to the anatomy being studied, and one or more acoustic detectors coupled acoustically to the outside surface of the anatomy. The typical acoustic detector is an ultrasound transducer, which is commonly comprised of piezo-electric material that converts detected pressure to an electrical signal. Thermoacoustic waves are induced within the anatomy wherever absorption takes place, the strength of which is proportional to the energy absorbed within a tissue volume. Some of these waves propagate through the anatomy over some time interval before being detected by one or more of the acoustic detectors. The exact time interval is proportional to the distance between an absorption site and a detector. For any given time interval (time of flight), each detector will receive the sum of the thermoacoustic waves originating at the same distance from the detector in question.