Thermoacoustic imaging

Thermoacoustic wave production

Sound, which propagates as a pressure wave, can be induced in virtually any material, including biologic tissue, whenever time-varying electromagnetic energy is absorbed. The stimulating radiation that induces these thermally generated acoustic waves may lie anywhere in the electromagnetic spectrum, from high-energy ionizing particles to low-energy radio waves. The term "photoacoustic" (see photoacoustic imaging in biomedicine) applies to this phenomenon when the stimulating radiation is optical, while "thermoacoustic" is the more general term and 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:

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 0.001 degree Celsius.
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 site 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 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. Frequencies greater than 1 MHz are referred to as ultrasonic, and are also associated with medical ultrasound applications.

Image formation principles

Any thermoacoustic imaging device requires 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, as is illustrated in Fig. 4.[[File:Time of Flight.jpg|200px|thumb|Fig. 5: For a given time of flight (t) acoustic waves will arrive at a transducer from all absorbers equidistant from the transducer (dotted blue line).]] The typical acoustic detector is an ultrasound transducer, which is commonly made of a piezo-electric material that converts detected pressure to an electrical signal. Thermoacoustic waves are induced within the anatomy wherever absorption takes place, and the strength of these thermoacoustic waves is proportional to the energy absorbed within the tissue. Some of these waves propagate through the anatomy over some time interval (time-of-flight) before being detected by one or more of the acoustic transducers. The exact time-of-flight is proportional to the distance between an absorption site and a transducer, assuming for the moment that each transducer is a point detector. For any given time-of-flight, each transducer will receive the sum of the thermoacoustic waves originating at the same distance from the detector in question as is illustrated in Fig. 5. For this reason, ambiguity arises when attempting to localize an absorption site with a point transducer. A variety of strategies have been employed to mitigate this ambiguity.

Detector geometries

Three generic detector configurations have been used: a spherically focused transducer; a linear (or curve-linear) array of transducers, focused in one dimension; or, a 2D array of unfocused transducers. In general, a single, focused transducer can image a single line through a 3D volume. A linear (1D) array, be it straight or curved, can image a 2D plane, but to image a full 3D volume requires a 2D array of transducers.

Focused Transducer

A spherically focused transducer is most sensitive to thermoacoustic waves originating along a line passing through its focal point. Time-of-flight information is used to estimate the thermoacoustic signal strength along this line. A 2D image can be assembled a line-at-a-time by translating the focused transducer laterally along a linear path. A 3D image can be built up by scanning the transducer along a rectilinear path within a 2D plane.http://labs.seas.wustl.edu/bme/Wang/epub/2006HZ-KM-PATfunct.pdf The ability to distinguish thermoacoustic signals along the line of focus (axial resolution) is superior to distinguishing thermoacoustic signals transverse to the line of focus (lateral resolution). For this reason the lateral spatial resolution is three- to four-times worse than the axial spatial resolution using this approach.

Linear array

2D array

Microwaves have also been used to form 3D thermoacoustic images of the human breast. One of the first devices to do so is depicted in Fig. 10. It consisted of an array of eight waveguides, which directed microwave energy into the breast. A transducer array was rotated in synchrony with the waveguides in order to acquire sufficient data to reconstruct the internal structures of the breast. Figure 11 shows an animation of the typical glandular tissue pattern in a normal breast.

References

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