Photoacoustic Tomography.pdf

Photoacoustic tomography (PAT) is an emerging biomedical imaging modality that
combines the high-contrast and spectroscopic-based specificity of optical imaging
with the high spatial resolution of ultrasound imaging in a single modality. By listening
to light, PAT detects tissue-absorbed photons ultrasonically through the photoacoustic
effect. Since ultrasonic scattering is two to three orders of magnitude
weaker than optical scattering in tissue, PAT breaks the 2–4 mm spatial resolution
limit associated with pure optical tomography such as diffuse optical tomography
(DOT) for deep tissue imaging, or the ~1 mm depth limit associated with confocal
and multiphoton microscopy and optical coherent tomography (OCT). In PAT,
tissue is excited with a short (typically a few nanoseconds) laser pulse (focused or
unfocused); the subsequent laser-induced transient photoacoustic waves in the range
of 1–100 MHz, due to the transient thermoelastic expansion of light-absorbing
components in tissue, are detected by wideband unfocused or focused ultrasound
transducer(s). Unique advantages of PAT are that functional or biochemical parameters
such as deoxy-hemoglobin (HbR), oxy-hemoglobin (HbO2), water (H2O), lipids,
and so forth along with vasculature and blood flow can be imaged in high resolution.
In addition, highly specific molecular PAT can be realized through the use of molecular
contrast agents. Finally, PAT can be made portable for bedside applications, is
economical, and uses non-ionization radiation.