Photoacoustic imaging

Schematic illustration of photoacoustic imaging.

Photoacoustic imaging (optoacoustic imaging) is a biomedical imaging modality based on the photoacoustic effect. In photoacoustic imaging, non-ionizing laser pulses are delivered into biological tissues (when radio frequency pulses are used, the technology is referred to as thermoacoustic imaging). Some of the delivered energy will be absorbed and converted into heat, leading to transient thermoelastic expansion and thus wideband (i.e. MHz) ultrasonic emission. The generated ultrasonic waves are detected by ultrasonic transducers and then analyzed to produce images. It is known that optical absorption is closely associated with physiological properties, such as hemoglobin concentration and oxygen saturation.[1] As a result, the magnitude of the ultrasonic emission (i.e. photoacoustic signal), which is proportional to the local energy deposition, reveals physiologically specific optical absorption contrast. 2D or 3D images of the targeted areas can then be formed.[2] Fig. 1 is a schematic illustration showing the basic principles of photoacoustic imaging.

Contents

1 Biomedical imaging
2 Photoacoustic computed tomography

2.1 General equation
2.2 Universal reconstruction algorithm
2.3 Simple system
2.4 Biomedical applications

2.4.1 Brain lesion detection
2.4.2 Hemodynamics monitoring
2.4.3 Breast cancer diagnosis

3 Photoacoustic microscopy
4 See also
5 References
6 External links

Biomedical imaging[edit]

Fig. 2. Absorption spectra of oxy- and deoxy-hemoglobin.

The optical absorption in biological tissues can be due to endogenous molecules such as hemoglobin or melanin, or exogenously delivered contrast agents. As an example, Fig. 2 shows the optical absorption spectra of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) in the visible and near infrared region.[3] Since blood usually has orders of magnitude higher absorption than surrounding tissues, there is sufficient endogenous contrast for photoacoustic imaging to visualize blood vessels. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, skin melanoma detection, methemoglobin measuring, etc.[2]

Δf
Primary contrast
Δz
δz
δx
Speed

Hz

mm
µm
µm
Mvx/s

Photoacoustic microscopy
50 M
Optical absorption
3
15
45
0.5

Photoacoustic tomography
5 M
Optical absorption
50
700
700
0.5

Confocal microscopy

Fluo
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