Abstract
In conventional photoacoustic tomography, several effects contribute to the loss of resolution, such as the limited bandwidth and the finite size of the transducer, or the space-dependent speed of sound. They can all be compensated (in principle) technically or numerically. Frequency-dependent acoustic attenuation also limits spatial resolution by reducing the bandwidth of the photoacoustic signal, which can be numerically compensated only up to a theoretical limit given by thermodynamics. The entropy production, which is the dissipated energy of the acoustic wave divided by the temperature, turns out to be equal to the information loss, which cannot be compensated for by any reconstruction method. This is demonstrated for the propagation of planar acoustic waves in water, which are induced by short laser pulses and measured by piezoelectric acoustical transducers. It turns out that for water, where the acoustic attenuation is proportional to the squared frequency, the resolution limit is proportional to the square root of the distance and inversely proportional to the square root of the logarithm of the signal-to-noise ratio. The proposed method could be used in future work for media other than water, such as biological tissue, where acoustic attenuation has a different power-law frequency dependence.
Highlights
IntroductionPhotoacoustic tomography (PAT), which is called optoacoustic or thermoacoustic tomography, is based on the generation of ultrasound following a temperature rise after the illumination of light-absorbing structures within a semitransparent and turbid material, such as a biological tissue
Photoacoustic tomography (PAT), which is called optoacoustic or thermoacoustic tomography, is based on the generation of ultrasound following a temperature rise after the illumination of light-absorbing structures within a semitransparent and turbid material, such as a biological tissue.It provides optical images with specific absorption contrast [1,2,3]
At depths larger than the range of the ballistic photons, i.e., more than a few hundreds of microns in tissue, light is multiply scattered, and the spatial resolution is limited by acoustics
Summary
Photoacoustic tomography (PAT), which is called optoacoustic or thermoacoustic tomography, is based on the generation of ultrasound following a temperature rise after the illumination of light-absorbing structures within a semitransparent and turbid material, such as a biological tissue It provides optical images with specific absorption contrast [1,2,3]. The physical reason for this ill-posedness is thermodynamics: acoustic attenuation is an irreversible process and the entropy production, which is the dissipated energy of the attenuated acoustic wave divided by the temperature, is equal to the information loss for the reconstructed image [16] This limits the spatial resolution, which correlates with the information content of the reconstructed image.
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