Resolution Limits in Photoacoustic Imaging Caused by Acoustic Attenuation

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.

State-of-the-Art Preclinical Photoacoustic Imaging in Oncology: Recent Advances in Cancer Theranostics

“…Even if PAI technology has reached preliminary applications in clinical studies, improvements are needed relatively to spatial resolution for visualization of smaller lesions in deeper tissues and temporal resolution for dynamic processes imaging [134]. Recent advances in laser technology, improving the pulse repetition rate and the optical penetration, have been proved highly effective to enhance PAI systems performances for oncological applications, allowing for in vivo monitoring of the metastatic process in a mouse model of melanoma and deep brain imaging in rodents [135].…”

Image Quality Improvement Techniques and Assessment Adequacy in Clinical Optoacoustic Imaging: A Systematic Review

“…The attainable resolution at depth is limited by frequency-dependent acoustic attenuation, caused by dissipation and scattering [ 34 ], and primarily affecting high-frequency signals. In addition to having lower amplitude, the attenuated waveforms broaden as their bandwidth becomes narrower [ 35 ], and the corresponding image features appear blurred ( Figure 4 c).…”

“…Many factors reduce or limit the quality of optoacoustic images, which, in turn, hinder the modality’s potential for clinical translation. These factors may emerge from the imaging hardware [ 25 , 26 , 27 , 28 , 29 , 30 ], the inexact or approximative image reconstruction algorithms [ 25 , 26 , 31 , 32 , 33 ], the attenuative properties and inhomogeneous nature of tissue (light-tissue interaction phenomena) [ 34 , 35 , 36 , 37 , 38 , 39 ], or particularities of the acquisition procedure [ 40 , 41 ], and manifest as image noise, artifacts, and poor overall image fidelity.…”

Breaking the resolution limit in photoacoustic imaging using positivity and sparsity

“…Non-equilibrium thermodynamics describes the connection between entropy production and information loss, e.g. [7], and as we have already elaborated for heat diffusion [8,9], we could determine a cut-off frequency also for damped acoustic waves in water [10]. Here, this approach is generalized for a frequency dependent acoustic attenuation described by a power-law using a general exponent which might be different from two in water.…”

“…In frequency space, the information content of wave components with frequencies above that cut-off frequency is so low that they cannot be statistically distinguished from the equilibrium distribution. This is equivalent that the acoustic wave amplitude on the sample surface is damped just below the noise level [10]. Consequently, the spatial resolution limit becomes diffraction limited and according to Nyquist it is half of the wavelength at this cut-off frequency [10].…”