Noncontact holographic detection for photoacoustic tomography

Abstract

A holographic method for high-speed, noncontact photoacoustic tomography is introduced and evaluated. Relative changes of the object's topography, induced by the impact of thermoelastic pressure waves, were determined at nanometer sensitivity without physical contact. The object's surface was illuminated with nanosecond laser pulses and imaged with a high-speed CMOS camera. From two interferograms measured before and after excitation of the acoustic wave, surface displacement was calculated and then used as the basis for a tomographic reconstruction of the initial pressure caused by optical absorption. The holographic detection scheme enables variable sampling rates of the photoacoustic signal of up to 50 MHz. The total acquisition times for complete volumes with 230 MVoxel is far below 1 s. Measurements of silicone and porcine skin tissue phantoms with embedded artificial absorbers, which served as a model for human subcutaneous vascular networks, were possible. Three-dimensional reconstructions of the absorbing structures show details with a diameter of 310 μm up to a depth of 2.5 mm. Theoretical limitations and the experimental sensitivity, as well as the potential for in vivo imaging depending on the detection repetition rate, are analyzed and discussed.

Heat transport considerations in the mathematical analysis of the photoacoustic and photothermal effects

“…Besides the complexity of the problem, we tried to keep the physical and mathematical sophistication to the minimum required, to asses how well the photoacoustic phenomena can be described in classical terms, avoiding the introduction of non-linear terms or even more realistic transport considerations than the ordinary Fourier law, showing that indeed our model can capture most of the main characteristics of the photoacoustic phenomena. Although the model we present can serve as a basic formalism on which more physics can be overlaid, it is far from capturing physical features reported in recent developments where is clear the impact of temperature distribution over the pulsed PA (or laser ultrasound) signal [12,33,34,[39][40][41][42][43].…”

“…This is the explicit form of the heat equation, with the heat source included, and decoupled from equation (12). Interestingly the heat source, related to the optically absorbed energy, preserves its analytic dependence with the Beer-Lambert law.…”

“…Since the time scale of these events is related to the time duration of the pulsed laser used as optical excitation source, then it is necessary to identify which are the main sources of heat transport during the phenomena at the interfaces to properly chose the boundary conditions of the modeling. A vast bibliography dealing with the mathematical analysis of these two modes of generating photoacoustic pulses can be found in the references [1][2][3][4][5][6][7][8][9][10][11][12].…”

Hand-held optoacoustic imaging: A review

“…For example, one such method measures the displacement of the sample tissue surface due to optoacoustic waves using low-coherence interferometry; it is limited, however, by the necessity of coating the sample in a layer of mineral oil [ 108 ]. Another non-contact method of detecting optoacoustic waves is performed by measuring the change in the refractive index of the sample surface caused by optoacoustic waves [ 48 ], while yet another method uses holography [ 109 ]. However, while non-contact methods have intriguing potential, their primary limitation is that they have not yet demonstrated the ability to image anything deeper than several millimeters in tissue [ 48 , 108 , 109 ].…”

“…Another non-contact method of detecting optoacoustic waves is performed by measuring the change in the refractive index of the sample surface caused by optoacoustic waves [ 48 ], while yet another method uses holography [ 109 ]. However, while non-contact methods have intriguing potential, their primary limitation is that they have not yet demonstrated the ability to image anything deeper than several millimeters in tissue [ 48 , 108 , 109 ]. In the following sections, we will explore common combinations of light sources and sensors for each OAI method that has been adapted for hand-held imaging, beginning with OACT.…”