KEYWORDS: Signal to noise ratio, Denoising, Imaging systems, Light emitting diodes, Interference (communication), Data acquisition, Yield improvement, Solid state lasers, Signal generators, Photoacoustic tomography
Usage of low energy alternatives, such as light emitting diode (LED) and pulsed laser diode (PLD), to solid state lasers as a source of illumination in photoacoustic tomography (PAT) is being actively investigated for several preclinical and clinical applications due to its inherent advantages such as portability, low cost and high pulse repetition frequency (PRF). However, the photoacoustic signal acquired from such low energy sources particularly LED, have weak light absorption-to-ultrasound conversion efficiency. Due to the weak nature of the generated PA signals, noises from external sources, such as, electronic noise, jitter, etc. have a more pronounced detrimental effect on the quality of the reconstructed LED-PAT images resulting in low Signal-to-Ratio (SNR) values. The high PRF of LED-PAT systems is usually exploited to improve the SNR by performing averaging of several hundreds of frames. However, this is a time-consuming procedure and decreases the frame rate achievable using the imaging system. In this work, we demonstrate that by employing signal recovery strategy at sub-λ array locations along with a denoising filter, it is feasible to improve image quality and reduce noise from LED-PAT images acquired using commercially available LED-PAUS imaging system (AcousticX, Cyberdyne Inc., Ibaraki, Japan). This method was tested on phantom and in-vivo data acquired using the above-mentioned system and was found to yield reduction in background noise of up to 12 dB compared to mere averaging Also, by applying the proposed method, we achieved two-fold improvement in lateral resolution.
Photoacoustic imaging is a molecular cum functional imaging modality based on differential optical absorption of the incident laser pulse by the endogeneous tissue chromophores. Several numerical simulations and finite element models have been developed in the past to describe and study Photoacoustic (PA) signal generation principles and study the effect of variation in PA parameters. Most of these simulation work concentrate on analyzing extracted 1D PA signals and each of them mostly describe only few of the building blocks of a Photoacoustic Tomography (PAT) imaging system. Papers describing simulation of the entire PAT system in one simulation platform, along with reconstruction is seemingly rare. This study attempts to describe how a commercially available Finite Element software (COMSOL(R)), can serve as a single platform for simulating PAT that couples the electromagnetic, thermodynamic and acoustic pressure physics involved in PA phenomena. Further, an array of detector elements placed at the boundary in the FE model can provide acoustic pressure data that can be exported to Matlab(R) to perform tomographic image reconstruction. The performance of two most commonly used image reconstruction techniques; namely, Filtered Backprojection (FBP) and Synthetic Aperture (SA) beamforming are compared. Results obtained showed that the lateral resolution obtained using FBP vs. SA largely depends on the aperture parameters. FBP reconstruction was able to provide a slightly better lateral resolution for smaller aperture while SA worked better for larger aperture. This interesting effect is currently being investigated further. Computationally FBP was faster, but it had artifacts along the spherical shell on which the data is projected.
During manual palpation of breast masses, the perception of its stiffness and slipperiness are the two commonly used information by the physician. In order to reliably and quantitatively obtain this information several non-invasive elastography techniques have been developed that seek to provide an image of the underlying mechanical properties, mostly stiffness-related. Very few approaches have visualized the "slip" at the lesion-background boundary that only occurs for a loosely-bonded benign lesion. It has been shown that axial-shear strain distribution provides information about underlying slip. One such feature, referred to as "fill-in" was interpreted as a surrogate of the rotation undergone by an asymmetrically-oriented-loosely bonded-benign-lesion under quasi-static compression. However, imaging and direct visualization of the rotation itself has not been addressed yet. In order to accomplish this, the quality of lateral displacement estimation needs to be improved. In this simulation study, we utilize spatial compounding approach and assess the feasibility to obtain good quality rotation elastogram. The angular axial and lateral displacement estimates were obtained at different insonification angles from a phantom containing an elliptical inclusion oriented at 45°, subjected to 1% compression from the top. A multilevel 2D-block matching algorithm was used for displacement tracking and 2D-least square compounding of angular axial and lateral displacement estimates was employed. By varying the maximum steering angle and incremental angle, the improvement in the lateral motion tracking accuracy and its effects on the quality of rotational elastogram were evaluated. Results demonstrate significantly-improved rotation elastogram using this technique.
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