Limited-view Photoacoustic Tomography Research Articles

Compensating unknown speed of sound in learned fast 3D limited-view photoacoustic tomography

Real-time applications in three-dimensional photoacoustic tomography from planar sensors rely on fast reconstruction algorithms that assume the speed of sound (SoS) in the tissue is homogeneous. Moreover, the reconstruction quality depends on the correct choice for the constant SoS. In this study, we discuss the possibility of ameliorating the problem of unknown or heterogeneous SoS distributions by using learned reconstruction methods. This can be done by modelling the uncertainties in the training data. In addition, a correction term can be included in the learned reconstruction method. We investigate the influence of both and while a learned correction component can improve reconstruction quality further, we show that a careful choice of uncertainties in the training data is the primary factor to overcome unknown SoS. We support our findings with simulated and in vivo measurements in 3D.

Flexible transparent CMUT arrays for photoacoustic tomography.

This paper reports the fabrication and characterization of the first flexible transparent capacitive micromachined ultrasound transducer (CMUT) array for through-illumination photoacoustic tomography. Fabricated based on an adhesive wafer bonding technique and a PDMS backfill approach, the array has a maximum transparency of 67% in visible light range and can be bent to a radius of curvature of less than 5 mm without the structural layers being damaged. With a center frequency of 3.5 MHz, 80% fractional bandwidth, and noise equivalent pressure (NEP) of 62 mPa/H z, the array was successfully used in limited-view photoacoustic tomography of a 100 µm wire target, demonstrating lateral and axial resolutions of 293 µm and 382 µm, respectively, with 46 dB signal-to-noise ratio. Additionally, deep tissue photoacoustic tomography was also demonstrated on a blood tube within a chicken tissue using the fabricated CMUT arrays.

Gradient minimization for limited-view photoacoustic tomography

Photoacoustic tomography (PAT) is an emerging and effective imaging technique, which offers high spatial resolution with high contrast. In particular, the acquired data is incomplete due to geometrical limitations or accelerating data acquisition by undersampling technology, thus some artifacts will be presented in the reconstructed image. To deal with limited-view PAT, we introduce a regularization scheme into PAT and propose a three-stage method. We first use the gradient descent method to obtain an initial solution, then project it onto a constrain set, and finally a proximal mapping scheme is used to further improve the reconstruction quality. Our simulation experiments on homogeneous medium are utilized to validate the effectiveness of the proposed method, and a discussion on the parameters of the proposed method is given. The experimental results reveal that the proposed method outperforms other classical methods, and it can further improve the reconstruction quality in terms of suppressing the noise and artifacts, and preserving the edge.

Improving limited-view photoacoustic tomography with an acoustic reflector.

The versatility and real-time imaging capability of commercial linear array transducers make them widely used in clinical ultrasound and photoacoustic imaging. However, they often suffer from limited detection view. For instance, acoustic waves traveling at a grazing angle to the transducer surface are difficult to detect. In this letter, we propose a simple and easy approach to ameliorate this problem by using a 45-deg acoustic reflector. The reflector forms a virtual array that is perpendicular to the physical array, thereby doubling the detection coverage. The improvement in image quality in photoacoustic tomography was demonstrated through a hair phantom, a leaf skeleton phantom, and an ex vivo mouse ear experiment.

Effects of size and arrangement of virtual transducer on photoacoustic tomography

In this paper, we investigate the effects of the relative size and arrangement of a virtual transducer on the image quality in limited-view photoacoustic tomography. A virtual transducer refers to the acoustic scatterers used to reflect photoacoustic waves and improve the images reconstructed from incomplete PA signal. Size and spatial arrangement determine the performance of the virtual transducer. In this study, the scatterers utilized as virtual transducers are arranged in different manners, such as on a straight line or on an arc line. We find that virtual transducers with a big distributing angle can provide more significant image improvement than with a small distributing angle, which is similar to the true transducers. We also change the size of virtual transducer and study its influence on image quality. It is found that the bigger scatterers provide better images than the smaller ones. Especially, when the size of scatterers is reduced to the wavelength of photoacoustic wave, the image quality observably decreases, owing to the strong diffraction effect. Thus, it is suggested that the size of the acoustical scatterers should be much larger than the photoacoustic wavelength. The simulations are conducted, and the results could be helpful for the application and further study of virtual transducer theory in limited-view photoacoustic tomography.

Limited-view photoacoustic tomography utilizing backscatterers as virtual transducers

In photoacoustic tomography, acoustic scattering is usually considered as a nuisance, because it distorts an incident wavefront and then induces artifacts and distortion. This work demonstrates that backscatterers could function as virtual transducers arranged behind the region of interest and are used to improve limited-view reconstruction. This idea is confirmed by both simulations and experiments. Our results suggest that enhanced photoacoustic tomography could be obtained by taking advantage of native scatterers in tissues or artificial ones injected around the region of interest.

Study of the resolution of limited-view photoacoustic tomography

In this study, numerical experiments are carried out to quantitatively investigate the effects of limited-view scanning on the resolution of photoacoustic tomography. An empirical formula is provided to approach our numerical results.The results of the study are helpful for the design of scanning trajectory and the evaluation of reconstruction.

Deconvolution reconstruction of full-view and limited-view photoacoustic tomography: a simulation study

Although many algorithms are available for full-view photoacoustic tomography (PAT), no exact and stable algorithm for limited-view PAT has been proposed. In this paper the deconvolution reconstruction (DR) algorithm is proposed for both full-view and limited-view PAT. In the DR algorithm, first a new function is constructed from detected photoacoustic signals and approximately simplified, and then the tissue's electromagnetic absorption is derived from this function on the basis of Fourier-based deconvolution. Computer simulations are carried out to compare the DR algorithm with two popular PAT algorithms, the time-domain reconstruction (TDR) and the filtered back projection (FBP). Although the error of the DR algorithm increases with the size of the detected object, it is shown that the DR algorithm has good precision and strong robustness to noise in the full-view PAT, nearly equivalent to the TDR and FBP. Yet the DR algorithm is more than ten times faster in computation speed. In the limited-view PAT, the DR is superior to the TDR and FBP in terms of both accuracy and robustness to noise.