Photoacoustic (PA) imaging with internal light illumination through optical fiber could enable imaging of internal organs at deep penetration. We have developed a transurethral probe with a multimode fiber inserted in a rigid cystoscope sheath for illuminating the prostate. At the distal end, the fiber tip is processed to diffuse light circumferentially over 2 cm length. A parabolic cylinder mirror then reflects the light to form a rectangular-shaped parallel beam which has at least 1 cm<sup>2</sup> at the probe surface. The relatively large rectangular beam size can reduce the laser fluence rate on the urethral wall and thus reduce the potential of tissue damage. A 3 cm optical penetration in chicken tissue is achieved at a fluence rate around 7 mJ/cm<sup>2</sup> . For further validation, a prostate phantom was built with similar optical properties of the human prostate. A 1.5 cm penetration depth is achieved in the prostate mimicking phantom at 10 mJ/cm<sup>2</sup> fluence rate. PA imaging of prostate can potentially be carried out in the future by combining a transrectal ultrasound transducer and the transurethral illumination.
In traditional photoacoustic tomography, external illumination is used to excite acoustic waves. However, with the assistance of fibre-transmitted light, multidirectional illumination or internal illumination can be achieved which can obtain a better image at a deeper depth. Laser pulses delivered by fibre are energy-limited by the fibre core size and damage threshold. To increase the amplitude of photoacoustic waves and their penetration, it is necessary to improve the fibre coupling energy and efficiency. To improve the coupling performance of single fibres, we use a cylindrical lens array to homogenize the incident beam before a coupling lens. Simulation in Zemax shows that this approach flattens the beam profile on the front surface of the fibre, decreasing the risk of fibre damage. Experimental results with fibre core diameters of 1mm and 1.5mm show that both types of fibre can output more than 50mJ energy per pulse at 700nm wavelength. The coupling efficiency is measured to be above 70% and even reaches 90% as the wavelength changes from 675nm to 900nm. This improvement of coupling energy in single fibres will benefit photoacoustic tomography applications using internal illumination.
Imaging complicated structures with photoacoustic (PA) modality when the field of view is limited can result in significant imaging artifacts or missing structures. Approaches to solve this problem include new reconstruction algorithms and specific transducer structures, such as hemi-spherical transducer arrays for breast cancer detection. However, most existing PA imaging techniques require either fullview complete projection data collection or complex and computationally-intensive reconstructions. Such approaches are not only time-consuming but also unsuitable for many clinical applications, because most clinical imaging hardware is constrained to limited reconstruction angles. In this paper, we present a method of using two commercial linear array transducers at different orientations to increase the view angle and thus improve the reconstruction of PA imaging. The method involves a two-step process. First, a calibration phantom is imaged to calibrate the relative position of these two linear transducers. Second, two PA images are obtained by a simple back projection algorithm and these images are registered using the information from the calibration process. The final registered image contains more detailed structures without the requirements of a specialized transducer or long processing time. Experimental results show that this method has the potential to provide good image quality using standard low-cost transducers.
An analytical apodization function of an elliptical mirror with an aperture angle greater than π is derived for the analysis of the focusing properties. The distribution of electric field intensity near the focal region is given using vectorial Debye theory. Simulation results indicate that a bone-shaped focal spot is formed under linearly polarized illumination, and a tight-circularly symmetric spot is generated under radially polarized illumination. The change in eccentricity causes such a change in the focusing pattern under radially polarized illumination, that a greater eccentricity causes a spot tighter in transverse direction but wider in axial direction. Under radially polarized illumination, the transverse and axial full-width-at-half-maximum will be 0.382λ and 0.757λ , respectively, and the conversion efficiency of the longitudinal component can go beyond 99%, when the semi-aperture angle is 2π/3 and the eccentricity is 0.6. It can, therefore, be concluded that the tight focusing pattern with strong and pure longitudinal field can be achieved under radially polarized illumination for particle acceleration, optical tweezers, and high-resolution scanning microscopy.