Slit-based photoacoustic tomography is a newly developed technique that improves the elevation numerical aperture of a linear array through acoustic diffraction. The slit, placed at the acoustic focus of a linear array, effectively forms an array of virtual detectors with high receiving angle, which subsequently improves the elevation resolution. However, due to the complex implementation, our original system could only image phantoms and sacrificed animals. In this report, the system has been significantly improved. In particular, we designed a slit holder that can be directly mounted to the transducer array for easy adjustment of slit width and simultaneous scanning of both the array and the slit. To enlarge the imaging field of view, we replaced the single circular optical fiber bundle with a bifurcated line fiber bundle which moved simultaneously with the array and the slit. The data acquisition system has also been updated to double the imaging speed. With these improvements, the new system can image a 3.8 × 4 cm2 region within 40 seconds and the object only needs to be coupled through ultrasound gel. We successfully used the system to image vasculatures in the palm and forearm of human volunteers.

Due to its unique capability of visualizing optical absorption in deep tissues, photoacoustic tomography is increasingly used in biomedical imaging. Among various types of transducer arrays, the linear array is perhaps the most widely used in photoacoustic tomography, because it is commercially available and readily allows ultrasound imaging. However, the three-dimensional imaging capability of a linear array is limited, due to its poor elevational resolution. While various scanning schemes have been proposed to address this problem, they all suffer from long scanning time. To address this issue, we introduce slit-enabled three-dimensional photoacoustic tomography. The metal slit, placed at the array focus, causes the incoming photoacoustic waves to diffract along the elevation direction and hence significantly improves the elevation detection aperture and resolution. We tested the new system in both phantoms and animals. The slit improves the elevation resolution by ten times without compromising scanning time.Photoacoustic (PA) tomography (PAT) is playing an increasingly important role in biomedical imaging. The hybrid nature of PAT allows acquisition of high-resolution images beyond the optical diffusion limit [1][2][3][4]. Among various transducer arrays used in PAT, linear transducer arrays are commonly seen due to their low cost, hand-held convenience and easy adaption to clinical applications [5][6][7][8]. However, the three-dimensional (3D) imaging capability of a linear array is limited, because its elevation resolution is much worse than the axial and lateral spatial resolutions. For instance, a Philips L7-4 array has 0.144 mm axial and 0.298 mm lateral resolutions, but only a 1.5 mm elevation resolution (at the acoustic focus). Over the past few years, multiple methods have been proposed to address this issue. Gateau et al. combined linear and rotational scanning to achieve nearly isotropic 3D spatial resolution [9]. Schwarz et al. proposed a bi-directional scan method with two array positions perpendicular to each other [10]. In principle, all these methods improve elevation resolution by converting the elevation direction into axial or lateral directions. However, such complicated scanning geometry often requires prolonged scanning times.In this letter, we propose a fundamentally different approach to improve elevation resolution. Our method is based on acoustic diffraction through a thin slit placed along the acoustic focus of the array (Fig. 1) Author Manuscript Author Manuscript Author ManuscriptAuthor Manuscript hence improves the receiving aperture along elevation direction. As shown in Fig. 1, in a conventional linear-array PAT system (Fig. 1a), due to elevation focus from the acoustic lens, photoacoustic waves coming out of the focal zone cannot be received by the transducer. The loss in elevation receiving aperture limits the corresponding spatial resolution. The metal slit (with a width close to the 300 µm central acoustic wavelength) eliminates the acoustic focus, allowing waves c...