Novel coaxial catheters in photoacoustic endoscopy

Abstract

Photoacoustic endoscopy (PAE) provides high sensitivity and spatial resolution of morphological information for intravascular (IV) and gastro intestinal (GI)applications. The IVPA catheter can detect the vulnerable plaque, intralipid and inflammation cells in the cardiac vessels; and the GIPA catheter is capable of imaging the vasculature of the rectum and its hemoglobin oxygen saturation. It is significant to improve the imaging efficiency of PAE. This study aims to expand the imaging range of PAE catheters and improve the efficiency of PA imaging, offering novel coaxial designs on both IV and GI endoscopic imaging. A PZT based ultrasound transducer (UST) is successfully developed with a high center frequency of 35 MHz, a dimension of 0.6 mm × 0.6 mm × 0.4 mm and a center orifice of 0.18 mm. This ring-shaped UST is a key element in the coaxial design, which enables a multimode fiber to get through, forming a long coaxial optical-acoustic overlapped area. Using the proposed coaxial setup, a novel miniature IV-PAE catheter is successfully fabricated with an outer diameter of 0.9 mm. The performance of the ring-shaped UST is tested by the pulse-echo experiment. The resolution of the new design is evaluated by the graphite rods. The IV-PAE catheter is demonstrated by imaging the nano particles in the ex-vivo tissue. To further explore the coaxial design in PAE catheters, transparent piezoelectric material is applied for developing a transparent ultrasound transducer (TUST). A 2 mm × 2 mm × 0.075mm lithium niobate (LN) single crystal wafer is used in a miniature PA probe, whose dimension was smaller than the previous LN-based TUSTs in the reported work. A Gradient-index (GRIN) lens is applied as the backing layer of the TUST, effectively shrinking the entire probe size. The proposed probe exhibits a high center frequency of 46.9 MHz and a -6 dB bandwidth of 29.4%. Experiments are further performed on the phantoms to demonstrate the multi-functions of the miniature probe on fluid flow sensing and imaging. Using the transparent piezoelectric material, a coaxial GI-PAE catheter is developed with a Bessel beam. The diameter of the catheter is 3.5 mm, which is compatible with the 3.7-mm instrumental channel of a conventional clinical optical endoscope. The Bessel beam is formed by a GRIN lens and an axicon, making a long depth of field. The Bessel beam patten is tested by a Charge Coupled Device (CCD) camera. The resolution of the proposed catheter is evaluated by the blade-edge method at different depths, showing its long DOF of ~4.5 mm. The phantom imaging is conducted at different depths to demonstrate the performance of the GI-PAE catheter in a large imaging range. In short, two novel coaxial PAE catheters are successfully designed and fabricated, both of which provide a large imaging range than the previous works with miniature dimensions. The imaging results successfully support the research goal. The improvements of imaging range brought by the coaxial configuration greatly enhance the PA imaging efficiency. This research suggests a bright future for both plaque evaluation and GI tumor detection.