Fiber-optic annular detector array for large depth of field photoacoustic macroscopy

Abstract

We report on a novel imaging system for large depth of field photoacoustic scanning macroscopy. Instead of commonly used piezoelectric transducers, fiber-optic based ultrasound detection is applied. The optical fibers are shaped into rings and mainly receive ultrasonic signals stemming from the ring symmetry axes. Four concentric fiber-optic rings with varying diameters are used in order to increase the image quality. Imaging artifacts, originating from the off-axis sensitivity of the rings, are reduced by coherence weighting. We discuss the working principle of the system and present experimental results on tissue mimicking phantoms. The lateral resolution is estimated to be below 200 μm at a depth of 1.5 cm and below 230 μm at a depth of 4.5 cm. The minimum detectable pressure is in the order of 3 Pa. The introduced method has the potential to provide larger imaging depths than acoustic resolution photoacoustic microscopy and an imaging resolution similar to that of photoacoustic computed tomography.

All-optical photoacoustic projection imaging

“…The resulting phase shift is then detected by interferometric means. A detailed explanation of the functionality of the used FOMZI can be found in Bauer-Marschallinger et al [8,30]. Graded-index polymer optical fibers (GIPOF) act as ultrasound sensitive elements in the measurement arms within these interferometers.…”

“…By a rule of thumb, the minimal spatial resolution is in the order of 1/200 of the imaging depth [5]. Various photoacoustic techniques were demonstrated, offering imaging depths ranging from millimeters in acoustic-resolution photoacoustic microscopy (AR-PAM) [6] up to several centimeters in photoacoustic scanning macroscopy (PASMac) [7,8] and photoacoustic computed tomography (PACT) [9]. AR-PAM and PASMac use focused element ultrasound transducers and the images are formed by a scanning technique.…”

“…The integrating line detectors of our O-PAPI prototype consist of fiber-optic interferometers [8,24]. An overview of different types of optical fiber based interferometers and other optical detection techniques used for photoacoustic imaging can be found in [27].…”

“…Additionally, integrating line detectors based on probe beam deflection have been demonstrated [28,29]. Receiving the photoacoustic signals with optical fibers instead of commonly used piezoelectric detectors offers various advantages [8,27]. Optical sensors exhibit low cross-talk and do not need electrical shielding.…”

“…Thereby, the detection bandwidth of the detector array can be adjusted by the used photodetectors without changing the actual detector elements. Moreover, fiber-optic ultrasound detectors offer higher sensitivity [8,27].…”