Photoacoustic computed tomography with lens-free focused fiber-laser ultrasound sensor

Abstract

Optical detection of ultrasound is attractive to photoacoustic imaging due to its high sensitivity per unit area, broad bandwidth, and electromagnetic immunity. To enhance the sensitivity, previous optical transducers commonly necessitate bulk acoustic lenses to achieve focused ultrasound detection. Here, we proposed and demonstrated a novel lens-free focused optical ultrasound sensor by mechanically bending a flexible fiber laser. At a curvature radius of 30 mm, the curved fiber laser well conformed to the spherical wavefront of ultrasound exhibiting ~5 times higher sensitivity compared with the straight one. The focused fiber laser ultrasound sensor (FUS) presented a minimum detectable pressure of ~36 Pa with a working distance equal to its curvature radius. The sensor was applied to circular scanning photoacoustic computed tomography (PACT), which showed a ~70 μm in-plane resolution and a ~500 μm elevational resolution. In vivo imaging of a zebrafish and mouse brain shows the potential of this focused FUS for photoacoustic imaging in biological/medical studies.

Fiber laser technologies for photoacoustic microscopy

“…Figures 9 (a)–(d) show the in vivo PAM images of a mouse ear and brain, using a fiber laser sensor for ultrasound detection [ 58 , 59 ]. During PAM, the focused 532-nm nanosecond laser beam raster scans as the sensor is mounted horizontally at 1.6 mm above the sample.…”

“…The step size in the lateral direction was 10 μm. Figure 9 (e) shows the photoacoustic computed tomography results of a mouse brain obtained by rotary scanning the sensor [ 59 ]. Figure 9 (f) shows the photoacoustic endoscopy results of a mouse rectum, taking advantage of the small size of the sensor.…”

“…9 In vivo PAI using the fiber-laser ultrasound sensor. ( a ): Fast-scanning optical-resolution PAM of mouse ear; ( b ) and ( c ): In vivo imaging of the mouse brain ( b ) without and ( c ) with intact skull; ( d ): Three consecutive snapshots showing the negative contrast flow in a blood vessel; ( e ): In vivo photoacoustic computed tomography of a mouse brain; ( f ): In vivo endoscope image of a rat rectum [ 58 , 59 ] …”

Flexible fiber-laser ultrasound sensor for multiscale photoacoustic imaging

“…tion facing the source z=0) with a length L eq . Based on the deduction above, we can write the equivalent interaction length as , which is ten to a hundred times shorter than the sensitive region (typically 2 to 10 mm) of a fiber laser sensor 28 . For a 50-MHz spherical ultrasound wave (wavelength: 30 μm; speed in water: 1500 m/s) with a source-detector distance d=500 μm, the equivalent interaction length is L eq =122 μm.…”

Improvement in resolution of fiber-laser photoacoustic tomography based on a virtual-point concept

“…We previously developed a curved fiber-laser ultrasound transducer for the circular-scanning photoacoustic computed tomography (PACT) imaging of a zebrafish and a mouse brain. The fiber was bent into an arc shape to realize ultrasound focusing with a centimeter-scale focal length for deep-tissue imaging [ 21 ]. The focal length of the fiber laser transducer was varied by tuning the fiber bending curvature with a lab-designed holder for the multi-depth linear-scanning PACT imaging of a rat abdomen [ 22 ].…”