Fig. 1. (a) Schematic of the detector response simulation. PD, point detector; AD, arc detector. (b) Point source signals received by point and arc detectors. Inset: the frequency spectrum. (c) The frequency response of the arc detector. Inset: the frequency response in logarithmic coordinates, slope: −0.497. (d) Schematic of the negatively focused fiber-laser ultrasound transducer. FBG: fiber Bragg grating. Inset: photograph of the fiber-laser ultrasound transducer. (e) The frequency of two orthogonally polarized laser modes. fb=|fx−fy|. (f) Comparison of the fiber-laser noise with different cavity lengths. (g) The time-domain PA signal acquired by the fiber-laser ultrasound transducer. Inset: the frequency spectrum.

Fig. 2. (a) PACT system based on the negatively focused fiber-laser ultrasound transducer. WDM, wavelength division multiplexer; SMF, single-mode fiber; DAQ, data acquisition unit; PC, personal computer. Inset: schematic of spatial location of the hair, carbon fiber, and leaf samples. (b) Schematic of working principle of the linear scanning PACT system using the negatively focused fiber-laser ultrasound transducer.

Fig. 3. (a) Simulated and (b) experimental results of the spatial response of the negatively focused fiber-laser ultrasound transducer. (c) Imaging result of the hair loop. Inset: photograph of the hair loop. (d) Photograph and (e) PA image of the leaf.

Fig. 4. Resolution results of the PACT system. (a) Reconstructed images of three 10-μm-diameter carbon fibers. Scale bar: 500 μm. GW, Gaussian window; CF, coherence factor. (b) Tangential and (c) axial profiles along the dashed lines in (a) at the maximum of the carbon fiber 2 of the original image, the image after Gaussian-window apodization, and the image after coherence-factor weighting. (d) Tangential and (e) axial resolutions versus the distance between the carbon fiber and the fiber-laser ultrasound transducer.

Fig. 5. Noninvasive PACT imaging results of the mouse brain. (a) Schematic of the mouse brain imaging plane. (b), (c) Two coronal images at two different locations of the mouse brain. Left: original reconstructed image; right: reconstructed image with Gaussian window. 1, the scalp; 2, the skull; 3, the cerebral cortex; 4, the thalamus or mesencephalon. (d) Tangential and axial resolutions of the vessel in the mouse scalp as indicated by the solid box in (c). Inset: enlarged vessel image after Hilbert transform. (e), (f) Plots of the vessel profiles along the dotted lines in the dotted boxes A and B in (c). Insets: enlarged vessel images after Hilbert transform.

Fig. 6. (a) Schematic of limiting the acceptance angle for the imaging reconstruction. (b) Reconstructed images by the PACT system for the same fiber-laser ultrasound transducer with acceptance angles of 30, 60, 90, and 120 deg. SSS, superior sagittal sinus; azPA, azygos pericallosal artery; RRV, rostral rhinal vein; AchA, anterior choroidal artery.

Fig. 7. (a) Schematic of the procedure for building an intracerebral hemorrhage model. SS, the sagittal suture; B, the bregma; L, the lambda. Brain images of the mouse before and after (b), (c) insertion of an ink-coated optical fiber and (d), (e) the blood injection for inducing the intracerebral hemorrhage. (f), (g) Brain images of the mouse before inserting and after withdrawing the empty syringe.