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High-frequency ultrasound imaging can provide exquisite visualizations of tissue to guide minimally invasive procedures.Here,we demonstrate that an all-optical ultrasound transducer,through which light guided by optical fibers is used to generate and receive ultrasound,is suitable for real-time invasive medical imaging in vivo.Broad-bandwidth ultrasound generation was achieved through the photoacoustic excitation of a multiwalled carbon nanotube-polydimethylsiloxane composite coating on the distal end of a 300-μm multi-mode optical fiber by a pulsed laser.The interrogation of a high-finesse Fabry-Pérot cavity on a single-mode optical fiber by a wavelength-tunable continuous-wave laser was applied for ultrasound reception.This transducer was integrated within a custom inner transseptal needle (diameter 1.08 mm;length 78 cm) that included a metallic septum to acoustically isolate the two optical fibers.The use of this needle within the beating heart of a pig provided unprecedented realtime views (50 Hz scan rate) of cardiac tissue (depth:2.5 cm;axial resolution:64 μm) and revealed the critical anatomical structures required to safely perform a transseptal crossing:the right and left atrial walls,the right atrial appendage,and the limbus fossae ovalis.This new paradigm will allow ultrasound imaging to be integrated into a broad range of minimally invasive devices in different clinical contexts.