The sharp inner needle (schematic and inset photo) used to puncture the cardiac septum
to gain access to the left atrium can be safely recessed within a blunt outer needle cannula. After puncturing, the dilator sheath is advanced over the needle into
the left atrium. The probe includes two optical fibers positioned within the inner needle for pulse-echo ultrasound imaging: one for transmission (Tx) with the
delivery of pulsed excitation light to an optically absorbing coating and one for reception (Rx) with the delivery of continuous-wave (CW) light to a Fabry-Pérot
cavity. Acoustic isolation between the Tx and Rx fibers is provided by a thin metal septum. Scale bar, 500 μm.