This paper proposed an oscillator-based ultrasonic sensor interface that can be applied to intravascular applications. The readout circuits are validated with a capacitive micromachined ultrasonic transducer (CMUT) and a current-to-frequency chip. The CMOS CMUTs are integrated with a current amplifier on the same chip and the current-to-frequency chip provides the current-to-frequency readout interface. Relaxation oscillators with injection locking are employed to achieve low-power consumption. Also, the time-based output signal can be further digitized with a time-to-digital converter. Both chips are fabricated in a 0.35μm CMOS MEMS process technology. The CMUTs are designed with 1MHz to 4MHz cells for intravascular diagnosis applications. The current amplifier consumes 281.7 μW and the power consumption of the current-to-frequency circuits is 25.4 μW.