FLASH radiation therapy (RT), an emerging technology delivering radiation at ultra-high dose rates, holds great potential to overcome the limitations of conventional RT. To accelerate wide clinical adoption of FLASH RT, it is essential to address the challenges associated with precise dosimetry and real-time beam monitoring for Ultra-High Dose Rate (UHDR) radiation treatments. We propose to solve this technical gap in FLASH RT by introducing a newly invented imaging concept, namely ionizing radiation acoustic imaging (iRAI). As validated in our preliminary studies, iRAI offers a unique and practical tool for mapping the dose deposition during RT, and holds a great potential to address the challenges associated with precise dosimetry and real-time beam monitoring for FLASH RT. The goal of this research is to develop an iRAI-based ultrafast dosimetry system for 3D mapping of accumulated radiation dose during FLASH RT in real time with single-pulse resolution and characterizing the time structure of the radiation beam. By leveraging the unique advantages of iRAI technology in mapping radiation beams, including high resolution, high imaging speed, and high accuracy, our proposed ultrafast volumetric dosimetry system, once validated, could provide a practical and cost-effective solution for quality assurance in FLASH RT.