Methods for measuring blood flow speed within blood vessels using a PA imaging system are disclosed. The methods include position a field of view of a PA imaging system along a section of a blood vessel, introducing a PA discontinuity into the blood flow within the blood vessel upstream of the FOV of the PA system, monitoring the movement of the PA discontinuity during movement through the FOV of the PA imaging device, and calculate the blood flow speed using at least two measured positions and corresponding times of the PA discontinuity within the FOV of the PA imaging system. PA discontinuities may include blood surges formed by compressing and releasing the blood vessel and/or boluses of a fluid with high PA contrast relative to the blood, such as saline solution.
