1. A method for spatially calibrating an ultrasound probe, comprising: placing the ultrasound probe in a first position and orientation relative to a phantom; measuring the first position and orientation of the ultrasound probe; acquiring a first ultrasound image of the phantom; determining a first spatial relationship between a phantom reference frame and a pixel reference frame, based on the first ultrasound image; repositioning the ultrasound probe in a second position and orientation relative to the phantom; measuring the second position and orientation of the ultrasound probe; acquiring a second ultrasound image of the phantom; determining a second spatial relationship between the phantom reference frame and the pixel reference frame, based on the second ultrasound image; and computing a probe calibration matrix based on the first position and orientation of the ultrasound probe, the first spatial relationship, the second position and orientation of the ultrasound probe, and the second spatial relationship.