B-mode US: Basic Concepts and New Technology. AAPM/RSNA Physics Tutorial for Residents
If you prefer having something to read rather than watching videos, this is 14-page publication provides an excellent overview of basic ultrasound physics as well some more advanced concepts.
If you’re starting from scratch it might be best to use something more basic initially and read this to advance you knowledge.
B-mode US: Basic Concepts and New Technology