Here is a good introductory 18 minute YouTube video, Gut Pocus by Dr. Baker, June 9, 2019:
And here are two resources Dr. Baker mentions in her video:
(1) Ultrasound assessment of gastric content and volume [PubMed Abstract] [Full Text HTML] [Full Text PDF]. Br J Anaesth. 2014 Jul;113(1):12-22. doi: 10.1093/bja/aeu151. Epub 2014 Jun 3.
The above article has been cited by 22 PubMed Central articles.
(2) How to perform gastrointestinal ultrasound: Anatomy and normal findings [PubMed Abstract] [Full Text HTML] [Full Text PDF]. World J Gastroenterol. 2017 Oct 14;23(38):6931-6941. doi: 10.3748/wjg.v23.i38.6931.
The above article has been cited by 6 PubMed Central articles.