Footnotes

• Handling editor Simon Carley

• Contributors EN and WS obtained ultrasound images jointly. EN conducted the primary literature review and authored the primary article. WS edited and provided advice on the article.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; externally peer reviewed.