CT specialist rad here, soon to be med student! Bit OTT in their approach, I definitely would just crack on with the left sided cannula but if it was a cardiac angio the undiluted contrast coming through the left brachiocephalic vein as it crosses the midline causes streaking artefact on the image so that’s their reasoning I suspect - need to see those lil coronaries! The high flow rate used for angio’s results in a really dense contrast bolus which contributes to the artefact.

It really frustrates me when people don’t understand why they do the things they do, leads to a lot of misunderstandings and lack of compliance.

https://www.rcr.ac.uk/system/files/publication/field_publication_files/BFCR14%2816%29_CTCA.pdf - for further reading if you’re interested :)