Simple really. Using the transverse view the needle tip can be hard to visualise. In the longitudinal view you don’t see the carotid artery. Applying an oblique view with an obliquely oriented needle “uses the superiority of the short axis view by visualizing all of the important surrounding structures (artery and vein) in an oblong view while allowing continuous real-time visualization of the long axis of the needle, therefore providing a larger, more easily visible target with a brighter more easily recognized needle.” The ultrasound probe is orientated at approximately 45° so that the medial end of the ultrasound probe aligns with the patient’s contralateral nipple or shoulder.

The oblique view: an alternative approach for ultrasound-guided central line placement
J Emerg Med. 2009 Nov;37(4):403-8
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