Children requiring veno-arterial extracorporeal membrane oxygenation (VA ECMO) or cardiac surgery often undergo cervical cannulation or carotid artery clamping, which can interrupt cerebral circulation. Inadequate collateral flow through the circle of Willis (CoW) may lead to cerebral ischaemia within the vascular territory and/or watershed regions. Pre-cannulation survey of the CoW using transcranial Doppler (TCD) ultrasound may be performed to predict and plan neuroprotection. It is important to note in this article TCD refers to Duplex or colour coded trans-cranial Doppler (TCCD) using radiology-based machines, which is distinct from the more traditional trans-cranial Doppler technique that does not incorporate a B-mode image. This article describes our technique, in use since 2019, to guide surgical approaches and neuroprotective measures when an incomplete CoW is identified. High-end radiology-based ultrasound platforms and various transducers are used to assess brain morphology and haemorrhage through the anterior fontanelle in neonates. TCD is performed with the highest frequency transducer possible, utilising Doppler imaging to visualise cerebral arteries. Manual carotid compression can be used to functionally assess collateral flow when segments appear aplastic or hypoplastic. Potential pitfalls include mistaking the anterior choroidal artery for a hypoplastic posterior communicating artery (PCommA). Since implementing this protocol 5 years ago, no catastrophic infarcts related to cervical cannulation have occurred. This technique provides a practical solution for pre-operative assessment of cerebral collateral circulation in children undergoing VA ECMO or cardiac surgery, allowing for consideration of neuroprotective measures and improving patient safety.