The helmet bounces microwaves off the brain to determine whether there has been a bleed or clot deep inside.
Researchers in Sweden, from Chalmers University of Technology, Sahlgrenska Academy and Sahlgrenska University Hospital, came up with the device that could be used by ambulance crew on the way to hospital.
The helmet uses microwave signals - the same as the ones emitted by microwave ovens and mobile phones but much weaker - to build a picture of what is going on throughout the brain.
When a person has a stroke, doctors must work quickly to limit any brain damage. If it takes more than four hours to get to hospital and start treatment, parts of their brain tissue may already be dying.
But to give the best treatment, doctors first need to find out if the stroke is caused by a leaky blood vessel or one blocked by a clot, 'BBC News' reported.
A computerised tomography (CT) scan will show this, but it can take some time to organise one for a patient.
The new helmet could speed diagnosis and treatment of stroke to boost chances of recovery.
Tests with an early prototype of the device - a refashioned bicycle helmet - found it could accurately distinguish between bleeds (haemorrhagic stroke) and clots (ischaemic stroke), although not 100 per cent of the time.