It could speed diagnosis and treatment of stroke to boost chances of recovery, the scientists say.
The wearable cap bounces microwaves off the brain to determine whether there has been a bleed or clot deep inside.
The Swedish scientists who made the device plan to give it to ambulance crews to test after successful results in early studies with 45 patients.
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.
A computerised tomography (CT) scan will show this, but it can take some time to organise one for a patient, even if they have been admitted as an emergency to a hospital that has one of these scanners.
Any delay in this “golden hour” of treatment opportunity could hamper recovery.
To speed up the process, researchers in Sweden, from Chalmers University of Technology, Sahlgrenska Academy and Sahlgrenska University Hospital, have come up with a mobile device that could be used 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.
Tests with an early prototype – a refashioned bicycle helmet – found it could accurately distinguish between bleeds (haemorrhagic stroke) and clots (ischaemic stroke), although not 100% of the time.
They have since built and tested a custom-made helmet to better fits skulls of different shapes and sizes, and they have tested it out with the help of nurses and patients at a local hospital ward.
Ultimately, they want to fit it into the pillow the patient rests their head on.
The researchers say their device needs more testing, but could be a useful aid in the future.