Microwave tomography · Grey Matter

Microwave tomography sends low-power microwaves through the head from a ring of antennas and reconstructs a map of how the tissue inside slows and absorbs them, and its main use is to tell a bleed from a clot in a suspected stroke with equipment that fits in an ambulance.

Microwave tomography. Microwave tomography sends low-power microwaves through the head from a ring of antennas and reconstructs a map of how the tissue inside slows and absorbs them, and its main use is to tell a bleed from a clot in a suspected stroke with equipment that fits in an ambulance.

Tissues differ in their dielectric properties, which depend mostly on how much water and salt they hold. Blood from a haemorrhage raises them, tissue starved of blood after an ischaemic stroke lowers them, and the difference is large enough to see. Antennas around the head take turns transmitting and receiving, and an algorithm searches for the map of properties that would explain every measured signal, an inverse problem much like that of EEG. Prototype systems work around 0.5 to 2.5 GHz, the compromise between frequencies that penetrate the head and those short enough to resolve detail, and reconstruct images on grids of a few millimetres.

It is a route that trades resolution for depth. Microwaves cross bone and reach the whole brain, but their wavelength in tissue is centimetres, so they see lesions, never cells.

It images what tissue is made of. Neural firing changes dielectric properties too little to detect this way, so microwave tomography is a candidate for monitoring and triage and has no route to decoding.

Its appeal is cost and size. A helmet of antennas is cheap and portable next to CT or MRI, which matters when the decision between a clot-dissolving drug and none depends on minutes.

Questions: Could microwaves passed through the head read what a person is thinking? With the physics as it is known, no. Microwave tomography maps how much water and salt tissue holds, at grids of a few millimetres and wavelengths of centimetres inside the head, which is enough to find a pool of blood after a stroke and far too coarse and slow to follow neural activity, whose effect on those properties is tiny. A worry about mental privacy is better spent on the methods that do read activity, such as EEG headbands, fMRI decoders and implants, where the data already exist.