(As published on physicsfocus)
I’m writing this post from Room 7 of the paediatric emergency ward of l’Hopitale Sud in Rennes, France. It’s my fifth day spent in the room, distantly separated from my holiday luggage. It might seem like a strange priority, writing a piece for physicsfocus at this juncture, but there’s not a lot to do in the evenings, other than watch a cathode ray tube plotting graphs of the electrical signals emanating from my four year old son in the bed next to mine. I’m fine. He will be too, thanks to several dozen outstanding French doctors and the miracles of medical physics.
It has been a week that we’ll never forget; a week that has tested our emotional stamina and my O-level French to the limit. I could write about the turmoil caused by the cruel and dangerous condition that suddenly afflicted my son, or about the sheer brilliance of the French doctors who managed to diagnose an illness that only affects one person in a hundred thousand, with unique symptoms each time. But in view of this blog’s remit, I’ll tell you about a physical phenomenon that helped to preserve my sanity by providing a distraction from an otherwise bleak day.
Some of the vital diagnostic clues were provided by an MRI machine. You might know that MRI (Magnetic Resonance Imaging) uses an incredibly strong magnetic field, only realisable by modern superconducting electromagnets that can carry high electric currents without the resistance that would make ordinary metal wires heat up to melting point. Consequently, I was told, before approaching the machine, to remove any metal objects including my belt, phone, etc, that might fly towards the magnet, causing injury. Surprisingly, I was allowed to keep my gold wedding ring.
Entering the MRI room, I noticed the heavy door was edged with copper contacts that meshed with similar contacts in the door frame, to complete a Faraday cage: a metal enclosure completely surrounding the machine, screening its sensitive magnetic probes from stray radio noise in the outside world, and also preventing its own radio signals spilling out into the hospital.
My little one lay in the machine’s central tunnel, and the compassionate technician in charge let me lean in to hold his hand. The technician was ad-libbing, since this was an adult hospital, to which we had been diverted due to a broken MRI in the local children’s hospital. So perhaps she had overlooked the fact that my left hand – complete with wedding ring – would be inside the high-field region. In the event, the ring remained obediently on my finger and caused no problems, but I was treated to the distracting sensation of the gold band dancing and vibrating on my ring finger as the magnetic field was switched back and forth to elicit informative radio broadcasts from all the atomic nuclei in my son’s brain.
By tugging magnetically on the tiny bar magnet that is an atomic nucleus, then nudging it with a radio wave, the machine makes it precess exactly like a wobbling spinning top. That wobbling nucleus makes its own magnetic field wobble, generating radio waves that are picked up by the machine. As a side effect, the switching magnetic field made electric currents flow around my gold ring, turning it into an electromagnet that pushed and pulled against the field. Understanding the process made it seem no less magical when an invisible force shook my hand.
I’m very glad to be living in an age when this incredible technology, which would have been science fiction only a few years ago, has developed out of the curiosity-driven research of academic physicists. The non-invasive MRI scan was able to rule out all the common ailments, leading to a swift diagnosis and treatment.
They tell me he’s going to be OK; we just have to wait. The French medical staff have been excellent and the medical physics has been state-of-the-art. Call me a harsh critic, but I’m afraid, all in all, the holiday still gets a thumbs down.