In Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss biomagnetism, magnetic resonance imaging, and the biological effects of electromagnetic fields. We don’t, however, talk about the safety of static magnetic fields. If you want to learn more about that topic, I suggest an article by John Schenck:
Schenck, J. F. (2005) “Physical interactions of static magnetic fields with living tissues,” Prog. Biophys. Mol. Biol. Volume 87, Pages 185–204.
Clinical magnetic resonance imaging (MRI) was introduced in the early 1980s and has become a widely accepted and heavily utilized medical technology. This technique requires that the patients being studied be exposed to an intense magnetic field of a strength not previously encountered on a wide scale by humans. Nonetheless, the technique has proved to be very safe and the vast majority of the scans have been performed without any evidence of injury to the patient. In this article the history of proposed interactions of magnetic fields with human tissues is briefly reviewed and the predictions of electromagnetic theory on the nature and strength of these interactions are described. The physical basis of the relative weakness of these interactions is attributed to the very low magnetic susceptibility of human tissues and the lack of any substantial amount of ferromagnetic material normally occurring in these tissues. The presence of ferromagnetic foreign bodies within patients, or in the vicinity of the scanner, represents a very great hazard that must be scrupulously avoided. As technology and experience advance, ever stronger magnetic field strengths are being brought into service to improve the capabilities of this imaging technology and the benefits to patients. It is imperative that vigilance be maintained as these higher field strengths are introduced into clinical practice to assure that the high degree of patient safety that has been associated with MRI is maintained.
The article discusses magnetic forces due to tissue susceptibility differences, magnetic torques caused by anisotropic susceptibilities, flow or motion-induced currents, magnetohydrodynamic pressure, and magnetic excitation of sensory receptors.
On the lighter side, below are excerpts from a 2015 General Electric press report that describes one of Schenck’s claims to fame: his brain was the first one imaged using a clinical 1.5 T MRI scanner.
Heady Times: This Scientist Took the First Brain Selfie and Helped Revolutionize Medical Imaging
Early one October morning 30 years ago, GE scientist John Schenck was lying on a makeshift platform inside a GE lab in upstate New York. The [lab itself] was put together with special non-magnetic nails because surrounding his body was a large magnet, 30,000 times stronger than the Earth’s magnetic field. Standing at his side were a handful of colleagues and a nurse. They were there to peer inside Schenck’s head and take the first magnetic resonance scan (MRI) of the brain…
[In the 1970s] GE imaging pioneer Rowland “Red” Redington… hired Schenck, a bright young medical doctor with a PhD in physics [to work on MRI]… Schenck spent days inside Redington’s lab researching giant magnets and nights and weekends tending to emergency room patients. “This was an exciting time,” Schenck remembers….
It took Schenck and the team two years to obtain a magnet strong enough to… achieve useful high-resolution images. The magnet… arrived in Schenck’s lab in the spring of 1982. Since there was very little research about the effects of such [a] strong magnetic field on humans, Schenck turned it on, asked a nurse to monitor his vitals, and went inside it for ten minutes.
The field did Schenck no harm and the team spent that summer building the first MRI prototype using [a] high-strength magnetic field. By October 1982 they were ready to image Schenck’s brain.
Many scientists at the time thought that at 1.5 tesla, signals from deep tissue would be absorbed by the body before they could be detected. “We worried that there would only be a big black hole in the center” of the image, Schenck says. But the first MRI imaging test was a success. “We got to see my whole brain,” Schenck says. “It was kind of exciting.”…
Schenck, now 76, still works at his GE lab and works on improving the machine. He’s been scanning his brain every year and looking for changes… “When we started, we didn’t know whether there would be a future,” he says. “Now there is an MRI machine in every hospital.”