I had been having palpitations, and managed to convince my PCP to order a heart test. She wanted me to get a Holter monitor but I insisted on an echocardiogram which turned out to have technical issues (due no doubt to a training situation: in a teaching hospital, an indigent will often be “teaching material”). This led to the sonographer recommending a cardiac MRI. Unlike a previous (spinal) MRI, the noise was tolerable, and the results were normal. I was glad to have gotten some basic personal health information without further exposure to ionizing radiation.

Nicola Tesla discovered the Rotating Magnetic Field way back in 1882, but the specific phenomenon on which MRI is based (nuclear magnetic resonance (NMR)) was not discovered until 1937, when Isidore I. Rabi, a physicist at the Pupin Physics Laboratory in New York, did experiments demonstrating the resonant interaction of magnetic fields and radio-frequency pulses. (“Resonance” in physics refers to a matching of frequencies; in this case, that of the radio waves and the vibrations of the protons from water molecules.) The idea for NMR actually came from an obscure Dutch physicist named Cornelius Gorter, in the previous year, but he could not demonstrate it experimentally due to a limited setup. Rabi, with his superior technological resources, was able to detect magnetic resonance in a ‘macular beam’ of lithium chloride. In practical terms, this meant that the structure of chemical compounds could now be identified spectroscopically. Several years later, Block and Purcell simultaneously demonstrated NMR in condensed matter (water and paraffin, respectively).

When a patient is placed in a MRI machine, a powerful magnet pulls the positively charged protons of the body’s water molecules into alignment, after which a radio wave pulse of the same frequency as the particles’ oscillation knocks them askew. When the radio frequency pulse is turned off, the protons relax and return to alignment, sending back ‘captured’ information about the structure of which they are a part. This signal appears as a diffuse, amorphous image called k-space. To get a coherent version of this image, redundant information must be subtracted from it via a computer algorithm called the Fast Fourier Transform. The result is the remarkably detailed pictures of the brain and other bodily organs we are used to seeing in reproductions. (The Fourier transformation decomposes time signals into sinusoidal components of varying frequency. Thus, it uses mathematics to simplify physical phenomena for technological applications.) A chemist and a physicist (Paul Lautebur and Peter Mansfield) were given the Nobel Prize for the invention of the MRI, but it was a physician, Raymond Damadian, who actually built the first NMR body scanner, which can be viewed in the Smithsonian.

Structural (diagnostic) MRI is generally considered more reliable than functional MRI. The latter has come under fire in recent years for a number of reasons, most notably the generation of spurious results, including brain activity in a dead fish which was said to be used as a control (a bit of humor, no doubt), since the machine required an object to avoid damaging g itself). Unlike diagnostic MRI, fMRI works on the BOLD principle as an indicator of neural activation. (The magnetic resonance signal is an indirect effect related to changes in blood flow from neuronal activation.) This assumption is controversial.

The obvious advantage of MRI as an imaging choice is the absence of ionizing radiation, with its potential to cause cancer.) There is a general professional consensus that ~ 100 milliSieverts puts one in a higher risk category, but for amounts below that (“low-level” radiation) the picture is unclear. And whether sporadic media reports about this over the past decade have had any effect is a matter of speculation. Some companies have provided dosage reduction software, and there are the Image Gently and Image Wisely campaigns to reduce exposure in children and adults respectively. But there is still too much ignorance about the risk, which is more serious for children (as well as for small, relatively young adults). And there is insufficient attention being given to multi phase exams, many of which are unnecessary. Finally, there is the matter of communicating the risk. In Europe, this is done clearly on complete consent forms. In the US. patients are generally not given any idea of the large amount of radiation they will receive from CT scans or nuclear medicine procedures.

-~ Rylan Dray, Ph.D. – December 2020