Kanishka: What is resonance and how does it work for Magnetic Resonance Imaging (MRI)?
To understand how novel functional MRI techniques can capture brief stimulus-response events in the brain, it’s helpful to first understand the basic principles behind MRI itself. Starting with the concept of resonance and how it’s used in MRI provides a foundation to appreciate how advances in this field have made it possible to image fast, event-related brain activity in real time.
Known from the Harry Potter franchise, The Fat Lady is a talking painting at Hogwarts School of Witchcraft and Wizardry. In one scene of the Harry Potter and the Prisoner of Azkaban (2004), she tells the students to wait as she tries to shatter a glass with her voice.
The Fat Lady was trying to witness the phenomenon of resonance, attempting to break the glass with her voice by matching its natural frequency. When two frequencies align, sound (for example here), can transfer energy to break the glass. And so, when energy finds the perfect frequency, it amplifies, transforms, and sometimes even breaks through.
From capturing local signals (LFPs) to whole brain signals (MRI)
In my last blog, I explained what local field potentials (LFPs) are and how they help us study stimulus-response events in the brain. However, because of their limited spatial coverage, LFPs can only capture signals from a small region—just a few millimeters around the electrode. While it’s possible to place electrodes across the entire scalp—known as scalp EEG—for broader coverage, recording local field potentials (LFPs) deep within the brain would require highly invasive procedures that are neither practical nor ethically feasible in most contexts. Nevertheless, the time resolving capabilities of LFPs and EEGs are commendable and thus makes them a gold standard for electrophysiological studies in neuroscience.
Now, when it comes to capturing responses to specific stimuli from the whole brain, there’s an exceptional non-invasive technique called Magnetic Resonance Imaging (MRI). It is built on the principles of Nuclear Magnetic Resonance (NMR) which describes how atomic nuclei behave in magnetic fields. MRI leverages this phenomenon to produce high-resolution, three-dimensional images, all without using harmful radiation. A step further is an advanced form called functional MRI (fMRI), which reveals real-time brain activity by detecting changes in blood oxygen levels. These changes reflect shifts in blood flow and volume that are linked to underlying neuronal activity.
Rise of the MRI: Not so boring history
Nuclear magnetic resonance (NMR) was first discovered as a phenomenon by Isidor Isaac Rabi (Nobel prize 1944), while exploring behaviors of isolated Hydrogen, Deuterium and Lithium nuclei under magnetic fields. The extension of NMR to solid and liquid systems came later from the work of Felix Bloch and Edward Purcell (Nobel Prize 1952). While Purcell used a quantum-based approach for measuring energy absorption, Bloch took a classical route by measuring current from a spinning magnetization vector.
Richard Ernst (Nobel prize 1991) revolutionized Magnetic Resonance by making it possible to observe all the nuclei in a molecule or the human body at the same time, instead of one by one—greatly improving sensitivity. He used a method called Fourier transformation to separate and analyze complex signals, helping scientists and doctors get much more detailed and useful images. Later, Paul Lauterbur and Peter Mansfield (Nobel prize 2003) turned what was once seen as a problem—small variations in the magnetic field (now called gradients)—into a powerful tool. By using these “Gradients”, they made it possible to figure out exactly where signals in the body were coming from, allowing Magnetic Resonance Imaging to create detailed images of internal structures.
It’s amazing how NMR as a phenomenon evolved over time into a powerful technique, even earning multiple Nobel Prizes along the way!
Resonance in MRI
Resonance is a fascinating concept. It happens when two bodies/systems can exchange energies when their frequencies match with each other. Just like the Fat Lady tried to demonstrate! Our body is 60% water and since each water molecule contains Hydrogen and oxygen atoms, most of the MRI signal comes from hydrogen. Interestingly, nearly all brain disorders lead to changes in water content, and these alterations are clearly visible in MRI scans.
But how does MRI actually scan water molecules? It all comes down to Hydrogen (H) nuclei. Due to their intrinsic properties, H nuclei behave in a unique way when placed in magnetic fields. They begin to precess—a motion similar to a spinning top—and the frequency at which they precess is determined by:
ω0 = -γ B0
where ω0 is the frequency of the precessing particle, B0 is magnetic field γ is the gyromagnetic ratio, a constant specific to each particle.
For example, at 9.4 Tesla field strength, the H protons precess at around ~400 MHz – this is their Larmor (resonant) frequency, meaning they will only absorb energy if the applied field matches this frequency. Once the protons are aligned to a static field B0,, they gain a tiny net magnetization in that direction. When a radio-frequency field is applied to this system, the protons now experience two magnetic fields – one is static (from the magnet), and a rotating field B1 generated by a radio-frequency (RF) coil.
Static field (B0) vs rotating field (B1)
The rotating field B1 is much weaker than the strong static field B0 but still manages to flip the protons from a lower energy state (also called thermal equilibrium state) to an excited state! This seemed puzzling to me at first to understand: how can such a weak field cause protons to flip?
The answer lies in resonance. This weak field is still “resonant” with the proton’s precessional frequency. Also, this RF field is oscillating in nature. As the spin precesses, the rotating RF field stays in sync with it. This continuous alignment allows even a weak RF field to transfer energy, eventually flipping the spins to a higher energy state. So, the key indeed is resonance!
Studying such rotating systems can be hard to imagine. One elegant trick used in MRI is the idea of “rotating frame”. Instead of watching the spin from a fixed outside perspective (where everything is spinning!), we shift to a rotating reference frame—as if we’re a part of the rotation, spinning with the proton. In that frame, the rotation disappears, and the problem becomes much easier to visualize and solve. I recently read a quote that fits here:
“Two monks were arguing about a flag. One said: “The flag is moving.” The other said: “The wind is moving.” The sixth patriarch happened to be passing by. He told them: “Not the wind, not the flag; mind is moving.”
-Mumon, “The Gateless gate”
All this might sound abstract, but that’s the way the quantum mechanics of MR are. Fortunately, most of the practical aspects of MRI can be understood without quantum philosophies!
In conclusion, resonance is key in MRI. By tuning the radio-frequency (RF) field to the proton’s Larmor frequency, MRI can reveal detailed structural and functional information within tissues.
This wraps our discussion on resonance in MRI. Stay tuned for my next blog, where we will explore more about studying brain activity via LFPs and MRI!
Further reading/references
Malcolm H. Levitt. Spin dynamics: basics of nuclear magnetic resonance. John Wiley & Sons, Chichester, UK, 2001, 686 pp. Price £34.95. ISBN 0-471-48921-2
Richard Ernst (1933–2021).Science 373,164-164(2021).DOI:10.1126/science.abj9824
https://mriquestions.com/quantum-reality.html
Is quantum mechanics necessary for understanding magnetic resonance?
https://www.tafce.com/index.php?title=Fat_Lady_in_Painting
https://mriquestions.com/fall-to-lowest-state.html
Research groups
https://uefconnect.uef.fi/muistin-neurobiologia/ led by Prof. Heikki Tanila
https://uefconnect.uef.fi/biolaaketieteellinen-mri/ led by Prof. Olli Gröhn
Kanishka works as a doctoral researcher in the Neuro-Innovation PhD Programme. Her research focuses on functional imaging of brain-wide networks associated with momentary events.