Section 1.1: Breaking Down Nuclear Magnetic Resonance (NMR)

Let’s dissect the terminology of this technique for clarity (similar to how I analyzed GC/MS). The term “nuclear” pertains to the nucleus of an atom, not nuclear energy or radioactivity; it involves “exciting” atomic nuclei. “Magnetic” is straightforward, as this method employs massive, powerful magnets to create intense magnetic fields. Lastly, “resonance” describes a resonant frequency: think of that experiment where you delicately rub a wine glass's rim to produce a sound…

This is akin to what we do with atoms! However, instead of creating music from glass, we’re generating “music” from hydrogen atoms through magnets, magnetic fields, and radio waves.

Section 1.2: The Physical Structure of an NMR

An NMR instrument resembles a large metal cylinder, with the core processes occurring inside. A sample is placed within this cylinder, nestled between powerful magnets made of superconductive materials that operate at extremely low temperatures. So, how cold is it? A standard NMR magnet must be cooled to 4 K (Kelvin), which equates to -452°F or -269°C. For context, 0 K represents the lowest conceivable temperature, meaning an NMR magnet functions just 4 degrees above that threshold. Naturally, you might wonder, “Why such low temperatures?” and “How is this achieved?”

The latter question is simpler to address: the magnet is surrounded by commercially available liquid helium, which boils at 4 K. Since liquid helium can be costly, it is kept from boiling off too quickly by surrounding it with an insulating layer of liquid nitrogen, which boils at 77 K and is more economical. Despite advancements in insulation, both liquids will eventually evaporate, necessitating regular refills—liquid nitrogen weekly and liquid helium monthly.

To succinctly answer the first question, the magnet must be cold to maintain superconductivity, which is crucial for generating a powerful magnetic field. The stronger the magnetic field, the more precise the information we glean from the analysis.

Section 2.1: The Relationship Between NMR and MRI

Remember—NMR and MRI are closely related. An MRI machine operates on identical principles, but instead of producing a molecular fingerprint, it translates the emitted signals into pixel values, generating an image. The hydrogen-containing molecules in your muscles, liver, and tendons emit distinct signals; through tuning and mathematical calculations, an image is produced.

NMR used to create an image is MRI | Source

You may be pondering: “Didn’t you mention that a sample is placed between superconducting magnets?” Yes! When you enter an MRI machine, you are positioned between powerful magnets. Have you ever noticed the ominous warnings against bringing metal into the MRI room? This stems from two main reasons. First, these magnets can physically pull metal objects towards them, and second, any metal present will disrupt the magnetic field, which is essential for accurate data. A uniform magnetic field yields the best results, and metal can compromise this uniformity, potentially leading to poor data quality.

Despite lying within a robust magnetic field and confined in a potentially claustrophobic space, the MRI technique has numerous benefits. It does not expose patients to radiation like x-rays, can visualize aspects that x-rays often miss, and is primarily non-invasive. Moreover, it delivers superior imaging quality compared to traditional x-rays; however, it comes with a higher price tag (MRIs are significantly more costly than x-rays). Due to this expense, x-rays are often more accessible, prompting providers to typically order an x-ray before considering an MRI.

This MRI is quite old, but you get the gist — they can be quite claustrophobic | Photo by National Cancer Institute on Unsplash

In conclusion, you now grasp the essential principles of magnetic resonance imaging. Remember—giant superconducting magnets and metal create a hazardous situation! They can also demagnetize the strips on your credit cards, so it’s advisable to leave all belongings outside the examination room. Many chemists have learned this the hard way, forgetting to empty their pockets before approaching the NMR instrument, resulting in malfunctioning credit cards.