What Is Magnetic Resonance?

Magnetic resonance is a physical phenomenon in which nuclei or electrons with nonzero spin absorb and re-emit electromagnetic radiation at a characteristic frequency determined by both the strength of an applied static magnetic field and the intrinsic magnetic moment of the particle. This resonance frequency, called the Larmor frequency, is proportional to the applied field strength: for protons in a 1.5 tesla field it falls at approximately 63.9 MHz, in the radio-frequency range of the spectrum. The phenomenon was first described experimentally for nuclear spins by Felix Bloch and Edward Purcell in 1946, work that earned them the 1952 Nobel Prize in Physics.

The field draws on quantum mechanics and classical electromagnetism. Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and the clinical imaging modality known as magnetic resonance imaging (MRI) are all expressions of the same underlying physics, differentiated by which type of spin is interrogated and at what scale.

Nuclear Magnetic Resonance Physics

The Bloch equations, introduced by Felix Bloch in 1946, describe the time evolution of bulk magnetization in a magnetic field under the influence of a radiofrequency pulse and subsequent relaxation. Two relaxation time constants characterize the return to equilibrium after a perturbation: T1, the spin-lattice relaxation time, describes the recovery of longitudinal magnetization as spin energy is transferred to the surrounding lattice; T2, the spin-spin relaxation time, describes the decay of transverse magnetization as local field inhomogeneities and spin-spin couplings dephase the precessing moments.

Different tissues and materials have characteristic T1 and T2 values determined by molecular dynamics, binding environment, and field strength. In NMR spectroscopy, the resonance frequency also depends on the local electronic environment of the nucleus (chemical shift), making NMR the primary tool for determining molecular structure in solution. A foundational treatment of the physical principles is provided in the Magnetic Resonance Imaging Part I article in the Western Journal of Medicine, which identifies spin density, T1, T2, flow, and spectral shift as the five tissue variables from which MRI images are constructed.

MRI and Clinical Imaging

Magnetic resonance imaging translates NMR physics into three-dimensional anatomical images by using spatially varying gradient fields to encode position information into the Larmor frequency. Slice selection, frequency encoding, and phase encoding gradients allow the received signal from a large volume of tissue to be decoded into individual voxel values via Fourier transformation. Clinical MRI scanners operate at field strengths from 1.5 to 7 tesla, with higher fields providing greater signal-to-noise ratio and spectral resolution at the cost of increased susceptibility artifacts and RF power deposition.

MRI produces unparalleled soft-tissue contrast without ionizing radiation, enabling differentiation of white and gray brain matter, cartilage and ligament in joints, and tumor boundaries in abdominal organs. NIST maintains calibration resources for MRI systems documented in NIST Special Publication 250-97 on Magnetic Resonance Imaging, covering measurement standards for field strength, gradient performance, and image quality phantoms. Functional MRI (fMRI) extends the technique to map brain activity by detecting the blood-oxygen-level-dependent (BOLD) signal, a T2* effect.

Electron Paramagnetic Resonance

Electron paramagnetic resonance (EPR), also called electron spin resonance (ESR), applies the same resonance principle to unpaired electron spins, which precess at microwave rather than radio frequencies in fields of comparable strength. EPR is used to characterize free radicals, transition metal complexes, and defect centers in solid materials. An accessible explanation of NMR precession and the Larmor frequency at the e-MRI educational resource demonstrates how the relationship between precession frequency and field strength underpins both NMR and EPR.

Applications

Magnetic resonance has applications in a wide range of fields, including:

• Clinical MRI for neurological, musculoskeletal, and oncological diagnosis
• Functional MRI for brain mapping in neuroscience and presurgical planning
• NMR spectroscopy for molecular structure determination in chemistry and biochemistry
• EPR for characterizing free radicals and paramagnetic species in materials science
• Magnetic resonance spectroscopy for in vivo metabolite quantification
• Diffusion-weighted MRI for acute stroke detection and white matter tractography