What Is Biomagnetics?

Biomagnetics is an interdisciplinary field concerned with the interaction between magnetic fields and biological systems, including both the magnetic fields that living organisms produce and the effects of external fields on biological tissue. The discipline spans passive measurement of biological magnetic activity, active stimulation of tissue using applied fields, and the use of engineered magnetic materials as therapeutic or diagnostic agents. Biomagnetics draws on physics, electrical engineering, materials science, and biomedical engineering, and the IEEE Magnetics Society identifies it as a distinct research domain at the convergence of magnetics, biology, and medicine.

Biological Magnetic Fields and Sensing

Living organisms generate weak magnetic fields as a byproduct of ionic currents in excitable cells. The human heart produces the strongest of these, with a peak field strength on the order of 100 picotesla, while the brain generates fields roughly a thousand times weaker, in the femtotesla range. Measuring these signals requires superconducting quantum interference devices (SQUIDs) operating in magnetically shielded rooms, because even geomagnetic field fluctuations would otherwise overwhelm the signal. Magnetocardiography (MCG) and magnetoencephalography (MEG) record the magnetic counterparts of the electrocardiogram and electroencephalogram respectively, providing complementary spatial resolution and requiring no physical contact with the skin. MEG is particularly valued in presurgical mapping of epileptic foci and functional brain areas because its spatial localization is less distorted by the resistive effects of the skull than that of electroencephalography.

Magnetic Stimulation

Transcranial magnetic stimulation (TMS) applies a brief, intense magnetic pulse to the scalp using a figure-eight coil carrying several thousand amperes for a few hundred microseconds. The time-varying magnetic field induces an electric field in the underlying cortex sufficient to depolarize neurons, allowing researchers and clinicians to temporarily activate or suppress cortical regions noninvasively. Repetitive TMS (rTMS) protocols produce lasting changes in cortical excitability and have received regulatory approval for treatment of medication-resistant major depressive disorder in the United States. The physical principles governing coil design, pulse shape, and induced field distribution are grounded in classical electromagnetic theory, and research on biomagnetic stimulation and imaging documents the clinical validation of these therapeutic protocols. Peripheral magnetic stimulation is applied in a similar manner to stimulate peripheral nerves and muscles for pain management and rehabilitation.

Magnetic Materials and Particles in Medicine

Engineered magnetic materials, particularly superparamagnetic iron oxide nanoparticles (SPIONs), have found extensive use in both diagnostic imaging and therapy. As MRI contrast agents, SPIONs shorten the transverse relaxation time (T2) of nearby water protons, producing dark regions in T2-weighted images that can be used to detect tumors, characterize liver lesions, and track labeled cells in vivo. In magnetic hyperthermia, nanoparticles concentrated at a tumor site are exposed to an alternating magnetic field at frequencies of 100 kHz to several hundred kHz, causing the particles to dissipate energy as heat through Neel relaxation and Brownian motion. The local temperature rise, targeted to the range of 42 to 45 degrees Celsius, damages cancer cells preferentially while sparing surrounding tissue. IEEE Xplore publications on recent advances in biomagnetics discuss the electromagnetic design parameters for hyperthermia systems and the challenge of achieving uniform field distribution over clinical tumor volumes. Research into magnetic particle imaging (MPI), a newer modality that images the distribution of SPIONs directly using a time-varying applied field, offers the potential for high-sensitivity, radiation-free tracer imaging.

Applications

Biomagnetics has applications in a range of fields, including:

• Clinical neurology through magnetoencephalography for epilepsy mapping
• Psychiatry through repetitive transcranial magnetic stimulation for depression treatment
• Oncology through magnetic hyperthermia cancer therapy
• Diagnostic radiology through MRI contrast enhancement with iron oxide particles
• Cell biology through magnetic labeling and sorting of cell populations