How MRI Works: Magnetic Resonance Imaging Explained Simply

Gradient coils are electrical coils that, when energized, create spatial variations in the magnetic field within the MRI scanner. Unlike the main magnet which produces a uniform field, gradient coils intentionally introduce controlled non-uniformities to enable spatial encoding of the MR signal. These spatial variations allow the system to determine where specific signals originate within the body, enabling the reconstruction of detailed anatomical images from the raw data collected during the scan.

Gradient coils apply a gradient (differential) along the main magnetic field, enabling spatial localization of MRI signals. These coils lie perpendicular to each other along the X, Y, and Z axes. By running current through gradient coils, magnetic fields are created that superimpose over the main B₀ field, either adding to or reducing its strength at different positions. This creates a linear variation in magnetic field strength along each axis. Since precessional frequency depends on magnetic field strength, different hydrogen protons experience different frequencies depending on their position. This differential magnetic field allows the MRI system to encode spatial information into the signal, forming the foundation for image reconstruction.

Gradient coils create linear variations in the main magnetic field along three perpendicular axes (x, y, z), enabling spatial encoding of the MRI signal. By applying gradients, the precession frequency of protons varies with position, allowing the system to determine the spatial origin of each signal. The gradient strength and direction are precisely controlled during image acquisition to encode spatial information into the frequency and phase of the detected signals.

This section explains how gradient coils enable spatial encoding in MRI. Gradient coils create controlled variations in the main magnetic field across different body regions, allowing the system to select imaging planes and encode spatial information. The three orthogonal gradients (X, Y, Z) work together: when Z gradient is selected, axial plane is imaged (X and Y perpendicular); when Y gradient is selected, coronal plane is imaged (X and Z perpendicular); when X gradient is selected, sagittal plane is imaged (Y and Z perpendicular). Gradient encoding involves two functions: phase encoding (gradiente codificador de fase) creates linear phase variation across the imaging plane, and frequency encoding (gradiente codificador de frequência) creates linear frequency variation. Together with slice selection, these gradients enable reconstruction of detailed anatomical images by encoding spatial location in the frequency and phase of received signals. The system turns these gradients on and off at specific times to encode spatial information, allowing the MRI to determine exactly where each signal originated within the body.

Gradient magnetic fields serve two critical purposes: slice selection and pixel localization. The Z-axis gradient determines slice thickness by selecting specific frequency ranges. The Y-axis gradient performs phase encoding, determining the direction of precession. The X-axis gradient performs frequency encoding, determining precession frequency based on local field strength. Gradient coils are pulsed intermittently for fractions of a second to create these spatial variations, allowing precise 3D localization of the MRI signal.

Functional MRI (fMRI) measures brain activity by detecting changes in blood oxygenation levels (BOLD signal), revealing which brain regions become active during specific tasks and mapping functional connectivity between regions; Diffusion Imaging tracks water molecule movement along axonal pathways to visualize white matter tracts and anatomical connectivity between brain regions, enabling researchers to reconstruct neural pathways such as motor pathways from the cortex through the internal capsule to the spinal cord.

Diffusion MRI applies strong gradient pulses to make signals sensitive to water molecule displacement. In free fluid, diffusion is isotropic; in structured tissue like white matter, diffusion is restricted along axons. Diffusion tensor imaging (DTI) measures diffusion in multiple directions, reconstructing 3D white matter tract orientations for brain connectivity visualization. fMRI uses BOLD contrast: activated brain regions have increased oxygenated blood, reducing deoxyhemoglobin and shortening T2* relaxation. This provides spatial resolution of a few millimeters and temporal resolution of a few seconds. Spectroscopy detects metabolites like choline (indicating tumors) and lactate (indicating anaerobic metabolism). Parallel imaging techniques (SENSE, GRAPPA) and compressed sensing reduce scan times. Real-time imaging captures cardiac and respiratory motion without breath-holding.

Diffusion-weighted imaging (DWI) measures water molecule movement at microscopic levels. Restricted diffusion (dark on ADC maps) indicates cellular swelling, high cellularity, or viscous fluids, crucial for early stroke detection and tumor characterization. Diffusion Tensor Imaging (DTI) measures water diffusion directionality in white matter tracts. Fractional Anisotropy (FA) values indicate tissue organization: high FA (0.7-0.9) in organized white matter, low FA (0.2-0.4) in gray matter. Tractography reconstructs white matter pathways for surgical planning. Functional MRI (fMRI) detects brain activity through blood oxygenation changes, showing activation during specific tasks. These techniques are essential for pre-surgical mapping of eloquent cortex and understanding brain connectivity.

Diffusion-weighted MRI measures water molecule Brownian motion, revealing cellular membrane integrity. Restricted diffusion appears dark; increased diffusion (in ischemia, tumors, inflammation) appears bright. Diffusion tensor imaging extends this by measuring directional diffusion, enabling reconstruction of white matter fiber tracts through mathematical tractography. Functional MRI detects neural activity via the BOLD effect: increased neural activity raises local blood flow, temporarily reducing deoxyhemoglobin (which is paramagnetic) and increasing signal. This enables mapping of brain regions activated by specific stimuli, revealing functional connectivity and neural circuit organization.

Modern neuroimaging relies on two complementary MRI-based technologies. Diffusion MRI tracks water molecule movement to infer neural connectivity, as water diffuses preferentially along axons rather than across myelin barriers. Functional MRI measures blood oxygen level dependent signals, where neural activity causes astrocytes to dilate nearby blood vessels, increasing blood flow. However, fMRI has severe limitations: each voxel pools signals from hundreds of cortical columns, and the BOLD response takes ~4 seconds to develop while neurons fire up to 100 times per second, capturing only about one-millionth of available neural information.

MRI produces multiple biological effects requiring safety protocols. Radiofrequency energy causes tissue heating, with safe limits of no more than 1°C peripheral increase and 37°C central temperature. Passive implants like aneurysm clips can move or heat, while active implants like brain stimulators may malfunction. Pacemakers are prohibited. Noise levels can exceed 99 decibels, requiring hearing protection. Claustrophobia management includes verbal communication and panic signaling devices. Safety screening requires informing patients the magnet is always on, using questionnaires about implants and ferromagnetic objects, and conducting oral interviews. Pediatric sedation follows guidelines from the American Academy of Pediatrics and American Society of Anesthesiologists, with greater vigilance for older children who may conceal ferromagnetic objects. Only one family member may accompany, who must also complete screening and wear hearing protection.

Certain patients are contraindicated for MRI due to the powerful magnetic field. Patients with aneurysms (particularly with metallic clips) and pacemakers cannot undergo MRI as the magnetic field can cause metallic objects to move or malfunction. Patients with metallic implants such as cochlear implants, metallic foreign bodies in the eyes or ears, and military personnel with retained metallic fragments cannot undergo MRI due to the risk of injury or death. Ferromagnetic objects in the MRI room can become dangerous projectiles. MRI coils and wires must be properly positioned and not in contact with the patient, with a distance of at least 3-4 fingers between the coil and patient.

Magnetic resonance imaging (MRI) uses electromagnetic fields and a giant magnet to produce high-quality images of soft tissues without ionizing radiation, making it superior to X-rays and CT scans for brain and joint imaging; however, patients must disclose all metallic implants, including cerebral aneurysm clips (absolute contraindication) and pacemakers (relative contraindication), as the MRI's powerful magnetic field (30,000 times Earth's strength) can cause ferromagnetic objects to move dangerously, though modern titanium/platinum clips and careful risk-benefit assessment allow some patients to safely undergo the exam.

MRI safety requires strict protocols due to the powerful magnetic field. Ferromagnetic objects become dangerous projectiles, necessitating screening of all visitors and equipment. Patient screening is critical for metal implants: ferromagnetic implants are contraindicated, while non-ferromagnetic materials (titanium) are generally safe. Absolute contraindications include pacemakers, cochlear implants, and certain aneurysm clips. Vascular stents require careful evaluation based on implant material and field strength. Post-surgical patients with metal hardware need assessment for safety.

MRI safety requires careful consideration of contraindications including metallic implants (pacemakers, aneurysm clips, cochlear implants) which can experience dangerous torque and heating effects in magnetic fields, and early pregnancy where limited safety data, potential contrast agent exposure, and thermal effects create theoretical risks to fetal development; healthcare providers must consult with specialists and verify device specifications before proceeding with MRI examinations.