imagingMRI

Magnetic resonance imaging (MRI) is a medical imaging modality that uses strong magnetic fields and radiofrequency pulses to produce detailed pictures of internal anatomy. Unlike X-ray–based techniques, MRI does not rely on ionizing radiation, enabling repeated imaging with excellent soft-tissue contrast.

Principle: A large external magnet aligns hydrogen protons in the body. Radiofrequency pulses disturb this alignment; as protons relax, they emit signals that are spatially encoded by gradient fields to form cross-sectional images. Differences in T1 and T2 relaxation times create tissue contrast.

Equipment and techniques: MRI systems typically operate at 1.5 to 3 tesla. Imaging uses coils to transmit and receive signals. Common sequences include T1-weighted, T2-weighted, FLAIR, diffusion-weighted imaging (DWI), and perfusion-related methods. Gadolinium-based contrast agents may enhance vascular or tissue properties when indicated.

Applications: MRI provides high-resolution images of the brain and spinal cord, joints and musculoskeletal tissues, and abdominal or pelvic organs. Functional MRI assesses brain activity by blood flow changes. Diffusion and other advanced techniques map white-matter pathways and tissue microstructure.

Safety and limitations: MRI requires screening for implants or devices that are unsafe in a magnetic field. Contraindications include certain pacemakers, aneurysm clips, and metal fragments. Claustrophobia and motion can degrade images. Contrast agents carry rare risks, particularly in patients with kidney impairment; new agents and protocols aim to reduce exposure. The modality is relatively expensive and time-consuming.

Advances: Ongoing improvements include faster scan methods, higher-field systems, and quantitative MRI biomarkers that standardize measurements across centers.