Magnetic resonance imaging

Magnetic resonance imaging is a non-invasive medical imaging technique used widely in clinical radiology to generate detailed visualisations of anatomical structures and physiological processes. Distinguished by its avoidance of ionising radiation, it has become an essential diagnostic tool across numerous medical specialties. MRI relies on the principles of nuclear magnetic resonance, particularly the behaviour of hydrogen nuclei within strong magnetic fields, to produce high-contrast images of soft tissues such as the brain, abdominal organs, and musculoskeletal structures.

Background and Development

The foundations of MRI lie in nuclear magnetic resonance, a physical phenomenon first observed in the mid-twentieth century. Although originally termed nuclear magnetic resonance imaging, the word nuclear was removed from clinical terminology to minimise public concern associated with radiation. The development of clinical MRI systems in the 1970s and 1980s enabled radiologists to generate high-resolution images without exposing patients to X-rays.
Hydrogen nuclei, prevalent in biological tissues owing to the high water and lipid content of the human body, form the basis of most clinical MRI examinations. When these nuclei are placed in an external magnetic field, they align with the field and behave like minuscule bar magnets. The application of specific radiofrequency pulses temporarily disturbs this alignment. As the nuclei return to equilibrium—a process governed by measurable relaxation properties—they emit signals that can be detected and mathematically reconstructed into cross-sectional images.

Principles of Magnetic Resonance

MRI relies on the interaction between nuclear spin, magnetic fields, and radiofrequency energy. In a strong magnetic field, hydrogen protons assume a preferred orientation, establishing a net magnetisation along the field axis. A radiofrequency pulse matching the resonance frequency of these protons excites them, tipping the magnetisation into the transverse plane. When the pulse is removed, the protons relax back to equilibrium, releasing energy detectable by radiofrequency coils.
The rate at which this relaxation occurs differs between tissues, producing intrinsic contrast. Two major relaxation mechanisms are central to image formation:

• T1 relaxation (spin–lattice relaxation): recovery of longitudinal magnetisation.
• T2 relaxation (spin–spin relaxation): decay of transverse magnetisation.

MRI sequences manipulate timing parameters such as repetition time (TR) and echo time (TE) to weight images towards T1 or T2 behaviour. T1-weighted images are valuable for depicting anatomy, fat-containing structures, and lesions enhanced by contrast agents. T2-weighted images highlight fluid, inflammation, and oedematous changes.

Scanner Components and Construction

An MRI system comprises several key components working in unison:

• Main Magnet: Generates a stable, strong magnetic field, typically between 0.2 and 7 tesla in clinical settings. Research systems may reach 14 tesla or higher. Most clinical units use superconducting magnets cooled with liquid helium.
• Shim Coils: Adjust the magnetic field to improve homogeneity, allowing more precise imaging.
• Gradient Coils: Create small, linearly varying magnetic fields that enable spatial localisation of signals. Their rapid switching produces the characteristic noise of an MRI scan.
• Radiofrequency System: Includes coils that transmit excitation pulses and receive emitted signals.
• Computer System: Coordinates pulse sequences, processes raw data, and reconstructs images.

Open MRI systems employing permanent magnets are sometimes used for patients with severe claustrophobia, although they provide lower field strengths and reduced image resolution.
Advances in technology have expanded MRI into ultralow-field domains using sensitive detectors such as superconducting quantum interference devices, enabling imaging in microtesla ranges through pre-polarisation techniques.

MRI Image Contrast and Pulse Sequences

The versatility of MRI stems from its capacity to alter image contrast by manipulating pulse sequence parameters. Different tissues exhibit distinct relaxation characteristics, allowing radiologists to fine-tune examinations for specific diagnostic purposes.
Commonly used contrasts include:

• T1-weighted sequences: suited for anatomical detail, fat detection, and post-contrast imaging.
• T2-weighted sequences: ideal for identifying oedema, inflammation, or tissue fluid changes.
• Proton-density sequences: highlight differences in hydrogen concentration.
• Diffusion-weighted imaging (DWI): detects molecular water movement, crucial in early stroke diagnosis.
• Functional MRI (fMRI): measures blood oxygenation changes to map brain activity during cognitive tasks.
• Magnetic resonance spectroscopy (MRS): analyses tissue metabolism by identifying chemical signatures.

By adjusting pulse sequence settings, MRI can differentiate subtle tissue variations, making it superior to CT for many soft-tissue conditions.

Clinical Applications

MRI is widely used across medical specialties for diagnosis, treatment planning, and monitoring disease progression. Key applications include:

• Neurological imaging: brain tumours, stroke, multiple sclerosis, epilepsy evaluation, and mapping neural pathways via diffusion tensor imaging.
• Musculoskeletal imaging: ligament tears, cartilage defects, spinal disc abnormalities, and bone marrow pathology.
• Abdominal and pelvic imaging: liver lesions, uterine and prostate anatomy, bowel disorders, and kidney abnormalities.
• Oncology: cancer staging, treatment response assessment, and whole-body imaging for metastasis detection.
• Cardiovascular MRI: evaluation of myocardial viability, cardiac function, and vascular abnormalities.

MRI is also used for imaging non-living subjects such as archaeological artefacts or mummified remains, demonstrating its value beyond clinical medicine.

Safety Considerations and Limitations

Although MRI does not emit ionising radiation, several safety factors must be considered. The strong magnetic field poses risks for individuals with ferromagnetic implants, such as some pacemakers, aneurysm clips, or metal fragments. Modern implant technologies increasingly incorporate MRI-conditional designs, but careful screening remains essential.
Noise generated by rapid gradient switching necessitates hearing protection. Some patients experience anxiety or claustrophobia in the confined scanning environment, though open configurations and shorter scan times have mitigated these concerns. In specific examinations, contrast agents containing gadolinium are administered to enhance vascular and tissue visibility; these are generally safe, though caution is required in patients with severe renal impairment.
MRI’s limitations include higher cost, longer scan durations compared with CT, and reduced accessibility in emergency settings. Increasing demand for MRI has led to discussions surrounding cost-effectiveness and potential overdiagnosis in certain clinical pathways.
MRI continues to evolve through innovations in hardware, ultrafast imaging, artificial intelligence-assisted reconstruction, and higher field strengths. Its capability to visualise soft tissues with exceptional clarity ensures its enduring role in modern medical diagnosis and research.