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An MRI scanner is a medical imaging device that uses a strong magnetic field, radiofrequency pulses, and computer processing to visualize internal body structures. It exploits the magnetic properties of hydrogen nuclei in water and fat to generate detailed soft tissue images without ionizing radiation.

A typical MRI system includes a powerful magnet (often superconducting and cooled with liquid helium), gradient coils for spatial encoding, and a radiofrequency RF coil that transmits pulses and detects signals. The patient lies on a motorized bed that slides into a bore within a shielded room to minimize external interference.

MRI signal generation relies on aligning protons with the main magnetic field and perturbing them with RF pulses. The emitted signals are spatially encoded by the gradient fields and reconstructed by computer algorithms into images. Various imaging sequences emphasize different tissue properties, including T1-weighted, T2-weighted, diffusion-weighted imaging, and functional MRI.

Clinical systems operate at field strengths from about 0.2 to 7 tesla, with 1.5T and 3T being common for routine exams. Higher-field scanners offer greater signal-to-noise and resolution but can increase artifact risk and safety considerations. Designs include closed-bore, wide-bore, and open configurations to accommodate claustrophobia or obesity. Some exams require contrast agents such as gadolinium-based compounds.

Advantages of MRI include excellent soft tissue contrast and absence of ionizing radiation. Limitations include sensitivity to motion and magnetic susceptibility, longer exam times, higher cost, and contraindications for patients with certain implants or devices. Safety practices mandate screening for ferromagnetic items, monitoring for heating, and adherence to restricted areas and safety procedures in case of emergencies.

In scientific research, functional MRI uses the BOLD signal to map brain activity, while advanced techniques extend diffusion, perfusion, spectroscopy, and quantitative imaging to various clinical applications.