MRI Scan — Purpose, Procedure & Normal Values | MyMedicPlus

An MRI (magnetic resonance imaging) scan uses a powerful static magnetic field, pulsed radiofrequency energy, and sophisticated computer processing algorithms to produce highly detailed, multiplanar cross-sectional images of the body's soft tissues, organs, blood vessels, and bony structures — all without using ionising radiation (X-rays). When placed in a magnetic field, hydrogen protons in the body's water molecules align with the field. Radiofrequency pulses briefly disrupt this alignment, and the energy released as protons return to their resting state is detected and converted into images. Different tissue types — fat, water, muscle, fibrous tissue — produce different signal intensities on different pulse sequences, enabling exquisite contrast between normal and pathological tissue. Gadolinium-based contrast agents (GBCAs) are injected intravenously in many studies to enhance the visibility of blood vessels, areas of active inflammation, disrupted blood-brain barrier, and tumours. MRI is the gold standard for brain, spinal cord, joint, and pelvic soft tissue evaluation.

MRI is ordered for a wide range of clinical indications across virtually every organ system. Brain conditions: ischaemic stroke (diffusion-weighted imaging detects infarction within minutes to hours), brain tumours (primary and secondary), multiple sclerosis (white matter plaques), dementia assessment, epilepsy (cortical dysplasia, mesial temporal sclerosis), pituitary lesions, and intracranial infections. Spinal disorders: disc herniation with nerve root or cord compression, spinal cord tumours, spinal cord injury, spinal infection (discitis, epidural abscess), and inflammatory myelopathy. Joint and musculoskeletal pathology: ligament and tendon tears (ACL, rotator cuff), cartilage damage, bone marrow oedema, stress fractures, and occult fractures not visible on X-ray. Abdominal and pelvic organs: liver lesions characterisation, MRCP for biliary and pancreatic disease, uterine fibroids, endometriosis, prostate cancer staging, ovarian masses, and rectal cancer staging. Cardiac MRI: myocarditis, cardiomyopathy, congenital heart defects, viability assessment. MRI is preferred over CT when repeated imaging is required or when radiation exposure must be minimised — especially in children, young adults, and pregnant women.

MRI requires rigorous safety screening because the powerful magnetic field can attract and heat metallic implants. MRI is absolutely or conditionally contraindicated for patients with: cardiac pacemakers or implantable cardioverter defibrillators (most modern devices manufactured after 2012 are MRI-conditional — contact the device manufacturer to confirm), cochlear implants (many are now MRI-conditional), certain intracranial aneurysm clips (older ferromagnetic clips), intraocular metallic foreign bodies (welders and metal workers must undergo orbital X-rays to exclude this), and some neurostimulators or deep brain stimulators. Remove all metal before entering the scan room — jewellery, piercings, hairpins, hearing aids, removable dental work, and clothing with metal fasteners, zips, or underwired bras. Patients undergoing contrast abdominal or pelvic MRI should fast for 4–6 hours. For contrast studies, kidney function (eGFR) must be checked beforehand. Inform staff of claustrophobia, tattoos, medication patches, and all implanted devices. Pregnant patients should avoid gadolinium contrast unless essential.

You change into a hospital gown and are screened again by the radiographer at the scan room door. You lie on a motorised padded table that slides into the bore (opening) of the cylindrical MRI magnet — typically 1.5 or 3 Tesla field strength. A radiofrequency coil (a specialised antenna) is placed over or around the body part being imaged to receive the signal. The scanner produces loud, repetitive knocking, thumping, and beeping sounds from rapid switching of the radiofrequency gradient coils — earplugs or noise-cancelling headphones with music are provided and mandatory. Multiple imaging sequences are acquired in succession; you must remain completely still during each sequence, which lasts 30 seconds to several minutes. For contrast-enhanced studies, a nurse inserts an intravenous cannula into an arm vein and injects gadolinium contrast at a specific point during the examination. A brain MRI typically takes 30–45 minutes; a lumbar spine MRI 20–30 minutes; a knee MRI 30–45 minutes; a cardiac MRI or liver MRI 60–90 minutes. An emergency call button is provided for reassurance.

A trained radiologist reviews and interprets the MRI images and dictates a structured report, typically available within 24–72 hours for routine studies and within 30–60 minutes for urgent emergency scans. Images are reviewed in multiple orthogonal planes — axial, coronal, and sagittal — and in multiple pulse sequences each sensitive to different tissue characteristics: T1-weighted sequences show fat as bright and fluid as dark, ideal for anatomy; T2-weighted sequences show fluid as bright (white), excellent for detecting oedema, inflammation, and most pathology; FLAIR (fluid-attenuated inversion recovery) suppresses cerebrospinal fluid signal and highlights white matter lesions; diffusion-weighted imaging (DWI) detects restricted diffusion in acute ischaemic stroke within minutes; post-contrast T1 sequences show areas of blood-brain barrier breakdown and tumour vascularity. The report describes the anatomy of all structures imaged, characterises any lesions by their size, location, margins, and signal properties, notes any mass effect or invasion, and provides an impression with differential diagnoses and recommendations for further management or follow-up imaging.

MRI produces no ionising radiation and is considered extremely safe for the vast majority of patients when contraindications are properly screened. Gadolinium-based contrast agents (GBCAs) are generally well tolerated but carry specific risks: nephrogenic systemic fibrosis (NSF) — a rare but serious fibrosing condition of skin and organs — can occur in patients with severely reduced kidney function (eGFR below 30 mL/min/1.73m²); eGFR should always be checked before administering gadolinium in at-risk patients. All GBCAs carry a small risk of acute allergic reaction (approximately 0.07–2.4% of injections, usually mild). Linear GBCAs are associated with gadolinium retention in brain tissue (particularly the dentate nucleus and globus pallidus) with repeated administrations; the clinical significance is uncertain but has prompted regulatory guidance to use macrocyclic GBCAs preferentially. MRI is contraindicated in patients with non-MRI-conditional ferromagnetic implants. Claustrophobia affects approximately 5% of patients and may prevent completion of the study. MRI is slower and more expensive than CT, and is not suitable for acute trauma triage or critically ill patients requiring intensive monitoring.