MRI scan

PHYSICS: Patient is placed in a strong static magnetic field (typically 1.5 or 3 Tesla), which aligns hydrogen protons (abundant in water and fat) along the field. A radiofrequency (RF) pulse tips the protons out of alignment. When the pulse stops, protons relax and emit signal, which is detected by coils and reconstructed into images. Two relaxation processes: T1 (longitudinal recovery, spin-lattice) and T2 (transverse decay, spin-spin). Gradient coils provide spatial localization. TR (repetition time) and TE (echo time) determine weighting.

SEQUENCES (high-yield): - T1-weighted: anatomy; fat is bright; water/CSF is dark. Gadolinium contrast is bright on T1. - T2-weighted: pathology; water/CSF is bright (edema, inflammation, tumor, infarct are bright). Fat is intermediate-bright. - FLAIR: T2 with CSF signal suppressed; best for periventricular lesions, e.g., multiple sclerosis plaques, and subarachnoid disease/edema. - DWI/ADC: detects restricted diffusion; earliest detection of acute ischemic stroke (within minutes to hours), also abscess and some tumors (e.g., epidermoid). - GRE/SWI: sensitive for hemorrhage, microbleeds, and calcification. - MRA/MRV: vascular imaging without ionizing radiation, with or without contrast. - Fat-suppressed sequences (STIR): useful for bone marrow edema, occult fractures, and osteomyelitis.

CONTRAST: Gadolinium-based agents (paramagnetic) shorten T1 and enhance areas with blood-brain barrier breakdown or increased vascularity (tumors, abscess, active MS plaques). Risk: nephrogenic systemic fibrosis (NSF) in patients with severe renal impairment (eGFR <30) with older linear agents; skin thickening/fibrosis. Gadolinium can also deposit in the brain (clinical significance uncertain). Use caution in pregnancy.

CLINICAL USES / PREFERRED OVER CT: - Brain: MS, posterior fossa lesions, acute ischemic stroke (DWI), pituitary tumors, acoustic neuroma (vestibular schwannoma), HSV encephalitis, brain tumors, demyelinating disease, Chiari malformation. - Spine: cord compression, disc herniation, epidural abscess, cauda equina, syringomyelia, metastases. - MSK: ligament/meniscal tears (ACL), rotator cuff, osteomyelitis, avascular necrosis, bone marrow pathology. - Others: cardiac MRI (myocarditis, infiltrative disease, viability), MRCP (biliary tree), pelvic imaging (cervical/prostate cancer staging), breast MRI (high-risk screening), liver lesions.

ADVANTAGES: No ionizing radiation; superior soft-tissue contrast; multiplanar imaging. DISADVANTAGES: Long scan time, expensive, claustrophobia, motion artifact, poor for acute trauma/hemorrhage triage vs CT (CT is first-line for acute head trauma and suspected acute intracranial hemorrhage), poor cortical bone and calcification detail.

CONTRAINDICATIONS/SAFETY: - Absolute/relative: non-MRI-conditional pacemakers/ICDs, cochlear implants, ferromagnetic intracranial aneurysm clips, metallic intraocular foreign bodies (projectile/heating risk), some neurostimulators. Many modern implants are MRI-conditional. - Projectile hazard from ferromagnetic objects (oxygen tanks, etc.). - Heating/burns from RF energy; loud noise (hearing protection). - Severe renal disease: gadolinium NSF risk. - Pregnancy: MRI without gadolinium is generally considered safe if clinically indicated; avoid gadolinium unless essential.

BOARD PEARLS: - Acute stroke: CT first to rule out hemorrhage; DWI MRI most sensitive for early ischemia. - Suspected cord compression (e.g., metastatic): urgent MRI of the whole spine. - MS: periventricular ovoid lesions ("Dawson fingers") perpendicular to ventricles on T2/FLAIR. - Mnemonic: "T1 = fat bright, anatomy; T2 = water bright, pathology." - CT is better for acute hemorrhage, bony detail, kidney stones, and in unstable patients; MRI is better for soft tissue and posterior fossa.