Radiology

MRI Scan

Magnetic resonance imaging — how protons in water make medical pictures

MRI (magnetic resonance imaging) generates high-resolution cross-sectional images by exploiting the behavior of hydrogen nuclei (protons) in tissue water under a strong magnetic field. Clinical scanners use 1.5 T or 3 T magnets (~30,000-60,000× Earth's field). When placed in the field, proton spins precess at the Larmor frequency (~64 MHz at 1.5 T, ~128 MHz at 3 T). A radiofrequency pulse tips them out of alignment; as they relax, they emit signal detected by coils. Two relaxation times — T1 (longitudinal recovery, ~600-1000 ms in tissue) and T2 (transverse decay, ~50-100 ms) — provide contrast that varies by tissue. Spatial localization comes from gradient coils. No ionizing radiation. Excellent soft tissue contrast — neuroimaging, MSK, abdomen, breast, cardiac, prostate. Costs ~$400-3,500/study (US).

  • Magnetic field1.5 T or 3 T (research up to 7 T)
  • Larmor frequency42.58 MHz/T for protons
  • T1 brain white matter at 1.5 T~510 ms
  • T2 brain white matter at 1.5 T~67 ms
  • Gadolinium contrast agentsUsed in ~30% of MRI exams
  • Earth's magnetic field comparison1.5 T = ~30,000× stronger

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Why MRI matters

  • Brain imaging. Stroke (DWI), MS (FLAIR), tumor (Gd), dementia.
  • Spine. Disc herniation, cord compression, infection.
  • Joints. Cartilage, meniscus, ligament, marrow edema.
  • Cancer staging. Liver, prostate, rectal, breast.
  • Cardiac. Function, viability, infiltrative disease.
  • No ionizing radiation. Safer for repeat imaging, pregnancy, pediatric.
  • Functional and diffusion mapping. Beyond anatomy — physiology and microstructure.

Common misconceptions

  • MRI uses radiation. Magnetic field and RF — no ionizing radiation.
  • Contrast is required for every MRI. About 30% — many studies (DWI, MSK, basic anatomy) don't need it.
  • Pacemakers always exclude MRI. Most modern devices are MR-conditional with protocols.
  • MRI is always better than CT. CT is faster, better for bone, acute bleed, lung, vascular emergencies.
  • Gadolinium is universally safe in CKD. NSF risk with older linear agents; macrocyclic preferred but not zero risk.
  • fMRI shows thinking directly. Indirect — measures hemodynamic correlate, not neural firing itself.

Frequently asked questions

How does MRI image protons?

External field B₀ aligns proton spins parallel/anti-parallel; slight excess parallel creates net magnetization. Larmor precession at γB₀ (γ for ¹H = 42.58 MHz/T). RF pulse at Larmor frequency tips magnetization (90° flip puts it transverse). After RF off, two relaxations: T1 — longitudinal recovery to B₀ alignment (energy returned to lattice); T2 — transverse dephasing (loss of coherence among spins). Gradient coils superimpose linear field variation, making Larmor frequency position-dependent — k-space encoding allows Fourier reconstruction into image.

What's T1 vs T2 weighting?

T1-weighted: short TR/TE (e.g., 500 ms / 15 ms). Fat is bright (short T1, recovers fast); water/CSF dark; good for anatomy. T2-weighted: long TR/TE (e.g., 4000 ms / 100 ms). Water bright (long T2, slow dephasing); fat moderate; good for pathology (edema, inflammation, tumor). FLAIR (fluid-attenuated inversion recovery): T2 with CSF suppressed — highlights periventricular MS lesions. STIR: short tau IR — fat-suppressed T2; bone marrow edema. DWI: detects restricted diffusion — bright in acute stroke (within minutes), abscess, hypercellular tumor.

What is gadolinium contrast?

Gd³⁺ chelates (gadobutrol, gadoteridol, gadobenate) shorten T1 of nearby water, brightening tissues with vascular/extracellular leak (tumor, inflammation, infection, infarct after BBB breakdown). Distribute in ECF; cleared by kidneys. NSF (nephrogenic systemic fibrosis) is severe complication in CKD/ESRD with older linear chelates — current macrocyclic agents (Group II) very low risk. Some Gd retains in brain (dentate nucleus T1 hyperintensity); clinical significance unclear. Avoid in pregnancy. Replacement for iodinated CT contrast in renal patients is a misconception — gadolinium can also be nephrotoxic.

When is MRI better than CT?

MRI for soft tissue contrast — brain (stroke, MS, tumors, dementia), spine (cord compression, disc disease, infection), MSK (cartilage, ligament, marrow), pelvis (prostate, gynecologic), liver lesion characterization, breast (high-risk screening). CT for: bone, acute hemorrhage (especially head trauma), lung parenchyma, calcifications, vascular emergencies (CTA for PE, AAA, dissection — fast and widely available). MRI is slower (30-60 min), claustrophobic, more expensive, and contraindicated for some implants.

What are MRI safety hazards?

Projectile injuries — ferromagnetic objects (oxygen tanks, scissors, wheelchairs, pens) become missiles within feet of magnet. Implant compatibility must be checked: pacemakers (now mostly MR-conditional), aneurysm clips, cochlear implants, neurostimulators, metallic foreign bodies (orbital metal contraindication). Burns from RF heating (tattoos with iron pigments, conductive loops with ECG leads). Acoustic noise (~110 dB) — mandatory hearing protection. Pregnancy — no known harm but limit elective scans in 1st trimester. Quench (rare): rapid helium boil-off if magnet fails — oxygen displacement risk.

What's functional MRI (fMRI)?

Detects neural activity indirectly via BOLD (blood oxygen level dependent) signal — active neurons consume oxygen, triggering local hyperemia that overshoots demand, increasing oxyhemoglobin (diamagnetic) relative to deoxyhemoglobin (paramagnetic). Net T2* signal increase ~1-5%. Used for presurgical mapping of language and motor cortex, research into cognition, network connectivity. Spatial resolution ~1-3 mm; temporal ~1-2 sec (limited by hemodynamic response). Resting-state fMRI maps default mode and other networks without task.

How does diffusion-weighted imaging work?

Sensitizes signal to random thermal motion of water molecules. In free water (CSF), diffusion is fast — signal attenuates greatly. In restricted environments (cytotoxic edema in acute stroke, abscess pus, dense tumor cells), diffusion is impaired — signal preserved (bright on DWI). ADC map quantifies — low ADC confirms true restriction (eliminating "T2 shine-through"). DWI is the most sensitive sequence for acute ischemic stroke (positive within minutes, before CT). DTI (diffusion tensor imaging) maps white matter tracts via anisotropic diffusion along axons.