M. R. I.

 Here is a detailed overview of MRI (Magnetic Resonance Imaging) including how it works, components, types of sequences and coils, indications, contraindications, and artifacts:

1. What is MRI?

MRI (Magnetic Resonance Imaging) is a non-invasive imaging technique that uses strong magnetic fields, radio waves, and a computer to produce detailed images of internal structures, especially soft tissues.

2. How MRI Works

MRI works based on the principles of nuclear magnetic resonance (NMR):

  1. Magnetic Field: Aligns hydrogen protons (mainly in water and fat) in the body.
  2. Radiofrequency (RF) Pulses: Applied to disturb the alignment of protons.
  3. Relaxation: After the RF pulse is turned off, protons return to their original alignment, releasing signals.
  4. Signal Detection: These signals are captured by coils and processed into images.

3. Components of MRI Machine

  • Main Magnet: Generates a strong magnetic field (usually 1.5T or 3T).
  • Gradient Coils: Used to spatially encode signals for image formation.
  • RF Coils (Radiofrequency Coils): Transmit RF pulses and receive returning signals.
  • Patient Table: Moves the patient into the bore of the scanner.
  • Computer System: Processes data to reconstruct images.
  • Shielding: Prevents external electromagnetic interference.

4. Types of MRI Sequences

Different sequences highlight different tissues:

  • T1-Weighted (T1W): Good for anatomy; fat appears bright, water is dark.
  • T2-Weighted (T2W): Good for pathology; fluid appears bright.
  • FLAIR (Fluid Attenuated Inversion Recovery): Suppresses CSF; useful in brain lesions.
  • DWI (Diffusion Weighted Imaging): Detects acute stroke and cellular injury.
  • GRE (Gradient Echo): Sensitive to blood products and micro-bleeds.
  • STIR (Short TI Inversion Recovery): Suppresses fat; useful in musculoskeletal imaging.
  • MR Angiography (MRA): Visualizes blood vessels.
  • MR Spectroscopy: Analyzes chemical composition.

5. Types of MRI Coils

  • Surface Coils: Placed over specific body parts for high-resolution imaging.
  • Body Coil: Built into the MRI bore; used for general imaging.
  • Head Coil: Designed for brain imaging.
  • Spine Coil: Used for spinal imaging.
  • Knee Coil: Designed specifically for knee studies.
  • Phased Array Coil: Combines multiple coils for better signal and coverage.

6. Indications of MRI

  • Brain and Spine: Stroke, tumors, multiple sclerosis, disc herniation.
  • Musculoskeletal: Ligament, tendon, and soft tissue injuries.
  • Cardiac MRI: Heart structure, perfusion, viability.
  • Abdomen and Pelvis: Liver lesions, prostate, uterus, ovaries.
  • Vascular Imaging: Aneurysms, stenosis (with MRA).
  • Oncology: Tumor staging and follow-up.

7. Contraindications of MRI

  • Absolute:

    • Pacemakers (non-MRI compatible)
    • Cochlear implants
    • Certain aneurysm clips
    • Metallic foreign bodies (e.g., in eye)

  • Relative:

    • Claustrophobia
    • Pregnancy (especially 1st trimester)
    • Renal failure (if contrast is needed - gadolinium)
    • Implanted devices (depending on MRI compatibility)

8. MRI Artifacts

  • Motion Artifact: Blurring from patient movement.
  • Metal Artifact: Signal loss and distortion near metallic objects.
  • Gibbs Artifact: Edge ringing near sharp transitions.
  • Susceptibility Artifact: From air-tissue or metal-tissue interfaces.
  • Chemical Shift Artifact: Misregistration between fat and water.
  • Zipper Artifact: Due to RF interference.


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