MRI Anatomy

MRI Principles - Signal Secrets

• Core Sequences & Tissue Signals:
  • T1-Weighted (T1W): Anatomy. Fat is bright (hyperintense); water/CSF is dark (hypointense). 📌 T1: 1 thing bright (fat).
  • T2-Weighted (T2W): Pathology. Fat and water/CSF are bright. 📌 T2: 2 things bright (fat, water).
  • Proton Density (PD): High signal where proton concentration is high (e.g., brain parenchyma).
• Key Specialized Sequences:
  • FLAIR (Fluid Attenuated Inversion Recovery): T2-like, but CSF signal is suppressed (dark). Highlights periventricular lesions (e.g., MS plaques).
  • DWI (Diffusion-Weighted Imaging): Shows restricted water movement. Crucial for acute stroke detection.
  • GRE (Gradient Recalled Echo) / SWI (Susceptibility Weighted Imaging): Detects hemorrhage (dark), calcification.
• Contrast Agents:
  • Gadolinium-based: Shortens T1 relaxation time. Brightens inflamed tissues, tumors, vessels. ⚠️ Risk of Nephrogenic Systemic Fibrosis (NSF) in severe renal impairment.

• Signal Characteristics Table:

Neuroaxis MRI - Brain & Spine Snapshots

• Brain MRI: Key Structures & Views
  • Standard Views (Axial, Sagittal, Coronal): Identify lobes (frontal, parietal, temporal, occipital), ventricles, basal ganglia (caudate, putamen, globus pallidus), thalamus, corpus callosum, brainstem (midbrain, pons, medulla), and cerebellum.
  • Dedicated Sequences: For pituitary fossa and Internal Auditory Canals (IACs).
• Spine MRI: Core Assessment
  • Vertebral Bodies: Check alignment and height.
  • Intervertebral Discs: Evaluate hydration (T2 bright signal) and identify herniation types (bulge, protrusion, extrusion, sequestration).
  • Spinal Cord: Assess signal intensity and morphology.
  • Neural Elements: Examine nerve roots and facet joints.

⭐ On T2-weighted images of the spine, normal intervertebral discs appear bright due to high water content; dessicated or degenerated discs lose this hyperintensity.

MSK & Body MRI - Joint & Organ Views

• MSK - Knee:
  • Menisci (medial C-shaped, lateral O-shaped): Normal low signal. Tears: ↑ signal reaching articular surface.
  • Ligaments: ACL, PCL, MCL, LCL - assess integrity, tears (↑ signal, discontinuity).
  • Cartilage: Smooth, intermediate signal. Defects, thinning.
  • Bone marrow: Edema (↑ T2 signal), effusions (fluid signal).
• MSK - Shoulder:
  • Rotator Cuff: Supraspinatus, infraspinatus, teres minor, subscapularis. Tears, tendinopathy (↑ signal, thickening).
  • Labrum: Glenoid labrum tears.
  • Biceps tendon: Tenosynovitis, tears.
• Basic Tissue Appearance (T1, T2, PD):
  • Muscle: Intermediate signal.
  • Tendon/Ligament: Normally dark (low signal) on all sequences.
  • Cartilage: Intermediate signal.
• Body MRI (Basic T1/T2 Appearance & Location):
  • Liver, Spleen, Kidneys, Pancreas: Assess signal, morphology.
  • Uterus, Bladder: Pelvic anatomy; bladder shows fluid signal.
• Common MRI Artifacts:
  • Motion: Blurring, ghosting. Cause: Patient movement.
  • Metal Susceptibility (Blooming): Signal loss/distortion near metal. Cause: Metallic implants.
  • Chemical Shift: Dark/bright bands at fat-water interfaces. Cause: Different resonant frequencies of fat/water protons.
  • Aliasing/Wrap-around: Anatomy outside Field of View (FOV) wraps into image. Cause: FOV too small.

⭐ Fluid-sensitive sequences like T2-weighted with fat suppression (T2 FS) or STIR are crucial in MSK MRI for detecting edema, inflammation, and fluid in ligamentous/meniscal tears.

High‑Yield Points - ⚡ Biggest Takeaways

• T1-Weighted (T1W): Fat is bright, water/CSF dark; excellent for anatomical detail.
• T2-Weighted (T2W): Water/fluid bright, fat also bright; highlights pathology like edema.
• FLAIR: CSF signal suppressed (dark); superior for periventricular lesions (e.g., MS plaques).
• DWI: Detects acute stroke (bright) by showing restricted water diffusion.
• Gadolinium contrast (T1W): Enhances areas of blood-brain barrier disruption (tumors, infections).
• MRI Safety: Paramount due to strong magnetic fields; screen for contraindications (pacemakers).