CT scan principles and interpretation

CT Physics - The Photon Phandango

• An X-ray tube rotates 360° around a patient, emitting a fan beam. Detectors measure the attenuated (absorbed) photons that pass through.
• This raw data is reconstructed by a computer into a cross-sectional image using mathematical algorithms.
• Hounsfield Units (HU): A standardized scale for radiodensity.
  • Formula: $HU = 1000 \times \frac{\mu_{tissue} - \mu_{water}}{\mu_{water}}$
  • Key Densities: Air (-1000), Fat (-100), Water (0), Bone (+1000).
  • 📌 Mnemonic: "All Fat Women Seem Big" (Air, Fat, Water, Soft Tissue, Bone)

⭐ Windowing is key for interpretation. The vast range of Hounsfield Units is compressed into ~256 visible gray shades. The window width controls contrast, while the window level sets the central brightness point.

Hounsfield & Windowing - Shades of Grey

• Hounsfield Unit (HU): A quantitative scale for radiodensity in CT scans, based on a standardized linear transformation of X-ray attenuation coefficients.
  • Formula: $HU = 1000 \times \frac{\mu_{tissue} - \mu_{water}}{\mu_{water}}$

-   **Window Width (WW):** The *range* of HU displayed. A narrow WW shows subtle tissue differences (e.g., grey vs. white matter).
-   **Window Level (WL):** The *center* of the HU range. Set to the HU of the tissue of interest.

⭐ Stroke Window: For non-contrast head CT, a narrow window (WW 80, WL 40) is crucial to differentiate early ischemic changes from normal brain parenchyma.

Contrast Agents - Glow Up Juice

• Types: Primarily iodine-based (IV/arterial) or barium-sulfate (oral/rectal) for GI tract.
• Mechanism: High atomic number materials that attenuate X-rays, ↑ tissue density & improving structure visualization.

• Risks:
  • Contrast-Induced Nephropathy (CIN): Acute kidney injury within 48-72 hrs. Prevent with IV fluids.
  • Allergic Reactions: Range from mild (urticaria) to severe (anaphylaxis).

⭐ Metformin & Contrast: Hold metformin for 48 hours post-procedure if eGFR < 60 mL/min/1.73m² or in patients with known hepatic dysfunction or alcoholism, due to risk of lactic acidosis.

Interpretation & Artifacts - Spot the Lesion

• Systematic Approach: Use a consistent pattern (e.g., ABCDEs for chest CT: Airway, Breathing/Lungs, Cardiac/Circulation, Diaphragm, Everything else).
• Hounsfield Units (HU): Quantifies radiodensity.
  • Air: -1000 (black)
  • Fat: -100 to -50
  • Water: 0
  • Soft Tissue: +40 to +80
  • Bone: +700 to +3000 (white)
• Common Artifacts:
  • Motion: Blurring from patient movement.
  • Beam Hardening: Dark streaks between dense objects.
  • Metallic Artifact: Severe streaking from implants.

⭐ Windowing is key. The same raw data is viewed with different brightness/contrast settings. A lung window (e.g., W:1500, L:-600) highlights parenchyma, while a mediastinal window (e.g., W:350, L:50) shows soft tissues.

High‑Yield Points - ⚡ Biggest Takeaways

• CT density is measured in Hounsfield Units (HU): bone is white (+1000), air is black (-1000), and water is 0.
• "Windowing" adjusts the greyscale to optimize viewing of specific structures like bone, lung, or soft tissue.
• IV contrast (iodine-based) makes vessels and vascular organs appear hyperdense (brighter).
• Acute hemorrhage is classically hyperdense on non-contrast CT, critical for initial stroke evaluation.
• Hypodense (darker) areas can represent fat, edema, cysts, or chronic infarcts.
• CT involves significant ionizing radiation, a key risk to consider, especially in pediatric patients.