Computed Tomography (CT) – Comprehensive Bullet-Point Study Notes

Terminology & Core Concept of Tomography

  • Root of the word
    • “Tomography” derives from Greek “tomo/tomos” meaning “to cut, section, or layer”.
  • Philosophy of CT
    • Uses a computer to transform x-ray data into cross-sectional “cuts”.
    • Each slice = an independent plane through the patient, analogous to a single slice of bread in a loaf.
  • Image Orientations generated
    • Transverse/axial (perpendicular to long axis).
    • From the transverse data set, sagittal and coronal planes can be reconstructed.
    • Newer scanners allow direct acquisition in multiple planes.

Historical Milestones

  • 1970 – Sir Godfrey Hounsfield demonstrates first CT technique.
  • 1979 – Nobel Prize in Physiology/Medicine shared by Godfrey Hounsfield & Alan Cormack (Tufts University).

Conventional Imaging vs. CT

  • Conventional Radiography
    • Area x-ray beam, screen-film receptor.
    • Low contrast due to Compton scatter, high-intensity beam, superimposition of anatomy → degraded images.
  • Conventional (Axial) Tomography
    • Selectively blurs anatomy above/below an “in-focus” plane.
    • Enhanced contrast of chosen plane, but overall image still dull/blurred using film-screen.
  • Digital Radiographic Tomosynthesis
    • Area beam + multiple projections → 3-D data set; reconstruction of any plane, improved contrast.
  • Computed Tomography (CT)
    • Produces true cross-sectional (transverse) images by reconstructing hundreds of projections collected as the x-ray tube/detectors translate & rotate.
    • Eliminates superimposition; delivers high spatial & low-contrast resolution.

Evaluation Criteria for CT Image Quality

  1. Spatial Resolution – Ability to distinguish small, closely spaced objects.
  2. Low-Contrast Resolution – Ability to differentiate objects with minimal density difference.
  3. Temporal Resolution – Speed of data acquisition; critical for moving organs (e.g., heart) to minimize motion artifacts.

CT Data Geometry

  • Voxel & Pixel Relationships
    • Each slice subdivided into volume elements (voxels). Width = XX axis, height = YY axis, thickness = ZZ axis.
    • In-plane grid = matrix (commonly 512×512512 \times 512). Each matrix cell = pixel.
  • Projections
    • One translate-rotate sweep → intensity profile (projection) dependent on attenuation pattern.
    • Multiple projections (often hundreds) are stored digitally; superimposed mathematically to reconstruct the slice.

Beam Attenuation Principles

  • Basic rule: More attenuation ⇒ lighter pixel; less attenuation ⇒ darker pixel.
  • Determinants of Attenuation
    • Atomic number ZZ
    • Photoelectric absorption probability ∝Z3\propto Z^{3}.
    • Mass density (kg m−3\text{m}^{−3}) – more matter per volume → more interactions.
    • Thickness traversed.
  • Linear Attenuation Coefficient (μ\mu)
    • Expresses fraction of beam attenuated per unit path length.
    • Example at 125 kVp125 \text{ kVp} for water: μ≈0.18  cm−1\mu \approx 0.18\; \text{cm}^{−1}.
    • General trend: μ\mu ↓ with ↑ photon energy, ↑ with ↑ ZZ or density.
  • Representative Equation (conceptual): I=I0e−μxI = I_{0} e^{−\mu x} (not in transcript but underlies mathematics).

Hounsfield Units (HU)

  • Quantifies relative attenuation on a scale where
    • Water = 0  HU0\; \text{HU}
    • Air = −1000  HU−1000\; \text{HU}
    • Dense bone/metal ≈ +1000  HU+1000\; \text{HU} or higher.
  • Purpose: Compare unknown tissue values to known ranges for diagnosis.
  • Typical Ranges (approx.)
    • White matter +20–+30  HU+20 \text{–} +30\; \text{HU}
    • Gray matter +30–+40  HU+30 \text{–} +40\; \text{HU}
    • Muscle +20–+40  HU+20 \text{–} +40\; \text{HU}
    • Hemorrhage +65–+95  HU+65 \text{–} +95\; \text{HU}
    • Fat −30–−70  HU−30 \text{–} −70\; \text{HU}

Polychromatic X-ray Beams & Beam Hardening

  • All diagnostic x-ray tubes emit a spectrum of photon energies.
  • Detectors treat all arrived photons equally; cannot adjust for low-energy photons that are preferentially absorbed.
  • Preferential absorption → beam hardening artifact (streaks, cupping), especially adjacent to dense bones (e.g., petrous ridges).
  • Filtration with materials (Teflon, Al) removes low-energy photons, narrowing energy spread, mitigating artifact & patient dose.

Volume Averaging

  • Voxels with mixed tissues display averaged HU → potential to obscure small or low-contrast lesions.
  • Influencing factors:
    • Slice thickness (Z-axis) – thicker slices ↑ averaging.
    • In-plane pixel size (X & Y) – larger pixels ↑ averaging.
  • Trade-off: Thin slices (< 1  mm1\; \text{mm}) ↓ averaging but ↑ patient dose; protocol balances diagnostic need vs. ALARA.

Data Types

  • Raw (Scan) Data – Detector samples prior to image formation; unsectioned, no HU assigned.
  • Image Data – After reconstruction; each pixel has HU; can be displayed or archived.
  • Retrospective Reconstruction – Reusing raw data with altered parameters (e.g., slice thickness, kernel) to generate new images without re-scanning.

Scan Modes

  1. Step-and-Shoot (Axial/Sequential)
    • 360° tube rotation per slice → table increments → repeat; dominated 1980s.
  2. Helical/Spiral
    • Gantry rotates continuously while table moves; path of tube traces a helix.
    • Enabled by slip-ring technology (cable-free continuous rotation).
  3. Multidetector Row CT (MDCT)
    • Multiple parallel detector rows (2 → 4 → 16 → 64 → 128+).
    • Captures multiple slices per rotation; facilitates sub-second volume coverage & thin collimation.

Imaging Planes & Patient Orientation

  • Standard Planes
    • Axial (transverse), Sagittal, Coronal.
  • Oblique Planes – Angled relative to standard planes.
  • Indications for Non-Axial Scans
    • Anatomy oriented vertically (e.g., spine, sinuses).
    • To minimize artifacts from adjacent structures.
  • Plane adjustment via patient positioning, gantry tilt, or multiplanar reformatting (MPR).

Basic Phases of Contrast-Enhanced Scans

  • Arterial phase: 20–40  s20\text{–}40\;\text{s} post-injection.
  • Portal venous phase: ≈ 80  s80\;\text{s}.
  • Delayed phase: 6–10  min6\text{–}10\;\text{min}.
  • Choice depends on organ/lesion characterization requirements.

CT System Components

  • Gantry – Houses rotating hardware: x-ray tube, collimators, detectors, DAS.
  • X-ray Tube
    • Filament heats → electrons emitted → accelerated across high kV → strike rotating anode.
    • Focal spot – Where electrons hit anode; size affects spatial resolution.
    • Tube current (mA) – Controls photon quantity.
    • >99%>99\% electron energy → heat; tube’s heat capacity & dissipation are design limits.
  • Generator – Supplies high voltage (kV) to tube.
  • Detectors
    • Solid-state scintillator (modern) or xenon gas (older).
    • Convert x-ray energy → light/electrical signal; high efficiency, low after-glow.
  • Data Acquisition System (DAS) – Samples detector signals, digitizes, forwards to CPU.
  • Central Processing Unit (CPU) – Executes reconstruction algorithms (e.g., filtered back-projection, iterative methods).

Three Segments of the CT Process

  1. Data Acquisition – Photons generated, modulated by patient, captured by detectors.
  2. Image Reconstruction – Sorting & mathematical processing assign one HU per pixel.
  3. Image Display – HU matrix converted to greyscale; output to monitors/film/PACS.

Summary of Key Numerical / Formula References

  • Matrix size: 512×512512 \times 512 common.
  • Typical HU scale: −1000(air)  →  0(water)  →  +1000(dense bone/metal)−1000 \text{(air)} \; \rightarrow \; 0 \text{(water)} \; \rightarrow \; +1000 \text{(dense bone/metal)}.
  • Linear attenuation coefficient example: μwater≈0.18  cm−1\mu_{\text{water}} \approx 0.18\; \text{cm}^{−1} at 125  kVp125\;\text{kVp}.
  • Photoelectric dependence: PPE∝Z3P_{\text{PE}} \propto Z^{3}.
  • Intensity law: I=I0e−μxI = I_{0} e^{−\mu x}.
  • Contrast-enhancement timings: 20–40  s20\text{–}40\;\text{s} (arterial),  80  s~80\;\text{s} (portal), 6–10  min6\text{–}10\;\text{min} (delayed).

Practical & Ethical Considerations

  • Radiation Dose Management
    • Thin slices & multiphase protocols increase dose; protocols must balance diagnostic benefit vs. ALARA principle.
  • Artifact Recognition & Mitigation
    • Techs must recognize beam hardening, motion, partial-volume artifacts and use filtration, faster acquisitions, or alternative planes to reduce them.
  • Manufacturer Variability
    • Similar features often branded differently (e.g., “spiral CT”, “Helical CT”, “Volume CT”); understanding core principles prevents confusion.

Connecting Concepts to Clinical Relevance

  • HU measurement allows identification of pathology (e.g., differentiating hemorrhage from calcification, fat characterization of adrenal lesions).
  • Multiplanar & 3-D reconstructions critical for surgical planning, trauma assessment, and interventional guidance.
  • Temporal resolution advances enable coronary CT angiography, reducing motion artifacts.