Cross Sectional Imaging - Scanner Operation and Parameter Selection

Module Learning Outcomes

  • Describe and explain the physical principles of cross-sectional imaging modalities.
  • Understand the impact of parameter selection on imaging outcomes.

Session Outcomes

  • Grasp principles of protocol selection.
  • Recognize key parameters affecting radiation dose and image quality.
  • Appreciate the importance of the clinical question in protocol selection.

Protocol Selection

  • What influences protocol design?
    • Consider the body area (e.g., chest, abdomen).
    • Define the field of view (area of interest).
    • Adjust parameters to ensure good image quality:
    • Sufficient radiation/photons reaching detectors.
    • Compare with conventional radiography: several parameters (kV, mA) plus timing for contrast.
    • Aim for minimum radiation dose.

Who Sets the Protocol?

  • Protocols are developed by radiologists/radiographers based on clinical info.
  • Use of acronyms to describe protocols (e.g., NCUA, CAP).
  • The vetter outlines body areas, specific phases for imaging.

Setting Up Protocols on the Scanner

  • After acquiring a new CT scanner, an applications specialist establishes baseline protocols.
  • Radiographers and medical physicists collaborate on protocol setup.
  • Protocols are consistent across patients managed by the applications specialist.

Image Quality

Types of Image Quality

  • Spatial Resolution: Ability to resolve closely spaced objects, measured in lp/cm.
  • Contrast Resolution: Ability to distinguish small differences in density.
  • Longitudinal Resolution: Resolution along the z-axis, crucial for slice thickness.
  • Temporal Resolution: Capability to capture fast-moving objects; crucial in cardiac CT.

Factors Affecting Image Quality

  • kV (Kilovoltage):
    • Higher kV increases photon penetration but raises radiation dose.
    • Enhances spatial resolution and detail.
  • mAs (Milliampere-second):
    • Refers to the total number of photons; low mAs leads to noisy images.
    • Higher mAs improves contrast resolution but increases dose.

Pitch and Rotation Time

  • Pitch: Distance the table travels per rotation divided by slice width.
    • Higher pitch reduces radiation dose; lower pitch improves image quality but increases dose.

Slice Thickness

  • Thinner slices mean more accurate data but higher radiation exposure.
  • Thicker slices aggregate information, sacrificing detail for a lower dose.

CT Image Matrix

  • A 2D grid of pixels that makes up the image; size typically 1024x1024 for modern scanners.
    • Optimizing matrix utilization is vital for better spatial resolution.

Kernels and Post-processing

  • Kernels: Algorithms for post-processing images to enhance resolution and contrast.
  • Post-processing techniques involve adjustments of slice thickness and algorithms to optimize viewing.

Clinical Challenges

  • Obese patients: Issues arise if anatomy falls outside the FOV, causing artifacts.
  • Elderly agitated patients: Movement artifacts impact image quality, especially in urgent scans.

Summary

  • The clinical question determines the appropriate protocol based on parameters affecting image quality and radiation dose.
  • Four key types of resolution to understand:
    • Spatial Resolution
    • Contrast Resolution
    • Longitudinal Resolution
    • Temporal Resolution

Additional Note

  • If any questions arise, feel free to email: h.k.Adamson@gmail.com for private queries.