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.