Cross-sectional Imaging 2: Scanner Operation and Parameter Selection

Module Learning Outcomes

• Describe and explain the physical principles behind cross-sectional imaging modalities.
• Explain the impact of different parameter selection options.

Session Outcomes

• Understand protocol selection.
• Understand key parameters affecting radiation dose and image quality.
• Appreciate the importance of the clinical question in protocol selection and determining dose and image quality.

Protocol Selection

• Factors Influencing Protocol Design:
  • Body Area: e.g., chest, abdomen, neck, head.
  • Field of View (FOV): Area of interest; coverage of whole organs.
  • Image Quality Parameters:
  • Adequate radiation/photons must reach detectors.
  • Comparison with conventional radiography; CT has more parameters.
  • Timing: Capture contrast in the appropriate phase for clinical needs.
  • Aim for the lowest possible radiation dose while ensuring proper imaging.

Who Sets the Protocol?

• Radiologist/Radiographer Role:
  • Review clinical information and enter protocol into CRIS/RIS or electronic systems.
  • Use acronyms like NCUA (non-con unless abnormal), CAP (chest, abdomen, pelvis).
  • Protocols must indicate body areas and scanning phases.

Setting Up Protocols on the Scanner

• Protocols are set by application specialists upon receiving a new CT scanner.
• Knowledge of specific scanner models is crucial for protocol fine-tuning.
• Collaboration among radiographers, team leaders, and medical physicists to ensure consistency across patients.

Clinical Questions and Protocols

• Importance of selecting the appropriate protocol for the clinical question.
• Challenges arise from inadequate clinical histories or conflicting clinical questions.

Image Quality

Types of Image Quality:

1. Spatial Resolution:

   • Ability to resolve small objects distinctly from the background.
   • Measured in line pairs per centimeter (lp/cm).

2. Contrast Resolution:

   • Differentiation of small differences in density between objects.

3. Longitudinal Resolution (z-axis):

   • Representation of depth in imaging (slice thickness matters).
   • Thinner slices provide more accurate information but result in increased radiation dose.

4. Temporal Resolution:

   • Ability to capture fast-moving objects (important for cardiac imaging).

Additional Considerations:

• Image Noise: Appears as mottle in images, affecting quality.

Parameters Affecting Image Quality and Radiation Dose

kV (Kilovoltage)

• Higher kV increases photon penetration but raises radiation dose.
• Enhanced detail due to more accurate attenuation profiles improving spatial resolution.

mAs (Milliampere-seconds)

• Represents the number of photons (mA over time).
• Needs to be adequate to prevent noise/mottle in images.
• Adjusted by Automatic Exposure Control (AEC) for balance between quality and dose.

Pitch & Rotation Time

• Pitch: Ratio of the distance the table travels per rotation to slice width. High pitch = faster scans and lower dose; low pitch = better overlapping data and higher dose.

Slice Thickness

• Thinner slices result in more accurate data but increase radiation dose.
• Slices can represent varying data based on thickness chosen.

CT Image Matrix

• Matrix: 2D grid of pixels for image representation (common size 1024x1024).
• Effective use of matrix optimizes spatial resolution.

Kernels & Post Processing

• Kernels: Algorithms used to refine images post-acquisition.
• Reconstruction allows for adjustments in thickness and noise reduction.
• Windowing adjusts visibility of specific tissues by modifying window width and level.

Summary

• Clinical questions guide appropriate protocol selection.
• Protocols are composed of multiple parameters that optimize image quality for specified body areas/pathologies.
• Includes more parameters compared to conventional radiography impacting image quality and radiation dose.
• Key types of image quality include spatial, contrast, longitudinal, and temporal resolution.