Cross Sectional Imaging Lecture 2: Scanner Operation and Parameter Selection

Module Learning Outcomes

• Describe and explain the physical principles behind cross-sectional imaging modalities.

• Understand the impact of different parameter selection options on imaging.

Session Outcomes

• Understand protocol selection.

• Recognize key parameters affecting radiation dose and image quality.

• Appreciate the importance of the clinical question in protocol selection.

Protocol Selection

• Factors influencing protocol design include:

  • Body area of interest (e.g., chest, abdomen).

  • Field of view (FOV) and the specific areas to be imaged.

  • Various imaging parameters to ensure quality (e.g., kV, mA).

  • More parameters than conventional radiography, including timings for contrast capture.

  • The clinical question guides protocol design (e.g., detecting PE).

  • Importance of minimizing radiation dose.

Protocol Control

• Set by the radiologist/radiographer based on clinical information.

• Common acronyms:

  • NCUA (non con unless abnormal)

  • CAP (chest abdomen pelvis).

• Protocols specify body areas and scan phases.

Scanner Protocol Setup

• Applications specialists set baseline protocols on new CT scanners.

• Input from radiographers, leaders, medical physicists ensures consistency.

• Protocols should remain unchanged unless necessitated by challenging patient scenarios.

Protocols and Clinical Questions

• The right protocol is crucial for addressing specific clinical inquiries.

• Challenges arise from incomplete clinical histories or conflicting questions.

Image Quality Types

1. Spatial Resolution:

   • Ability to distinguish small, separate objects against a background.

   • Measured in line pairs per centimetre (lp/cm).

2. Contrast Resolution:

   • Differentiating small density differences between objects.

3. Longitudinal Resolution (Z-axis):

   • Describes pixel attenuation in a 3D voxel.

   • Isotropic imaging aims for equal resolution across dimensions.

   • Thinner reconstructed images can either enhance or hinder detail.

4. Temporal Resolution:

   • Ability to resolve fast-moving objects, akin to camera shutter speed.

   • Important in specific applications like cardiac CT.

Noise in Imaging

• Image noise appears as mottle, affecting clarity and quality.

Key Parameters Affecting Image Quality and Radiation Dose

• kV (Kilovoltage):

  • Higher kV enhances photon penetration and detail but increases radiation dose.

• mAs:

  • Continuous scan representation; directly correlates with photon quantity.

  • Insufficient mAs leads to noise and lower quality.

  • Modulation through Automatic Exposure Control (AEC).

• Pitch:

  • Distance table moves per rotation divided by slice thickness.

  • Affects radiation dose and image overlap, with lower pitch offering better quality but requiring higher dose.

• Slice Thickness:

  • Thinner slices provide greater accuracy but increase radiation exposure.

  • Thick slices average out voxel data, compromising detail.

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CT Image Matrix and Kernels

• Matrix:

  • 2D grid for pixel arrangement; typically 1024x1024.

  • Scrutinizing matrix usage improves spatial resolution.

• Kernels:

  • Algorithms applied for enhancing image quality post-acquisition.

  • Different kernels target specific tissues (e.g., high resolution for lungs).

Post Processing Techniques

• Effective in visualizing and optimizing acquired images.

  • Includes reconstruction for varying slice thickness and density adjustments.

  • Adjusting the window level and width enhances visibility of regions of interest.

Summary of Key Points

• Clinical questions are decisive for appropriate protocol selection.

• Protocols encompass parameters defined to optimize image quality.

• Imaging in CT involves more parameters compared to conventional radiography.

• Four main types of resolution affect how images are interpreted:

  • Spatial, contrast, longitudinal, and temporal resolution.