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
Spatial Resolution:
Ability to distinguish small, separate objects against a background.
Measured in line pairs per centimetre (lp/cm).
Contrast Resolution:
Differentiating small density differences between objects.
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.
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.