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Noise reduction by frame averaging: a numerical simulation for portal imaging systems
W Swindell1, M A Mosleh-Shirazi
1Physics Department, Institute of Cancer Research, Sutton, Surrey, United Kingdom.
Medical Physics
|September 1, 1995
Summary
Frame summing before and after the Analog-to-Digital Converter (ADC) can reduce noise in portal imaging. However, excessive pre-ADC frame summing may unexpectedly decrease signal-to-noise ratio (SNR).
Area of Science:
- Medical Imaging
- Radiological Physics
- Image Processing
Background:
- Portal imaging systems are crucial for radiotherapy.
- Quantum and digitization noise affect image quality.
- Frame summing is a technique to reduce noise.
Purpose of the Study:
- To evaluate pre- and post-Analog-to-Digital Converter (ADC) frame summing for noise reduction in portal imaging.
- To identify optimal summing strategies for improving signal-to-noise ratio (SNR).
Main Methods:
- Analysis of a fluorescent-screen video-camera portal imaging system.
- Mathematical modeling to predict SNR based on frame summing parameters.
- Investigation of noise levels at the ADC input and quantization intervals.
Main Results:
- Pre- and post-ADC frame summing can increase SNR when ADC input noise is significant.
- An unexpected operating region exists where increasing pre-ADC frames (M1) decreases SNR.
- This anomalous behavior is linked to ADC input noise relative to the quantization interval (sigma_in/q).
- Post-ADC frame summing (M2) may not always improve image quality beyond a certain point.
Conclusions:
- Careful consideration of pre-ADC frame summing is necessary to avoid SNR degradation.
- System operating parameters, particularly ADC input noise, dictate the effectiveness of frame summing techniques.
- Optimizing frame summing requires balancing noise reduction with potential SNR penalties.