FRCR: Physics Lectures Diagnostic Radiology. Lecture 3 Image quality, scatter and contrast agents Dr Tim Wood Clinical Scientist. What is image quality? Contrast What is contrast? Contrast agents Scatter vs contrast Spatial Resolution What is limiting spatial resolution?
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Image quality, scatter and contrast agents
Dr Tim Wood
Image quality describes the overall appearance of the image and its fitness for purpose
Remember: There is always a play-off between image quality and patient dose
We only need images that are of diagnostic quality (fit for purpose) – not pretty pieces of art!
The main factors to consider are:
Low Energy (kV) = Low Penetration = High Contrast = High Patient Dose
High Energy (kV) = High Penetration = Low Contrast = Low Patient Dose
Optimal Situation = kV that gives adequate contrast for an acceptable patient dose – will vary for the clinical application
% contrast reduced
Scatter = 4 x Primary (PA Chest)
S+P contrast = (10-9)/10 x 100 = 10%
Primary contrast = (2-1)/2 x 100 = 50%
Spatial resolution describes the ability to see fine detail within an image
Fine detail is clearer when the contrast is high
May be expressed as the smallest visible detail, but most common descriptor is the highest frequency of lines that can be resolved in a high-contrast bar pattern
Ideal point source of X-rays
Focal spot of finite size, f
Reduced Exposure = Low Patient Dose= Increased Q-Noise = Reduced Image Quality
Increased Exposure = High Patient Dose = Reduced Q-Noise = Improved Image Quality
Optimal Situation = Exposure high enough to give a diagnostic image with acceptably low quantum noise, whilst maintaining an acceptable patient dose.