Module 7 - Week 11 Flashcards
What is attenuation?
Attenuation is the reduction of x-ray photons in the beam as it passes through a material, due to interactions like the photoelectric effect and Compton scattering.
What factors influence attenuation?
Tissue thickness, density, atomic number, and kVp settings.
What is the 5cm tissue thickness rule?
For every 5cm increase in tissue thickness, x-ray photon transmission is reduced by 50%, requiring a doubling of exposure (mAs). Conversely, for every 5cm decrease, exposure can be halved.
How does subject contrast affect an x-ray image?
Subject contrast determines how different tissues appear on an image due to variations in absorption, scatter, and transmission of x-rays.
What three factors determine subject contrast?
- Tissue thickness
- Tissue density
- Atomic number
How does tissue thickness impact subject contrast?
Thicker tissues attenuate more x-rays, reducing the amount that reaches the receptor, leading to increased contrast in the image.
How does tissue density affect subject contrast?
Denser tissues have more atomic packing, leading to increased attenuation and greater subject contrast.
How does atomic number impact subject contrast?
Higher atomic number tissues absorb more x-rays through the photoelectric effect, increasing subject contrast.
What is the effect of contrast media on subject contrast?
Contrast media (e.g., iodine, barium) have high atomic numbers, enhancing subject contrast by increasing x-ray absorption.
How does scatter radiation affect image contrast?
Scatter reduces contrast by introducing unwanted radiation, making structures appear more uniform and less distinct.
How does kVp affect subject contrast?
- Low kVp → More photoelectric absorption, higher contrast
High kVp → More x-ray penetration, increased Compton scatter, lower contrast
What happens to Compton and photoelectric interactions as kVp increases?
- Photoelectric interactions decrease significantly
Compton interactions also decrease but at a slower rate
How can scatter radiation be reduced to improve contrast?
Using collimation, grids, and lower kVp settings.
What is the effect of increasing kVp on attenuation?
Higher kVp increases photon penetration, reducing attenuation and decreasing subject contrast.
What is the relationship between tissue thickness and radiation transmission?
As tissue thickness increases, fewer x-ray photons reach the receptor, resulting in higher contrast areas (whiter on the image).
