Radiology: Differential Absorption Concepts CH 10

Differential Absorption

• X-rays interact with matter in five ways: coherent scattering, Compton scattering, photoelectric effect, pair production, and photodisintegration
• Only two interactions are critical to radiology: Compton scattering and the photoelectric effect
• Two x-ray production methods are crucial: bremsstrahlung and characteristic x-rays.
• X-rays that pass through the body without interaction are more significant for creating an image than those that interact through scattering or absorption
• Image formation depends on the difference between x-rays absorbed photoelectrically by the patient and those transmitted to the image receptor. This difference is called differential absorption
• Approximately 1% of incident x-rays reach the image receptor
• Radiographic images come from about 0.5% of the x-rays emitted
• Differential absorption increases as the kVp decreases
• Higher quality radiographs require proper kVp selection for optimal x-ray energy for maximum differential absorption
• Reducing kVp to enhance differential absorption increases patient radiation dose

X-Ray Interactions

• Compton scattering contributes no information to the image; it results in image noise, making the image less clear
• Techniques and apparatus are used to reduce scattered x-rays reaching the image receptor
• X-rays that undergo photoelectric interaction provide diagnostic information
• Photoelectric absorption produces the light areas in a radiograph, corresponding to areas with high x-ray absorption
• These areas are radiopaque, meaning they absorb more x-rays
• Transmitted x-rays create dark areas in a radiograph, representing structures through which the x-rays pass, which are called radiolucent

Dependence on Atomic Number

• The probability of photoelectric effect is proportional to the third power of the atomic number of the tissue
• Bones have a higher atomic number, causing more x-rays to be absorbed via the photoelectric effect, resulting in lighter areas in the image

Dependence on Mass Density

• Mass density is the quantity of matter per unit volume
• Mass density is sometimes measured in g/cm³
• X-ray interaction with tissue is directly proportional to the tissue's mass density, regardless of the type of interaction

Summary Table(Mass Density)

Explore the essential interactions of X-rays with matter that impact radiology, focusing on Compton scattering and the photoelectric effect. Understand the significance of differential absorption in image formation and the factors that affect radiographic quality. This quiz is designed to assess your knowledge on these fundamental concepts in radiology.