Week 3 - Interactions Flashcards
Why are the processes of interaction between radiation and matter important?
Because a basic understanding is necessary to set technical exposure factors when imaging
What are the important technical exposure factors worth knowing?
kVp (peak kilovoltage) and mAs (milliampere-seconds)
What is kVp?
Highest energy level photons in the x-ray beam equal to highest voltage established across the tube
What does kVp control?
Quality/penetrating power of the photons in the beam and to some degree the quantity of photons
What is mAs?
The product of milliampere and the amount of time in seconds the tube is activated
What is mA?
The current of the electron tube
What is the formula for mAs?
mA x s = mAs
What does mAs control?
The Quantity of radiation that is directed toward the patient during exposure
What is the radiographer responsible for when imaging?
- Setting the technical exposure factor that control quality/quality of the beam
- Selecting the dose the patient receives
What can selecting appropriate techniques achieve with imaging?
- Minimized dose to patients
- Images of optimal-quality
What do x-ray carry?
Manmade electromagnetic energy
What are the 3 ways x-rays enter a material such as human tissues?
- Interact with atoms of the tissue and are absorbed
- Interact with atoms of the tissue and are scattered, causing indirect transmission
- Pass through without interaction
What is absorption?
When electromagnetic energy is transferred from the x-rays to the atoms of a patient’s tissue
What is absorbed dose?
The amount of energy per unit mass
How can biologic damage be minimized?
By keeping the amount of electromagnetic energy transferred to patient as small as possible
Why is absorption important to diagnostically useful images?
Helps distinguish anatomic structures in images
What are the benefits to keeping patient dose minimal?
- Less radiation is scattered from the patient
- Reduces occupational hazards to the radiographer
How is a diagnostic x-ray beam produced?
When a stream of high-speed electrons bombard a positively charged target in a evacuated glass tube
What are anodes composed of?
Tungsten (metal) or Tungsten rhenium (metal alloy)
Why are tungsten and tungsten rhenium used as target materials?
- Have high melting points
- High atomic numbers
Do all photons in an x-ray beam have the same energy?
No, they have variable energy levels
What is the max energy a photon can have?
No more energy that the electrons that bombard the target
What is the energy of the electrons inside the tube specified in?
In terms of electrical voltage applied across the tube
In diagnostic radiology, what is the energy of electrons in the tube is expressed in?
kV, but because the voltage fluctuates its usually expressed in kVp
What is attenuation?
When an x-ray beam passes through a patient
What is direct transmission?
When primary photons traverse the patient without interacting and hit the image receptor
What is indirect transmission?
When primary photons traverse the patient with interacting and hit the image receptor
What is attenuation?
The reduction in intensity of the x-ray beam after its passed through a patient
What two mechanisms produce attenuation?
Absorption and Scattering
What is scattering?
A change in direction of a photon after interaction with an atom
What is small- angle scatter?
When the beam changes direction only slightly so that its still directed towards the IR
How does small-angle scatter interfere with image production?
It produces radiation fog
When is an optimal x-ray image formed?
When only direct transmission x-rays photons reach the IR
What methods can be used to limit the effects of indirect transmission photons?
Radiographic grids and air gap techniques
What types of transmission form radiographic images?
Direct and Indirect transmitted x-ray photons
What problems are caused by radiographic fog?
Interferes with the radiologists ability to distinguish different structures in the image
How can radiographic fog be reduced?
By reducing the amount of tissue irradiated, which is done via collimation
How can the probability of photon interactions with biologic matter be described?
Random
What can be predicted when photons enter tissue?
What will happen on average when a large number of photons enter the body
What does predicting photon interactions determine?
The characteristics of the image that results from these types of interactions
When primary radiation emerges from the x-ray tube what properties do the photons have?
X-ray photons of various energies
What are primary photons?
Photons that emerge from the x-ray source before they hit the body
What are exit, or image-formation, photons?
Photons that pass through the patient being radiographed and reach the image receptor
What are attenuated photons?
Photons that have interacted with the patient and have been scattered so that they don’t reach the IR