Principles of radiography Flashcards
2 types radiographic image
Bones dark (positive image) or bones white (negative image)
Key requirements for x-ray production
electron source, method for accelerating electrongs, a heavy metal target (tungsten)
How are x-rays produced?
when fast electrons collide with atoms of a heavy metal such as tungsten.
X-ray absorption depends on ? 3 Effect of increasing any of these?
tissue density, atomic number of tissue elements, tissue thickness (increasing any of these increases the number of electrons in the x-ray path).
What is Bremsstrahlung radiation?
Fast electrons are slowed as they approach a tungsten nucleus –> loss of energy = x-rays. Bremsstrahlung results in a broad continuous range of x-ray energies.
Is air or metal more radiolucent on an x-ray?
Radiolucent (=black), air is more radiolucent than metal (opaque/white on an x-ray)
How do you distinguish parenchymal organs, muscle and fluids on an x-ray?
You can’t unless you add a contrast media
What is opacity?
A white or light grey area on a radiograph corresponds to a body part that absorbs x-rays (i.e. is opaque to x-rays)
What is lucency?
A black or dark grey area on a radiograph represents a part of the body that allows more x-rays to pass through (i.e. is lucent to x-rays)
Equation for mAs
= milliamps (mA) * time (seconds)
Define milliamperes (mA)
The current (number of electrons) passing across the x-ray tube. (time in the equation for mAs isthe duration of this current)
How do you increase the number of electrons passing through the x-ray tube/increase the number of x-rays produced/make the radiograph darker?
Increasing the current and/or increasing the time
Typical range for radiographing small animals
25-300mA and 0.01-0.3s
When to increase the mA setting? 2
High mA settings are used to minimise the exposure time and avoid motion blurr.
Define kVp
Kilovoltage peak - the voltage applied across the x-ray tube
Effect of increasing the kVp - 2
Increased the number of x-rays produced AND increases the x-ray beam energy which increasing the penetrating power of the x-ray beam (the higher the voltage, the faster the electrons travel)
Use - low kVp exposure
Very small body parts (e.g. extremities)
Use - high kVp exposure
Thicker body parts (e.g. thorax)
Types of positive contrast media - 2
- Iodide ions - high atomic number therefore absorbs x-rays and appear white (IV or bladder imaging)
- Barium (sulphate) - to look at GIT disease.
In an x-ray tube, what is the function of the copper that the tungsten is mounted on?
When the copper heats up (conducts heat from electrons), prevents the tungsten heating up too much and splitting
Why is the tungsten, copper and aluminium cup put in a high vaccuum?
To stop air disrupting the path of electrons
Describe the electromagnetic spectrum
Low energy = AC power, radio, television, infrared, radar, visible, ultraviolet, gamma rays, x-rays
Define KeV
kiloelectron volts (90= peak energy/highest)
Why don’t low x-rays escape?
They are filtered/absorbed by 2mm aluminium.
How does changing the kVp affect the shape of the x-ray spectrum?
It changes the shape of the x-ray spectrum - peak and median energies change.
Are there standard kVp size guidelines
kVp numbers vary between x-ray machines. Increase the kVp the thicker the animal’s body part.
