CSB012 WEEK 2 WED

X-ray tube components

Cathode

Anode

Two types of filaments in x-ray tubes

1. fine focus

2. Broad focus

Fine focus function

1. Focus accurately on anode target from cathode

2. Limited by amount of current due to blowout

Broad focus functions

Allows for increase of current for higher exposure and intensity

Less focus- less spatial resolution

Focusing cup

Allows for a more accurate focus on anode target, effectiveness dependent on size, shape and position of cup

Anode functions

1. Electrical conductor

2. Mechanical support for the target

3. Thermal Dissipater

X-ray formation

Electrons from cathode filament through thermionic emission are accelerated to high speeds by a potential difference between the cathode and anode.

X-ray spectrum

X-ray beam typically consists of a continuous spectrum of x-ray photon energies (bremsstrahlung) with a peak (characteristic)

What can affect the x-ray spectrum?

1. Changing kVp or current

2. Removing low energy x-rays through filters

Explain Bremsstrahlung X-rays

Electrons travel close to atom nucleus

Protons in nucleus cause electrons to change directions, losing energy and speed (electrostatic force)

A loss of KE reappears as Bremsstrahlung x-rays

EM radiation travels how?

Straight lines (central beam) and diverge from their source in a fan beam (divergent beam). They continue to travel until they interact with matter.

Energy transfer during an x-ray

Electrical energy (x-ray machine) → EM energy (when x-rays are produced at the anode → Chemical energy (converted to electrical signal at IR)

What happens in medical imaging?

X-rays pass through patient and is captured on IR creating a latent image that is processed through chem energy to produce a 2D x-ray image.

Main X-ray interactions (3)

1. Compton Scatter

2. Photoelectric effect

3. Transmitted to IR

Other interactions (3)

Pair production

Coherent scattering

Photodisintegration

Explain Compton Scatter

Interaction between incident x-ray and outer shell electron, where the atom is ejected and the x-ray is redirected with less energy.

Wavelength of the scattered x-ray is longer than the incident

Negative impact for diagnostic imaging, creates image noise and reduces contrast for image

Photoelectric effect

interaction x-ray with inner shell electron, the atom is ejected and the x-ray is fully absorbed.

The x-ray does not reach IR therefore represents anatomical structures with high x-ray absorption - radiopaque

Transmitted xray

Penetrates the body and reaches the IR

Produces dark areas of x-ray with higher density than radiopaque

Anatomical structures are radiolucent, e.g. air in lungs

Primary radiation

Radiation existing in x-ray tube

Scatter radiation

Non diagnostic radiation (compton scatter)

Absorbed radiation

Radiation absorbed by the patient (photoelectric effect)

Remnant radiation

Radiation transmitted to IR

Prime exposure factors

1. kVp (kilovoltage peak)

2. mA (milliamperes)

3. S (variable seconds)

4. SID (source image distance)

Effect of kVp on resultant x-ray image (primary and secondary control)

Primary control (effect) - quality (kVp) energy/penetrability

Secondary control - Quality (KeV) - intensity of the beam

factors affected by kVp

Compton scatter interactions

Photoelectic effect (absorption) is less frequent

Affects image contrast

SID

Source image distance (from x-ray tube to IR)

mAs controls _____?

Radiographic density (blackening of image)

X-ray beam quantity (intensity)

X-ray intensity quantity) is directly proportional to mAs, explain?

2x mAs, 2x electrons at anode, 2× 2x x-ray emitted

mA and S are____ (relationship).

Inversely proportional, 2x mA and half seconds creates the same mAs (quantity)

What does quantum mottle produce, how is it created?

Image noise, grainy speckled images as a result of insufficient mAs.