27 : Medical imaging Flashcards
1
Q
X-rays
A
- EM- waves 10-10000 times more energy than a photon of visible light
- harmful to living cells and can kill them
2
Q
Production of x-rays
A
Fast moving electrons are decelerated by interaction with atoms of a metal
- external power supply is used to create a large p.d between these electrodes.
- cathode produces electrons through thermionic emission
- they are accelerated towards the anode = target metal with high melting points
- x-ray photons are produced when the electrons are decelerated by hitting the anode
- energy output is less than 1% and rest is transformed into thermal energy
- anode is shaped so that the x-rays are emitted in the desired direction through a window
3
Q
The shortest wavelength
A
- electron accelerated through a potential difference gains kinetic energy eV
- max energy of x-ray photon = max kinetic energy of electron
Hf = eV
4
Q
Piezoelectric effect
A
- when external p.d. Is applied across the opposite faces of the crystal he electric field can compress or stretch the crystal
- the crystal can produce an e.m.f. When they are distorted
5
Q
Ultrasound transducer
A
- Alternating p.d. Is applied across crystal which repeatedly compresses and expands the crystal
- frequency is the same ad the natural frequency of p.d. The crystal resonates and produces intense ultrasound signal
- ultrasound incident on the crystal will make it vibrate, so crystal compresses and expanded which generate an alternating e.m.f across ends of crystal which can be detected
6
Q
Acoustic matching
A
- transducer placed on skin air pockets will be trapped
- the air skin boundary means that 99% of incident ultrasound will be reflected
- a coupling gel is placed which has similar acoustic impedance similar to skin which ensures that almost all the ultrasound enters the patients body
7
Q
Speed of blood
A
- shifts in frequency is directly proportional to the speed of blood flow
- blood moving towards means increase in frequency
