medical physics Flashcards
The nature of xrays
- can be polarised
- EM waves
- highly ionising
Production of xrays
Fast moving electrons are decelerated after interaction with atoms of am metal.
The kientic energy difference of the electrons is transformed into x ray photons
the tube itself? power supply…
X ray tube
Evacuated tube (electrons pass through without interacting with the atoms)
Power supply creates a large p.d between 2 electrodes.
The cathode is heated which produces electrodes by thermionic emission
Electrons accelerted towards anode (metal with high melting)
max energy of x-ray photon =
max KE of electron
What is atTENuation
why does bone attenuate matter
The decrease in the intensity of EM radiation as it passes through matter
Because as the radiation passes through it and its intensity decreases
4 attenuation mechanisms
- Simple scattering
- Photoelectric effect
- Compton scattering
- Pair production
Simple scattering attenuation
The photon is scattered elastically by an electron
(bounces off)
No change to energy
Photoelectric effect attenuation
Photon absorbed by electron
Electron escapes atom
Compton scattering attenuation
X ray photon interacts with electon
Electron released
Photon scattered with reduced energy
ENERGY AND MOMENTUM CONSERVED
Pair production attenuation
Photon interacts then disappears
ELECTRON AND A POSITRON released
example of a contrast medium
iodine
Theraputic use of x rays
Special machines accelerate photons to kill off cancerous cells
What is a cat scan
Records a large number of xv ray images from different angles and assembles them into a 3D image using technology
Advantages and disadvantages of CAT scans
+ 3D images
- Xrays are quicker and cheaper
- higher amounts of radiation
- patients have to remain very still
example of a medicalo tracer
fluorine 18
What is a PET scan
gpt
3D image produced using gamma rays rather than xrays
Tracer injected into patient
Accumulates in places with high levels of respiration
tracer emits positrons which anihilate with electrons ad release two photons
Patient surrounded by ring of detectors
what does a gamma camera do
detects gamma photons from emitted from the medical tracer
How does a gamma camera work
- Travels to collimator (long thin lead tubes to absorb photons that hit it at an angle)
- Reaches scintillator which turns the photons into visible light photons
- Visible light photons travel into photomultiplier tubes where photons converted into voltage
- Connected to a computer to be processed and create an image
What is Ultrasound
Benefits of using ultrasound to form images
What is ultrasound transducer
Sound waves with frequency above 20kHz
- not ionising
- non invasive
- quick
Device that generates and receives
What is the piezoelectric effect
The production of EMF by some crystals (quartz) when compressed , stretched twisted or distorted
(Reversible process)
How ultrasounds are produced
The ultrasound transducer contains contains piezoelectric crystal which when an alternating p.d is applied to it, compresses and stretches it
- resonates
- emits pulses of ultrasound
- crystal also detects the ultrasound
- generates emf
- detected by circuit
Ultrasound A Scan
what is on graph?
- 1 dimensional scan
- sends a single line of ultrasound into body
- records the time it takes for the echoes to return
- echoes received will have less energy because of energy losses within the body
- graph of time against voltage produced
Ultrasound B scan
- 2 Dimensional scan
-sends out waves in multiple directions - the brightness of each dot produced is proportional to the intensity of the reflected ultrasound
What is acoustic impedance
the product of the density of a substance and the speed of ultrasound in that substance
What is acoustic matching/ impedance matching
When two substances (coupling gel and skin) have similar acoustic impedance so negligible reflection occurs at the boundary between the two substances.
how is the doppler effect used in ultrasound
Ultrasound transducer above blood vessel
- Ultrasound reflected off tissues will return with the same frequency and wavelength as not moving
- the returned pulses from blood cells will have changed frequency
- transducer connected to computer which shows colour coded image of direction and speed of blood flow
How does frequency change depending on direction of blood flow
Frequency increases when blood moving towards transducer
Δf ∝ v
