6.5 - Medical Imaging Flashcards
How were x-rays discovered?
- 1895, Wilhelm Roentgen
- he was working with fluorescent tubes which glow when a high voltage is put across them
- he was investigating what happens at low pressures
- he found that when the tube went dark, a screen about a metre away was fluorescing
- when the tube was shielded with plack paper, the screen was still fluorescing
- he had discovered an invisible ray capableof passing through the heavy paper - an x-ray
What are some further discoveries about x-rays?
- 1906 -charles barkkla established waves could be polarised
- 1912 - arnold sommerfeld estimated their wavelength. (10^-12 - 10^-9 m)
- Max von Laue proved that x-rays could be diffracted by crystals
How are X-Rays produced?
- an X-ray tube
- inside the machine there is a vaccuum which contains a cathode
- the cathode is heated which produces a beam of electrons (thermionic emission)
- using very high voltages (thousands/millions), electrons are accelerated to very high velocities
- accelerated electrons collide with a metal target (eg tungsten) called the anode
- electrons decelerate rapidl and convert some of their KE to X-Rays (Bremmsstrahlung- Braking radition). The rest heats the anode
- anode may rotate to spread the heat out
What is the energy of an X-ray photon?
E(max) = eV
Where:
• E = energy
• e = charge on electron
• V = potential difference
The energy of emittedphotons depends on he energy of invident electrons
This is a maximum value bcs some of the energy will be converted into heat
How can you caluclate wavelength of an X-ray?
E =hc/λ
λ = hc/eV
K line??
- Brehmssrahlung radiation: X-rays produced due to electrons slowing doen and losing energy
- K lines: X-rays produced due to electrons being excited and dexcited, resulting in energy being lost in the form of X-rays.
What are some problems encountered in X-ray production?
Only 1% of the electron enegry is used to produce X-rays - the rest heats up the anode
• the anode is cooled by oil flowing arund it
• it also has a high SHC
Operators need to be protected from the radiation
• x-ray tube is surrounding by lead shielding
• anode is shaped so that the x-rays are directed through a window
What is attenuation?
- X-rays are scattered and absorbed by matter
- X-rays interact with atoms in different ways:
- simple scattering
- photoelectric absorption
- compton scattering
- pair production
• attenuation is the effect of the intensity of X-ays being reduced as they pass through matter
What is simple scattering?
- occurs for x-ray energies 1-20keV
- x-ray interacts with an electron in the wtom but doesnt ionise the atom
- energy of scattered xray doesnt change
- hospital X-ray machines generate xrays >20kEV so this effect is insignificant in radiogaphy
What is photoelectric absorption?
- incident X-ray photon ejects an orbital electron from an atom in the absorbing material
- an electron from a higher sgell may drop down to fill this hole, giving out energy in the form of an X-ray photon
- significant for photon energies less than 100keV
What is compton scattering?
- significant for photon energies in the range 0.5 - 5.0MeV
- incident photon is scattered by an orbital electron in the absorbing material
- the electron takes some of the energy of the photon and moves off in a different direction
- compton discovred that some of the deflected photon had a longer wavelength than the initial wavelength bcs it has less energy
- the greater the angle of deflection, the greater the change in wavelength
What is pair production?
- at higher photon energies, the X-ray photons can cause pair production in the absorbing material
- an X-ray photon collides with a nucleus within the material, causing the production of an electron-positron pair
- the positron then annihilates with an electron and produces a pair of photons
Why are there always 2 photons produced in pair production?
To conserve momentum
What is the equation for attenuation?
I = I0 e^(-µx)
Where: •I = attenuated intensity • I0 = original intensity • µ = attenuation (or absorbtion) coefficient • x =thickness of medium
What are contrast media?
- soft tissues don’t show up well in x-rays bcs they have low absorption coefficients
- therefore a contrast medium (eg iodine, barium) is used
- these have large atomic numbers and absorb x-rays via photoelectric effect (for this to happen, µ must be proportional to atomic number cubed)
- iodine used as contrast medium for liquids eg blood
- barium is used for the digestive system
What is the inverse square law?
Intensity ∝ 1/r²
Why are x-rays collimated?
Sothat the intensity of x-rays is unchanged (if travelling through a vacuum)
What is the half-value thickness?
The distance in a medium over which x-ray attenuated to half its original value (I = I0 e^(-µx))
What are the problems with x-ray imaging?
- considerable exposure time required (dangerous due to ionising)
- difficult to imagingsoft tissue due to lack of contrast
- limited to still imaging
Why is gadolinium phosphate used to absorb x-rays?
- photographic film is a poor absorber of x-rays, but a good absorber o visible light
- gadolinium phophate absorbs x-rays and emits light, so the photographic film makes a clearer image
How are x-ray images intensified?
- a intensifying screen is placed wither side of the photographic film
- when x-ray photons hit crystals in this screen, the atoms become excited
- they then relax into their ground state an fluoresce (emit visible light photons)
- photographic film is much more efficient at absorbing visible light photons than x-ray photons. They develop the film, turning it black (fogging it)
- this gives more concise but not clearer image
- fewer x-rays are required so less exposure for patiet and the imae is formed faster
How can you increase the contrats of an x-ray?
- use a contrast medium
* use gadolinium phosphate
