Chapter 27 - Medical imaging Flashcards
how are x-rays produced
by rapidly accelerating or decelerating charged particles, their kinetic energy transfers to high-energy photons
why are x-rays referred to as soft
because they have energies lower than that of gamma rays
structure of an x-ray tube
heated filament (cathode) and tungsten anode with p.d. between them up to 200kV, and sealed in a vacuum tube
how does an x-ray tube work
electrons emitted from cathode and drawn towards anode. they collide with the anode and some KE is released as x-rays in all directions
why do x-ray tubes need a vacuum
to prevent electrons from colliding with molecules of air before enough energy is gained to release x-rays
how is the anode prevented from overheating
water is used as a coolant, circulating through the anode
importance of x-rays being focused at one point
- minimise patients exposure
- can be directed at specific areas (broken bone)
what is x-ray attenuation
when a material absorbs x-rays, exponentially decreasing the intensity
what does a greater attenuation coefficient mean
material will absorb more incident x-rays
what happens to the kinetic energy of electrons during the x-ray process
- less than 1% is emitted as x-rays
- the rest is transferred to thermal energy in the anode
photoelectric effect (x-rays)
- less than 100 keV
- x-rays absorbed by electrons, photoelectron is released when absorbed, and another electron is moved down an energy level
Compton effect (x-rays)
- 0.5 to 5 MeV
- inelastic collision between photon and electron
- x-rays lose a fraction of their energy to electrons in the absorbing material
- scattered x-ray photon has less energy than before
Pair production (x-rays)
- energy greater than 1.02 MeV
- x-ray passes through the electric field of an atom
- produces an electron-positron pair
- positron collides with another electron and annihilate produced photons
CAT
computerised axial tomography
ultrasound frequency
greater than 20kHz
two invasive processes
x-rays
surgery
3 non-invasive processes
MRI
ultrasound
endoscopy
benefits of PET
non-invasive
versatile diagnoses
downsides of PET
expensive
complex diagnoses only
not all hospitals have access to them
pros of ultrasound
no ionising radiation
quick and affordable
can distinguish between tissues
cons of ultrasound
high intensity US can cause potential tissue damage
acoustic impedance
the resistance to sound travelling through a substance
pulse repetition frequency
time taken for pulse to travel to and from a direction
a-scans
no image produced
measurements can be taken
shows reflected intensities over time
b-scans
images produced
many echoes collected and collated
what is a transducer
electronic device which converts energy from one form to another
why is gel used during ultrasound
the gel has similar acoustic impedance to the skin, meaning hardly any ultrasound is reflected, so that the transducer can only target the desired area
two main properties of ultrasound
-cant travel through a vacuum
-vibrate at a frequency greater than the human hearing range - more than 20kHz
difference between a-scans and b-scans
a-scans don’t produce an image but b-scans do
two factors effecting the value of acoustic impedance
-density of tissue
-velocity of sound wave transmitted through boundary
two properties of x-rays
-can travel in a vacuum
-produce images of human bones
what does I0 stand for in intensity equation
intensity of incident beam
difference between MRI scans and PET scans
PET scans use a radioactive tracer to display areas, MRI scans use radio waves and magnetism to display areas
advantages of CAT scans compared to an x-ray image
CAT scans show internal organs as well as bones whereas x-rays only display bones
why is a contrast medium used in x-ray imaging
helps radiologists identify between normal and abnormal conditions
