Chapter 27 - Medical imaging

Question 1

how are x-rays produced

Answer 1

by rapidly accelerating or decelerating charged particles, their kinetic energy transfers to high-energy photons

Question 2

why are x-rays referred to as soft

Answer 2

because they have energies lower than that of gamma rays

Question 3

structure of an x-ray tube

Answer 3

heated filament (cathode) and tungsten anode with p.d. between them up to 200kV, and sealed in a vacuum tube

Question 4

how does an x-ray tube work

Answer 4

electrons emitted from cathode and drawn towards anode. they collide with the anode and some KE is released as x-rays in all directions

Question 5

why do x-ray tubes need a vacuum

Answer 5

to prevent electrons from colliding with molecules of air before enough energy is gained to release x-rays

Question 6

how is the anode prevented from overheating

Answer 6

water is used as a coolant, circulating through the anode

Question 7

importance of x-rays being focused at one point

Answer 7

• minimise patients exposure
• can be directed at specific areas (broken bone)

Question 8

what is x-ray attenuation

Answer 8

when a material absorbs x-rays, exponentially decreasing the intensity

Question 9

what does a greater attenuation coefficient mean

Answer 9

material will absorb more incident x-rays

Question 10

what happens to the kinetic energy of electrons during the x-ray process

Answer 10

• less than 1% is emitted as x-rays
• the rest is transferred to thermal energy in the anode

Question 11

photoelectric effect (x-rays)

Answer 11

• less than 100 keV
• x-rays absorbed by electrons, photoelectron is released when absorbed, and another electron is moved down an energy level

Question 12

Compton effect (x-rays)

Answer 12

• 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

Question 13

Pair production (x-rays)

Answer 13

• 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

Question 14

CAT

Answer 14

computerised axial tomography

Question 15

ultrasound frequency

Answer 15

greater than 20kHz

Question 16

two invasive processes

Answer 16

x-rays
surgery

Question 17

3 non-invasive processes

Answer 17

MRI
ultrasound
endoscopy

Question 18

benefits of PET

Answer 18

non-invasive
versatile diagnoses

Question 19

downsides of PET

Answer 19

expensive
complex diagnoses only
not all hospitals have access to them

Question 20

pros of ultrasound

Answer 20

no ionising radiation
quick and affordable
can distinguish between tissues

Question 21

cons of ultrasound

Answer 21

high intensity US can cause potential tissue damage

Question 22

acoustic impedance

Answer 22

the resistance to sound travelling through a substance

Question 23

pulse repetition frequency

Answer 23

time taken for pulse to travel to and from a direction

Question 24

a-scans

Answer 24

no image produced
measurements can be taken
shows reflected intensities over time

Question 25

b-scans

Answer 25

images produced many echoes collected and collated

Question 26

what is a transducer

Answer 26

electronic device which converts energy from one form to another

Question 27

why is gel used during ultrasound

Answer 27

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

Question 28

two main properties of ultrasound

Answer 28

-cant travel through a vacuum -vibrate at a frequency greater than the human hearing range - more than 20kHz

Question 29

difference between a-scans and b-scans

Answer 29

a-scans don't produce an image but b-scans do

Question 30

two factors effecting the value of acoustic impedance

Answer 30

-density of tissue -velocity of sound wave transmitted through boundary

Question 31

two properties of x-rays

Answer 31

-can travel in a vacuum -produce images of human bones

Question 32

what does I0 stand for in intensity equation

Answer 32

intensity of incident beam

Question 33

difference between MRI scans and PET scans

Answer 33

PET scans use a radioactive tracer to display areas, MRI scans use radio waves and magnetism to display areas

Question 34

advantages of CAT scans compared to an x-ray image

Answer 34

CAT scans show internal organs as well as bones whereas x-rays only display bones

Question 35

why is a contrast medium used in x-ray imaging

Answer 35

helps radiologists identify between normal and abnormal conditions