medical physics

Question 1

non invasive techniques

Answer 1

techniques which involve no cutting or incision of the body

Question 2

an example and advantage of non-invasive technique

Answer 2

X-rays

it has a lower risk of infection

Question 3

How does a medical tracer work

Answer 3

a radioactive substance is injected or ingested
Technetium-99 is commonly used
most tracers emit gamma radiation
the tracer is absorbed by the organ of interest
a gamma camera is used to image the organ

Question 4

collimator

Answer 4

a block of lead drilled with narrow tubes
gamma ray photons travel along the axis of the tubes
longer and narrower tubes give a sharper image

Question 5

scintillator

Answer 5

can be made from sodium iodide
a gamma ray photon strikes the scintillator
producing many visible light photons

Question 6

photomultiplier

Answer 6

electrical pulse produced by each photon of visible light

Question 7

what does the computer do in the gamma camera

Answer 7

connected to the photomultiplier

software uses electrical signals to generate an image

Question 8

4 parts of a gamma camera

Answer 8

collimator
scintillator
photomultiplier
computer

Question 9

how to PET scans work

Answer 9

A positron-emitting tracer is placed into the body
Flourine-18 can be used
each positron annihilates a single electron
producing two gamma ray photons travelling in opposite directions
the patient is surrounded by a ring of gamma detectors
a computer connected to the detectors works out the arrival times of the photons indicates the location of emission
the computer uses the information to create a 3d image

Question 10

advantage of PET scans

Answer 10

can be used to reveal the function of the brain or organs

Question 11

disadvantage of PET scans

Answer 11

uses ionising radiation which causes a small risk

Question 12

How does MRI work

Answer 12

Nuclei behave like tiny magnets

nuclei precess about the strong magnetic field provided by electromagnets

Question 13

Properties of ultrasound

Answer 13

longitudinal wave with a frequency higher than 20kHz

Question 14

piezoelectric effect

Answer 14

a potential difference is applied across a piezoelectric crystal
the crystal expands and contracts

Question 15

producing an ultrasound wave

Answer 15

an alternating potential difference is applied across the piezoelectric crystal
the crystal vibrates at a high frequency and emits ultrasound waves

Question 16

detecting ultrasound waves

Answer 16

an ultrasound wave is incident on a piezoelectric crystal

the cases the crystal to vibrate and produce an alternating potential difference

Question 17

How do ultrasound scans work

Answer 17

transducer with piezoelectric crystal emits and detects ultrasound waves
transducer sends pulses of ultrasound into the body
ultrasound is reflected at boundaries between tissues
reflected signals are detected by the transducer
time delay used to determine the depth of boundaries
intensity of reflected signal is used to identify the tissues

Question 18

what are A scans

Answer 18

A single transducer is used
in one direction only
allows depth or distance to be found

Question 19

what are B scans

Answer 19

an array of transducers are used
in multiple directions
allows 2d and 3d images to be produced

Question 20

how does the wavelength affect the image produced

Answer 20

higher frequency waves have a shorter waves length to give finer detail images

Question 21

how to work out acoustic impedance

Answer 21

density x wave speed

Question 22

why is impedance matching important

Answer 22

if there is a large difference in Z a large fraction of intensity is reflected
air and skin have very different Z’s so most of the intensity is reflected at the skin
gel with similar Z to skin is used
so very little of the intensity is reflected

Question 23

Doppler effect

Answer 23

a change in frequency and wavelength when waves are emitted from a moving source
for a source moving towards the observer the wavelength is smaller and the frequency is higher

Question 24

Doppler scans

Answer 24

used to measure the speed of blood flow in an artery
a transducer is placed at an angle to the artery
pulses of ultrasound are reflected by moving blood cells
the frequency of the ultrasound is changed because of the Doppler effect
the change in frequency is related to the speed of the blood

Question 25

Nature of xrays and xray photons

Answer 25

can travel in a vacuum at c
no charge and no mass
highly ionising

only waves
transverse and em waves with wavelength of 10^-10

Question 26

production of xrays

Answer 26

produced in an xray tube
electrons are accelerated across a high voltage
the fast moving electrons hit a metal target
the kinetic energy of the electrons is transferred into xray photons

Question 27

photoelectric effect

Answer 27

one xray photon is absorbed by an electron in an atom

an electron is ejected from the atom

Question 28

Compton effect

Answer 28

one xray photon collides with on electron in an aom
an electron is ejected from the atom
and a photon is scattered with a lower energy

Question 29

pair production

Answer 29

one xray photon collides with the nucleus of an atom

the photon disappears and an electron positron pair is produced

Question 30

what are image intensifiers

Answer 30

used because most xrays are not absorbed by the photographic film
scintillator is placed in front of the film
it absorbs an xray photon and releases many visible light photons
visible light photons are absorbed by the film
producing a higher contrast image with a lower exposure of xrays to the patient

Question 31

how does contrast media work

Answer 31

different soft tissues have very little difference in attenuation
barium can be digested into the digestive system
iodine can bee injected into capillaries and blood vessels
they have high atomic numbers so they have high attenuation coefficients
they absorb xrays producing a shadow on an xray image allowing the soft tissue to be outlined

Question 32

how does computerised axial tomography work

Answer 32

a thin fan shaped beam of xrays is used
the tube and detectors are rotated around the patient
so beam passes through the patient at different angles
computer software produces images of slices through the patient
the tube and detectors are moved along the patient
to produce the slices
computer using information from the detectors to calculate the 3d image

Question 33

how are CAT scans different to Xray images

Answer 33

Cat scans use many different angles

uses a computer to produce an image

Question 34

advantages of CAT scans

Answer 34

produces a 3d image

gives better contrast between different soft tissues

Question 35

attenuation

Answer 35

a reduction in intensity by absorption scattering or reflection

Question 36

how do you find the energy of xrays when they’re being produced

Answer 36

work done by pd = maximum kinetic energy of x-ray

QV=hf=hc/λ

Question 37

how does one x-ray photon produce many visible photons

Answer 37

x-rays have a higher energy which can form many visible light photons

Question 38

how is an x-ray image of bone produced

Answer 38

fewer photons pass through bone to the film under the patient
the bone casts a shadow on the film
metal absorbs x-rays better than bones so show up bright on x-rays