medical physics Flashcards
non invasive techniques
techniques which involve no cutting or incision of the body
an example and advantage of non-invasive technique
X-rays
it has a lower risk of infection
How does a medical tracer work
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
collimator
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
scintillator
can be made from sodium iodide
a gamma ray photon strikes the scintillator
producing many visible light photons
photomultiplier
electrical pulse produced by each photon of visible light
what does the computer do in the gamma camera
connected to the photomultiplier
software uses electrical signals to generate an image
4 parts of a gamma camera
collimator
scintillator
photomultiplier
computer
how to PET scans work
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
advantage of PET scans
can be used to reveal the function of the brain or organs
disadvantage of PET scans
uses ionising radiation which causes a small risk
How does MRI work
Nuclei behave like tiny magnets
nuclei precess about the strong magnetic field provided by electromagnets
Properties of ultrasound
longitudinal wave with a frequency higher than 20kHz
piezoelectric effect
a potential difference is applied across a piezoelectric crystal
the crystal expands and contracts
producing an ultrasound wave
an alternating potential difference is applied across the piezoelectric crystal
the crystal vibrates at a high frequency and emits ultrasound waves
detecting ultrasound waves
an ultrasound wave is incident on a piezoelectric crystal
the cases the crystal to vibrate and produce an alternating potential difference
How do ultrasound scans work
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
what are A scans
A single transducer is used
in one direction only
allows depth or distance to be found
what are B scans
an array of transducers are used
in multiple directions
allows 2d and 3d images to be produced
how does the wavelength affect the image produced
higher frequency waves have a shorter waves length to give finer detail images
how to work out acoustic impedance
density x wave speed
why is impedance matching important
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
Doppler effect
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
Doppler scans
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
Nature of xrays and xray photons
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
production of xrays
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
photoelectric effect
one xray photon is absorbed by an electron in an atom
an electron is ejected from the atom
Compton effect
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
pair production
one xray photon collides with the nucleus of an atom
the photon disappears and an electron positron pair is produced
what are image intensifiers
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
how does contrast media work
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
how does computerised axial tomography work
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
how are CAT scans different to Xray images
Cat scans use many different angles
uses a computer to produce an image
advantages of CAT scans
produces a 3d image
gives better contrast between different soft tissues
attenuation
a reduction in intensity by absorption scattering or reflection
how do you find the energy of xrays when they’re being produced
work done by pd = maximum kinetic energy of x-ray
QV=hf=hc/λ
how does one x-ray photon produce many visible photons
x-rays have a higher energy which can form many visible light photons
how is an x-ray image of bone produced
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
