G485 - Medical Imaging Flashcards
How are x-rays produced?
- high voltage applied between anode and cathode
- electrons are pushed on to the cathode by the negative terminal of the battery
- p.d accelerates electrons across the gap between the cathode and the anode
- when the electron collides with the anode it loses a lot of kinetic energy which cannot be easily dissipated through the anode
- some energy heats the anode and some is released as an x-ray photon
Why are x-rays produced in a vacuum?
so that air particles don’t deflect the electrons and disrupt the current
What is the maximum energy of a produced x-ray photon equal to?
the kinetic energy of one electron = WQ
Emax = hfmax = hc/λmin
What material is the anode made from and why?
tungsten
- high melting point
- durable
How is the anode kept cool?
The anode rotates so that the same area isn’t constantly heated
each section has time to cool while it rotates outside of the electron beam
Gamma Wavelength
0.0001nm
X-Ray Wavelength
0.1nm
Visible Wavelength
600nm
Radio Wavelength
1m
Intensity Definition
energy incident per unit area per unit time
X-ray Spectra - Characteristic Peaks
- specific to anode metal
- electrons are stripped out of shells in the anode atoms by incident electrons
- atoms from higher energy levels drop down to replace them
- difference in energy when they drop down is released as a photon
- so some specific wavelengths are produced in greater numbers
Work Function Definition
minimum energy required to release a photoelectron from the surface of a metal
X-ray Photon Energy Equation
Work Function + KEmax
Attenuation Definition
decrease in intensity
decrease in the number of photons in the x-ray beam
Attenuation - Absorption
- all energy from the photon is transferred to the atom
- some of that energy is used to liberate an electron
- a special case is pair production
Attenuation - Pair Production
- all energy from photon transferred to atom
- electron and positron emitted
- for this to happen the energy of the incident photon has to be greater than or equal to mc², where m is the mass of the electron and the positron
- this can only occur at an atom as linear momentum has to be conserved (the atom is deflected to balance the movement of the electron and the positron away from the atom)
Attenuation - Scattering
photon from the beam continues but changes direction so that is no longer part of the beam i.e. the intensity if the beam is decreased
in elastic scattering the energy of the photon is conserved
Attenuation - The Compton Effect
the photon transfer some of its energy to an electron ionising it
the remaining energy continues moving as a photon but in a different direction
What happens as an object approaches the speed of light?
mass decreases
What is the function of a collimator?
a series of parallel tubes
incident radiation can only travel through the collimator if it is parallel to the tubes
How can sharpness be increased using a collimator?
longer tubes
thinner tubes
How can the radiation absorbed by a patient during an x-ray be minimised?
- collimator
- aluminium sheet
- shielding surrounding tissue
What is the purpose of an aluminium sheet in x-ray production?
- the aluminium sheet absorbs low energy photons after they have passed through the collimator
- the low energy photons would not have enough energy to pass through the patient and produce an image but are still ionising
- this has little to no effect on useful high energy photons
How can the intensity of the produced x-rays be increased?
- increased p.d.
- increased current
How is an image produced using an x-ray?
- x-ray radiation is passed through the patient
- different materials absorb more or fewer photons
- photons that do pass through hit a photographic film where they trigger a chemical reaction causing it to change colour
Image Enhancement - Fluorescent Plate
- absorbs x-ray photons and emits visible light photons
- one x-ray photon causes lots of visible light photons to be emitted as x-ray photons are much higher energy than visible light photons
- the emitted visible light photons hit the photographic film
Image Enhancement - Barium Meal
- a liquid suspension of barium sulphate is ingested
- barium sulphate is a relatively large molecule and insoluble so it doesn’t enter the blood
- it has a high attenuation coefficient
- greater contrast between different soft tissues
Angiogram
- a normal x ray is taken of the organ
- a contrast medium is injected into the blood stream
- same x ray is taken again
- the two images are subtracted from each other
- differences stand out and similarities fade
- blood vessels stand out as they only contained the contrast medium in the second image
What does CAT stand for?
Computerised Axial Tomography
How is a CAT scan taken?
- an x-ray emitter rotates on a ring around the patient
- radiation is detected by a detector which rotates opposite the emitter or by a stationary ring of detectors
- data is processed by a computer
- a series of cross sectional images are produced which can be compiled to produce a 3D image
What tracer is used for a gamma camera?
technetium-99m
What is the half life of technetium-99m?
6 hours
Why is technetium-99m used as a tracer?
it is a metastable isomer - a high energy arrangement which can decay to a lower energy arrangement releasing a gamma photon
it can be attached to many different molecules e.g. glucose to detect a tumour
How does a gamma camera work?
- tracer injected / ingested
- patient is put under a gamma camera
- photons are emmited from the body and travel along the axis of the lead tubes in a collimator to ensure that the detected radiation corresponds to the part of the body directly below
- a layer of sodium iodide crystal absorbs the gamma photons and remits visible light photons
- for every gamma photon that is aabsorbed, thousands of visible light photons are emmited
- photomultiplier tubes detect the visible light photons and emit an electrical impulse relative in magnitude to the intensity of the light
- the data is processed by a computer and an image is produced
What does PET stand for?
Positron Emission Tomography
How does a PET scan work?
- glucose is labelled with a radioactive isotope which undergoes beta decay to emit positrons
- the glucose is injected into the blood stream
- once inside the body, when a positron is emitted it collides with an electron already in the body and annihilates
- two gamma photons are released in opposite directions
- the photons are detected by a ring of detectors around the patient, as they are moving at the same speed, the time between detection of opposite sides of the ring allows the position they were emitted from to be determined
- this information is processed by a computer to produce a 3D image
What does MRI stand for?
Magnetic Resonance Imaging
Nuclear Precession
- nuclei spin on an axis
- when they are placed in a magnetic field the spin axis lines up with the field and the axis of spin precesses
- the position of one end of the axis is fixed and the other end draws out a circular path
- single protons (hydrogen nuclei) precess more easily than those that are in a nucleus with other nucleons
Larmor Frequency Definition
frequency of precession
the number of circles drawn out by the spin axis per second
dependent on the flux density of the magnetic field
How does an MRI machine work?
- patient is placed in a strong magnetic field
- half of the protons in the body line up with the field half line up against
- however a few protons in every million are unpaired and also line up with the field but are free to switch alignment
- gradient coils are used to make small adjustments to the field at specific point which also makes small changes to the larmor frequency of the protons in those areas
- radio transmitting coils emit pulses of radio waves at a a specific frequency, they will only be absorbed by protons with a matching larmor frequency
- absorption of radio waves causes protons to flip on their vertical spin axis to a high energy state where they are lined up against the field
- after a specific relaxation time the protons flip back to their original state and reemit the radio photon they absorbed
- this photon is detected and the data is processed to locate the position in the body it was emitted from an produce an image
What are ultrasound waves?
high frequency sound waves
How are ultrasound waves produced?
- a high frequency alternating p.d. is applied across a piezoelectric crystal
- this causes it to oscillate at the same frequency as the alternating p.d.
- this high frequency vibration produces ultrasound waves
How are ultrasound waves used to produce an image?
- ultrasound waves are emitted in pulses by an ultrasound transducer
- the waves enter the body
- at every tissue boundary a proportion of the wave is reflected back towards the transducer
- the waves are emitted in pulses so that the transducer can be used as a detector inbetween pulses
- when the ultrasound wave reaches the piezoelectric crystal it causes it to oscillate which induces a measurable alternating p.d.
What is the purpose of the backing block in an ultrasound transducer?
it stops the piezoelectric crystal from vibrating as soon as the p.d. is switched off
this means that the reflected ultrasound waves can be detected
if the crystal is already oscilating then the echos can’t be detected
Why is gel required to do an ultrasound scan?
the gel acts as a coupling medium to allow the ultrasound waves to enter the body
there is a much smaller difference in acoustic impedence between the gel and the skin than there is between the air and the skin
this allows almost 100% of the ultrasound waves to enter the body
What is a type A ultrasound scan?
- produces a graph
- the time between emission and detection and the speed of the ultrasound waves in the tissue is used to calculate the distance between tissue boundaries (s=d/t/)
- the magnitude of the induced p.d. corresponds to the height of the peaks on the graph
What is a type B ultrasound scan?
- produces a sonogram, a 2D image
- each returning signal is plotted as a shaded pixel where the darkness is relative to the induced p.d.
The Doppler Effect
If a source is moving towards the detector, the frequency increases
If a source is moving away from the detector the frequency decreases
Acoustic Impedence Equation
Z = pc
Acoustic Impedence = density x speed
Fraction of Reflected intensity Equation
Ir/Io = (Z2-Z1)² / (Z2+Z1)²
The Doppler Effect Equation
f’ = cf / (c-v)
f = initial frequency c = wave speed v = speed of source f' = new frequency
Frequency of a wave reflected off a moving object
Equation
f’ = cf / (c-2v)
f = initial frequency c = wave speed v = speed of object f' = new frequency
Advantages of MRI
- detailed soft tissue images
- 3D image
- no ionising radiation
Disadvantages of MRI
- very expensive
- takes a long time
- dense tissue (bones) not as detailed
- requires a lot of energy
- magnetic metals can’t be taken near the machine
Advantages of CAT
- 3D image
- can be taken in a few minutes
- cheaper than MRI
- good for imaging bones
- most hospitals have one
Disadvantages of CAT
- increased exposure to high frequency x-rays
- ionising radiation
X Rays
Properties
Can travel in a vacuum
Travels at 3x10^8 m/s in a vacuum
Highly Ionising
No charge / rest mass
Ultrasound
Properties
High frequency, higher than 20Hz
Longitudinal Wave
