Medical Imaging Flashcards
Everything you need to know to get A* in OCR Physics A Level, closely following the specification and the textbook
What is an X-ray?
Short wavelength electromagnetic waves with wavelengths from 10^-8 to 10^-13
What are the components of an X-ray tube?
Hot filament (cathode), anode, target metal (tungsten), and a high voltage supply
What are X-ray tubes used for?
Produce X-rays
How are X-rays produced in an X-ray tube?
An external power supply is used used to create a large p.d.
Cathode is a heater which produces electrons by thermionic emission.
Electrons accelerated towards anode.
Anode is made from target metal (tungsten).
X-ray photons produced when electrons are decelerated by hitting anode.
Oil is circulated to cool the anode, or the anode is rotated.
What percent of the kinetic energy of the electrons is converted into X-rays?
Less than 1% of the kinetic energy of the incident is converted into X-rays. The rest is converted into thermal energy.
How is a radiographer protected from the X-rays produced in an X-ray tube?
The X-ray tube is lined with a lead shield.
What is the kinetic energy of an electron accelerated through a p.d. of V?
eV
How many X-ray photons does 1 electron release?
1 X-ray photon
What does the conservation of energy say about the max energy of a photon from X-ray tube and the max energy of a single electron?
Max energy of X-ray photon = max energy of electron
Write equation combining E=hc/λ and E=eV
λ=hc/eV
What is attentuation?
The decrease in the intensity of electromagnetic radiation as it passes through matter and/or space
Name the 4 attenuation mechanisms
Simple scattering, Photoelectric effect, Compton scattering and Pair production
At what energy range does simple scattering occur?
1-20 keV
What is simple scattering?
The X-ray photon interacts with an electron in the atom, but it has less energy than the energy required to remove the electron, so the X-ray photon just bounces off without any change to its energy
In what energy range does the Photoelectric effect occur?
Less than 100 keV
What is Photoelectric effect?
The X-ray photon is absorbed by one of the electrons in the atom. The electron uses this energy to escape.
In what energy range does Compton scattering occur?
0.5-5 MeV
What is Compton scattering?
The incoming X-ray photon interacts with an electron within the atom. Electron ejected from atom, but X-ray photon doesn’t completely disappear - it is scattered with reduced energy
In what energy range does pair production occur?
energy equal to or greater than 1.02 MeV
What is pair production?
An X-ray photon interacts with the nucleus of the atom. It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle, a positron
What is the attenuation coefficient/absorption coefficient?
A measure of the absorption of X-ray photons by a substance
What is the equation for the transmitted intensity I?
I = I(initial) e^-μx, where μ is attenuation coefficient and x is the thickness of the material
Describe the difference in attenuation coefficient between bone and muscle
Bone is a better absorber of X-rays than muscle, so bone has a larger value of μ than muscle
Why are contrast mediums used?
Soft tissues have low μ values, so contrast medium can improve visibility of internal structures in X-ray images
What are two common contrast mediums?
Iodine and Barium compounds
What is the predominant interaction mechanism for X-ray imaging?
Photoelectric effect
How is iodine used as a contrast medium?
It is used in liquids to view blood flow. An organic compound of iodine is injected into blood vessels to diagnose blockades in blood vessels and and the structure of organs such as heart from X-ray image
How is barium used as a contrast medium?
Barium sulfate is swallowed as a barium meal before X-ray is taken to image digestive systems.
What else can X-rays be used for?
They can be used to kill off cancerous cells
What mechanism do X-rays use to kill off cancerous cells?
Compton scattering and pair production
What does CAT scan stand for?
Computerised axial tomography scanning
What are the components of a CAT scan?
Rotating X-ray tube, ring of detectors, computer software and a display
What does the X-ray tube do in a CAT scan?
It produces fan-shaped beams of X-rays that irradiate a thin slice of the patient.
What do the ring of detectors do?
The X-rays are attenuated by different amount by different tissue. The intensity of the transmitted X-rays are recorded by the detector, which send electrical signals to computers
What is the computer software and display used for?
Used to produce a 3D image
Once the first X-ray slice of the patient has been taken, what happens in a CAT scan?
Each time, the X-ray tube and detectors make a 360 degree turn, and another slice is taken. The table with the patient also moves forwards by 1cm for each revolution.
What are some advantages of a CAT scan over an X-ray?
CAT scans can produce 3D images to help assess the size, shape and position of tumours. They also can distinguish between soft tissue of similar μ values
What are some disadvantages of CAT scans over an X-ray?
X-ray scan is quicker, cheaper and there is less exposure to X-rays in an X-ray scan
What radiaton do the best medical tracers emit?
Gamma radiation
Why are gamma-emitting sources ideal in medical imaging?
As gamma photons are the least ionising and can penetrate through the patient to be detected outside the body
What kind of half life must radioisotopes have, why, and how is it beneficial to the patient?
They must have a short half life to ensure high activity from the source so only a small amount is required to form the image.
Benefit: Patient is not subjected to high dosage of radiation that continues long after the procedure
What are some examples of suitable radioisotopes for medical tracing?
Fluorine-18, Technetium-99m
How is Fluorine-18 produced?
It must be produced on site using a particle accelerator in the hospital
How is Technetium-99m produced?
From the beta minus decay of Molybdenum-99, which has a half life of 67 hours
What is the half life of Technetium-99m?
6 hours
What does the m in Technetium-99m stand for?
Metastable, meaning the nucleus stays in a high energy state, with more energy than the stable nucleus, for a longer period than expected
What is a radiopharmaceutical?
A radioisotope chemically combined with elements that will target a particular tissue in order to ensure the radioisotope reaches the correct organ or tumour for diagnosis or treatment
What does a gamma camera do?
It detects the gamma photons emitted from the medical tracer injected into the patient, and an image is constructed indicating the concentration of the tracer within the patients body
What are the components of a gamma camera?
A collimator, a scintillator, a light guide, and photomultiplier tubes
What is a collimator, and what does it do?
It consists of long tubes made of lead, and it absorbs any photons travelling at an angle to the axis of the tubes, so only those travelling along the axis of the tubes reach the scintillator
What is a scintillator, and what does it do?
It is often made from sodium iodide. When a single gamma photon hits it, the scintillator produces thousands of photons of visible light. Not all gamma photons produce this light, as chance of photon interacting with scintillator is 1 in 10
What is a photomultiplier tube, and what does it do?
An apparatus that converts a photon of visible light into an electrical pulse, for example as part of a gamma camera
How is a gamma camera different from an X-ray?
It produces an image that shows the functions and processes of the body rather than its anatomy
What does PET scan stand for?
Positron emission tomography scanning
What type of decay does fluorine-18 undergo?
Beta plus decay, so it emits a positron
What medical tracer do most PET scanners use?
fluorodeoxyglucose, FDG
What does a PET scanner look like?
Patient is placed on a table surrounded by a ring of gamma detectors, all connected to a high speed computer
How does a PET scanner work?
Patient is injected with FDG
PET scanner detects gamma photons emitted when positrons from decaying fluorine-18 annihilate with electrons inside patient
2 gamma photons are produced travelling in opposite directions
How is the image produced from a PET scan?
Computer can detect point of annihilation.
Computer analyses voltage signals from detectors and produces an image
Different concentrations of the tracer show up as areas of different colour and brightness
What can a PET scan be used for?
Can diagnose types of cancers, can help plan heart surgery, observe the function of the brain, assess effect of new medicine
What are advantages of a PET scan?
It is non-invasive
What are disadvantages of a PET scan?
It is expensive, due to the facilities required to make the medical tracers
What is ultrasound?
A longitudinal wave with frequency above 20kHz
What is the frequency range for human hearing?
20Hz-20kHz
What is an ultrasound transducer?
A device that emits and receives ultrasound
What is the piezoelectric effect?
The production of an electromotive force by crystals when they are compressed, stretched, twisted or distorted
How is ultrasound generated?
A high frequency alternating p.d. is applied across opposite faces of a crystal, which repeatedly expands and compresses the crystal. The frequency chosen is the same as the natural frequency of the crystal. This causes the crystal to resonate, and an intense ultrasound crystal is produced
What are the 2 types of ultrasound scans?
A scan and B scan
What can an A scan be used for?
To determine the thickness of bone or the distance between the lens and retina in the eye
How does an A scan work?
Transducer sends ultrasound pulse
Each pulse is partially reflected and partially transmitted at boundary between any 2 tissue
reflected pulse is received at transducer, and it has less energy than original pulse
How is A scan image produced?
The pulsed voltage at the ultrasound transducer is displayed on oscilloscope screen as voltage against time plot.
What can a B scan be used for?
A scan that produces a 2D image
How does a B scan work?
Transducer is moved over patient’s skin
Output of transducer in connected to high speed computer
For each position, row of dots is produced on screen, which show the boundaries between tissues
How is a B scan image produced?
For the dots produced, the brightness of the dot is proportional to the intensity of the reflected ultrasound pulse. The collection of dots produced correspond to the different positions of the transducer over the patient, making a 2D image
What is acoustic impedance?
The product of the density of the substance and the speed of ultrasound in that substance
What is the equation for acoustic impedance?
Z=ρc
Where Z = acoustic impedance
ρ = density and c = speed of ultrasound
What is the SI unit for acoustic impedance?
kgm^-2s^-1
What is the equation for reflected intensity?
Ir/I0 = ((Z2-Z1)/(Z2+Z1))^2
Where I = reflected intensity, I0 = initial intensity
What is the intensity reflection coefficient?
The ratio of reflected intensity over incident intensity for ultrasound incident at a boundary
What is a coupling gel?
A gel with an acoustic impedance similar to that of skin smeared onto the transducer
Why is a coupling gel used?
When transducer is placed on skin, there will be air pockets. The air-skin boundary means lots of ultrasound is reflected. Coupling gel is smeared on the skin as it has a similar acoustic impedance to skin, and it fills the air gaps between the transducer and the skin
What can doppler imaging be used for?
Evaluate blood flow through major arteries and veins
What happens in Doppler imaging?
Transducer pressed to skin above blood vessel
Ultrasound pulse sent, and reflected pulse is received
There will be a change in the frequency.
What does the change in frequency show about the blood flow?
Frequency increases when blood is moving towards the transducer, frequency decreases when blood moves away from transducer
How do you calculate the change in frequency of the ultrasound?
Δf = (2fvcosx)/c where f is original frequency, v is speed of blood cells, c is speed of ultrasound
What is the relationship between change in frequency and speed of blood?
They are directly proportional
