Physics - Medical Imaging
Flashcards
2022-04-05
Who discovered X-rays?
Wilhelm Röntgen.
When are X-ray photons released?
When fast-moving electrons are slowed down by interaction with a metal.
What is a common metal used to decelerate electrons in order to emit X-rays?
Tungsten.
Where does the energy come from that makes X-rays?
The loss of kinetic energy in an electron.
In an X-ray tube, how are lots of electrons made?
A hot filament releases electrons via thermionic emission.
Is the cathode positive or negative?
Negative.
Is the anode positive or negative?
Positive.
In an X-ray tube, is the hot filament the cathode or the anode?
The cathode.
What is the metal plate called that electrons hit in an X-ray tube?
The target metal.
2022-04-06
Can you briefly visualise the layout of an X-ray tube?
Why is it important that the metal in an X-ray tube has a high melting point?
Because most of the energy from electrons becomes thermal energy.
Why is an X-ray tube kept in a vacuum?
So the electrons don’t collide with anything.
How can you increase the energy of X-ray photons in an X-ray tube?
Increase the potential difference across the anode and cathode.
How can you increase the number of X-ray photons emitted in an X-ray tube?
Increase the potential difference across the hot filament.
How can you work out the maximum energy of the X-ray photons in an X-ray tube?
Equate $eV$ and $hf$.
Why do bones show up in X-rays more than soft tissue?
Because bone attenuates X-rays more.
What is attenuation?
The decrease in intensity of electromagnetic radiation as it passes through matter.
What type of attenuation mechanism is this?
What type of attenuation mechanism is this?Simple scattering.
What type of attenuation mechanism is this?
What type of attenuation mechanism is this?The photoelectric effect.
What type of attenuation mechanism is this?
What type of attenuation mechanism is this?Compton scattering.
What type of attenuation mechanism is this?
What type of attenuation mechanism is this?Pair production.
2022-04-09
What is the equation for the intensity $I$ of X-rays as they pass through matter with an attenuation coefficient $\mu$?
What are the units of the attenuation coefficient of a material?
What is a contrast medium?
Something with a high attenuation coefficient that is taken in order to make certain parts of an X-ray show up more.
For what energies does simple scattering occur for X-rays?
For what energies does the photoelectric occur for X-rays?
For what energies does Compton scattering occur for X-rays?
For what energies does pair production occur for X-rays?
Why does scattering of X-rays due to pair production only occur for X-ray photons above $1.02 \text{MeV}$?
Because that is the energy contained in the combined mass of an electron and positron.
What type of collision with a material occurs in simple scattering?
Elastic.
What is true about the energy of an X-ray photon before and after simple scattering?
It is the same.
What is true about the energy of an X-ray photon before and after Compton scattering?
It is not the same.
What type of collision with a material occurs in Compton scattering?
Inelastic.
When taking X-ray images, what are the main types of scattering that occur due to the energy range?
Photoelectric and Compton scattering.
When using X-rays for radiotherapy, what are the main types of scattering that occur due to the energy range?
Compton scattering and pair production.
What are two popular contrast mediums?
- Barium sulphate
- Iodine
How does a patient take a contrast medium of barium sulphate?
It is eaten.
How does a patient take a contrast medium of iodine?
It is injected.
What sort of X-ray images is barium sulphate good for getting?
Images of the digestive system.
What sort of X-ray images is iodine good for getting?
The circulatory system.
Why aren’t attenuation coefficients solely a property of the material?
They are different for different types of scattering.
2022-04-14
CAT Scans
What does “CAT” stand for?
Computerised axial tomography.
What is a major issue with normal X-rays?
Overlapping bones and tissues cannot be differentiated.
What technique does a CAT scanner use for taking picture?
X-rays.
Why is a CAT scan “computerised”?
Software is used to stitch X-ray images together.
Why is a CAT scan “axial”?
Images are taken along the axial plane which give cross-sections of the patient.
What does tomography mean?
“slice-record”
What is the large ring called in a CAT scanner?
The gantry.
How does the X-ray tube move in a CAT scanner?
It rotates around the gantry.
How does a CAT scanner scan a patient?
Successive X-ray slices are made as the machine slowly slides the patient along the gantry.
How long is a typical CAT scan?
10-30 minutes.
How thick is the fan-shaped beam of X-rays produced in a CAT scanner?
Can you briefly visualise the layout of a CAT scanner?
What are some disadvantages of a CAT scan?
- Expensive
- Long
- More radiation for the patient
What happens if a patient moves during a CAT scan?
It will blur the image.
Gamma Camera
Why are gamma-emitting radioisotopes best for medical imaging?
They are the least ionising and won’t be absorbed, which makes detection easier.
What is important about the half-life of injected radioisotopes used for medical imaging?
It is short.
Where are the radioisotopes typically made for medical imaging?
On-site in a particle accelerator.
What is the name of the substance that a radioisotope is combined with in order to target the desired tissues during medical imaging?
Radiopharmaceutical
What’s another word for a radiopharmaceutical?
A medical tracer.
What is a collimator in a gamma camera?
A honeycomb of long, thin tubes that prevent photons travelling at an angle.
What is a scintillator in a gamma camera?
A material that will produce flashes of visible light when interacting with gamma photons.
What is a scintillator typically made out of?
Sodium iodide.
What is the rough chance of a gamma photon interacting with the scintillator in a gamma camera?
Once photons of visible light have been produced by a scintillator in a gamma camera, what happens?
They travel through photomultiplier tubes to be converted into an electrical pulse.
What is the name of the component in a gamma camera that converts light into electricity?
Photomultiplier tubes.
Can you briefly visualise the layout of a gamma camera?
What is the important difference between devices like X-ray or CAT scanners compared to a gamma camera?
X-ray scanners show information about anatomy, whereas gamma cameras show information about the function and processes of the body.
How could you tell that the kidney on the right was damaged with a gamma camera?
How could you tell that the kidney on the right was damaged with a gamma camera?It is left active and so has taken up less of the radioisotope.
2022-04-28
Is the gamma camera a diagnostic or therapeutic technique?
A diagnostic technique.
Is radiotherapy a diagnostic or therapeutic technique?
A therapeutic technique.
Other than a short half-life leading to a short exposure time, what’s another advantage of a short-lived radioisotope for a patient?
You get more activity for less mass.
What are the two most common medical tracers?
- Fluorine-18
- Technetium-99m
What is the half-life of fluorine-18?
2 hours.
What is the half-life of technetium-99m?
6 hours.
What is the combined radioisotope and active biological substance called?
A medical tracer.
Why is a collimator made out of lead?
Lead is a good absorber of X-rays.
How many visible light photons will be produced by a gamma photon interacting with a scintillator?
Thousands.
PET Scans
If a PET scan is to the gamma camera what a CAT scan is to an X-ray machine, what are the X-ray detectors replaced by?
Gamma detectors.
Can you briefly visualise the layout of a PET scanner?
What is fluorodeoxyglucose?
A medical tracer that is picked up where the body respirates.
What medical tracer is typically used for imaging brain function and detecting tumours?
Fluorodeoxyglucose.
What medical tracer is typically used for imaging blood flow?
Carbon-11 monoxide.
How can a computer build up a 3D image of the organs of a patient in a PET scanner?
How can a computer build up a 3D image of the organs of a patient in a PET scanner?Photons are released in opposite directions, and the position of the emitter can be estimated by looking at the differences in the time it took to reach the detectors.
Where do the gamma photons come from that are detected by a PET scanner?
The annihilation of a electron and positron.
What is the main advantage of a PET scan?
It is non-invasive.
What is the main disadvantage of a PET scan?
It can be very expensive.
Ultrasound
What is “ultrasound”?
A longitudinal sound wave with a frequency above the range of human hearing.
What is “infrasound”?
A longitudinal sound wave with a frequency below the range of human hearing.
What is the main advantage of ultrasound compared to techniques like PET scans?
It is non-ionising.
What are the three main things that can happen to ultrasound waves?
- Refracted
- Reflected
- Diffracted
Roughly what is the wavelength of ultrasound used in imaging?
What is an ultrasound transducer?
A device used to generate and receive ultrasound.
What is the piezoelectric effect?
Where crystals produce an electromotive force in response to mechanical stress.
What happens when you apply a force to a piezoelectric crystal?
An electromotive force is induced.
What happens when you apply a voltage to a piezoelectric crystal?
It stretches and squishes.
What is true about compressing compared to stretching a piezoelectric crystal?
The polarities will be opposite.
How are ultrasound pulses created?
By applying an alternating potential difference across a piezoelectric crystal.
Can you briefly visualise the layout an ultrasound transducer?
How can a transducer determine where a boundary between two materials is?
It can time the differences in amplitudes of reflected ultrasound waves.
What is an ultrasound A-scan?
Where a single transducer is used to record along a straight line through a patient.
What is the always true about the total distance travelled by an ultrasound pulse when it “picks up” a feature?
It will be twice the distance to that feature, as it’s had to come back.
What does “A” in “A-scan” stand for?
Amplitude.
What must you consider carefully in ultrasound calculations?
Whether there is a factor of two.
What is an ultrasound B-scan?
Where a transducer is moved across a patient’s skin to build up a 3D picture.
2022-05-08
What does “B” in “B-scan” stand for?
Brightness.
What is the formula for acoustic impedance $z$?
\[z = \rho c\]
What does $c$ represent in the acoustic impedance equation?
The speed of sound in the material.
For a sound wave of intensity $I _ 0$ incident on a surface, what is true about the reflected intensity $I _ r$ and the intensity through the surface $I _ t$ when there is no absorption?
For a sound wave of intensity $I _ 0$ incident on a surface, what is the formula for the intensity reflection coefficient in terms of the reflected intensity $I _ r$ and the intensity through the surface $I _ t$?
For a sound wave travelling through a medium with acoustic impedance $z _ 1$ into a material with acoustic impedance $z _ 2$, what is the formula for the intensity reflection coefficient?
\[\alpha = \frac{(z _ 2 - z _ 1)^2}{(z _ 2 + z _ 1)^2}\]
From this equation, what can you say about when there will be the biggest reflection coefficient?
When there is the largest difference in impedances.
\[\alpha = \frac{(z _ 2 - z _ 1)^2}{(z _ 2 + z _ 1)^2}\]
From this equation, what can you say about when there will be the smallest reflection coefficient?
When the impedances are roughly equal.
What is impedance matching?
Using materials with a similar impedance in order to reduce reflections.
What is a common example of impedance matching?
The gel they apply for ultrasounds.
When does Doppler imaging occur?
When ultrasound waves are reflected off something moving.
Why is blood particularly good for Doppler imaging?
Because it contains iron.
What is the formula for the change in perceived ultrasound frequency $\Delta f$ when using Doppler imaging?
\[\Delta f = \frac{2fv\cos \theta}{c}\]
What does $\theta$ represent here?
The angle from the blood vessel to the transducer.
\[\Delta f = \frac{2fv\cos \theta}{c}\]
What typical values of $\theta$ are used here?
2022-06-05
Must a contrast material have a short half life?
Not necessarily.
2022-06-09
What must you specify about the beam of X-rays used in a CAT scan?
It is thin and fan-shaped.
What term do you have to use for when the acoustic impedances are equal and hence the reflection intensity coefficient is zero?
There is impedance matching.
Is it better to say non-ionising or non-invasive for ultrasound?
Non-invasive.
When describing PET scans, should you say “gamma rays” or “gamma photons”?
Gamma photons.