27) medical imaging Flashcards
What provides the energy of the X-rays in an X-ray tube?
The kinetic energy of the accelerated electrons.
What is an X-ray?
A photon produced by the deceleration of a fast moving electron
What is the typical p.d. of an X-ray tube for medical imaging?
30-100kV
What causes the emission of electrons from the cathode in an X-ray tube?
It is heated to produce thermionic emission.
What name is given to the anode in an X-ray tube?
The target metal.
What is an essential property for the target metal in an X-ray tube?
High melting point.
What causes the production of X-rays in an X-ray tube?
The deceleration of electrons when they hit the target metal/anode.
In an X-ray tube what happens to most of the kinetic energy lost by the electrons when they hit the anode?
It is transferred to thermal energy of the anode.
What measures can be taken to prevent an anode melting in an X-ray tube?
It is cooled by oil or rotated
What feature of X-ray tubes protects the radiographer?
Lead lining.
What determines the maximum frequency of X-rays produced in an X-ray tube?
The maximum kinetic energy of a single electron.
When is the equation λ = hc/eV used?
To find the minimum wavelengths of X-ray produced (by an X-ray tube)
What is V in the equation λ = hc/eV?
The accelerating potential difference.
Describe the beam used by CAT scanners.
A thin (1-10mm) fan.
What happens to the position of the X-ray tube during a CAT scan?
It rotates around the patient.
What are the key advantages of CAT scans over conventional X-rays?
They produce a 3D image and can distinguish between tissues with similar attenuation coefficients.
What are the disadvantages of CAT scans compared to conventional X-rays?
The radiation dose is greater and they are more expensive and take more time.
How does a CAT scan produce a 3D image?
A series of slices are imaged and processed via a computer to produce a 3D image.
What is X-ray attenuation?
The decrease in intensity as X-rays pass through matter.
What are the four mechanisms of X-ray attenuation?
Simple scatter Photoelectric Effect Compton Scattering Pair Production
What occurs during simple scatter of X-rays?
The X-ray interacts with an electron in an atom and changes direction but not energy.
Which method of attenuation is most significant with X-rays with energy in the range 1-20keV?
Simple scatter
What occurs during the photoelectric effect attenuation of X-rays?
The X-ray is absorbed by an electron in an atom which gains the energy of the photon and leaves the atom.
Which method of attenuation is significant with X-rays with energies up to 100keV?
Photoelectric effect
Which method of attenuation is most significant to medical X-ray imaging?
The photoelectric effect.
What occurs during Compton scattering attenuation of X-rays?
The X-ray interacts with an electron in the atom causing the electron to be ejected and the photon to be scattered with reduced energy.
Which method of attenuation is significant with X-rays with energy in the range 0.5-5MeV?
Compton scattering.
What occurs during pair production with X-rays?
An X-ray interacts with the nucleus of an atom converting the energy of the photon to and electron and a positron.
Which method of attenuation occurs with X-rays with energies 1.02MeV?
Pair production
What is I in the equation I = I₀e^(-µx)?
Transmitted intensity
What is I₀ in the equation I = I₀e^(-µx)?
Intensity before absorption
What is µ in the equation I = I₀e^(-µx)?
Attenuation/absorption coefficient.
What is x in the equation I = I₀e^(-µx)?
Thickness of the absorbing substance
What are the SI units of attenuation coefficient?
m⁻¹
What is a contrast medium in X-rays?
A material with a high attenuation coefficient used to image soft tissue.
What contrast medium is typically used to examine blood flow?
Iodine
What contrast medium is typically used to imagine the digestive system?
Barium sulfate
How are X-rays used for cancer therapy?
A linear accelerator produces high energy X-rays that destroy cells.
Which radionuclide is used for imaging with a gamma camera?
Technitium-99m
Which radionuclide is used in PET scanning?
Fluorine-18
Why are gamma emitting sources useful for imaging inside the body?
They are the least ionising and can penetrate through a patient to be detected externally.
Why are short half-lives necessary for the radioisotopes used for medical imaging?
To ensure a large activity from a small amount and to minimise radiation dose after the procedure.
What is a medical tracer?
A compound containing a radioisotope that is put into the patients body.
What is the function of a collimator in a gamma camera?
To absorb any photons not travelling along the axis of the tubes so the source location of gamma radiation can be idenitifed.
How is a collimator in a gamma camera constructed?
It is a honeycomb of long, thin lead tubes.
What is the function of a scintillator in a gamma camera?
It produces many photons of visible light when a gamma photon interacts with it.
What is the function of the photomultiplier tubes in a gamma camera?
They convert visible light photons to an electrical pulse.
What is the function of the computer in a gamma camera?
It processes the signals from the photomultipliers to locate where the gamma ray originated to produce an image showing the concentrations of medical tracer.
What type of radiation is detected in a PET scan?
Gamma
What type of radiation is emitted by Fluorine-18 nucleus in a PET scan?
Beta plus/positron
What process produces gamma photons in a PET scan?
Electron and positron annihilation.
How is the the source of the gamma photons located in a PET scan?
By the time delay between the arrival of diametrically opposite gamma photons.
What is the typical time delay between the arrival of gamma photons in PET scan?
Nanoseconds
Why is medical imaging using gamma emitting tracers useful?
It can measure the function of organs in the body rather than just the structure.
What are the disadvantages of a PET scan?
They are expensive and require facilities to prepare tracers local to the scanner.
What determines the acoustic impedence of a substance?
The density and the speed of sound in the substance.
What are the units of acoustic impedance?
kgm⁻²s⁻¹
What is the intensity reflection coefficient for ultrasound?
The ratio (reflected intensity of ultrasound ÷ incident intensity of ultrasound)
What factor determines the refelction intensity coefficient for ultrasound at a boundary?
The difference between the acoustic impedances of the two substances.
Why does a coupling gel need to be used when performing an ultrasound?
Air pockets between the transducer and skin cause reflection of nearly all of the ultrasound at the skin-air boundary.
What is an ultrasound coupling gel?
A substance applied to the skin and transducer with an impedance similar to skin.
What is impedance/acoustic matching?
When two substances havce similar acoustic impedances so negligible reflection of ultrasound occurs.
What causes ulatrasound to undergo doppler shift?
Reflection from moving blood cells.
What is doppler ultrasound used for?
Measuring the speed (and volume) of blood flow.
What is measured in doppler ultrasound to determine the velocity of the blood?
The observed change in frequency of the reflected ultrasound pulse.
How is blood flow viewed during a doppler ultrasound?
The signal is processed by computer and displayed as a colour coded image on a screen.
Why does the transducer have to placed away from the normal during a doppler ultrasound?
To ensure there is a component of the blood flow’s velocity in the direction of the ultrasound pulse.
What is ∆f in the equation? ∆f = (2fvcosθ)÷c
Change in the observed frequency of the reflected ultrasound.
What is v in the equation? ∆f = (2fvcosθ)/c
Blood flow speed.
What is θ in the equation? ∆f = (2fvcosθ)/c
The angle between the tranducer axis and the blood flow.
What is c in the equation? ∆f = (2fvcosθ)/c
The speed of ultrasound in blood.
