past papers Flashcards
Name the process by which Kv X-ray photons are produced
Bremstrahlung
Give a brief description of the bremstrahlung process, mentioning an important property of the target material used in the process
Electrons are decelerated in the influence of the coulomb field of the nucleus. Breaking radiation. The electron loses energy and this is transferred to the photon. The target has a high Z value.
Charged and uncharged particles interact in different ways with tissue: mention 3 important general characteristics of each particle type
Uncharged particles (photons and neutrons) rarely interact with matter. They display exponential attenuation: a fraction of the incident particles interact with the material, being traversed and lost from the beam.
During interactions, charged particles are set in motion (electrons), which themselves cause ionization.
Charget particles undergo many interactions with matter. The continuously lose small amounts of energy at each interaction. They have a well-defined range, which no particles can travel beyond.
What reference is used to measure beam quality at ortho voltage energies? Name two materials used to do this
Half Value Layer (thickness) HVL - aluminium and copper used.
Explain why the attenuation factor of any pair of detectors in PET scanner is independant of the position of the source along the line joining the detectors.
The total attenuation factor for the line of response is given by AF1 x AF2. e(^-ux)x e^u(a-x) = e(-ua).
i.e. if the annihilation event takes place towards one side of the obect. The distance x between it and the closest surface is not relevent to the total attenuation for that line of response.
Describe the process by which a CT scan can be used to correct for the effect of attenuation in a PET image.
A CT scan acquired just before or after PET scan. CT is a map of density and therefore attenuation within a patient.
Bi-linear scaling is applied to the CT pixel values to account for the energy dependance of the attenuation (CT photons vs 511keV photons) to give an attenuation map which is incorperated into the PET reconstruction.
What causes the partial volume effect in a PET image? In what way does this effect the image
PVE, is caused by the finite spatial resolution of the scanner. It causes blurring that results in a reduction in apparent activities from hot objects that are smaller than twoce the spatial resolution of the image
IN MRI where do you experience the largest
1. translational force
2. Torque
- Edge of the bore. The product of the field strength and spatial gradient is greatest.
- In the centre of the bore, where field strength is greatest.
Why and in what circumstances might the control of noise at work regulations 2005 be of interest to a MRI physicist?
Gradient coils of MRI vibrate due to Lorentz forces and so generate a loud noise. If staff are inside the room during imaging they will be experienced to a noise loud enough to exceed the aloud threshold.
These activities would require a ris assessment and staff would be advised to wear hearing protection.
MR safety.
1. what are current loops?
2. Why do they present a hazard?
3. How do you minimise them from ocurring?
- RF field generate a loop of current in the body.
- Low magnitude currents - can receive burns to two parts of the body
- When the patient is being positioned on the scanner, avoid points of contact by using foam pads to seperate areas of the body from touching themselves or the scanner bore.
Name the different weighting factors required to convert absorbed dose into effective dose
Radiation weighting factor (Wr) and tissue weighting factor (Wt).
Describe 3 different possible outcomes after a cell is exposed to radiation. What type of damage does each outcome cause?
- No damage / cell repaird
- Cell death - Fatally damaged and cannot repair
- Mutation - Partially damaged, mutation occurs which may lead to cancer.
Cell death leads to deterministic effects e.g. skin erythema, wheras mutations can lead to stochastic effect e.g. cancer
Define lifetime additional risk of harm following a radiation exposure and state how this changes with age. and why?
Lifetime additional risk takes into account fatal and non-fatal cancers and heritable effects, and is weighted by life and lost severity. Lifetime additional risk is highest in children and decreases with age. This is because children’s cells are dividing more/more quickly, and because children are most likely to live long enough to survive the latency period for effects to take place.
What is the current average risk of lifetime additional harm ?
5% per Sv.
Why is it beneficial for an X-ray beam to be filtered?
Filtered means hardened because the lower energy X-rays have been removed. These energies would otherwise be absorbed by the patient therefore not contributing to the image but contributing heavily to the absorbed dose.
HVL is measure to be 1cm. QVL is to be done which is it likely to be
1. 2cm
2. <2cm
3. >2cm
> 2cm. HVL filtration hardens the beam increasing the average energy. therefore a greater thickness would be required to reduce beam intensity by another 50%.
Describe the process of annihilation
- Proton-rich nucleus in the radiopharmaceutical decays and emits a positron
- Positron travels a short distance
- Positron annihilates with an electron and two collinear 51kEv photons are emitted.
- Gamma photons exit the body with an electron and are detected in the PET ring.
Explain 3 physical processes that affect spatial resolution on a PET scanner.
- Positron range
- Acollinearity of the annihilation gammas
- Physical dimensions of the detector ring
How will TOF info improve SNR for PET
TOF is where the small difference in arrival time of collinear photons is measured. This helps to provide information about the distance each annihilation photon travelled to reach their responective detectors.
This helps to localise where along the line of response the event took place, thus reducing noise.
What property of PET crystals is essential for TOF imaging
Good temporal resolution / short decay time.
To what extent will the affects associated with
1. static mag field
2. switched field gradients
3. RF field
Affect the development of high field MRI. How might they be mitigated?
- -Increased projectile ris. –Magnetohaemodynamic effect of reducing blood flow / increasing blood pressure becomes significant at 10T.
- Some unknown effects including long term adverse effects.
- No real mitigation possible - Limitation of gradient performance to reduce impact of PNS. but this is not directly related to field strength.
- RF heating increases as the square of the static field strength, all else being equal. Mitigation includes using less RF-intensive sequences, using local transmit coils.
- Mitigated through RF shimming and parallel imaging.
Outline the decay mode of a radionuclide and how it is related to the moethod used for its production.
Radionuclides produced in a cyclotron tend to decay by B+ emission as the targets are bombarded with positively charged particles so the products have excess protons. e.g. 18O+1H = 18F+1n.
Radionuclides produced in a reactor tend to decay by B- emission because products of neutron activation and fission products have an excess of neutrons e.g. U235 + 1n = 137Cs+99Mo.
What does the line integral represent in
1. CT
2. PET
reconstruction
- In CT line integral represents the total attenuation of the X-ray beam as it travels through the scanned object.
- In PET it represents the sum of the radioactivity along the line of response.
Excessive blurring is usually observed in images resconstructed with simple back projection.
1. How does deconvolution
2. FBP
methods attempt to eliminate blurring
Blurring is assumed to be described by the 1/r function with r = disatance from the centre of the object of interest.
- Back-projected image can be described as the convolution of the original image with the blurring function 1/r. By deconvolving the blurred image, the de-blurred image is recovered.
- A high-pass filter (usually a ramp) is applied on the projection data in order to remove the low frequencies that correspond to background noise before back-projecting the data.
What is the system matrix?
System matrix describes the relationship between image space and projection space and it can be applied on the data to move them from one space to the other.
List 4 factors that could be included in the estimation of the system matrix
Objection motion
positron range
TOF
attenuation
detector non-uniformaties
scatter
detector response models
Name and describe the most popular algorithm used in iterative reconstruction
Maximum likelihood expectation maximisation algorithm (MLEM).
The algorithm makes use of the fact that measured photons follow a Poisson distribution and Bayes’ rule. It tries to maximise the probability of observing certain counts given an already known distribution (Poisson) or the reverse
What are the main disadvantages of Thermoluminescent TLD dosimeters vs film doseimeters
Storage instability
Readout destroys signal
Fading
Light sensitivity
Can’t identify type of radiation
Can’t identify gross non-uniformaties. e.g. one large exposure.
Briefly describe how a DAP meter works
An ionisation chamber. With Incident photons interact within the gas filled chamber creating positive ions that are attracted towards the cathode. The created charge is proportional to the ionisation that has occured, as ionisation chamber 200-400V.
DAP, is the dose multiplied by the exposed area.
Give 3 differences between narrow and broad beam conditions setup or properties
- in narrow beam, the beam is highly collimated prior to the attenuator. This results in the beam remaining concentrated over longer distances providing higher spatial resolution.
Broad beam - has little or no collimation at this point. allowing the beam to cover a much larger area.
- Intensity and Dose distribution - X-ray beam is concentrated over a smaller area focussing the beams energy and depositing a higher dose. This can enhance image quality but may require more precise alignment and setup.
Broad - Dose spread over a larger area resulting in more uniform dose distribution and lower peak intensity. - Scatter and image contrast -
Narrow beam - less scattered radiation because there is less interaction with tissue. Results in higher image contrast and better image quality, because scattered photons reaching detector is reduced thus reducing noise.
Broad - More scatter degrades contrast and image quality. Therefore requiring anti-scatter grids in front of the detector to maintain image quality.
What two conditions are required for Fanos theory to apply to a measuring device?
- Cavity is homogeneous
- The cavity is of the same elemental composition
Define collisional and radiative stopping power
Collisional - Stopping power due to the inelastic colisions with atomic electrons of the medium resulting in excitation and ionisations.
Radiative - Stopping power due to electron interactions with the electric field of the nucleus in the production of bremstrahlung.
Why is radiative stopping power negligable for protons but significant for electrons
Because radiative stopping power is inversely proportional to the square of the mass of the particle. Protons habe a much higher mass and therefore radiative stopping power is less significant when compared to electrons.
Collisional stopping depends on what?
Not the mass but the charge of the particle. Rate of energy loss is proportional to the square of the charge on the ion.
Rate of energy loss is inversely proportional to the square of the velocity.
Describe radiative and collisional stopping power
Why does a bragg peak occur
As heavy ion slows down it’s rate of energy loss increases leading to more absorbed dose in the medium. depositing more energy, this occurs exponentially until a peak at which point all the energy is deposited.
Why does electron depth dose curve not exhibit a bragg peak
Each electron will exhibit a braag peak. But at each interaction scatter will occur. This acts to smooth out the energy deposition. Scattering occurs due to small mass. Bragg peak is smeared.
What is meant by list mode. state one advantage and one disadvantage
- is where events are tagged sequentially and seperated by timing tags. It stores all acquisition data such as detector position, photon energy, distance from COR.
- advantage, sinograms can be generated from it, allowing different reconstructions to be done.
- Disadvantage, it uses a large amount of storage space.
Briefly describe how FBP works
Each projection angle is distributed evenly across the FOV for every angle in the acquisition. The data is filtered prior to this step.
Apply a high pass ramp filter, this sharpens the image by removing low frequencies
What do elements in the columns and rows mean in a system matrix?
Columns: Represent sinograms, corresponding to every possible point source.
Rows: The probability that an emission from a pixel gives rise to a detection in a given sinogram element.
In medical applications what are the 3 main mechanisms by which energy is removed from a radiation photon beam as it passes through tissue for energies between 0.01 and 10MeV
- Compton scatter - X-ray photon collides with an outer shell electron. causing it to be ejected from the atom, the photon continues in a different trajectory
- Photoelectric effect - Photon is completely absorbed causing ejection of a tightly bound electron.
- Pair production - High energy photon >1.022MeV interacts with the electric field of a nucleus and transforms into an electron-positiron pair. Only occurs in the nucleus due to the conservation of momentum.
Which interaction is most relevent to radiotherapy
compton scatter
What implications does compton scatter have for imaging, treatment and dose calculation in radiotherapy?
Imaging - Effects image contrast by creating noise in the image.
What us the equations to calculate mass attenuation coefficient. include units
um (cm2/g) = u / p
What are the two main issues with a GM counter
Dead time - After an avalanche the voltage accross the unit must recover, this means there is a waiting time during which any interaction with the tube will not be recorded. This can lead to it becoming paralyzed.
Cannot give pulse height data - Meaning cannot identify the energy of the radiation being monitored.
Name the interaction type that occurs between the incident electron and a k-shell electron
photoelectric affect
Describe two other types of photon interaction
Rayleigh scatter - A coherent scatter. photon is scattered without losing energy. More likely as energy decreases and increasing atomic number.
Photonuclear reactions - Photon interacts with nucleus of an atom which emits energy usually in the form of neutrons or protons.
A trolley is built from plastic with some metal componenants what MR safety rating would you give it?
Summerise the effects of the static field in MRI
Sensory affects
Cardiovascular effects become more significant at higher strengths
Unknown effects of cognitive and neurobehavioural effects
Epidemiological studies to look at long-term effects
Summerise the main effects of gradient fields hazards
Direct effects - peripheral nerve stimulation, ventricular fibrilation (although PNS would be intollerable by this point)
Indirect - Acoustic noise, can be minimised with sequence choice. e.g.
Summerize RF hazards
Direct effects - Tissue heating
Indirect - Induction of currents in wires causing other heating within the patient
Describe the fourier slice theorum and how it could be utilised in the context of image reconstruction.
What are some of the pros and cons
Obtain 1D projections = sinogram
fourier transform each to produce 1-D datasets
Use results to fil in a 2-D fourier space
Convert to a cartesian grid
Apply an inverse 2-D fourier transform to produce the image.
This can be utilised to reconstruct the cross-sectional images from x-ray projections.
pros - robust to noise
cons- streak artefacts occur due to undersampling
Describe the type of target material of the orthovoltage unit and charged particle interaction processes that occur within it
- High Z atomic number material like tungston. Because it produces a high intensity bremsstrahlung radiation.
- Brem production, high energy electrons are decelerated by the atomic nuclei in the target material resulting in the emission
- Photon absorption and scattering - Produced photons will undergo this as they move through the material.
- Pair production - High energy photons can interact with atomic nuclei to produce electron - electron pairs.
These interactions contribute to the production of the theraputic radiation beam used in orthovoltage therapy.
Compare the charged particle interactions that occurr between a 6MeV electron and 25MeV proton beam
Electron beams primarily interact with matter through brem radiation and collisional ionisation. while proton beams interact mainly through Coulomb interactions.
The differences arise primarily through the distinct mass, charge and energy loss mechanisms of electrons and protons. Leading to variations in their stopping power, range and dose distribution in the medium.
Discus the basic properties of scintillation detectors. Include a basic description of the two effects phosphorescence and fluresence. Explain the function of a photomultiplier.
What are the basic tests of a gamma camera
uniformity, resolution (spatial and energy), linearity (spatial distortion) sensitivity (efficiency) count rate performance
In a gamma cameras what are spatial non-linearities caused by. How is it tested
The structured pattern of the PMts giving a non-uniform response to the scintillation light for different positions of events. Causes a local count compression near the centre of the PMts. Digital linearity corrections available on modern cameras.
Assessed through observing the straightness of a line source for distortion.
