Past paper questions Flashcards
List the performance metrics of an ionising radiation imaging system
appropriate half-life
type of emitted radiation
chemical properties
availability
detectionDe
Define the performance metric of appropriate half-life
half-life should match the intended application
why is a shorter half-life preferred in medical imaging?
to minimize patient exposure
why is a longer half-life necessary for radiometric dating?
necessary to measure ancient events
define the performance metric of type of emitted radiation
the type of radiation emitted affects the radioisotope’s suitability for specific applicationswh
why are gamma emitters useful for imaging and non-destructive testing?
due to their high penetrating power and lower ionisation power (dose)
deifne the performance metric of chemical properties
the radioisotope should have suitable chemical properties that allow it to react, bond or accumulate in the desired target or system
define the performance metric of availability
a useful radioisotope should be readily available or can be produced in sufficient quantities for the intended application
define the performance metric of detection
the emitted radiation should be easily detectable and measurable using standard equipment
What kind of technique is gamma camera?
Non-invasive diagnostic
Where is gamma camera used?
diagnostic technique
used in nuclear imaging
How is gamma camera used?
patient given small amount of radiopharmaceutical
accumulates selectively in certain tissues/organs
emits gamma photons
radiation is detected by a gamma camera
What does the gamma camera consist of?
collimator
scintillation crystal
photomultiplier tubes
What signal do gamma cameras receive?
the emitted gamma radiation from the radiopharmaceutical which accumulated inthe tissue/organ/tumour
What do the gamma cameras do with the signal?
processes the detected signal to create a 2D image
What does the scintigram reveal?
The scintigram, or the processed gama signal from the radiopharmaceutical detected by the gamma camera, reveals the distribution and concentration of the radiopharmaceutical within the body
What does collimator serve as?
radiation shield and guide for gamma photons
What does the collimator ensure?
only those photons traveling in linear directions from the source reach the gamma camera’s scintillation crystal
What is the collimator typically made of?
a dense, high atomic number material, i.e lead or tungsten
Why is the collimator usually made of a high atomic number material?
It is more effective at absorbing gamma photons
What does the collimator consist of, structurally?
large number of narrow, parallel channels
What do the narrow channels making up the collimator allow?
gamma photons traveling in a line to pass through, while blocking or absorbing those coming from other angles
What does the collimator help ultimately produce?
a more precise and accurate representation of the source of radiation within the patient’s body
What are the advantages of PET scans over SPECT scans?
significant gain in SNR due to removing the collimator
What are the advantages of SPECT over PET ?
Larger variety of radio-isotopes available, easier to create a nuclear tracker
What is the disadvantage of PET over SPECT?
- limited number of radioisotopes produced beta + emission
- small range of beta+ which limits the spatial resolution
What is the disadvantage of SPECT?
There is a significant amount of noise in the image, and it is difficult to become quantitative
What are the shared disadvantages between PET and SPECT?
Everything that appears bright is not necessarily a tumour.
List all the possible things that a SPECT/PET detector would incorrectly determine to be a tumour?
- physiological uptake
- pathway of excretion
- uptake in macrophages
- post treatment
Describe what physical property x-ray CT is measuring. Explain what it means to record a high or low value in the image
captures the tumour and surrounding anatomical structures, it is used for target tumor volume delineation
Describe what physical property PET is measuring. Explain what it means to record a high or low value in the image.
Detects and identifies simultaneous gamma photon pairs that result from the same annihilation event. (coincidence event)
Describe what physical property gamma cameras are measuring. Explain what it means to record a high or low value in the image.
The gamma rays emitted from the radiopharmaceutical. High value means that it is metabolically active region.
describe an example of a clinical case for which x-ray CT, gamma camera, PET and image intensifier may be used. Explain what makes that imaging system an appropriate choice
x-ray CT: detailed cross-sectional images of the abdomen, acquires detailed images
gamma camera: functional imaging, where the focus is on physiological processes, can detect / localize sources of bleeding or assess organ function
PET: detecting metabolic and molecular changes within tissues, so used for cancer staging, target delineation
image intensifier: in procedures requiring real-time imaging, in case there is a need for dynamic assessment, an image intensifier provides continuous, high-quality images in real-time
The radiologist suspects a very narrow fracture. What factors within the x-ray imaging
system must be considered to produce a high resolution image? Give a brief reason for
each
X-ray tube voltage needs to be lower as they enhance image contrast by increasing the absorption difference between bone and surrounding tissues
X-ray tube current
Focal spot size: smaller focal spots to produce sharper images by reducing geometric unsharpness
Detector resolution: decrease the pixel size for higher spatial resolution
Briefly define lateral resolution in the context of ultrasound imaging, and explain why it may change with distance from the transducer
The resolution between adjacently placed objects along the detector
With distance from the transducer, the lateral resolution may change due to beam divergence.
List 3 different medical imaging scans that a patient is likely to receive at any stage from initial diagnosis to follow-up after treatment. Name the imaging modality in each case and carefully explain the purpose of each scan.
- X-ray CT: initial stages of the simulation/planning phase. Used to image the tumour and surrounding anatomical structures. Also used for target delineation of the tumour ( Gross Tumour Volume, Clinical Target Volume, Hypoxic regions)
- Ultrasound: used during the treatment phase to determine the location of the radioactive seeds
- PET: used for target delineation and allows for the preferential killing of the tumour by exploiting the difference in responses between tumours and healthy cells
Explain how a PET imaging system works. Include a labelled diagram of a PET scanner in your answer and an explanation of the physics taking place in the patient and in the PET scanner.
positrons are emitted
annihilation occurs
2 gamma photons are produced
each gamma photon has an energy of 511keV
Gamma photons are detected by the PET scanner
1. coincidence detection
2. time of flight reconstruction
Describe how radiation is detected in the PET scanner and how the image is formed.
gamma photons emitted from annihilation event
a coincidence ring detector surrounds the patient
Line of response formed as the most likely path taken by the 2 gamma photons
Several LORs collected and reconstruct 3D image of the radiopharmaceutical distribution within the patient’s body
X-rays are produced in an x-ray tube when a fast moving electron beam strikes a target. Describe the processes taking place that lead to x-ray production.
1.characteristic x-ray production
2. bremsstrahlung x-ray production
- electron makes direct hit on inner shell electron
incident electron has enough energy to eject the inner electron from its orbital shell
outer electron falls down energy level
energy released in the form of x-ray, corresponding to the difference in energy levels of the 2 orbits - electron approaches close to the nucleus
strong attraction between the positively charged nucleus and the negatively charged electrons
large acceleration
electron moves in partial orbit around the nucleus
electron loses energy in the interaction which is emitted as an x-ray photon
the photon energy depends on the degree of acceleration
Explain, with the use of a diagram, what is meant by the term geometric unsharpness as applied to an x-ray source. Describe the effect it will have on an x-ray image.
Geometric unsharpness is the blurring effect caused by the fact that the source of radiation has dimensions and is practically not a point source.
Describe the basic principles of B-mode ultrasound imaging performed with conventional focused beams
Brightness mode
The received echo amplitude is used to form 2D grayscale images made from scan lines
Describe the basic principles of B-mode ultrasound imaging performed with plane wave imaging
Plane wave imaging: rather than using separate focused beams to make up images using scan lines, images are obtained with higher frame rates using plane wave imaging.
All the elements are fired together and beamforming is performed on receive, focusing at each point in the image
Describe the advantages of plane wave imaging over conventional focused beam imaging
increased SNR
enhancing boundaries lying at an angle to the transducer face
Define the amplitude reflection coefficient in the context of ultrasound imaging
the ratio of the pressure of the incident wave and the pressure of the reflected wave at the PZT - matching element boundary
Explain how image artefacts come to be, and describe their appearance in a B-mode image
Speed of sound range errors and distortion
speed of sound defocusing errors
refraction artefacts
attenuation artefacts
acoustic shadowing
reverberation artefacts
specular reflection
mirror image artefact
Provide a sketch of a typical diagnostic x-ray emission spectrum. Describe the contribution of the 2 types of electron interaction occurring within the target of an x-ray tube ,and explain the effect of varying the kVp and the filter window
With the aid of a diagram, briefly describe the construction of a typical magnet used for
magnetic resonance imaging (MRI) of patients. Explain why and how the magnetic coils
are cooled.
main field coil along the z-axis
gradient coils along the y and x axes
gradient coils are saddled around the inner cylindrical surface of the magnet
main field coil is around the cylinder
Also contains RF coils
through the magnet bores there are solenoid wires which shim the magnetic field and make it homogenous
stainless steel housing
filled with liquid helium
vaccum shielding
surrounded with liquid nitrogen
the magnetic coils need to be prevent overheating of the patient
Explain why it is necessary for the magnetic field to be highly homogeneous, and how the
homogeneity of the field is adjusted when a patient is inside the magnet.
so that the intrinsic transverse relaxation time is long enough to be measured by the electronic system
Describe, with the aid of a sketch, how the recordings for a EEG recording change thorughout an hour as a volunteer is awake and alert and progressively falls asleep.
Awake and alert: beta waves
relaxed and drowsy: alpha waves
stage 1 sleep: Theta waves
stage 2 sleep: reduced theta activity
Briefly describe what happens in the brain during epileptic activity, and how such activity is typically exhibited in an EEG recording
Abnormal spontaneous and excessive electrical activity in the brain
Very large number of neurons are activated simultaneously
Brief, high-amplitude spikes or sharp waves between seizures
Rhythmic, high-amplitude waves or spiked
EEG suppression, or slowing
Briefly describe what a MEG system measures and how it is typically measured. Describe the principal advantage of MEG compared to EEG, and 2 disadvantages.
Recoding the magnetic fields associated with the electrical currents flowing in the dendrites of neurons during synaptica activity
Arrays of several SQUIDs cover the entire scalp, and measurements are performed within a magnetically shielded room
Principal advantage:far less influenced by the overlying layers of tissue
Disadvantages: challenge to convert measurements of magnetic field strength. A detectable signal requires thousands of neurons to activate.
Using a classical interpretation of a hydrogen nucleus (proton), briefly explain why the
nucleus is associated with a tiny magnetic field (known as a magnetic moment). With the
aid of a diagram, describe what influence the rules of quantum mechanics have on the
orientation of the proton magnetic moment when placed in an external magnetic field B0.
The nucleus of a hydrogen atom, proton, possesses a magnetic moment due to spin
The proton’s magnetic moment can only align in specific quantized orientations relative to external magnetic field B_0
The interaction between the proton’s magnetic moment and the external magnetic field leads to lower energy and higher energy states
Describe what is meant by the Larmor frequency of the nucleus in the external field
The frequency at which a magnet moment precesses around an external applied field
Explain why a sample containing many hydrogen nuclei can have a net magnetisation M
which aligns with an external field B0, even though individual magnetic moments cannot.
while individual proton magnetic moments may not align with the external field, statistical effects and quantum mechanical interactions lead to a net alignment at the macroscopic level, resulting in the observed net magnetization of the sample.
X-rays can be produced at a variety of energies to image the human body (e.g. kV or MV
imagers). Assuming an ideal detector, explain why kV imaging may provide the best image
quality, considering the dependence of contrast, spatial resolution, and penetration depth on
photon energy. Provide a sketch of the total cross-section plotted against energy to support
your explanation
high contrast between soft tissue structures as they are lower energy
shorter wavelengths, enable higher spatial resolution
lower energy and limited penetration depth, useful for imaging superficial structures and soft tissues
Explain, with reasons, the
advantages and disadvantages of using the following as the basis of the oedema monitoring method:
i) microwaves
ii) optical imaging
iii) ultrasound
iv) magnetic resonance imaging
v) thermography
include factors such as depth sensitivity, and any practical difficulties and/or safety issues involved in
implementing the method in a residential home`
i)
1. do not produce significant heating effect
2. penetration depth falls with decrease in microwave wavelength
3. significant diffraction occurs
4. requires index matching medium
ii)
diffuse optical tomography allows deep penetration due to low absoption by the blood
source-detector separations are small to ensure strong signals so measurements can be acquired quickly
iii)
non-invasive
real-time imaging
portability
cost-effective
disadvantages: operator dependency, limited tissue penetration, limited field of view
iv)
high resolution
multiplanar imaging
excellent tissue contrast
no radiation exposure
disadvantages: cost and availability only in healthcare facilities, longer acquisition time, contraindications (contraindicated in patients with metallic implants or claustrophia or inability to lie still for extended periods)
v)
non-invasive
quick and easy
no radiation exposure
disadvantages:
limited depth of assessment, variability, limited specificity
Line of response
the line connecting the 2 detectors that recorded the gamma photons
represents the most probable path of the annihilated positron-electron pair
How is a 3D image of the radiopharmaceutical distribution within the patient’s body reconstructed?
the PET scanner registers the LOR rom several annihilation processes
TOF reconstruction
the time difference between when the 2 gamma photons are detected by the coincidence detector allows the localisation of the annihilation event
EEG what are the frequencies and states and associated types of wavelengths?
States: asleep, drowsy, awake, excited, concentrated
Frequency ranges: 0-4,4-8,8-13,13-30
Wavelengths: delta, theta, alpha, beta, gamma
