Past paper questions

Question 1

List the performance metrics of an ionising radiation imaging system

Answer 1

appropriate half-life
type of emitted radiation
chemical properties
availability
detectionDe

Question 2

Define the performance metric of appropriate half-life

Answer 2

half-life should match the intended application

Question 3

why is a shorter half-life preferred in medical imaging?

Answer 3

to minimize patient exposure

Question 4

why is a longer half-life necessary for radiometric dating?

Answer 4

necessary to measure ancient events

Question 5

define the performance metric of type of emitted radiation

Answer 5

the type of radiation emitted affects the radioisotope’s suitability for specific applicationswh

Question 6

why are gamma emitters useful for imaging and non-destructive testing?

Answer 6

due to their high penetrating power and lower ionisation power (dose)

Question 7

deifne the performance metric of chemical properties

Answer 7

the radioisotope should have suitable chemical properties that allow it to react, bond or accumulate in the desired target or system

Question 8

define the performance metric of availability

Answer 8

a useful radioisotope should be readily available or can be produced in sufficient quantities for the intended application

Question 9

define the performance metric of detection

Answer 9

the emitted radiation should be easily detectable and measurable using standard equipment

Question 10

What kind of technique is gamma camera?

Answer 10

Non-invasive diagnostic

Question 11

Where is gamma camera used?

Answer 11

diagnostic technique
used in nuclear imaging

Question 12

How is gamma camera used?

Answer 12

patient given small amount of radiopharmaceutical
accumulates selectively in certain tissues/organs
emits gamma photons
radiation is detected by a gamma camera

Question 13

What does the gamma camera consist of?

Answer 13

collimator
scintillation crystal
photomultiplier tubes

Question 14

What signal do gamma cameras receive?

Answer 14

the emitted gamma radiation from the radiopharmaceutical which accumulated inthe tissue/organ/tumour

Question 15

What do the gamma cameras do with the signal?

Answer 15

processes the detected signal to create a 2D image

Question 16

What does the scintigram reveal?

Answer 16

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

Question 17

What does collimator serve as?

Answer 17

radiation shield and guide for gamma photons

Question 18

What does the collimator ensure?

Answer 18

only those photons traveling in linear directions from the source reach the gamma camera’s scintillation crystal

Question 19

What is the collimator typically made of?

Answer 19

a dense, high atomic number material, i.e lead or tungsten

Question 20

Why is the collimator usually made of a high atomic number material?

Answer 20

It is more effective at absorbing gamma photons

Question 21

What does the collimator consist of, structurally?

Answer 21

large number of narrow, parallel channels

Question 22

What do the narrow channels making up the collimator allow?

Answer 22

gamma photons traveling in a line to pass through, while blocking or absorbing those coming from other angles

Question 23

What does the collimator help ultimately produce?

Answer 23

a more precise and accurate representation of the source of radiation within the patient’s body

Question 24

What are the advantages of PET scans over SPECT scans?

Answer 24

significant gain in SNR due to removing the collimator

Question 25

What are the advantages of SPECT over PET ?

Answer 25

Larger variety of radio-isotopes available, easier to create a nuclear tracker

Question 26

What is the disadvantage of PET over SPECT?

Answer 26

1. limited number of radioisotopes produced beta + emission
2. small range of beta+ which limits the spatial resolution

Question 27

What is the disadvantage of SPECT?

Answer 27

There is a significant amount of noise in the image, and it is difficult to become quantitative

Question 28

What are the shared disadvantages between PET and SPECT?

Answer 28

Everything that appears bright is not necessarily a tumour.

Question 29

List all the possible things that a SPECT/PET detector would incorrectly determine to be a tumour?

Answer 29

1. physiological uptake
2. pathway of excretion
3. uptake in macrophages
4. post treatment

Question 30

Describe what physical property x-ray CT is measuring. Explain what it means to record a high or low value in the image

Answer 30

captures the tumour and surrounding anatomical structures, it is used for target tumor volume delineation

Question 31

Describe what physical property PET is measuring. Explain what it means to record a high or low value in the image.

Answer 31

Detects and identifies simultaneous gamma photon pairs that result from the same annihilation event. (coincidence event)

Question 32

Describe what physical property gamma cameras are measuring. Explain what it means to record a high or low value in the image.

Answer 32

The gamma rays emitted from the radiopharmaceutical. High value means that it is metabolically active region.

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

Briefly define lateral resolution in the context of ultrasound imaging, and explain why it may change with distance from the transducer

Answer 35

The resolution between adjacently placed objects along the detector
With distance from the transducer, the lateral resolution may change due to beam divergence.

Question 36

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.

Answer 36

• 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

Question 37

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.

Answer 37

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

Question 38

Describe how radiation is detected in the PET scanner and how the image is formed.

Answer 38

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

Question 39

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.

Answer 39

1.characteristic x-ray production
2. bremsstrahlung x-ray production

1. 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
2. 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

Question 40

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.

Answer 40

Geometric unsharpness is the blurring effect caused by the fact that the source of radiation has dimensions and is practically not a point source.

Question 41

Describe the basic principles of B-mode ultrasound imaging performed with conventional focused beams

Answer 41

Brightness mode
The received echo amplitude is used to form 2D grayscale images made from scan lines

Question 42

Describe the basic principles of B-mode ultrasound imaging performed with plane wave imaging

Answer 42

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

Question 43

Describe the advantages of plane wave imaging over conventional focused beam imaging

Answer 43

increased SNR
enhancing boundaries lying at an angle to the transducer face

Question 44

Define the amplitude reflection coefficient in the context of ultrasound imaging

Answer 44

the ratio of the pressure of the incident wave and the pressure of the reflected wave at the PZT - matching element boundary

Question 45

Explain how image artefacts come to be, and describe their appearance in a B-mode image

Answer 45

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

Question 46

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

Question 47

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.

Answer 47

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

Question 48

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.

Answer 48

so that the intrinsic transverse relaxation time is long enough to be measured by the electronic system

Question 49

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.

Answer 49

Awake and alert: beta waves
relaxed and drowsy: alpha waves
stage 1 sleep: Theta waves
stage 2 sleep: reduced theta activity

Question 50

Briefly describe what happens in the brain during epileptic activity, and how such activity is typically exhibited in an EEG recording

Answer 50

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

Question 51

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.

Answer 51

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.

Question 52

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.

Answer 52

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

Question 53

Describe what is meant by the Larmor frequency of the nucleus in the external field

Answer 53

The frequency at which a magnet moment precesses around an external applied field

Question 54

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.

Answer 54

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.

Question 55

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

Answer 55

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

Question 56

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`

Answer 56

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

Question 57

Line of response

Answer 57

the line connecting the 2 detectors that recorded the gamma photons
represents the most probable path of the annihilated positron-electron pair

Question 58

How is a 3D image of the radiopharmaceutical distribution within the patient’s body reconstructed?

Answer 58

the PET scanner registers the LOR rom several annihilation processes

Question 59

TOF reconstruction

Answer 59

the time difference between when the 2 gamma photons are detected by the coincidence detector allows the localisation of the annihilation event

Question 60

EEG what are the frequencies and states and associated types of wavelengths?

Answer 60

States: asleep, drowsy, awake, excited, concentrated
Frequency ranges: 0-4,4-8,8-13,13-30
Wavelengths: delta, theta, alpha, beta, gamma