Section 1 (Radiation Biology and Safety)
Question 1
a) Regarding ICRP recommended dose limits:
i. What is the occupational dose limit for effective dose? (1)
A limit on effective dose of 20 mSv/year, averaged over 5 years (i.e., a limit of 100 mSv in 5 years) with the further provision that in any single year:
– The effective dose should not exceed 50 mSv
– The equivalent dose should not exceed 20 mSv for the lens of the eye, 500 mSv for the skin, hands and feet
ii. What is the occupational dose limit for the lens of the eye and why is the dose to the lens of the eye a concern in radiology practice? (2)
Limit of 20 mSv per year (averaged over 5 years, with no single year exceeding 50 mSv).
Increased risk of radiation-induced cataractogenesis suggested from epidemiological evidence.
iii. Explain why it is inappropriate to set dose limits for medical exposures. (1)
Dose limits only apply to occupational and public exposure. It does not apply to medical exposure where radiation exposure is intended for the diagnostic or therapeutic benefit of the patient.
Patients often have concurrent chronic, severe, or even life-threatening medical conditions that are more critical than radiation exposure. No practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individuals or to society to offset the radiation detriment it
causes.
Read more: ICRP Publication 105
b) Identify and outline the two key principles of radiation protection for medical exposure. (2)
– Justification: Any decision that alters the radiation exposure situation should do more good than harm
– Optimisation of Protection: Doses should all be kept as low as reasonably achievable, taking into account economic and societal factors
Read more: ICRP Publication 103
Other accepted answers:
Dose limitation: The total dose to any individual … should not exceed the appropriate limits
c) You would like to compare the doses for CT chest examinations at your hospital with the national Diagnostic Reference Levels (DRL).
i. State two quantities, with units, indicative of patient dose to include in your dose audit. (2)
CTDIvol (CT Dose Index Volume) – mGy
– a measure of exposure per slice in a phantom
– represents the local absorbed dose
DLP (Dose Length Product) – mGy.cm
– measure of total radiation exposure to a phantom for the whole series of images
– DLP = CTDIvol x irradiated length
– represents total absorbed dose or relative risk
ii. If either of your survey dose quantities are higher than the DRL, describe two actions that should be taken. (2)
If the facility DRL exceeds the national DRL for a particular imaging protocol, this indicates that the facility is delivering a dose higher than 75% of other imaging facilities for the same procedure.
Action 1
Review of the imaging protocol and equipment with a qualified medical physicist to determine whether optimisation of protection and safety for patients is adequate
Action 2
Developing strategies to reduce dose for that procedure or protocol
Question 2
a) Ionising radiation exposure may be expressed as absorbed dose, equivalent dose or effective dose. Define each of these quantities and give their units. (3)
Absorbed dose – the quantity of energy deposit in matter, measured in Gy
Equivalent dose – the amount of radiation dose to tissue which accounts for the biological effects of different types of ionising radiation by multiple the absorbed dose by a radiation weighting factor. Measured in Sv.
Effective dose – the tissue-weighted sum of the equivalent dose in all specified tissues and represents an overall stochastic health risk to the whole body. Measured in Sv.
b) Ionising radiation can cause harmful effects to persons irradiated.
i. Briefly describe stochastic effects and tissue reactions (deterministic effects). (2)
Deterministic effects
Results from radiation-induced cell loss or tissue damage, where the damage does not occur in a non-random pattern and severity increases with dose. Deterministic effects only occur above a certain thershold.
Stochastic effects
Stochastic effects are random or probabilistic. The probability of stochastic effects does increase with higher doses but the severity does not.
ii. Describe two possible effects for medical irradiation at low doses (<100 mGy). (1)
The threshold for deterministic effects is said to be 100mGy. Hence the radiation effects at low dose will be stochastic in nature.
1. Carcinogenesis
The currently accepted model for cancer induction is the linear-no-threshold model, where the risk of cancer induction increases linearly with no threshold. The approximate risk is 5% per Sv.
2. Hereditary effects
Irradiation of germ cells involved in reproduction can result in hereditary effects (although currently there is no epidemiological evidence of such in exposed humans, only from animal/experimental research). Radiation increases the incidence of mutation that occurs spontaneously.
iii. Describe two possible effects of high doses (>2 Gy) in interventional radiology. (1)
Erythema
At 2 Gy, early transient erythema may be observed in a matter of hours
At 6 Gy, erythema can occur 1 to 2 weeks following exposure
At >10 Gy dry desquamation can occur
At >15 Gy, moist desquamation occurs
Epilation
Between 3 to 5 Gy, temporary epilation can occur after 2 to 3 weeks.
AT >15 Gy, permanent epilation can occur
c) A patient has received a considerable dose of ionising radiation over the course of many years as a result of multiple CT scans. Their GP believes that they should have another CT scan, but is concerned about the previous dose received. What would your advice be concerning the advisability of further CT scans? How would this advice differ if the patient had no previous ionising radiation exposure? (3)
As per the ICRP fundamental principle of radiation protection, diagnostic procedures must be justified on an individual basis a
justification process should examine whether the diagnostic information sought can be obtained using a
lower dose imaging modality;
The risk is extremely low (~5%/Sv, given the effective dose is 5 mSv – 10 mSv, therefore the theoretical
risk (probability of cancer induction) is a small fraction of a percent (%).
At these exposures the risk is a stochastic effect, any multiple imaging procedures over time are deemed
independent from each other in terms of their individual risk (probability) of cancer. However, the
lifetime cancer risk for the individual receiving multiple imaging procedures will increase. If further
imaging procedures are required and the additional procedure is justified on an individual basis, the
advice wouldn’t differ from one patient to the next.
Question 3
a) Identify the major study which has contributed most to our knowledge of the carcinogenic effects of radiation. (1)
The Life Span Studies of the Japanese survivors of the atomic bomb blast in 1945.
b) Name four organs or tissues most sensitive to radiation. (2)
Breast
Lung
Colon
Stomach
c) The total air kerma incident on an area of a patient’s skin during a lengthy vascular procedure is estimated to be 5Gy. State two possible tissue reactions the patient may experience, and their approximate timeframes. What should be the response of the clinical centre to such an event? (3)
Erythema
At 2 Gy, early transient erythema may be observed in a matter of hours
Epilation
Between 3 to 5 Gy, temporary epilation can occur after 2 to 3 weeks.
Monitoring the patient for such effects over this time period should occur, as well as documentation of the procedure and dose time. Should further complications arise from these deterministic effects
d) It is a general observation that reducing peak skin dose (PSD) to the patient is associated with reduced occupational dose to the operator. List four practical measures, each of which would ensure minimized patient PSD, and therefore operator dose. (4)
1. Collimating to region of interest
Reducing the field of view will result in less skin area being irradiated and subsequently the volume of tissue producing backscatter radiation to the operator
2. Projection geometry – SID
Reducing the subject to image detector and increasing the source to image detector improves the radiation incident upon the imaging detector. This means less radiation dose is required to produce the same diagnostic image using the Automatic Brightness Control.
3. Projection geometry – AP/lateral
Avoid oblique and lateral projections through body regions where it will result in a thicker volume of tissue being irradiated. Thicker tissue results in increase attenuation and therefore increase dose as the ABC compensates by increasing output. Operator dose increases due to increase back scatter radiation.
4. Increasing off-screening time or using low-dose/pulsed fluoroscopy mode
The simplest way is to ensure that screening, particularly high-dose/boost/magnification mode is only performed when necessary. Low-dose screening can be performed for initial wire guidance for example.
Section 2 (Basic Physics & Technology including Mammography, Fluoroscopy & DSA)
Question 1
In the context of digital projection radiographic imaging:
a) Define the term ‘contrast resolution’ and provide one example of a medical imaging examination where contrast resolution is critical. (2)
Contrast resolution is defined as the ability of a imaging modality to distinguish between differences in image intensity.
Contrast resolution is critical in angiography where normally radiolucent vessels are opacified using contrast media.
b) Define the term ‘spatial resolution’ and provide one example of a medical imaging examination where spatial resolution is critical. (2)
Spatial resolution is defined as the ability of an imaging modality to differentiate between two near-by objects.
Spatial resolution is critical in mammography where closely-clustered, subtle microcalcifications that can be indicative of cancer may be missed.
c) Identify, and briefly describe the impact of, three factors having a substantial effect on:
i. contrast resolution (3)
1. Contrast media
– Contrast media utilises the k-edge phenomenon of the photoelectric effect to maximise the difference in attenuation coefficient.
– counter-strategy: nil, this is a desirable effect
2. Volume of irradiated tissue
– large field size/thick body parts increases scatter (predominantly Compton), which reduces contrast
– counter-strategy: collimate to area of interest, compress tissue where permissible
3. Decreasing kVp where permissible
– decreasing kV will increase contrast at the expense of increased patient dose
ii. spatial resolution (3)
1. Patient motion
– movement artifact blurs fine detail on images and therefore reduces spatial resolution
– counter-strategy: breath-hold technique, sandbag/restraint immobilisation, cardiac/respiratory gating imaging techniques
2. Focal spot size
– large focal spot sizes increase image penumbra
– counter-strategy: reduce focal spot size if the imaging protocol allows for
3. Detector elements
– the smaller the size of the detector element, the better the
– counter-strategy: remove binning or upgrade imaging equipment
Question 2
a) Describe, with reasons, the effects on:
• beam quantity,
• beam quality, and
• the expected changes to the shape of the X-ray spectrum (see diagram) when the following parameters are changed:
i. the added filtration is increased. (3)
– Beam quantity: reduced. Filtration removes low-energy photons and thereby reduces the total number of photons as well.
– Beam quality: increased. Removing low-energy photons increases the average energy of the beam in a process called ‘hardening’
– Changes to the spectrum: The total area under the curve is reduced, the amplitude is reduced and the lower energy cut-off is increased
ii. the kVp is reduced to 60 kVp. (4)
– Beam quantity: increased.
– Beam quality: increased.
– Changes to the spectrum: The maximum photon energy is 60kVp
b) Explain the role of the Automatic Exposure Control (AEC) system used in Digital Radiography. A detailed description of the technology is not required. (3)
The role of the automatic exposure control
Question 3
a) Name one filter material that is used in conjunction with a:
i. Molybdenum (Mo) target (1)
Molybdenum (Mo)
Other answers
Rhodium (Rh)
ii. Tungsten (W) target (1)
Rhodium (Rh)*
Other answers
Silver (Ag)*
Aluminium (Al)*
* used in digital mammography
b) Briefly describe four differences between the magnification technique and the contact technique used in mammography. (4)
1. Focal spot size
Contact: broad focus (0.3mm)
Magnification: fine focus (0.1mm)
2. Grid
Contact: grid is used for scatter removal
Magnification: air gap is used for scatter removal
3. mA
Contact: typically 100 mA
Magnification: typically 25 mA
4. Field of view
Contact: whole breast imaged (CC and MLO views)
Magnification: particular region of breast tissue imaged
Other answers
5. Purpose
Contact: screening and diagnostic
Magnification: diagnostic only
6. Subject/Object to Image Detector distance
Contact: 65cm from focal spot
Magnfication: 35cm from focal spot
c) Describe, with reasons, four advantages associated with breast compression during mammography. (4)
1. Spread dense tissue out
– this helps overcome the issue of superimposing tissue obscuring subtle lesions
2. Tissue immobilisation
– reduced chance of patient movement decreases the risk of motion blur artefact which can degrade spatial resolution
3. Reduced dose
– attenuation/absorption of radiation dose is a function of tissue thickness. Compression reduces the thickness through which the primary beam traverses, therefore decreasing attenuation
4. Allows use of lower voltages
– use of lower voltages improves subject contrast
Other answers
5. Reduces the object to image receptor distance
– improves spatial resolution by reducing geometric unsharpness due to the penumbra effect
6. Reduced exposure time
– results in reduced chance of patient blur artifact
Section 3 (CT, MRI, US & Nuclear Medicine)
Question 1
a) The image shows a CT slice containing beam hardening and an additional artefact. Name this additional artefact and briefly explain why it has occurred. (2)
The image is required to answer this question, however possible answers based on common CT artefacts include:
Partial Volume
– occurs when several objects are averaged together in a voxel, thereby reducing spatial resolution. This particularly occurs with large voxels, i.e. large slice thickness
Streak
– a loss of line integral information, occurs due to attenuation by an object (fillings, pacemaker, prosthetic devices) exceeding the dynamic range of the detector.
Ring artefact
– concentric rings, occurs due to imperfect detector elements result in loss of signal
Stair Step
– occurs in Sagittal/Coronal MPR images due to straight structures that are oblique to the reconstruction
View Aliasing
– occurs when too few projections are used for reconstruction of high-frequency objects
Helical Windmill
– occurs when helical data interpolation leads to periodic dark and light streaks (‘windmill’) around high contrast edges
b) A CT exam is performed where the exposure factors are 120 kV, 150 mAs and a pitch of 1:1. Automatic current modulation is not employed. The reconstruction is performed using filtered back projection with a bone filter. The reconstructed slice width is 1 mm. The signal to noise ratio (SNR) of the resultant images is too low.
i. If the patient is to be rescanned, discuss how you would change the mAs to increase the SNR, explaining how the change you suggest leads to the desired improvement. (2)
Increase the mAs
Increasing the mAs increases the photon quantity reaching the detectors, therefore increasing the signal.
It also reduces the quantum mottle (the predominant contributor to CT image noise)
ii. State the effect of this mAs change on the effective dose received by the patient and explain why it occurs. (2)
Increasing the mAs increases the effective dose as more photons are being absorbed by the patient.
iii. List two ways that the initial data set could have been reconstructed to improve SNR without having to rescan the patient. For each, briefly explain why the SNR is improved. (4)
1. Reconstructing large slice thickness (vs. thin 1mm slices)
2. Soft tissue kernel (vs. bone filter)
Other answers
3. Iterative reconstruction (vs. filtered back projection)
Question 2
a) For a basic MRI spin echo pulse sequence, voxels within a particular slice are encoded with unique spatial information. Describe the key steps of this encoding process (NB. you do NOT need to address slice selection). (4)
1. Phase encoding
Following the simultaneous excitation pulse and application of the slice selection gradient perpendicular to the plane of interest (suppose the Z-axis), a phase encoding gradient is applied in the Y-axis. This results in protons in the selected slice precessing in the same frequency but in different phases.
2. Frequency encoding
This is the final step in spatial encoding. A frequency encoding gradient is applied when the signal is received. This modifies the Larmor frequencies in the X-axis.
b) Fast pulse sequences sometimes employ a reduced flip angle for the initial RF pulse. Explain why this allows for faster image acquisition compared to a conventional spin echo pulse sequence. (3)
The FSE/TSE pulse sequence resembles a conventional spin-echo (CSE) sequence (i.e. uses a series of 180º-refocusing pulses after a single 90º-pulse to generate a train of echoes), however, changes the phase-encoding gradient for each of these echoes. As a result of changing the phase-encoding gradient between echoes, multiple lines of k-space (i.e., phase-encoding steps) can be acquired within a given repetition time (TR) thus significantly reducing imaging time.
A major limitation of FSE sequences is excessive heating of tissues due to high concentration of 180°-pulses (a 180°-pulse deposits 4x the energy of a 90°-pulse). By reducing the RF-flip angles from 180º to the 50º−150º range, thermal energy deposition can be significantly reduced with minimal impact on the MRI signal.
c) For each patient device listed below, list one reason why it may contraindicate an MRI:
i. Cardiac pacemaker (1)
Strong radiofrequencies may induce currents in coils/cables causing malfunction or potential thermal burns
ii. Aneurysm clip (1)
Strong fixed magnetic field can display ferromagnetic objects, including implanted metallic devices
iii. Intracardiac pacing leads (1)
Strong radiofrequencies may induce currents in coils/cables
Question 3
c) Diagnostic ultrasound images can be created using a pulse echo mode. Describe the key principles of how pulse echo mode is employed to form a single line of the ultrasound image. Your answer should include how depth and brightness are determined. (3)
Brightness: The displayed brightness of an USS image is determined by the amount of echo reflected back towards the transducer. This depends on the difference in acoustic impedence between tissue – large differences in acoustic impedance will reflect more energy i.e. echo.
Depth: The echo arrival time can determine the depth. It is simply the time between the transmitted pulse and received echo divided by 2. This assumes that the ultrasound propagates at 1540m/s in tissue.
d) A fluid filled cyst is surrounded by normal liver tissue. Describe the appearance of the tissue immediately deep to the cyst, and explain why this occurs. (2)
Acoustic enhancement
Fluid serves as a good acoustic window as it neither attenuates nor reflects the transmitted pulse. Structures deep to a fluid-filled structure will have increase echogenicity (appear brighter) due to overcompensation by the TGC (time-gain compensation).
e) Explain how the beam is swept through the field of view by a linear array transducer. (2)
Simultaneously fires approximately 20 elements to produce the ultrasound beam and then moves down the array
f) Modern diagnostic ultrasound machines provide a thermal index value to the operator.
i. Define this term. (1)
The thermal index refers to the amount of heating the absorption of ultrasound waves causes in tissue.
ii. Suppose you are overseeing a fetal scan and the displayed TIS (thermal index of soft tissue) reaches 2.4. The sonographer asks if it is safe to continue scanning. What would you advise? (2)
Color and spectral Doppler imaging use increased levels of ultrasound output power, thus will further increase the TIS, hence should be discouraged when imaging an embryo to avoid crossing a theoretical thermal index threshold. Current guidelines recommend a thermal index <1.0 for first-trimester screening (11 weeks to 13 weeks, 6 days) with exposure time to Doppler imaging for as little as possible, preferably less than 5-10 minutes.
Question 4
a) Gamma cameras are the basic imaging device for much of nuclear medicine imaging. The diagram (FIG. 1) depicts a schematic of the major components of a gamma camera detector head. For the components labelled A and B in the diagram, name each component and briefly describe its function in the detector head’s operation. (3)
b) The image (FIG. 2) is a static nuclear medicine bone scan image that is of poor quality.
i. Give the most likely reason for the poor image quality. (1)
Insufficient photon count
ii. Discuss one method by which the image quality may be improved and detail any potential drawbacks associated with your suggestion (NB. assume that the gamma camera’s collimator was positioned as close to the patient as possible). (3)
Increasing radioisotope dose will increase the count density and therefore improve contrast
This is at the expense of increased patient dose
c) 99mTc is the most widely used radioisotope in Nuclear Medicine (NM) imaging. List two physical or chemical properties of 99mTc that make it ideally suited for NM imaging and explain why each property is desirable. (3)
99mTc half-life is 6 hours: Short half-life compatible with the duration and objectives of the nuclear medicine study. Half-life should be long enough to complete the study with an adequate concentration in the organ of interest, but short enough to minimise patient dose.
99mTc decays by an isomeric transition,
and approximately 10% of the gamma photons undergo internal conversion producing internal conversion electrons; the resulting vacancies in the K shells cause emission of characteristic x-rays and
Auger electrons
Produce monochromatic gamma rays with energies between 100-300 keV.
Photons with too low energies are largely attenuated by the body, increasing patient dose without contributing to image formation.
Photons with too high energies escape the body but have poor detection efficiency and easily penetrate collimator septa.
Minimise production of particulate radiation, such as beta particles, internal conversion electrons, and Auger electrons.
Localise in organ or tissue of interest i.e. have a high “target/non-target activity”.
Be non-toxic and contain no chemical or radionuclide contaminants.
Low toxicity is enhanced by the use of high-specific-activity, carrier-free radionuclides that also facilitate radiopharmaceutical preparation and minimise the required amount of the isotope.
Ideally should be readily and economically available.
Radionuclides should have a chemical form, pH, concentration, and other characteristics that facilitate rapid complexing with the pharmaceutical under normal laboratory conditions.