PPQ Flashcards
Give two conditions of compliance that are included in an EPR permit for radioactive substances
Use of Best Available Techniques (BAT):
The permit requires that all activities involving radioactive substances use the Best Available Techniques to minimize both the volume and activity of radioactive waste, and its environmental impact
- Record-keeping and auditing: Operators must keep accurate records of radioactive material use and waste disposal, and report regularly to the Environment Agency.
Give one condition that still applies for substances that are exempt from the need for an environmental permit
Even if substances are exempt from needing a permit, there is still a requirement to keep adequate records and ensure that the limits for exemption are not exceeded. For example, solid waste disposed of as Very Low Level Waste (VLLW) must comply with activity limits (e.g. ≤40 kBq per item, ≤400 kBq per 0.1 m³) and must be traceable with records
Briefly state how exemption categories can be used for a site with an environmental permit
Even on a site with an Environmental Permit, exemption categories can still be applied to specific materials or waste streams that meet the exemption thresholds. This allows the site to manage low-activity substances outside the constraints of the permit, provided the exemption conditions are fully met (e.g. limits on activity and storage).
In CT, describe the relationship beyween image noise and number of photons (not considering iterative reconstructions methods. In order to obtain an image by half noise how would you need to increase the dose by?
Noise is inversely proporitonal to the sqaure root of the number of photons. Noise = 1/root N.
To reduce noise by a factor of 2 you need to increase N by a factor of 4.
1/root(4N) = 1/2root(N)
Give the definition of helical pitch and explain how the pitch influences the patient dose and image quality for non-modulated protocols
Pitch = Table movement per roatation / beam width
Low pitch < 1 = Overlapping slices = higher dose with better image quality
High pitch > 1 - gaps between slices = lower dose & reduced image quality
Briefly describe two different dose modulation methods in CT
Automatic Tube Current Modulation (ATCM):
Adjusts the tube current (mA) in real time based on patient size or density to maintain consistent image quality.
Angular (x-y) Modulation:
Varies the tube current as the tube rotates around the patient, reducing dose where less is needed (e.g., lateral projections).
Calculate the CTDIvol of a CT scanner with Pitch 0.5, if the
CTDI100 of a body phantom shows 10 mGy at the centre and
20 mGy at the periphery of the phantom.
33.3mGy
In PET imaging what is meant by list-mode acquisition (2 marks)
and what is meant by sinogram mode acquisition (2 marks)? Give
one advantage of list-mode acquisition over sinogram mode
acquisition (1 mark)
List-Mode - Each detected coincidence is recorded individually and stored with detailed information. Time, location of detectors. Data is stored chronologically.
Sinogram - Detected coincidence events are sorted immediately into projection bins based on the detector pair geometry.
The data is stored in a 2D sinogram matrix, where each bin corresponds to a specific line of response (LOR) and angle.
An advantage of list mode - Greater flexibility – List-mode allows retrospective reconstruction for specific time frames (e.g., dynamic imaging), respiratory/cardiac gating, or motion correction, which sinogram mode doesn’t support easily
Describe the 3-compartment model of FDG
Compartment 1: Plasma – FDG circulating in the blood.
Compartment 2: Tissue (unmetabolised FDG) – FDG that has been transported into cells but not yet metabolised.
Compartment 3: Metabolised FDG (FDG-6-phosphate) – FDG that has been phosphorylated by hexokinase and is trapped in the cell (cannot be metabolised further).
Describe the time course of the 18F concentration in each
compartment.
Plasma (Compartment 1):
Rapid peak after injection, then decreases over time as FDG moves into tissues and is cleared.
Unmetabolised tissue FDG (Compartment 2):
Rises quickly as FDG enters cells, then may plateau or decrease depending on phosphorylation rate.
Phosphorylated FDG (Compartment 3):
Increases more gradually, reflecting ongoing trapping in cells. This compartment retains FDG-6-phosphate, so its concentration builds up over time, especially in high-glucose-using tissues (e.g., tumours).
Explain how the kinetics of FDG determine the standard
clinical FDG-PET protocol.
Because FDG is trapped in tissues after phosphorylation (Compartment 3), waiting ~60 minutes post-injection allows enough time for background activity (blood pool) to clear and for optimal tumour uptake to be established.
Describe the general radiation safety precautions that must
be taken when working with and administering
radionuclides.
Time
Minimise time spent near radioactive sources to reduce exposure.
Plan procedures in advance to work efficiently.
Distance
Maximise distance from the source whenever possible.
Use tongs, shielding, and remote handling tools to avoid direct contact.
Shielding
Use appropriate shielding materials based on the radionuclide type:
Lead or tungsten for gamma/positron emitters
Describe how and why different precaution may be required
for working with single-photon and positron emitting
radionuclides.
Positron emitters produce higher energy photons, which result in increased radiation exposure and require stricter safety measures compared to single-photon emitters.
Main parameters used to define the performance of a PET scanner
Spatial Resolution, Sensitivity, Time resolution
What is meant by normalisation of a PET scanner
Normalisation is the process of correcting for variations in detection efficiency across the different detector elements in a PET scanner. Normalisation ensures that all detectors contribute equally and accurately to image formation, improving quantitative accuracy and image uniformity.
Why can’t a CT scan acquired in PET-CT scanner be used
directly to correct a registered PET acquisition for the
effects of attenuation?
A CT scan measures attenuation at lower X-ray energies and not at the 511 keV energy relevant for PET. Therefore, it must be converted using a calibration algorithm before it can be used for attenuation correction in PET.
What operations are applied to the CT scan so it can be used
to correct for PET photon attenuation in a PET-CT scanner?
How could MR images be used for attenuation correction in
PET-MR scanner and what is the main difficulty with this
approach?
Briefly describe 4 of the key physical characteristics that
make a NaI(Tl) crystal suitable for imaging single photons.
Crystals, light yeild, like output consistency and optical clarity impact how well it can convert gamma photon energy into visible light. More light = better SNR and better potential energy resolution. Na(Tl) has about 6% energy resolution.
Fast scintillation decay time, allows rapid detection. Mechanically easy to shape and grow.
High atomic number leads to high density and therefore high stopping pwer / efficiency.
The time constant for the NaI(Tl) is 230 ns. Explain why this
is a limiting factor for the use of this crystal in PET while it
is acceptable for use in single photon imaging.
The decay time is the time it takes for the light pulse to decay after absorbing a photon.
A much fast decay time is required in PET…PET requires high timing resolution. This is in order to accurately determine the coincidence events.
In the context of gamma camera quality control, define energy resolution.
In the context of gamma camera quality control, energy resolution refers to the ability of the gamma camera to accurately measure the energy of incident gamma photons. It is defined by the Full Width at Half Maximum (FWHM) of the photopeak in the pulse height spectrum, which represents the distribution of detected energies.
The energy resolution is influenced by the finite energy resolution of the system, which leads to a blurring of the pulse height spectrum due to variations in the number of photoelectrons released from the photocathode. These variations cause a spread in the measured energy, making it harder to distinguish between gamma photons of different energies.
how would you conduct energy resolution test?
Energy resolution is typically measured using the same setup as intrinsic uniformity tests. The FWHM is analyzed and expressed as a percentage relative to the energy of the photopeak, using the formula:
Energy resolution = (FWHM/Energy of the Photopeak) * 100 = %
A lower percentage indicates better energy resolution, meaning the gamma camera is more capable of distinguishing between different photon energies. This is an important aspect of gamma camera performance, as it affects the quality of imaging and the accuracy of quantifying radiation in nuclear medicine procedures
What is meant by the term temporal resolution for a PET
scanner?
Temporal resolution refers to the scanner’s ability to distinguish between two events occurring closely in time. In PET, it’s the minimum time difference between two detected photons that the system can still identify as a coincidence (typically measured in nanoseconds).
How does the PET scanner identify a true coincidence, and how does this depend on temporal resolution?
A true coincidence is identified when two photons are detected within a very short predefined time window, known as the coincidence timing window.
This window is determined by the scanner’s temporal resolution: better (shorter) temporal resolution allows a narrower window, which helps in rejecting random coincidences and improves image quality.
How do ‘random coincidences’ arise?
Random coincidences occur when two unrelated annihilation events produce photons that are detected within the coincidence time window. The system mistakenly records them as a single true event
What aspects of scanner design influence sensitivity to true, scattered, and random coincidences?
Detector Efficiency: High-efficiency detectors (e.g., LYSO) increase sensitivity to true events.
Coincidence Timing Window: Narrow windows reduce random coincidences; wider windows increase them.
Crystal Size and Thickness: Thicker crystals detect more gamma rays (increasing true and scattered coincidences).
Energy Resolution: Better resolution helps distinguish scattered photons (lower energy) and reject them.
Axial Field of View (FOV): Longer FOVs increase true sensitivity but also increase random and scattered coincidences due to more detected events.
Septal-less Design (e.g., 3D PET): Increases sensitivity but also increases scatter and randoms, compared to 2D PET with se
What aspects of clinical scanning protocols influence the rates of true, scattered, and random coincidences?
Injected Activity: Higher activity increases true, scattered, and random rates.
Scan Duration: Longer acquisitions improve true event statistics but may also accumulate more randoms and scatter.
Patient Size and Composition: Larger patients cause more attenuation and scattering, increasing scatter rates.
Relationship of coincidence rates to activity, with diagram
True coincidences increase linearly with activity.
Scatter coincidences increase approximately linearly, though more complex due to photon interactions.
Random coincidences increase quadratically with activity, since the chance of unrelated photons coinciding increases with the square of the count rate.
) A point source in air gives a count rate of 20,000 cps, as
measured by a radiation detector, for example a gamma
camera. When the point source is placed at the centre of a
5 cm radius sphere of unknown material the count rate is
reduced to 10,000 cps. What is the half-value layer for this
material? What would you expect the counts rate to be if the
same source is placed in the centre of a 15 cm radius cylinder
of the same material?
Some of the photons emitted by the above mentioned point
source have undergone Compton scattering loosing 20% of
their initial energy of 140 keV. Briefly describe a technique
to exclude these photons from being registered into the final
image.
Use energy Windowing: Set narrow energy window (e.g., 126–154 keV) around 140 keV photopeak
Rejects lower-energy scattered photons like the 112 keV ones
A CT scan of the phantom has also been acquired on the
hybrid SPECT/CT system. Briefly describe two operations
where the CT can be used to help obtain accurate
quantitative information from the SPECT data.
Attenuation Correction (AC):
CT provides attenuation map used to correct for photon loss through tissue.
Anatomical Localisation:
CT helps define exact location and shape of organs or lesions for accurate ROI-based quantification.
Briefly explain how the choice of parameters of the iterative
reconstruction, for example with OSEM (ordered subsets
expectation maximisation) can in general affect the
accuracy of quantification of the SPECT data.
Key Parameters:
Number of iterations: More iterations → better convergence but can amplify noise
Number of subsets: Affects speed and smoothness of convergence
Effects:
Too few iterations → underestimation of activity
Too many iterations → noisy, overfitted images
Balanced settings yield optimal quantification and image quality
) What is the main objective of IR(ME)R and who is the
regulator in England?
To protect patients and CQC
Other than patients undergoing diagnosis or treatment,
name two other situations where IR(ME)R applies.
** Carers and comforters and
A patient, in England, Joe Bloggs, is referred for a Tc99mMAG3 renogram (effective dose according to ARSAC: 0,7mSv)
after attending clinic. Joe is imaged as requested. It later
transpires that the referrer made an error and had intended
the exposure for another patient with similar clinical history.
Should this be reported to the regulator (for England), state
the reasons for your answer?
Yes because it was an unnessessary exposure to a radiopharmaceutical:
Therefore it was an exposure without valid justificiation. Exposing the wrong person is alwasy reportable despite the low dose.
Describe the purpose of local diagnostic reference levels:
what do they represent and how should they be utilised and
managed?
Purpose:
Serve as benchmarks for typical doses in common procedures
Aim to identify unusually high or low doses that may indicate suboptimal practice
Represent:
The median or 75th percentile dose values from a local sample of patients
Not dose limits, but guidance levels for optimisation
Use and Management:
Compare actual patient doses to LDRLs
Investigate when doses consistently exceed the LDRL
Review and update LDRLs periodically based on clinical data and equipment
How many ARSAC licences should a hospital Trust with a
nuclear medicine department possess and what is the role of
ARSAC?
Explain the essential difference between additive and
multiplicative models used in the estimation of radiation
risk.
The additive model (Excess Absolute Risk, EAR) estimates radiation risk as an absolute increase in cancer incidence regardless of the baseline cancer rate in the population. In contrast, the multiplicative model (Excess Relative Risk, ERR) estimates risk as a proportional increase relative to the baseline rate. While these models may yield similar results in the original population used to develop them, they can diverge significantly when applied to populations with different baseline cancer rates
