PPQ Flashcards
(38 cards)
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
Angular (x-y) modulation - tube current is modulated as the X-ray tube rotates around the patient. (Reduces dose in AP to lateral)
Z-axis - adjust along the body length. Less dose to neck and legs
Combined
AEC - system automatically varies based on density
Organ - based - operator might select where the scanner should reduce dose
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.
CTDIw = 1/3CTDIcentre + 2/3 CTDI periphery
CTDI = CTDIw / pitch
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.
