Design of PET-CT facilities Flashcards
What are some of the main issues that need to be considered when designing a PET-CT facility?
- High energy and high dose rates.
- Multiple sources of exposure (i.e. patient and CT scanner), some of which are mobile.
- Expanding service which could mean potentially high workloads.
- Multiple patients at the facility at once.
- Long stay on the premises for each patient.
What are the sources of exposure in PET-CT?
- Fluorine-18 radionuclide which is a positron emitter resulting in annihilation and 511 keV gamma emissions. 400 MBq a typical adult administration.
- CT typically uses 120 kV x-rays.
What is the typical patient journey through a PET-CT department? What is the staff journey?
- Arrival & clerking: This may take place in a reception area or maybe an uptake bay. A member of staff will be with the patient.
- Administration: The patient will be transported to the uptake bay by a member of staff. A member of staff will perform the radiopharmaceutical injection.
- Rest period: This will likely take place in the uptake bay. A staff member may be required to be with the patient here but this will be avoided if possible.
- Use of toilet: The patient will be taken by a member of staff to use the hot toilet. The member of staff may need to wait with/help the patient in some cases.
- Scanning: The patient will be transported to the scan room by a member of staff. Staff will remain in the control room when the scan is taking place except in some cases.
- Waiting post scan: This may take place in the scan room or a staff member may transport the patient to some other ‘hot wait area’.
- Discharge: The patient will then be discharged.
What is the typical pharmaceutical journey through a PET-CT department?
- Receipt into department.
- Preparation of patient dose (automated devices used to minimise staff exposure).
- Administration (automated devices used to minimise staff exposure).
- Patient.
- Waste disposal.
How can the typical dose rate from a PET-CT patient be estimated?
- Published data (e.g. BIR, AAPM) provide dose rates per MBq administered activity at 1 m immediately following administration. Uncertainties are involved with this where actual values will depend on patient build and pathology. Some values also include patient attenuation and some do not.
- Dose reduction factors must then be applied to account for the exponential reduction in dose rate with time and the excretion factor associated with voiding the bladder prior to a scan. Typical dose rates from the patients at each phase of the patient journey can then be determined.
What are some general considerations for a PET-CT department to protect staff?
- Minimise distance between hot toilet and scan room.
- Maximise distance between uptake bays and control room.
- Eliminate direct line of site of resting patients to staff (e.g. through nibs, baffles, using cameras etc.).
- Site ‘hot rooms’ next to low occupancy areas.
- Systems of work (e.g. signs to indicate where waiting should be avoided) to reduce occupancy of hot areas.
What are some general considerations for shielding material selection?
- Cost.
- Thickness required.
- Mass required, especially when considering departments that are not on the ground floor.
How is required transmission determined?
Required transmission = Dose constraint * occupancy corrected/Unattenuated dose rate @ 1 m * ISL correction * workload.
What dose constraint is used when determining shielding for a PET-CT department? How can multiple sources of exposure (e.g. from patient in scan room and patient in uptake bay) be considered for shielding calculations?
- 0.3 mSv annualdose constraint would be applied for staff/public outside of the facility.
- Higher could be used for staff within the department as the majority/whole of the department will be a controlled area. However, would still aim for 0.3 mSv, where possible, when determining shielding requirements.
- A significant component of staff exposure will be from direct patient contact which is difficult to quantify and not usually included in calculations.
What are some typical occupancy factors for the different areas of a PET-CT department?
- Control room: 100%.
- Scan room: 20%.
- Internal corridors: 10%.
- Uptake bays: 5-20% depending on whether used for clerking, injection only and use of automated injection devices.
- Prep room: 10%.
What is a typical workload for a PET-CT department? What additional factors may need to be considered in terms of workload when determining shielding?
- Scan time typically 20 mins => Typically 15-20 patients per day (8 hour working day with patient setup etc. included).
Other considerations:
- Potential for a 7-day working week.
- Staff working hours may vary and some staff may rotate through different departments. This may have to be considered for PET-CT to allow for shielding savings.
How are the transmission curves associated with gamma rays from PET-CT different to general x-rays?
The gamma rays emitted by the patient are monochromatic. This results in varying amounts of forward scatter depending on the shielding material. This means the transmission curve for most materials is initially dominated by scatter buildup.
What considerations are required for CT shielding in PET-CT facilities?
- CT shielding needs to extend to the full height of the room due to the potential for tertiary scatter contribution. For PET shielding, 2 m is sufficient.
- Patient throughput lower than for conventional CT scanners.
- Low dose scans likely to be used if only for attenuation correction.
What general things may considered to avoid over-specification of shielding for a PET-CT department?
- Realistic estimates of workload (e.g. variations in staff working hours and rotation of staff through different departments).
- Latitude in dose constraint used.
