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  • Overview Traditional Shielding Issues Shielding Common Isotopes Shielding Issues Exposure Rates

    PET Shielding Issues

    Preparing a Lab for F-18 FDG Imaging Patient Issues

    Recommended Shielding Portable Shielding PET/ CT Issues Meeting regulatory requirements can be expensive Work with equipment vendor, facility architect, and qualified medical physicist

    Traditional Shielding Issues

    Shielding Common Isotopes:Tc-99m, Ga-67, In-111, Xe-133, I-123, Tl-201 Most isotopes require only 1/8 to 1/4 inch of lead Shields are not bulky Distance is a good ALARA practice also Only other problematic isotope is I-131.

    1. Partially because of higher energy (0.365 MeV) 2. Partially because of high activity (10 300 mCi)

    Additional shielding was needed in Hot Lab Shielded syringes are useful Some institutions added - inch of shielding in walls We all became fairly comfortable...

  • PET Shielding Issues ...Changes Occurred

    Prior to ~ 2000, PET had been limited primarily to only institutions with a cyclotron on site

    Technology has improved PET has become much more common Shielding changes were needed to accommodate PET scanning

    Positron Emission Tomography

    PET uses annihilation radiation, produced when a positron collides with a negatron (electron), for Nuclear Medicine imaging

    This collision always produces two 0.511 MeV photons emitted at a 180o angle to each other

    Shielding 0.511 MeV Photons

    Photons with energy this high are difficult to shield Higher penetrating ability Increased radiation (Compton) scatter

    Radiation sources requiring shielding

    Doses Patients

    Preparing a Lab for F-18 FDG Imaging

    We all know that exposures from PET doses have the potential to be high... How high is high??

    Dose Rate from Injection

    Positron-emitting isotopes have high gamma ray dose constants (G) Hand dose can be very high Dose rate 5 cm from unshielded syringe with 555 MBq (15 mCi) of F-18 is 33

    mSv/hr (3300 mrem/hr)

  • Reducing Hand Dose

    Tungsten syringe shield can reduce hand dose by 85%. Downside: extra weight (0.75 kg) can make it difficult to inject

    Lead glass window in syringe shield can increase whole body dose

    Other options:

    Automatic dispensing systems Divide responsibilities among staff

    Many good products exist: Good ideas can come from vendors and other PET facilities.

  • Radiation Safety and PET: The $20K Solution

    Dose Rates from Patient

    American Association of Physicist in Medicine (AAPM) task group published the patient dose rate for F-18:

    The expected dose rates from a patient are relatively high

    =3.4Rxcm2mCixhr

  • PET Shielding Considerations: F-18 Injection & Uptake

    Patient Injection: either in Hot Lab or Uptake Room with a typical dose: 370 -740 MBq (10-20 mCi)

    FDG Uptake: Patient waits 60-90 minutes post injection before imaging is initiated Activity will decay while waiting. Remaining activity 0.83, 0.68 and 0.57 for wait

    times of 30, 60 and 90 minutes, respectively

    F-18 Scanning

    Patient Void: Patient clears ~15% just before scanning Patient Scan: Whole-body PET scan for 30-60 minutes

    Radiation Safety: Areas of Concern

    Adjacent offices Adjacent hallways Rooms with radiation sensitive equipment (i.e. cameras, uptake systems) Storing Patients:

    Waiting rooms will probably need additional shielding

    Dose to employees Dose to visitors High background for imaging Patients wait 30-90 minutes Transient Patients Some departments do not have a waiting area near injection room Radiation from patient may have effect on radiation sensitive equipment (increased

    background)

    1. Scintillation well counters 2. Thyroid uptake systems 3. Scintillation cameras

  • How Much Shielding Is Needed For PET? ALARA considerations

    Many facilities have set ALARA limits to be 10% of the applicable limits for radiation workers, 5 mSv/yr (500 mrem/yr)

    The More Lead, the Better?? Not exactly true- consider the law of diminishing returns Yesit will reduce exposure rates Yesit is $$$$

    Factors Affecting Radiation Protection

    Each institution must find a happy medium between safety and cost Number of patients imaged Activity administered per patient Length of time patient remains in facility Location of department within facility

    Physicist performs calculations based on:

    Number of patients imaged Activity administered per patient Length of time patient remains in facility Location of facility relative to controlled and uncontrolled areas

    Suggestions for amount of shielding needed to keep doses ALARA

    Imaging room will most likely need additional shielding Many facilities add inch lead in walls facing occupied areas (offices, busy

    hallways) Some could add only inch if they do not expect a large patient load (realistic??) Some facilities only add inch lead in walls facing semi-occupied areas

    Dont forget floors and ceilings- If patient load is high enough, shielding is needed

  • Photograph illustrating sheets of lead installed during construction of imaging room.

    Photograph illustrating shielded space for Nuclear Medical Technologists in PET Imaging facility- Leaded Glass Windows and Lead in Walls.

    Portable Shielding:

    Pb shields 2.5 cm and 5.0 cm thick are available providing dose reduction factors of 40 and 1900, respectively

    \

  • Problems Patient may move in relation to gantry Shield can limit access to patient

    Portable Shield Warning

    Traditional portable shields are not adequate- only have a few mm of lead Wont do much to stop 0.511 MeV photons

    PET/CT Shielding

    PET uses high levels of high energy photons CT uses higher levels of lower energy photons (x-rays) If shielded for PET no additional shielding needed If shielded for CT only additional shielding will be necessary

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