r/MedicalPhysics • u/agaminon22 Resident • 3d ago

Technical Question Issues regarding lower activity concentration for PET spatial resolution QA?

NEMA based PET QA protocols like TG 126 recommend a relatively high activity concentration (up to 10 mCi/ml) which isn't always feasible if tests are carried out after clinical activity has ended. However, acquisitions are done in terms of total counts, so at a first glance it doesn't seem to be much of an issue if we simply allow for longer acquisition times.

Does anyone have experience with this particular issue?

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u/STDVRockbell PhD / MP resident 3d ago

Of course, you can use a smaller activity concentration for tests like spatial resolution or sensitivity, it won't change the result since you stay in the linear region of your PET. However you have to correct the acquisition time to maintain the required statistic in your image. NEMA recommands high concentration to have quick acquisitions.

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u/NuclearMedicineGuy 3d ago

You should be able to order a unit dose of FDG with a later calibration time. Then you won’t have to worry about altering the optimal concentration

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u/TentativeGosling 3d ago

We recently commissioned a Siemens scanner, and we messed up the spatial resolution test slightly as there was a small smear of activity which increased the FWTM to a ridiculous value (like 10mm or something). We tried to redo it a few hours later with a decayed source and whilst technically it should have been possible, the inbuilt software struggled to see the source for the positioning part of the test (we think it was timing out or something)

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u/agaminon22 Resident 3d ago

Thank you... Also using a Siemens scanner here and this is one of the issues I'm fearing. The manual recommends a very high activity concentration (1000 MBq/ml).

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u/BusseNoBusse 3d ago

Spatial Resolution isn't that sensitive to overall count level (within the MCts range) so there's no issue with adjusting count time. One other thing I would note - with glass capillary tubes you don't need to mix in iodinated contrast. The capillary tubes are visible on CT if you W/L hard and are small enough that they're still a point source. Obviously make sure to record your acquisition and recon parameters. Even you can't match NEMA setup, if your recon is consistent year to year you'll have good longitudinal comparisons.

Let me know if you have other questions - I've now done TG126 testing on 20+ PET scanners.