[Gate-users] Greater number of delayed coincidences than random coincidences for PET data

[Twyman, Robert]

Hi Luca,

Thank you for your response. I found no true events in the delayed window data. The delayed offset is 200ns so this makes sense to me. I had not thought about the multiple coincidences policy, I do not set it in my macros and therefore I think it is the default “keepIfAllAreGoods”. This would be the case for both the Coincidence data and Delayed and so I would not expect a difference.

I also noticed the broken link in the original email, here is the updated: https://github.com/UCL/STIR-GATE-Connection/blob/master/ExampleScanners/D690/digitiser_D690.mac#L39-L54

Robert

On 14 Apr 2021, at 03:56, PRESOTTO LUCA <presotto.luca at hsr.it<mailto:presotto.luca at hsr.it>> wrote:

Hi,
Not really sure, I'm just guessing. I'm not using delayed window as our clinical scanners correct randoms using single rates.
I'm wondering 2 options, both related to the count-rate. Is it possible that your delayed window might contain some true coincidences? Also, which is your multiple coincidences policy?

Luca

________________________________________
Da: Gate-users <gate-users-bounces at lists.opengatecollaboration.org<mailto:gate-users-bounces at lists.opengatecollaboration.org>> per conto di Twyman, Robert <robert.twyman.18 at ucl.ac.uk<mailto:robert.twyman.18 at ucl.ac.uk>>
Inviato: martedì 13 aprile 2021 19:03
A: Gate-users at lists.opengatecollaboration.org<mailto:Gate-users at lists.opengatecollaboration.org>
Oggetto: [Gate-users] Greater number of delayed coincidences than random coincidences for PET data

Dear GATE users,

I have been simulating PET acquisitions using GATE on an XCAT phantom for reconstruction using STIR. The reconstruction processes is iterative and therefore I have need to estimate the contributions due to Random and Scattered events in the measured data. I am estimating the Randoms from the Delayed coincidence data output by GATE using a 200ns Delayed offset. Both the Coincidence and Delayed windows are set to 4.9ns.

Please see my digitiser on GitHub: https://github.com/UCL/STIR-GATE-Connection/blob/master/ExampleScanners/D690/digitiser_D690https://github.com/UCL/STIR-GATE-Connection/blob/master/ExampleScanners/D690/digitiser_D690.mac#L39-L54
.mac#L39-L54

The problem is as follows. I unlist (using STIR) all valid events in the Delayed ROOT file. I unlist all random events (defined as the photons having different eventIDs) from the Coincidence ROOT file. The result is, ~10% more events in the Delayed data, which has knock on effects to my Randoms estimation and into the Scatter estimation…

Does anyone have any idea why I might have more events in my Delayed data than Random events in my Coincidence data?

Regards,
Robert Twyman
Ph.D Student
UCL



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