[Gate-users] Issues with pinhole collimator

[Matthew Strugari]

Hi everyone!

I would be very grateful if someone would be willing to discuss my issue with me or offer potential solutions. The issue that I'm facing is degraded resolution and enhanced sensitivity of my simulated SPECT scanner when compared to experiment for a 1.0 mm pinhole collimator (see attached plots). These issues suggest that the modelled pinhole is larger than it should be, but I have verified the geometry of the modelled collimator.

Using the NEMA NU-1 2018 protocol, I have fully validated the intrinsic resolution (0.85 mm) and count rate (attached) of the scanner, among other intrinsic parameters, so I would expect comparable results between experimental and simulated system measurements when a collimator is included. However, I find that the system resolution is degraded by ~7%-20% across the FOV. Also, note that the experimental resolution at the face of the collimator is less than 1.0 mm which exceeds the theoretical resolution. I attribute this to a vendor-specific PSF resolution enhancement during data processing and I was willing to accept the discrepancy until I simulated the system sensitivity.

The simulated sensitivity results are ~ 35% greater than experiment as shown in the attachment. This is not what I expected since I have already validated the intrinsic count rate and verified the pinhole geometry. I seem to have run out of tunable parameters in the digitizer and adjusting the scanner's efficiency to match simulated and experimental sensitivities will invalidate the intrinsic count rate measurements.

Considering that most of the system and SPECT tests for NEMA are based on resolution and sensitivity measurements, I would like to further tune the simulation, but I am starting to walk down a dangerous path of trying to perfect things in my model. My current approach is to reduce the pinhole diameter which will improve resolution and decrease sensitivity accordingly, so my question to you is: do you think this is a suitable solution? I am hesitant to do this since the model geometry will then be incorrect although the simulation output will be more comparable to experiment. Do you have any specific ideas on what could be causing these discrepancies, namely the sensitivity issue and what would be your approach to address the issue without altering the pinhole diameter?

Thank you for taking the time to read this and providing your input.

Positively,
Matthew


--

Matthew Strugari

Biomedical Translational Imaging Centre - BIOTIC,
5890 University Ave,
Halifax, NS, B3K 6R8
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