[Castor-users] CASToR TOF reconstruction

[Philip Kalaitzidis]

Dear CASToR developers and users,

I have a question regarding an issue with CASToR TOF reconstruction.

I will give a brief background of the issue: I have modelled the G.E.
Discovery MI (DMI) PET/CT with GATE and simulated a PET acquisition with
the NEMA IQ phantom. I have converted the data to the CASToR list-mode
format and so far, there are generally no problems. However, when including
TOF in the data file I get an increased signal in the reconstructed image
in places where I do not expect, e.g., in the lung insertion, but also a
quite reduced signal in the spheres.

The TOF information for each event that I write to the binary file I simply
take the arrival time difference between crystal1 and crystal2, i.e.,
time1-time2 and then multiply the difference by 1e+12 to get the delta time
in ps. I then save the crystals in the binary file as Crystal ID1 (c1)
corresponding to GATE crystal1 and Crystal ID2 (c2) corresponding to GATE
crystal2 (since it is mentioned in the general documentation that the TOF
delta time is positive when the emission occurs closer to c2.) In the
header file I set the TOF resolution to 380 ps and the TOF measurement
range to 4900 ps. I decided to only write the true coincidences in the
binary file (together with attenuation, normalization, and the inclusion of
a span of 2 for indirect slices for segment 0) so that I could efficiently
look at the effect of TOF without having to bother with random- and scatter
correction.

If I then reconstruct the data, I notice an increased signal in the
insertion compartment, as well as a notable reduction in signal between
some of the spheres (especially visible between the largest spheres), as
opposed to if I reconstruct the data using the *-ignore-TOF *option (or
without including TOF information in neither the binary- nor the header
file.) If I reconstruct the same data but increase the TOF resolution to,
e.g., 700 ps, the signal in the insertion compartment is reduced and the
reduced signal between spheres are no longer present. (I also tested using
the GATERootToCastor converter with the TOF resolution to 380 ps yielding
similar results.)

I have attached images showing a slice of the three examples that I
mentioned (TOF included with 380 ps TOF resolution, TOF included with 700
ps TOF resolution, and with the *-ignore-TOF *option used.) I have also
included profiles through the same slices passing through two of the
spheres and the insertion compartment.

Intuitively, this seems to yield erroneous results (with 380 ps TOF
resolution), but I cannot seem to understand why this effect occurs. I was
hoping someone could help me understand what the issue might be, if I have
missed something, or whether I am on the wrong track and have misunderstood
the *“TOF-induced”* results.

I hope to hear from you.

Best regards,
*Philip*
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