Twilite Calibration

In experiments using the twilite and a PET system is necessary to cross-calibrate the two devices. This means, that the count-rate of coincidences which the twilite measures for a tracer concentration in the catheter needs to be converted into the activity concentration which the PET measures for the same tracer concentration in the field-of-view.

In principle, the following is done for the calibration:

1. PET tracer is added to water such that the activity concentration is in the range of 200-500 kBq/cc. Note that the same isotope should be used as in the actual live experiments. The reason is the variability of the branching factor, which is accounted for during the calculation of the tracer concentration by the PET system. While the standard PET isotopes have branching factors above 0.95, is is lower for Cu-64 (0.174) and Ga-68 (0.891).
2. A catheter of the type used in the live experiment is filled with the fluid.
3. A PET phantom is filled with the same fluid.
4. The catheter is measured in the twilite, producing the rate of coincidences in counts/sec. The geometry of the catheter loop in the measuring head needs to be exactly the same as in the experiment. This is achieved by inlaying the catheter in a precision templates which Swisstrace delivers for the different catheter diameters.
5. At the same time the phantom is measured in the PET system, the data corrected and reconstructed in the same was as in the live experiment, resulting in a tracer concentration in kBq/cc.
6. The calibration factor is finally calculated by diving the PET concentration by the twilite count-rate:
   F = C_PET/R_twilite
   and has units (kBq/cc)/(counts/sec).

In practice, the calibration has to cope with the following challenges:

• The twilite has an intrinsic number of background counts due to the radioactivity in the LYSO scintillators. This background has to be subtracted from the measurement. The background can either be measured in a separate twilite acquisition with an empty catheter in place, or by a step acquisition which starts with an empty catheter, which is later filled during the acquisition by flushing the catheter.
• The radioactivity decays continually, which has to be compensated for. PET systems correct this decay to the start of the acquisition. Therefore, the twilite activity also needs to be corrected to the same time. This is particularly relevant if there is an offset between the PET and the twilite calibration measurements.

Recommendation: Although it was found that the calibration factor is quite stable over time, it is recommend to perform a new calibration for every live experiment.