SIMULATING THE PROBE-AND-RING SYSTEM 87

(a) (b)

Figure 3.2: The proposed system with a Derenzo-like phantom, drawn to scale, showing a) the full probe and ring setup and b) a close up of the probe. The Si layers are shown to be transparent in b) and their pixelization is not shown.

simplifying read out of events at each of these levels. Furthermore, GATE is capable of modeling time-dependent phenomena such as de-tector movement and source decay kinetics.

GATE is intended for simulating more conventional geometries such as SPECT and PET systems and its use for simulating the full ring-and-probe system is a novel application. When simulating the probe geometry special care had to be taken when defining the GATE system and assigning its associated levels to the appropriate physical volumes. The addition of the probe to the typical PET geometry, consisting of rings of detector modules composed of scintillation crys-tals, required the use of the scanner (or PETscanner ) GATE system, which has no fixed geometry.

In GATE, the basic volume elements where information about the interactions is saved must be attached to a crystalSD (SD for

“sensitive detector”) and be part of the chosen system. It is in each of these sensitive volumes that individual interactions, or hits, are summed into a single pulse. These basic detector elements can be grouped into a volume with a single readout common to all, just as crystals in a block detector whose combined signal is read out by a group of PMTs. By attaching this volume to GATE’s hierarchal structure of levels, the summing of the signal can be performed using a single command line. The resulting pulse contains the total energy of the block and the location of the crystal with the highest energy deposit (winner-takes-all). Although only an approximation of block-detector readout, GATE’s winner-takes-all approach was used in the Biograph simulation for its convenience. At this stage, the common pulse from each block was used as the GATE singles output. The spectral and temporal resolutions of the Biograph detectors, as well as the energy thresholds, were applied to the singles output using dedicated software.

In the probe, which does not fit into GATE’s hierarchal structure, pulses from the Si-pixel level were used as output singles. No common readout was necessary in the probe simulation since every pixel in a Si pad detector is read out individually. Like in the Biograph, the spectral and temporal response of the Si detector, as well as its energy threshold, were applied to the GATE output. No model of dead time was incorporated into the simulation for either detector.

Modeling detector response outside of GATE provided the addi-tional flexibility needed to apply separate time resolutions to LSO and Si detectors. Furthermore, the application of a constant energy uncertainty to events detected in the probe (as explained in section 3.3.3) was made possible. The block effect in the Biograph detectors was applied to the singles output data in the same post-simulation process as time and energy resolutions.

3.5. SUMMARY 89 Because the probe could not be attached to the hierarchy of levels, the GATE coincidence sorter could not be used to create probe-ring coincidence events and a dedicated sorting algorithm was required.

Coincidence sorting was performed using the method by which a time window cannot be opened until the previous one has been closed. Ran-dom coincidences and those which underwent scatter in the phantom or probe are tagged and included or removed from reconstruction, according to the specific study.

3.5 Summary

In this chapter, the PET probe system used in simulation studies was described. The simulated geometry consists of a whole-body clinical PET scanner, based on the Siemens Biograph64, and a layered Si detector probe. Si was chosen due to a number of properties which make the material suitable for such an application. The geometry of the scanner, as well as energy and timing settings, were taken from the Biograph specifications and simulated using GATE, as well as post-processing of the simulated data. The probe design was based on a prototype constructed by the MADEIRA collaboration. The timing and energy resolutions of the existing device were simulated, but the number of pixels in a single detector and the number of Si layers was modified. GATE was used in a novel way to simulate the probe-and-ring system. The processing of the simulated data was also briefly described.

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Part II