title: Depth of interaction determination in monolithic scintillator with double side SiPM readout.
abstract: BACKGROUND
  Monolithic scintillators read out by arrays of photodetectors represent a promising solution to obtain high spatial resolution and the depth of interaction (DOI) of the annihilation photon. We have recently investigated a detector geometry composed of a monolithic scintillator readout on two sides by silicon photomultiplier (SiPM) arrays, and we have proposed two parameters for the DOI determination: the difference in the number of triggered SiPMs on the two sides of the detector and the difference in the maximum collected signal on a single SiPM on each side. This work is focused on the DOI calibration and on the determination of the capability of our detector. For the DOI calibration, we studied a method which can be implemented also in detectors mounted in a full PET scanner. We used a PET detector module composed of a monolithic 20 × 20 × 10 mm3LYSO scintillator crystal coupled on two opposite faces to two arrays of SiPMs. On each side, the scintillator was coupled to 6 × 6 SiPMs. In this paper, the two parameters previously proposed for the DOI determination were calibrated with two different methods. The first used a lateral scan of the detector with a collimated 511 keV pencil beam at steps of 0.5 mm to study the detector DOI capability, while the second used the background radiation of the176Lu in the scintillator. The DOI determination capability was tested on different regions of the detector using each parameter and the combination of the two.
  
  
  RESULTS
  With both parameters for the DOI determination, in the lateral scan, the bias between the mean reconstructed DOI and the real beam position was lower than 0.3 mm, and the DOI distribution had a standard deviation of about 1.5 mm. When using the calibration with the radioactivity of the LYSO, the mean bias increased of about 0.2 mm but with no degradation of the standard deviation of the DOI distribution.
  
  
  CONCLUSIONS
  The two parameters allow to achieve a DOI resolution comparable with the state of the art, giving a continuous information about the three-dimensional interaction position of the scintillation. These results were obtained by using simple estimators and a detector scalable to a whole PET system. The DOI calibration obtained using lutetium natural radioactivity gives results comparable to the other standard method but appears more readily applicable to detectors mounted in a full PET scanner.
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy. matteo.morrocchi@pi.infn.it.
authors.forename: Matteo
authors.identifier: http://orcid.org/0000-0001-6636-3849
authors.initials: M
authors.lastname: Morrocchi
authors.affiliation: INFN, Sezione di Perugia, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Ambrosi
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Maria Giuseppina
authors.initials: MG
authors.lastname: Bisogni
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Filippo
authors.initials: F
authors.lastname: Bosi
authors.affiliation: University of Torino and INFN, Sezione di Torino, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Boretto
authors.affiliation: INFN, Sezione di Torino, Italy.
authors.forename: Piergiorgio
authors.initials: P
authors.lastname: Cerello
authors.affiliation: INFN, Sezione di Perugia, Italy.
authors.forename: Maria
authors.initials: M
authors.lastname: Ionica
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Ben
authors.initials: B
authors.lastname: Liu
authors.affiliation: University of Torino and INFN, Sezione di Torino, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Pennazio
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Maria Antonietta
authors.initials: MA
authors.lastname: Piliero
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Pirrone
authors.affiliation: INFN, Sezione di Perugia, Italy.
authors.forename: Vasile
authors.initials: V
authors.lastname: Postolache
authors.affiliation: INFN, Sezione di Torino, Italy.
authors.forename: Richard
authors.initials: R
authors.lastname: Wheadon
authors.affiliation: University of Pisa and INFN, Sezione di Pisa, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Del Guerra
keywords: Depth of interaction; Monolithic scintillator; Positron emission tomography; Silicon photomultiplier