3. A Depth-of-Interaction Detector for PET with GSO Crystals Doped with Different Amounts of Ce

Naoko Inadama, Hideo Murayama and Hideyuki Kawai

Keywords: depth-of-interaction detector (DOI), positron emission tomography (PET), pulse shape discrimination, GSO crystal

As one way to obtain high resolution and high sensitivity in 3D mode acquisition on PET, the concept for a depth-of-interaction (DOI) detector was proposed. In this paper, the new 4-stage DOI detector which was developed from the 3-stage DOI detector reported previously is introduced. Like the previous one, it is composed of one 16-channel (4-by-4 matrix anodes) position-sensitive photomultiplier tube (PS-PMT) and four scintillator blocks. The PS-PMT is optically coupled to the blocks by silicone oil. One block is four stage deep and one stage consists of a 2-by-2 array of rectangular Gd₂SiO₅ (GSO) crystal elements sized 2.9mm by 2.9mm by 7.5mm. Except for the fourth stage, there is a reflector between elements in the same stage; the fourth stage has an air gap instead. Each block is also wrapped in a reflector and optically isolated. Therefore scintillation light is spread over the block from the site of the interaction through the fourth stage crystals, and it enters the photo cathode of the PS- PMT. Because incident photons have a distribution corresponding to the interaction site, outputs of the four PS-PMT anode signals under the block indicate the crystal of interaction. It is identified on a 2-dimensional positioning image histogram mapped by an Anger-type position arithmetic calculation using the signals.

The 4-stage DOI detector is achieved by using two kinds of GSO crystals doped with different amounts of Ce, 0.5mol% and 1.5mol%. The scintillation decay time constant of 1.5mol% GSO crystal is 35ns and 0.5mol% GSO is 60ns. From pulse shape discrimination, it can be recognized in which kind of crystals the interaction takes place. The Anger-type position arithmetic calculation is applied to anode signals of each event data after classifying them according to the pulse shape discrimination, and two positioning contour image histograms are obtained. Setting these GSO crystal stages alternately as a 4-stage block, the performance of the detector is evaluated by irradiating a gamma ray uniformly from a 0.1mCi ¹³⁷Cs point source.

Fig. 3 (a) is the positioning image histogram of the measurement for 750k events before pulse shape discrimination and pulse height selection. Conducting pulse shape discrimination and setting windows on the energy spectra of each crystal, we obtain two 2-dimensional histograms as a result (Figs. 3 (b), (c)). The crystal of interaction can be easily identified in both histograms compared to the histogram without selecting by pulse shape and height. The accuracy of crystal identification with this 4-stage block detector is checked by scanning collimated gamma rays along the detector face. The results show that there is no significant problem for crystal identification by this method.

We conclude that the performance tests verify the 4-stage DOI detector is reliable enough for crystal identification. The detector will be investigated further while changing detector parameters such as crystal surface and size, reflector arrangement, and arrangement of two kinds of crystals in order to realize a large number of reliable detectors for the next generation PET.

Publications:
Inadama, N., et al.: 2001 IEEE NSS & MIC Conf. Rec., M2-3, 2001.

Fig.3. 2-dimensional positioning image histograms obtained in the uniform irradiation experiment of the 4-stage DOI detector with one block. The upper histogram, (a), represents all crystal data before the selection by pulse shape and height. The lower histograms express data from (b) 0.5mol% GSO crystals and (c) 1.5mol% GSO crystals.