One of the first attempts to obtain DOI information was using pulse shape discrimination (PSD): two or more scintillation crystals with different decay times are coupled together. Based on the pulse shape observed, estimations are made whether the interaction happened in the top or the bottom crystal. The design is schematically represented in Fig. 28.5a. These configurations are also known as “phoswich” configurations, a combination of “phosphor” and “sandwich”. Phoswich designs originate from X- and gamma-ray astronomy experiments [27].

The PSD approach is still being investigated as a method to improve DOI in the next-generation scanners. Mosset et al. [28] built an LSO/LuYAP combination which is 98 % efficient in determining the correct interaction location. As shown in simulations by Jung et al. [29], identification may be improved depending on the optical coupling between different crystals. More recently, Vaquero et al. [30] investigated a combination of LYSO and GSO coupled to a matrix of silicon photomultipliers (Si-PMs, see Sect. 28.4.3) and evaluated a delayed charge integration method.

While several research groups have reported low misidentification and high efficiency, the PSD approach has a number of drawbacks. One problem is that the pulse shape needs to be determined, which increases requirements on the readout electronics. Another disadvantage is that optical photons created in the first scintillator need to propagate through the second scintillator in order to be detected. This may cause a dispersion in pulse shape and degrade the timing resolution achievable with this design. The different light output and associated differences in energy resolution of the crystals used in a PSD design may also introduce nonuniformities in scatter rejection capabilities.

It is worth noting that PSD has also been used in the LabPET scanner [31] in order to reduce the number of electronic readout channels. In this design, two crystals with different decay times are positioned next to each other and are coupled to the same photosensor, thus multiplexing the readout by a factor of two.

In order to circumvent the discrete nature of the phoswich approach, Du et al. [32] proposed a continuous phoswich crystal, whereby the far end and part of the sides of a crystal are coated with a phosphor powder. The phosphor re-emits absorbed scintillation light at a later time point and at a lower frequency. Using different pulse shape discrimination and advanced analysis techniques [33], continuous DOI can be obtained. The authors obtained an 8 mm FWHM DOI resolution for a 2-cm-thick array.

A solution to the different response from the different scintillators in a phoswich design is to use a stack of the same scintillator material, albeit with an offset. The offset causes the optical light created in the different layers to follow different paths, resulting in different patterns on the photodetector surface, as shown in Fig. 28.5b. The location of the focal light spot on the photodetector surface thus gives DOI information. Liu et al. [34] stacked a 6 × 6 array of 1.8 × 1.8 × 10 mm³ LSO crystals on top of a 7 × 7 array of the same crystals. The lower array was coupled to a position-sensitive PMT (PS-PMT). Using this approach, the authors observed an improvement from 4.5 mm FWHM to 3.4 mm FWHM at 30° photon incidence, with uniform energy and timing resolution. Zhang et al. [35] stacked a 9 × 9 layer of 2.1 × 2.1 × 6.5 mm³ BGO crystals on top of a 10 × 10 array of 2.1 × 2.1 × 11.5 mm³ BGO crystals, though with moderate crystal identification performance. Ito et al. [36] used a similar approach to stack 4 layers of 29 × 29, 29 × 28, 28 × 28, and 28 × 27 LSO crystals of 1.5 × 1.5 × 7 mm³ each. The second layer was offset by half a pitch in the X direction, the third by half a pitch in the Y direction, and the fourth by an additional half pith in the X direction. Even and odd rows in the flood histogram would thus correspond to even and odd layers. Each individual crystal was wrapped with a reflector. Crystal resolving power in individual layers was excellent, yet the different DOI layers overlapped at the edges of the flood and thus were inseparable, presumably due to loss of photon collection efficiency at the edge. In these designs, the array closest to the photodetector is referred to as the “frontal” layer, and the one further away from the photodetector is the “distal” layer.

While the crystal offset method bypasses some of the disadvantages of the phoswich design, it has its own drawbacks. An offset method will result in nonuniform sensitivity as inevitably the second “distal” layer has less solid angle coverage than the first. A bigger problem is the light collection problem from the distal layer, similar to that of the phoswich design, but worse since light collection for the back crystal elements are split between two front element crystals. In addition, it is often hard to reliable distinguish both layers, in particular for incident photon angles larger than 0°, the angles for which DOI improvement is relevant.

In an attempt to improve the nonuniform sensitivity caused by the crystal offset method, various researchers have experimented using different reflector arrangements for different layers, instead of physically offsetting different crystal layers. This method also results in a distinct pattern on the photodetector surface, as shown in Fig. 28.5c. Different reflector configurations are schematically drawn with full and dotted lines in the figure. The crystal location pattern observed on the photodetector is not equidistant in each row, as opposed to the offset design.

Using the light sharing method, Tsuda et al. [37] made a stack of 4 layers of GSO crystals each measuring 1.42 × 1.42 × 4.5 mm³. Each stack contained a different arrangement of light reflectors, so that a different pattern appears on the surface of the photodetector for each individual layer. They were able to achieve separation between the four layers, but with quite significant overlap, leading to layer misidentification.

Inadama et al. [38] used a combination of the light sharing and the phoswich approach to build an 8-layer DOI detector, organized as a 4-layer stack of a phoswich of arrays of GSO crystals with 0.5 mol% Ce dopant and GSO crystals with 1.5 mol% Ce. Each phoswich layer had a different reflector orientation. The authors were able to separate all layers, but also observed large overlap and hence misidentification.

The previously discussed DOI approaches all feature the drawback that the dispersion in the optical photons from the most distal layer will be larger than the dispersion from the proximal crystals, potentially compromising energy and timing resolution of those distal layers. A solution to this problem could be to collect the scintillation light on both sides of the crystal using two photodetectors. Depth information is obtained by the ratio of the signal collected in both photodetectors, as shown in Fig. 28.5d. Dual-ended readout was already proposed in 1988 [39]. These authors placed PMTs in the transaxial direction on both ends of a array of BGO crystals. The setup thus had a radial length of . They were able to obtain a FWHM of 9.5 mm across the 50 mm transaxial direction when irradiating at 0.5 mm intervals.

The development of small silicon-based avalanche photodiodes (APDs) as a replacement for PMTs allowed dual readout in the radial direction. Such an approach was used by the ClearPEM group [40] for use in a dedicated breast PET camera. These researchers coupled two APDs to a array of LSO crystals. The introduction of the APD and a thin circuit board between the LSO crystals and the 511-keV source only introduces minimal scatter.

James et al. [41] used position-sensitive APDs (PSAPDs) to read out both sides of an array comprising LSO crystals. A DOI resolution of was obtained at a temperature of 0 °C in a design intended to be used for small animal imaging.

The dual-ended readout seems a promising solution to estimate DOI. However, it comes with a number of drawbacks: the number of photodetectors needs to be doubled and positioning these inside the photon path will introduce additional scatter. Moreover, the continuous approach currently only estimates where the interaction happened, which yields an uncertainty in the DOI coordinate and hence a potentially large misidentification.

Instead of making an estimate of the interaction location using the ratio of the charge collected at both ends of a scintillation crystal, the AXPET design [42] placed a number of wavelength shifting strip (WLS) underneath an array of LYSO crystals. The WLS measured . Both scintillation crystal and WLS are read out by a Si-PM. The authors measured an axial resolution of less than 1.7 mm. A similar approach for a small animal PET camera was used in the COMPET design [43]. The drawback of this approach is the need for photodetectors on four sides of the “block,” and the low light yield of the WLS, potentially reducing the efficiency of the method.

In order to obtain direct interaction location measurements in three dimensions, Yamaya et al. [44] built a cube of LGSO crystals (X’tal cube). Each of the six faces of the cube was coupled to a array of Si-PMs. Almost all of the 4,096 crystals were identified in the flood map, albeit with cross talk at the edge and cross talk in the center. The fabrication was improved using a laser to cut microcracks in an LYSO block, resulting in a 3D grid [45]. A grid with 2 mm pitch was produced using this method. An intrinsic resolution of 2.1 mm was obtained. The design of the X’tal cube is shown in Fig. 28.5e. The obtained flood histogram is actually a 3D histogram.

Another approach for 3D imaging was first proposed conceptually by Levin [46]. In this approach, an array of crystals is coupled to a thin APD. Every layer is directly read out by a photodetector, as shown in Fig. 28.5f. A stack of these detectors forms a 3D block detector. In this particular design, scintillation photons are collected from the sides rather than ends of long scintillation crystal elements, increasing the light yield and removing the dependence of light collection on DOI. Vandenbroucke et al. [47] measured an intrinsic spatial resolution of , using an array of LSO crystals coupled to a PSAPD. The PSAPDs themselves are mounted on a thin Kapton readout circuit in order to decrease the dead area [48]. Because of the small crystals used in this design, and their readout in small cohorts of two arrays, each individual interaction location of a multiple photon interaction events (MIPE) can be identified. These are events whereby the 511 keV photon Compton scatters once or more than once before it undergoes photoelectric absorption. In order to include these MIPE, the readout electronics need to be carefully designed, so that the trigger threshold can be set as low as possible. A feasibility study of measuring these MIPE events in this stacked design is shown in Ref. [49]. This design is currently incorporated in the construction of a high-resolution breast PET camera [50].