Effects of multiple-interaction photon events in a high-resolution PET system that uses 3-D positioning detectors

Abstract

Purpose: The authors' laboratory is developing a dual-panel, breast-dedicated PET system. The detector panels are built from dual-LSO-position-sensitive avalanche photodiode (PSAPD) modules-units holding two 8 x 8 arrays of 1 mm3 LSO crystals, where each array is coupled to a PSAPD. When stacked to form an imaging volume, these modules are capable of recording the 3-D coordinates of individual interactions of a multiple-interaction photon event (MIPE). The small size of the scintillation crystal elements used increases the likelihood of photon scattering between crystal arrays. In this article, the authors investigate how MIPEs impact the system photon sensitivity, the data acquisition scheme, and the quality and quantitative accuracy of reconstructed PET images.

Methods: A Monte Carlo simulated PET scan using the dual-panel system was performed on a uniformly radioactive phantom for the photon sensitivity study. To establish the impact of MIPEs on a proposed PSAPD multiplexing scheme, experimental data were collected from a dual-LSO-PSAPD module edge-irradiated with a 22Na point source, the data were compared against simulation data based on an identical setup. To assess the impact of MIPEs on the dual-panel PET images, a simulated PET of a phantom comprising a matrix of hot spherical radiation sources of varying diameters immersed in a warm background was performed. The list-mode output data were used for image reconstruction, where various methods were used for estimating the location of the first photon interaction in MIPEs for more accurate line of response positioning. The contrast recovery coefficient (CRC), contrast to noise ratio (CNR), and the full width at half maximum spatial resolution of the spheres in the reconstructed images were used as figures of merit to facilitate comparison.

Results: Compared to image reconstruction employing only events with interactions confined to one LSO array, a potential single photon sensitivity gain of > 46.9% (> 115.7% for coincidence) was noted for a uniform phantom when MIPEs with summed-energy falling within a +/- 12% window around the photopeak were also included. Both experimental and simulation data demonstrate that < 0.4% of the events whose summed-energy deposition falling within that energy window interacted with both crystal arrays within the same dual-LSO-PSAPD module. This result establishes the feasibility of a proposed multiplexed readout of analog output signals of the two PSAPDs within each module. Using MIPEs with summed-energy deposition within the 511 keV +/- 12% photopeak window and a new method for estimating the location of the first photon interaction in MIPEs, the corresponding reconstructed image exhibited a peak CNR of 7.23 for the 8 mm diameter phantom spheres versus a CNR of 6.69 from images based solely on single LSO array interaction events. The improved system photon sensitivity could be exploited to reduce the scan time by up to approximately 10%, while still maintaining image quality comparable to that achieved if MIPEs were excluded.

Conclusions: MIPE distribution in the detectors allows the proposed photodetector multiplexing arrangement without significant information loss. Furthermore, acquiring MIPEs can enhance system photon sensitivity and improve PET image CNR and CRC. The system under development can therefore competently acquire and analyze MIPEs and produce high-resolution PET images.

Figure 1

Detector system overview; shown are (a) the fully assembled system comprising two opposing and parallel detector panels, (b) individual 8×8 arrays of 1 mm×1 mm×1 mm LSO crystals covered with a reflecting polymer, (c) coupling of the arrays to PSAPDs, (d) a dual-LSO-PSAPD module, (e) a sensor card assembling many adjacent modules, and (f) a detector head/panel assembled by stacked sensor cards, which, in turn, are coupled to readout boards. In this manner, incoming photons encounter a minimum of ∼2 cm thick LSO crystal.

Figure 2

Proposed analog signal multiplexing scheme (right) for the dual-LSO-PSAPD module (left) (Ref. 14).

Figure 3

Simulation breast slab phantom geometry with 4 cm panel separation.

Figure 4

Photon interaction category definitions. Note that specific “O” arrays, designating “Other crystal arrays” involved in the event, are highlighted for illustration only in defining the categories.

Figure 5

Edge-on ²²Na irradiation of a one dual-LSO-PSAPD module. Each LSO array comprises an 8×8 matrix of 1 mm×1 mm×1 mm crystal elements.

Figure 6

Phantom used for reconstructed image quality and quantification study. The ratio of the activity concentration in the spheres to that of the surrounding water is 10:1, and the total activity is 800 μCi.

Figure 7

Percentage of single photon events interacting with a dual-LSO-PSAPD module at the panel corner and center (see Fig. 4) with the interaction energies summing to a 24% window about 511 keV acquired as a function of minimum energy per interaction needed to trigger an acquisition. Data plotted as a fraction of (a) all single photons that interacted at least once with the LSO detector volume and (b) events with sum of energy depositions falling within the 511 keV photopeak.

Figure 8

Experimental vs GRAY simulation result comparison of photon interaction categories (see Fig. 4 for the definitions, categories E and D are not applicable here for the case of edge-on irradiation of one dual-LSO-PSAPD module). Note that the vertical axis is in logarithmic scale, and 100% corresponds to all events with sum energy within a 511 keV±20% window. The statistical uncertainty in the results is shown as error bars on each bar, indicating the 90% confidence interval.

Figure 9

Population makeup of coincident events used in image reconstruction. The numbers inside the bars represent counts of that event type in millions. The lighter patches represent coincident photons pairs where either or both individual photon events are a MIPE.

Figure 10

Contrast recovery coefficient and noise of reconstructed volume over successive MLEM iterations up to a maximum of 100 iterations. Subplots (a), (b), (c), and (d) correspond to 1, 2, 4, and 8 mm diameter spheres in the phantom, respectively.

Figure 11

Contrast to noise ratio as a function of number of MLEM iterations. Subplots (a), (b), (c), and (d) correspond to CNR derived from 1, 2, 4, and 8 mm diameter spheres in the phantom, respectively.

Figure 12

Comparison of images reconstructed using (a) only single LSO array events that deposit energy within the 511 keV±12% window, (b) events used in (a) as well as MIPEs using the energy-weighted spatial mean positioning method for placing the LOR, and (c) the same event set as (b) but using the ML algorithm for estimating the initial interaction in each MIPE for positioning the LOR. (d) shows the reconstructed volume of (c) in a slice orthogonal to the panels, spanning the 4 cm panel separation. Orientation of the axes corresponds to that of Figs. 13. The sphere diameters are 1, 2, 4, and 8 mm, with mutual separation of at least twice the diameter, and have activity concentration ten times that of the surrounding medium. The total activity of the entire volume is 800 μCi and all images were normalized to account for system photon sensitivity nonuniformity.

Figure 13

(a) Contrast recovery coefficient-noise curves of images reconstructed using the single LSO array data for 100% scan time compared to those based on the MIPEs-ML method over 90% scan time. Graph shows the CRC and noise values derived from the 2 mm diameter spheres. Curves for the 4 and 8 mm diameter spheres exhibit similar behavior. (b) Contrast to noise ratio as a function of the number of MLEM iterations. Plotted are curves derived from the 2 mm diameter spheres reconstructed using the single LSO array data for the full scan time and using the MIPEs-ML method over 90% of the scan time.

Figure 14

Confounding factors to the MIPEs-ML LOR positioning algorithm. Note that occurrences in only two dimensions are shown for simplicity.