Initial results of a mouse brain PET insert with a staggered 3-layer DOI detector.

Objective.Small animal positron emission tomography (PET) requires a submillimeter resolution for better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution while maintaining the sensitivity. Recently, we developed a staggered 3-layer DOI detector with 1 mm crystal pitch and 15 mm total crystal thickness, but we did not demonstrate the imaging performance of the DOI detector with full ring geometry. In this study we present initial imaging results obtained for a mouse brain PET prototype developed with the staggered 3-layer DOI detector.Approach.The prototype had 53 mm inner diameter and 11 mm axial field-of-view. The PET scanner consisted of 16 DOI detectors each of which had a staggered 3-layer LYSO crystal array (4/4/7 mm) coupled to a 4 × 4 silicon photomultiplier array. The physical performance was evaluated in terms of the NEMA NU 4 2008 protocol.Main Results.The measured spatial resolutions at the center and 15 mm radial offset were 0.67 mm and 1.56 mm for filtered-back-projection, respectively. The peak absolute sensitivity of 0.74% was obtained with an energy window of 400-600 keV. The resolution phantom imaging results show the clear identification of a submillimetric rod pattern with the ordered-subset expectation maximization algorithm. The inter-crystal scatter rejection using a narrow energy window could enhance the resolvability of a 0.75 mm rod significantly.Significance.In an animal imaging experiment, the detailed mouse brain structures such as cortex and thalamus were clearly identified with high contrast. In conclusion, we successfully developed the mouse brain PET insert prototype with a staggered 3-layer DOI detector.

Biological washout modelling for in-beam PET: rabbit brain irradiation by 11C and 15O ion beams.

Positron emission tomography (PET) has been used for dose verification in charged particle therapy. The causes of washout of positron emitters by physiological functions should be clarified for accurate dose verification. In this study, we visualized the distribution of irradiated radioactive beams, 11C and 15O beams, in the rabbit whole-body using our original depth-of-interaction (DOI)-PET prototype to add basic data for biological washout effect correction. Time activity curves of the irradiated field and organs were measured immediately after the irradiations. All data were corrected for physical decay before further analysis. We also collected expired gas of the rabbit during beam irradiation and the energy spectrum was measured with a germanium detector. Irradiated radioactive beams into the brain were distributed to the whole body due to the biological washout process, and the implanted 11C and 15O ions were concentrated in the regions which had high blood volume. The 11C-labelled 11CO2 was detected in expired gas under the 11C beam irradiation, while no significant signal was detected under the 15O beam irradiation as a form of C15O2. Results suggested that the implanted 11C ions form molecules that diffuse out to the whole body by undergoing perfusion, then, they are incorporated into the blood-gas exchange in the respiratory system. This study provides basic data for modelling of the biological washout effect.

Compartmental analysis of washout effect in rat brain: in-beam OpenPET measurement using a (11)C beam.

In-beam positron emission tomography (PET) is expected to enable visualization of a dose verification using positron emitters (ß+ decay). For accurate dose verification, correction of the washout of the positron emitters should be made. In addition, the quantitative washout rate has a potential usefulness as a diagnostic index, but modeling for this has not been studied yet. In this paper, therefore, we applied compartment analyses to in-beam PET data acquired by our small OpenPET prototype, which has a physically opened field-of-view (FOV) between two detector rings. A rat brain was located at the FOV and was irradiated by a (11)C beam. Time activity curves of the irradiated field were measured immediately after the irradiations, and the washout rate was obtained based on two models: the two-washout model (medium decay, k2m; slow decay, k2s) developed in a study of rabbit irradiation; and the two-compartment model used in nuclear medicine, where efflux from tissue to blood (k2), influx (k3) and efflux (k4) from the first to second compartments in tissue were evaluated. The observed k2m and k2s were 0.34 and 0.005 min(-1), respectively, which was consistent with the rabbit study. Also k2m was close to the washout rate in cerebral blood flow (CBF) measurements by dynamic PET with (15)O-water, while, k2, k3, and k4 were 0.16, 0.15 and 0.007 min(-1). Our present work suggested the dynamics of (11)C might be relevant to CBF or permeability of a molecule containing (11)C atoms might be regulated by a transporter because the k2 was relatively low compared with a simple diffusion tracer.