Reducing Calibration Time in PET Systems Based on Monolithic Crystals

In the past years, the gamma-ray detector designs based on the monolithic crystals have demonstrated to be excellent candidates for the design of high-performance PET systems. The monolithic crystals allow to achieve the intrinsic detector resolutions well below state-of-the-art; to increase packing fraction thus, increasing the system sensitivity; and to improve lesion detectability at the edges of the scanner field of view (FOV) because of their intrinsic depth of interaction (DOI) capabilities. The bottleneck to translate to the clinical PET systems based on a large number of monolithic detectors is eventually the requirement of mechanically complex and time-consuming calibration processes. To mitigate this drawback, several methods have been already proposed, such as using non-physically collimated radioactive sources or implementing the neuronal networks (NN) algorithms trained with simulated data. In this work, we aimed to simplify and fasten a calibration process of the monolithic based systems. The Normal procedure consists of individually acquiring a 11 × 11 22Na source array for all the detectors composing the PET system and obtaining the calibration map for each module using a method based on the Voronoi diagrams. Two reducing time methodologies are presented: (i) TEST1, where the calibration map of one detector is estimated and shared among all others, and (ii) TEST2, where the calibration map is slightly modified for each module as a function of their detector uniformity map. The experimental data from a dedicated prostate PET system was used to compare the standard calibration procedure with both the proposed methods. A greater similarity was exhibited between the TEST2 methodology and the Normal procedure; obtaining spatial resolution variances within 0.1 mm error bars and count rate deviations as small as 0.2%. Moreover, the negligible reconstructed image differences (13% deviation at most in the contrast-to-noise ratio) and almost identical contrast values were reported. Therefore, this proposed method allows us to calibrate the PET systems based on the monolithic crystals reducing the calibration time by approximately 80% compared with the Normal procedure.

RADT-12. USE OF FDOPA PET FOR RADIATION THERAPY TARGETING IN GRADE 2 IDH MUTANT GLIOMA

BACKGROUND Low-grade, IDH mutant (IDHmt) gliomas typically do not enhance on MRI complicating radiotherapy (RT) target delineation. Amino acid PET using 3,4-dihydroxy-6-[18F]-fluoro-L-phenylalanine (FDOPA) has demonstrated avidity in IDHmt gliomas and may assist in RT planning for non-enhancing tumors. This study aims to compare conventional and FDOPA-defined target volumes in grade 2 IDHmt gliomas. METHODS In a prospective pilot study, patients underwent MRI and FDOPA PET using a 3T MRI/PET system followed by standard therapy. Gross tumor volumes (GTV) included the T2/FLAIR abnormality and surgical cavity; clinical target volumes (CTV) included a 1 cm expansion constrained anatomically. Metabolic target volumes (MTVs) were generated using the FDOPA SUV > 1.5-fold normal brain isocurve. Union of GTV and MTV generated a fusion GTV (fGTV); expanding fGTV by 1 cm yielded the fusion CTV (fCTV). Target volumes were compared volumetrically with overlap (Dice coefficient) and surface metrics (Hausdorff distance). Medians are reported with ranges. RESULTS Four patients with grade 2 IDHmt glioma (3 1p/19q codeleted oligodendrogliomas, 1 non-codeleted astrocytoma) received MRI/PET before treatment. All oligodendrogliomas exhibited FDOPA avidity; the astrocytoma showed no avidity. GTV and CTV measured 16.1 cc (4.9 - 82.2 cc) and 76.7 cc (29.5 - 256.1 cc), respectively. The MTV volume outside of GTV was 0.8 cc (0.2 – 6.1 cc), but was covered in each case by the CTV. Addition of FDOPA increased fGTV and fCTV volumes by 5.4% and 17.5%, respectively. Dice coefficient and Hausdorff distances for GTV vs fGTV were 0.96 (0.95 - 0.99) and 11.2 mm (10.0 – 11.9 mm), respectively, and for CTV vs fCTV were 0.87 (0.81 – 0.95) and 10.2 mm (10.0 - 11.0), respectively. CONCLUSIONS FDOPA PET identified tracer-avid regions outside of MRI-defined GTVs in a group of IDHmt gliomas. FDOPA PET provides useful metabolic information for RT planning and warrants further investigation.

[18F]FDG-labelled stem cell PET imaging in different route of administrations and multiple animal species

Stem cell therapy holds great promise for tissue regeneration and cancer treatment, although its efficacy is still inconclusive and requires further understanding and optimization of the procedures. Non-invasive cell tracking can provide an important opportunity to monitor in vivo cell distribution in living subjects. Here, using a combination of positron emission tomography (PET) and in vitro 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) direct cell labelling, the feasibility of engrafted stem cell monitoring was tested in multiple animal species. Human mesenchymal stem cells (MSCs) were incubated with phosphate-buffered saline containing [18F]FDG for in vitro cell radiolabelling. The pre-labelled MSCs were administrated via peripheral vein in a mouse (n = 1), rats (n = 4), rabbits (n = 4) and non-human primates (n = 3), via carotid artery in rats (n = 4) and non-human primates (n = 3), and via intra-myocardial injection in rats (n = 5). PET imaging was started 10 min after cell administration using a dedicated small animal PET system for a mouse and rats. A clinical PET system was used for the imaging of rabbits and non-human primates. After MSC administration via peripheral vein, PET imaging revealed intense radiotracer signal from the lung in all tested animal species including mouse, rat, rabbit, and non-human primate, suggesting administrated MSCs were trapped in the lung tissue. Furthermore, the distribution of the PET signal significantly differed based on the route of cell administration. Administration via carotid artery showed the highest activity in the head, and intra-myocardial injection increased signal from the heart. In vitro [18F]FDG MSC pre-labelling for PET imaging is feasible and allows non-invasive visualization of initial cell distribution after different routes of cell administration in multiple animal models. Those results highlight the potential use of that imaging approach for the understanding and optimization of stem cell therapy in translational research.

Prospective study of dynamic whole-body 68Ga-DOTATOC-PET/CT acquisition in patients with well-differentiated neuroendocrine tumors

To present the feasibility of a dynamic whole-body (DWB) 68Ga-DOTATOC-PET/CT acquisition in patients with well-differentiated neuroendocrine tumors (WD-NETs). Sixty-one patients who underwent a DWB 68Ga-DOTATOC-PET/CT for a histologically proven/highly suspected WD-NET were prospectively included. The acquisition consisted in single-bed dynamic acquisition centered on the heart, followed by the DWB and static acquisitions. For liver, spleen and tumor (1–5/patient), Ki values (in ml/min/100 ml) were calculated according to Patlak's analysis and tumor-to-liver (TLR-Ki) and tumor-to-spleen ratios (TSR-Ki) were recorded. Ki-based parameters were compared to static parameters (SUVmax/SUVmean, TLR/TSRmean, according to liver/spleen SUVmean), in the whole-cohort and according to the PET system (analog/digital). A correlation analysis between SUVmean/Ki was performed using linear and non-linear regressions. Ki-liver was not influenced by the PET system used, unlike SUVmax/SUVmean. The regression analysis showed a non-linear relation between Ki/SUVmean (R2 = 0.55,0.68 and 0.71 for liver, spleen and tumor uptake, respectively) and a linear relation between TLRmean/TLR-Ki (R2 = 0.75). These results were not affected by the PET system, on the contrary of the relation between TSRmean/TSR-Ki (R2 = 0.94 and 0.73 using linear and non-linear regressions in digital and analog systems, respectively). Our study is the first showing the feasibility of a DWB 68Ga-DOTATOC-PET/CT acquisition in WD-NETs.