Evaluation of image quality with four positron emitters and three preclinical PET/CT systems.

BACKGROUND: We investigated the image quality of 11C, 68Ga, 18F and 89Zr, which have different positron fractions, physical half-lifes and positron ranges. Three small animal positron emission tomography/computed tomography (PET/CT) systems were used in the evaluation, including the Siemens Inveon, RAYCAN X5 and Molecubes ß-cube. The evaluation was performed on a single scanner level using the national electrical manufacturers association (NEMA) image quality phantom and analysis protocol. Acquisitions were performed with the standard NEMA protocol for 18F and using a radionuclide-specific acquisition time for 11C, 68Ga and 89Zr. Images were assessed using percent recovery coefficient (%RC), percentage standard deviation (%STD), image uniformity (%SD), spill-over ratio (SOR) and evaluation of image quantification. RESULTS: 68Ga had the lowest %RC (< 62%) across all systems. 18F had the highest maximum %RC (> 85%) and lowest %STD for the 5 mm rod across all systems. For 11C and 89Zr, the maximum %RC was close (> 76%) to the %RC with 18F. A larger SOR were measured in water with 11C and 68Ga compared to 18F on all systems. SOR in air reflected image reconstruction and data correction performance. Large variation in image quantification was observed, with maximal errors of 22.73% (89Zr, Inveon), 17.54% (89Zr, RAYCAN) and - 14.87% (68Ga, Molecubes). CONCLUSIONS: The systems performed most optimal in terms of NEMA image quality parameters when using 18F, where 11C and 89Zr performed slightly worse than 18F. The performance was least optimal when using 68Ga, due to large positron range. The large quantification differences prompt optimization not only by terms of image quality but also quantification. Further investigation should be performed to find an appropriate calibration and harmonization protocol and the evaluation should be conducted on a multi-scanner and multi-center level.

Intubation-free in vivo imaging of the tracheal mucosa using two-photon microscopy.

The mucosal layer of conducting airways is the primary tissue exposed to inhaled microorganisms, allergens and pollutants. We developed an in vivo two-photon microscopic approach that allows performing dynamic imaging studies in the mouse trachea, which is a commonly used in vivo model of human small-diameter bronchi. By providing stabilized access to the tracheal mucosa without intubation, our setup uniquely allows dynamic in vivo imaging of mucociliary clearance and steady-state immune cell behavior within the complex airway mucosal tissue.

64Cu- and 68Ga-labelled [Nle(14),Lys(40)(Ahx-NODAGA)NH2]-exendin-4 for pancreatic beta cell imaging in rats.

PURPOSE: Glucagon-like peptide-1 receptor (GLP-1R) is a molecular target for imaging of pancreatic beta cells. We compared the ability of [Nle(14),Lys(40)(Ahx-NODAGA-(64)Cu)NH2]-exendin-4 ([(64)Cu]NODAGA-exendin-4) and [Nle(14),Lys(40)(Ahx-NODAGA-(68)Ga)NH2]-exendin-4 ([(68)Ga]NODAGA-exendin-4) to detect native pancreatic islets in rodents. PROCEDURES: The stability, lipophilicity and affinity of the radiotracers to the GLP-1R were determined in vitro. The biodistribution of the tracers was assessed using autoradiography, ex vivo biodistribution and PET imaging. Estimates for human radiation dosimetry were calculated. RESULTS: We found GLP-1R-specific labelling of pancreatic islets. However, the pancreas could not be visualised in PET images. The highest uptake of the tracers was observed in the kidneys. Effective dose estimates for [(64)Cu]NODAGA-exendin-4 and [(68)Ga]NODAGA-exendin-4 were 0.144 and 0.012 mSv/MBq, respectively. CONCLUSION: [(64)Cu]NODAGA-exendin-4 might be more effective for labelling islets than [(68)Ga]NODAGA-exendin-4. This is probably due to the lower specific radioactivity of [(68)Ga]NODAGA-exendin-4 compared to [(64)Cu]NODAGA-exendin-4. The radiation dose in the kidneys may limit the use of [(64)Cu]NODAGA-exendin-4 as a clinical tracer.

Effect of polystyrene microsphere surface to fluorescence lifetime under two-photon excitation.

Molecular assays such as immunoassays are often performed using solid carriers and fluorescent labels. In such an assay format a question can be raised on how much the fluorescence of the label is influenced by the bio-affinity binding events and the solid carrier surface. Since changes in fluorescence intensity as labels bind to surfaces are notoriously difficult to quantify other approaches are preferred. A good indicator, independent of the fluorescence intensity of the label, is the fluorescence lifetime of the marker fluorophore. Changes in fluorescence lifetime reliably indicate the presence of dynamic quenching, energy transfer or other de-excitation processes. A microsphere based assay system is studied under two-photon excitation. Changes in fluorescence lifetime are studied as labeled protein conjugates bind on microsphere surfaces--both direct on the surface and with a few nanometer distance from the surface. Fluorescence signal is measured from individual polystyrene microspheres and the fluorescence lifetime histogram is simultaneously recorded. The results indicate that self-quenching and quenching by the polystyrene surface are both present in such a system. However, the effect of the surface can be avoided by increasing the distance between the surface and the label. Typical distances achieved by a standard sandwich type of assay, are already sufficient to overcome the surface induced quenching in fluorescence detection.

Two-photon excited fluorescence energy transfer: a study based on oligonucleotide rulers.

The use of two-photon excitation of fluorescence for detection of fluorescence resonance energy transfer (FRET) was studied for a selected fluorescent donor-acceptor pair. A method based on labeled DNA was developed for controlling the distance between the donor and the acceptor molecules. The method consists of hybridization of fluorescent oligonucleotides to a complementary single-stranded target DNA. As the efficiency of FRET is strongly distance dependent, energy transfer does not occur unless the fluorescent oligonucleotides and the target DNA are hybridized. A high degree of DNA hybridization and an excellent FRET efficiency were verified with one-photon excited fluorescence studies. Excitation spectra of fluorophores are usually wider in case of two-photon excitation than in the case of one-photon excitation. This makes the selective excitation of donor difficult and might cause errors in detection of FRET with two-photon excited fluorescence. Different techniques to analyze the FRET efficiency from two-photon excited fluorescence data are discussed. The quenching of the donor fluorescence intensity turned to be the most consistent way to detect the FRET efficiency. The two-photon excited FRET is shown to give a good response to the distance between the donor and the acceptor molecules.

Two-photon excitation in fluorescence polarization receptor-ligand binding assay.

Fluorescence polarization is one of the most commonly used homogeneous assay principles in drug discovery for screening of potential lead compounds. In this article, the fluorescence polarization technique is combined with 2-photon excitation of fluorescence. Theoretically, the use of 2-photon excitation of fluorescence increases the volumetric sensitivity and polarization contrast of fluorescence polarization assays. The work in this report demonstrates these predictions for an estrogen receptor ligand binding assay.