Jurgen Seidel

Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [(18)F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [(18)F]tetrabutylammonium fluoride ([(18)F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [(18)F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH9) for 15 min at 37-40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18-35% (n=30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [(18)F]RSA is an effective blood pool imaging agent in rats and might, as [(18)F]HSA, prove similarly useful as a clinical imaging agent.

Performance characteristics of position-sensitive photomultiplier tubes combined through common X and Y resistive charge dividers

Combining signal channels from detector arrays can improve event throughput and minimize cost but, potentially, at the expense of other performance parameters. In this work, the authors evaluated a method to reduce signal number by combining the anode outputs of up to three position-sensitive photomultiplier tubes (PSPMTs) through a common X and a common Y resistive charge divider. Field flood images of these combined modules were compared to those obtained with independent detectors illuminated individually. No significant reduction in position detection accuracy was observed at low count rates when up to three tubes were combined. Event mis-positioning was also minimal for total count rates <100,000 cps. At higher rates, pulse pileup degraded accuracy. These results suggest that this method of combining PSPMTs may be useful in reducing signal number while maintaining good event localization at reasonable count rates.

A depth-encoding PET detector module with improved spatial sampling

Detector modules in small ring diameter PET scanners must possess depth-of-interaction (DOI) encoding, increased spatial sampling, high sensitivity and the ability to handle high photon input rates without excessive pulse pileup or random coincidences. We created such a module by optically coupling an entrance array of individual LGSO crystals to an exit array of individual GSO (and other) crystals that was, in turn, optically and directly coupled to a miniature PSPMT. DOI was determined for each event by delayed charge integration (DCI), a technique that exploits differences in light decay time between GSO and LGSO. Spatial sampling in 3D was increased by introducing a half crystal pitch spatial offset between the entrance and exit arrays in both the X and Y directions. Position detection accuracy in both the LGSO and GSO layers, and the accuracy of DOI assignment of events to either layer was high. These results suggest that this combination of scintillators and acquisition/processing methods may be particularly useful in the design of high performance, small ring diameter PET scanners for small animal imaging.

One-pot synthesis and biodistribution of fluorine-18 labeled serum albumin for vascular imaging

INTRODUCTION Equilibrium single-photon radionuclide imaging methods for assessing cardiac function and the integrity of the vascular system have long been in use for both clinical and research purposes. However, positron-emitting blood pool agents that could provide PET equivalents to these (and other) clinical procedures have not yet been adopted despite technical imaging advantages offered by PET. Our goal was to develop a PET blood pool tracer that not only meets necessary in vivo biological requirements but can be produced with an uncomplicated and rapid synthesis method which would facilitate clinical translation. Herein, albumin labeled with fluorine-18 was synthesized using a one-pot method and evaluated in vitro and in vivo in rats. METHODS A ligand (NODA-Bz-TFPE), containing NODA attached to a tetrafluorophenylester (TFPE) via a phenyl linker (Bz), was labeled with aluminum fluoride (Al[18F]F). Conjugation of the serum albumin with the ligand (Al[18F]F-NODA-Bz-TFPE), followed by purification (size exclusion chromatography), yielded the final product (Al[18F]F-NODA-Bz-RSA/HSA). In vitro stability was evaluated in human serum albumin by HPLC. Rat biodistributions and whole-body PET imaging over a 4 h time course were used for the in vivo evaluation. RESULTS This synthesis exhibited an overall radiochemical yield of 45 ± 10% (n = 30), a 50-min radiolabeling time, a radiochemical purity >99% and apparent stability up to 4 h in human serum. Blood had the highest retention of Al[18F]F-NODA-Bz-RSA at all times with a blood half-life of 5.2 h in rats. Al[18F]F-NODA-Bz-RSA distribution in most rat tissues remained relatively constant for up to 1 h, indicating that the tissue radioactivity content represents the respective tissue plasma volume. Dynamic whole-body PET images were in agreement with these findings. CONCLUSIONS A new ligand has been developed and radiolabeled with Al[18F]F that allows rapid (50-min) preparation of fluorine-18 serum albumin in one-pot. In addition to increased synthetic efficiency, the construct appears to be metabolically stable in rats. This method could encourage wider use of PET to quantify cardiac function and tissue vascular integrity in both research and clinical settings.

A simple device to convert a small-animal PET scanner into a multi-sample tissue and injection syringe counter

INTRODUCTION We describe a simple fixture that can be added to the imaging bed of a small-animal PET scanner that allows for automated counting of multiple organ or tissue samples from mouse-sized animals and counting of injection syringes prior to administration of the radiotracer. The combination of imaging and counting capabilities in the same machine offers advantages in certain experimental settings. METHODS A polyethylene block of plastic, sculpted to mate with the animal imaging bed of a small-animal PET scanner, is machined to receive twelve 5-ml containers, each capable of holding an entire organ from a mouse-sized animal. In addition, a triangular cross-section slot is machined down the centerline of the block to secure injection syringes from 1-ml to 3-ml in size. The sample holder is scanned in PET whole-body mode to image all samples or in one bed position to image a filled injection syringe. Total radioactivity in each sample or syringe is determined from the reconstructed images of these objects using volume re-projection of the coronal images and a single region-of-interest for each. We tested the accuracy of this method by comparing PET estimates of sample and syringe activity with well counter and dose calibrator estimates of these same activities. RESULTS PET and well counting of the same samples gave near identical results (in MBq, R2=0.99, slope=0.99, intercept=0.00-MBq). PET syringe and dose calibrator measurements of syringe activity in MBq were also similar (R2=0.99, slope=0.99, intercept=- 0.22-MBq). CONCLUSION A small-animal PET scanner can be easily converted into a multi-sample and syringe counting device by the addition of a sample block constructed for that purpose. This capability, combined with live animal imaging, can improve efficiency and flexibility in certain experimental settings.

Comparison of planar, PET and well-counter measurements of total tumor radioactivity in a mouse xenograft model

INTRODUCTION Quantitative small animal radionuclide imaging studies are often carried out with the intention of estimating the total radioactivity content of various tissues such as the radioactivity content of mouse xenograft tumors exposed to putative diagnostic or therapeutic agents. We show that for at least one specific application, positron projection imaging (PPI) and PET yield comparable estimates of absolute total tumor activity and that both of these estimates are highly correlated with direct well-counting of these same tumors. These findings further suggest that in this particular application, PPI is a far more efficient data acquisition and processing methodology than PET. METHODS Forty-one athymic mice were implanted with PC3 human prostate cancer cells transfected with prostate-specific membrane antigen (PSMA (+)) and one additional animal (for a total of 42) with a control blank vector (PSMA (-)). All animals were injected with [18F] DCFPyl, a ligand for PSMA, and imaged for total tumor radioactivity with PET and PPI. The tumors were then removed, assayed by well counting for total radioactivity and the values between these methods intercompared. RESULTS PET, PPI and well-counter estimates of total tumor radioactivity were highly correlated (R2>0.98) with regression line slopes near unity (0.95<slope≤1.02) and intercepts near zero (-0.001MBq≤intercept ≤0.004MBq). CONCLUSION Total mouse xenograft tumor radioactivity can be measured with PET or PPI with an accuracy comparable to well counting if certain experimental and pharmacokinetic conditions are met. In this particular application, PPI is significantly more efficient than PET in making these measurements.

Investigation of factors affecting a potential worldwide network of medical PET scanners to monitor the decay rate of Lu-176 and detect global radiation events

Medical PET scanners now exist in relatively large numbers around the world suggesting that certain physics experiments might be carried out during off-hours with these machines if organized into a global array of interlinked devices. Workers at Purdue, for example, have claimed to detect a possibly neutrino-induced annual periodicity in the decay rate of Cl-36 as the Earth traverses its elliptical orbit around the Sun but these claims remain controversial. A subset array of Lu-based PET scanners might help resolve this matter by measuring for an extended period (years) the decay rate of Lu-176 (38 billion year half life), a radioactive contaminant (2.6% abundance) present in the scintillation crystals of these machines. Night-side PET scanners in time synchrony might also detect and directionally locate other previously unrecognized global radiation events of astronomical origin. The present work identifies several sources of error that could affect such measurements.

A recovery coefficient study using pre-clinical hollow spheres in clinical PET/CT scanners

Aim: To measure and study the recovery coefficients of pre-clinical hollow spheres in clinical PET/CT scanners. Materials and Methods: Seven small hollow spheres with inner diameters ranging from 3.95mm up to 15.43mm, were imaged in a Jaszczak ECT Phantom, in air and with sphere to background activity concentration ratios of 15 to 1 for 16 minutes. The images were reconstructed to 2×2×2 mm voxel size images using the default clinical reconstruction method supplied by the scanner. The imaging protocol was performed on five different PET/CT scanners; the Gemini TF and Vereos by Philips, the mCT and HRRT by Siemens, and the Discovery 710 by General Electric. Results: The NEMA NU2 recovery coefficient for the 15.43 mm and 12.43 mm diameter spheres were about the same within each scanner. The Philips Vereos and HRRT scanners had RC values of 108% and 104% respectively, while the GE Discovery 710 was 118% and the Siemens mCT was 126%. The Gemini TF was 76%. For the smallest hollow sphere of 3.95 mm, the RC were 6.6%, 3.6%, 13%, 8.9% and 6.1% for the Gemini TF, Vereos, Discovery 710, mCT and HRRT respectively. An anomalous increase in the RC for the Discovery 710 and mCT was found for sphere sizes 9.89 mm diameter spheres where the RC was 161% and 145% for each respectively. A related recovery coefficient parameter called RCI which measures the recovery coefficient of the integrated activity in the spheres did not show this anomaly of increased recovery above 100% at the 9.89 mm sphere diameter measurement. Conclusion: The recovery coefficient data resulting from scanning five different scanners is presented. An increase in the recovery coefficient is seen in two scanners which is possibly due to over correction by the iterative reconstruction used to generate the images. This effect is not present when measuring the total activity in the sphere.

An inexpensive phantom for evaluating gated blood pool data acquisition/processing systems at heart rates above 400/min

Equilibrium gated blood pool imaging of the heart is a common diagnostic procedure for visualizing cardiac function in human subjects. Recently, this procedure has been modified to evaluate cardiac function in mice. However, the high heart rates encountered in these animals (often greater than 400 beats/min) can confound R-wave trigger devices, acquisition systems and image processing software containing default conditions tailored specifically to the lower heart rates of human subjects. In order to determine whether data acquisition and processing components of a commercial or self-generated gated blood pool imaging procedure are performing properly, input of known timing and imaging signals that mimic those generated during high heart rate gated blood pool imaging is required. Here, the authors describe an inexpensive phantom that is suitable for initial evaluation of an unknown system or for ongoing QC of a previously verified system.