Julien Guglielmi

Distribution and Dynamics of 99mTc-Pertechnetate Uptake in the Thyroid and Other Organs Assessed by Single-Photon Emission Computed Tomography in Living Mice

BACKGROUND (99m)Tc pertechnetate is a well-known anion, used for clinical imaging of thyroid function. This gamma emitter is transported by the sodium iodide symporter but is not incorporated into thyroglobulin. Scintigraphy using (99m)Tc pertechnetate or (123)iodide represents a powerful tool for the study of sodium iodide symporter activity in different organs of living animal models. However, in many studies that have been performed in mice, the thyroid could not be distinguished from the salivary glands. In this work, we have evaluated the use of a clinically dedicated single-photon emission computed tomography (SPECT) camera for thyroid imaging and assessed what improvements are necessary for the development of this technique. METHODS SPECT of the mouse neck region, with pinhole collimation and geometric calibration, was used for the individual measurement of (99m)Tc pertechnetate uptake in the thyroid and the salivary glands. Uptake in the stomach was studied by planar whole-body imaging. Uptake kinetics and biodistribution studies were performed by sequential imaging. RESULTS This work has shown that thyroid imaging in living mice can be performed with a SPECT camera originally built for clinical use. Our experiments indicate that (99m)Tc pertechnetate uptake is faster in the thyroid than in the salivary glands and the stomach. The decrease in (99m)Tc pertechnetate uptake after injection of iodide or perchlorate as competitive inhibitors was also studied. The resulting rate decreases were faster in the thyroid than in the salivary glands or the stomach. CONCLUSIONS We have shown that a clinically dedicated SPECT camera can be used for thyroid imaging. In our experiments, SPECT imaging allowed the analysis of (99m)Tc pertechnetate accumulation in individual organs and revealed differences in uptake kinetics.

99mTcO4−-, Auger-Mediated Thyroid Stunning: Dosimetric Requirements and Associated Molecular Events

Low-energy Auger and conversion electrons deposit their energy in a very small volume (a few nm3) around the site of emission. From a radiotoxicological point of view the effects of low-energy electrons on normal tissues are largely unknown, understudied, and generally assumed to be negligible. In this context, the discovery that the low-energy electron emitter, 99mTc, can induce stunning on primary thyrocytes in vitro, at low absorbed doses, is intriguing. Extrapolated in vivo, this observation suggests that a radioisotope as commonly used in nuclear medicine as 99mTc may significantly influence thyroid physiology. The aims of this study were to determine whether 99mTc pertechnetate (99mTcO4 −) is capable of inducing thyroid stunning in vivo, to evaluate the absorbed dose of 99mTcO4 − required to induce this stunning, and to analyze the biological events associated/concomitant with this effect. Our results show that 99mTcO4 −–mediated thyroid stunning can be observed in vivo in mouse thyroid. The threshold of the absorbed dose in the thyroid required to obtain a significant stunning effect is in the range of 20 Gy. This effect is associated with a reduced level of functional Na/I symporter (NIS) protein, with no significant cell death. It is reversible within a few days. At the cellular and molecular levels, a decrease in NIS mRNA, the generation of double-strand DNA breaks, and the activation of the p53 pathway are observed. Low-energy electrons emitted by 99mTc can, therefore, induce thyroid stunning in vivo in mice, if it is exposed to an absorbed dose of at least 20 Gy, a level unlikely to be encountered in clinical practice. Nevertheless this report presents an unexpected effect of low-energy electrons on a normal tissue in vivo, and provides a unique experimental setup to understand the fine molecular mechanisms involved in their biological effects.

Normalisation to blood activity is required for the accurate quantification of Na/I symporter ectopic expression by SPECT/CT in individual subjects.

The utilisation of the Na/I symporter (NIS) and associated radiotracers as a reporter system for imaging gene expression is now reaching the clinical setting in cancer gene therapy applications. However, a formal assessment of the methodology in terms of normalisation of the data still remains to be performed, particularly in the context of the assessment of activities in individual subjects in longitudinal studies. In this context, we administered to mice a recombinant, replication-incompetent adenovirus encoding rat NIS, or a human colorectal carcinoma cell line (HT29) encoding mouse NIS. We used 99mTc pertechnetate as a radiotracer for SPECT/CT imaging to determine the pattern of ectopic NIS expression in longitudinal kinetic studies. Some animals of the cohort were culled and NIS expression was measured by quantitative RT-PCR and immunohistochemistry. The radioactive content of some liver biopsies was also measured ex vivo. Our results show that in longitudinal studies involving datasets taken from individual mice, the presentation of non-normalised data (activity expressed as %ID/g or %ID/cc) leads to ‘noisy’, and sometimes incoherent, results. This variability is due to the fact that the blood pertechnetate concentration can vary up to three-fold from day to day. Normalisation of these data with blood activities corrects for these inconsistencies. We advocate that, blood pertechnetate activity should be determined and used to normalise the activity measured in the organ/region of interest that expresses NIS ectopically. Considering that NIS imaging has already reached the clinical setting in the context of cancer gene therapy, this normalisation may be essential in order to obtain accurate and predictive information in future longitudinal clinical studies in biotherapy.

High-Frequency Quantitative Ultrasound Spectroscopy of Excised Canine Livers and Mouse Tumors Using the Structure Factor Model

Three scattering models were examined for characterizing ex vivo canine livers and HT29 mouse tumors in the 10-38- and the 15-42-MHz frequency bandwidth, respectively. The spherical Gaussian model (SGM) and the fluid sphere model (FSM) that were examined are suitable for dealing with sparse media, whereas the structure factor model (SFM) is adapted for characterizing concentrated media. For the canine livers, the scatterer radius and the acoustic concentration estimated with the three models were similar and matched well the nuclear structures obtained from histological analysis (with relative errors less than 7%). These results show that the livers could be considered as a diluted medium and that the nuclei in liver could be a dominant source of scattering. For the homogeneous mouse tumors, containing mostly viable HT29 cells, scatterer radius and volume fraction estimated with the SFM showed good agreement with the whole cell structures obtained from histological analysis (with relative errors less than 15%), whereas the sparse models (the SGM and the FSM) gave no consistent quantitative ultrasound parameters. This suggests that the viable HT29 cell areas have densely packed cellular content and that the whole HT29 cell could be responsible for scattering. For the heterogeneous tumors, the hyperechogenic zones observed in the B-mode images were linked to the presence of small necrotic areas surrounded by viable HT29 cells. Comparison between sparse and concentrated models shows that these hyperechogenic zones could be considered as a concentrated medium.

Evaluation of tetrafunctional block copolymers as synthetic vectors for lung gene transfer

In the present study, we evaluated, in mice, the efficacy of the tetrafunctional block copolymer 704 as a nonviral gene delivery vector to the lungs. SPECT/CT molecular imaging of gene expression, biochemical assays, and immunohistochemistry were used. Our dataset shows that the formulation 704 resulted in higher levels of reporter gene expression than the GL67A formulation currently being used in a clinical trial in cystic fibrosis patients. The inflammatory response associated with this gene transfer was lower than that induced by the GL67A formulation, and the 704 formulation was amenable to repeated administrations. The cell types transfected by the 704 formulation were type I and type II pneumocytes, and transgene expression could not be detected in macrophages. These results emphasize the relevance of the 704 formulation as a nonviral gene delivery vector for lung gene therapy. Further studies will be required to validate this vector in larger animals, in which the lungs are more similar to human lungs.

Amplitude-based data selection for optimal retrospective reconstruction in micro-SPECT.

Respiratory motion can blur the tomographic reconstruction of positron emission tomography or single-photon emission computed tomography (SPECT) images, which subsequently impair quantitative measurements, e.g. in the upper abdomen area. Respiratory signal phase-based gated reconstruction addresses this problem, but deteriorates the signal-to-noise ratio (SNR) and other intensity-based quality measures. This paper proposes a 3D reconstruction method dedicated to micro-SPECT imaging of mice. From a 4D acquisition, the phase images exhibiting motion are identified and the associated list-mode data are discarded, which enables the reconstruction of a 3D image without respiratory artefacts. The proposed method allows a motion-free reconstruction exhibiting both satisfactory count statistics and accuracy of measures. With respect to standard 3D reconstruction (non-gated 3D reconstruction) without breathing motion correction, an increase of 14.6% of the mean standardized uptake value has been observed, while, with respect to a gated 4D reconstruction, up to 60% less noise and an increase of up to 124% of the SNR have been demonstrated.

Iodinated contrast agents perturb iodide uptake by the thyroid independently of free iodide

Perturbation of thyroid iodide uptake is a well-documented side effect of the use of iodinated contrast media (ICM) administered intravenously. This side effect is thought to be mediated by free iodide in ICM formulations, but this hypothesis has never been formally proven. The aim of the present study was to assess the validity of this hypothesis. Methods: We used mass spectrometry analysis to quantify free-iodide contamination in ICM. Established cell lines expressing the Na/I symporter (NIS) were used to quantify the effect of ICM on iodide uptake. SPECT/CT was used to measure the in vivo uptake of 99mTc-pertechnetate and 123I in 2 NIS-expressing mouse tissues, thyroid and salivary glands. Scintiscans of ICM-naïve and ICM-administered patients were compared. Immunohistologic and Western blot analyses were performed to evaluate NIS protein expression in these organs. Results: Although free iodide was present in ICM formulations, in vitro uptake of iodide by NIS-expressing cells was not significantly affected by ICM. In mice, intravenous or sublingual administration of ICM led to a reduction in radiotracer uptake by the thyroid, accompanied by a dramatic reduction in NIS protein expression in this tissue. In the salivary glands, neither radiotracer uptake nor NIS protein expression was affected by ICM. The thyroid-selective effect of ICM was also observed in humans. Administration of potassium iodide as a source of free iodide led to a diminution of 99mTc-pertechnetate uptake in both mouse thyroid and mouse salivary glands. Altogether, these data rule out a direct intervention of free iodide in the perturbation of thyroid uptake and suggest a direct and selective effect of ICM on the thyroid. Conclusion: We demonstrated that ICM reduce thyroid uptake of iodide independently of free iodide. This effect is due to a specific and dramatic decrease in NIS expression in thyrocytes. These data cast serious doubt on the relevance of measuring urinary iodide concentration to evaluate the delay between ICM administration and radioiodine therapy in patients with differentiated thyroid carcinoma. Finally, the ability of ICM to perturb iodide uptake in the thyroid may be used in radioprotection.

DO MULTIPLE ADMINISTRATIONS OF STABLE IODINE PROTECT POPULATION CHRONICALLY EXPOSED TO RADIOACTIVE IODINE: WHAT IS PRIODAC RESEARCH PROGRAM (2014–22) TEACHING US?

Single dose of potassium iodide (KI) is recommended to prevent the risk of thyroid cancer during nuclear accidents. However in the case of repeated/protracted radioiodine release, a unique dose of KI may not protect efficiently the thyroid against the risk of further developing a radiation-induced cancer. The new WHO guidelines for the use in planning for and responding to radiological and nuclear emergencies identify the need of more data on this subject as one of the four research priorities. The aims of the PRIODAC project are (1) to assess the associated side effects of repeated intakes of KI, (2) to better understand the molecular mechanisms regulating the metabolism of iodine, (3) to revise the regulatory French marketing authorization of 65-mg KI tablets and (4) to develop new recommendations related to the administration of KI toward a better international harmonization. A review of the literature and the preliminary data are presented here.

Comparison between sparse and concentrated scattering models for characterization of cell-pellet biophantoms and excised mouse tumors

Three scattering models were examined for characterizing in vitro biophantoms and ex vivo colon adenocarcinoma (HT29) mouse tumors in the 10-42 MHz and the 15-42 MHz frequency bandwidth, respectively. The Spherical Gaussian Model (SGM) and the Fluid-Filled Sphere Model (FFSM) that were examined are suitable for dealing with sparse media, whereas the Structure Factor Model (SFM) is adapted for characterizing concentrated media. Biophantoms results demonstrated the superiority of the SFM for all investigated volume fractions (i.e., from 0.006 to 0.30). In particular, the sparse SGM and FFSM underestimated scatterer size and overestimated acoustic concentration when the volume fraction was greater than 0.12. The SFM-based tumor results yielded scatterer radius and volume fraction in good agreement with the expected whole cell structures obtained from histological analysis (with relative errors less than 15%), whereas the sparse models gave inconsistent QUS estimates. These results suggest that the whole HT29 cells could be responsible for scattering. Moreover, the concentrated SFM provides better fit to the BSC data when compared with classical models (SGM, FFSM) for both experiments on cell-pellet biophantoms and mouse tumors. This study demonstrates that the SFM is the most-suitable model for densely packed scatterers, e.g., tissue.

Image-based motion detection in 4D images and application to respiratory motion suppression

Respiratory motion may blur the tomographic reconstruction of PET or SPECT images, which subsequently impairs quantitative measurements, e.g. in the upper abdomen area. Respiratory phase-based gated reconstruction addresses this problem for CT images, but deteriorates the signal-to-noise ratio and other intensity-based quality measures for ET images. This article describes an image-based motion detection method for dynamic images, which works for both 4D-CT and 4D-SPECT images. This method allowed us to identify the motionless phases from the image-based motion signal. Interestingly, we found that the peak of motion according to the pressure signal, recorded during the acquisition and considered as a reference, is temporally shifted compared to the motion-phases identified by the proposed method. Moreover, these observations permit to reconstruct a breath hold like 3D SPECT image improving the quality of the 3D reconstruction compared to both 4D reconstruction and standard 3D reconstruction.