Localization of regions with increased uptake of radiotracer in small-animal SPECT is greatly facilitated when using coregistration with anatomic images of the same animal. As MRI has several advantages compared with CT (soft-tissue contrast and lack of ionizing radiation) we developed a SPECT/low-field MRI hybrid device for small-animal imaging. Methods: A small-animal single-pinhole g-camera (pinhole, 1.5 mm in diameter and 12 cm in focal length) adjacent to a dedicated low-field (0.1 T) small MR imager (imaging volume, 10 · 10 · 6 cm 3 ) was used. The animal was placed in a warmed nonmagnetic polymethyl methacrylate imaging cell for MR acquisition, which was followed immediately by SPECT after translation of the imaging cell from one modality to the other. 3-Dimensional T1-weighted sequences were used for MRI. Phantom studies enabled verification of a low attenuation (10%) for 99m Tc and 201 Tl and a very slight increase in Compton scattering due to the radiofrequency coil and polymethyl methacrylate imaging cell. Results: SPECT/ MRI data acquisition and image coregistration of selected examples using different radiotracers for lungs, kidneys, and brain were obtained in 3 nude mice with isotropic spatial resolutions of 0.5 · 0.5 · 0.5 mm 3 for MRI and 1 · 1 · 1 mm 3 for SPECT. The total acquisition time for combined SPECT and MRI lasted 1 h 45 min. Conclusion: A low-magnetic-field strength of 0.1 T is a simple and useful solution for a small-animal dual-imaging device combining pinhole SPECT with the adjacent MR imager. Among small-animal imaging techniques, SPECT provides a unique possibility to follow and measure in vivo, and noninvasively, the biodistribution of a 10 29 molar concentration of a wide range of radiolabeled biomolecules commonly available in nuclear medicine departments (1). However, one essential drawback in SPECT images is the lack of anatomic references related to the tissue uptake of tracer. Therefore, localization of regions with increased uptake of radiotracer is greatly facilitated when using coregistration of SPECT with anatomic images of the same animal, from CT or MRI. Over the last few years, SPECT/ CT dual modality has been widely proposed by manufacturers but MRI presents specific advantages compared with CT, including lack of ionizing radiation, high soft-tissue contrast, and sensitivity to tissue alterations evidenced by specific imaging sequences (2,3). Yet, compared with CT, the use of MRI for coregistration of both functional and structural information has, to our knowledge, essentially served to demonstrate the potential interest of using coregistration of images after data acquisition in separate nuclear and MR rooms. However, as experienced for rat and mice, the strategy of pinhole SPECT followed by MRI in a clinical scanner is a long and complicated task (4-7), requiring a careful transfer of the animal in a specially designed bed equipped with multimodality fiducial markers helping in the coregistration of images (4,5). In addition, separate dual-modality scans (follow...

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We describe the efficient algebraic reconstruction (EAR) method, which applies to cone-beam tomographic reconstruction problems with a circular symmetry. Three independant steps/stages are presented, which use two symmetries and a factorization of the point spread functions (PSFs), each reducing computing times and eventually storage in memory or hard drive. In the case of pinhole single photon emission computed tomography (SPECT), we show how the EAR method can incorporate most of the physical and geometrical effects which change the PSF compared to the Dirac function assumed in analytical methods, thus showing improvements on reconstructed images. We also compare results obtained by the EAR method with a cubic grid implementation of an algebraic method and modeling of the PSF and we show that there is no significant loss of quality, despite the use of a noncubic grid for voxels in the EAR method. Data from a phantom, reconstructed with the EAR method, demonstrate 1.08-mm spatial tomographic resolution despite the use of a 1.5-mm pinhole SPECT device and several applications in rat and mouse imaging are shown. Finally, we discuss the conditions of application of the method when symmetries are broken, by considering the different parameters of the calibration and nonsymmetric physical effects such as attenuation.

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The use of iterative algorithms in tomographic reconstruction always leads to a frequency adapted rate of convergence in that low frequencies are accurately reconstructed after a few iterations, while high frequencies sometimes require many more computations. In this paper, we propose to build frequency adapted (FA) algorithms based on a condition of incomplete backprojection and propose an FA simultaneous algebraic reconstruction technique (FA-SART) algorithm as an example. The results obtained with the FA-SART algorithm demonstrate a very fast convergence on a highly detailed phantom when compared to the original SART algorithm. Though the use of such an FA algorithm may seem difficult, we specify in which case it is relevant and propose several ways to improve the reconstruction process with FA algorithms.

La crise de la COVID-19 s’est ouverte sur un déficit de moyens thérapeutiques permettant de lutter efficacement contre cette maladie pour les cas les plus graves et sur de nombreuses initiatives visant à mettre en évidence un traitement efficace. Si différentes options ont été explorées, parmi lesquelles la transfusion de plasma de patients guéris, la lutte contre les mécanismes immunitaires déclenchant une réponse trop forte (les orages cytokiniques) ou, à moyen terme, la vaccination, ce sont dans un premier temps surtout les médicaments antiviraux qui ont nourri les espoirs. Après l’identification d’un certain nombre de principes actifs montrant des effets in vitro, il s’agissait d’obtenir rapidement des réponses quant à leurs effets bénéfiques in vivo et aux risques induits. Aussi, s’est-on senti en droit d’attendre que la science nous apporte les connaissances nécessaires sur ces médicaments, qu’elle nous parle, une fois sollicitée, de manière claire et d’une seule voix ; et cela, dans une situation d’urgence où la synchronisation des rythmes de la recherche et du soin n’a rien d’une évidence.

Background The weekend effect has been extensively observed for different diseases and countries and recognized as a fact but without obvious causes. Objectives In this paper we first aimed at investigating the existence of a periodicity in the death count due to Covid-19, and second, at opening the discussion concerning the reality of this effect in this particular context. Methods Daily statistics of deaths due to the Covid-19 pandemic were subjected to a discrete Fourier transform spectral analysis for France and the world, over the periods from March 29 to May 16, 2020 and from January 22 to May 18, 2020 respectively. Results In both cases, a frequency peak of one harmonic corresponding to a period of 7.11 days was observed for France and the world. In France, this weekly frequency corresponds to a decrease in deaths every Sunday, whereas for the world the systematic decrease is shifted on average by 1.5 days and corresponds to Saturday or Friday. Conclusion At the world scale and for the epidemic period we confirm the existence of a consecutive weekend effect in the context of the Covid-19 pandemic.

An automated, level-set based, segmentation framework is proposed in this work for computation of tumoral volumes on mice brain bearing gliomal tumors. T1 and T2 weighted MRI images were acquired to monitor tumor growth, at different time points. We developed an original multi-phase and multi-channel segmentation method, based on the level set framework of Chan and Vese, to facilitate the estimation of tumoral volumes. A clinical study comparing manual and segmented volumes on 18 mice demonstrate the adequacy of the multi-channel segmentation and its superiority over single-T1 channel automated segmentation in terms of measurement accuracy and correlation.