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steady state free precession sequence repeated at different echo times (TE 30, 60, 90, 120, 150 msec). For in-vitro measurements blood samples from the fetal carotid artery with different oxygenation levels were measured. For in-vivo measurements eight sheep fetuses were examined during a control, hypoxic and recovery phase to perform T2-weighted scans of the fetal blood in the heart. Signal intensities in the left (LV) and right ventricle (RV) were measured to calculate the MR blood oxygen saturation (MR-sO2). During each phase fetal carotid artery sO2 was directly measured and correlated with MR-sO2. A Bland-Altman plot was performed. Results: Fetal carotid artery-sO2 was 69% sO2% during control, 16% sO2% during hypoxemia and 67% sO2% during recovery. Mean values of the MR-sO2 were 49% sO2% and 40% sO2% for control, 6% sO2% and 3% sO2% for hypoxemia, 51% sO2% and 43% sO2% for recovery in LV and RV, respectively. Mean values of fetal carotid artery-sO2 and MR-sO2 were highly correlated (LV: r=0.87, p=0.000002 and RV: r=0.89, p=0.000001). According to the Bland-Altman plot MR-sO2 was lower compared to fetal carotid artery-sO2 (15% in LV and 20% in RV). Conclusions: Based on our preliminary results it seems to be possible to assess fetal oxygen saturation with MR oximetry. However, for practical application measurements have to be more accurate.

Purpose: Dedicated small‐animal imaging devices are being used more frequently for translational molecular imaging studies. However, few studies have investigated the magnitude of animal motion during extended dynamic imaging studies or the precision of animal repositioning in multi‐modality and/or serial imaging protocols. The objective of this work was to determine the positional accuracy and precision with which tumors in situ can be reliably and reproducibly imaged on dedicated small‐animal imaging equipment. Method and Materials: A custom rodent animal cradle with a stereotactic template served to define a coordinate system and to facilitate rigid‐body image registration. Attached to the template were fiduciary markers containing PET tracer and MRI and CTcontrast media for visualization on the respective scanners. To quantify animal tumor motion during imaging protocols, “gold standard” point markers were inserted into tumors grown on the hind limb of nude rats. Three types of imaging examination were performed with the animals continuously anesthetized and immobilized: (i) single‐modality imaging (microPET and MRI) in which the animals remained in the same scanner continuously for 2 hours, (ii) multi‐modality imaging studies in which the animals were transported from a microPET to an MR scanner located in another building, and (iii) serial microPET scans in which the animals were removed from the scanner, then re‐positioned and scanned. Results: The animal tumor moved by less than 0.2–0.3 mm over two‐hour microPET or MR imaging sessions. Transporting the animal between instruments introduced additional error of ∼0.2 mm. In serial animal imaging studies, in which the animal was returned to its cage and subsequently re‐positioned, reproducibility within ∼0.8 mm could be obtained. Conclusion: To our knowledge, this is the first study systematically and rigorously evaluating the accuracy and precision with which tumors can be repeatedly imaged in small‐animal imaging devices.

Purpose: To develop a method for PET hypoxia tracer validation based on statistical analysis of the spatial correspondence between the intratumoral distributions of tracer uptake (assessed by digital autoradiography) and pimonidazole, an independent marker of hypoxia (assessed by immunofluorescent microscopy). The utility of the method was demonstrated by applying it to three PET hypoxia tracers, IAZGP, FMISO and Cu‐ATSM. Method and Materials: Eight rats bearing R3327‐AT tumors were divided into four groups of two animals each. Group♯1 was injected with 18F‐FMISO and sacrificed 2hr later. Group♯2 was injected with 124I‐IAZGP and sacrificed 3hr later. Groups♯3 and ♯4 were injected with 64Cu‐ATSM and sacrificed, respectively, 24hr or 1hr later. Pimonidazole was administered to all animals 2hr before sacrifice. Tumors were excised, frozen and sectioned. Digital autoradiograms of the tracer distribution were obtained from selected sections and co‐registered with images of pimonidazole‐associated immunofluorescence derived from adjacent sections. The statistical analysis of association between PET tracer uptake and pimonidazole immunofluorescent staining intensity was then performed on a pixel‐by‐pixel basis. Results: For rats from group♯1 correlation coefficients between pimonidazole‐associated immunofluorescent staining intensity and 18F‐FMISO uptake were 0.84 and 0.75. For group♯2 rats correlations between pimonidazole and 124I‐IAZGP were 0.85 and 0.77. For group♯3 (64Cu‐ATSM administered 24hr prior to sacrifice), the correlations with pimonidazole were 0.61 and 0.64. However, for group♯4 (64Cu‐ATSM administered 1hr prior to sacrifice), the correlation coefficients were −0.76 and −0.77, demonstrating that in this tumormodel64Cu‐ATSM uptake was not indicative of hypoxia at early times post injection. Conclusion: The proposed method enables evaluation of the degree of spatial correspondence between distributions of a PET tracer and alternative markers (either endogenous or exogenous) or tracers of the biological process under study. This method can be used for the in‐vivo validation of any nuclear medicine tracer.

Resume L’irradiation en modulation d’intensite est utilisee en routine dans les tumeurs des voies aerodigestives superieures a l’Universite de Gand. La distribution de dose souhaitee est determinee au prealable comme une fourchette de doses acceptables pour chaque volume d’interet. Il est important de specifier separement ces contraintes de dose pour les differentes regions du volume cible previsionnel : en dehors et dans les zones de gradient, dans les zones d’intersections ou non avec les organes a risques. Pour eviter des doses elevees a distance du volume cible previsionnel, la creation d’une « zone d’enveloppe », definie comme le volume couvert par le scanner minore du volume cible previsionnel, est une methode efficace, surtout si des sous-volumes, a l’interieur de la « zone d’enveloppe », ont ete definis a des distances croissantes du volume cible previsionnel. En attribuant des contraintes de dose maximale a ces sous-volumes, decroissantes avec la distance au volume cible previsionnel, la conformation est assuree. La creation de ces volumes cibles previsionnels additionnels et de ces sous-volumes d’organes a risque permet une definition claire et sans ambiguite des limites de doses acceptables et evite d’avoir a utiliser des ponderations de contraintes, utilisateur-dependantes, pour la procedure d’optimisation. L’efficacite de la planification inverse s’en trouve grandement renforcee. Son resultat est previsible, l’evaluation d’un plan de traitement devient objective puisque le plan repond ou non aux contraintes de doses predefinies. La description d’un plan de traitement est facilitee par l’expression des differentes doses a tous les volumes. Le cout de cette procedure est modere et reside principalement : 1. dans la creation des sous-volumes, qui peut-etre semi-automatique grâce aux outils modernes de segmentation des images ; dans l'inclusion des contraintes a tous les sous-volumes dans la fonction objective.