¹⁸F, ¹¹C and ⁶⁸Ga in small animal PET imaging

 

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Summary

Aim: The partial volume effect (PVE) significantly affects quantitative accuracy in PET. In this study we used a micro-hollow sphere phantom filled with ¹⁸F, ¹¹C or ⁶⁸Ga to evaluate different partial volume correction methods (PVC). Additionally, phantom data were applied on rat brain scans to evaluate PVC methods on in vivo datasets. Methods: The four spheres (7.81, 6.17, 5.02, 3.90 mm inner diameter) and the background region were filled to give sphere-to-background (sph/bg) activity ratios of 20 : 1, 10 : 1, 5 : 1 and 2 : 1. Two different acquisition and reconstruction protocols and three radionuclides were evaluated using a small animal PET scanner. From the obtained images the recovery coefficients (RC) and contrast recovery coefficients (CRC) for the different sph/bg ratios were calculated. Three methods for PVC were evaluated: a RC based, a CRC based and a volume of interest (VOI) based method. The most suitable PVC methods were applied to in vivo rat brain data. Results: RCs were shown to be dependent on the radionuclide used, with the highest values for ¹⁸F, followed by ¹¹C and ⁶⁸Ga. The calculated mean CRCs were generally lower than the corresponding mean RCs. Application of the different PVC methods to rat brain data led to a strong increase in time-activity curves for the smallest brain region (entorhinal cortex), whereas the lowest increase was obtained for the largest brain region (cerebellum). Conclusion: This study was able to show the importance and impact of PVE and the limitations of several PVC methods when performing quantitative measurements in small structures.

Zusammenfassung

Ziel: Der Partialvolumeneffekt (PVE) beeinflusst die quantitative Genauigkeit bei der PET. In dieser Studie wurde ein Phantom mit kleinen Kugeln verwendet, die mit ¹⁸F, ¹¹C oder ⁶⁸Ga gefüllt wurden um verschiedene PVE-Korrekturmethoden zu testen. Danach wurden die ermittelten Faktoren auf einen In-vivo-Datensatz basierend auf Rattenhirn-Scans angewendet. Methoden: Die vier kleinen Kugeln (Innendurchmesser: 7.81, 6.17, 5.02, 3.90 mm) und die Hintergrundregion wurden mit verschiedenen Aktivitätskonzentrationen gefüllt um folgende Ratios zwischen Kugeln und Hintergrund zu erhalten: 20 : 1, 10 : 1, 5 : 1 und 2 : 1. Zwei Akquisitions- und Rekonstruktionsprotokolle sowie drei Radionuklide wurden mit einem microPET-Scanner gemessen. Die Recovery-Koeffizienten (RC) und Kontrast-Recovery-Koeffizienten (CRC) wurden an Hand der Bilder berechnet. Drei Korrekturmethoden wurden getestet: eine Methode basierend auf dem RC, eine basierend auf dem CRC und eine basierend auf dem Volumen von Interesse (VOI). Danach wurden die Methoden auf den In-vivo-Datensatz angewendet. Ergebnisse: Die RCs zeigten eine starke Abhängigkeit vom Radionuklid mit den höchsten Werten für ¹⁸F, gefolgt von ¹¹C und ⁶⁸Ga. Die berechneten CRCs waren im Allgemeinen niedriger als die entsprechenden RCs. Die Anwendung der verschiedenen Korrekturmethoden auf den Rattenhirn Scan führte zu einem starken Anstieg in den Zeit- Aktivitätskurven von kleinen Hirnregionen (entorhinaler Kortex), während für größere Hirnregionen (Zerebellum) nur kleine Anstiege erhalten wurden. Schlussfolgerung: Diese Studie zeigt die Wichtigkeit und Auswirkung vom PVE sowie die Einschränkungen diverser PVE-Korrekturen bei der quantitativen Auswertung kleiner Strukturen in der präklinischen Bildgebung.

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