¹⁸F-FDG-PET and MRI in patients with malignancies of the liver and pancreas

Summary

Purpose: To evaluate the accuracy of retrospective rigid image registration and fusion between F-18 fluorodeoxyglucose positron emission tomography (FDG-PET) and magnetic resonance imaging (MRI) of the upper abdomen. Patients, material, methods: Image fusion of PET and MRI was performed in 30 patients with suspected malignancy of the liver or pancreas. Using a commercially available image fusion tool capable of rigid manual point-based registration, PET-Images were retrospectively registered and fused by matching eight homologous points in the 3D spoiled gradient echo (GRE) MRI sequences acquired in portal venous phase and in the CT-component of PET/CT. Two separate observers (R1, R2) assessed accuracy of image registration by determining the distances in the x-, y- and z-axis as well as the absolute distance between anatomical landmarks which differed from the landmarks chosen for registration. Quality of fusion was graded using a three point grading scale (1 poorly fused; 2 satisfactory fused; 3 correctly fused) and compared to hybrid PET/CT fusion. Results: Mean time of registration per patient was less than 2 minutes. Objective registration assessment showed errors between 2.4–6.3 mm in x-axis: mean 3.6 mm (R1); 4.6 mm (R2), 2.3–9.3 mm in y-axis (mean 5.1 mm; 5.5 mm) and 3.3–12.0 mm in z-axis (mean 5.9 mm; 5.9 mm.) The mean error in absolute distance between points was 6.0–16.8 mm (mean 9.9 mm; 10.6 mm). In visual assessment, most fusions were graded to be satisfactory or correctly fused: R1, R2: grade 3, 11/30 (36.7%), 22/30 (73.3%); grade 2, 13/30 (43.3%), 8/30 (26.7%); grade 1, 6/30 (20%), 0/30 (0%). Fusions were mostly comparable to hybrid PET/ CT fusions. All of the fusions were defined as diagnostically relevant by both observers. Conclusion: Retrospective rigid image fusion of FDGPET and MRI of the upper abdomen using the CT-component of PET/CT for registration is feasible without adaptation in image acquisition protocols and shows sub-centimeter registration errors in most cases.

Zusammenfassung

Ziel: Evaluation der Genauigkeit der retrospektiven rigiden Bildregistrierung und -fusion von ¹⁸F-Fluordeoxyglukose Positronenemissions-tomographie (FDG-PET) und Magnetresonanztomographie (MRI) im Oberbauch. Patienten, Material, Methoden: Die Bildfusion von PET und MRI wurde in 30 Patienten mit Verdacht auf maligne Leber- oder Pankreasläsionen durchgeführt. Mit Hilfe einer frei erhältlichen Software, welche die rigide, manuelle und punktbasierte Registrierung ermöglicht, wurden PET-Bilder mittels acht homologer Punkten einer 3D Gradienten - echosequenz der MRI sowie der CT-Komponente der PET/CT registriert und fusioniert. Zwei Radiologen (R1, R2) evaluierten die Genauigkeit der Bildregistrierung und -fusion durch Bestimmung der Distanzen in der x-, yund z-Achse sowie durch Bestimmung der absoluten Distanz zwischen anatomischen Landmarken welche sich von den Landmarken, welche zur Registrierung benutzt wurden unterschieden. Die Qualität der Fusion wurde subjektiv mittels einer Drei-Punkte-Skala bewertet (1, ungenügend fusioniert; 2 zufriedenstellend fusioniert; 3 korrekt fusioniert) und mit der Hybrid PET/CT Fusion verglichen. Resultate: Die durchschnittliche Zeit, welche zur Fusion pro Patient aufgewendet wurde betrug weniger als zwei Minuten. Die Messung der Distanzen zwischen homologen Punkten zeigte Registrierungsfehler zwischen 2.4–6.3 mm in x-Richtung: Durchschnitt 3.6 mm (R1); 4.6 mm (R2), 2.3–9.3 mm in y-Richtung (5.1 mm; 5.5 mm) und 3.3–12mm in z-Richtung (5.9 mm; 5.9 mm). Der durchschnittliche Registrierungsfehler der absoluten Distanz zwischen zwei homologen Punkten betrug 6.0–16.8 mm (9.9 mm; 10.6 mm). In der subjektiven, visuellen Auswertung der Fusionsgenauigkeit zeigten die meisten Fusionen ein zufriedenstellendes oder korrektes Resultat: R1, R2: Grad 3, 11/30 (36.7%), 22/30 (73.3%); Grad 2, 13/30 (43.3%), 8/30 (26.7%); Grad 1, 6/30 (20%), 0/30 (0%). Die meisten Fusionen waren subjektiv vergleichbar mit denen des Hybrid PET/CT Systems. Sämtliche Fusionen wurden von beiden Radiologen als diagnostisch bewertet. Schlussfolgerung: Die retrospektive, rigide Bildfusion von FDG-PET und MRI im Oberbauch durch Benutzung der CT-Komponente der PET/CT für die Registrierung ist ohne Anpassung der Akquisitionsprotokolle durchführbar und zeigt in den meisten Fällen Registrierungsfehler im Sub- Zentimeter-Bereich.

Keywords

Magnetic resonance imaging - positron emission tomography - PET/MR; image fusion - multimodality imaging - PET/CT

Schlüsselwörter

Magnetresonanztomographie - Positronenemissionstomographie - PET/MR - Bildfusion - PET/CT

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