Evaluation of PET quantification accuracy in vivo

Summary

Quantitative positron emission tomography (PET) requires accurate scanner calibration, which is commonly performed using phantoms. It is not clear to what extent this procedure ensures quantitatively correct results in vivo, since certain conditions differ between phantom and patient scans. Aim: We, therefore, have evaluated the actual quantification accuracy in vivo of PET under clinical routine conditions. Patients, methods: We determined the activity concentration in the bladder in patients undergoing routine [¹⁸F]FDG whole body investigations with three different PET scanners (Siemens ECAT EXACT HR+ PET: n = 21; Siemens Biograph 16 PET/CT: n = 16; Philips Gemini-TF PET/CT: n = 19). Urine samples were collected immediately after scan. Activity concentration in the samples was determined in well counters cross-calibrated against the respective scanner. The PET (bladder) to well counter (urine sample) activity concentration ratio was determined. Results: Activity concentration in the bladder (PET) was systematically lower than in the urine samples (well The patient-averaged PET to well counter ratios for the investigated scanners are (mean ± SEM): 0.881 ± 0.015 (ECAT HR⁺), 0.898 ± 0.024 (Biograph 16), 0.932 ± 0.024 (Gemini-TF). These values correspond to underestimates by PET of 11.9%, 10.2%, and 6.8%, respectively. Conclusions: The investigated PET systems consistently underestimate activity concentration in the bladder. The comparison of urine samples with PET scans of the bladder is a straightforward means for in vivo evaluation of the expectable quantification accuracy. The method might be interesting for multi-center trials, for additional quality assurance in PET and for investigation of PET/MR systems for which clear proof of sufficient quantitative accuracy in vivo is still missing.

Zusammenfassung

Für die Bestimmung quantitativer Parameter mittels Positronenemissionstomographie (PET) ist eine genaue Kalibrierung des PET-Scanners mit Hilfe geeigneter Phantommessungen notwendig. Auf Grund der offensichtlichen Unterschiede zwischen Phantom- und Patientenmessungen bestehen jedoch Unsicherheiten im Bezug auf die In-vivo-Genauigkeit einer solchen phantom-basierten Kalibrierung. Ziel dieser Studie war es daher die Genauigkeit einer solchen Kalibrierung mittels klinischer Routinemessungen in vivo zu evaluieren. Patienten, Methoden: Wir bestimmten die Aktivitätskonzentration in der Blase bei Patienten die an unterschiedlichen PET-Scannern eine [¹⁸F]FDG-Ganzkörperuntersuchung erhielten (Siemens ECAT EXACT HR+ PET: n = 21; Siemens Biograph 16 PET/CT: n = 16; Philips Ge- mini-TF PET/CT: n = 19). Unmittelbar nach Messung der Blasenregion gaben alle Patienten Urinproben ab. Die Aktivitätskonzentration der Urinproben wurde mit Hilfe eines kreuzkalibrierten Bohrlochzählers bestimmt. Im Anschluss wurde das Verhältnis zwischen Aktivitätskonzentration PET (Blase) zu Bohrlochzähler (Urinprobe) berechnet. Ergebnisse: Die Aktivitätskonzentration in der Blase (PET) war systematisch niedriger als die der Urinproben (Bohrlochzähler). Das über die jeweiligen Patienten gemittelte Verhältnis zwischen PET und Bohrlochzähler war für die untersuchten Scanner (Mittelwert ± SEM): 0,881 ± 0,015 (ECAT HR+), 0,898 ± 0,024 (Biograph 16), 0,932 ± 0,024 (Gemini-TF). Diese Werte entsprechen Unterschätzungen der Aktivitätskonzentration durch die PET von jeweils 11,9%, 10,2% und 6,8%. Schlussfolgerungen: Die untersuchten PET- Systeme unterschätzen die Aktivitätskonzentration in der Blase. Der direkte Vergleich von Urinproben und PET-Bildern der Blase stellt eine einfache Art der In-vivo-Evaluation der zu erwartenden Quantifizierungsgenauigkeit eines PET-Scanners dar. Die vorgestellte Me- thode kann für Multicenterstudien, für eine zusätzliche Qualitätssicherung in der PET sowie für eine Untersuchung von PET/MR- Systemen interessant sein, für die es hinsichtlich einer ausreichenden Quantifizierungsgenauigkeit in vivo noch keine belastbaren Zahlen gibt.

Keywords

PE - quantification - accuracy - in vivo - multicenter - QA

Schlüsselwörter

PET - Quantifizierung - Genauigkeit - in vivo - Multicenter - QA

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