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The assessment of time-of-flight on image quality and quantification with reduced administered activity and scan times in 18F-FDG PET

Armstrong, Ian S.a,b; James, Jackie M.a; Williams, Heather A.a; Kelly, Matthew D.c; Matthews, Julian C.b

Nuclear Medicine Communications: July 2015 - Volume 36 - Issue 7 - p 728–737
doi: 10.1097/MNM.0000000000000305

Objectives The last decade has seen considerable technological innovations in PET detectors with the availability, among other advances, of time-of-flight (TOF). TOF has been shown to increase the signal-to-noise ratio (SNR), which should allow for a reduction in acquired counts while maintaining image quality.

Methods Fifty-eight patients referred for routine 18F-flurodeoxyglucose (18F-FDG) oncology PET studies were included in this study. Patients with weight below or above 100 kg were prescribed 350 or 400 MBq of 18F-FDG, respectively. Listmode data were acquired for 2.5 min per bed position and reconstructed with ordered-subset expectation maximization (OSEM) reconstruction. TOF reconstruction was performed on reduced-count data, with two levels of reduction (−20 and −40% for patients <100 kg and −16 and −30% for patients >100 kg) achieved by clipping the listmode data. Liver SNR, mediastinum mean standardized uptake value (SUVmean), and lesion maximum standardized uptake value (SUVmax) were measured in all images. All images were visually assessed as adequate or suboptimal.

Results No significant difference was seen in mediastinum SUVmean or lesion SUVmax when comparing reduced-count TOF with full-count OSEM images. Compared with the original OSEM images, liver SNR was higher for TOF images using the more conservative −20% reduction of counts (P<0.001, Wilcoxon’s signed-rank test), whereas no significant statistical difference was seen with −40% reductions.

Conclusion Incorporation of TOF allows for a reduction in acquired counts; this method has been implemented at our institution, with administered activity reduced for all patients to 280 MBq and a reduction in scan times for all but the largest patients. This has significantly reduced the patient radiation dose and improved scanner flexibility and throughput.

aNuclear Medicine Centre, Central Manchester University Hospitals NHS Foundation Trust

bInstitute of Population Health, MAHSC, University of Manchester, Manchester

cSiemens PLC, Healthcare Sector, Molecular Imaging, Oxford, UK

Correspondence to Ian S. Armstrong, MPhys, MSc, Nuclear Medicine Centre, Central Manchester University Hospitals NHS Foundation Trust, Oxford Road, Manchester M13 9WL, UK Tel: +44 161 276 4785; fax: +44 161 276 8023; e-mail:

Received December 10, 2014

Received in revised form January 30, 2015

Accepted January 30, 2015

Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.