8th Baltic Congress of Radiology

Nuclear Medicine

oral abstract

The recent introduction of digital PET/CT scanners for routine clinical use represents a significant milestone for nuclear medicine and molecular imaging. Although PET/CT scanners have always been truly “digital” insofar as their outputs were in the form of digital signals, the replacement of analogue photomultiplier tubes with solid-state detection systems resulted in the first fully “digital” PET/CT, and has surmounted many of the inherent physical limits placed by previous-generation analogue technologies. Whereas the current iteration of the European Asociation of Nuclear Medicine (EANM) guidelines report examination protocols for analogue scanners and for “step-and-shoot” type acquisitions, no current guidelines are yet available for systems with digital acquisition. Although the performance characteristics, and EANM Research Ltd (EARL) compliance of such scanners have been reported, the full clinical potential of digital scanners is yet to be fully characterized. In particular, whereas the manufacturers’ literature report shorter-acquisition times and/or reduced activity as potential improvements of such systems, the advantage for the patient needs to be investigated and verified in a clinical setting. We share our experience in single centre in Latvia. The aim of this presentation is to show the effect of shorter acquisition times and reduced injected activity on small lesion detectability as assessed by one nuclear medicine physician, followed by lesion uptake quantitation in a digital scanner.

In total 2191 exams were performed. 301 exams in 2019, 756 exams in 2020 and 1134 PET/CT exams were performed in 2021. Patients underwent oncological [18F] FDG PET/CT on a digital PET/CT Philips Vereos system and were retrospectively evaluated. Scan data were acquired for a standard 2 min/bed position (bp), 1 min/bp and 15 s/bp. Scans were read by one nuclear medicine physician. Scans were evaluated for the visual tissue/background ratio and size of pathological lesions detected. Measured uptake for lesions was evaluated by maximum standardized uptake value SUVmax.

Based on nuclear medicine physician experience we reduced the injected dose and shortened the acquisition time as otherwise the images were with too much signal and noise and not optimal. Our chosen acquisition protocol was introduced early in practice and we optimized it for clinical setting as follows: • Injected Dose reduction based on body weight / 2.0-2.5 Mbq/kg (<70kg), 2.0-2.1Mbq/kg (≥70kg), • Exam time reduction - 60sec/bed for body region, 15sec/bed for legs • Post processed reconstruction from 5mm to 1mm for region of small lesion. Our physician found this protocol as most valuable for image quality (visual tissue/background ratio) and reduced dose as well as faster acquisition time. Lesion SUVmax even for small lesions 2-3 mm were detected and variation between SUVmax 1.0 – 4.5 were found. It grew gradually with small lesion size to SUVmax 13.2 when reaching 5 mm. Mean background activity in liver was SUVmax 2.5-3.9 representing small lesion detectability. Our experience and data show that with reduced dose and exam time reduction we can acquire high quality images in clinical setting.

Digital PET/CT represents a step forward in molecular imaging, with increasing numbers of publications confirming the favourable performance characteristics for such scans in comparison to previous-generation analogue systems, including increased lesion detection and sensitivity. These new fully digital systems exhibit a plethora of technical advantages, which include a better coupling between the crystal and photodetectors, improved background-to-noise, faster time-of-flight (TOF) and associated advanced TOF reconstruction. In addition, state-of-the-art digital systems often include longer axial coverage, smaller crystals, and more advanced electronics, which lead to higher sensitivity, higher spatial resolution, and shorter deadtime. These improved performance characteristics have been confirmed by a number of publications, which correspond to improvements in image quality and lesion detection. Our single center experience confirms these advantages. Given the known relationship between applied radiopharmaceutical dose, acquisition time and count statistics, by analogy, our data can also be interpreted as supportive of a reduction in applied activity and shorter acquisition times in digital PET/CT. Indeed, with a mean effective dose (excluding the CT component) of 4.9 mSv the 60 s/bp reconstructions are equivalent to a 75% reduction in dose, or 3.7 mSv. Wider clinical experience and studies must be performed to confirm this for all cancer types and in all patient groups, we continue to work with.  

The aim of this presentation is to show the effect of shorter acquisition times and reduced injected activity on small lesion detectability as assessed by one nuclear medicine physician, followed by lesion uptake quantitation in a digital scanner. 1134 PET/CT exams in 2021. Injected Dose reduction based on body weight / 2.0-2.5 Mbq/kg (<70kg), 2.0-2.1Mbq/kg (≥70kg),Exam time reduction - 60s/bed body , 15s/bed for legs. Post processed reconstruction from 5mm to 1mm for region of small lesion.

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