Impact of acquisition time and penalizing factor in a block-sequential regularized expectation maximization reconstruction algorithm on a Si-photomultiplier-based PET-CT system for 18F-FDG

Abstract

Background Block-sequential regularized expectation maximization (BSREM), commercially Q. Clear (GE Healthcare, Milwaukee, WI, USA), is a reconstruction algorithm that allows for a fully convergent iterative reconstruction leading to higher image contrast compared to conventional reconstruction algorithms, while also limiting noise. The noise penalization factor β controls the trade-off between noise level and resolution and can be adjusted by the user. The aim was to evaluate the influence of different β values for different activity time products (ATs = administered activity × acquisition time) in whole-body 18 F-fluorodeoxyglucose (FDG) positron emission tomography with computed tomography (PET-CT) regarding quantitative data, interpretation, and quality assessment of the images. Twenty-five patients with known or suspected malignancies, referred for clinical 18 F-FDG PET-CT examinations acquired on a silicon photomultiplier PET-CT scanner, were included. The data were reconstructed using BSREM with β values of 100–700 and ATs of 4–16 MBq/kg × min/bed (acquisition times of 1, 1.5, 2, 3, and 4 min/bed). Noise level, lesion SUV max , and lesion SUV peak were calculated. Image quality and lesion detectability were assessed by four nuclear medicine physicians for acquisition times of 1.0 and 1.5 min/bed position. Results The noise level decreased with increasing β values and ATs. Lesion SUV max varied considerably between different β values and ATs, whereas SUV peak was more stable. For an AT of 6 (in our case 1.5 min/bed), the best image quality was obtained with a β of 600 and the best lesion detectability with a β of 500. AT of 4 generated poor-quality images and false positive uptakes due to noise. Conclusions For oncologic whole-body 18 F-FDG examinations on a SiPM-based PET-CT, we propose using an AT of 6 (i.e., 4 MBq/kg and 1.5 min/bed) reconstructed with BSREM using a β value of 500–600 in order to ensure image quality and lesion detection rate as well as a high patient throughput. We do not recommend using AT < 6 since the risk of false positive uptakes due to noise increases.

Reconstructed spatial resolution and contrast recovery with Bayesian penalized likelihood reconstruction (Q.Clear) for FDG-PET compared to time-of-flight (TOF) with point spread function (PSF)

“…The authors reported superior SNR at matched noise in a liver VOI with Q.Clear (β of 133 to 533) compared to PSF+TOF (3 it, 16 ss, 5.0 mm filter), and clinical whole-body FDG-PET data with β of 267 to 533 achieved the highest score in subjective image quality [8]. Trägårdh et al found a β of 500 to 600 to be optimal for whole-body FDG-PET with the 4-ring detector GE Discovery MI if the product "AT" of injected activity per kilogram and acquisition time per bed position was 6 (i.e., 4 MBq/kg and 1.5 min acquisition) [18]. The same group recommended a β of 400 to 550 for 18F-fluorocholine PET if "AT" was 6 [19].…”

Image reconstruction methods affect software-aided assessment of pathologies of [18F]flutemetamol and [18F]FDG brain-PET examinations in patients with neurodegenerative diseases

“…The commercially available block-sequential regularized expectation maximization (BSREM) (Q.Clear; GE Healthcare) reconstruction method implements regularization by a relative difference penalty ( De Pierro and Yamagishi, 2001 , Nuyts et al, 2002 , Ross, 2014 ), and allows the user to adjust the strength of the activity-dependent smoothing. Studies addressing whole body [ 18 F]FDG on various PET/CT scanners have shown for BSREM to reduce image noise without impeding the tumor detectability, or vice versa, to improve tumor detectability at similar noise levels as ordered-subsets expectation maximization (OSEM) ( Teoh et al, 2016 , Sah et al, 2017 , Lindström et al, 2018 , Trägårdh et al, 2019 , Bjöersdorff et al, 2019 ). Other studies have also shown the need to adjust the setting of β depending on the tracer and application being used ( Lindström et al, 2019 , Lindström et al, 2020 ).…”

Optimization of [18F]PSMA-1007 PET-CT using regularized reconstruction in patients with prostate cancer

“…Under these conditions, they found higher SUV and tumour-to-background ratios for BSREM compared to OSEM, especially for small and highly avid lesions. Optimization of BSREM has also been published on other radiopharmaceuticals, which include [ 18 F]fluorodeoxyglucose [17][18][19][20], [ 18 F]fluorocholine [13], and [ 18 F]fluciclovine [21], with different optimal β values and acquisition times. The differences depend on different bio-kinetics of the radiopharmaceutical, and different amounts of administered activity, which cause different amounts of signal in the images.…”