Optimization of a Bayesian penalized likelihood algorithm (Q.Clear) for 18F-NaF bone PET/CT images acquired over shorter durations using a custom-designed phantom

Abstract

Background: The Bayesian penalized likelihood (BPL) algorithm Q.Clear (GE Healthcare) allows fully convergent iterative reconstruction that results in better image quality and quantitative accuracy, while limiting image noise. The present study aimed to optimize BPL reconstruction parameters for ¹⁸F-NaF PET/CT images and to determine the feasibility of ¹⁸F-NaF PET/CT image acquisition over shorter durations in clinical practice.

Methods: A custom-designed thoracic spine phantom consisting of several inserts, soft tissue, normal spine, and metastatic bone tumor, was scanned using a Discovery MI PET/CT scanner (GE Healthcare). The phantom allows optional adjustment of activity distribution, tumor size, and attenuation. We reconstructed PET images using OSEM + PSF + TOF (2 iterations, 17 subsets, and a 4-mm Gaussian filter), BPL + TOF (β = 200 to 700), and scan durations of 30-120 s. Signal-to-noise ratios (SNR), contrast, and coefficients of variance (CV) as image quality indicators were calculated, whereas the quantitative measures were recovery coefficients (RC) and RC linearity over a range of activity. We retrospectively analyzed images from five persons without bone metastases (male, n = 1; female, n = 4), then standardized uptake values (SUV), CV, and SNR at the 4th, 5th, and 6th thoracic vertebra were calculated in BPL + TOF (β = 400) images.

Results: The optimal reconstruction parameter of the BPL was β = 400 when images were acquired at 120 s/bed. At 90 s/bed, the BPL with a β value of 400 yielded 24% and 18% higher SNR and contrast, respectively, than OSEM (2 iterations; 120 s acquisitions). The BPL was superior to OSEM in terms of RC and the RC linearity over a range of activity, regardless of scan duration. The SUV_max were lower in BPL, than in OSEM. The CV and vertebral SNR in BPL were superior to those in OSEM.

Conclusions: The optimal reconstruction parameters of ¹⁸F-NaF PET/CT images acquired over different durations were determined. The BPL can reduce PET acquisition to 90 s/bed in ¹⁸F-NaF PET/CT imaging. Our results suggest that BPL (β = 400) on SiPM-based TOF PET/CT scanner maintained high image quality and quantitative accuracy even for shorter acquisition durations.

Keywords: 18F-NaF; BPL; Q.Clear; Quantitation; SiPM; TOF.

Fig. 1

Custom-made thoracic spine phantom. Simplified schema (a), CT image (b), and photograph (c) of phantom. Example of setup shows vertebral body phantoms with tumors of 10, 13, 17, 22, and 28 mm in diameter. Vertebral body phantom without tumors is at the bottle. Vertebral body, tumor, processus, and sternum contain K₂HPO₄ solution with density equivalent to that of bone and ¹⁸F-NaF. Elliptical body phantom contained ¹⁸F-NaF

Fig. 2

Sample PET images acquired from a 13-mm sphere with different acquisition durations and reconstructed using OSEM (2 iterations) (a) and BPL (β value, 400) (b)

Fig. 3

PET image quality of 10-mm spheres as a function of β value (range 200–700) for each acquisition duration using BPL. a SNR_10mm. b Contrast. c CV

Fig. 4

Relationship between contrast and CV curves of hot spheres with 10-mm diameter using OSEM and BPL reconstructions after each acquisition duration. Plots of OSEM correspond to 2 iterations. Curves for BPL run from left to right with decreasing β values, respectively. Unfilled and filled symbols represent OSEM and BPL, respectively

Fig. 5

Recovery coefficient as a function of sphere size with different methods of reconstructing images acquired over 120 s

Fig. 6

Correlation between true and measured activity concentrations. The RC linearity over a range of activity was measured using five spheres with diameters of 13 mm containing TNR of 1, 2, 4, 8, and 16 at the normal spine activity concentration of 15.6 kBq/mL. Unfilled symbols and dotted lines, OSEM; filled symbols and solid line, BPL. AC, activity concentration

Fig. 7

¹⁸F-NaF PET images of a 77-year-old female (weight 63 kg). Maximum intensity projection (MIP) and axial PET images reconstructed by a, c OSEM (2 iterations) and b, d BPL (β value, 400), respectively. The hotspots of osteoblastic activity in MIP images (a, b) show the degenerative bone changes and bone bruises

Fig. 8

Clinical comparisons of a SUV_max, b SUV_mean, c CV, and d vertebral SNR under optimal reconstruction conditions (OSEM, 2 iterations; BPL, β value, 400)