Phantom and Clinical Evaluation of the Bayesian Penalized Likelihood Reconstruction Algorithm Q.Clear on an LYSO PET/CT System

Abstract

Q.Clear, a Bayesian penalized-likelihood reconstruction algorithm for PET, was recently introduced by GE Healthcare on their PET scanners to improve clinical image quality and quantification. In this work, we determined the optimum penalization factor (beta) for clinical use of Q.Clear and compared Q.Clear with standard PET reconstructions.

Methods: A National Electrical Manufacturers Association image-quality phantom was scanned on a time-of-flight PET/CT scanner and reconstructed using ordered-subset expectation maximization (OSEM), OSEM with point-spread function (PSF) modeling, and the Q.Clear algorithm (which also includes PSF modeling). Q.Clear was investigated for β (B) values of 100-1,000. Contrast recovery (CR) and background variability (BV) were measured from 3 repeated scans, reconstructed with the different algorithms. Fifteen oncology body (18)F-FDG PET/CT scans were reconstructed using OSEM, OSEM PSF, and Q.Clear using B values of 200, 300, 400, and 500. These were visually analyzed by 2 scorers and scored by rank against a panel of parameters (overall image quality; background liver, mediastinum, and marrow image quality; noise level; and lesion detectability).

Results: As β is increased, the CR and BV decreases; Q.Clear generally gives a higher CR and lower BV than OSEM. For the smallest sphere reconstructed with Q.Clear B400, CR is 28.4% and BV 4.2%, with corresponding values for OSEM of 24.7% and 5.0%. For the largest hot sphere, Q.Clear B400 yields a CR of 75.2% and a BV of 3.8%, with corresponding values for OSEM of 64.4% and 4.0%. Scorer 1 and 2 ranked B400 as the preferred reconstruction in 13 of 15 (87%) and 10 of 15 (73%) cases. The least preferred reconstruction was OSEM PSF in all cases. In most cases, lesion detectability was highest ranked for B200, in 9 of 15 (67%) and 10 of 15 (73%), with OSEM PSF ranked lowest. Poor lesion detectability on OSEM PSF was seen in cases of mildly (18)F-FDG-avid mediastinal nodes in lung cancer and small liver metastases due to background noise. Conversely, OSEM PSF was ranked second highest for lesion detectability in most pulmonary nodule evaluation cases. The combined scores confirmed B400 to be the preferred reconstruction.

Conclusion: Our phantom measurement results demonstrate improved CR and reduced BV when using Q.Clear instead of OSEM. A β value of 400 is recommended for oncology body PET/CT using Q.Clear.

Keywords: Bayesian penalized likelihood; NEMA; image quality; image reconstruction; optimization; positron emission tomography.

FIGURE 1

Graphs show mean contrast recovery and background variability for hot spheres with diameter 10mm(A), 13(B), 17(C), 22(D) (cold spheres in Supplementary Figure 1). These are shown for OSEM (2i 24ss 6.4mm filter), OSEM PSF (3i 24ss 2mm filter), and Q.Clear (beta = 100-1000, as labelled on the points). Error bars shown are one standard deviation.

FIGURE 2

Graph mean residual lung error (LE) for OSEM (2i 24ss 6.4mm filter), OSEM PSF (3i 24ss 2mm filter), and Q.Clear (beta = 100-1000. Error bars shown are one standard deviation.

FIGURE 3

Graphs showing mean contrast recovery (A), background variability (B), and contrast to noise ratio (C) for all spheres in the NEMA IQ phantom (10, 13, 17, 22 were hot and 28, 37 cold). These are shown for OSEM (2i 24ss 6.4mm filter), OSEM PSF (3i 24ss 2mm filter), and Q.Clear (beta = 200-500). Error bars shown are one standard deviation.

FIGURE 4

Coronal PET images demonstrating an ¹⁸F-FDG-avid liver metastasis across OSEM and Q.Clear reconstructions. The smooth homogeneous appearance of the background liver on Q.Clear reconstruction renders the metastasis more conspicuous compared to OSEM PSF. Conversely, there is also a risk of false positive findings for small foci on the OSEM PSF reconstruction due to the high level of background noise. All images are displayed on SUV scale 0-6.

FIGURE 5

Axial PET images demonstrating an ¹⁸F-FDG-avid 7mm right upper lobe lung nodule. This was mildly ¹⁸F-FDG-avid on OSEM but the degree of uptake was shown to be higher using Q.Clear reconstruction. Despite a similar degree of uptake on OSEM PSF, there was still a low level of background noise from within the lungs, which was reduced even on B200. The improvement in background noise within the mediastinum compared to OSEM PSF is also illustrated. All images are displayed on SUV scale 0-6.