Quantitative analysis of image metrics for reduced and standard dose pediatric 18F-FDG PET/MRI examinations

Abstract

Objective:: The study performs a comprehensive analysis of image metrics to objectively support the reduction of injected activity in pediatric oncology ¹⁸F-FDG PET/MR (¹⁸F-fludeoxyglucose PET/MR) examinations. Contrast-to-Noise Ratio (CNR), Normalized Noise (NN), tumor burden, and standardized uptake value (SUV) parameters stability were investigated to robustly define the acceptable reduced activity level that preserves the clinical utility of images, considering different PET applications.

Methods:: 21 PET/MRI examinations performed on a 3-Tesla Biograph mMR scanner were analyzed. Tracer activity reduction was stimulated by decreasing the count statistics of the original list-mode data (3 MBq kg^-1). In addition to the already studied SUV metrics and subjective scoring on lesion detectability, a thorough analysis of CNR, NN, Metabolic Tumor Volume (MTV), and Total Lesion Glycolysis (TLG) was performed.

Results:: SUVmax and SUVmean increased more than 5% only in 0.6 MBq kg^-1 reconstructed images (+10% and +9%, respectively), while SUVpeak was almost unaffected (average variations < 2%). The quantified CNR, NN, MTV, and TLG behavior with the decrease of the injected activity clearly defines 1.5 MBq kg^-1 as a threshold of activity after which the quality of the image degrades. Subjective and objective analyses yielded consistent results. All 56 lesions were detected until activity of 1.2 MBq kg^-1, whereas five lesions were missed on the 0.6 MBq kg^-1 image. Perceived image quality (IQ) decreased in Lower Tracer Activity (LTA) images but remained acceptable until 1.5 MBq kg^-1.

Conclusion:: Results about the stability of image metrics beyond the semi-quantitative SUV parameters and subjective analysis, rigorously proves the feasibility of the reduction of injected activity to 1.5 MBqkg^-1 for pediatric patients aged between 7 and 17 years.

Advances in knowledge:: This is the first report on the quantitative evaluation of the effect of activity reduction on image quality in pediatric PET/MR. The findings offer objective corroboration to the feasibility of a significant dose reduction without consequences on clinical image reading and tumor burden metrics.

Figure 1.

Coronal view of a patient for standard injected activity of 3MBq kg^–1 and simulated activity of 1.5 and 0.6 MBq kg^–1 (from left to right). The subjective score was 5, 3, and 1, respectively. In the 0.6 MBq kg^–1 image, the lung lesion was missed.

Figure 2.

Quality grade distributions for all tracer activity images. FTA, full tracer activity; LTA, lower tracer activity; SUV, standardized uptake value.

Figure 3.

Boxplots of the ratios between LTA and FTA images of SUVmax, SUVmean, SUVpeak, and Normalized Noise. FTA, full tracer activity; LTA, lower tracer activity; SUV, standardized uptake value.

Figure 4.

Bland-Altman plots of CNR (activity) / CNR (3 MBq kg^–1) for activity = 2.4, 1.5 and 0.6 MBq kg^–1. CNR, contrast-to-noise ratio.

Figure 5.

Boxplots of the ratios between LTA and FTA images for CNR, MTV, and TLG and Bland-Altman plot of MTV(1.5 MBqkg^-1)/MTV(3 MBqkg^-1). CNR, contrast-to-noise ratio; FTA, full tracer activity; LTA, lower tracer activity; MTV, metabolic tumor volume; TLG, total lesion glycolysis.

Figure 6.

Boxplots of the ratios between LTA and FTA images for SUVmean^fixedVOI and CNR^{fixed VOI}. CNR, contrast-to-noise ratio; FTA, full tracer activity; LTA, lower tracer activity; SUV, standardized uptake value.