Comparison of semi-quantitative parameters of 18F-FDG digital PET/CT and simultaneous PET/MRI in lymphoma patients

Introduction: Recently, integrating MRI with PET delivers morphological information with excellent soft tissue contrast, as well as minimizes a significant amount of radiation dose compared with the standard PET/CT modality. Whilst, the impact of Standardized uptake value (SUV) derived from PET/MRI on the interpretation and disease assessment in lymphoma still needs to be validated. Thus, this retrospective study aimed to evaluate the semi-quantitative analysis between the simultaneous PET/MRI and state-of-art digital PET/CT.

Methods: One hundred and six patients with known lymphoma were enrolled and divided into 2 different paths. On the first method, fifty-one patients underwent 18F-FDG whole-body PET/CT, followed by simultaneous PET/MRI with a single intravenous injection (PET/CT_first). Whereas, the other fifty-five patients received PET/MRI, followed by PET/CT (PET/MRI_first). The semi-quantitative imaging analysis based on SUV measurement was performed within lesions, liver, and mediastinal blood pool. Finally, all measured SUVs were statistically compared to examine the differences as well as correlations between PET/CT and PET/MRI.

Results: PET/MRI SUV_max and SUV_mean performed on both orders of acquisition in lesion and liver, and mediastinal blood pool, were statistically significantly lower than PET/CT (p<0.05) except for the SUV_mean of the mediastinal blood pool which was performed on PET/MRI_first showed no significant difference (p<0.09). The excellent correlation between both modalities from both methods using Spearman's correlation was represented in SUV_mean and SUV_max of lesions with Spearman's rho 0.90-0.96. Moreover, the high correlation presenting spearman's rho between 0.71-0.77 was demonstrated at the following examined parameters, including SUV_max and SUV_mean of liver under PET/CT_first, SUV_mean of the liver, and mediastinal blood pool for PET/MRI_first. While the moderate correlation indicating rho 0.56-0.69 were observed at the mediastinal blood pool's SUV_max acquired from both means and the SUV_max in the liver from PET/MRI_first method, the lowest correlation was presented at mediastinal blood pool's SUV_mean with rho 0.48. Remarkably, there was also a good agreement between modalities for the calculation of the Deauville five-point scale (D5PS) (&kappa;=0.6566 in PET/CT_first and &kappa;=0.4846 in PET/MRI_first, p<0.001).

Conclusions: PET/MRI can be considered an effective tool for lymphoma patients due to the considerable correlation of SUV quantification as well as a good agreement with D5PS acquired from a digital PET/CT system. However, the quantitative comparison of PET imaging from both digital PET/CT and PET/MRI should be concerned owing to the different system designs and methods of gamma attenuation correction resulting in variations of PET imaging. Hence, the response assessment should be recommended on the same machine for a follow-up examination towards a more precise and accurate evaluation of quantitative changes in tracer uptake.