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Background  In patients with differentiated thyroid cancer (DTC), serial  124 I PET/CT imaging is, for instance, used to assess the absorbed (radiation) dose to lesions. Frequently, the lesions are located in the neck and they are close to or surrounded by different tissue types. In contrast to PET/CT, MR-based attenuation correction in PET/MR may be therefore challenging in the neck region. The aim of this retrospective study was to assess the quantitative performance of  124 I PET/MRI of neck lesions by comparing the MR-based and CT-based  124 I activity concentrations (ACs). Sixteen DTC patients underwent PET/CT scans at 24 and 120 h after administration of about 25 MBq  124 I. Approximately 1 h before or after PET/CT examination, each patient additionally received a 24-h PET/MR scan and sometimes a 120-h PET/MR scan. PET images were reconstructed using the respective attenuation correction approach. Appropriate reconstruction parameters and corrections were used to harmonize the reconstructed PET images to provide, for instance, similar spatial resolution. For each lesion, two types of ACs were ascertained: the maximum AC (max-AC) and an average AC (avg-AC). The avg-AC is the average activity concentration obtained within a spherical volume of interest with a diameter of 7 mm, equaling the PET scanner resolution. For each type of AC, the percentage AC difference between MR-based and CT-based ACs was determined and Lin’s concordance correlation analysis was applied. Quantitative performance was considered acceptable if the standard deviation was ± 25% (precision), and the mean value was within ± 10% (accuracy).    Results  The avg-ACs (max-ACs within parentheses) of 74 lesions ranged from 0.20 (0.33) to 657 (733) kBq/mL. Excluding two lesions with ACs of approximately 1 kBq/mL, the mean (median) ± standard deviation (range) was − 4% (− 5%) ± 14% (− 28 to 29%) for the avg-AC and − 9% (− 11%) ± 14% (− 33 to 33%) for the max-AC. Lin’s concordance correlation coefficients were ≥ 0.97, indicating substantial AC agreement.    Conclusions  Quantification of lesions in the neck region using  124 I PET/MR showed acceptable quantitation performance to  124 I PET/CT for AC above 1 kBq/mL. The PET/MRI-based  124 I ACs in the neck region can be therefore reliably used in pre-therapy dosimetry planning.

Quantitative performance of 124I PET/MR of neck lesions in thyroid cancer patients using 124I PET/CT as reference