¹²⁴ I Quantification with LAFOV PET: Calibration, Bias and Systematic Performance Evaluation

Quantitative ¹²⁴I PET imaging is challenged by low positron branching ratio, prompt gamma emissions, and limited count statistics. Long axial field-of-view (LAFOV) PET systems provide substantially increased sensitivity, potentially enabling more robust imaging in terms of quantitative accuracy and noise mitigation under these conditions. This study aimed to systematically evaluate image quality, quantitative accuracy, and sources of bias in low-count ¹²⁴I PET for varying acquisition times across the preparation and imaging pipeline, with particular focus on scatter/prompt gamma correction and dose calibrator calibration.

Methods

Multiple phantoms were employed in the current study, namely the NEMA IQ phantom (sphere-to-background ratio 20:1) and cylindrical phantoms of different sizes representing different scatter geometries. All phantoms were filled with low activity concentrations typical of clinical imaging (about 0.4 kBq/mL background, corresponding to a 37 MBq ¹²⁴ I administration in a 70 kg patient imaged 24 h post-injection). Contrast recovery, recovery coefficients, image noise as coefficient of variation (CV), and lung residual error were assessed. Data were acquired on a LAFOV PET/CT scanner (Biograph Vision Quadra, Siemens Healthineers) and reconstructed using single scatter simulation with tail fitting (SSS-TF) and an alternative maximum-likelihood scatter scaling approach (SSS-MLSS). The impacts of acquisition time (15 min vs. 30 min) and object size on image quality and quantification were evaluated. Dose calibrator performance and inter-device consistency were assessed across 0.5–60 MBq range of activity. A representative ¹²⁴ I PET scan of a patient with metastatic differentiated thyroid cancer (DTC) was included to assess lesion detectability and quantification at reduced scan durations.

Results

Image quality remained robust under low-count ¹²⁴ I conditions (0.4 kBq/mL) with a CV of 15.8% at 30 min, which is consistent with EANM/EARL recommendations and comparable to matched low-count ¹⁸F acquisitions (15.4% at 30 min). Reducing acquisition time to 15 min increased noise but preserved contrast and recovery (≤2.1% and <±2%). Using SSS-TF, activity concentration was underestimated for ¹²⁴ I, particularly in the background of the NEMA IQ phantom (83.0% for ¹²⁴I vs. 102.7% for ¹⁸F). SSS-MLSS improved background recovery (95.8%) while maintaining sphere recovery, yielding more consistent quantification. Size-dependent effects were observed, with underestimation in larger objects (86.1% phantom ø=8 cm vs. 81.4% phantom ø=20 cm using SSS-TF), which was reduced using SSS-MLSS (87.0% vs. 96.1%, respectively). Dose calibrator measurements showed high stability and low inter-device variability (≤2.3%). In the patient dataset, lesion detectability and quantification remained stable across reconstruction methods and scan durations down to 5 min.

Conclusion

LAFOV PET enables robust low-count ¹²⁴I imaging with preserved image quality and quantification, allowing the reduction of acquisition times to ≤15 min. Quantitative accuracy is primarily impaired by scatter including prompt gamma coincidence correction and object geometry, while calibration-related effects are minor under controlled conditions.