The accurate assessment of PET/CT scanner performance is crucial for ensuring reliable clinical imaging and diagnosis. Sensitivity and scatter fraction are two key parameters that influence the quality of PET images. This study aimed to validate the performance of in-house developed sensitivity and scatter phantoms by comparing their results with those obtained using NEMA NU-2 standard phantoms and also to the vendor-prescribed standard values. Sensitivity, defined as the number of counts recorded per unit activity, is a measure of a PET system's ability to detect radiation. The theoretical estimation of sensitivity depends on various factors, including detector type, decay time, light output, and the performance of photomultiplier tubes and amplifiers.

During alpha testing, the NU-2IndiSen and NU-2IndiSc phantoms demonstrated no significant differences when compared to the NU-2Sen and NU-2Sc phantoms. The dimensions of both phantoms showed no significant variation (p-value > 0.05) relative to the commercially available NEMA NU-2 phantoms. Similarly, the radiation exposure test revealed no significant difference in the performance of the NU-2IndiSen and NU-2IndiSc phantoms when compared to the Nu-2Sen and Nu-2Sc phantoms. These results confirm the development of both phantoms was successful. Additionally, both phantoms were successfully validated in beta testing as well.

The NU-2IndSen phantom was evaluated by measuring the system sensitivity and comparing it to the NEMA NU-2 standard sensitivity phantoms and also to the vendor-specified value. The results demonstrated excellent agreement between the two, with an average system sensitivity of 6724 cps/MBq, well within the vendor's acceptable range and comparable to that of standard NEMA-NU-2 sensitivity phantom. The sensitivity value obtained by NU-2IndSen phantom was also consistent with the previously published literature [8,9,10]. This finding indicates that the NU-2IndSen phantom accurately reflects the system's sensitivity performance.

The scatter phantom was used to assess the system's ability to differentiate between true and scattered radiation. The scatter fraction, which is the ratio of scattered counts to total counts, is a critical parameter in PET imaging. The study evaluated the noise equivalent count rate (NECR) and scatter fraction at various activity concentrations. The maximum NECR observed with the NU-2IndSc was 106.4, which was more than the vendor's acceptance criteria for NECR and also comparable to that of the value obtained using the standard NEMA NU-2 scatter phantom. Additionally, the mean scatter fraction at peak NECR was 25.9%, well within the vendor-specified limit of 28%. Furthermore, the maximum true count rate measured during the acquisition was 243.2, which was consistent with the vendor's performance specifications for PET/CT systems. Scatter fraction, NECR and peak count rate values obtained by NU-2IndiSc phantom were consistent with that of the values published in the literature [8,9,10]. These results collectively demonstrate that the NU-2IndiSc phantom can accurately assess the system's scatter performance.

The validation study revealed that the in-house developed NU-2IndiSen and NU-2IndiSc phantoms accurately reflect the performance characteristics of the Phillips PET/CT scanner. The obtained results for sensitivity, scatter fraction, and count rate were in excellent agreement with the vendor-specified limits, validating the use of these phantoms as reliable quality assurance tools.

While the development and validation of in-house NEMA NU-2 phantoms i.e. NU-2IndiSen and NU-2IndiSc phantom represent a significant achievement, it is important to acknowledge potential limitations and biases associated with their use. Material variability can influence the accuracy of measurements due to variations in material density, purity, and homogeneity, which can lead to systematic errors. Manufacturing precision is essential to ensure that the phantom’s dimensions and geometry conform to NEMA standards, as any deviations can affect the accuracy of measurements, particularly for spatial resolution and sensitivity. Accurate calibration of the radioactive sources used in the phantom is crucial, as errors in source activity or distribution can lead to inaccurate measurements of sensitivity and scatter fraction. Over time, the phantom’s physical and radiological properties may change due to factors such as radiation damage, temperature fluctuations, and material degradation, affecting its long-term reliability. Additionally, in-house phantoms may have limitations in terms of the range of parameters they can assess, making them potentially unsuitable for evaluating the performance of PET systems under extreme conditions or specific clinical protocols.

Potential biases include operator bias, where the accuracy of measurements can be influenced by the skill and experience of the operator, leading to inconsistent measurement techniques or data analysis procedures. System-specific factors, such as detector efficiency, energy resolution, and reconstruction algorithms, can also introduce bias in the evaluation, as in-house phantoms may not fully account for these factors. Comparisons with commercial phantoms can help validate the in-house phantom, but differences in design, materials, and manufacturing processes must be considered, as they can introduce bias into the comparison. To mitigate these limitations and biases, it is essential to implement rigorous quality control measures, periodically calibrate the phantom’s radioactive sources, adhere to standardized measurement protocols, cross-validate results with other reference phantoms or independent measurements, and continuously evaluate and improve the performance of the in-house phantom to maintain its accuracy and reliability.