Validation of quantitative accuracy of 99mTc-pertechnetate SPECT/CT: a phantom study

Objectives: To investigate the quantitative accuracy of 99mTc-pertechnetate SPECT/CT in a phantom under different conditions of acquisition and reconstruction.

Methods: The NEMA IEC Body Phantom (Data Spectrum Corp, USA) with six fillable spheres of different sizes (diameter of 37mm, 28mm, 22mm, 17mm, 13mm, 10mm) was used in this experiment to validate uptake quantification accuracy of 99mTc-pertechnetate SPECT/CT imaging. The sphere-to-background ratio of average activity concentration was proximal 10:1. Discovery 670 SPECT/CT and Q.metrix software (GE Healthcare, USA) were used for imaging data acquisition, attenuation correction, scatter correction, resolution recovery, OSEM reconstruction and quantitative analysis. The volumes of interest (VOI) and radioactive concentrations of six spheres were measured and quantitative accuracy of each sphere was computed as [1-(Loaded activity-Measured activity)/Loaded activity][asterisk]100% . A series of experiments has been performed in order to check the impact of activity concentration, reconstruction scheme and the way of VOI delineation. Acquisitions were repeated for 12 times with different radioactivity concentration in the phantom. Imaging data were processed with 2 or 5 iterations for OSEM reconstruction and 2 different delineation methods using either CT VOI or NM threshold. Inter-group difference was statistically analyzed using one-way ANOVA test.

Results: Comparing two methods of VOI delineation (CT VOI and NM threshold), in addition to 17mm sphere with significant difference (p<0.01), the rest spheres had no difference (p>0.05). Besides, the 5 iterations OSEM reconstruction group showed significant (17mm sphere p<0.05, others p<0.01) higher accuracy than that of 2 iterations group. Finally, the experiment showed the average accuracy under 12 different radioactive concentration in order from large to small spheres were 102%, 95%, 80%, 66%, 27%, 20%, respectively (Fig 1).

Conclusion: Significant differences of quantitative accuracy were found in volumes and reconstruction methods. In general, spheres smaller than 17mm were also significantly affected by partial volume effect. In addition, using NM threshold method to delineation a volume of interest was not statistically different from CT VOI method, but might be more suitable for clinical application.

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