A phantom study: Should 124I-mIBG PET/CT replace 123I-mIBG SPECT/CT?

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Abstract

Purpose

The isotope 123I is commonly labeled with meta-iodobenzylguanidine (mIBG) for imaging of neuroendocrine tumors, such as pheochromocytomas and neuroblastomas. 123I-mIBG SPECT/CT imaging is performed for staging, follow-up and selection of patients for treatment with 131I mIBG. As an alternative to 123I, 124I-mIBG PET/CT may be used, potentially taking advantage of the superior PET image quality. The purpose of this study was to investigate whether 124I PET/CT improves image quality as compared with 123I SPECT/CT for equal patient effective radiation dose (in mSv).

Methods

Phantom measurements were performed using the NEMA-2007 image quality phantom. SPECT and PET reconstruction settings were used with and without time-of-flight (TOF) and point-spread-function (PSF) modeling. As a measure of image quality, the contrast-to-noise ratio (CNR) was calculated. The ratio of the 123I to 124I activity concentration was determined at which the contrast-to-noise ratio was equal for both modalities. This metric was defined as the contrast equivalent activity ratio (CEAR).

Results

CEARs of 47.7, 25.6, 23.1, 14.6, 10.0, and 9.1 were obtained for a TOF and PSF modeled 124I reconstruction method and an attenuation and scatter-corrected 123I reconstruction method for sphere sizes of 10 to 37 mm, respectively. As the effective radiation dose of 124I-mIBG is higher than of 123I-mIBG (in mSv/MBq), an equal effective dose corresponds to a CEAR of 5 to 10. Therefore, CEARs higher than 5 to 10 indicate that 124I PET/CT outperforms 123I SPECT/CT in the sense of image quality for equal patient effective radiation dose.

Conclusion

The CEAR is much larger than a factor of 5 to 10 (needed for equal patient effective radiation dose) for most of the reconstruction methods and sphere sizes. Therefore, 124I-mIBG PET/CT is expected to improve image quality and/or may be used to reduce effective patient dose as compared with 123I-mIBG SPECT/CT.

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