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

PURPOSE

The isotope I is commonly labeled with meta-iodobenzylguanidine (mIBG) for imaging of neuroendocrine tumors, such as pheochromocytomas and neuroblastomas. I-mIBG SPECT/CT imaging is performed for staging, follow-up and selection of patients for treatment with I mIBG. As an alternative to I, I-mIBG PET/CT may be used, potentially taking advantage of the superior PET image quality. The purpose of this study was to investigate whether I PET/CT improves image quality as compared with I 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 I to I 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 I reconstruction method and an attenuation and scatter-corrected I reconstruction method for sphere sizes of 10 to 37 mm, respectively. As the effective radiation dose of I-mIBG is higher than of I-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 I PET/CT outperforms I 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, I-mIBG PET/CT is expected to improve image quality and/or may be used to reduce effective patient dose as compared with I-mIBG SPECT/CT.