A phantom study: should (124) I-mIBG PET/CT replace (123) I-mIBG SPECT/CT?

Beijst, Casper, de Keizer, Bart, Lam, Marnix G E H, Janssens, Geert O, Tytgat, Godelieve A M, de Jong, Hugo W A M


Medical Physics 44 (5), p. 1624–1631


PURPOSE: The isotope (123) I is commonly labeled with meta-iodobenzylguanidine (mIBG) for imaging of neuroendocrine tumors, such as pheochromocytomas and neuroblastomas. (123) I-mIBG SPECT/CT imaging is performed for staging, follow-up and selection of patients for treatment with (131) I mIBG. As an alternative to (123) I, (124) 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 (124) I PET/CT improves image quality as compared with (123) 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) modelling. As a measure of image quality the contrast-to-noise ratio (CNR) was calculated. The ratio of the (123) I to (124) 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 (124) I reconstruction method and an attenuation and scatter corrected (123) I reconstruction method for sphere sizes of 10 to 37 mm, respectively. As the effective radiation dose of (124) I-mIBG is higher than of (123) 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 (124) I PET/CT outperforms (123) 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, (124) I-mIBG PET/CT is expected to improve image quality and/or may be used to reduce effective patient dose as compared with (123) I-mIBG SPECT/CT. This article is protected by copyright. All rights reserved.