Quantitative Comparison of 124I PET/CT and 131I SPECT/CT Detectability.
Radioiodine therapy with (131)I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic (124)I PET/CT with a low activity (~1% of (131)I activity) can be performed to determine whether uptake of (131)I, and thereby the desired therapeutic effect, may be expected. However, false-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT have been reported by several groups. The purpose of this study was to investigate whether the reported discrepancies may be ascribed to a difference in lesion detectability between (124)I PET/CT and (131)I SPECT/CT and, hence, whether the administered (124)I activity is sufficient to achieve equal detectability.


METHODS
Phantom measurements were performed using the National Electrical Manufacturers Association 2007 image-quality phantom. As a measure of detectability, the contrast-to-noise ratio was calculated. The (124)I activity was expressed as the percentage of (131)I activity required to achieve the same contrast-to-noise ratio. This metric was defined as the detectability equivalence percentage (DEP).


RESULTS
Because lower DEPs were obtained for smaller spheres, a relatively low (124)I activity was sufficient to achieve similar lesion detectability between (124)I PET/CT and (131)I SPECT/CT. DEP was 1.5%, 1.9%, 1.9%, 4.4%, 9.0%, and 16.2% for spheres with diameters of 10, 13, 17, 18, 25, and 37 mm, respectively, for attenuation- and scatter-corrected SPECT versus point-spread function (PSF) model-based and time-of-flight (TOF) PET. For no-PSF no-TOF PET, DEP was 3.6%, 2.1%, 3.5%, 7.8%, 15.1%, and 23.3%, respectively.


CONCLUSIONS
A relatively low (124)I activity of 74 MBq (~1% of (131)I activity) is sufficient to achieve similar lesion detectability between (124)I PSF TOF PET/CT and (131)I SPECT/CT for small spheres (≤10 mm), since the reported DEPs are close to 1%. False-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT may be ascribed to differences in detectability for large lesions (>10 mm) and for no-PSF no-TOF PET, since DEPs are greater than 1%. On the basis of DEPs of 3.5% for lesion diameters of up to 17 mm on no-PSF no-TOF PET, (124)I activities as high as 170 MBq may be warranted to obtain equal detectability.
Department of Radiology and Nuclear Medicine, UMC Utrecht, Utrecht, The Netherlands Image Sciences Institute, UMC Utrecht, Utrecht, The Netherlands cbeijst@umcutrecht.nl.
Casper
C
Beijst
Department of Nuclear Medicine, Antoni van Leeuwenhoek Hospital, The Netherlands Cancer Institute, Amsterdam, The Netherlands Department of Surgical Oncology and Endocrine Surgery, UMC Utrecht, Utrecht, The Netherlands; and.
Jakob W
JW
Kist
Department of Radiology and Nuclear Medicine, UMC Utrecht, Utrecht, The Netherlands.
Mattijs
M
Elschot
Image Sciences Institute, UMC Utrecht, Utrecht, The Netherlands.
Max A
MA
Viergever
Department of Radiology and Nuclear Medicine, VU University Medical Center, Amsterdam, The Netherlands.
Otto S
OS
Hoekstra
Department of Radiology and Nuclear Medicine, UMC Utrecht, Utrecht, The Netherlands.
Bart
B
de Keizer
Department of Radiology and Nuclear Medicine, UMC Utrecht, Utrecht, The Netherlands.
Hugo W A M
HW
de Jong
D007091:Image Processing, Computer-Assisted; D007457:Iodine Radioisotopes; D064847:Multimodal Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D059629:Signal-To-Noise Ratio; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*
D007457:Iodine Radioisotopes
124I PET/CT; 131I SPECT/CT; detectability; phantom; radioiodine