Document Type : Original Article
Department of Nuclear Medicine, Royal North Shore Hospital St Leonards NSW 2065
Sydney Vital Translational Cancer Research Centre, Sydney, Australia
Objective(s): The spatial resolution of emission tomographic imaging systems can
lead to a significant underestimation in the apparent radioactivity concentration in
objects of size comparable to the resolution volume of the system. The aim of this
study was to investigate the impact of the partial volume effect (PVE) on clinical
imaging in PET and SPECT with current state-of-the-art instrumentation and the
implications that this has for radionuclide dosimetry estimates.
Methods: Using the IEC Image Quality Phantom we have measured the
underestimation in observed uptake in objects of various sizes for both PET and
SPECT imaging conditions. Both single pixel measures (i.e., SUVmax) and region of
interest mean values were examined over a range of object sizes. We have further
examined the impact of the PVE on dosimetry estimates in OLINDA in 177Lu SPECT
imaging based on a subject with multiple somatostatin receptor positive
paragangliomas in the head and neck.
Results: In PET, single pixel estimates of uptake are affected for objects less than
approximately 18 mm in minor axis with existing systems. In SPECT imaging with
medium energy collimators (e.g., for 177Lu imaging), however, the underestimates
are far greater, where single pixel estimates in objects less than 2-3×the resolution
volume are significantly impacted. In SPECT, region of interest mean values are
underestimated in objects less than 10 cm in diameter. In the clinical case example,
the dosimetry measured with SPECT ranged from more than 60% underestimate
in the largest lesion (28×22 mm in maximal cross-section; 10.2 cc volume) to >99%
underestimate in the smallest lesion (4×5 mm; 0.06 cc).
Conclusion: The partial volume effect remains a significant factor when estimating
radionuclide uptake in vivo, especially in small volumes. Accurate estimates of
absorbed dose from radionuclide therapy will be particularly challenging until
robust solutions to correct for the PVE are found.