Partial volume effect (PVE)

The partial volume effect is the systematic underestimation of measured activity concentration in objects that are small relative to the spatial resolution of a PET or SPECT imaging system, caused by two combined phenomena: the finite spatial resolution of the scanner blurs the true activity distribution (a 3D convolution with the system's point-spread function), causing signal to 'spill out' of small hot lesions and background signal to 'spill in'; and the sampling of a continuous activity distribution onto a discrete voxel grid means boundary voxels contain a mixture of tissues (the tissue-fraction effect). Both effects become significant when an object's smallest dimension is below roughly two to three times the system's full-width-at-half-maximum (FWHM) spatial resolution, corresponding to roughly 6-15 mm for typical PET systems and roughly 14-25 mm for SPECT systems, which have poorer intrinsic resolution.

The practical consequence is a size-dependent, reproducible bias, not random noise: the smaller the lesion, the greater the underestimation of measured uptake (SUVmax or SUVmean) or, for post-therapy dosimetry, of the true absorbed dose. In one worked clinical example, SPECT-based lesion dosimetry for 177Lu therapy underestimated the true absorbed dose by more than 60% in a roughly 1 cm lesion and by more than 99% in a lesion only a few millimetres across.

Partial volume effect matters for radiopharmaceutical therapy dosimetry because small metastases, exactly the lesions most likely to be missed or under-dosed, are the ones most affected; it also affects diagnostic PET quantification, where small lymph nodes below about 10 mm read out with substantially lower SUV than their true uptake. Partial volume correction methods (recovery-coefficient-based, MRI- or CT-guided) can partly compensate but rarely restore full accuracy for sub-centimetre structures, and current guidance recommends excluding lesions below about 10 mm from quantitative analysis unless a validated correction method is applied.