# Partial volume effect (PVE)

Source: https://nuclide.cc/terms/partial-volume-effect/  
Nuclide record `partial-volume-effect` (Term). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

A PET or SPECT scan can make small tumours or lesions look fainter than they really are, simply because the scanner cannot see fine detail perfectly; the true 'brightness' gets smeared out over a wider area, especially in objects smaller than about 1-2 cm.

## Summary

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.

## Fields

- Kind: Term
- Last checked: 2026-09-22

## Sources

- PMC: partial volume effect in SPECT and PET imaging and impact on dosimetry: https://pmc.ncbi.nlm.nih.gov/articles/PMC9803618/
- Journal of Nuclear Medicine: partial-volume effect in PET tumor imaging: https://jnm.snmjournals.org/content/48/6/932

## Connected records

- terms: [Attenuation correction](https://nuclide.cc/terms/attenuation-correction/), [Standardised uptake value (SUV)](https://nuclide.cc/terms/suv/), [Time-of-flight (TOF) PET](https://nuclide.cc/terms/time-of-flight-pet/)
- isotopes: [Lutetium-177](https://nuclide.cc/isotopes/lu-177/)
- technologies: [PET (positron emission tomography)](https://nuclide.cc/technologies/pet/), [Quantitative imaging biomarkers (RECIST, PERCIST, SUV, ADC)](https://nuclide.cc/technologies/quantitative-imaging-biomarkers/), [Radioligand dosimetry](https://nuclide.cc/technologies/radioligand-dosimetry/), [SPECT & bone scan](https://nuclide.cc/technologies/spect/)

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