{"entity":{"id":"partial-volume-effect","kind":"term","name":"Partial volume effect (PVE)","aka":[],"tldr":"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.\n\nThe 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.\n\nPartial 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.","asOf":"2026-09-22","links":[{"label":"PMC: partial volume effect in SPECT and PET imaging and impact on dosimetry","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC9803618/"},{"label":"Journal of Nuclear Medicine: partial-volume effect in PET tumor imaging","url":"https://jnm.snmjournals.org/content/48/6/932"}],"tags":[],"related":["attenuation-correction","suv","time-of-flight-pet","lu-177"],"indications":[],"sections":[],"technologies":["pet","spect","quantitative-imaging-biomarkers","radioligand-dosimetry"],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"category":"physics"},"route":"/terms/partial-volume-effect/","neighbours":{"term":[{"id":"attenuation-correction","kind":"term","name":"Attenuation correction","route":"/terms/attenuation-correction/"},{"id":"suv","kind":"term","name":"Standardised uptake value (SUV)","route":"/terms/suv/"},{"id":"time-of-flight-pet","kind":"term","name":"Time-of-flight (TOF) PET","route":"/terms/time-of-flight-pet/"}],"isotope":[{"id":"lu-177","kind":"isotope","name":"Lutetium-177","route":"/isotopes/lu-177/"}],"technology":[{"id":"pet","kind":"technology","name":"PET (positron emission tomography)","route":"/technologies/pet/"},{"id":"quantitative-imaging-biomarkers","kind":"technology","name":"Quantitative imaging biomarkers (RECIST, PERCIST, SUV, ADC)","route":"/technologies/quantitative-imaging-biomarkers/"},{"id":"radioligand-dosimetry","kind":"technology","name":"Radioligand dosimetry","route":"/technologies/radioligand-dosimetry/"},{"id":"spect","kind":"technology","name":"SPECT & bone scan","route":"/technologies/spect/"}]}}