Partial volume (imaging)

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Partial volume effect is when a imaging system with limited resolution makes small structures appear less active than they really are. It occurs in medical and biological imaging, such as PET and SPECT. If the object is smaller than about twice the system's resolution (2 × FWHM, the full width at half maximum) in any direction, its measured activity is underestimated. Higher resolution reduces this effect. The loss is most likely when the surrounding area has little or no activity, and it is often accompanied by spillover of activity into nearby regions. For very small objects, the observed activity equals the true activity minus the partial volume loss plus spillover from outside. Partial volume correction techniques seek to fix this bias.