The partial volume effect can be defined as the loss of apparent activity in small objects or regions because of the limited resolution of the imaging system. It occurs in
medical imaging and more generally in
biological imaging Biological imaging may refer to any imaging technique used in biology.
Typical examples include:
* Bioluminescence imaging, a technique for studying laboratory animals using luminescent protein
* Calcium imaging, determining the calcium status of a ...
such as
positron emission tomography
Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, ...
(PET) and
single-photon emission computed tomography
Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera (that is, ...
(SPECT). If the object or region to be imaged is less than twice the
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
(
FWHM
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
) resolution in x-, y- and z-dimension of the imaging system, the resultant activity in the object or region is underestimated.
A higher resolution decreases this effect, as it better resolves the tissue.
Partial volume loss alone occurs only when the surrounding activity of the object or region is zero,
or less or more than the object. And the loss of activity in the object generally involves an increase in activity in adjacent regions, which are considered outside the object (i.e.,
spillover). For a small object (e.g., a
voxel
In 3D computer graphics, a voxel represents a value on a regular grid in three-dimensional space. As with pixels in a 2D bitmap, voxels themselves do not typically have their position (i.e. coordinates) explicitly encoded with their values. ...
) or an object of size comparable to the spatial resolution of the imaging system, the observed activity is the sum of activity due to partial volume loss plus spillover from adjacent regions. The method to correct for the partial volume effect is referred to as partial volume correction (see
).
See also
*
Spillover (imaging)
References
Neuroimaging
Cardiac imaging
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