Basics of PET and SPECT Imaging - Correction Techniques in Emission Tomography - page 60

the activity distribution [2] [7]. Two different partial volume effects can be
distinguished: \spill in" and \spill out."

4.1.6.1 Spill out

Usually, an image volume element contains the measured activity concen-
tration. If the object of interest is smaller than such a volume element, the
activity of this object is assigned to the total volume element. Hence, although
the total measured activity is correct, the measured activity concentration is
smaller than in the original structure. Consequently the measured activity
concentration and hence the intensity in the reconstructed image of objects
is getting smaller with the size of the object. This eect is called \spill out"
as the activity of the object is spilled out into the surrounding volume. This
effect can be seen in Figure 4.7 which shows the PET image of a phantom
with different sized spheres which all contain the same activity concentra-
tion but appear in the PET image as smaller spheres containing less activity
concentration.

4.1.6.2 Spill in

Accordingly, activity of surrounding structures can be assigned to the ob-
ject of interest and hence lead to a false high measured activity concentration.
This is of special importance if the object of interest has lower activity con-
centration than the volume around, for example if the blood activity concen-
tration is measured and the surrounding tissue has higher activity than the
blood pool. This is done, for example, for kinetic modeling studies. Often the
lumen of the left ventricle of the heart is used for the measurement of blood
pool activity; however, for some tracers like FDG there can be high tracer
uptake in the surrounding myocardium, especially for later time frames. This

FIGURE 4.7: Partial volume effect: PET image of a phantom filled with
18 F-FDG and containing spheres with inner diameters of 37, 28, 22, 17, 13,
and 11 mm, respectively. These were filled with 18 F-FDG having a 10 times
higher activity concentration than the background (left). Corresponding re-
covery coecients for the different spheres (right).