Image Processing Techniques in Emission Tomography - Correction Techniques in Emission Tomography - page 139

(a) Original signal (b) Signal with TF

FIGURE 7.11: (a) Signal with true borders before sampling. (b) Signal
intensities after sampling at the voxel grid (with tissue fraction).

systems have a FWHM, i.e., spatial resolution of approximately 4{6 mm, small
animal PET systems in preclinical research achieve a FWHM of approximately
1{2 mm.
The blurring induced by PVE is in fact caused by two different effects,
which are often generalized as partial volume effect in the literature. These two
effects, tissue fraction and spill-over, are described in the following paragraphs.

Tissue fraction
The causes of tissue fraction (TF) can be traced back to the process of sam-
pling. The measured continuous signal is sampled at a finite voxel grid to
create a three-dimensional PET image. Whenever the border of two adjacent
tissues with different tracer concentrations lies inside a voxel of this grid, one
can expect an erroneous contour. The intensity of such voxels is an average
value of the different tracer concentrations inside the respective voxels.
Figure 7.11 shows a two-dimensional example to illustrate the impact of
TF at the borderline of two neighboring tissues. Figure 7.11(b) shows the re-
sult of sampling the measured tracer concentrations indicated by the circle
in Figure 7.11(a). The tissue is blurred and pixels at the tissue border show
averaged intensity values, since they contain the signal of different compart-
ments.
To summarize, the TF causes blurred edges but is significant only at tissue
borders, since these voxels often contain strongly varying intensities. Thus, it
is considered less severe compared to the spill-over effect.

Spill-over
The largest contribution to PVE is caused by the spill-over effect. As the
name suggests, it can be described as a spilling of the measured tracer con-
centration of a voxel into its surrounding region. This leads to larger and
dimmer structures in the PET image. One reason for this blurring is the finite