Biomedical Engineering Reference
In-Depth Information
In this section, we briefly describe some of the major factors that directly relate
to the spatial resolution of PET. Spatial resolution is defined as the ability of the
scanner to depict small objects and is limited by a number of factors:
distance the positron travels before it annihilates;
annihilation photon noncollinearity due to residual momentum of the
positron;
intrinsic resolution and size of the detectors;
stopping power (and material) of the detector,
sampling requirements, and
image reconstruction parameters (e.g., reconstruction filter, matrix size,
reconstruction algorithm, etc).
The finite distance travelled by the positron before annihilation also has adverse
effects on the spatial resolution of PET scanner [52]. This distance is referred
to as the
positron range
which varies from fraction of a millimeter to several
millimeters, depending on the density of the tissue in which the emission occurs
and the maximal positron energy of the radionuclide (Eq. (2.3) and Table 2.1). It
is apparent that a positron with higher energy can travel farther from the nucleus
before annihilation occurs. This effect leads to a blurring of the data which is
characterized by an exponential function with a FWHM of the order of 0.2-3 mm
for most positron-emitting isotopes.
Another factor which can degrade the spatial resolution is caused by the
residual kinetic energy and momentum possessed by the positron and the elec-
tron (because both of them are moving) when they annihilate. The apparent
angle between the two emitted photons deviates slightly from 180
◦
for about
0
.
5
◦
FWHM. The degradation in resolution due to this photon noncollinearity ef-
fect depends on the diameter of the detector ring of the PET scanner. This effect,
and the positron range, imposes a lower limit of the spatial resolution which is
approximately 3 mm for human PET imaging and 1 mm for a small-diameter
animal PET system.
The intrinsic resolution of the detectors is the crucial factor which deter-
mines the spatial resolution of modern PET scanners. For arrays of a single-
element detector of width
D
, the resulting coincidence point spread function is
