Biomedical Engineering Reference
In-Depth Information
FIGURE 5.3
SPECT nonuniform artifacts resulting from poor uniformity correction are shown. The
gamma camera was a dual-headed device with the heads opposed at 180
to each other.
The reconstructed images show a 20 cm diameter cylinder which has been filled with water
containing
99m
Tc and uniformly mixed. The nonuniformities exhibit as a “hot-spot” in the
center of the image matrix (not the center of the object) seen in all images in the top row
and the two right-most images in the bottom row, and a number of “cold” ring artifacts in
the other images at varying radii. Note that the nonuniformities are centerd on the image
matrix, not on the object. The object center is displaced above the image center.
system for the reconstruction algorithm, is simply the distance from the center
of the tomograph to the crystal face. However, the mean free path (i.e., the
average distance traveled) of the 0.511 MeV photons in the detector is around
1 cm for BGO scintillators (density(
7.13 g · cm
3
). Therefore, the
effective
radius
of the PET system is increased by this amount, and this affects the
voxel spatial dimensions. In practice, a suitable method to avoid this effect is
to measure the dimensions accurately with a calibrated test object. A circular
disk of perspex which contains an array of fine holes drilled very accurately
can be used. Into this tubing containing a radioactive solution is introduced.
An example from a CT scan is shown in Figure 5.4. The same test object can be
used in SPECT, PET, CT, and MRI with appropriate contrast (
99m
Tc in SPECT,
18
F in PET, air in CT, and CuSO
4
for MRI) for the same purpose.
)
5.4.1.2
Physical Effects Causing Intensity Artifacts
and Spatial Distortions
5.4.1.2.1 Attenuation Effects
Some physical effects cause changes in the spatial distribution of the recon-
structed signal. Neglecting to correct for attenuation, for example, renders the
projection data inconsistent at different radial angles, and this causes a num-
ber of distortions in the data. First, as the attenuation is greatest towards the
center of the body, the reconstructed concentration in a homogenous structure
