Biomedical Engineering Reference
In-Depth Information
FIGURE 11.7
18
An example of coregistration of [
F]-DG scans obtained with a gamma camera PET system is
shown. The top row shows the raw, reconstructed data before realignment and the bottom
row shows the studies on days 28 and 56 realigned to the baseline scan. The data were realigned
with a fully automated algorithm using mutual information criteria for matching.
23
The patient
had a primary cancer of the bowel with a large (2 cm) secondary lesion in the left lung (indicated
by the crosshairs on the baseline scan). The lung lesion showed a 20% increase in [
18
F]-DG
uptake at day 28 compared with the baseline study and 50% increase at day 56. Note the
differences in the images in the bladder (days 28 and 56) and the starting and stopping points
for the scans. In spite of these differences, the automated routine performed extremely well.
images can help to correct a number of these processes and are discussed in
this section.
11.4.1
Scatter and Attenuation Correction
Photons emitted from radionuclides
have a relatively high likelihood
of undergoing attenuation or scattering within the body. In emission tomo-
graphy, where the task is to reconstruct a three-dimensional volume from
multiple one- or two-dimensional projections, photon attenuation and scat-
tering are confounding factors in the projection data which need to be cor-
rected in order to produce accurate, distortion-free quantitative images. The
origin of attenuation and scattering is the interaction of the photons with the
tissues of the body, and this is dependent in part on the electron density of
the tissues. Electron density can be measured with x-rays or gamma rays.
Consequently, many investigators have used planar, line, or point sources of
radionuclides and the gamma camera or PET camera to measure photon
transmission
in vivo
7,9,10,24,25
to provide correction factors to be used with the emission
data. The use of x-ray CT scans has also been suggested.
26
Fleming et al. used
x-ray CT images to produce attenuation correction factors for attenuation
correction of SPECT emission data.
27
For a recent review of transmission
scanning in nuclear medicine, see Bailey.
28
