Biomedical Engineering Reference
In-Depth Information
FIGURE 5.11: Bilinear transformation of CT attenuation values given in
HU to PET attenuation values. Note that CT values between -1000 and 0 HU
are assumed to represent basically a mixture of air and water, while values
above 0 HU are considered to be water/bone mixtures.
based transmission scan techniques described above. It is especially valuable
in clinical daily routine as attenuation data can be obtained in the order of a
few seconds due to fast scanning and reconstruction that is possible in modern
CT systems. Due to very high photon flux during CT scans, noise is usually
no problem in CT data; besides, this high flux easily allows post-injection
transmission scans. Furthermore, X-ray tubes do not decay as radionuclide
sources do, thus minimizing replacement costs.
CT images basically are linear attenuation coecient maps CT (r) of the
scanned objects for the utilized X-ray beams. It is important to notice that
the used radiation is not monochromatic in nature as in PET (511 keV), but
polychromatic. For example, radiation released during a typical CT scan by
the X-ray tube that is run at a tube voltage of 130 kV has a continuous spec-
trum of energies of up to 130 keV Bremsstrahlung in addition to characteristic
peaks, altogether with an effective energy of around 70 to 80 keV. Values in
CT images are given in Hounsfield units (HU). These are attenuation values
that are scaled to the linear attenuation coecient of water at the effective
CT energy:
CT
C H 2 O
!
HU = 1000
1
:
(5.17)
Thus, water in CT images has 0 HU, while air is at -1000 HU.
The main challenge in PET attenuation correction on PET/CT systems is
 
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