Corrections for Physical Factors - Correction Techniques in Emission Tomography - page 78

thus avoiding excessive radiation events seen by a single detector element.
The released annihilation radiation of these sources is registered just as in the
emission scan, resulting in transmission sinograms ti i that contain information
about the attenuation along all lines of response i. To derive attenuation
correction factors, a second positron source-based scan, the so-called blank
scan, has to be performed without any object inside the scanner's eld of
view, resulting in blank sinograms bi. i . In its simplest version, the attenuation
correction factor for a line of response i is then given by b i

t i , which leads to

the corrected emission data g AC
i

:

= b i g AC

i

t i ·

g i (5.15)

(see Figure 5.8). The main advantage of this method as opposed to other
approaches where assumptions about attenuation values are made is the direct

FIGURE 5.8: Measured attenuation correction in stand-alone PET systems.