transmission scan is performed before the emission scan to avoid interfer-
ence of radiations from the administered radioactivity. Other approaches
include post-injection transmission scanning (transmission scan after the
emission scan) and simultaneous emission/transmission scanning, but each
method suffers from various disadvantages of its own.
In PET/CT, the CT transmission scan is utilized for attenuation correc-
tion, which takes less than a minute thus improving the patient throughput.
As mentioned in the SPECT/CT technique, typically at the beginning of the
day, a blank CT scan without the patient in the scanner is obtained, which
is later used for subsequent patient studies for the day. Next, the CT trans-
mission scan of each patient is taken with the patient in the CT scan field
before the emission scan, and an attenuation correction map is generated
from the ratios of counts of each pixel (i.e., LOR) of the blank scan and the
transmission scan. Because the PET and CT units are fixed on the same
gantry, the patient remains in the same position on the table, which is then
moved to the PET scan field for the emission scan. Factors from the atten-
uation map are subsequently applied to each LOR in the patient’s emission
scan. The CT transmission method provides essentially noiseless images.
Attenuation depends on photon energy; therefore correction factors
derived from ~70-keV CT x-ray scans must be scaled to the 511-keV
198 13. Positron Emission Tomography
* a
bDDetector 2
Detector 1Fig. 13.10. Two 511-keV annihilation photons traverse thicknesses aand bof tissues
of an organ. However, attenuation of the two photons depends on the total thick-
ness Dof the organ regardless of aand b.