(13.3)
where Ameanis the mean of all pixel counts and Aiis the counts in the ith
pixel. The observed count Ciin the ith pixel from the patient is then nor-
malized by
Cnorm,i=Ci×Fi (13.4)where Cnorm,iis the normalized count in the ith pixel. The normalization data
collection requires a long time (~6–8 hrs) and is normally carried out
overnight. These factors are obtained weekly or monthly and most vendors
offer algorithms to obtain them routinely.
Photon Attenuation Correction
The two 511-keV photons in PET can traverse different thicknesses of
tissues before detection and are attenuated to a different degree similar to
situations discussed under SPECT. If the two photons traverse aand b
thicknesses of tissues of an organ, then the attenuation correction Pfor each
pixel (i.e., each LOR) is given by
P=e−ma×e−mb=e−m(a+b)=e−mD (13.5)where mis the linear attenuation coefficient of 511-keV photons in tissue
and Dis the total thickness of the organ (Fig. 13.10). When photons tra-
verse various organs, differences in linear attenuation coefficients and organ
thicknesses must be taken into consideration. As in SPECT, Eq. (13.5) has
been employed to correct for attenuation in brain PET imaging, based on
the assumption of uniform density of tissue and a constant mfor 511-keV
photons in tissue (Chang method). However, the method tends to cause
artifacts due to underestimation of attenuation in the thorax area.
The transmission method is widely used for attenuation correction of
PET emission scan data. In this method, normally a^68 Ge source is used to
obtain the transmission scan in dedicated PET imaging. The source is placed
in a holder mounted at the edge of the scanner bore and the holder is
rotated by a motor so that data detected by all detector pairs can be
acquired. Normally, a blank scan is obtained at the beginning of the day
without any object or patient in the scanner. These blank scan data are used
for all subsequent patients for the day. Next a transmission scan is obtained
with the patient in the scanner for each patient. The ratios of the counts in
each pixel (i.e., each LOR) between the blank scan and the transmission
scan are calculated for each patient. The emission scan is taken with the
patient in the same position as in the transmission scan to minimize errors
in correction factors, and then each LOR datum is corrected for attenua-
tion by using the corresponding ratio. It takes 20 to 40 min for acquisition
of the transmission scan depending on the source strength. Normally the
F
A
A
i
mean
i=
Factors Affecting PET 197