Equation (12.9) is the attenuation correction factor that is applied to the
geometric mean counts to obtain the unattenuatted counts.
Attenuation Correction
There are two methods of attenuation correction: the Chang method and
the transmission method.
Chang Method.The application of Eq. (12.9) is called the Chang method.
In this method, an attenuation map is generated from individual pixel
values based on the estimated thickness of an organ of interest and the
assumption of a constant m. Emission data are obtained by Eq. (12.7) and
attenuation correction is applied using the factors from the map. This
method works reasonably well for organs such as the brain and abdomen,
where the attenuating tissue can be considered essentially uniform.
However, the situation is complicated in areas such as the thorax, where
mvaries due to close proximity of various organs, and the Chang method
is difficult to apply.
Transmission method.The most acceptable method for attenuation cor-
rection in SPECT is the transmission method. Several SPECT systems
currently use a transmission source of a radionuclide that is mounted
opposite to the detector such as is an x-ray tube in computed tomogra-
phy. The detector collects the transmission data to correct for attenua-
tion in emission data. For 99 mTc imaging, common transmission sources
are gadolinium-153 (^153 Gd) (48 keV, 100 keV) and^57 Co (122 keV),
whereas for^201 Tl imaging, americium-241 (^241 Am) (60 keV) and^153 Gd are
used in different configurations. In one common configuration, a well-
collimated line source is mounted that is translated across the plane
parallel to the detector face to collect transmission data. The line source
is scanned at each angular stop during the SPECT data collection to
apply attenuation correction to each angular projection.
Typically, a blank scan is obtained without the patient in the scanner. The
data from this scan are used for all subsequent patients for the day. Then a
transmission scan is obtained with the patient in the scanner before the
emission scan is acquired. The ratio of counts of each pixel between the
blank scan and the transmission scan is the attenuation correction factor
for the pixel, which is applied to the emission pixel data obtained next.
This is done for each patient for the day. Because the patient is positioned
separately in the two scans, error may result in the attenuation correction.
Because the transmission and emission photons have different energy, it
is possible that SPECT cameras can be used to collect both transmission
and emission data simultaneously using separate discriminator settings.
However, one should keep in mind that there is spillover of scattered high-
energy photons (i.e., with reduced energy) into the low-energy photopeak
window. The transmission data are used to calculate the attenuation factors,
which are then applied to the emission data. It should be noted that in the
Single Photon Emission Computed Tomography 173