iterative reconstruction method, the attenuation factor is taken into con-
sideration in the estimated image and it is perhaps the best approach for
attenuation correction in SPECT.
In SPECT/CT, the CT transmission data are utilized for attenuation cor-
rection, with an advantage of fast data collection in less than a minute thus
improving the patient throughput. In the CT technique, typically a blank
CT scan is taken without the patient in the scanner at the beginning of the
day and is used for subsequent patient studies for the day. Next, the CT
transmission scan of the patient is taken and an attenuation correction map
is generated from the ratios of counts of each pixel of the blank and the
patient transmission scans. After the CT scan, the scanning table with
the patient in the same position is moved to the SPECT scanning field and
the emission scan is obtained. Factors from the map are then applied to the
corresponding pixels in the patient’s emission scan for attenuation correc-
tion. A typical patient scan is shown in Figure 12.17 with and without atten-
uation correction.
Note that the attenuation depends on photon energy and, therefore, cor-
rection factors derived from ~70 keV CT x-ray scans must be scaled to the
energy of single photons of the radionuclide used (e.g., 140 keV of 99 mTc)
by applying a scaling factor defined by the ratio of mass attenuation coef-
ficient of the photons in tissue to that of 70 keV photons. This factor is
assumed to be the same for all tissues except bone, which has a slightly
higher mass attenuation coefficient. Because the position of the patient
does not change in CT transmission and PET emission scans, the error in
positional misalignment of pixels between the two scans is minimized. The
CT transmission method provides essentially noiseless images.
Several factors introduce errors in CT attenuation correction factors. One
is the respiratory motion of the thorax during scanning that causes mis-
match in the fusion of SPECT and CT images. These effects are minimized
by multislice CT scanning and a short scanning time (~25 s), with a breath
hold at end expiration. Also, contrast agents affect the CT attenuation
174 12. Single Photon Emission Computed Tomography
Fig. 12.17. Illustration of attenuation correction of cardiac SPECT image recon-
structed by filtered backprojection. A. Cardiac SPECT images without attenuation
showing deficient activity in the inferior wall. B. The same images with attenuation
correction using the CT transmission method showing improvement in count
density in the inferior wall.