patient’s organ of interest, and the transmitted beam is detected by an array
of detectors. The acquired data are used to reconstruct images using re-
construction methods described for SPECT images. Currently multislice
CT scanners are available, which are fast and provide 6, 16, or 64 slices
of the organ in seconds. These scanners have produced high-resolution
diagnostic-quality images and reduced the imaging time significantly thus
improving the patient throughput.
Either CT or SPECT imaging can be performed first, followed by the
other. For example, CT images are taken first with the organ of interest in
the CT field of view. Next, the scan table with the patient in the same posi-
tion is moved to the center of the SPECT FOV and images are taken. Both
CT (anatomical) and SPECT (functional) images are reconstructed and
then fused together by applying appropriate alignment algorithms. Various
vendors provide commercially available fusion software, namely, Syngo by
Siemens, Syntegra by Philips Medical, MIM of MIMVISTA, and Volumetrix
of GE Healthcare. Because the position of the patient on the table does not
change, both CT and SPECT images are aligned very accurately and the
overall accuracy is improved by 20 to 25% compared to either modality
alone.
A major advantage of including CT in the dual-modality is that the CT
data can be utilized in attenuation correction of SPECT data, which is par-
ticularly useful in cardiac perfusion imaging. Apparent perfusion defects are
often seen in the anterior wall in women due to breast position and in the
inferior wall in men, and soft-tissue attenuation also shifts between rest and
stress images. As will be described later, attenuation correction using CT
transmission data compensates for these artifacts more accurately in a
shorter time than using the conventional sealed source transmission data.
Such CT transmission attenuation correction can be applied to other organ
imaging as well.
GE Healthcare pioneered the first commercial SPECT/CT system inte-
grating its Infinia SPECT camera with the Hawkeye CT scanner. Currently,
Symbia Truepoint SPECT/CT by Siemens and Precedence by Philips
Medical have been added to the commercial market.
Factors Affecting SPECT
Photon Attenuation
g-Ray photons are attenuated in body tissue while passing through a
patient. Attenuation causes less count density generating artifacts particu-
larly at the center of the image. The degree of attenuation depends on the
photon energy, the thickness of tissue, and the linear attenuation coefficient
of the photons in the tissue. If I 0 is the number of photons emitted from
an organ and Iis the number of photons detected by the gamma camera,
then
I =I 0 e-mx (12.6)Single Photon Emission Computed Tomography 171