Biomedical Engineering Reference
In-Depth Information
the emerging radiation and the formation of tomographic images are similar for
both PET and SPECT but the underlying physics and the instruments employed
are completely different. In SPECT, nuclear decay results in the emission of one
or a few uncorrelated photons in the 100-200 keV energy range. A lead colli-
mator drilled with small holes is used to mechanically collimate the incoming
photons by allowing those traveling in one particular direction to interact with
the scintillator, while all others are absorbed. Rotating gamma cameras with
single or multiple crystal detectors are used to form a tomographic image. Here
lies the sensitivity differences between PET and SPECT, and partly explains why
PET has received much more attention than SPECT for
in vivo
assessment and
quantification of physiologic functions in the body.
Despite the fact that both PET and SPECT suffer from attenuation and Comp-
ton scattering of the photons inside the body which can result in image artifacts
and loss of quantitative accuracy, SPECT has been largely considered to be
nonquantitative and limited to providing qualitative or relative functional im-
ages. This is because correction of attenuation and scatter in SPECT are not
easy as compared to PET, where attenuation correction is routine (with the
exception of whole-body PET). In addition, the spatial resolution of SPECT is
inferior to that in PET. Even with triple-headed gamma cameras, the resolution
is approximately 8-10 mm FWHM but the theoretical limit of 1-2 mm FWHM
can be achieved for PET with new generation of detector technology. Further,
typically higher signal-to-noise ratio and lower scatter with PET also helped
establish PET as the favorable method for quantitative measurements of physi-
ological parameters.
Although PET will continue to provide insights into biochemical and physio-
logical processes
in vivo
, access to PET is limited due to the requirement of a cy-
clotron and high operation costs. Recent advances in quantitative SPECT and the
widespread application of multidetector SPECT systems with improved sensitiv-
ity and dynamic imaging capabilities have made absolute physiological parame-
ter estimation possible with the much more widely available SPECT. One of the
major applications of dynamic SPECT is to quantify myocardial perfusion, which
is important for the diagnosis and clinical management of patients with coronary
artery disease where a perfusion defect after an intervention may indicate in-
complete reperfusion or persistent coronary occlusion. Similar to dynamic PET,
compartmental modeling is used in dynamic SPECT to quantify physiologic
