Biomedical Engineering Reference
In-Depth Information
a source of noise, the rate of which is approximately proportional to the square
of the activity in the field of view (FOV). The performance of PET scanner for
high count rate studies is degraded and therefore, correction for randoms is
necessary.
Multiple
events are similar to random events, except that three photons
originated from two positron annihilations are detected within the coincidence
timing window. Because of the ambiguity in positioning the event, these coinci-
dences are normally discarded.
A
single event
for which only one photon is emitted is also possible due to
some physical factors. The single events are usually rejected by the coincidence
detection circuit since detection of only one event within the timing window
violate the condition of coincidence. Yet in practice, about 1-10% of single events
are converted into paired coincidence events.
2.9 Data Acquisition
Most of the modern PET tomographs are capable of acquiring data in two
different modes: two-dimensional (planar) acquisition with septa in-place and
three-dimensional (volumetric) acquisition with septa retracted, exposing the
detectors to oblique and transaxial annihilation photon pairs. Both modes of
configuration for data acquisition are shown in Fig. 2.5. In two-dimensional
imaging, each ring of detectors is separated by septa made of lead or tungsten.
The main aim is to keep the scatter and random coincidence event rates low so
as to minimize the cross-talk between rings. However, in doing so, the sensitivity
of the scanner is drastically reduced. Three-dimensional acquisition can be used
to improve the sensitivity by removing the interplane septa, thus allowing coin-
cidences that happened within all rings of detector to be detected. Although the
sensitivity of the scanner is increased, higher fraction of scattered and random
coincidences and substantial dead time are more apparent.
In a tomograph, each detector pair records the sum of radioactivity along
a given line (i.e. the
line integral
or
projection
) through the body. The data
recorded by many millions of detector pairs in a given ring surrounding the body
is stored in a two-dimensional (projection) matrix called
sinogram
, as shown
in Fig. 2.6(B) and Fig. 2.6(A) shows how data is acquired in two-dimensional
mode. Each point in the sinogram represents the sum of all events detected with
