Biomedical Engineering Reference
In-Depth Information
from the four PMTs. Anger-logic is used to obtain the
X
and
Y
positions based
on the four PMT outputs
P
i
:
P
1
+
P
2
−
P
3
−
P
4
P
1
+
P
2
+
P
3
+
P
4
X
=
(2.4)
P
1
−
P
2
+
P
3
−
P
4
P
1
+
P
2
+
P
3
+
P
4
Y
=
(2.5)
The combined BGO block/photomultiplier system has an approximately cubic
spatial resolution of 4 mm full-width-at-half-maximum and coincidence timing
window of approximately 12 ns.
As seen from Fig. 2.2, the probability that the annihilation event occurs ex-
actly within the region of coincidence detection and is recorded by the detectors
is very small because most gamma rays may travel out of the region of coinci-
dence detection even if the annihilation event occurs within that region. This
probability can be increased by using a ring of detectors within which any detec-
tor is in coincidence with all other detectors located at the opposite side of the
ring. With the use of multiple rings of detectors, the probability of coincidence
detection is further increased because coincidences can be detected by other
rings of detectors if they cannot be recorded by the plane of the ring within
which the annihilation events occur. The device that used to detect the 511 keV
gamma rays emitted from annihilation and construct a map of radiopharma-
ceutical distribution inside the body is called
tomograph
(or
scanner
), which
usually has multiple rings of detectors surrounding the patient.
2.8 Detected Events
Ideally the only prompt events registered by the detectors are those which arise
from “real” positron annihilation. However, a number of other unwanted events
that satisfied the coincidence criteria are also registered. The detection of un-
wanted events causes noise and degradation in spatial resolution. Therefore,
their correction is essential to improve the quantification. In general, five types of
event can be detected by PET scanner, and four of them are illustrated in Fig. 2.4.
A
true coincidence
refers to an event that two photons are emitted back-
to-back from a single positron-electron annihilation, and are detected simulta-
neously by opposing detectors within the region of coincidence detection and
within the coincidence timing window of the system.
