Biomedical Engineering Reference
In-Depth Information
two gamma rays, time taken to produce scintillation light in the detector, and
time delays in the electronic devices in the PET system.
Once the signal leaves the detector module, it is processed by several elec-
tronic circuits. The choice of components depends upon the application and,
therefore, there are many ways to implement the coincidence detection circuitry.
A simplified schematic representation of detecting coincidence events with two
detectors is also shown in Fig. 2.2. The output signal from each detector is fed
into a pulse generator. Note that the signal amplitude from the two detectors (
V
A
and
V
B
) may not be the same due to incomplete deposition of photon energies
or variation in efficiency among the detectors. In addition, there exists a time
difference between the detectors to react upon the photons arrival, and a finite
reaction time for the electronic devices to response, resulting in difference in
the time
t
1
and
t
2
at which the amplitude of the signal crosses a certain fixed
voltage level (
V
T
), which triggered the pulse generator to produce a narrow
pulse. The narrow pulse is then fed into the gate-pulse generator where a pulse
of width 2
τ
(coincidence timing window) is generated for individual detectors.
A coincidence detection circuit is then used to check for a logical
AND
between
the incoming pulses. For the example shown in Fig. 2.2, there is a pulse over-
lap between two signals produced by the gate-pulse generators. Therefore, the
event is a true coincidence which is regarded as valid and is registered. It is
easy to see that if
t
2
−
t
1
≥
2
τ
, the event is not in coincidence, and thus it is not
recorded by the coincidence detection circuit.
2.6 Coincidence Criteria
In general, an event (positron annihilation/photon emissions) is regarded as valid
and is registered by the coincidence detection circuit if the following criteria
are satisfied [26, 53]:
two photons are detected within a predefined coincidence window,
the LoR formed between the two photons is within a valid acceptance
angle of the tomograph, and
the energy deposited in the crystal of the detector by both photons is within
the selected energy window.
Such coincident events are often referred to as
prompt
events.
