4.2. Quantitative Imaging at Higher Count-rates
4.2.1. Scenario 1: Effect of gating image intensifier on count-rate
According to the vendor, Proxivision, these gating signal parameters are controlled via an external TTL input pulse (“Pulse Follow mode”) connected to the photocathode. This was experimentally achieved by using a waveform generator to set the required imaging parameters.
The first imaging parameter, gating repetition rate or frequency, should match the CMOS camera's frame-rate speed of the large-area iQID. For these measurements, the gating frequency will be 20 Hz and gating times similar to the scintillator, Transcreen LE, of 1 µs to 1 ms. An example of the recommended TTL input signal/waveform is expressed in Figure 30, where gating at the photocathode occurs after the TTL input signal is applied.
Figure 30 : (Top) Recommended Vendor Waveform from Proxivision vendor with TTL signal trigger voltages of +5V and 0V. These correspond to photocathode (PC) voltages of +12V and - 200V of closed and open, respectively.
The second gated imaging parameter is amplitude, which is the maximum displacement of a wave measured from its equilibrium position [62]. As shown in Figure 30, the gating amplitude is +5Vpp (peak-to-peak) and equal to the microchannel plate's control voltage. If the TTL input pulse is +5V, then the gated photocathode (PC) is set to an input voltage of +12V. In other words, the gated photocathode at +12V is not sensitive to light. The gated photocathode is only sensitive to light at -200V after receiving a TTL input pulse of 0V. The third gated imaging parameter, DC offset, is the mean amplitude displacement of a waveform gating signal from zero. For these gated large-area iQID measurements, the DC offset will be constant at +2.5V.
The fourth gated imaging parameter, the duty cycle, is the focus of this section. The duty cycle is the percentage of time when the gating signal is “ON” and can be defined by Equation 24 below.
As shown in Equation 24 below, the duty cycle is determined by pulse width and period. Pulse width is the time from a 50% threshold of the pulse's rising edge to a 50% threshold of the next falling edge. The period is the reciprocal of the gating frequency (20Hz), as previously discussed above [63].
𝐷𝑢𝑡𝑦 𝐶𝑦𝑐𝑙𝑒 [%] = 100 𝑥 (𝑃𝑢𝑙𝑠𝑒 𝑊𝑖𝑑𝑡ℎ)
𝑃𝑒𝑟𝑖𝑜𝑑 Equation 24
In this first scenario, the duty cycle was adjusted according to the CMOS shutter's pulse width of 50 ms. The gating signal was repeated at the same gating frequency (20 Hz) as the CMOS frame-rate (20 FPS). At a 50 ms CMOS shutter, the duty cycle was 100% by using Equation 24 above. In other words, the gating frequency of the image intensifier matched the frame-rate of the CMOS sensor. To have the image intensifier “on” for half the CMOS shutter (i.e., 25 ms), the duty cycle would be reduced to 50%. An example image of a square wave at normal polarity and 50% duty cycle is expressed in Figure 31.
This idea of lowering the duty cycle of the gated large-area iQID down to 0.01 % was further investigated below. This demonstrated the effectiveness of the gated image intensifier on the count-rates of the large-area iQID. Hence, providing insight into the signal generation process of image intensifier and CMOS sensor.
Figure 31: Example of a square waveform at normal polarity and 50% duty cycle
4.2.1.A Methods
In this second scenario, gated measurements were tested with 99Tc at constant frame-rates (20 FPS) and CMOS shutter times (50 ms). First, characterization measurements were conducted in alpha and beta imaging mode. Scintillators used for alpha and beta imaging mode were
ZnS:Ag and TranScreen LE scintillator, respectively. The alpha characterization measurements consisted only of background measurements. On the other hand, beta characterization
measurements consisted of background and source measurements with 99Tc. This was achieved by calculating three different metrics for the non-gated and gated background and source (99Tc) baseline measurements (beta imaging mode only). Metrics included count-rate, source and non- source ROI analysis, and data cuts using cluster intensity and area. New non-gated large-area iQID baseline background (Figure 36) and 99Tc baseline measurements (Figure 39) were first conducted. This preliminary step was required because gated background count-rates were currently unknown since this was a new configuration for large-area iQID imager. To match the specifications of the Vendor (Proxivision), characterization of gated 99Tc baseline measurements occurred at an inverted polarity and 99% duty cycle. The remaining gated imaging parameters were fixed at a gating frequency of 20 Hz, TTL gating amplitude of +5Vpp, and a DC offset of +2.5V. Since the waveform generator could introduce normal or inverted wave polarities, both polarities were tested to determine their effect on count-rate. This step was used as an operational validation because it was hypothesized that the inverted polarity would produce the highest count-rates. I hypothesized this because Proxivison vendor stated the recommended TTL signal was inverted polarity (Figure 30). Waveforms of the different polarities and duty cycles for the newly source, non-gated, and gated backgrounds were tested.
Figure 32 shows the waveform for the newly non-gated baseline background measurements and Figure 33 displays the waveform for baseline measurements conducted at an inverted polarity with a 99% duty cycle.
Figure 32: Schematic of the new baseline (non-gated) signal within the gated image intensifier's microchannel plate.
The gated LAiQID is a new iQID imager configuration so the absolute efficiency of 99Tc must be re-calculated as the new characterized baseline for the gated LAiQID. The purpose of this
experiment was to determine the impacts of the gated imaging parameters on count-rate capability. As for the new baseline measurements, examples of the different duty cycles and wave polarities to characterize the gated image intensifier are expressed in Figure 34 and Figure 35.
Figure 33: Schematic of the new baseline (gated) signal within the microchannel plate of the gated image intensifier at 99% duty cycle and an inverted polarity.
Figure 34: Schematic of the gating signal within the microplate at 10 % duty cycle and normal polarity (top) and inverted polarity (bottom).
Figure 35: Schematic of the gating) the signal within the microchannel plate at 10 % duty cycle and normal polarity (top) and inverted polarity (bottom).