High Count-Rate Systems

The TCSPC chaimels of this system have 100 ns dead time. The maximum useful (reeorded) count rate of each individual channel is 5 10 s The system ean be used at a total recorded count rate up to 20T0 s or a total detector count rate of 40T0 s A typical result is shown in Fig. 5.84. 

A potential application of multimodule systems is high-speed two-photon multibeam scaiming systems [53, 77]. FLIM systems with 4, 8 or even 16 beams and the same number of parallel TCSPC channels appear feasible. The problem is to direct the fluorescence signals from the individual beams to separate PMTs or separate charmels of a multianode PMT. If this problem is solved, two-photon lifetime images can be recorded with unprecedented speed and resolution. 

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The shape of the detector pulse and the charge collection time have an influence on the way in which the pulse processing electronics is set up. In particular, long charge collection times, especially if extended by trapping (see Section 3.6.6), have important implications for counting rates. These matters will be discussed more fully when the electronic parts of the system (Chapter 4) and high count rate systems (Chapter 14) are considered. 

The essential functions of an amphfier were discussed in Chapter 4, Section 4.4. I suggested there that pulse processor would be a more appropriate name for this item than the historic amplifier . This is particnlarly true when considering high count rate systems. The data in Table 14.1 showed us that the pulse processor is the critical restraint on pulse throughput, mainly dne to pulse pUe-up (random summing) within it. The high cost of... 

RESISTIVE FEEDBACK PREAMPLIFER. The conventional preamplifier in which the input voltage step is reset by a feedback resistor. For high count rate systems the Transistor Reset... 

Fig. 3.7. The detection efficiency of a system with a dead-time 350 ns as a function of the incident count rate. At high count rates the detection efficiency reduces due to pileup effects.

In the first case — to which belong the gas filled detector systems — the position of each absorbed photon has to be determined and the result has to be stored in the data acquisition system. In high counting rate experiments, these operations have to be carried out in very short times, in order to minimize dead time losses. 

The resolution of the wire per wire readout is restricted to one wire distance, which for many applications is not sufficient. However, the high count rate capability is certainly an advantage where no high spatial resolution is necessary. The encoding time can be made as short as 50 nsec with standard circuits (Schottly TTL or ECL). The storage of the large amount of data in a memory systems however, is not trivial, and is certainly accompanied by high expenses. 

The total time required to complete the above steps is defined as the dead time (r) and is related to the signal integration time that depends on the electronics and the scintillation decay time. During this time the detection system is unable to process a second event, which will be lost. This loss (called the dead-time loss) is a serious problem at high count rates and varies with different PET systems. It is obvious that the dead-time loss can be reduced by using detectors with shorter scintillation decay time and faster electronics in the PET scanners. 

With a dead time of 100 ns per TCSPC channel, total useful count rates of the order of 20 MHz can be achieved. All four channels can be used for multidetector operation. The high count rate and the high number of channels make multimodule TCSPC systems exceptionally useful for diffuse optical tomography [34], and high count rate applications in laser scanning microscopy [39]. Details are described under Sect. 5.5, page 97 and Sect. 5.7, page 129. 

Conventional TCSPC in combination with slow-scan systems has been used in [74, 76]. However, high count rates and high scan rates cannot be achieved with this technique. 

These results show that MCP PMTs can be used for eount rates up to the maximum useful count rate of currently available TCSPC systems. It should be noted, however, that at a count rate of 3.3 MHz and an operating voltage of 3 kV the output current is considerably higher than the specified maximum of 100 nA. This is certainly not a problem in applications where high count rates appear only temporarily, e.g. in scanning microscopy. The lifetime of the MCP is not known at a continuous count rate of more than 3 MHz. 

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