Imaging by Position-Sensitive Detection

Imaging by seanning teehniques has its benefits, sueh as relatively simple implementation of multiwavelength deteetion and optieal seetioning by eonfoeal deteetion or two-photon exeitation. Nevertheless, parallel aequisition of time-resolved images is desirable for a number of applieations. 

Parallel image aequisition is, in prineiple, possible by two-dimensional multianode PMTs. The anode elements are eonneeted to a standard routing deviee, and the photons are reeorded as deseribed above under Seet. 3.1, page 29. However, the number of amplifiers and diseriminators in the router inereases with the number of anode elements. Power eonsumption and spaee requirements restriet the number of pixels that ean be obtained this way. The praetieal limit is somewhere between 16 and 64. An array of 64 pixels eannot really be eonsidered an image. 

Almost continuous position information can be obtained from an MCP-PMT with a resistive anode or with a micromachined wedge-and-strip geometry [247, 248, 262, 312] (see also Sect. 6, page 213). The detector principles are shown in Fig. 3.11. 

The position is calculated by the ratio of the pulse amplitudes (or charge) measured at the outputs of the anode stracture. Timing information is available from the pulse at the last dynode or at the low-voltage side of the multichannel plate (see also Sect. 6.1.3, page 215). Once the position information is available, the task is reduced to multidetector operation in a large number of detector channels. 

A seeond approaeh is to use FIFO operation (see Sect. 3.6, page 43). The signals A1 to A4 are eonverted direetly by four ADCs. The converted values for A1 to A4 and the TAC times for the individual photons are stored in a file. The image is built up off-line by analysing the recorded data. The problem of this approach is the huge amount of data that has to be transferred over the computer bus and stored on a hard dise. Given a data word size of 12 bit for A1 through A4, and 12 bit for the TAC time, 60 bits or 8 bytes per photon are required. 

---