Studio delle prestazioni di un sistema a fosfori per mammografia ...

eading system and of the IP under investigation are describe more in detail in Chapter 3.
1.3 The digitization process
Digitization is a two step process of converting an analog signal into a discrete digital value.
The signal must be sampled and quantized. Sampling determines the location and size of the
PSL signal from a specific area of the PSP receptor, and quantizing determines the average
value of the signal amplitude within the sample area. The output of the PMT is measured at
a specific temporal frequency coordinated with the laser scan rate, and quantized to a discrete
integer value dependent on the amplitude of the signal and the total number of possible digital
values by the Analog to Digital Converter (ADC). The ADC converts the PMT signals at a rate
much faster than the fast scan rate of the laser (on the order of 2000 times faster, corresponding
to the number of pixels in the scan direction). A pixel clock coordinates the time at which a
particular signal is encoded to a physical position on the scan line. Therefore, the ratio between
the ADC sampling rate and the fast scan (line) rate determines the pixel dimension in the scan
direction. The translation speed of the phosphor plate in the sub scan direction coordinates with
the fast scan pixel dimension so that the width of the line is equal to the length of the pixel (i.e.
the pixels are ”square”).
ADC
sample
Figure 1.7: Schematic diagram of the digitization process.
The pixel size is typically between 100 and 200 �m but in mammographical applications a
pixel size of 50 �m is preferred. Typically the ADC converts the signal to a 10,12 or 16 bits
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