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

centers activated in the illuminated area and thus to the dose exposure on the plate, with a linear
dependence. The laser power determines the ratio of the information released by the F-centers
and consequently, the reading time and the remaining unused information.
The radiation emitted by the F-center from both sides of the IP is isotropic. In order to
collect most of the emitted radiation from the upper part of the plate, a light guide receives the
direct light and the light deflected by a mirror that has the goal of cover most of the solid angle
as possible. The lower side of the IP is coupled to a second light guide and a second PMT. The
signal from the two sides, properly weighted, are added together. The photostimulated emis-
sion is centered at about 400 nm, a wavelength which differs deeply from the laser stimulating
radiation, so to avoid interference between the stimulating and detection process. In fact the
photomultiplier tubes have their sensitivity peak at 400 nm.
An analog amplifier stage that applies a logarithmic conversion follows the PMT. This is
necessary to recover the linearity between the photostimulated light and the electrical signal
which had been lost in the PMT photo-electron amplification process [17]. Then the analog to
digital conversion takes place.
The system under investigation digitize the total X-ray induced signal over a range of in-
cident exposures four orders of magnitude wide (from 0.01 mR to 100 mR) keeping a linear
relation between exposure dose and ADC counts. The 12 bit ADC produces 11 bits of effec-
tive quantization levels prior to normalization of the image 10 bit depth. The output data for a
typical high resolution image used during this analysis are stored in 32 MBytes files.
In order to perform a complete cancellation of the information from the IP after the reading
process, the plate is exposed to an intense light that removes the remaining image causing the
decay of the metastable sites still present. Due to the dual side reading technology, the erasure
lamps act on both the surfaces of the imaging plate.
3.6 Histogram analysis and IP sensitometric curve
The digital output represents the pre-processed data. Histogram analysis is applied to the pre-
processed data to define the wanted versus unwanted signals in a scanned image plate for a
particular incident exposure and examination type. As the linear exposure latitude for the imag-
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