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

4.1 Experimental characteristic curve
The response curve of the imaging system, in terms of PV as a function of exposure dose, has
been evaluated by exposing the IP at several dose levels with a fixed tube voltage of 28 kVp.
The experimental setup is the same as the one described in Fig. 3.1 since we want to use the
mAs vs. the air kerma calibration curve previously measured. The only difference is that the
ionization chamber was replaced by the IP cassette, while the 30 mm thick PMMA external
filter, close to the beam exit window, remained the same. The cassette was placed just above
the cassette holder, and therefore it was exposed without the anti-scatter grid in between.
The IP has been exposed to the uniform X-ray beam for a set of dose values, the operator
being careful to maintain the reproducibility of the process. After the exposure, the IP was
immediately read, in order to minimize and to keep the temporal decay effect constant for every
measure, and then cancelled. The acquired data were processed in the linear fix mode, allowed
by the FUJI system software, in order to fully control every step of the image processing mode.
The system was operating in High Resolution mode, with a pixel size of 50 �m. The sensitivity
S was set to 200 and the latitude L to 4.0, corresponding to a linear processing curve over the
entire dynamic range of the system. Each DICOM image was read by an IDL software code in
order to access the raw data. The IP is exposed so that its shorter side is parallel to the direction
where the heel effect is bigger. In Fig. 4.1 a row parallel to the direction where the heel effect is
present is shown. In this case the IP was exposed to a uniform beam with a dose of about 676
�Gy.
In order to analyze a uniformly exposed region (ROI) of the image, and to be allowed
to forget the heel effect, a strip 400 pixels wide (2 cm) and 2000 pixels height (10 cm) was
selected. Furthermore the ROI was centered around a point distant 4 cm from the thorax side
laterally centered. The ionization chamber was positioned in the same point during dose-mAs
calibration measurements. The selected area is shown in black in the typical uniformly exposed
image reported in Fig. 4.2.
For each dose, the mean PV has been computed as the average value of the PVs of the ROI
previously described. The corresponding standard deviation SD has been calculated on the ROI
after a plane subtraction, in order to eliminate the first order contribution. If we consider the
plane P that best fits the ROI under investigation and we address each ROI pixel by the indexes
47