# Radiochromic Films for the Two-Dimensional Dose Distribution Assessment
## 3 Instruments for Dose Reading
### Flatbed Scanners
In order to have the highest reproduction quality of the original image, it is saved in TIFF format and digitized without the options for color correction usually present in scanner software. The spatial resolution of a dosimetry system comprising of RCFs and flatbed-scanners is determined by the intrinsic spatial resolution of the RCF, as well as by the scanner spatial resolution. The user can select different scanning resolutions in units of dot per inch (DPI). Scanning resolutions of 1200 DPI (0.02 mm/pixel) are typical with modern flat-bed scanners. However, it should be noted that RCF images digitized at high resolution feature high noise and large uncertainties. In addition, the time to scan and process such large images increases with the increasing DPI. [PERSON] et al. evaluated the actual spatial resolution of a scanner used for Gafchromic XR-QA2 films by means of the Modulation Transfer Function (MTF) method [83]. They found that values higher than 150 DPI (0.17 mm/pixel) do not give significantly improve the spatial resolution. Spatial resolutions of <10 um in combination with small uncertainties can be achieved with reading techniques, such as the Confocal Raman Spectroscopy [84]. These studies are crucial for the precise dosimetry in small radiation fields or in regions with high dose gradients [85]. The study of the works in the literature suggests that the use of 72 dpi (0.35 mm/pixel) or 127 dpi (0.20 mm/pixel) in combination with 48-bit depth color (16 bit per color channel) are the most common options [16; 77], although scanning at higher DPI is possible. The digitized image is then analyzed as a matrix of the spatial coordinates and of the Pixel Values (PVs). As an example, Figure 3 shows the two-dimensional dose profile of an RCF, in this case, an HD-V2 Gafchromic(tm) film exposed to 126 Gy. The gray PV is plotted as a function of the distance, expressed in pixels. It should be noted that in this scale 255 corresponds to the maximum light and 0 to no light transmission. The plot of Figure 3 shows the characteristic profile of the particular radiation field analyzed (\({}^{90}\)Sr/\({}^{90}\)Y beta source) featuring a 15 \(\times\) 15 mm\({}^{2}\) square-shaped collimator. For dosimetry measurements, it is necessary to connect a dose value to a single piece of RCF, for example for calibration purposes. To this aim, a region of interest (ROI) is selected around the center of the irradiation of the digitized image, as shown in Figure 4a. From the histogram (Figure 4b) of the PVs of the selected ROIs, the average PV and its standard deviation are evaluated. The inset of Figure 4b reports the statistical parameters of the selected ROI. On the same line, more than one ROIs can be selected and the average PV calculated as the average of the PVs of each ROI. For enhancing the accuracy of dosimetry measurements, the use of Wiener filter to further reduce the measurement uncertainties is recommended [87]. The RCF color image can be analyzed either in gray scale or RGB  
Figure 3: Typical two-dimensional dose distribution measured with RCFs. Adapted from Reference [86]. scale on the basis of the dynamic range and of the precision required for the specific measurement. Figure 5 shows the typical response of a set of GafchromicTM HD-V2 films to different doses (up 1000 Gy) as a function of the color channel. Similar plots are reported from the manufacturer also for other GafchromicTM films [60]. The red channel is better for discriminating low doses. The dynamic range can be increased by considering the green channel and (even more) the blue channel, at the cost of a loss of sensitivity. The gray PV has characteristics intermediate among the three color channels, with being PVgray the average of the PVs of the other three channels:  
\[PV_{gray}=\frac{PV_{red}+PV_{green}+PV_{blue}}{3}.\]  
Once the dynamic range, the precision and the application field are defined, more than one choices are available for the calibration. This makes RCF dosimetry very flexible.