Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry.

doi:10.21203/rs.3.rs-2448459/v1

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Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry.

https://doi.org/10.21203/rs.3.rs-2448459/v1

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Radiochromic film is a good dosimeter choice for patient QA for complex treatment techniques because of its near tissue equivalency, very high spatial resolution and established method of use. Epson scanners are widely used for film dosimetry. Previous studies reported that different components in the construction of flatbed scanners introduce light polarisation which is responsible for enhancing the Lateral Response Artefact (LRA) effect. The purpose of this study is to investigate the contribution of the scanner bed and mirror system of EPSON flatbed scanners to this effect. EBT3 films were irradiated with 40x40cm² field size in 6 MV beams. Films were analysed using images captured by a Canon 7D camera utilising a 50mm focal length lens to evaluate the polarization introduced by various combinations of mirrors making up the mirror system. An EPSON V700 scanner is used to investigate the effect of the scanner bed. The magnitude of polarization introduced by the mirror system is found to be up to 14%, whereas the path length effect, from the difference in refractive index of film and scanner bed, is found to be insignificant (at less than 2%).

Radiotherapy dosimetry

Radiochromic Film

GafChromic

EPSON scanner

Film dosimetry

mirror effect

scanner bed effect

Radiochromic film is often used for two-dimensional dose measurement in radiotherapy. Its high spatial resolution, weak energy dependence and near tissue equivalency make it popular for patient-specific QA of complex radiotherapy treatment techniques (e.g. IMRT, VMAT, SABR/SBRT)[1]–[5]. Digitization of optical density of irradiated films is commonly done using commercial flatbed scanners, as they are inexpensive and can produce very high-resolution images. However, the dosimetry system of radiochromic film coupled with a commercial flatbed scanner has some drawbacks like any other dosimetry system The orientation effect, of film to scanner bed, and the lateral response artefacts (LRA) are the two main issues associated with the system [6]–[15]. A strict protocol of marking and placing the film in the same orientation throughout the dosimetry process eliminates the orientation effect. The LRA effect is the change of measured optical density from middle to side of the film, orthogonal to the scanner’s light source travel direction[8], [11], [16]–[19]. It remains as a main issue which has been investigated widely[11], [17]–[21]. The origin of the LRA effect comes from two different sources, the film itself and some components of the flatbed scanner. The needle-like crystal structure[11], [22], [23] in the active layer of radiochromic films introduces anisotropic scattering and polarization of light[23]. Upon irradiation, the neighbouring polymers create bonds and turn into even longer rods, which enhances both of these phenomena. The lens system, scanner bed and mirror system of flatbed scanners[13] are the scanner components that contribute towards LRA in different ways. The lens system fails to collect all the light from distant parts of the film[10], [15], the difference of refractive indices of film and scanner bed causes a path length effect which reduces optical densities at the distant part of the film[12] and the mirror system introduces light polarization[12], [13]. The magnitude of light polarization, introduced by the scanner and the film, increases with increasing lateral distance from the centre of the scanner [12]. The effect of the lens system was investigated by Shameem et al [15]. Van Buttom et al [12] investigated the path length effect and light polarisation caused by the scanner. They stated that “ The film-induced optical path length variation becomes relevant if its refraction index differs from that of the glass plate of the flatbed scanner”. Shoenfeld et al [13] confirmed these two phenomena and also provided the theoretical background of the pathlength effect and of the introduction of light polarization caused by reflection of light by the mirrors, hence affecting the LRA.

The purpose of this work is to investigate the path length effect caused by the scanner bed from having a different refractive index from that of film and also the effect of the mirror system used on an EPSON scanner, following a different approach from that of Van Buttom et al [12] and Schoenfeld et al[13], by investigating each of these components individually. This is part of a wider study considering each component of the scanning system separately, with a view to optimising scanner design and procedures for film dosimetry.

Path Length Effect: The film preparation, handling and irradiation methods were similar to those described in our previous work [15], [24]. Full detail can be obtained there, but essential detail is repeated here for completeness. EBT3 films were cut into 3cm x 20.3cm strips along the short side of the film. The light source of the scanner is across the short side of the scanner bed and the longer side of a film strip is positioned on the scanner bed to be parallel to the light source, perpendicular to scan direction. Film was irradiated at 10cm depth of plastic water with 10cm backscatter in a phantom that was 30 cm x 30 cm area presented to large area beams to achieve uniform dose across the film. Film pieces were irradiated on an Elekta Synergy linear accelerator (linac) using a 6MV beam and a 40cm x 40 cm field size, giving doses to the film of 0MU, 500MU and 1000MU which is equivalent to 0Gy, 5.64Gy and 11.28Gy respectively. An EPSON V700 flatbed scanner was used to scan the film. The scanner bed was then removed and the film was scanned using as support (substitute ‘scanner bed’) variously, a piece of acrylic sheet, an opaque laminating paper, another piece of EBT film and finally with no bed at all. For the latter, a hole was cut in the middle of a scanner bed and the film piece was placed over the hole to scan it with no bed directly beneath the area of interest.

Irradiated EBT3 films were left in the box for two hours[25], [26] and then scanned using the EPSON V700 scanner with the above ‘beds’. The film piece was taped down to the bed to flatten the curvature. Gloves were used all the time during handling the film to avoid any contamination from fingerprint marks. The following settings were used for the scanner

Mode: Professional
Document Type: Film (with Film Area Guide); this setting allows transmission scanning
Film Type: Positive Film
Image Type: 48-bit colour
Resolution: 508dpi
No Colour Correction was applied

The film piece was placed at the central position on the scanner bed and on the other materials used as substitute scanner beds to ensure the whole film piece is in the scanning area. Each film piece was scanned 5 times. The scanned images were saved as *.tiff (tagged image file format) which were read and separated into three colour channels in ImageJ V1.49 software. The film pieces were rotated by 180⁰ and the whole process was repeated.

In ImageJ an average profile was generated across each film for each combination using a rectangular ROI cropped 1mm in from the film edge [15, 24]. The average profiles for each dose level, each orientation, each bed type and each colour channel were analysed in MS Excel where they were normalised to the mean of the central 100 data points. For each bed type and each dose level there are 10 scans, 5 in initial orientation and 5 in 180⁰ rotation. Averaging these 10 scans eliminated any asymmetry in profiles resulting from the scanner or from the film itself. Since, the aim of this experiment is only to compare the relatively small effects of different bed materials, making the profile symmetric not only does not affect the result but also helps to clearly visualise these small effects when presented graphically.

Mirror effectSchoenfeld et al[10] showed the schematic diagram of the mirror system used in a 10000XL EPSON scanner which uses five mirrors. An Epson V700 scanner was dismantled and its mirror system was found to have four mirrors. Five mirrors were used to represent the larger number and were placed at angles as described by Schoenfeld et al[10] to simulate the scanner mirror positions and angles between the light source and the camera (Fig. 1). A piece of linear polariser sheet was stuck on the Epson V700 scanner bed placed in front of the light source and 5 photos were taken using a Canon 7D camera fitted with a 50mm lens. The linear polariser was rotated by 10⁰ increments until it reached 180⁰ from its original position. 5 photos were taken for each of its positions. The mirrors were then taken out one by one starting from mirror 5 until no mirrors were left and the whole process was repeated, with the camera re-positioned for each mirror arrangement. The images were captured as RAW, which were converted to *.tiff format using Canon software and then were read and separated into three colour channels in ImageJ V1.49 software. A rectangular ROI was selected covering only the part of the photo illuminated by the V700 light source. The mean pixel values of each colour channel of the selected ROI were measured using ImageJ V1.49 software and recorded in an MS Excel spreadsheet. The pixel values were normalised as a percentage of the average of 0⁰ and 180⁰ polariser sheet angles Results were plotted as normalised pixel values (Y axis) against linear polariser angle (X axis). The uncertainty was calculated as the standard deviation of pixel values of five photos as a percent of mean pixel values.

The following settings were used for the Canon 7D camera

ISO: 100
Shutter speed; 1/500
Aperture: f4.5
The room light was on

No colour correction was applied when converting the RAW image to .tiff image

Path length effect: Figure 2 shows the profiles of the red channel of 500MU dose level across film pieces with different scanner ‘bed’ materials below them. The differences found between the bed materials used in this investigation were up to approximately 2%. Similar differences were found for other dose levels and also for all dose levels in the green channel. Uncertainty, as standard deviation of the pixel values of 10 scans, was found to be less than 0.5% at any data points.

Mirror effect: Figure 3 shows the change of pixel values with the change of polariser angle, normalised to the average pixel values with polariser angles of 0⁰ and 180⁰, both for having the mirror system present, with varying number of mirrors, and without any mirrors. The pixel values do not change significantly when there are no mirrors, showing no significant polarisation of light occurred while passing through the film only. With the addition of one mirror the results didn’t change significantly from the no mirror result. With two and three mirrors the results are almost identical and showed about 5% change at the polariser angle of 90⁰. With the addition of the 4^th and 5^th mirrors the pixel values changed about 14% at the polariser angle of 90⁰. The results indicate that the mirrors introduce polarisation, and the degree of polarisation depends on the number of mirrors used.

Path length effect: Figure 2 shows profiles of irradiated film pieces when using different materials as the ‘scanner bed’ to support the film, including no bed, i.e. equivalent to suspending the film piece in air. The results do not agree with the findings reported in a previous study[12], where the theory behind this path length effect was discussed. According to that theory any material placed between the light source and imaging system would introduce some degree of path length effect unless the material has similar refractive index to air. So, a piece of film will always have some degree of pathlength effect. This work aimed to investigate if the scanner bed enhances that effect and if any material other than glass changes that effect. To do that, we compared the profiles using different materials as scanner bed to that acquired with the normal glass scanner bed supplied with the scanner. Though we found up to about 2% difference for using different bed materials, the findings do not support the theory. The maximum difference was found between an acrylic sheet bed, showing the smallest LRA variation overall, and opaque laminating paper bed, showing the greatest LRA variation, with the other three bed materials being in between these. The other three (no bed, glass and film) are insignificantly different from each other. Since another film piece used as ‘bed’ has exactly the same refractive index as the experimental film, this was expected to show the least variation, or alternatively the ‘no bed’ option as this introduces nothing else to contribute to the path length effect. The film has rod like crystals and the laminating pouches use EVA glue, which are solid particles at room temperature. Polarization of light by scattering is a known phenomenon[27], [28], which may have occurred here in the cases of film or laminating paper, respectively, as bed.

Mirror Effect: Previous studies [12], [13] showed the polarization caused by the scanner. Schoenfeld et al [13] explained the theory behind the polarization resulting from the mirror system. This depends on mirror angle and light incident angle on each mirror. In the current study, the polarization caused by varying numbers of mirrors was investigated and compared with the absence of any mirror system. Van Buttum et al [12] found the polarization caused by an Epson 10000XL scanner, which has five mirrors in the whole system, is 16% (change of relative OD was from 0.42 to 0.50). In the current study, the normalized pixel values varied by 14% with the presence of 4 or 5 mirrors in the system, in good agreement with van Buttum [12], and negligible polarization was found with the absence of any mirror system or with only one mirror. Polarization by reflection from a mirror is very complex. There are a number of other variables impacting this, including mirror coating material and thickness, refractive index and also even the presence of dust on the mirror [27]–[29]. Modelling the polarization by the mirrors used in this investigation is beyond the study’s scope.

Two components of an EPSON V700 scanner were investigated for their contribution to the LRA effect of a radiochromic film dosimetry system. The scanner bed shows a relatively small contribution (at less than approximately 2%), but the mirror system introduces significant light polarisation. The degree of polarisation depends on the number of mirrors used. Conventionally EPSON flatbed scanners use four or five mirror systems, both of which were observed to introduce a similar degree of light polarisation of about 14%. With a smaller number of mirrors, the effect was observed to be less; two or three mirror systems, showing about 5% light polarisation, whereas a one mirror system had no significant effect on light polarisation. This further supports the conclusions from the work evaluating lens effects in film dosimetry [15], that a re-designed direct imaging system could potentially improve overall film dosimetry by significantly reducing the need for LRA effect corrections, although this would increase system size.

Funding: The authors declare that no funds, grunts or other support were received during the preparation of this manuscript.

Competing Interests: The authors have no relevant financial or non-financial interests to disclose

Author Contribution: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Tarafder Jahangir Shameem. The first draft pf the manuscript was written by Tarafder Jahangir Shameem and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript

Ethical Approval: this article does not contain any studies with human participants or animal experiments performed by any of the authors.

Consent to participate: Not applicable

Consent to publish: Not applicable

Availability of data and material: All the data are available from the corresponding author by request.

Code availability: Not applicable

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Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry.

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