Validez del CEE

Image

Mattress type (Bi- Flex/Standard)

Platform position

(elevated/not elevated) mAs

Grid

(Y/N) SID (cm)

Ref Tabletop n/a 16 Y 110

1 St elevated 16 Y 110 2 St not elevated 16 Y 110 3 St elevated 20 Y 110 4 St not elevated 20 Y 110 5 St elevated 25 Y 110 6 St not elevated 25 Y 110 7 St elevated 32 Y 110 8 St not elevated 32 Y 110 9 St elevated 16 Y 120 10 St not elevated 16 Y 120 11 St elevated 20 Y 120 12 St not elevated 20 Y 120 13 St elevated 25 Y 120 14 St not elevated 25 Y 120 15 St elevated 32 Y 120 16 St not elevated 32 Y 120 17 St elevated 16 Y 130 18 St not elevated 16 Y 130 19 St elevated 20 Y 130 20 St not elevated 20 Y 130 21 St elevated 25 Y 130 22 St not elevated 25 Y 130 23 St elevated 32 Y 130 24 st not elevated 32 Y 130

Image

Mattress type (Bi- Flex/Standard)

Platform position

(elevated/not elevated) mAs

Grid (Y/N) SID (cm) 25 Bi elevated 16 Y 110 26 Bi not elevated 16 Y 110 27 Bi elevated 20 Y 110 28 Bi not elevated 20 Y 110 29 Bi elevated 25 Y 110 30 Bi not elevated 25 Y 110 31 Bi elevated 32 Y 110 32 Bi not elevated 32 Y 110 33 Bi elevated 16 Y 120 34 Bi not elevated 16 Y 120 35 Bi elevated 20 Y 120 36 Bi not elevated 20 Y 120 37 Bi elevated 25 Y 120 38 Bi not elevated 25 Y 120 39 Bi elevated 32 Y 120 40 Bi not elevated 32 Y 120 41 Bi elevated 16 Y 130 42 Bi not elevated 16 Y 130 43 Bi elevated 20 Y 130 44 Bi not elevated 20 Y 130 45 Bi elevated 25 Y 130 46 Bi not elevated 25 Y 130 47 Bi elevated 32 Y 130 48 Bi not elevated 32 Y 130

Table 4 - the acquisition conditions for all images within the main method (with images highlighted in blue demonstrating the images acquired using the same acquisition parameters as the reference image).

Once the reference image was acquired, the experimental images were acquired on the Lifeguard 50 trolley in various imaging conditions. Images were acquired using two mattresses (standard 65mm and Bi-Flex 130mm) for comparison. The trolley also has an image receptor holder (platform) which should be elevated prior to an exposure to reduce object to image distance (OID). See section 3.1.4 above on page 20 for clarification. In clinical practice, this platform should always be elevated, however, after conducting the

current practise questionnaire (discussed in section 3.2 on page 28), it was apparent that radiographers do not always ensure that the platform is elevated. For this reason, images were acquired with and without the elevation of the platform for comparison. All images were acquired with a commercially available stationary grid (Lysholm, Sweden) with a grid ratio of 10:1 and strip density of 40 lines/cm (Sandborg et al., 1993). This was

employed due to the unavailability of the oscillating grid (situated within the x-ray tabletop Bucky) when imaging on the trolley. Initially, images were to be acquired with and without a grid for comparison however this idea was eliminated following the preliminary

experiments. Table 4 highlights (blue) four images which were acquired using the same acquisition parameters as the reference image. These four images were analysed separately in order to explore whether the imaging acquisition parameters used for x-ray tabletop imaging were directly transferable to trolley imaging for AP pelvis.

The current practices questionnaire by Tugwell (2014) revealed that some radiographers for trolley imaging would double their mAs for chest and pelvis examinations from the exposure factors recommended for x-ray tabletop imaging. For this reason, it was decided that four different mAs settings would be used for the main method in order to evaluate the effect of this practice highlighted by Tugwell on image quality and radiation dose. The initial mAs used was derived from the AEC reading of the acquisition parameters used to acquire the reference image which was 16. Lanca et al. (2014) and Tugwell et al. (2014) used the same principle for their AP pelvis studies where the average mAs value derived from the AEC system was used for their initial manual exposure (reference). The other mAs values used for this thesis were 20, 25 and 32 which are the customary increments found on control panels with 32mAs being double the mAs used to acquire the reference image (16mAs).

Three different SIDs of 110cm, 120cm and 130cm were used for this study, the latter two were used to compensate for increased OID and thus reduce magnification on the resultant images. According to Carver and Carver (2012) increasing SID to compensate for OID reduces image magnification and improves geometric unsharpness. By keeping the three SID’s identical for all imaging conditions allowed for easier comparison of results. A 110cm SID was used the same as for the reference image with a 120cm and 130cm used to reduce magnification but also to help reduce radiation dose as found by Heath et al. (2011), Woods and Messer (2009) and Tugwell et al. (2014). A 130cm SID was considered the

maximum practical and achievable SID to use due to the effective range of the stationary grid with regards to grid cut off. Also Heath et al. (2011) and Tugwell et al. (2014) found that image quality started to deteriorate at higher SID values. Initially, only two SID values were going to be used: a 110cm, being the same as the reference image and one other SID where the OID of each trolley imaging condition was added to the 110cm SID in order to compensate for each condition independently. It was already established before the pilot study that this method would not compensate for magnification in some conditions because OID was too large and it would be physically impossible to achieve the required SID for some conditions to keep magnification identical to the reference image. Nevertheless, it was still considered and experimented upon during the pilot phase (see section 5.10 for further details).

Collimation was adjusted to the region of clinical interest for each SID to include the iliac crest, greater trochanters and proximal one third of the femora in accordance with Carver and Carver (2012) as demonstrated on the resultant image in figure 20. This area of clinical interest was marked with tape in order to maintain the collimation size for all exposures (figure 16). This allowed for the same area of coverage at the surface of the phantom to ensure the collimation did not affect radiation dose or image quality. Collimation was therefore adjusted accordingly when SID was increased because the amount of scattered radiation hence patient dose varies when different volumes of tissue are irradiated (Davey & England, 2015).

Figure 20 -figure demonstrating the anatomy included within the collimation borders (greater trochanters, iliac crest and upper third of femora)