Exclusion of the mobile x-ray unit

For the pilot study, both the mobile unit and the x-ray room unit were used to acquire images. The mobile unit has a 12.5kW single phase generator, a total filtration of 2.5mmAl, 1.2mm focal spot and an anode angle of 16° (Shimadzu Corporation, Japan). This mobile unit is a state of the art system that has been designed according to radiology requirements including easy manoeuvrability and rapid examination in a variety of

situations. The differences between the ceiling suspended x-ray tube and the mobile unit x- ray tube is reflected in table 7. Although the differences in the characteristics of both these units are small, these differences may still cause variation in image quality and radiation dose. For example, a generator controls radiation output and the high frequency generator as seen for the Philips Diagnost x-ray tube will have a more efficient output compared to

the Shimadzu mobile unit and therefore would require lower exposure factors (Johnston & Fauber, 2015). In addition, the difference in total filtration between both units should have minimal impact on image quality however 3.2mmAl filtration would absorb more low energy photons than 2.5mmAl resulting in less absorbed dose by the patient (Trapp & Kron, 2008; Johnston & Fauber, 2015).

Philips Diagnost Shimadzu mobile Unit

Generator 50kW high frequency 12.5kW single phase

Broad focal Spot 1mm 1.2mm

Total Filtration 3.2mm Al 2.5mm Al

Anode angle 17° 16°

Table 7 - table demonstrating the difference between the x-ray tube characteristics of the x-ray room and mobile unit

When conducting the pilot study, it was difficult to acquire the images consecutively using the mobile unit due to the high clinical demand of this machine. This study was carried out within a busy district hospital therefore it seemed impossible during the pilot study to acquire all images uninterruptedly and successively without the mobile unit being needed for emergencies. This had major time implications on the pilot study but it also had the potential to cause inconsistency in the imaging conditions as the room and experimental requirements had to be set up repeatedly instead of the images being acquired in one undisrupted session. For this reason the mobile unit was effectively excluded from the main study.

In addition to the above problem, a further rationale for excluding the mobile unit from the main study was because of an additional implication on resources. From a radiation

protection requirement the experiments using this mobile unit would need to be conducted within an x-ray room because they are lead lined. This means that two pieces of equipment would be put out of action for the conduction of this study (both the x-ray room and the mobile unit) which would be unreasonable due to their high demand.

Lastly, another reason for excluding the mobile unit was the lack of control over important variables, which may result in error. When undertaking trolley imaging, SID has to be set manually using a tape measure at the side of the trolley (see figure 8). This means the

radiographer has to measure SID at the side of the trolley and then move both the x-ray tube and image receptor in alignment with the patient’s area of interest. This was proven difficult with the Shimadzu mobile unit as the button that releases the transverse movement of the tube also realised the up and down movement. This meant that the accuracy and consistency of maintain identical SID for all imaging conditions was difficult as the up and down movement may alter the SID when the tube is re-positioned over the phantom. At this point there is no method available to re-check and determine the SID since the image receptor is in the image receptor holder.

In order to provide evidence for the above problem, a small experiment was conducted following the pilot study to explore the error in achieving a constant SID using the mobile unit and to justify the exclusion of this unit from the main study. Three radiographers were asked to measure an SID of 130cm at the side of the trolley (as they would in clinical practice), and subsequently move the x-ray tube to the desired location over the phantom (centring point). The accuracy of the SID would then be determined by moving the trolley and re-measuring the SID without moving the tube (see figure 24 for results). Carlton and Adler (2013) suggested that radiographers who estimate SID must be within 15 percent to avoid producing a significant exposure difference emphasising the importance of being able to produce consistent acquisition parameters for comparison to be made between the imaging conditions.

Average, minimum and maximum values of 3 radiographers estimating 130cm SID on a x-ray mobile

unit 126 127 128 129 130 131 132 participants SI D (c m )

Figure 24 - figure demonstrating the 10 attempt of 3 radiographers to measure 130cm using the Shimadzu mobile unit

Figure 24 highlights the error in achieving an accurate SID using a mobile unit that has a multi release function (button that releases both the transverse and the up and down movement of the tube). Due to the inconsistency seen in figure 24, this was deemed another reasonable factor to exclude the mobile unit from the main study and justify using only the ceiling suspended x-ray tube to acquire all images.