Overview

The research method utilised for this thesis was an experimental approach, which was carried out under controlled conditions in an attempt to maximize objectivity and increase the study’s validity and reliability. The study manipulated independent variables in order to evaluate their effect upon the dependent variables. An experimental approach was deemed the most suitable method in order to address the aims and objectives of the study. This type of method offers precision and control where objectives are met through a deductive approach thus generating quantitative data allowing for statistical analysis (Burns, 2000). The aim of the experiment was to discover new knowledge surrounding image quality and radiation dose optimisation when imaging trolley bound patients since there is limited previous evidence surrounding this topic. Data from the literature and current technological developments were considered when deciding upon the most appropriate and robust

equipment, parameters and techniques to use for this experimental study.

The main objective of the thesis was to assess and evaluate whether the same acquisition parameters used for AP pelvis on the x-ray tabletop can be used for trolley imaging bearing in mind the differences between imaging on the tabletop and trolley. The remaining

objectives aimed to optimise image quality and radiation dose on the trolley by exploring the effect of the mattresses, platform position, mAs and SID on image quality and radiation dose.

To achieve these objectives; the method involved imaging an anthropomorphic pelvis phantom in the AP position using a computed radiography (CR) system. The use of an anthropomorphic phantom allowed for repeated exposures to be made which would have been unethical to achieve on humans. All images were acquired using a ceiling suspended x-ray tube in a standard trauma x-ray room. A reference image was acquired on the x-ray tabletop using standard acquisition parameters for AP pelvis whereas the experimental images were acquired on a commercially available trolley suitable for imaging. The reference image was acquired to allow for visual comparison to all the experimental images. The experimental images were acquired in different imaging conditions on the trolley where four independent variables were manipulated. These included the use of two

different mattresses (standard and Bi-Flex), two different image receptor holder positions (elevated and not elevated), three SID values, and four mAs values. Prior to the

commencement of the main experiment, a pilot study was conducted to assess and determine the suitability of the proposed method and to ensure that no problems were encountered with data analysis.

Five dependent variables were used for this experiment in order to evaluate the effect of the independent variables on image quality and radiation dose. These included two image quality measures, two radiation dose measures and the magnification level of the acquired images. Image quality was determined by calculating contrast to noise ratio (CNR) and also by visual evaluation of image quality using a 2 alternative forced choice (2AFC) method. Radiation dose was established by using Monte Carlo simulation software to derive effective dose (mSv) and also by using an ionising chamber to establish ESD. The magnification level of the images was assessed by measuring the right femoral head diameter of each image. All of these outcome measures generated quantitative primary data enabling statistical analysis of the data.

As previously stated, two different image quality measures were used for this thesis (physical and visual) in order to provide complementary information allowing for correlation measures

to be performed. Special emphasis and attention was given to the visual evaluation method due to its significance in simulating image evaluation within clinical practice. This visual image quality assessment was conducted using a 2 alternative forced choice (2AFC) method (Yu, Carter & McCollough, 2013; Abbey & Eckstein, 2002; Ulrich & Miller, 2004) and bespoke software (Hogg & Blindell, 2012). Five observers were asked to visually assess the clarity of anatomical structures of images acquired on the trolley (experimental images) in comparison to a reference image. This was done using dual monitors, side by side, and a validated psychometric image criteria scale for AP pelvis. Nevertheless, the objective physical measure was useful to compliment and support the visual evaluation, mainly because the experiments used anthropomorphic phantoms (i.e. no anatomical variation) and digital images (i.e. pixel based).CNR was the physical measure of image quality where the mean pixel value and standard deviation of that pixel using a specific and consistent region of interest was measured and calculated using ImageJ (National Institutes of Health, Bethesda,MD) (ImageJ, 2014).

Two radiation dose measures were also attained in order to compare and complement each other. Effective dose is the principle dose quantity for this thesis as it is a useful measure of the overall risk to the patients from ionising radiation. It is based on a mathematical model which takes into account the type and amount of exposed tissue in order to calculate the risk associated with the radiation dose to the patient. ESD derived with an ionising chamber is the second measure of radiation dose used for this thesis. This is measured in order to support the data from effective dose because ESD provides a direct measure of the radiation dose entering the patient at skin level as oppose to a mathematical simulation calculation. ESD is also useful since it will directly demonstrate the variation that exists between different imaging conditions especially since OID hence SOD varies between these conditions (Tootell et al., 2014).