Image quality assessmentThe evaluative panel consisted of five final year radiographystudents, who at the time of the study were <6 months away fromqualification. Each of the raters had previously participated in visualgrading analysis (VGA) experiments and were deemed sufficientlyexperienced to undertake image analysis. Images wereassessed under standardised viewing conditions using twoEA243WM MultiSync (NEC Corporation, Tokyo, Japan) 2.3 megapixelmonitors. Ambient lighting, less than 50 lx15 and the distanceof the chair from the monitor were kept constant. Details of howthe images were acquired were blinded to all raters.Two-alternative forced choice (2AFC) software16 was used topresent the acquired images to the raters. This allowed thepresentation of the reference image concurrently alongside thecomparator images on the monitor but in a randomised order. Afurther advantage of this software was that it prohibited zoomingand window width or level adjustments. Previous research hasreported on the benefits of 2AFC in that it permits easier detectionof differences in quality when compared to an absolute methodwhere observers are asked to evaluate images utilising criteriawithout a comparison reference image.17 Raters were invited toevaluate the images using image quality criteria adapted from theGuidelines from the Commission of European Communities (CEC)(Table 1).7 Definitions regarding the visibility of anatomical structureswere compared to the reference image and evaluated using a
5 point Likert scale (Table 2). These guidelines were deemed
appropriate for VGA and have been successfully employed in a
range of previous studies.4,9 Weighting factors previously
employed by Brennan and Madigan4 were applied to each
anatomical criterion based on their level of importance as outlined
in Table 1. In the report by Brennan and Madigan4 weighting factors
had been established by three clinicians, with a minimum of five
years clinical experience. Brindhaban et al. (2005) also employed
weighting factors in their research in order to account for the significance
of visually sharp reproduction of anatomy compared to
simple reproduction.9
Finally, magnification was assessed and compared between the
AP and PA projections using the software program Image J (National
Institute of Health, Bethesda, MD). This was assessed in the same
manner as that employed by Heriard, Terry & Arnold (1993) who
determined the magnification differences between the two projections
by measuring the transverse diameter of the vertebral
body of L3.16
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