The unavoidable distance between the cervical spine and the image receptor presents measurable levels of geometric
unsharpness, which hinders arthritic scoring. The current work explores the impact on the visualisation of important
arthritic indicators by increasing the distance between the X-ray source and image detector (SID) from the commonly
employed 150cm. Lateral cervical spine images were acquired of an osteoarthritic human cadaver using a DR imaging
system. All exposures were taken at 65kVp using automatic exposure control and various SID distances from 150 to
210cm. Four experienced clinicians assessed the images by means of visual grading analysis, using objective criteria
based on normal anatomic features and arthritic indicators. A statistically significant improvement in image quality was
observed with images acquired at 210cm compared with those acquired at 150cm and 180cm (p<0.05), with values of
56.0 (SE=1.105), 50.85 (SE=1.415) and 65.35 (SE=0.737) respectively. All images with a SID of 210cm scored higher
for visually sharp reproduction of the spinous processes, facet joints, intervertebral disc spaces and trabecular bone
pattern compared with both 180cm and 150cm. Results indicate that total image quality and visualisation of specific
anatomical features is improved in cervical spine radiographs when traditionally employed SID distances are increased.
Purpose
Detection of low-contrast details is highly dependent on the adaptation state of the eye. It is important therefore that the
average luminance of the observer's field of view (FOV) matches those of softcopy radiological images. This study
establishes the percentage of FOV filled by workstations at various viewing distances.
Methods
Five observers stood at viewing distances of 20, 30 and 50cm from a homogenous white surface and were instructed to
continuously focus on a fixed object at a height appropriate level. A dark indicator was held at this object and then
moved steadily until the observer could no longer perceive it in his/her peripheral vision. This was performed at 0°, 90°,
180° and 270° clockwise from the median sagittal plane. Distances were recorded, radii calculated and observer and
mean FOV areas established. These values were then compared with areas of typical high and low specification
workstations.
Results
Individual and mean FOVs were 7660, 15463 and 30075cm2 at viewing distances of 20, 30 and 50cm respectively. High
and low specification monitors with respective areas of 1576.25 and 921.25cm2 contributed between 5 to 21% and 3 to
12% respectively to the total FOV depending on observer distance. Limited inter-observer variances were noted.
Conclusions
Radiology workstations typically comprise between only 3 and 21% of the observer's FOV. This demonstrates the
importance of measuring ambient light levels and surface reflection coefficients in order to maximise adaptation and
observer's perception of low contrast detail and minimise eye strain.
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