Keywords:
Dosimetric comparison, Radiation safety, Equipment, Dosimetry, Digital radiography, Radioprotection / Radiation dose, Radiographers
Authors:
E. Higgins, K. Matthews; Dublin/IE
DOI:
10.26044/ecr2019/C-1982
Results
24 of the 45 pelvis and 23 of the 45 lumbar spine projections had DAP and mAs values less than or equal to the values from images taken with the AEC set as in current practice.
The quality of these 47 images was analysed,
whilst the 43 images with DAP values exceeding those in clinical practice were discarded.
Influence of Altering AEC Settings on the Dose Constant
The effects on DAP of altering the DS and ECF settings on the AEC controls are shown in Fig. 3 and Fig. 4.
Predictably similar changes in mAs were recorded.
For the digital system under test,
at any DS setting, modifying the ECF by ±0.5 results in an mAs and DAP change of 10.16±1.54%.
At any given ECF value,
selection of the next DS setting e.g.
400 to either 560 or 280,
results in an mAs and DAP change of 26.8±1.45%.
Comparison of Image Quality and DAP
Indicative Noise Analysis
The amount of noise present in an image should decrease as the x-ray tube output increases [12],
and this was shown in the current study (see Fig. 5).
The indicative noise level of 34.7 associated with the reference pelvis image underwent a 57.9% increase to 54.8 at the lowest DAP dose recorded.
The AP lumbar spine image with the lowest recorded DAP had an indicative noise level of 79.5,
representing only a 15% increase from the indicative noise value of 69.1 of the reference image. Indicative noise levels were gathered from images which had been post-processed immediately after exposure [13]: it was not possible to retrieve images which contained no post processing.
It is therefore important to consider the indicative noise results in relation to the VGA scores.
VGA Scores
Fig. 6 shows an example of the average observer VGA scores presented in comparison with DAP values for AP lumbar spine at DS 800.
As in this example,
of the 47 images appraised,
none attained a less than diagnostic VGA score (<3). It is evident in this example that as DAP values decrease there is no matching trend in VGA scores - this pattern repeated for all combinations of AEC settings.
Overall,
a perfect VGA score of 6 was achieved by 13 of the 23 lumbar spine and 9 of the pelvis images with a DAP dose below that of the reference image.
The AEC settings associated with these images were further considered for the recommendation of more optimal AEC settings at the clinical site.