Seven X-ray table mattresses, each of 2.5cm thickness of different ages and different manfacturers, were examined in terms of their influence on IQ and radiation dose. The CDRAD 2.0 phantom (Artinis Medical System, The Netherlands) (Figure 1) combined with 17.5 cm of polymethyl methacrylate (PMMA) slabs was used for IQ evaluation.
CDRAD phantom is widely used in the literature for image quality evaluation [1–3] and it was found to have a good correlation with visual image quality and lesion visibility [4,5].
The ‘CDRAD+PMMA’ phantom was imaged using a commercially x-ray machine (Wolverson X-ray Ltd, Willenhall, West Midlands, UK) with and without x-ray table mattresses (Figure 2), to determine the impact on IQ. For image acquisitions, manual exposure control was used with different kVp and mAs values (Table 1), SID=100 cm, broad focal spot size and anti-scatter grid. For each set of acquisition conditions, three images were acquired of the CDRAD phantom; this repeated imaging comes from a recommendation of CDRAD 2.0 phantom manufacturer [6].
Table 1: Exposure parameters used for image acquisition and radiation dose measurements.
|
Protocol No.
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
kVp
|
65
|
70
|
75
|
80
|
85
|
90
|
95
|
100
|
105
|
110
|
mAs
|
40
|
25
|
20
|
14
|
10
|
8
|
6.3
|
5.6
|
4.5
|
4
|
Image quality evaluation
IQ was measured physically from CDRAD phantom images using CDRAD analyser software and it was represented by image quality figure inverse (IQFinv) which is calculated using the following equation:
IQFinv=∑15i=1 1/CiD(i,th)
Where:
D(i, th): the lowest diameter in column (i) that has a visible hole correctly detected.
Ci : the contrast of the threshold visible hole in the column (i).
The total IQFinv is represented by taking the average of the three IQFinv values for the three repeated images for each exposure.
Radiation dose measurements.
Radiation dose was represented by Incident Air Kerma (IAK) and was measured using a solid-state dosimeter (RaySafe X2, Unfors Ray Safe AB, Billdal, Sweden). IAK was measured with and without mattress (Figure 3), and to reduce random error three repeated exposures for each protocol were measured and the average was calculated.
Statistical Analysis
Image quality and radiation dose data were analysed using SPSS version 22.0 (IBM Inc, Armonk, New York, US). Shapiro-Wilk test was used to investigate the normality of the data prior for using any statistical test to select the valid test. Both IQ and radiation dose data were found to be normal distributed data (p > 0.05), so a Paired T-Test was used to compare IQ and radiation dose with and without a mattress.