Dose Indices rating
This study evaluated the dose rate for the routine Thorax protocol,
focusing only on the comparison of the dose received by the phantom without and with protection in different configurations.
The values of CTDI_Vol,
DLP and standard deviation from the value without protection are presented in Table 1.
Configuration
|
CTDIvol
(mGy)
|
Standard Deviation
to the value without protection
|
DLP (mGy.cm)
|
Standard Deviation
to the value without protection
|
Without protection
|
2,52
|
0,0%
|
5,04
|
0,0%
|
With protection (no sponge)
|
1,95
|
-22,6%
|
3,90
|
-22,6%
|
With protection (1 sponge)
|
2,02
|
-19,9%
|
4,04
|
-19,9%
|
With protection (2 sponges)
|
2,08
|
-17,6%
|
4,15
|
-17,6%
|
With protection (coupled to gantry)
|
1,81
|
-28,2%
|
3,62
|
-28,2%
|
Table 1: Results for the CTDIvol and DLP in different settings and comparison with the value without protection for the Thorax CT protocol.
The values of CTDI _Vol for the different settings without protection were 2.52 mGy; with protection but without sponge 1.95 mGy; with protection and with a sponge 2.02 mGy; with protection and with two sponges 2.08 mGy; and finally with the protection coupled to the gantry 1.81 mGy.
DLP values in different configurations (Figure 2) show different variations such as without protection 5.04 mGy.cm; with protection but without sponge 3.90 mGy.cm; with protection and with a sponge 4.04 mGy.cm; with protection and with two sponges 4.15 mGy.cm; and finally with the protection coupled to the gantry 3.62 mGy.cm.
The percentage of deviation from the DLP relative to the value without protection was: with protection but without sponge -22.6%; with protection and with a sponge 19.9%; with protection and with two sponges 17.6% and with the protection coupled to the gantry 28.2% ( Fig. 2 ).
It was found that without the use of sponges the percentage of dose reduction was higher compared to the percentage obtained with sponges.
When coupled to the gantry,
the bismuth protection has reduced the dose in the phantom in 28.2%.
In all measurement with bismuth protection (no sponges) we observed dose decrease by 22.6%,19.9% with protection (one sponge),
17.6% with protection (two sponges) and 28.2% with protection coupled to the gantry.
Image Quality Control
The evaluation of image quality control in this study aims to make a comparison of the images obtained without the protection in the phantom and with the protection for different configurations.
In the module 1 it was found that with the bismuth shield,
the HU values related to polyethylene,
acrylic,
air and water are not within the tolerance range (Fig. 3; Fig. 4; Fig. 5; Fig. 6 Fig. 7; Fig. 8).
However,
the bone was the only material that was within that range.
In Module 2,
the CNR values without protection was 0.8 and 1.5 for the chest and abdomen protocol.
Small changes in CNR values were observed with protection in different settings and both protocols: with no sponge protection 1.2 and 2.0; with protection with a sponge 1.1 and 1.6; with protection and two sponges 0.9 and 1.4; and the protection attached to the gantry 0.7 and 1.1 (Fig. 9). In the module 3,
it was observed that without the phantom protection over all ROIs (12H,
9H,
6H,
3H and heart) were within the tolerance range.
When placed on the phantom shield with and without sponge,
there was no uniformity of ROIs from the periphery to the central.
Once the protector coupled to the gantry,
it was found that the values of the ROIs to the periphery were within the tolerance range,
with the exception of the central ROI (Fig. 10).
In module 4,
for both protocols,
were identified 7 lp/mm in all configurations using a kernel B41s (soft tissue) and 9 lp / mm for B50s kernel (lung parenchyma).
Regarding the image quality,
with bismuth protection,
these have improved when compared to scans with and without sponges.