General considerations about the results
In order to compare the results measured by both the ionization chamber and the optically stimulated dosimeters (OSL),
a convertion from mGy to mSv had to be done,
through the quality factor of the x-ray beam (1 in this research).
About the quality control,
the phantom GAMMEX 464® was used.
This phantom is composed by 4 different modules that allow the evaluation of several aspects of image quality such as hounsfield units (HU) values,
low contrast resolution,
image uniformity and spatial resolution.
In this research the sensitivity of OSL dosimeters was better for both primary radiation and secondary radiation and,
for this reason,
only the values obtained with this devides were used.
Dosimetry done in routine head CT examinations
At a depth of 10mm (Hp10),
results show that lens was the organ that had received more radiation,
37.854mSv due to the fact tha it is exposed to the primary x-ray beam.
The barium protection reduced the dose to 36.732mSv.
Thyroid received a dose of 3.568mSv and breast received a dose of 0.792mSv.
After protection of lead on thyroid and bismuth on breast the doses received were 2.772mSv and 0.274mSv,
respectively.
The Fig. 14 presents a graph with these values.
At a depth 0.07mm (Hp0.07),
results show that lens was the organ that had received more radiation,
35.962mSv due to the fact tha it is exposed to the primary x-ray beam.
The barium protection reduced the dose to 34.896mSv.
Thyroid received a dose of 3.522mSv and breast received a dose of 0.792mSv.
After protection of lead on thyroid and bismuth on breast the doses received were 2.772mSv and 0.554mSv,
respectively.
The Fig. 15 presents a graph with these values.
Dosimetry done in routine thorax CT examinations
At a depth of 10mm (Hp10),
results show that thyroid was the organ that had received more radiation,
18.790mSv due to the fact tha it is exposed to the primary x-ray beam.
The lead protection reduced the dose to 5.162mSv.
Breast received a dose of 14.410mSv and lens received a dose of 0.676mSv.
After protection of bismuth on breast and barium on lens the doses received were 9.540mSv and 0.348mSv,
respectively.
The Fig. 16 presents a graph with these values.
At a depth 0.07mm (Hp0.07), results show that thyroid was the organ that had received more radiation,
17.852mSv due to the fact tha it is exposed to the primary x-ray beam.
The lead protection reduced the dose to 7.788mSv.
Breast received a dose of 13.690mSv and lens received a dose of 0.694mSv.
After protection of bismuth on breast and barium on lens the doses received were 9.062mSv and 0.438mSv,
respectively.
The Fig. 17 presents a graph with these values.
Dosimetry done in routine abdomen CT examinations
In abdomen CT examinations,
the dose was check just at the level of the breast.
At a depth of 10mm (Hp10),
results show that breast received a dose of 20.038mSv due to the fact that it is exposed to the secondary x-ray beam.
The bismuth protection reduced the dose to 14.128mSv.The Fig. 18 presents a graph with these values.
At a depth 0.07mm (Hp0.07),
results show that breast received a dose of 20.936mSv due to the fact that it is exposed to the secondary x-ray beam.
The bismuth protection reduced the dose to 13.422mSv.The Fig. 19 presents a graph with these values.
Dosimetry done in routine lower limb CT examinations
In lower limb CT examinations,
the dose was check just at the level of the female gonads at the pelvis.
The examined anatomic part was the left knee.
At a depth of 10mm (Hp10),
results show that the pelvic area received a dose of 0.140mSv due to the fact that it is exposed to the secondary x-ray beam.
The lead apron protection increased the dose to 0.162mSv.The Fig. 20 presents a graph with these values.
At a depth 0.07mm (Hp0.07),
results show that the pelvic area received a dose of 0.134mSv due to the fact that it is exposed to the secondary x-ray beam.
The lead apron protection increased the dose to 0.166mSv.The Fig. 21 presents a graph with these values.
Image Quality control
MODULE 1: Housnfield units (HU) tolerance values are calculated by the sum and by the subtraction of the standard deviation to the each mean value observed in the region of interest (ROI) evaluated.
If the values obtained are included in the indicated tolerance interval,
then the HU calibration is acceptable to the evaluated materials.
In the Fig. 22 we can see that,
without protection,
poliethilene HU are between -95.28 and -82.06,
bone HU are between 866.29 and 878.47,
acrilic HU are between 117.55 and 129.27,
air HU are between -988.9 and -993.99 and water HU are between -5.3 and 4.64.
We can see the acquisition of module 1 without protection in Fig. 23,
were all the materials have acceptable HU values.
In the Fig. 22 Fig. 24 we can see that,
with bismuth protection,
poliethilene HU are between -46.89 and -32.73,
bone HU are between 855.73 and 873.77,
acrilic HU are between 133.73 and 147.77,
air HU are between -972.64 and -958.46 and water HU are between 28.6 and 50.26.
We can see the acquisition of module 1 without protection in Fig. 23 Fig. 25,
were poliethilene and water does not have acceptable HU values.
MODULE 2: Low contrast resolution is verified if the contrast to noise ratio (CNR) is above 1.
This value is calculated by the subtration of the mean HU of ROI A by the mean value of ROI B and then by dividing this result by th standard deviation of ROI B. Without bismuth protection,
the CNR is 1.173,
which is acceptable,
as shown in Fig. 26.
In Fig. 27,
we can see the ROI placed in position B.
With the bismuth protection,
CNR is 0.764,
as shown in Fig. 28, which is unacceptable. In Fig. 29,
we can see the ROI placed in position B.
Despite the sponge placed between the protection and the phantom,
the beam ardening artifacts increased the HU of both A and B ROI placed,
which affected negatively the CNR.
MODULE 3: Image uniformity is calculated by checking the mean HU at the ROI placed in the center of the image.
The tolerance values are calculated by adding and subtracting 5 HU to that mean.
Then,
the mean HU values of the for ROI placed at 12 o'clock,
3 o'clock,
6 o'clock and 9 o'clock can be evaluated in order to check if the image is uniforme.
Without the bismuth protection,
all the HU values are in the tolerance interval.
The ROI placement can be seen in Fig. 30 and the values can be checked in Fig. 31.
With the bismuth protection,
none of the HU values are in the tolerance interval.
The ROI placement can be seen in Fig. 32 and the values can be check in Fig. 33 and it shows that the bismuth protection have a negative effect on image uniformity.
MODULE 4: Spatial resolution is verified by simply counting the pair of lines.
To have good spatial resolution,
7 pairs of lines should be visible.
In Fig. 34 (without the bismuth protection) and in Fig. 35 (with bismuth protection) is possible to count 7 pairs of lines,
which means that the bismuth protection does not affect negatively the spatial resolution.