The Fig. 5 represents the tendency of the dose rate (mSv/s) of the scattered radiation according to the orientation of the detector and comparison between mean dose rates with and without lead protection.
We observed an abrupt decrease of the dose rate values when applied the lead protection (grey line).
The highest value of the dose rate was measured without lead protection and with the detector directly faced to the gantry,
in 0º position.
With the lead protection the major contribution of the scattered radiation came from the sides,
when the detector was placed in 90º and -90º.
Regarding to the lowest dose rate,
the lower contribution of the scattered radiation occurs when the detector is orientated backwards,
in the 180° position.
Table 1 shows the ratio as a percentage between the tertiary radiation and total scattered radiation (secondary and tertiary radiation) at the set location previously defined in the CT room for the various directions of the radiation detector.
Table 1 - Ratio between the tertiary and secondary radiation,
expressed in percentage.
Ratio of tertiary radiation and the total scattered radiation
|
Orientation detector
|
Percentage (%)
|
0º
|
3,4%
|
90º
|
4,6%
|
-90º
|
6,9%
|
180º
|
42,4%
|
Up
|
4,1%
|
Down
|
4,6%
|
Considering the experimental phase with the radiation detector placed inside of the lead cylinder,
the dose rate of the scattered radiation was measured and we observed a considerable decrease of the dose rate values when a lead “cover” is applied on the top of the cylinder (grey column) (Fig. 6)
It was also measured the background radiation contribution with the detector completely sealed.
it is always present with a 1,61 × 10-8 mSv/s value,
however,
it is such a low dose that its contribution is insignificant.
With the obtained results it can also be calculated a better estimative of the tertiary radiation coming from the roof,
crossing the results of the dose rate measurement in the different settings of the experimental setup.
In the cylinder setting of the experimental setup,
with the detector directed upwards inside the cylinder,
it was verified that the tertiary radiation has a 88.4% contribution.
So,
in a rough estimate,
the tertiary radiation coming from the roof had a 3.6% contribution in that defined place of the CT room.
Simulation with a Full body anthropomorphic phantom
In table 2,
there are presented the results referring to the personal equivalent dose (H) (mSv/s) with a 0,07 mm and a 10,00mm depth,
product of the measurements made in the CT room using TLD dosimeters.
Table 2 - Measurements of the equivalent dose according to the TLD dosimeters.
Measurement with dosimeter TLD
|
Dosimeters
|
Localization
|
Lead protection
|
Hp(0,07) (mSv)
|
Hp (0,07) (mSv/s)
|
Hp(10) (mSv)
|
Hp(10) (mSv/s)
|
1
|
Skull (top)
|
No
|
0,11
|
5,98 × 10-4
|
0,11
|
5,98 × 10-4
|
2
|
Right Ear
|
No
|
0,22
|
1,20 × 10-3
|
0,23
|
1,25 × 10-3
|
3
|
Left Ear
|
No
|
0,22
|
1,20 × 10-3
|
0,23
|
1,25 × 10-3
|
4
|
Skull (back)
|
No
|
0,05
|
2,72 × 10-4
|
0,05
|
2,72 × 10-4
|
5
|
Right Eye
|
Yes
|
0,01
|
5,44 × 10-5
|
0,01
|
5,44 × 10-5
|
6
|
Left Eya
|
No
|
0,29
|
1,58 × 10-3
|
0,29
|
1,58 × 10-3
|
7
|
Chest
|
Yes
|
0,03
|
1,63 × 10-4
|
0,03
|
1,63 × 10-4
|
8
|
Right thigh (front)
|
Yes
|
0,00
|
0,00
|
0,00
|
0,00
|
9
|
Back
|
No
|
0,00
|
0,00
|
0,00
|
0,00
|
10
|
Right thigh (back)
|
No
|
0,01
|
5,44 × 10-5
|
0,01
|
5,44 × 10-5
|
11
|
Right hand
|
No
|
0,37
|
2,01 × 10-3
|
0,40
|
2,18 × 10-3
|
Through the analysis of the results,
it can be verified that the highest values of H correspond to the right hand,
left eye,
right and left ear.
The table 3 refers to the exposition of an individual occupationally exposed according to the conditions used in the simulation of a real situation,
in comparison with the effective dose described by the guidelines for this type of procedures.
Table 3 – Individual exposure occupationally exposed to simulate a real situation
CT-guided fluoroscopy (130 kV,
25 mA)
|
Occupationally exposed individual
|
Dose rate at the entrance skin
|
2 × 10-3 mSv/s (in hand out of the primary beam)
|
Threshold of deterministic effects on skin (2 Sv)
|
16667 min (outside the primary beam)
|
Whole body effective dose rate
|
1,6 × 10-4 mSv/s
|
Whole body effective dose limit (20 mSv/year)
|
2083 min/year with apron 0,35 mm Pb
|
Limit on equivalent dose in hand (500 mSv/year)
|
4167 min/year (outside the primary beam)
|
Equivalent dose limit in the lens (20 mSv/year)
|
211 min/year (without protective glasses)
|
It is possible to estimate the total duration that a interventional CT guided procedure can have in a year,
so as not to reach the thresholds and limits for occupationally exposed individuals.