Instrumentation
The materials used in order to conduct this research were:
- 1 computed tomography (CT) equipment SIEMENS® SOMATOM®EMOTION® 16
- 1 water phanton (25 liters)
- 1 lead shield (thickness of 20 mm)
- 1 radiation detector (Atomtex AT1123)
- 11 thermoluminescent dosimeter
- 1 Full body anthropomorphic phantom
Variables
In this study,
namely in the experimental fase,
there were considered two types of variables:
· Independent variables: acquisition parameters of the image (mAs,
exposure time,
cut thickness),
distance to isocenter,
thickness and composition of the phantom.
· Dependent variables: energy (kV) and dose rate of tertiary radiation.
Literature Review
A literature review was done to gather information about the patient positioning,
the health professionals positioning inside the TC room and the acquisition parameters used in this type of procedures (Fig. 1).
Quantification of Tertiary Scatter Radiation
After the data collection,
we proceeded to the tertiary radiation measurement fase through the simulation of an interventional procedure,
with the aim of understanding the behavior of the scatter radiation.
We used the following protocol for the tertiary radiation measurement:
1. Simulating a chest CT examination with 25 mA,
130 kVp and 18,45 s; a 5 mm thickness,
a B41s kernel,
a 0,9,
DLP pitch of 91,08 mGy.cm and CTDIvol of 1,78 mGy.
2. The water phantom was placed in the CT equipment table;
3.
Detector’s position in the room were defined: 0º directed to the gantry,
90º to the right side,
-90º to the left side,
180º backwards and also directed up and down;
4. The detector was turned on and 10 minutes later the maximum value of the background radiation was measure as well the equipment error rate;
5.
Six measurements were made with the detector in each position (0º,
90º,
-90º,
180º,
up and down) without lead protection (Fig. 2).
The results were registered.
6. It was placed a lead barrier between the phantom and the detector in order to make the measurements with protection;
7. Six measurements were made with the detector in each position (0º,
90º,
-90º,
180º,
up and down) with lead protection (Fig. 2Fig. 3).
The results were registered;
8. In order to isolate the detector from the tertiary radiation,
a cylinder lead was used (Fig. 4);
9. Two measurements were made with the detector directed to the roof in the interior of the lead cylinder,
with and without top lead protection on the cylinder.
The results were registered;
10. The measurements were concluded and 10 minutes later the maximum value of the background radiation was measure as well the equipment error rate.
Simulation with a full body anthropomorphic phantom
After the understanding of the distribution of the tertiary radiation in the CT room,
we proceeded to the real situation simulation fase.
The radiation detector was replaced by a full body anthropomorphic phantom,
in order to measure the scattered radiation an TLD dosimeters were placed on it.
We used the following experimental protocol:
1.
Simulating a chest CT examination with the same previously parameteres;
2.
The water phantom was placed in the CT equipment table to simulate the patient;
3.
The anthropomorphic phantom was used to simulate the doctor in the CT room (according to A in Fig. 1 ).
4. Eleven TLD dosimeters were placed in specific regions in the anthropomorphic phantom (1.
Skull (superior part); 2.
Right ear; 3.
Left ear; 4.
Skull (posterior part); 5.
Right eye; 6.
Left eye; 7.
Thorax; 8.
Right thigh (anterior part); 9.
Back; 10.
Right thigh (posterior part); 11.
Right hand.);
5. The lead apron was placed in the anthropomorphic phantom,
as recommended in this type of interventional procedures,
covering the TLD dosimeters placed in the thorax and in the anterior right femur;
6. A 2 mm lead protection was placed above the right eye covering the dosimeter placed in that area;
7. Ten scans were performed on the water phantom,
to make sure that the minimum sensitivity was reached,
so the TLD dosimeters reading would be possible.