Analysis of the relationship between field size and change in radiation dose to patient and staff based on radiographic technique in fluoroscopy and radiography

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INTRODUCTION
Fluoroscopy is used in a wide variety of medical examinations and procedures to evaluate, in real time, the anatomy, physiology and possible pathologies of internal organs or treat patients.It employs ionizing radiation and implies radiation exposure of patients and staff with doses that can be rather high.Due to the importance of this issue many documents of various international organizations discuss guidelines for optimization and radiological protection in fluoroscopy [1][2][3][4][5][6][7].The inadvertent irradiation of the medical staff performing this procedure in examination room is another concern.It is possible to reduce the dose values optimizing various technical parameters of fluoroscopic equipment.
One of such parameters is the radiation field size.This parameter is usually discussed in connection with unnecessary irradiation of the patient's body parts that do not need to be imaged [8].However, excessive expansion of the field size leads also to some other effects: • Fluoroscopic examinations are usually accompanied by production of various radiographic images.In radiographic mode, the unnecessary irradiated patient's body parts turn into new sources of scattered radiation, which degrades image quality and irradiates assistants who remain inside the examination room during the procedure.
• In fluoroscopic mode, the automatic brightness control changes the dose rate at the entrance of the patient's body.Even modern detectors used in automatic brightness control are not capable of evaluating the intensity of radiation in this or that part of the image.They assess the total intensity of radiation passing through the patient and this intensity increases proportionally to the irradiated area.To maintain the same brightness (same intensity of radiation passing through the patient) the equipment has to change the dose rate at the entrance.

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This work continues our research into the influence of various technical parameters of fluoroscopic equipment on patient and occupational dose begun earlier [9,10].In the present work, our goals were to analyse how the mention above effects depend on irradiation field size and primary X-ray spectrum.First, we studied the change in dose rate at the exit of the patient's body and in occupational dose rate in radiographic mode.Then, we investigated the change in dose rate at the entrance of the patient's body and in occupational dose rate in fluoroscopic mode.

MATERIALS AND METHODS
The study was carried out in a University Hospital in the city of Curitiba (Brazil), in the Department of Radiology, in January 2023.The research was not submitted to the Institutional Review Board for not involving human beings.
The fluoroscopy equipment used was the remote-controlled system AXIOM ICONOS MD, Siemens, 2006 A detailed description of the equipment can be found in Nunes et al [9].
To measure the dose rate of ionizing radiation, RAYSAFE solid-state detectors were used together with a Base X2 unit to read the results [11].The X2 RF Sensor was used to measure the dose rate at the entrance and exit surface of the simulator and the specific model for measuring dispersion, the X2 Survey sensor, was used to measure the occupational dose rate.The calibration certificates for the both detectors were issued by the Unfors Raysafe AB Laboratory on 22 June 2022.
To simulate the patient body, 5 acrylic plates measuring 30 cm high x 30 cm long x 3 cm thick each were used, totalling 15 cm thick.This thickness was adopted considering, for example, skull of a typical adult patient [12] or thorax of a ~10-year-old paediatric patient from the same hospital [13].
The measurements were carried out in conditions close to those of barium meal test routinely performed in this hospital.Fig. 1 and 2 shows the experimental setup.The center of the X-ray beam was positioned exactly at the center of the patient simulator.The focus of the X-ray tube and the center of the patient simulator were located at 102 cm from the floor.
The table was arranged vertically and the patient simulator was positioned on a support against the examination table with a horizontal distance of 90 cm from the focus of the Xray tube.The image intensifier was at 115 cm from the focus.The detectors at the entrance and exit of the simulator were positioned at the center of the irradiation field.The occupational dose rate detector was located on the lead apron positioned at 50 cm from the center of the patient simulator, representing the position that assistant usually maintains to support the patient's head and/or to administer the contrast.In the vertical direction, the detector was positioned in three ways to measure dose rates on the face (its center was 140 cm from the floor), thorax (110 cm from the floor) and pelvis (80 cm from the floor).
In radiographic mode, the dose rate measurements were performed for different kVp, filters and field sizes (Table 1).In fluoroscopic mode, the kVp values were automatically chosen by the equipment.To estimate the statistic errors, three dose rate measurements were performed for each adopted parameter, totalling 1272 tests.We analysed the increase factor of the dose rate: the ratio between the dose rate for a particular field size and for the smallest one.

Change in dose rate at the exit of the patient's body in radiographic mode
The mean values and standard deviations of the increase factor of the exit dose rate are presented in Table 2 as a function of kVp, field size and filter thickness.The exit dose rates were normalized with respect to the entrance dose rate.The increase reaches 85% and does not depend on either the kVp or the filter thickness, that is, the primary X-ray spectrum.Possible difference between increase factors does not exceed 4% which matches the accuracy of the detector.
Fig. 3 shows the increase factors for the two opposite situations: the primary X-ray spectra of highest and lowest average energies.The primary X-ray spectrum for 70 kVp without filter has the lowest average energy and the spectrum for 100 kVp and the filter thickness of 0.3 mm has the highest average energy.The other spectra gave the results between these two.The lines and formulas in Fig. 3 are the results of fitting of a secondorder polynomial to the results of all measurements.This formula allows evaluating the increase factors in relation to the 10 cm field.It can be rewritten for any field as a reference point.For that, it is necessary to calculate the value of the increase factors for the new reference point using this formula and then divide the coefficients in it by a value equal to this increase factor.p. 10

Change in occupational dose rate in radiographic mode
The increase factors of the dose rate in the various regions of the professional's body are shown in Tables 3-5 as a function of kVp, field size and filter thickness.The dose rates were normalized with respect to the entrance dose rate.The increase of the dose rate is significant and may be greater than 850%.There are signs of an increase in this factor with a decrease in kVp and an increase in the thickness of the filter.But this effect does not exceed 10% and was statistically confirmed only for some kVp and filter thicknesses in the face and pelvic regions.
Analysing the dependence on kVp, the statistically significant differences were observed: p. 13 1.In the facial region only for field size 28 x 28 cm 2 and filter thickness of 0.3 mm between 70 and 90 or 70 and 100 kVp; 2. In the pelvic region, for field size 28 x 28 cm 2 and filter thicknesses of 0.3 and 0.2 mm between 70 and 100 kVp; 3. In the pelvic region, for field size 22 x 22 cm 2 and filter thicknesses of 0.3 mm between 70 and 100 kVp or 81 and 100 kVp.
All these differences are more than two but less than three errors and cannot be considered statistically detected.The only differences that exceed three errors are observed in the pelvic region for field size 22 x 22 cm 2 and filter thicknesses of 0.2 mm between 70 and 81 or 90 or 100 kVp.
Considering the dependence on filter thickness, all observed differences greater than two errors do not exceed three errors: 1.In the facial region, for field size 28 x 28 cm 2 and 70 kVp between filter thicknesses of 0.3 and 0 mm; 2. In the pelvic region, for field size 28 x 28 cm 2 and 70 kVp between filter thicknesses of 0.3 and 0.1 mm; 3. In the pelvic region, for field size 22 x 22 cm 2 and 81 kVp between filter thicknesses of 0.3 and 0.2 mm; The increase factors values in the face and thorax regions are almost identical when those in the pelvic region are smaller, but this difference does not exceed 15% and decreases with decreasing field size.Unlike the dose rate at the exit of the patient's body, in the case of the occupational dose, the dose rate is linearly proportional to the field area.Possible absorption effects of scattered photons do not exceed 1%.Change of scattering angle in this case is very small and it makes no sense to consider it.

Change in dose rate at the entrance of the patient's body in fluoroscopic mode
Fig. 6 shows the increase factors of the entrance dose rate as a function of field side length for radiographic, fluoroscopic with grade and fluoroscopic without grade modes.
p. 15 In radiographic mode, where automatic brightness control is not used, the dose rate practically does not change.In fluoroscopic mode, it decreases linearly with the increase of field side length.This effect is almost two times larger without grid and reaches 25% with grid and 50% without grid.To maintain the same brightness with increasing field size, the automatic brightness control has to decrease dose rate at the entrance.In fluoroscopic mode with grade, the part of radiation that was eliminated by grid and that did not reach the detector regulating the activity of the automatic brightness control is not constant.It depends on field size.
Tables 6 and 7 show how the increase factor depends on filtration.As was expected, the dose rate growth is significant, but it is lower than that in radiography mode and can reach 500%.As already discussed, such reduction in the increase factor is caused by automatic brightness control that decreases the dose rate at the body entrance with increasing field size.The effect of filter thickness was not observed.The lines in Fig. 7 give the impression that the occupational dose rate is highest for thorax and lowest for pelvis.However, analysing the errors of the fitting coefficients in Fig. 7, it is clear that this difference was not statistically confirmed.Because of the automatic brightness control, the increase of the dose rate with increasing field size is slower than in the case of radiography mode.

Figure 3 :
Figure 3: Increase factor (D/D10) of the exit dose rate as a function of field side length (L) and area (S) (radiographic mode).

Fig. 5 Figure 5 :
Fig. 5 shows the results of fitting the straight line to the results obtained for the regions of face, thorax, and pelvis separately.The formulas allow evaluating the increase factors in relation to field size.

Figure 6 :
Figure 6: Increase factor (D/D12) of the entrance dose rate as a function of field side length (L) for radiographic, fluoroscopic with grade and fluoroscopic without grade modes.

Fig. 7
Fig.7shows the increase factor of the occupational dose rate as a function of field side length and area in fluoroscopic mode with and without grid.The curves shown are the results of fitting the second-order polynomial to the results obtained for the regions of face,

Figure 7 :
Figure 7: Increase factor (D/D10) of the occupational dose rate as a function of field side length (L) and area (S) (fluoroscopic mode).

Table 1 :
Technical parameters

Table 2 :
Mean values and standard deviations of the increase factor of the exit dose rate as a function of kVp, field size and filter thickness (radiographic mode).

Table 3 :
Mean values and standard deviations of the increase factors of the dose rate in the facial region as a function of kVp, field size and filter thickness (radiographic mode).

Table 4 :
Mean values and standard deviations of the increase factors of the dose rate in the thorax region as a function of kVp, field size and filter thickness (radiographic mode).

Table 5 :
Mean values and standard deviations of the increase factors of the dose rate in the pelvic region as a function of kVp, field size and filter thickness (radiographic mode).

Table 6 :
Mean values and standard deviations of the increase factor of the entrance dose rate as a function of field size and filter thickness.Fluoroscopic mode with grid.

Table 10 :
Mean values and standard deviations of the increase factors of the dose rate in the pelvic region as a function of field size and filter thickness (fluoroscopic mode with and without grid).