This study describes a novel method for the simultaneous characterization of the angular MOSFET dependency in three orthogonal directions both free-in-air and in PMMA using 5° increments. The largest variations in sensitivity were observed free-in-air for normal-to-axial and tangent-to-axial rotations, especially for the beam directed towards the MOSFET distal tip and lead wire base. Figures –6 seemed to indicate a periodic oscillation behaviour in each 5° step. However, performing a fast fourier transformation to the data did not reveal any significant amplitude corresponding to the 5° frequency. The visual effect is most probably due to the measurement noise, since our measurements were done with 5°-increments.
In PMMA, the sensitivity variations at different angles were considerably smaller due to the smoothing effect of the scattered radiation, as the scattered radiation approaches the dosimeter mainly from directions other than that of the primary beam. Also, in PMMA, the smallest variations were registered in the axial rotation. It is worth noticing that PMMA is not exactly soft-tissue or water equivalent material, especially in the lower photon energy range. However, it still provides a good reference material for studies in radiological beam qualities, as in this case. The mean sensitivity and relative ± relative standard deviation in PMMA is illustrated in Fig. .
The mean sensitivity and ±relative standard deviation in PMMA. (±SD shown as shaded line.)
The angular dependencies observed in this study were subsequently compared with the results of other researchers who had applied similar X-ray tube potentials. Since the other researchers had used 22.5° or 30° angular increments, the values for plotting were obtained by linear interpolations. The original values of the other researchers are shown with markers in Figs –.
Roschau et al
] measured the isotropic response of a Thomson-Nielsen TN-502RDI dosimeter in free air, and used a soft-tissue equivalent cylinder 5.2 cm in diameter. A 3-phase generator with 3.0 mm Al HVL was applied to produce 70 kVp X-rays. The study was made for axial and normal-to-axial rotations with the flat side of the dosimeter directed towards the beam for 0º orientation [8
For free-in-air axial rotation Roschau et al
. observed a nearly isotropic response with a standard deviation of 3.0% of the mean value [8
]. This value is substantially lower than the 12.3% value observed in our study.
Similarly, Pomije et al
] studied the axial rotation sensitivity of a (TN-1002RD) dosimeter in soft-tissue equivalent material using 22.5° angle increments and 60 kVp, 90 kVp and 120 kVp tube voltages. A 0° angle was defined as having the epoxy side towards the beam source. The 90 kVp values demonstrated a 9.7% standard deviation (Fig. b). The measured 4.7% standard deviation of our study was in between the findings of Roschau et al
. (2.7% SD) [8
] and Pomije et al
(9.7% SD) [9
The free-in-air normal-to-axial rotation response was investigated by Pomije et al
. (TN-1002RD) [9
], Roschau et al
. (TN-1002RD) [8
] and Dong et al
. (TN-1002RDI) [9
]. The X-ray tube potential and HVL used for making the comparison were for Pomije et al
. 90 kVp, HVL not known, and for Dong et al
. 90 kVp, HVL 3.49 mm Al.
The standard deviations of the mean value were 12.5% (Pomije), 14.7% (Roschau) and 9.8% (Dong). In similar conditions, our study yielded a 12.3% standard deviation of the mean value. The greatest differences in the values were observed at 270°, with 29% and 44% response drops for Pomije and Roschau, respectively. For Dong et al. the greatest drop occurred at 90° (30% response drop). Similarly this study showed the largest response drop at 90° (55%).
Roschau et al
] and Pomije et al
] also studied the normal-to-axial rotation response in PMMA, yielding 7.4% and 11.6% standard deviations of the mean values, respectively (Fig. b). The results from our measurements were non-isotropic, with a 15.8% standard deviation of the mean. Pomije et al
. observed the largest difference at the 270° angle (an almost 40% response drop from the normalized value). Similarly, Roschau et al
. observed the largest response drop (20%) at the 270° angle. In our study, however, the largest deviation was observed at the 90° angle (24% response drop).
Our results indicated that the relative standard deviation of the mean in the tangent-to axial rotation was 17.6% free-in-air (Fig. a) and 7.1% in PMMA (Fig. b).
The main reason for this non-isotropic response lies in the technical design and manufacturing method of the MOSFET dosimeter. Epoxy resin is commonly used for encapsulating, and hence shields the MOSFET dosimeters. The thickness of 25 dosimeters were measured to evaluate the size and thickness variation in the epoxy, which was assumed to cause the differences in the dose readings when the dosimeter was irradiated from different angles of incidence. The average thickness of the epoxy bulb, excluding the flexprint, was 1.02 mm with 0.03 mm standard deviation. The most significant effect of the epoxy thickness on sensitivity was estimated to occur between the angles of 0° and 180°, which would represent the minimum and maximum epoxy layer thicknesses.
The variation in the dimensions was considered to be a negligible source of the sensitivity differences in the air at 0° and 180º angles. At these angles the response differences were 2% for axial rotation, 6% for normal-to-axial rotation, and 0% for tangent-to-axial rotation. Thus, the epoxy bulb shape has a much smaller effect on the sensitivity than do the distal structure and the lead-wire base.
When performing rotational exposures, as in partial arc CBCT examinations, the use of angle-specific sensitivity correction factors for MOSFET measurements should be emphasized. However, this would require correction of the MOSFET voltage signal synchronized with the angular movement of the CBCT X-ray tube, which is not applicable with current technology. Furthermore, anisotropic exposure near attenuation interfaces, e.g. bones, could nevertheless cause errors in the dose reading (i.e. if anisotropy of the exposure incidentally matches the anisotropy of the dosemeter sensitivity in a certain location). This cannot be corrected.
The method of measuring the MOSFET dosimeter angular response in 5° steps used in this study is capable of revealing local response variations, especially free-in-air at 90° and 270° degree angles. This study also included the tangent-to axial measurements free-in-air and in PMMA using dental photon energy, which to our knowledge has not been formerly studied before.
This study confirms the need to take angular dependency into account when using multiple angles of irradiation in a single examination. Fortunately, the presence of scattered radiation reduces the angular variation produced when using MOSFET dosimeters in anthropomorphic phantoms for organ dosimetry, one of the best uses for MOSFET dosimeters in clinical practice. The observed angular dependence shows that MOSFET dosimeters provide a versatile dosimetric method for dental radiology applications, such as CBCT. However, due to the observed variation in angular sensitivity, MOSFET dosimeters should always be calibrated in the actual clinical settings, taking into account the beam geometry and angular range of the CBCT exposure.