- Email: [email protected]
Calculation of radiation dose to the lens of the eye using Monte Carlo simulation
Applied Radiation and Isotopes 55 (2001) 557–560
Calculation of radiation dose to the lens of the eye using Monte Carlo simulation Niyazi Meric¸* Department of Engineering Physics, Faculty of Science, University of Ankara, 06100, Bes¸evler - Ankara, Turkey Received 3 May 2000; received in revised form 16 November 2000; accepted 18 December 2000
Abstract The radiation dose to the lens of the eye of patients undergoing diagnostic and interventional radiological procedures of the lacrimal drainage system has been calculated using a Monte Carlo technique. The technique has also been suggested for the retrospective estimation of the lens dose; when applied to individual patients, good correlation is obtained. In such study, data is required for image acquisition frame numbers and fluoro on-time, mean exposure values for these parameters, and the ratio of lens-to-air dose (viz. the head factor, HF) derived for a standard adult head. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radiation; Lens dose; Monte Carlo; Exposure to patients
1. Introduction In interventional radiology and cardiology complex procedures have been introduced, often requiring long periods of fluoroscopy and thus relatively high doses of radiation to patients. Particularly for systems working in high-rate fluoroscopy modes, doses may even reach therapeutic levels: serious radiation-induced skin injuries to patients have been reported which have resulted from fluoroscopically guided therapeutic interventional procedures (Christopher, 1991; Huda and Peters, 1994). However, assessing the dose from fluoroscopy is difficult, primarily due to the range of several parameters (kVp , mA, field site and size, number of exposures, fluoro time). Nevertheless, many techniques have been developed for this purpose (Suleiman et al., 1991). Skull examinations provide a case of special concern due to the deterministic effect of radiation on the lens of the eye, with the eye typically remaining in the field of the primary X-ray beam. One of the specific techniques in which lens dose becomes important is transluminal balloon dilatation of the lachrymal drainage system *Fax: +90-312-2232-395. E-mail address: [email protected] (N. Meric¸).
under fluoroscopic guidance for treatment of epiphora. The procedure begins with diagnostic imaging of the lachrymal drainage system (LDS) using digital subtraction dacryoiystography (DS-DCG) of the patient presenting with epiphora in order to determine the site and degree of the obstruction. Following confirmation of the site of obstruction, the procedure continues with treatment of the LDS obstruction (Ilgit et al., 1995). Although the X-ray beam is attenuated by the skull in both projections, the patient’s eye receives a considerable amount of radiation especially for the lateral projections. The Monte Carlo method has been widely used in medical radiation dosimetry because of the technical difficulties involved in experimental studies. In the present study, we have developed computer programs, based on the Monte Carlo method, to calculate the head factor (HF) this being the ratio of the lens-to-air dose. The calculated factors can be used for estimation of doses to the lens of the eye of patients who undergo diagnostic and interventional radiological procedures of the LDS. When using this value for patient dose estimation it is necessary to know patient-specific parameters that include total fluoroscopy on-time and DS-DCG frame number, as well as air dose values for
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 9 5 - X
558
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560
the mean fluoroscopy time and DS-DCG frame at the related exposure parameters. We have also compared calculated results with related experimental data.
2. Material and methods The Monte Carlo technique and a mathematically described adult head phantom were used for the theoretical estimation of HF value. The head is represented by an elliptical cylinder topped by a halfellipsoid and is assumed to consist of a skull (cranium and facial skeleton), brain and eye lenses. The cranium and facial skeleton are represented by the total and partial volumes between two concentric ellipsoids. The brain is represented by an ellipsoid and eye lenses are defined as portions of the volume between two concentric elliptical cylinders (Drexler et al., 1984; Kramer et al., 1982). For the selection of the size of the long and short axes of ellipsoids the dimensions of the Rando Phantom (Alderson Research Laboratories, Stamford, USA) were used as a reference. The densities and the elemental composition of the skin, soft tissue and bone were taken from the literature (ICRP, 1975). In the Monte Carlo calculations, photoelectric absorption, coherent and incoherent scattering of a polychromatic broad beam of X-rays in the simulated head were considered. In the case of the photoelectric interaction, the photon was assumed to be totally absorbed at the interaction site, terminating the history of the photon. In the cases of coherent scattering, the scattering angles of the photons were calculated by sampling the Thomson differential cross-section, corrected by the relativistic atomic form factors (Hubbell and Overbo, 1979). For incoherent scattering, the scattering angles were calculated by using Klein–Nishina differential cross section corrected for electron binding energy (Hubbell et al., 1975). The history of the photon was terminated when it escaped out of the phantom or its energy had decreased to less than 5 keV (Boone and Seibert, 1988; Chan and Doi, 1984). In the simulation, the mathematically described adult head phantom is defined in terms of Cartesian coordinates. The X-ray source was assumed to be located at the point x ¼ y ¼ z ¼ 0 point in air at a distance of z ¼ 65 cm (for posteroanterior projections) and z ¼ 76 cm (for lateral projection) from the phantom. The X-rays were uniformly directed to each point on the phantom within the limit of the X-ray field. Divergence of the beam through the depth of the phantom was arranged for by defining the direction of each photon at the focal point. The X and Y coordinates of each photon were randomly selected and the angle between the initial direction of the photons at the phantom entrance and zaxis was calculated. Ten runs each having 105 photons were used in each iteration and from this statistical
uncertainties associated with HF values were also estimated (Knoll, 1979). The details of our own code and its validity have been described previously (Meric¸ et al., 1998, 1999). Each transit of a single photon through the head was followed and only the energy deposition (E) of the photons absorbed by the lens was considered for the lens dose (D) calculation D ¼ E=M;
ð1Þ
where M is the mass of the lens. Suitable data were selected from the literature for simulation of the spectral output of the X-ray tube used in patient studies (data were interpolated for intermediate values). The air dose at the phantom entrance was also determined for each spectrum (Birch et al., 1979). The calculated lens and air doses were then used for the assessment of the theoretical HF value. Prediction of each patient dose was carried out using this factor in Eq. (2). D ¼ ½HFF ðDF TF Þ þ HFD ðDD TD ÞLAT þ ½HFF ðDF TF Þ þ HFD ðDD TD ÞPA ;
ð2Þ
where DF and DD are the respective air doses for one minute of fluoroscopic exposure and one frame of DSDCG exposure. These values were measured with the ion chamber at the point in air corresponding to the beam entrance position on the patient. TF and TD are the fluoro on-time and the total number of DS-DCG frames recorded for each patient, respectively. LAT is the lateral view and PA is the posteroanterior view. The calculated values of patient doses were individually compared with the patient TLD measurements using the mean relative deviation given by ( )1=2 n X 2 c ¼ 1=n ððDl =DTLD Þ 1Þ ; ð3Þ i¼1
where DTLD is the patient lens dose measured directly with a TLD and Dl is the patient lens dose obtained with the application of HF values.
3. Results and discussion The HF values, obtained for 702100 kVp from the Monte Carlo calculations, are given in Table 1 for the PA and the LAT projections. In the literature, a number of results of studies exist regarding the lens radiation dose in diagnostic X-ray examinations. In a study, by Ilgit et al. (2000), the radiation dose to the lens of the eye during fluoroscopically guided transluminal balloon dilation of the lacrimal drainage system (balloon dacryocystoplasty) was measured in 10 consecutive patients with use of thermoluminescent dosimeters placed on the eyelids of both eyes as close as possible to the lenses. The total number of frames acquired during digital subtraction dacryocystography,
559
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560 Table 1 The HF values obtained from Monte Carlo calculations for PA and LAT projections kVP
PA
70 75 80 85 90 100
LAT
Untreated eye
Treated eye
Untreated eye
Treated eye
0.018 0.004 0.017 0.007 0.027 0.005 0.026 0.008 0.025 0.010 0.026 0.011
0.015 0.003 0.016 0.006 0.027 0.005 0.027 0.007 0.024 0.009 0.026 0.010
1.438 0.093 1.444 0.094 1.472 0.170 1.342 0.190 1.271 0.266 1.379 0.196
0.208 0.064 0.218 0.063 0.239 0.068 0.143 0.073 0.182 0.056 0.198 0.100
Table 2 The comparison of Monte Carlo calculations with related experimental data (Ilgit et al., 2000). The mean relative deviation (C) for Monte Carlo calculations is 12% Patient no./age (yr)
1/36 2/36 3/61 4/48 5/36 6/53 7/27 8/65 9/22 10/52
Fluoroscopy time (s)
No. of digital images acquired
Lens dose (mGy)
Monte Carlo calculations (mGy)
PA (TF )
LAT (TF )
PA (TD )
LAT (TD )
Treated eye
Untreated eye
Treated eye
Untreated eye
1 1 264 1 1 1 108 204 126 1
120 210 96 258 246 12 270 228 36 180
24 28 12 26 31 11 9 35 18 9
26 0 11 12 0 10 47 0 34 0
5.1 6.1 4.6 9.1 6.8 2.8 2.1 3.2 1.9 4.3
39.0 50.4 24.5 67.8 60.1 14.7 47.1 29.8 20.5 30.9
6.8 7.6 6.0 9.2 8.7 1.9 4.6 4.8 3.3 5.1
41.6 55.2 27.2 61.0 63.1 12.0 50.4 33.2 24.0 34.0
Table 3 The calculation of the radiation dose to the lens of examined eye LAT
PA
Fluoroscopy HFF DF TF D (mGy)
Digital images 0.239 0.1 120 2.9
HFD DD TD +
Fluoroscopy 0.143 0.7 24 2.4
fluoroscopy times and measured radiation doses to the lens of the eyes for all ten patients have been taken from this study and are given in Table 2. This facilitates a comparison of measurements and calculations. To illustrate the use of Eq. (2), we have chosen the first patient in Table 2 and calculated the radiation doses to the lens of the treated and untreated eye in Tables 3 and 4, respectively. In Tables 3 and 4 we have used DF and DD data, the values of which are presented in Table 5. Similarly, we have applied the same equation to each patient in Table 2 and given the results in the last two columns of the same table.
Digital images
HFF DF TF
0.027 0.3 1
HFD DD TD
+
0.0
+
0.027 2.1 26 1.5
=6.8
The mean relative deviation given by Eq. (3) was used for the comparison of patient TLD measurements with Monte Carlo calculations. This has been found for therapeutic applications to be 12%. Beside the statistical fluctuations, a further factor which may account for small discrepancies between the results of Monte Carlo calculations and patient TLD measurements concerns the use of a constant HF value for a patient whose head size differs from the standard adult head. In Ilgit’s study, since the examined eye close to the image intensifier and the untreated eye next to the X-ray tube are placed in lateral projection, the untreated eye
560
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560
Table 4 The calculation of the radiation dose to the lens of other eye LAT
PA
Fluoroscopy HFF DF TF D (mGy)
Digital images 1.472 0.1 120 17.7
Fluoroscopy
HFD DD TD
1.342 0.7 24
HFF DF TF
0.027 0.3 1
HFD DD TD
+
22.5
+
0.0
+
Table 5 Air dose values in mGy for one frame of DS-DCG and 1 min of fluoroscopy
DS-DCG/frame (DD ) Fluoro (time/s) (DF )
Digital images
kVp
Lateral
PA
85 80
0.7 0.1
2.1 0.3
receives a higher dose than the examined one. However, no greatly significant differences was found between the absorbed dose values of the two eyes in the PA projection. Although the dose for the extremity studies is directly measured with the TLD method, this study suggests that useful information for dose determination can be obtained by use of a simple factor such as HF. HF should be determined with a sensitive method for each X-ray beam quality and anatomical projection used in patient studies. Monte Carlo calculations can be used for this purpose and subsequent patient dose assessments can be attained using the data on fluoroscopy time and number of DS-DCG frames. Recording these patient data also provides for retrospective dose assessment. Application of this technique to other angiographic studies will be limited when the tube output changes with the thickness and composition of the body part under investigation during a specific examination. The addition of more projections also increases the complexity of the technique.
References Birch, R., Marshall, B., Ardran, G.M., 1979. Catalogue of Spectral Data for Diagnostic X-rays. HPA, London, SRS 30. Boone, J.M., Seibert, J.A., 1988. Monte Carlo Simulation of the scattered radiation distribution in diagnostic radiology. Med. Phys. 15, 713–720. Chan, H.P., Doi, K., 1984. Radiation dose in diagnostic radiology: Monte Carlo simulation studies. Med. Phys. 11, 480–490. Christopher, H., 1991. Exposure rates in high-level-control fluoroscopy for image enhancement. Radiology 178, 643.
0.026 2.1 26 1.4
=41.6
Drexler, G., Panzer, W., Widenman, L., Williams, D., Zankl, M., 1984. The calculation of dose from external photon exposures using reference human phantoms and Monte Carlo methods, Part lll: organ doses in X-ray diagnosis. Mu¨nchen, Rep GSF. Brich S-1026. Hubbell, J.H., Overbo, I., 1979. Relativistic atomic form factors and photon cohorent scattering cross section. J. Phys. Chem. Ref. Data 8, 69–105. Hubbell, J.H., Veigele, W.J., Briggs, E.A., Brown, R.T., Cromer, D.T., Howerton, R.J., 1975. Atomic form factors, incoherent scattering functions, and photon scattering cross sections. J. Phys. Chem. Ref. Data 4, 471–538. Huda, W., Peters, K.R., 1994. Radiation-induced temporary epilation after a neuroradiologically guided enbolization procedure. Radiology 193, 642–644. Ilgıt, E.T., Meric¸, N., Bor, D., O¨znur, I.I., Konus, O¨., Is¸ık, S., 2000. Lens of the eye: radiation dose in balloon dacryocystoplasty. Radiology 217, 54–57. Ilgıt, E., Yuksel, D., Unal, M., Akpek, S., Isik, S., Hasanreisoglu, B., 1995. Transluminal balloon dilatation of the lacrimal drainage system for the treatment of epiphora. AJR 165, 1517–1524. International Commission on Radiological Protection (ICRP), 1975. ICRP Publication 23, Reference Man: Anatomical, Physiological and Metabolic Characteristics. Oxford, Pergamon Press. Knoll, G.F., 1979. Radiation Detection and Measurement. Wiley, New York, pp. 104–147. Kramer, R., Zankl, M., Williams, G., Drexler, G., 1982. The calculation of dose from external photon exposures using reference human phantoms and Monte Carlo methods, Part 1: the male (Adam) and female (Eva) adult mathematical phantoms. Mu¨nchen, Rep GSF. Brich S-885. Meric¸, N., Bor, D., Bu¨get, N., O¨zkırlı, M., 1998. The use of Monte Carlo technique for the determination of tissue–air ratios (TAR) in diagnostic range. Phys. Med. 14, 3–8. Meric¸, N., Bor, D., Buget, N., 1999. Determination of scatter fractions of some materials by experimental studies and Monte Carlo calculations. Appl. Radiat. Isot. 52 (2), 161–167. Suleiman, O.H., Anderson, J., Lones, D., Rao, G., Rosensttein, M., 1991. Tissue doses in the upper gastrointestinal fluoroscopy examination. Radiology 178, 653.
Calculation of radiation dose to the lens of the eye using Monte Carlo simulation Niyazi Meric¸* Department of Engineering Physics, Faculty of Science, University of Ankara, 06100, Bes¸evler - Ankara, Turkey Received 3 May 2000; received in revised form 16 November 2000; accepted 18 December 2000
Abstract The radiation dose to the lens of the eye of patients undergoing diagnostic and interventional radiological procedures of the lacrimal drainage system has been calculated using a Monte Carlo technique. The technique has also been suggested for the retrospective estimation of the lens dose; when applied to individual patients, good correlation is obtained. In such study, data is required for image acquisition frame numbers and fluoro on-time, mean exposure values for these parameters, and the ratio of lens-to-air dose (viz. the head factor, HF) derived for a standard adult head. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radiation; Lens dose; Monte Carlo; Exposure to patients
1. Introduction In interventional radiology and cardiology complex procedures have been introduced, often requiring long periods of fluoroscopy and thus relatively high doses of radiation to patients. Particularly for systems working in high-rate fluoroscopy modes, doses may even reach therapeutic levels: serious radiation-induced skin injuries to patients have been reported which have resulted from fluoroscopically guided therapeutic interventional procedures (Christopher, 1991; Huda and Peters, 1994). However, assessing the dose from fluoroscopy is difficult, primarily due to the range of several parameters (kVp , mA, field site and size, number of exposures, fluoro time). Nevertheless, many techniques have been developed for this purpose (Suleiman et al., 1991). Skull examinations provide a case of special concern due to the deterministic effect of radiation on the lens of the eye, with the eye typically remaining in the field of the primary X-ray beam. One of the specific techniques in which lens dose becomes important is transluminal balloon dilatation of the lachrymal drainage system *Fax: +90-312-2232-395. E-mail address: [email protected] (N. Meric¸).
under fluoroscopic guidance for treatment of epiphora. The procedure begins with diagnostic imaging of the lachrymal drainage system (LDS) using digital subtraction dacryoiystography (DS-DCG) of the patient presenting with epiphora in order to determine the site and degree of the obstruction. Following confirmation of the site of obstruction, the procedure continues with treatment of the LDS obstruction (Ilgit et al., 1995). Although the X-ray beam is attenuated by the skull in both projections, the patient’s eye receives a considerable amount of radiation especially for the lateral projections. The Monte Carlo method has been widely used in medical radiation dosimetry because of the technical difficulties involved in experimental studies. In the present study, we have developed computer programs, based on the Monte Carlo method, to calculate the head factor (HF) this being the ratio of the lens-to-air dose. The calculated factors can be used for estimation of doses to the lens of the eye of patients who undergo diagnostic and interventional radiological procedures of the LDS. When using this value for patient dose estimation it is necessary to know patient-specific parameters that include total fluoroscopy on-time and DS-DCG frame number, as well as air dose values for
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 9 5 - X
558
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560
the mean fluoroscopy time and DS-DCG frame at the related exposure parameters. We have also compared calculated results with related experimental data.
2. Material and methods The Monte Carlo technique and a mathematically described adult head phantom were used for the theoretical estimation of HF value. The head is represented by an elliptical cylinder topped by a halfellipsoid and is assumed to consist of a skull (cranium and facial skeleton), brain and eye lenses. The cranium and facial skeleton are represented by the total and partial volumes between two concentric ellipsoids. The brain is represented by an ellipsoid and eye lenses are defined as portions of the volume between two concentric elliptical cylinders (Drexler et al., 1984; Kramer et al., 1982). For the selection of the size of the long and short axes of ellipsoids the dimensions of the Rando Phantom (Alderson Research Laboratories, Stamford, USA) were used as a reference. The densities and the elemental composition of the skin, soft tissue and bone were taken from the literature (ICRP, 1975). In the Monte Carlo calculations, photoelectric absorption, coherent and incoherent scattering of a polychromatic broad beam of X-rays in the simulated head were considered. In the case of the photoelectric interaction, the photon was assumed to be totally absorbed at the interaction site, terminating the history of the photon. In the cases of coherent scattering, the scattering angles of the photons were calculated by sampling the Thomson differential cross-section, corrected by the relativistic atomic form factors (Hubbell and Overbo, 1979). For incoherent scattering, the scattering angles were calculated by using Klein–Nishina differential cross section corrected for electron binding energy (Hubbell et al., 1975). The history of the photon was terminated when it escaped out of the phantom or its energy had decreased to less than 5 keV (Boone and Seibert, 1988; Chan and Doi, 1984). In the simulation, the mathematically described adult head phantom is defined in terms of Cartesian coordinates. The X-ray source was assumed to be located at the point x ¼ y ¼ z ¼ 0 point in air at a distance of z ¼ 65 cm (for posteroanterior projections) and z ¼ 76 cm (for lateral projection) from the phantom. The X-rays were uniformly directed to each point on the phantom within the limit of the X-ray field. Divergence of the beam through the depth of the phantom was arranged for by defining the direction of each photon at the focal point. The X and Y coordinates of each photon were randomly selected and the angle between the initial direction of the photons at the phantom entrance and zaxis was calculated. Ten runs each having 105 photons were used in each iteration and from this statistical
uncertainties associated with HF values were also estimated (Knoll, 1979). The details of our own code and its validity have been described previously (Meric¸ et al., 1998, 1999). Each transit of a single photon through the head was followed and only the energy deposition (E) of the photons absorbed by the lens was considered for the lens dose (D) calculation D ¼ E=M;
ð1Þ
where M is the mass of the lens. Suitable data were selected from the literature for simulation of the spectral output of the X-ray tube used in patient studies (data were interpolated for intermediate values). The air dose at the phantom entrance was also determined for each spectrum (Birch et al., 1979). The calculated lens and air doses were then used for the assessment of the theoretical HF value. Prediction of each patient dose was carried out using this factor in Eq. (2). D ¼ ½HFF ðDF TF Þ þ HFD ðDD TD ÞLAT þ ½HFF ðDF TF Þ þ HFD ðDD TD ÞPA ;
ð2Þ
where DF and DD are the respective air doses for one minute of fluoroscopic exposure and one frame of DSDCG exposure. These values were measured with the ion chamber at the point in air corresponding to the beam entrance position on the patient. TF and TD are the fluoro on-time and the total number of DS-DCG frames recorded for each patient, respectively. LAT is the lateral view and PA is the posteroanterior view. The calculated values of patient doses were individually compared with the patient TLD measurements using the mean relative deviation given by ( )1=2 n X 2 c ¼ 1=n ððDl =DTLD Þ 1Þ ; ð3Þ i¼1
where DTLD is the patient lens dose measured directly with a TLD and Dl is the patient lens dose obtained with the application of HF values.
3. Results and discussion The HF values, obtained for 702100 kVp from the Monte Carlo calculations, are given in Table 1 for the PA and the LAT projections. In the literature, a number of results of studies exist regarding the lens radiation dose in diagnostic X-ray examinations. In a study, by Ilgit et al. (2000), the radiation dose to the lens of the eye during fluoroscopically guided transluminal balloon dilation of the lacrimal drainage system (balloon dacryocystoplasty) was measured in 10 consecutive patients with use of thermoluminescent dosimeters placed on the eyelids of both eyes as close as possible to the lenses. The total number of frames acquired during digital subtraction dacryocystography,
559
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560 Table 1 The HF values obtained from Monte Carlo calculations for PA and LAT projections kVP
PA
70 75 80 85 90 100
LAT
Untreated eye
Treated eye
Untreated eye
Treated eye
0.018 0.004 0.017 0.007 0.027 0.005 0.026 0.008 0.025 0.010 0.026 0.011
0.015 0.003 0.016 0.006 0.027 0.005 0.027 0.007 0.024 0.009 0.026 0.010
1.438 0.093 1.444 0.094 1.472 0.170 1.342 0.190 1.271 0.266 1.379 0.196
0.208 0.064 0.218 0.063 0.239 0.068 0.143 0.073 0.182 0.056 0.198 0.100
Table 2 The comparison of Monte Carlo calculations with related experimental data (Ilgit et al., 2000). The mean relative deviation (C) for Monte Carlo calculations is 12% Patient no./age (yr)
1/36 2/36 3/61 4/48 5/36 6/53 7/27 8/65 9/22 10/52
Fluoroscopy time (s)
No. of digital images acquired
Lens dose (mGy)
Monte Carlo calculations (mGy)
PA (TF )
LAT (TF )
PA (TD )
LAT (TD )
Treated eye
Untreated eye
Treated eye
Untreated eye
1 1 264 1 1 1 108 204 126 1
120 210 96 258 246 12 270 228 36 180
24 28 12 26 31 11 9 35 18 9
26 0 11 12 0 10 47 0 34 0
5.1 6.1 4.6 9.1 6.8 2.8 2.1 3.2 1.9 4.3
39.0 50.4 24.5 67.8 60.1 14.7 47.1 29.8 20.5 30.9
6.8 7.6 6.0 9.2 8.7 1.9 4.6 4.8 3.3 5.1
41.6 55.2 27.2 61.0 63.1 12.0 50.4 33.2 24.0 34.0
Table 3 The calculation of the radiation dose to the lens of examined eye LAT
PA
Fluoroscopy HFF DF TF D (mGy)
Digital images 0.239 0.1 120 2.9
HFD DD TD +
Fluoroscopy 0.143 0.7 24 2.4
fluoroscopy times and measured radiation doses to the lens of the eyes for all ten patients have been taken from this study and are given in Table 2. This facilitates a comparison of measurements and calculations. To illustrate the use of Eq. (2), we have chosen the first patient in Table 2 and calculated the radiation doses to the lens of the treated and untreated eye in Tables 3 and 4, respectively. In Tables 3 and 4 we have used DF and DD data, the values of which are presented in Table 5. Similarly, we have applied the same equation to each patient in Table 2 and given the results in the last two columns of the same table.
Digital images
HFF DF TF
0.027 0.3 1
HFD DD TD
+
0.0
+
0.027 2.1 26 1.5
=6.8
The mean relative deviation given by Eq. (3) was used for the comparison of patient TLD measurements with Monte Carlo calculations. This has been found for therapeutic applications to be 12%. Beside the statistical fluctuations, a further factor which may account for small discrepancies between the results of Monte Carlo calculations and patient TLD measurements concerns the use of a constant HF value for a patient whose head size differs from the standard adult head. In Ilgit’s study, since the examined eye close to the image intensifier and the untreated eye next to the X-ray tube are placed in lateral projection, the untreated eye
560
N. Meric¸ / Applied Radiation and Isotopes 55 (2001) 557–560
Table 4 The calculation of the radiation dose to the lens of other eye LAT
PA
Fluoroscopy HFF DF TF D (mGy)
Digital images 1.472 0.1 120 17.7
Fluoroscopy
HFD DD TD
1.342 0.7 24
HFF DF TF
0.027 0.3 1
HFD DD TD
+
22.5
+
0.0
+
Table 5 Air dose values in mGy for one frame of DS-DCG and 1 min of fluoroscopy
DS-DCG/frame (DD ) Fluoro (time/s) (DF )
Digital images
kVp
Lateral
PA
85 80
0.7 0.1
2.1 0.3
receives a higher dose than the examined one. However, no greatly significant differences was found between the absorbed dose values of the two eyes in the PA projection. Although the dose for the extremity studies is directly measured with the TLD method, this study suggests that useful information for dose determination can be obtained by use of a simple factor such as HF. HF should be determined with a sensitive method for each X-ray beam quality and anatomical projection used in patient studies. Monte Carlo calculations can be used for this purpose and subsequent patient dose assessments can be attained using the data on fluoroscopy time and number of DS-DCG frames. Recording these patient data also provides for retrospective dose assessment. Application of this technique to other angiographic studies will be limited when the tube output changes with the thickness and composition of the body part under investigation during a specific examination. The addition of more projections also increases the complexity of the technique.
References Birch, R., Marshall, B., Ardran, G.M., 1979. Catalogue of Spectral Data for Diagnostic X-rays. HPA, London, SRS 30. Boone, J.M., Seibert, J.A., 1988. Monte Carlo Simulation of the scattered radiation distribution in diagnostic radiology. Med. Phys. 15, 713–720. Chan, H.P., Doi, K., 1984. Radiation dose in diagnostic radiology: Monte Carlo simulation studies. Med. Phys. 11, 480–490. Christopher, H., 1991. Exposure rates in high-level-control fluoroscopy for image enhancement. Radiology 178, 643.
0.026 2.1 26 1.4
=41.6
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