British Journal of Radiology 74 (2001),358-367 © 2001 The British Institute of Radiology
The effect of beam tube potential variation on gonad dose to patients during chest radiography investigated using high sensitivity LiF:Mg,Cu,P thermoluminescent dosemeters
K K L Fung, PhD, FIR, MSc
1 and
W B Gilboy, PhD, FInstP, CPhys
2
1 Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
2 Department of Physics, The University of Surrey, Guildford GU2 7XH, UK
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Abstract
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Optimization of X-ray beam tube potential (kVp) in radiological examinations can minimize patient dose. This research aims to investigate the effect of tube potential variation on gonad doses to patients during posteroanterior (PA) chest radiography examinations. This study was carried out using a Toshiba general purpose X-ray unit and a Rando phantom. Dose measuring equipment included an ion chamber system, a dose–area product (DAP) meter and a thermoluminescent dosemeter (TLD) reader system with high sensitivity TLD pellets of LiF:Mg,Cu,P for low level gonad dose measurement. PA chest exposures of the phantom to produce a constant exit dose were made using a standard low tube potential (range 60–100 kVp) non-grid technique and a high tube potential (range 95–150 kVp) grid technique. Entrance surface doses (ESDs) and DAPs were also included in the measurements. Effective doses (EDs) were computed from ESD and DAP measurements using NRPB-SR262 and Xdose software. Results show that with the low tube potential technique both ovary dose and testes dose increase with increasing tube potential; statistically significant correlations of r=0.994 (p=0.0006) and r=0.998 (p=0.001), respectively, were found. For both organs, doses increase at a rate of approximately 2% per kVp. With the high tube potential technique there is insignificant correlation between gonad doses and tube potential. When comparing patient doses from typical exposures made at 70 kVp (low tube potential non-grid technique) with doses from exposures made at 120 kVp (high tube potential grid technique), the high tube potential technique delivers significantly higher values for ESD, and ovary, testes and effective doses by factors of 1.7, 5.2, 5.5 and 2.7, respectively.
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Introduction
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In the UK, man-made radiation accounts for 14% (0.37 mSv) of the average annual dose to the population from all sources of radiation [1]; diagnostic radiography examinations of patients comprise the largest contribution to various medical uses of radiation [1–3]. Investigation of dose reduction methods in diagnostic radiology has always been a primary aim of radiation protection research. The National Radiological Protection Board (NRPB) in the UK has proposed 28 methods for reducing doses to patients from radiography examinations, such as using the most sensitive film–screen combination, optimization of exposures, use of digital imaging equipment and improved procedural techniques [4]. The NRPB has recently introduced the new "achievable dose" concept to supplement the previous suggestion of "reference dose level" to promote further dose reduction in diagnostic radiology [5].
Chest radiography examination is the most frequently performed radiological procedure, accounting for 25% of all radiological examinations in the UK. Although the patient dose is relatively low, a large population is involved and chest radiography contributes 2% of the UK collective effective dose from all medical and dental X-ray examinations [3]. Previous surveys have shown a very wide range of patient dose from chest radiography examinations [3]. The reference dose level set by the NRPB for chest radiography is 0.3 mGy for entrance surface dose (ESD) and this is selected with the intention of improving the situation by recommending this as the investigation dose level for clinical centres [5]. In the past, many studies have demonstrated various ways of reducing patient dose in chest radiography examinations, such as the appropriate use of X-ray tube filters and fast film–screen systems [4]. Careful manipulation of X-ray tube potential (kVp) for patient exposure can reduce patient dose. Both high and low tube potential techniques are commonly used in chest radiography examinations. The Commission of the European Communities recommends an applied voltage in the range of 100–150 kVp with an anti-scatter grid and an automatic exposure control chamber for chest radiography [6, 7]. The high tube potential technique is more commonly used in the USA and European countries, but the low tube potential technique is still widely practised in many centres in the UK [8, 9].
Interpretation of the final chest radiograph is usually performed by radiologists and there is still no consensus among them regarding the appropriate tube potential to be used for chest radiography. There are advantages and disadvantages of both techniques. A low tube potential exposure produces a high contrast film image, with soft tissue pathological shadow and calcification being more clearly seen than on a high tube potential film. On the other hand, although a high tube potential exposure produces a lower contrast image, it also results in increased visualization of hidden areas of the lung owing to better penetration of overlying structures [10]. In chest radiography, with the use of a higher tube potential the X-ray photons are more penetrating, enabling more photons to reach the film, thus causing an increase in overall film density; to correct this, the integrated tube current (mAs) needs to be reduced to maintain a similar diagnostic film density. This results in reduced skin dose but, depending on the site involved, critical organ doses may increase due to scattered photons that are of higher energies. These scattered photons could reach remote out-of-beam tissues/organs to deliver an unintended dose [11]. Since there is a gradual loss of contrast in the final image with increasing tube potential, Seeram [12] suggested selecting the highest tube potential consistent with an acceptable image quality. During radiography of subjects using a high tube potential (e.g. >100 kVp) setting, more forward scattered photons are generated, which would cause deterioration of image quality, so it is common practice to use an anti-scatter grid in front of the film to remove this scattered radiation. However, the use of a grid blocks part of the transmitted primary beam, which therefore requires a larger quantity of X-ray photons to be generated from the X-ray tube. This is accomplished by increasing the integrated tube current (mAs), usually by a factor of between 3 and 6 (grid factor), to attain the normal diagnostic film density [13], which results in a 3- to 6-fold increase in patient dose.
Since at present both high and low tube potential chest radiographs are acceptable to radiologists, with a trend towards using the high tube potential technique, careful selection of the appropriate tube potential to deliver a lower patient dose is crucial, in line with the optimization principle in radiation protection of patients as advocated by the International Commission on Radiological Protection [14, 15]. Gonads (testes and ovary) are deemed the most radiosensitive tissues and consequently have the highest tissue weighting factor (0.2) in the calculation of whole body ED from radiation exposures [15]. It is thus essential to minimize any radiation dose to these tissues. The use of a higher tube potential and lower mAs values reduces patient dose [16], since the photons have higher energies to penetrate the tissue and reach the film, with a lower dose deposited in the tissue [12]. Hence, there is a significant reduction of skin and gonad doses under the direct X-ray beam, with increasing tube potential for constant exit dose. Gonads are outside the exposed field in chest radiography and it would be very useful for clinical radiographers to know how and to what extent the gonad (ovary and testes) doses vary with tube potential in chest radiography using both the high tube potential and low tube potential techniques. This knowledge will lead to appropriate exposure selection consistent with acceptable image quality.
In 1985, the NRPB published report NRPB-R186 [17], which uses the Monte Carlo technique to provide useful data for the estimation of organ doses from a knowledge of ESDs for common diagnostic radiological examinations. This was followed by the more recent publication NRPB-SR262 [18], which provides conversion coefficients for the estimation of EDs from ESD and dose–area product (DAP) measurements in common radiological examinations [18]. However, data generated from these documents are based on a hypothetical model, with the limitation of rigidly defined beam field sizes and focus-to-object distances. In addition data are provided only up to 120 kVp. In realistic clinical situations for chest radiography, beam collimation and focus-to-subject distance usually vary and nowadays tube potentials up to 150 kVp may be used with modern X-ray units. The phantom study presented here provides a realistic update of gonad dose (GD) measurements with varying tube potential settings. No similar research has been performed in the past in this field of study, mainly owing to the difficulty of measuring the low level peripheral doses encountered in diagnostic radiology.
LiF:Mg,Ti is the most commonly used conventional thermoluminescent dosemeter (TLD) for dose measurement in this field on account of its good dosimetric characteristics. However, its reliable dose detection threshold is relatively high at about 50 µGy [19]. This present study has become feasible with the advent of a recently developed high sensitivity TLD material LiF:Mg,Cu,P, which has very favourable dosimetric characteristics such as much higher light output per unit dose, linear dose response, low fading and low dose detection threshold [20]. This TLD material has become commercially available in recent years. It can be placed inside a body phantom in the form of TLD pellets for deriving low level organ doses.
The main aim of this research was to measure and quantify the variation of gonad (testes and ovary) doses with different realistic tube potential settings using low and high tube potential techniques for posteroanterior (PA) chest radiography. Other dose measurements for each corresponding tube potential were also made and these include ESD and DAP. Effective dose (ED) is a more meaningful dose parameter to estimate cancer risk [15] and this was computed from the measured ESDs and DAPs using the NRPB-SR262 [21] and Xdose software [22].
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Materials and method
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This work was carried out at the X-ray Laboratory of the Hong Kong Polytechnic University. A Rando phantom (Radiology Support Devices Inc., Long Beach, CA, USA) was used in this study instead of real patients. The Rando phantom is an anthropomorphic phantom consisting of a human skeleton embedded in synthetic tissue-equivalent material forming the natural body contours. It has no limbs and is cut into 36 sequential numbered slices. Each slice contains a regular matrix of 5 mm diameter holes, 3 cm apart. The holes are normally filled with plugs of the same material, which can be removed and replaced by TLDs for dose measurement at selected locations. Figure 1
shows the set-up for the chest projection at the X-ray Laboratory. The Rando phantom was placed in front of the vertical bucky stand, which holds the moving anti-scatter grid, and the flat transparent ionization chamber of the DAP meter was attached at the light beam diaphragm for DAP measurement at each exposure.
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Figure 1. Rando phantom placed against the vertical bucky in the standard chest radiograph projection. The ionization chamber of the dose–area product meter was attached at the light beam diaphragm.
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Chest radiograph projections from the phantom were taken with a medium frequency general purpose X-ray unit (Toshiba, KXO-30R Minato-ku, Tokyo, Japan), which was installed in 1997. Quality assurance tests for this equipment are performed regularly with a non-invasive X-ray test device (4000M Victoreen, Cleveland, OH). These tests include output reproducibility, and tube potential accuracy and reproducibility. Recent results confirmed the very stable performance of this X-ray source, with the coefficient of variation for all tests less than 2%. The beam-limiting collimator was also tested and showed very good alignment with the X-ray beam. The X-ray tube has an inherent filtration of 0.7 mm Al, together with beam-limiting device filtration of 1.7 mm Al plus an additional 1 mm Al filter, giving a total filtration of 3.4 mm Al. A DAP meter (Model M2, PTW Co., Freiburg, Germany) was mounted at the tube collimator to measure DAPs for all the exposures.
The film–screen combination used in this study was the Kodak X-omatic cassette with the Lanex standard screen and the Kodak general purpose green (MXG-1) film (Eastman Kodak Co., Rochester, NY). A 90 s automatic processor (Fuji FPM-2800, Minato-ku, Tokyo, Japan) was used to process the exposed phantom chest films. A densitometer (Model 07-443, Victoreen Co., Cleveland, OH) was used to measure the optical density of the phantom chest X-ray films. An air ionization chamber (MDH 2025 RadCal, Monrovia, CA) with a volume of 3 cm3 was used for ESD measurement and for calibration of TLDs over the diagnostic range of X-ray tube potentials. This ion chamber system was calibrated annually by the manufacturer with a technique traceable to US national standards. Calibration accuracy, energy dependence and repeatability are claimed to be within ±4%, ±5% and ±1%, respectively.
A TLD reader system (Rialto, NE Technology, Reading, UK) installed in 1994, together with some high sensitivity TLDs of LiF:Mg,Cu,P, were used for dose measurement. TLDs were supplied by Harshaw Co. in the form of round sintered disc-shaped pellets of 4.5 mm diameter and 0.8 mm thickness. The linearity (dose–response) of these TLDs was tested in previous experiments over the range from a few µGy to a few mGy, and the results showed a good linear fit with a coefficient of determination (R2) of 0.99. A new batch of these TLDs was initialized by 10 cycles of thermal treatment in an oven (Thermolyne Furnace 47900, Thermolyne Co., Dubuque, IA). For each thermal treatment the TLDs were heated to 240 °C and maintained at that temperature for 10 min, followed by cooling to room temperature in a natural draught. Energy response tests for the TLDs were carried out over the diagnostic X-ray tube potential range of 40–150 kVp using the X-ray unit and the ionization chamber system. The calculated mean response of the TLDs from 50–150 kVp was taken as the calibration factor in this study, since in this case the TLDs received X-ray doses mainly from scattered radiation, with the peak voltage set between 60 kVp and 150 kVp for all the projections.
The measured air dose was converted to tissue dose using a conversion factor of 1.06; this is the ratio of the mass energy absorption coefficients of tissue and air over the range of photon energies used [17]. The TLD reader for these high sensitivity TLDs was set to pre-heat at 135 °C for 8 s, followed by read-out for 19 s at 240 °C and a heating rate of 12 °C s-1. This was followed by a machine anneal at 240 °C for 90 s. 50 TLDs were selected from the initialized batch, all with homogeneity (sensitivity variation) within 5% of the mean response. The relative sensitivity of these TLDs was determined and individual calibration factors were calculated. Dose readings from these individually calibrated TLDs were estimated to be reliable to within ±10%.
Chest X-ray exposures were performed with the low tube potential (non-grid) technique using the Rando phantom in the standard PA position. The chest front of the phantom was placed up against the film-loaded cassette, which was held in a vertical position by a cassette holder. The focus-to-film distance used was 180 cm and the X-ray beam was collimated to 31 cmx33 cm size on film to just include the normal lung field area, as in a standard chest radiograph projection. A tube potential of 70 kVp was set initially to achieve a chest image of normal diagnostic density, which corresponds to a film optical density of 1.7 over the lower lung field of the chest. This was done by trial and error, with different exposures made by adjusting the X-ray tube integrated current (mAs value) at the control panel. After achieving good quality images at 70 kVp, the ESD and the exit dose of the phantom were measured along the central X-ray beam using the 3 cm3 ionization chamber. The DAP reading was taken from the DAP meter that was mounted at the tube collimator.
GD was measured with LiF:Mg,Cu,P TLD pellets placed at appropriate sites within the phantom. For testes dose determination, the TLDs were put on the surface of the phantom at the testes location. For dose measurement at the ovary, TLDs were inserted by vacuum tweezers in a sequential order of labelled TLDs at the pre-determined ovarian site in slice 29 of the phantom. This position is determined by the fact that the ovary is situated at the level of anterior–superior iliac spine above the mid inguinal point [23] and within the true pelvis against its lateral wall in the ovarian fossa [24]. Figure 2 is a radiograph of the pelvis of the phantom, indicating the position of the ovarian site on the left-hand side of the figure. Five TLD pellets were used at each measurement site. Owing to the very low level GDs expected, and to deliver accurately measurable doses to the TLDs, five sequential exposures were made each time using ten times the normal mAs value; thus, in total, 50 times the normal patient exposure was used for each simulated GD.
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Figure 2. Radiograph of the pelvis of the Rando phantom. The black arrowhead indicates the position of the ovarian site on the right side of the pelvis.
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Similar exposures were made up to 100 kVp in 5 kV steps for ESD and DAP measurements, and 10 kVp steps for GD measurement. Each measurement is made for the same phantom exit dose, achieved by appropriate integrated tube current (mAs) adjustment. A similar procedure was followed for the high tube potential technique using the vertical bucky (moving anti-scatter grid with grid ratio of 12:1). The automatic exposure control (AEC) was used for the high tube potential technique so that constant exit dose (determined by the AEC) was used for each dose measurement. Initially, the sensitivity control of the AEC was adjusted to produce a phantom chest film with the same normal density as in the low tube potential technique (i.e. film optical density equals 1.7). Exposures were then made using the fixed AEC sensitivity setting from 95 kVp to 150 kVp in steps of 5 kVp for ESD and DAP measurements and in steps of 10 kVp for GD measurements.
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Results
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Chest radiographs of the Rando phantom with the standard low and high tube potential techniques were produced with the same normal diagnostic film optical density of 1.7 measured at the same region of the lower lung field by the densitometer. The exit air dose required to produce the same film density for the low and high tube potential techniques was determined to be 4 µGy and 19 µGy, respectively. Table 1 shows the effect of tube potential on various doses using the low tube potential technique in the 60–100 kVp range. The measured doses include the ESD, DAP and GD. EDs were computed from ESD and DAP measurements using NRPB-SR262 and Xdose software; GDs were also computed for comparison with the measured values. Table 2 shows the effect of tube potential on these doses using the high tube potential technique in the 95–150 kVp range. The effect of tube potential variations on these dose parameters is better illustrated by graphical presentation of the data in Tables 1 and 2. The computer program computes the ovary and testes doses, rounding to the nearest 0.1 µGy. This accounts for the consistent values of 0.1 and 0 in Tables 1 and 2.
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Table 1. Effect of tube potential (kVp) variation on posteroanterior chest radiography doses using low tube potential (non-grid) technique for constant exit dose of 4 µGy
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Table 2. Effect of tube potential (kVp) variation on posteroanterior chest radiography doses using high tube potential (with grid) technique for constant exit dose of 19 µGy
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Figures 3–8 and Figures 9 and 10 illustrate the results for the low and high tube potential techniques, respectively. Linear and non-linear regression curves are generated where appropriate by Sigma Plot software Ver. 4 (SPSS Inc. Chicago, IL) using the curve fit function. Statistical correlation tests were performed using the SPSS software Ver. 9 (SPSS Inc., IL). For the low tube potential technique, the curves shown in Figures 3–5 have similar shapes and these show the fall in mAs, ESD and DAP values with increasing kVp; this effect is most marked for lower tube potential settings. A similar trend is achieved for the high tube potential technique. Figure 6 shows the variation of GD with kVp for the low tube potential technique; kVp shows positive linear correlations with GDs (ovary, r=0.994, p=0.0006; testes, r=0.998, p=0.001). Both ovary and testes doses increase at a rate of approximately 2% per kVp. For the high tube potential technique there is only a weak correlation between kVp and GDs (ovary, r=0.767, p=0.0745; testes, r=0.812, p=0.05), and doses for both organs vary little over the whole tube potential range, as illustrated in Figure 9.
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Figure 3. Variation of X-ray tube current (mAs) with tube potential (kVp) for constant exit dose (low tube potential technique).
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Figure 4. Variation of entrance surface dose (ESD) with tube potential (kVp) (low tube potential technique).
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Figure 5. Variation of dose–area product (DAP) with tube potential (kVp) (low tube potential technique).
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Figure 7. Variation of effective dose with tube potential (kVp) (low tube potential technique). ESD, Entrance surface dose; DAP, dose–area product.
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Figure 8. Variation of dose–area product (DAP) with entrance surface dose (ESD) (low tube potential technique).
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Figure 10. Variation of effective dose with tube potential (kVp) (high tube potential technique). ESD, Entrance surface dose; DAP, dose–area product.
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Figure 7
illustrates the effect of tube potential on EDs with the low tube potential technique. There are fairly weak negative correlations between tube potential and EDs (derived from ESD and DAP measurements): (ED [ESD], r=-0.9, p=0.001; ED [DAP], r=-0.887, p=0.01). Figure 10 shows more significant negative correlations between tube potential and EDs using the high tube potential technique (ED [ESD], r=-0.997, p=0.000; ED [DAP], r=-0.998, p=0.000). DAPs and EDs show very marked linear variations with kVp for the low tube potential technique (r=1, p=0.00), as illustrated in Figures 5 and 7. The result for the high tube potential technique shows a similar graphical pattern and correlation (r=0.997, p=0.00001).
Table 3 extracts data from Tables 1 and 2 to compare the various dose parameters for typical exposures made at 70 kVp (low tube potential non-grid technique) and 120 kVp (high tube potential grid technique). From this table, the ESD, ovary dose, testes dose and ED (from ESD data) for the 70 kVp technique were found to be 98.8 µGy, 0.073 µGy, 0.019 µGy and 10.5 µGy, respectively, whereas for the 120 kVp technique these were 168 µGy, 0.38 µGy, 0.105 µGy and 28.2 µGy, respectively; the corresponding dose ratios are 1.7, 5.2, 5.5 and 2.7. The relative ED/ESD ratio between 120 kVp and 70 kVp is 1.6.
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Table 3. Dose comparision of high and low tube potential techniques at two typical tube potentials for posteroanterior chest radiography (data extracted from Table 1 and 2)
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Discussion
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For the low tube potential technique, Figures 3–5 show mAs, ESD and DAP falling with increasing tube potential in a similar non-linear manner. Similar results were achieved for the high tube potential technique. Fits to these data yield equations that can be used to extrapolate or predict the mAs, EDS and DAP from any selected tube potential in standard situations. This may prove useful for estimating these parameter values for a given tube potential for a given technique, equipment and film processing situation.
This study has provided useful information for the selection of the appropriate technique (high or low tube potential) and for tube potential selection in realistic situations. As shown in Figure 6, for the low tube potential technique, both the ovary and testes doses increase with tube potential at the rate of about 2% per kVp. So a higher tube potential selection will produce a lower ESD (Figure 4) but a higher ovary or testes dose (Figure 6) and a slightly lower ED (Figure 7). However, there is minimal overall benefit to the patient since the decrease in ESD is compromised by the increase in GD. However, the final decision should be made by reference to the actual ED received at a particular tube potential, since ED is the best indicator for cancer risk estimation. From the shape of the ED curve shown in Figure 7, ED values are somewhat higher below 70 kVp, so the selection of a tube potential below 70 kVp is not recommended since this will deliver a high ESD and overall higher ED to the patient. A compromise is to select the tube potential between 70 kVp and 90 kVp. However, other factors need to be considered, such as the film contrast requirement, since a higher tube potential will produce a lower contrast film, thus deteriorating the diagnostic value.
The situation is totally different when the high tube potential technique is employed. As shown in Figure 9, there is little variation in the GD over the whole tube potential range, so selection of tube potential will be dictated instead by the ED variation with tube potential shown in Figure 10. There is a difference in ED values extracted from DAP and ESD, as shown in Figures 7 and 10. This may be due to the difference in the X-ray beam field size on film (31 cmx33 cm) used in this study as compared with that from the Monte Carlo mathematical simulation projection using 32 cmx40 cm [17, 18]. The discrepancy of ESD values between the beam size used in this study and that used by the NRPB calculation would be small. However there will be a larger discrepancy in the DAP read-out if there is a substantial difference in the beam area. The beam area ratio between that used by the NRPB calculation and that used by this study is 1.25, so the calculated ED will be proportionally smaller since DAP is a reading from the air dose recorded multiplied by the beam area. In addition, the NRPB quoted a potential error of 17% in converting DAP read-out to ED for PA chest radiography examinations [18]. In this mode, a higher tube potential selection is recommended since this will deliver both a lower ED and a lower ESD. Since a grid is always used in the high tube potential technique and the film contrast could be maintained throughout the tube potential range, this suggests using the highest possible tube potential available from the X-ray units, such as 150 kVp.
The next important practical question to be addressed is "which is the better technique, a high or low tube potential?" If the film image quality is equally acceptable from both techniques, the answer is to take the low tube potential technique, since this will definitely deliver significantly lower patient doses. Table 3 compares patient doses with typical exposures made at 70 kVp (low tube potential non-grid technique) and 120 kVp (high tube potential grid technique) for ESD, ovary dose, testes dose and ED. In this typical case, from a radiation protection point of view, the high tube potential technique is not recommended in chest radiography since it delivers a significantly higher ESD, and ovary, testes and effective doses by factors of 1.7, 5.2, 5.5 and 2.7, respectively, as shown in Table 3. In a practical scenario when a pregnant woman requires a chest radiograph, probably the best choice is to use the low tube potential technique at 70 kVp, which minimizes the overall dose to the fetus.
In this study GDs were obtained both by TLD measurements and by computation from ESD and DAP measurements; this latter method is only valid up to 120 kVp. The experimentally derived testes doses were very small under all exposure conditions. With the low tube potential technique (Table 1) the ovary dose computed from ESD measurements is 0.1 µGy from 60–100 kVp compared with a range of 0.064–0.116 µGy from the TLD measurements. The computer program probably rounds results to the nearest 0.1 µGy. However, for such small doses, this is quite acceptable. A similar situation also occurs with the high tube potential technique results shown in Table 2. This study was able to differentiate very low dose levels below 0.1 µGy with the thermoluminescent dosimetry technique. There is a discrepancy between the EDs computed from EDSs and DAPs, as shown in Tables 1 and 2. This may be due to the difference in beam field size used in this study compared with the field size specified by software using the Monte Carlo technique.
There are many factors that affect patient doses in chest radiography, such as the film–screen combination, film processor developer temperature, types of filters, and anti-scatter grids with different grid ratios [4]. For example, if a different film–screen combination system had been used for this study, such as the Agfa Curix Universal standard screen with the Agfa STG2 film also available in this laboratory, the dose levels would have been doubled due to the slower speed of the recording system, which is about half that of the Kodak system actually used. The various doses recorded in this study result from an integrated image recording system that is a combination of different equipment (X-ray unit, grid and screen–film type, and film processor) with a commonly used exposure factor. Dose values may be used as a reference for chest radiography in future, quoting the equipment and exposure technique used. In this study, the ESDs for low and high tube potential techniques (from Tables 1 and 2) ranged from 0.06–0.13 mGy and 0.1–0.25 mGy, respectively. These dose values are in line with the NRPB reference dose of 0.3 mGy and 0.2 mGy, which was the third quartile dose interval from national surveys performed in 1983–85 and 1988–95, respectively [25]. The ED from this study ranged from 0.01–0.036 mSv for both techniques and this also agrees well with the published typical ED of 0.02 mSv in the UK [26].
The use of high sensitivity TLDs of LiF:Mg,Cu,P (also known as GR200 or TLD100H) enabled the estimation of very low ovary and testes doses from chest radiography. Previous tests in this laboratory showed that these TLDs have a minimum detection limit of 0.6 µGy (at 95% confidence) and that phosphor has a sensitivity more than 30 times that of the conventional LiF:Mg,Ti, commonly known as TLD100. Individual calibration used in this study enabled the dose measurement accuracy to be improved by a few per cent. 50 exposures for each projection delivered enough dose to these TLDs for accurate light output measurements during read-out. For example, the testes site at 120 kVp exposure receives about 5 µGy after 50 exposures in the PA projection, and each exposed TLD produced an output reading of about 600 units inthe TLD reader compared with a background of 50.
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Conclusions
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The very low level GDs in PA chest radiography were successfully measured using the high sensitivity TLD pellets of LiF:Mg,Cu,P, which have a very low dose detection threshold. The effect of tube potential variation on GDs was shown in tables and illustrated graphically in both high and low tube potential techniques.
The ovary:testes dose ratio falls within a narrow range of 3.6–4.2 for any tube potential in both techniques. With the low tube potential technique, tube potential correlates well with both ovary and testes doses and both these doses increase with tube potential at a rate of 2% per kVp in the range of 60–100 kVp. With the high tube potential technique, there is no appreciable variation of GDs in the range of 100–150 kVp. ESDs and DAPs have shown a very significant correlation in both low and high tube potential technique ranges. Measured ESDs and EDs in this study agree with UK reference doses.
Dose comparison with two typical exposures at 120 kVp and 70 kVp show that the high tube potential technique delivers a higher ESD, ovary dose, testes dose and ED by a factor of 1.7, 5.2, 5.5 and 2.7, respectively. Hence, from a radiation protection perspective, a high tube potential technique (with grid) for PA chest radiography is not recommended.
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Footnotes
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This research project is funded by the University Research Grant of the Hong Kong Polytechnic University (A/C code G-333). 
Received for publication August 29, 2000.
Revision received October 31, 2000.
Accepted for publication November 27, 2000.
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References
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Abstract
Introduction
