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The effect of phantom type, beam quality, field size and field position on X-ray scattering simulated using Monte Carlo techniques

Joint Department of Physics, The Royal Marsden NHS Trust, Fulham Road, London SW3 6JJ, UK

	Abstract

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 
Determining the amount of scatter inside and outside a diagnostic X-ray room is important for evaluating the dose to staff and the public. The amount of scatter is affected by many physical factors including beam quality and field size. However, there is little published data on patient scatter and there are large differences between the available data sets. Hence, a Monte Carlo code was developed to allow a systematic study of the factors affecting patient scatter. A voxel phantom was used to provide a realistic model of the patient. The variation of scatter with different phantom types was investigated to show the effect of patient inhomogeneities and obliquities. The effect of altering tube voltage, filtration, voltage ripple, field size and position on patient scatter was studied. A larger than expected variation in the patient scatter was observed with increasing field area due to the proximity of the field borders with the patient obliquities. The effect of the tube voltage ripple on the patient scatter was also calculated. This showed that there would be little effect on the scatter levels within X-ray rooms if ageing X-ray generators, which produce substantial voltage ripple, were replaced by X-ray tubes with modern medium frequency generators. Recommendations are made on the choice of published scatter data for X-ray room design.

	Introduction

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 
Scatter is produced by all materials in a diagnostic X-ray room, with the main source of scattered radiation being the interaction of X-rays with the patient [1]. However, there is a limited amount of scatter data available for use in X-ray room design. The data were obtained from measurements undertaken with tissue equivalent slab phantoms [2]; with human-shaped homogeneous phantoms [3] and with heterogeneous phantoms such as the RANDO phantom [4–6]. Since these studies used a variety of phantom types and technique parameters, there was a large variation in the scatter values reported. An alternative solution is to use Monte Carlo computer simulations of the scatter produced by a model of human anatomy. This has enabled three systematic studies of the effect of different parameters on scatter from patients undergoing chest posteroanterior (PA), lumbar spine anteroposterior (AP) and lumbar spine lateral (LAT) radiographic examinations.

For the first study, four voxel phantoms (P1 to P4) have been used to simulate the patient as shown in Figure 1. These different approaches have been followed to investigate the effect of patient obliquities and inhomogeneities on scatter. The first approach was to use a voxel phantom (P1) reconstructed from CT data. It was developed by Zubal et al [7, 8] and was recently used in a Monte Carlo model to optimize image quality and patient dose in chest and lumbar spine radiography [9, 10]. Dance et al [11] showed that Zubal's voxel phantom was representative of a patient undergoing chest and lumbar spine radiographic examinations. The second approach was to use the voxel phantom developed by Zubal and change all the voxels inside the patient contour to be soft tissue and those outside to be air (P2). The third approach was to use the voxel phantom with all the voxels within the phantom to be soft tissue (P3). The fourth voxel phantom was developed as a block of soft tissue specified by the average dimensions of Zubal's voxel phantom (P4).

View larger version (28K): [in this window] [in a new window]   Figure 1. The simulation model used to calculate the scatter from a patient undergoing, for example, a chest posteroanterior X-ray examination. The position of the detector is shown at a scattering angle of 135°. The three other phantoms used in the calculations are also shown below.

The third study used the Monte Carlo code to calculate the effect on the scatter from the patient model (P1) of varying the field area (25–1225 cm²) and the position of the field on the patient. This study generalized the results for the chest and lumbar spine regions so that the data may be interpreted for other X-ray examinations. The calculated scatter values obtained in this work may be used to aid the design of X-ray rooms, but they may also assist in the analysis of the doses received by staff who undertake and assist with interventional radiological examinations.

	Methods and materials

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 
Voxel Monte Carlo code
The Monte Carlo code is similar to that used previously to study image quality and patient dose in radiographic examinations [9, 10], but was extended to simulate the scatter surrounding a voxel phantom. The program transports the photons through the voxel phantom; a collision density estimator [12] is used to provide an efficient method of calculating scatter. The model calculates the air kerma at points 1 m from the phantom surface for scattering angles between 30° and 150°. Scatter ratios were determined by the air kerma at each of these points divided by the incident air kerma without backscatter. The scatter ratios are expressed as percentages. A large number of photon histories were used to calculate this parameter so its uncertainty was less than ±1% (1 standard deviation).

The patient model was a voxel phantom (P1) derived from segmented CT data [7, 8]. Each voxel belonged to 1 of 55 organs [10]. The tissue type of each organ was specified as one of average soft tissue, healthy lung, bone or bone spongiosa. The calculations used tissue densities and compositions taken from the International Commission on Radiation Units and Measurements (ICRU) Report No. 46 [13], except for bone which was taken from Kramer [14]. The patient support device, i.e. the chest stand for the chest examination or couch top for the lumbar spine examinations, was included in the voxel phantom by the addition of an extra layer of voxels. Table 1 shows the thickness and composition of the chest stand and couch top. The dimensions of the voxel phantom were 89.9 cm long, 35.6 cm wide and 21.4 cm thick. As the lower limbs were not present in the phantom, its length was determined to be equivalent to the height of the average European male in sitting position. The shoulder width and chest thickness were determined after an initial study [11, 15] which compared calculations with measurements of patient entrance air kerma.

Table 1 shows the imaging system parameters. These parameters was found to provide good image quality in a recent EU clinical trial [17, 18] and were thus used as a reference system to observe the differences in patient scatter when the imaging system parameters were varied.

In the first study, the effect of the patient heterogeneities and obliquities on scatter were investigated. Therefore, in addition to the patient model (P1) described above, the other three phantoms shown in Figure 1 were used to simulate scatter from a patient undergoing chest PA, lumbar spine AP and lumbar spine lateral examinations. Phantom P2 was defined with the voxels inside the patient's surface set to average soft tissue and those outside the surface set to air. Phantom P3 was a slab phantom defined with all voxels, apart from the chest stand or couch top, set to be average soft tissue. Phantom P4 was also a slab phantom defined by the average thickness (z direction) and width (y direction) of the patient model within the field borders for each projection. Table 2 shows the dimensions of the P4 phantoms including the chest stand or couch top. This was undertaken as the shoulder width was considerably larger than the width further down the phantom's body outline. Hence, it was interesting to determine which slab phantom (P3 or P4) scatter approximated the scatter from the patient model most closely. The patient model (P1) was used for the other two studies described in the introduction.

Simulation of Williams' scattering experiment
This section describes the method used to compare the scatter calculated using the voxel Monte Carlo code with the scatter measured by Williams [5]. This was carried out to validate the calculations against recent independently published values and also to check the reliability of Williams' measured values.

Williams measured the scatter from the abdominal and pelvic sections of a RANDO phantom. Therefore, a voxelized cylinder of Alderson Muscle A material was used to simulate the RANDO phantom of dimensions 50.0 cm long, 25.0 cm wide and 21.5 cm thick. Its composition and density were obtained from ICRU Report No. 44 [19]. Williams [5] measured the scatter in terms of air kerma normalized to the dose–area product (DAP). Therefore, a DAP meter and the air between it and the phantom surface were included in the voxel model. McVey [20] showed that these materials produce a significant amount of scatter. The DAP meter was modelled as a solid block of Perspex with dimensions: 16.4 cm length, 18.1 cm width and 1.7 cm thickness. An average density of 0.315 g cm^–3 was used for the Perspex as the DAP meter was constructed from layers of Perspex 0.2 cm thick with an air gap between them. The DAP meter was assumed to be at a distance of 26.6 cm from the X-ray focus and the focus to surface distance (FSD) was 80 cm. The incident field at the phantom surface was 22 cm long and 17.5 cm wide. The scatter was calculated at points 1 m from the centre of the phantom for scattering angles between 30° and 150°.

	Results and discussion

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 
Validation of the scatter calculations
The scatter calculations were validated by comparing the scatter at points surrounding a block of solid water calculated with the voxel Monte Carlo code to the values calculated by an EGS4 Monte Carlo code for the same geometry as described by McVey [20]. Good agreement (within 2%) was shown between the values calculated by the two codes for photon energies between 20 keV and 150 keV and for tube voltages between 50 kV and 120 kV.

The voxel Monte Carlo code could not be used to simulate the scatter measurements previously carried out in a clinical X-ray room as detailed by McVey and Weatherburn [1]. This was due to the geometrical limitations of the code and the size of the simulated model. McVey and Weatherburn [1] used the EGS4 Monte Carlo code to calculate the scatter from solid water blocks placed within a simulated X-ray room and showed reasonable agreement with the measured scatter. For this simulation, the percentage scatter contribution from the X-ray room walls to the total calculated scatter was determined to be small, being 3.7% [20]. Thus, the scatter calculated by the voxel Monte Carlo code should be reasonably representative of the scatter levels found in clinical X-ray rooms.

Dependence of percentage scatter on phantom type
Figure 2 shows the scatter from the four phantoms calculated for the chest PA, lumbar spine AP and lumbar spine LAT projections, respectively. The scatter for the patient (P1) and the contoured phantom (P2) lie between the values for the thick slab phantom (P3) and the slab phantom with average dimensions (P4) below scattering angles of 62° for the lumbar spine AP view, below 108° for the chest PA view and below 125° for the lumbar spine LAT view. Therefore, the phantom with the average dimensions (P4) can provide a conservative estimate of the scatter from a patient (P1) for all scattering angles for the chest PA and lumbar spine LAT exams and for scattering angles less than 67° and greater than 131° for the lumbar spine AP exam.

View larger version (25K): [in this window] [in a new window]   Figure 2. The variation of the percentage scatter with different phantom types(P1 to P4) for (a) the chest posteroanterior projection, (b) the lumbar spine anteroposterior projection and (c) the lumbar spine lateral projection.

The largest differences between the different phantom types occurred between scattering angles of 30° and 87°. The largest difference was 77% for the lumbar spine AP projection at 87°. Table 3 shows the scatter for all the phantoms relative to the patient model for 45° and 120° scattering angles as examples of forward and backward directed scatter. The thick slab phantom (P3) produced the least scatter in the forward direction as it had the largest thickness and width and, therefore, greatly attenuated the scattered X-rays. The contoured phantom (P2) produced more scatter than the thick slab phantom in the forward direction. The phantom obliquity attenuated the scattered X-rays less. The contoured phantom's thickness and width varied along its length, and in places these were larger than the phantom with average dimensions (P4). Thus, the contoured phantom produced less scatter than the phantom with average dimensions.

For different examinations, Table 3 shows that the differences between the phantom types were larger in the forward direction for the lumbar spine AP projection compared with those for the chest examination as a lower tube voltage was used for the lumbar spine AP projection. For the lumbar spine LAT examination, the differences were not as large as would be expected for the large thickness of the patient in the lateral projection. The field edge was close to the phantom boundary. Therefore, there was less tissue to attenuate the forward scattered photons.

Figure 2 shows that the phantom type had less effect in the backward direction than in the forward direction for the frontal projections. The scatter was produced near the entrance surface of a phantom [20]. For the lateral projection, changes in the phantom width were more significant. The effect of the tissue inhomogeneities in the backward direction was similar to their effect in the forward direction for the chest PA and lumbar spine LAT projections. For the lumbar spine AP projection, there was a larger difference between the forward and backward directed scatter as bone volume occupied a greater proportion of the irradiated volume than for the other projections.

Dependence of percentage scatter on tube voltage
Table 4 shows the variation of scatter normalized to the reference system values for X-ray tube voltages between 60 kV and 150 kV for the different examinations. The largest variation was for forward directed scatter in all projections. The forward directed scatter became more penetrating with increasing tube voltage. The majority of backward scattered X-rays were produced close to the entrance surface [20]. Therefore, there was less of an effect for increasing tube voltage.

The lumbar spine LAT projection had a smaller variation with tube voltage than the AP projection. The LAT projection had a reference system tube voltage of 77 kV and the field was positioned closer to the patient's edge than the AP projection. The chest PA projection had a smaller variation with tube voltage than both the lumbar spine projections. This was due to the high reference system tube voltage of 141 kV and because the lungs attenuated the scattered X-rays less than tissue in the chest PA projection.

Table 5 compares the variation of the lumbar spine AP and LAT values with those from McVey and Weatherburn [1], Trout and Kelley [3] and Williams [5]. There were large differences in the variations of forward directed scatter with tube voltage due to differences in the attenuating properties of the phantoms used. The differences in the variations were less in the backward direction as the scatter was produced close to the entrance surface of the phantom [20]. The variation of Trout's scatter values was considerably larger than the other values. This was due to the 70 kV values being produced by a self-rectified X-ray tube which had a low beam quality. The variation of Williams' scatter values was slightly less than the other values. This was due to the significant amount of scatter produced by the DAP meter which was independent of tube voltage [1].

The percentage scatter for the lumbar spine AP projection had the largest variation with filtration and voltage ripple. The scatter in the forward direction decreased by 15% if the filtration decreased by 1.2 mmAl or the voltage ripple decreased by 20%. The variations in percentage scatter for the lumbar spine LAT and chest PA projections were less due to their higher beam qualities.

Similar variations of scatter with different filtrations and voltage ripples were observed at 102 kV, 90 kV and 95 kV for the chest PA, lumbar spine AP and LAT projections, respectively. Therefore, the variations shown in Table 6 are applicable over a large range of tube voltages.

Dependence of percentage scatter on field area
Table 7 shows the percentage scatter for a 100 cm² square field area which was used to normalize the scatter values for the different field areas and field positions shown in Table 8. All the reference system parameters for each projection, as shown in Table 1, were employed except for the field area. Table 7 shows the percentage scatter in the lumbar spine LAT projection calculated with the field centre at two positions. First, the field centre was positioned at the same place as the reference system, which was close to the patient's lateral boundary. Second, the field centre was positioned at the centre of the patient's width. In both cases, the scatter was calculated at points on the side of the patient where the field was off-centre. Table 7 shows that the position of field centre had a considerable effect. The percentage scatter was larger for all scattering angles with the field centred on the patient's obliquity as there was less tissue to attenuate the scattered radiation. The scatter for the field centred on the patient obliquity was 2.3 times greater than for the field centred on a thicker part of the patient for a scattering angle of 45°. Trout and Kelley [3] found the scatter for the field centred at the phantom's edge was between 6.5 and 1.8 times larger than the scatter for the field positioned at the centre of the phantom's width for tube voltages between 50 kV and 150 kV at a 45° scattering angle.

View this table: [in this window] [in a new window]   Table 8. Percentage scatter for different square field areas relative to the percentage scatter for the 100 cm2 square field areas given in Table 7

The variation of scatter with field area shown in Table 8 was due to the patient obliquities. A field width of 25 cm covered the majority of the patient's trunk. The field edges were incident on the patient obliquities and thus, the scattered X-rays were less attenuated. The scatter was therefore larger than expected in the forward direction. The patient's obliquity produced a small reduction in the amount of scatter produced in the backward direction due to there being less tissue. There was a greater difference for the lumbar spine view than the chest PA view due to the lower tube voltage.

A smaller number of field areas were investigated for the lumbar spine LAT exam due to the small width of the patient in this orientation. Table 8 shows the normalized scatter values with the field centre close to the patient obliquity (reference system position) and the field positioned at the centre of the patient's width. With the 225 cm² field centred medially on the patient, the normalized values were 3.38 and 2.29 for scattering angles of 45° and 120°. With the field moved laterally by 3 cm, the normalized scatter values were smaller and thus, there was a larger variation if the field was centred over a thick part of the patient than if the field was centred on the patient obliquity. However, the actual scatter values tended to be larger for the field positioned at the patient obliquity than at the patient centre as there was less attenuation of the scattered X-rays (Table 7). If the scatter was calculated at points on the other side of the patient than the field centre, then the increased attenuation would substantially reduce its amount.

Figure 3 shows the variation of scatter with square, rectangular and equivalent square field areas between 25 cm² and 900 cm² for a 45° scattering angle for the lumbar spine AP and chest PA projections. The rectangular field length was kept constant at 30 cm and the width increased from 5 cm to 30 cm. In X-ray room design it is difficult to account for the variation of field dimensions used in the clinical situation. Therefore, one method investigated was to calculate the equivalent square field area (s²) using the equivalent square field length (s) and the rectangular field dimensions (x and y) shown in Equation (1) [22].

Figure 3a shows large differences in the variation of scatter between square and rectangular field sizes for the lumbar spine AP projection as a low tube voltage was employed. There was an 82% difference between the rectangular and square fields with areas of 300 cm² (i.e. 10 cm x 30 cm for the rectangular field). For the rectangular field, the scattered X-rays were more attenuated as the smaller width covers thicker parts of the patient. For the square field, the scattered X-rays were less attenuated as the field edge was closer to the patient obliquity. Figure 3a shows that the equivalent square field approximated the variation of scatter for square fields well when the smallest rectangular field dimension was greater than or equal to 10 cm. For the smallest field dimension of 5 cm, the equivalent square field overestimated the scatter for the square field.

View larger version (13K): [in this window] [in a new window]   Figure 3. The variation of the normalized percentage scatter at a 45° scattering angle with square, rectangular and equivalent square field sizes between 25 cm2 and 900 cm2 for (a) the lumbar spine anteroposterior projection and (b) the chest posteroanterior projection.

Comparison of scatter values in the literature
Table 9 shows a summary of the imaging parameters stated in the literature which were used to obtain the scatter values shown in Table 10. All the imaging system parameters were selected to be as similar as possible for this comparison. The range of tube voltages studied was restricted to be from 50 kV to 100 kV. All scattering materials used were tissue equivalent. The phantoms used by Trout and Kelley [3] and Williams [5] had a human body contour. The voxel Monte Carlo code was used to model the experimental set up for Williams scatter measurements (as detailed above).

Table 10 shows that there was good agreement between the calculated and measured scatter values reported by McVey and Weatherburn [1] and the calculated values given in the previous section and with the measured values given by Williams [5]. These agreements give confidence in both the calculations and measurements. Both studies [1, 5] employed modern equipment, including a DAP meter, in the experimental set up. A FSD of 100 cm was used in the work of McVey and Weatherburn and a FSD of 80 cm was used in the work of Williams (Table 9). Williams' values were considerably greater than those of McVey and Weatherburn for scatter in the backward direction. The difference in FSD produced a difference in the position of the DAP meter which resulted in large differences in the scatter in the backward direction. McVey and Weatherburn's values were greater than Williams' values in the forward direction due to the smaller phantom thickness. Scatter in the backward direction was less affected by changes in phantom thickness than scatter in the forward direction [20].

The patient scatter values, calculated for the lumbar spine AP view at 72 kV, were similar to Williams' values in the forward direction and less than Williams' values in the backward direction (Table 10) as a DAP meter was not included in the scatter calculations.

Table 10 shows that there was poor agreement in the forward directed scatter values reported by Bomford and Burlin [2] and Trout and Kelley [3]. For example at a 30° scattering angle, Bomford's values were 0.2 times smaller than Trout's. It is difficult to understand the reason for the differences between Trout's and Bomford's results as their phantoms had similar thicknesses (Table 9). Bomford and Burlin had corrected their values for scatter from the surroundings and leakage from the X-ray tube head. These contributions were a large proportion of the total reading as the scatter from the phantom was small in the forward direction. McVey [20] calculated that the scatter from the collimators, ceiling, floor and walls varied between 0.03% and 0.05%. This accounted for some of the differences which were between 0.045% and 0.101%, but also suggested that the masonite and MixD phantoms may have substantially different attenuating properties. The scatter values were similar in the backward direction for the two phantoms as the scatter was less dependent on the phantom thickness or density.

	Conclusions

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 
Accurate determination of the scatter in X-ray rooms is important for designing shielding to meet the desired radiation protection requirements. Previous studies have used a variety of phantoms to estimate these scatter levels and, as a review of the literature has shown, there are large differences in the published scatter values. The work in this paper has determined the magnitude of scatter from patients undergoing diagnostic X-ray procedures with the imaging system parameters varied in a systematic manner to provide a comprehensive data set.

The voxel Monte Carlo calculations have demonstrated that the linear relationship between scatter and field area, as used in shielding reports [6, 21], is not valid for patient irradiation. For example, the scatter was 91% larger than the expected value for increasing the area of a square field from 100 cm² to 625 cm² for a patient undergoing an X-ray examination in the lumbar spine region. The position of the field on the patient in relation to the calculation points also had an effect. The scatter from a patient undergoing a lumbar spine LAT exam increased by 2.3 times for the centre of the field being moved from the centre of patient's width closer to the patient's obliquity with the calculation points on the same side of the patient. If scatter was calculated on both sides of the patient and the field centre moved laterally, then the scatter distribution would become asymmetric i.e. the scatter would be higher on one side compared with the other. Thus, the calculation points on the same side of the patient as the lateral shift would provide a conservative estimate of the scatter. As well as X-ray room design, this work can be applied to estimate the doses received by staff who undertake interventional procedures.

The Monte Carlo calculations have also demonstrated small variations in patient scatter, in particular for changing the tube voltage ripple. For example, the scatter from a patient undergoing a lumbar spine AP exam increased by 22% if an X-ray generator with a voltage ripple of 50% was replaced by a constant potential X-ray generator, whereas the scatter increased by 97% if the tube voltage increased by 38 kV. Therefore, replacing X-ray generators with substantial voltage ripple by medium frequency units would not produce sufficiently more scatter to warrant a change to the X-ray room design (using the same dose constraint).

A review of published scatter values [1, 5] has suggested that there was a FSD dependence on backward directed scatter when a DAP meter was present. However, Marshall and Faulkner [4] found no FSD dependence for air kerma measured adjacent to the couch for a 90° scattering angle, i.e. at a position forward of the DAP meter. Marshall and Faulkner imply that the FSD was simply increased, leading to an increase in field size incident on the phantom, which may explain the constancy in their scatter measurements. McVey and Weatherburn [1] showed that there was a large variation in backward directed scatter from the DAP meter. Therefore, further work is necessary to investigate these effects, their influence on patient scatter and their possible impact on shielding barrier calculations.

For X-ray room design, the largest scatter values provided by either McVey and Weatherburn [1] or Williams [5] are recommended to provide a conservative dose estimate at any FSD. The scatter from the patient detailed in this paper may also be considered. In this case, the significant scatter from the surroundings (e.g. the DAP meter and X-ray collimators) [1] should be taken into account. The inconsistent forward scatter values given by Trout and Kelley [3] and Bomford and Burlin [2] are not recommended for use even for X-ray units which have significant voltage ripple. In this case, the scatter data from this work, McVey and Weatherburn [1] or Williams [5] should be used and modified by the ratios detailed in this paper depending on the amount of voltage ripple. The work in this paper can also be used to study the effect of changing tube voltage, filtration, voltage ripple, field area and field position on the patient scatter. For example, in X-ray room design, these factors can be used to increase the recommended scatter values or independently measured scatter values to provide a conservative dose estimate as appropriate to the clinical situation.

	Acknowledgments

 
I would like to thank Dr David Dance, Prof. Gudrun Alm Carlsson and Dr Michael Sandborg for providing the voxel Monte Carlo code which was the basis of the scatter calculations. I also acknowledge the use of the computer facilities at the Physics Department, Royal Marsden Hospital, London, where all the Monte Carlo simulations described in this paper were undertaken.

	Footnotes

 
Current address: North Wales Medical Physics, Glan Clwyd Hospital, Bodelwyddan, Denbighshire LL18 5UJ, UK.

This work was supported by a grant from Anglia and Oxford Health Authority.

Received for publication October 25, 2004. Revision received June 7, 2005. Accepted for publication June 15, 2005.

	References

Top Abstract Introduction Methods and materials Results and discussion Conclusions References

 

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