A simple method to retrospectively estimate patient dose-area product ...

A simple method to retrospectively estimate patient dose-area product for chest tomosynthesis examinations performed using VolumeRAD Magnus Båth, Christina Söderman, and Angelica Svalkvist Citation: Medical Physics 41, 101905 (2014); doi: 10.1118/1.4895002 View online: http://dx.doi.org/10.1118/1.4895002 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/41/10?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Comment on “Comparison of patient specific dose metrics between chest radiography, tomosynthesis, and CT for adult patients of wide ranging body habitus” [Med. Phys. 41(2), 023901 (12pp.) (2014)] Med. Phys. 42, 2094 (2015); 10.1118/1.4914374 Comparison of patient specific dose metrics between chest radiography, tomosynthesis, and CT for adult patients of wide ranging body habitus Med. Phys. 41, 023901 (2014); 10.1118/1.4859315 Reply to “Comment on the ‘Report of AAPM TG 204: Size-specific dose estimates (SSDE) in pediatric and adult body CT examinations’” [AAPM Report 204, 2011] Med. Phys. 39, 4615 (2012); 10.1118/1.4725757 Determination of dose-area product from panoramic radiography using a pencil ionization chamber: Normalized data for the estimation of patient effective and organ doses Med. Phys. 31, 708 (2004); 10.1118/1.1650686 An automated patient recognition method based on an image-matching technique using previous chest radiographs in the picture archiving and communication system environment Med. Phys. 28, 1093 (2001); 10.1118/1.1373403

A simple method to retrospectively estimate patient dose-area product for chest tomosynthesis examinations performed using VolumeRAD Magnus Båtha) Department of Radiation Physics, Institute of Clinical Sciences, The Sahlgrenska Academy at University of Gothenburg, Gothenburg SE-413 45, Sweden and Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, Gothenburg SE-413 45, Sweden

Christina Söderman Department of Radiation Physics, Institute of Clinical Sciences, The Sahlgrenska Academy at University of Gothenburg, Gothenburg SE-413 45, Sweden

Angelica Svalkvist Department of Radiation Physics, Institute of Clinical Sciences, The Sahlgrenska Academy at University of Gothenburg, Gothenburg SE-413 45, Sweden and Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, Gothenburg SE-413 45, Sweden

(Received 4 April 2014; revised 30 June 2014; accepted for publication 18 August 2014; published 16 September 2014) Purpose: The purpose of the present work was to develop and validate a method of retrospectively estimating the dose-area product (DAP) of a chest tomosynthesis examination performed using the VolumeRAD system (GE Healthcare, Chalfont St. Giles, UK) from digital imaging and communications in medicine (DICOM) data available in the scout image. Methods: DICOM data were retrieved for 20 patients undergoing chest tomosynthesis using VolumeRAD. Using information about how the exposure parameters for the tomosynthesis examination are determined by the scout image, a correction factor for the adjustment in field size with projection angle was determined. The correction factor was used to estimate the DAP for 20 additional chest tomosynthesis examinations from DICOM data available in the scout images, which was compared with the actual DAP registered for the projection radiographs acquired during the tomosynthesis examination. Results: A field size correction factor of 0.935 was determined. Applying the developed method using this factor, the average difference between the estimated DAP and the actual DAP was 0.2%, with a standard deviation of 0.8%. However, the difference was not normally distributed and the maximum error was only 1.0%. The validity and reliability of the presented method were thus very high. Conclusions: A method to estimate the DAP of a chest tomosynthesis examination performed using the VolumeRAD system from DICOM data in the scout image was developed and validated. As the scout image normally is the only image connected to the tomosynthesis examination stored in the picture archiving and communication system (PACS) containing dose data, the method may be of value for retrospectively estimating patient dose in clinical use of chest tomosynthesis. C 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1118/1.4895002] Key words: chest tomosynthesis, dosimetry, dose-area product (DAP), radiation exposure

1. INTRODUCTION Chest tomosynthesis refers to the technique of acquiring a number of discrete projection radiographs of the chest over a limited angular range and using these radiographs for reconstructing section images.1–4 It is well known that in chest radiography, the superimposed anatomy may be the dominating factor negatively affecting the possibility of detecting relevant pathology.5–8 In tomosynthesis, the superimposed anatomy is reduced substantially in the reconstructed section images, and several investigations have shown that this reduction leads to improved possibilities of detecting pathology, compared to conventional chest radiography.9–16 As opposed to computed tomography (CT), the radiation dose from a chest tomosynthesis examination is comparable to that from a conventional 101905-1

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chest radiography examination and—depending on the system used—effective doses in the order of 0.05–0.2 mSv have been reported.9,12,14,17–20 Additionally, as the financial cost of a chest tomosynthesis examination usually is much lower than of a corresponding CT examination15 and the patient throughput is higher,21,22 it may be beneficial for healthcare if chest tomosynthesis could be used for certain tasks for which CT is used today. Taking into account recent evaluations showing the clinical potential of chest tomosynthesis,23–26 it can therefore be anticipated that the frequency of the examination will increase in the near future. In order to simplify the estimation of the radiation dose from a chest tomosynthesis examination, Båth et al.18 proposed a conversion factor of 0.26 mSv Gy−1 cm−2 between the total dose-area product (DAP) of the tomosynthesis

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Båth, Söderman, and Svalkvist: Retrospectively estimating patient dose in chest tomosynthesis

examination and the effective dose. The conversion factor was determined for the standard-sized patient27 (∼70 kg) using Monte Carlo simulations of VolumeRAD (GE Healthcare, Chalfont St. Giles, UK), the system presently dominating scientific and clinical evaluations of chest tomosynthesis. If a more patient-specific estimation is desired, Svalkvist et al.28 presented similar conversion factors that also take the patient size into account. More recently, Vult von Steyern et al.29 also calculated conversion factors that can be used for pediatric chest tomosynthesis using this system, and Zhang et al.30 performed a thorough investigation of the effect of the patient size on the radiation dose for conventional chest radiography, chest tomosynthesis, and chest CT. A complicating factor for estimating the radiation dose to a patient undergoing a tomosynthesis examination using the VolumeRAD system is that although the total DAP of the examination is presented at the work station, no dose data are stored in the reconstructed section images in the picture archiving and communication system (PACS). Although the DAP for each projection radiograph acquired during the tomosynthesis examination is stored in the digital imaging and communications in medicine (DICOM) header (as with conventional radiographs), these raw data projection radiographs are normally not stored in the PACS. If no dose surveillance software is used to store the dose data and if the data provided by modality performed procedure step (MPPS) cannot be stored, it is therefore not possible to directly retrieve the dose data in retrospect. However, the exposure parameters used for the acquisition of the projection radiographs of the tomosynthesis examination are determined by an initial scout image using a known algorithm for VolumeRAD.17,31 This scout image, an ordinary chest radiograph, is normally stored in the PACS. Using knowledge about how the exposure parameters for the tomosynthesis examination are determined by the scout, in combination with a correction for the field size changes with projection angle that occur during the acquisition, it should therefore be possible to determine the DAP of the tomosynthesis examination from the registered DAP of the scout image. Therefore, the purpose of the present work was to develop and validate a method of retrospectively estimating the DAP of a chest tomosynthesis examination performed using the VolumeRAD system from DICOM data available in the PACS for the scout image. 2. MATERIAL AND METHODS 2.A. Data acquisition and establishment of DAP reference value

The image data for the present work consisted of 40 clinical chest tomosynthesis examinations that were acquired at two xray labs (20 examinations at each lab) with equipment for chest tomosynthesis (GE Definium 8000 with VolumeRAD option, GE Healthcare, Chalfont St. Giles, UK). The mass, height, and age of the patients (17 male and 23 female) were 70 ± 6 kg, 171 ± 10 cm, and 61 ± 16 yr, respectively (mean ± 1 SD). During a chest tomosynthesis examination using VolumeRAD, the detector position is fixed, whereas the x-ray tube performs Medical Physics, Vol. 41, No. 10, October 2014

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a vertical, continuous sweeping movement from −17.5◦ to +17.5◦ relative to the standard horizontal projection. During this sweep, 60 low-dose projection radiographs of the patient are acquired in the angular interval of −15◦ to +15◦ during a time period of approximately 10 s. The focus-detector distance (FDD) was 180 cm in the horizontal direction and a tube voltage of 120 kV and a filtration of 3 mm Al + 0.1 mm Cu were used for all patients, following clinical routine. For the VolumeRAD system, the exposure parameters for the tomosynthesis examination are determined by an initial scout image (a conventional chest radiograph) of the patient, acquired using automatic exposure control (AEC).17,31 The AEC-determined tube load for the scout is multiplied by a dose ratio, which in the present work was 10:1 (the default setting), and equally distributed over the 60 angular projections. The resulting tube load for each projection is then rounded down to the closest milliampere second setting possible, using the constraints of the system of a minimum tube load of 0.25 mA s per projection and tube load steps distributed according to an R10 Renard series32 (i.e., 0.25, 0.32, 0.40, etc.). The acquired projection radiographs are used to reconstruct tomosynthesis section images, which are stored in the PACS along with the scout image. However, in the present work, the originally acquired low-dose projection radiographs were additionally retrieved from the VolumeRAD system for analysis, and the registered DAP for each of these projection radiographs was obtained from the DICOM header. By summing the DAP from all low-dose projection radiographs, the actual total DAP for each examination was determined. The summed DAP of all projections was used as the reference value against which the total DAP estimated using the developed method was tested. 2.B. Estimation of the total tube load and determination of the field size correction factor

The method described in the present work is based on retrospectively estimating the DAP of the tomosynthesis examination from the exposure data registered for the scout image. The method uses the above-described knowledge about how the tube load for the tomosynthesis examination is determined from the AEC-determined tube load used for the scout in combination with a field size correction factor. A data set consisting of the 20 examinations from one of the two labs were used for determining the field size correction factor. For these examinations, the tube load registered for the scout was multiplied by 10 (the dose ratio used) and divided by 60 to obtain the desired tube load for each tomosynthesis projection radiograph. From this desired tube load, an estimation of the tube load actually used was obtained by rounding down the desired tube load to the closest Renard step, with a minimum tube load constraint on the generator of 0.25 mA s. This estimated tube load was then multiplied by 60 to obtain an estimation of the total tube load for the tomosynthesis examination. As a first estimate of the total DAP for the tomosynthesis examination, the estimated total tube load was multiplied by the ratio between the registered DAP and the registered tube load for the scout. A linear relationship between the

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estimated total DAP and the actual total DAP (the reference value described above) was then assumed, and linear regression was used to determine a field size correction factor accounting for the change in field size and distance with projection angle. 2.C. Validation of the method

The developed method of estimating the total DAP for the tomosynthesis examination from the DAP for the scout was validated by its application to the data set consisting of the 20 additional tomosynthesis examinations acquired on the second lab. For each of these examinations, the DAP and tube load for the scout were extracted from the DICOM header. These data were used to determine the first estimate of the total DAP of the tomosynthesis examination as described above. Finally, the previously determined field size correction factor was applied to obtain the final estimated total DAP. This final estimated total DAP was then compared with the reference value (obtained by adding the registered DAP values of all 60 projection radiographs of each examination) and the relative error was computed. 3. RESULTS In Fig. 1, the total registered DAP for the tomosynthesis examinations in the first data set is plotted against the registered DAP for the scout. There was a limited correlation between the data, due to the rounding procedure used by the system to determine the tube load for the tomosynthesis projections from the tube load of the scout image. However, by taking this known procedure into account, a strong correlation between predicted and actual DAP for the tomosynthesis examination was obtained. In Fig. 2, the actual total DAP is plotted against the DAP predicted by (1) multiplying

F. 2. Plot of the total registered DAP for a tomosynthesis examination against the DAP estimated from the scout by using the knowledge about how the exposure parameters of the tomosynthesis examination are determined by the scout. From the linear relationship, a field size correction factor of 0.935 was determined.

the scout tube load by the dose ratio (for this system 10:1) and dividing by 60, (2) rounding the result to the closest Renard step (using the lower tube load limit of 0.25 mA s), (3) multiplying this rounded tube load by 60, and (4) multiplying the result by the ratio between the registered DAP and tube load for the scout. An R2 > 0.99 was obtained, even after forcing the linear regression through the origin. From the data in Fig. 2, a field size correction factor of 0.935 was determined. Figure 3 is a plot of the results of applying the developed method on the second data set, consisting of tomosynthesis examinations performed at a different lab. As can be seen, the error in estimating the total DAP of the tomosynthesis examination from the DAP of the scout using the proposed method was minor. The average difference between the estimated DAP and the actual DAP was 0.2%, with a standard deviation of 0.8%. However, the difference was not normally distributed and the maximum error was only 1.0%. The validity and reliability of the presented method were thus very high. In Fig. 4, a flow chart of the developed method is presented. The data provided are examples for one specific patient. In the example, an estimated total DAP from the scout image of 0.487 Gy cm2 was obtained, to be compared with the actual total DAP of 0.491 Gy cm2 obtained by adding the registered DAP values of all 60 projection radiographs. 4. DISCUSSION

F. 1. Plot of the total registered DAP for a tomosynthesis examination against the registered DAP for the scout image for 20 chest tomosynthesis examinations performed using VolumeRAD. Medical Physics, Vol. 41, No. 10, October 2014

In the present work, a method of estimating the total DAP for a chest tomosynthesis examination using the GE VolumeRAD system from DICOM data available in the corresponding scout image has been presented and validated. The method was shown to be able to predict the actual DAP

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F. 3. Plot of the total registered DAP for 20 chest tomosynthesis examinations against the total DAP estimated from the scout image using the method described in the present paper with the field size correction factor 0.935.

with very high accuracy. As the actual DAP used for the tomosynthesis examination is not stored in the reconstructed section images in the PACS, the method may be of value for retrospectively estimating patient dose. As can be seen in Fig. 1, the variation in DAP between patients was much smaller for the tomosynthesis examination than for the scout image. This is mainly due to the limitation of a minimum tube load of 0.25 mA s per exposure. For a majority of the cases in the present work, this limit determined the tube load for the tomosynthesis examination. As this affects only the lower end of the tube load distribution, the

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result is a narrower DAP distribution for the tomosynthesis examinations than for the scout images. This effect has been seen previously33 and may partly explain the small variation in reported dose values for adult tomosynthesis examinations using the VolumeRAD system at the standard dose setting.9,16–18,22 The method presented here is based on the assumption of a constant field size correction factor. This assumption may have a limited validity, as the collimation is restricted by the size of the detector. Thus, it is conceivable that the change in field size (and thus, the value of the field size correction factor) may show a dependency on the patient size. Similarly, the effect of this restriction may be dependent on the centering of the field on the scout image. However, the high correlation coefficient in Fig. 3 indicates that the effect of this potential limitation is small and that the reported field size correction factor of 0.935 can be used with high validity. A second limitation of the method is that it is based on the assumption that the scout image stored in the PACS is the scout image actually used to determine the exposure conditions of the tomosynthesis acquisition. However, unless retakes of the scout are common at a specific lab (and the rejected scout image is nevertheless stored in the PACS), the effects of the uncertainty should be small. (In the present work, it was known that all stored scout images were used for the tomosynthesis examinations. This is reflected by the strong linearity in Figs. 2 and 3.) Additionally, since a retake may lead to both higher and lower exposure values, it should not introduce any bias. Thus, for a group of patients, the effects of the uncertainty should be negligible. The present work was based on chest tomosynthesis examinations performed using VolumeRAD with the default

F. 4. A flow chart describing the developed method of estimating the total DAP of a chest tomosynthesis examination using VolumeRAD from DICOM data in the scout image. The data provided are examples for one specific patient for which the actual total DAP was 4.91 dGy cm2. The given field size correction factor has been determined for the default setting of 60 acquired projections over an angular range of ±15◦ at a FDD of 180 cm. Note that the units used in the DICOM header (µA s and dGy cm2) are also used in the figure. Medical Physics, Vol. 41, No. 10, October 2014

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settings (60 projection radiographs acquired over an angular range of ±15◦ at a FDD of 180 cm and using a dose ratio of 10:1). However, as the exposure parameters for the tomosynthesis examination are determined from an initial scout in a similar way for all VolumeRAD examinations, it should be possible to use the developed method to estimate the total DAP also for other tomosynthesis examinations, after adjustment of the parameters used by the method (the number of projection radiographs and the dose ratio, see Fig. 4). Also, the field size correction factor should be determined for each specific protocol, as it can be expected to depend on, e.g., both angular range and FDD. Finally, it should be noted that most conversion factors between total DAP and effective dose that have been reported have taken the variation in DAP with projection angle into account. For example, the chest tomosynthesis conversion factor of 0.26 mSv Gy−1 cm−2 proposed by Båth et al.18 for a standard-sized patient examined using the VolumeRAD system is based on the total DAP of the examination. Thus, the method presented in the present work can be used together with previously published conversion factors to estimate the effective dose of the examination.

5. CONCLUSIONS A method of estimating the DAP of a chest tomosynthesis examination performed using the VolumeRAD system from DICOM data available in the PACS for the scout image was developed and validated. As the scout image normally is the only image connected to the tomosynthesis examination stored in the PACS containing dose data, the method may be of value for retrospectively estimating patient dose in clinical use of chest tomosynthesis.

ACKNOWLEDGMENTS The authors are grateful to Gerhard Brunst for valuable input. This work was supported by grants from the Swedish Research Council (2011/488, 2013/3477), the Swedish Radiation Safety Authority (2008/2232, 2009/1689, 2010/4363, 2012/2021, 2013/2982, 2014/2641), the Swedish Federal Government under the LUA/ALF agreement (ALFGBG136281), and the Health & Medical Care Committee of the Region Västra Götaland (VGFOUREG-12046, VGFOUREG27551, VGFOUREG-81341). The authors report no conflicts of interest.

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