Optimizing radiation exposure for CT localizer radiographs

ZEMEDI-10687; No. of Pages 14

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ORIGINALARBEIT

Optimizing radiation exposure for CT localizer radiographs Evelyn Bohrer 1,∗ , Stefan Schäfer 2 , Ulf Mäder 1 , Peter B. Noël 3 , Gabriele A. Krombach 2 , Martin Fiebich 1 1

Institute of Medical Physics and Radiation Protection-IMPS, University of Applied Sciences, Gießen, Germany Department of Radiology, Justus-Liebig University, University Hospital Gießen, Gießen, Germany 3 Department of Diagnostic and Interventional Radiology, Technische Universität München, Germany 2

Received 3 March 2016; accepted 16 September 2016

Abstract Introduction: The trend towards submillisievert CT scans leads to a higher dose fraction of localizer radiographs in CT examinations. The already existing technical capabilities make dose optimization of localizer radiographs worthwhile. Modern CT scanners apply automatic exposure control (AEC) based on attenuation data in such a localizer. Therefore not only this aspect but also the detectability of anatomical landmarks in the localizer for the desired CT scan range adjustment needs to be considered. Materials and methods: The effective dose of a head, chest, and abdomen-pelvis localizer radiograph with standard factory settings and user-optimized settings was determined using Monte Carlo simulations. CT examinations of an anthropomorphic phantom were performed using multiple sets of acquisition parameters for the localizer radiograph and the AEC for the subsequent helical CT scan. Anatomical landmarks were defined to assess the image quality of the localizer. CTDIvol and effective mAs per slice of the helical CT scan were recorded to examine the impact of localizer settings on a helical CT scan. Results: The dose of the localizer radiograph could be decreased by more than 90% while the image quality remained sufficient when selecting the lowest available settings (80 kVp, 20 mA, pa tube position). The tube position during localizer acquisition had a greater impact on the AEC than the reduction of tube voltage and tube current. Except for the use of a pa tube position, all changes

Optimierung der Strahlenbelastung von CT-Übersichtsaufnahmen Zusammenfassung Einleitung: Der Trend zu dosisoptimierten CT-Scans bis in den Submillisievert-Bereich führt zu einem höheren relativen Dosisanteil der Übersichtsaufnahmen bei CT-Untersuchungen. Dies macht eine Dosisoptimierung der CT-Übersichtsaufnahmen erstrebenswert. Moderne CT-Geräte verwenden eine Belichtungsautomatik mit Röhrenstrommodulation (AEC) die auf den Schwächungswerten der Übersichtsaufnahme basiert. Daher muss dieser Aspekt ebenso berücksichtigt werden, wie die Erkennbarkeit von anatomischen Landmarken in der Übersichtsaufnahme. Material und Methoden: Die effektive Dosis einer Kopf-, Thorax- und Abdomen-Becken-Übersichtsaufnahme wurde bei Standardeinstellungen und anwenderbezogenen Einstellungen mit einer Monte-Carlo-Software berechnet. An einem anthropomorphen Phantom wurden CT-Untersuchungen durchgeführt. Dabei wurden verschiedene Aufnahmeparameter für die Übersichtsaufnahme und die Belichtungsautomatik für den Spiral-CT-Scan verwendet. Um die Bildqualität der Übersichtsaufnahme zu beurteilen, wurden anatomische Landmarken definiert. CTDIvol und effektive mAs pro Schicht wurden aufgenommen um die Auswirkung der optimierten Übersichtsaufnahmen auf den Spiral-CT-Scan zu untersuchen.

∗ Corresponding author: Evelyn Bohrer, Institute of Medical Physics and Radiation Protection-IMPS, University of Applied Sciences, Gießen, Wiesenstr. 14, 35390 Gießen, Germany. E-mail: [email protected] (E. Bohrer).

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of acquisition parameters for the localizer resulted in a decreased total radiation exposure. Conclusion: A dose reduction of CT localizer radiograph is necessary and possible. In the examined CT system there was no negative impact on the modulated helical CT scan when the lowest tube voltage and tube current were used for the localizer.

Keywords: Localizer radiograph, Tube current modulation, Automatic exposure control, Monte Carlo simulation, Computed tomography, Dose reduction

1 Introduction The scientific community is aware that patient exposure in computed tomography can reach high dose levels and makes a substantial contribution to the collective effective dose [1,2]. Up to now great effort has been made to minimize the dose of CT scans. Available techniques such as automatic exposure control (AEC) could decrease the dose up to 60% [3–6]. Also iterative reconstruction decreased the dose up to 74% [7–9] and z-axis collimation up to 55% [10,11]. Tube potential selection and beam shaping filters are further dose reduction techniques, each with a reduction potential up to 50% [12,13]. Taking all these techniques into account, the dose of a single-phase abdomen/pelvis CT scan could be minimized up to 70% to 2.8 mSv [14], for example. Further new technical advances show the potential to achieve submillisievert CT scanning for routine CT examinations [14]. In optimized CT scanning, doses below 1 mSv are already achievable [15–19]. Frequently in reports and studies about dose and dosereduction capabilities in computed tomography the localizer dose is disregarded [17,20–23]. However, Schmidt et al. showed that the effective dose of localizer radiographs reaches values up to 0.12 mSv (head), 0.39 mSv (chest), and 0.42 mSv (abdomen-pelvis) [24]. Compared to typical doses for routine head (2.3 mSv), chest (8 mSv), and abdomen-pelvis CT scans (10 mSv), the localizer dose is about 4–5% of the related CT scan dose [25]. Considering lung cancer screening protocols with an effective dose lower than 1 mSv [26], the localizer dose could be

Ergebnisse: Die effektive Dosis der CT-Übersichtsaufnahme konnte mit ausreichender Bildqualität um mehr als 90% reduziert werden, wenn die niedrigsten wählbaren Einstellungen (80 kV, 20 mA, pa-Röhrenposition) verwendet wurden. Die Röhrenposition in der Übersichtsaufnahme zeigte eine größere Auswirkung auf die Röhrenstrommodulation des SpiralCT-Scans, als die Reduzierung der Röhrenspannung und des Röhrenstroms. Außer bei der Verwendung einer pa-Röhrenposition führten alle veränderten Aufnahmeparameter der Übersichtsaufnahme zu einer Verringerung der Dosis des Spiral-CT-Scans. Schlussfolgerung: Eine Dosisreduzierung bei der Übersichtsaufnahme ist notwendig und möglich. Bei dem untersuchten CT-System gab es keine negative Auswirkung auf den röhrenstrommodulierten Spiral-CT-Scan, wenn geringste Röhrenspannung und geringster Röhrenstrom für die Übersichtsaufnahme verwendet wurden. Schlüsselwörter: Übersichtsaufnahme, Röhrenstrommodulation, Belichtungsautomatik, Monte Carlo Simulation, Computertomographie, Dosisreduzierung

more than 40% of the CT scan dose. Furthermore, optimized axial/helical CT scans with a short scan range such as cardiac imaging with a dose lower than 0.1 mSv [17] the localizer dose is higher than the axial/helical CT scan dose. This is due mainly to a longer scan length of the localizer radiograph. As vertical centering becomes more widely recognized as being critical for the AEC function to work properly (by avoiding magnification and reduction effects) [27,28], we can expect more repeated localizers to be obtained. This also adds to the justification of understanding the dosimetry effects of localizers. Former studies about the localizer radiograph recommend a lower tube current, tube voltage, and a posterior-anterior (pa) tube position for the localizer radiograph, especially for children due to their smaller body size and higher radiosensitivity [24,29–31]. Incident air kerma on central beam [29], effective dose [24,30,31], and image quality [31] of the localizer radiograph have been examined in former studies. However, to the best of our knowledge a complete investigation of the effective dose and optimization capabilities of a modern CT scanner with the tube current modulation technique has not been done. CT manufacturers use automatic exposure control (AEC) techniques that are essentially based on the attenuation recorded in the localizer radiograph. Therefore, the impact of the localizer acquisition parameters on the AEC must be considered before implementing radical localizer dose reductions. These findings encourage the investigation of doseoptimization capabilities for the CT localizer radiograph. In this study, the effective dose of localizer radiographs for head,

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chest, and abdomen-pelvis examinations was estimated with Monte Carlo simulations for standard localizer scan protocols provided by the manufacturer and for optimized localizer scan protocols. The detectability of anatomical landmarks in localizers for CT scan range adjustment was assessed as well as the influence of the optimized localizer radiograph on the AEC to determine the usability of optimized localizers.

2 Materials and methods To determine the effective dose of standard protocol localizers (factory settings provided by the manufacturer) and the dose-saving capabilities of optimized localizer protocols, measurements on a CT scanner and Monte Carlo simulations were performed. For optimizing the effective dose of localizer radiographs, different tube voltages, tube currents, and tube positions were investigated. As the CT scanner uses automatic exposure control software based on localizer image information, it has to be verified that the optimization does not affect the subsequent helical CT scan (for example a deterioration of the image quality). For this reason, CT examinations of an anthropomorphic phantom were performed with optimized parameters for the localizer radiograph to assess the detectability of anatomical landmarks typically used for CT scan range adjustment. Additionally, the impact of optimized localizer radiographs on the tube current modulation of the helical CT scan was examined. 2.1 CT scanner All measurements were performed on a SOMATOM Definition AS CT scanner (Siemens Healthcare, Forchheim, Germany). The automatic exposure control software “CARE Dose 4D” was implemented for tube current modulation of CT scans. The detector of the CT scanner was a solid state array composed of Ultra Fast Cereamic (UFC). The calculated modulation is partly based on the attenuation values from the localizer radiograph along the z-axis. The “CARE Dose 4D” software also offers the possibility for automatic tube-voltage optimization based on patient-size information (CARE kV on/off mode). For localizer the lowest available acquisition parameters are 80 kVp and 20 mA. The CT scanner offers an anteriorposterior (ap), posterior-anterior (pa) or lateral (lat) tube position, and a fixed scan length of 128 mm, 256 mm, 512 mm,

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768 mm, 1024 mm, 1536 mm, and 2000 mm for the localizer. Its beam collimation is fixed to 6 x 0.6 mm and the “FLAT” bowtie filter is used. 2.2 Localizer scan protocol Localizer radiographs were performed with factory settings and with lowered settings for head, chest, and abdomen-pelvis examination to investigate possible dose reduction. The tube voltage and tube current were therefore decreased towards the lowest selectable settings. The tube position was varied for each tube voltage and tube current combination. The used scan protocols are summarized in Table 1. The used scan length were the shortest selectable scan length that enabled the scanrange adjustment for the helical CT scans. These scan length were also recommended by the manufacturer. 2.3 Effective dose of localizer radiograph 2.3.1 Monte Carlo framework and models To calculate the effective dose for the localizer radiograph the software “GMctdospp” [32] was used, which provides a graphical user interface (GUI) for the Monte Carlo user code EGSnrc [33]. This software was designed to simulate CT examinations and enables a calibration of simulated doses in Gray per particle to absolute doses in Gray [32,34]. A source model of the Siemens Somatom Definition AS CT scanner is implemented in GMctdospp and validated for 80 kVp and 120 kVp [32]. The anthropomorphic ICRP voxel phantoms adult male (am) and adult female (af) [35] with organ masses based on ICRP publication 89 [36] were used for dose calculations. The height and weight was 176 cm and 73.0 kg for the male phantom and 163 cm and 60.0 kg for the female phantom, respectively. In these phantoms the arms were removed to follow clinical practice guidelines [37,38]. The CT scanner table affects patient dose in localizer radiographs, therefore it was included in the Monte Carlo setup. It was created from two arcs of different radii in cross-section [39]. The material carbon was chosen for the table surface and polyurethane for its filling. The material densities were adjusted to obtain the attenuation of a real CT scanner table. Simulations were done with 108 particles to reach a statistical uncertainty lower than 2% for the dose in the directly

Table 1 Scan protocols of the localizer radiograph for different anatomical scan regions. Values printed in bold represent the factory settings while the other values represent the optimization capabilities. For each scan region scan length and scan time were fixed. Scan region

Tube voltage (kVp)

Tube current (mA)

Tube position

Scan length (mm)

Scan time (s)

Head Chest Abdomen-pelvis

120, 100, 80 120, 100, 80 120, 100, 80

35 35, 20 35, 20

ap, pa, lat ap, pa, lat ap, pa, lat

256 512 512

2.7 5.3 5.3

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irradiated organs. Since the dose contribution of the indirectly irradiated organs will be very low the uncertainty of the effective dose will also be lower than 2%. 2.3.2 MC simulation protocols Localizer radiographs of head, chest, and abdomen/pelvis were simulated using factory and optimized settings for localizers (see Table 1) to calculate the effective dose and determine dose-saving capabilities. Since some organs in the body had no symmetric alignment, lateral localizer radiographs in the right-lateral position were simulated in addition to the leftlateral localizer for chest and abdomen-pelvis examinations. 2.3.3 Dose calibration of MC simulations and calculation of effective dose Calibration measurements on the CT scanner were done to achieve absolute organ doses in mSv. For each tube voltage (120 kVp, 100 kVp, 80 kVp) a tube current of 100 mA was used to measure kerma in air at the isocenter of the gantry during a head localizer with a CTDI chamber (RaySafe Xi, RaySafe, Billdal, Sweden). The CTDI chamber was therefore fixed on a tripod which was placed on the floor behind the CT scanner and positioned in the isocenter. The head and neck support of the scanner table was removed and a scan length of 128 mm (scan time 1.4 s) was selected to make sure that the scanner table was outside the measurement field during the localizer. The measurements were repeated three times and the mean value was used for calibration. For calibration a MC simulation was performed without the scanner table and a dose region at isocenter that described the CTDI chamber to match scan parameters as the measurements. After the calibration of the simulated organ doses, the effective dose was estimated by using weighting tissue factors from ICRP publication 103 [40]. For each tube voltage and tube position of the localizer the effective dose was calculated for one tube-current value. Then the effective dose was interpolated along different tubecurrent values as there is a linear relationship between dose and tube current. The interpolation was interrupted at 20 mA because this is the lowest tube current that can be selected on this CT scanner. 2.3.4 Influence of optimized localizer settings on the detectability of anatomical landmarks and on automatic exposure control The male anthropomorphic Alderson Radiation Therapy (ART) phantom (Radiology Support Devices, Inc., California, USA) was used to assess the impact of the optimized localizer settings on the detectability of anatomical landmarks used for scan range adjustment and on tube current modulation. With the ART phantom various CT examinations could be performed without a change of its position on the scanner table. This makes the different CT examinations comparable.

Specific anatomical landmarks (Table 2) which are existent in the ART phantom and widely used in CT scan planning were selected from CT guidelines [41–43]. The detectability assessment for these landmarks was performed in a qualitative way by non-radiologists (two physicists). Furthermore, a real clinical case with two CT examinations (initial CT study and follow-up after four months) was evaluated. An abdomen multiphase scan protocol with AEC was used. The maximal cross sectional lateral and ap- dimensions of the patient was about 395 mm and 333 mm in the initial CT study and about 390 mm and 325 mm in the follow-up study, respectively. The localizer settings of the follow-up study were optimized to 80 kVp, 20 mA, and pa tube position. Both helical CT scans (initial and follow-up) used 100 kVp, 220 quality reference mAs with a total collimation of 19.2 mm and a pitch of 0.6. Additionally, the impact of optimized localizer parameters on tube current modulation was investigated since its calculation is based on localizer attenuation information. The ART phantom was centered in the field of view by using its surface markers for centering. After each abdomen-pelvis localizer, done with varied localizer settings (Table 1), a subsequent helical “Abdomen Routine” CT scan was performed. The settings of the helical CT scan with AEC remained unchanged. Formerly done acquisitions showed that in CT examinations with more than one localizer the previously acquired localizer could affect the tube current modulation of the CT scan, too. Therefore for each localizer variation a new patient examination protocol was utilized. Furthermore, the CT examinations were all done in CARE kV on and in CARE kV off mode to examine how tube current modulation and automatic tube potential selection were affected by the localizer. In the CARE kV on mode the slider settings was 7 in all scans as given by the used Abdomen Routine scan protocol. The CT scanner reported mean CTDIvol and mean effective mAs (eff. mAs) of each helical CT scan were used as evaluation parameters for patient exposure. The effective mAs per slice obtained from the DICOM data of the helical CT scans were used to analyze the influences on the AEC. The CT examination with factory settings and with 80 kVp, 20 mA, and ap tube position for the localizer were repeated three times and the standard deviation was calculated to determine the system’s influence on the variability of the evaluation parameters (eff. mAs, CTDIvol , mAs per slice). This information was used

Table 2 Selected anatomical landmarks for different scan regions, used to assess image quality of localizer radiograph. Scan region

Anatomical landmarks

Chest

Lung apices, posterior costophrenic sulci, heart Dome of diaphragm, sacroiliac joint, iliac crest, ischial tuberosity

Abdomen-pelvis

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to assess whether potential distinctions between the helical CT scans were caused by varying localizer settings.

3 Results 3.1 Effective dose for different localizer settings The calculated effective dose of the localizer radiograph for the ICRP adult male and ICRP adult female voxel phantom and different body regions are shown in Figs. 1–3. Due to the linear relationship between dose and tube current, the calculated doses for 35 mA (dotted line) were interpolated down to the minimum tube current of 20 mA that can be selected on the CT scanner used. The calculated effective dose for CT localizer radiographs of the ICRP adult male and adult female voxel phantom is shown for the body regions head (Fig. 1), chest (Fig. 2), and abdomen-pelvis (Fig. 3) for varied acquisition parameters. A reduction of the tube current from factory settings (35 mA) to the lowest selectable tube current (20 mA) of the used CT scanner decreased the dose for the head localizer (Fig. 1) by about 43% from 0.027 mSv to 0.015 mSv (male) and from 0.045 mSv to 0.026 mSv (female). A tube voltage of 80 kVp instead of 120 kVp decreased the dose by about 72% to 0.007 mSv (male) and 71% to 0.013 mSv. The localizer radiograph with lateral or pa- tube position had a lower dose than a localizer radiograph in the ap tube position. A painstead of an ap tube position caused a dose reduction of 38% to 0.016 mSv (male) and 44% to 0.026 mSv (female). Chest localizer radiographs (Fig. 2) in the left-lateral tube position showed higher doses than localizers in the

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right-lateral tube position. The dose for the right-lateral localizer was slightly higher than for the pa localizer. Compared to the localizer with 120 kVp, 35 mA, and ap tube position, a change of tube position decreased the dose by up to 56% from 0.227 mSv to 0.099 mSv (male) and up to 49% from 0.255 mSv to 0.130 mSv (female). Decreasing just the tube current to 20 mA resulted in an effective dose of 0.130 mSv (male) and 0.145 mSv (female), while decreasing just tube voltage to 80 kVp resulted in an effective dose of 0.061 mSv (male) and 0.070 mSv (female). Here, the relative dose reduction is similar to the results of the head localizer radiograph. The different scan parameter settings for the abdomenpelvis localizer radiograph (Fig. 3) showed a similar relative dose reduction as for the chest localizer radiograph. Unlike the chest localizer (Fig. 2) the dose of the female phantom for the right-lateral localizer was slightly lower, but that of the male phantom slightly higher than for the pa localizer. The effective dose of the abdomen-pelvis localizer (Fig. 3) with factory settings was 0.176 mSv (male) and 0.251 mSv (female). For the male voxel phantom the dose decreased by up to 0.088 mSv (pa-tube position), 0.100 mSv (20 mA), and 0.046 mSv (80 kVp) and for the female voxel phantom by up to 0.140 mSv (pa-tube position), 0.143 mSv (20 mA), and 0.068 mSv (80 kVp). Table 3 summarizes the effective dose for all body regions for factory (120 kVp, 35 mA, ap tube position) and optimized (80 kVp, 20 mA, pa tube position) localizer settings and the achievable relative dose reduction. In all localizer radiographs the female voxel phantom showed higher dose values than the male voxel phantom.

Fig. 1. Effective dose of the male (am) and female (af) head localizer radiograph for different scan parameters (kVp, mA, and tube position). The point where the dotted vertical line hits the dotted horizontal line represents the factory settings (120 kVp and 35 mA). The interpolated lines are truncated at the lowest tube current (20 mA) the CT scanner enables to select.

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Fig. 2. Effective dose of the male (am) and female (af) chest localizer radiograph for different scan parameters (kVp, mA, and tube position). The point where the dotted vertical line hits the dotted horizontal line represents the factory settings (120 kVp and 35 mA). The interpolated lines are truncated at the lowest tube current (20 mA) the CT scanner enables to select.

3.2 Detectability of anatomical landmarks in optimized localizer radiograph Images of localizer radiographs from the ART phantom are shown in Figs. 4 and 6 and from a real patient in Fig. 5.

In the abdomen-pelvis localizer radiograph in the ap tube position all osseous structures could be identified when 20 mA and 80 kVp were used instead of 35 mA and 120 kVp (Fig. 4e). An enlarged region of interest (ROI) of the pubic symphysis is shown in Fig. 4c and d. This ROI is marked in the localizer

Table 3 Effective dose of the localizer radiograph for different scan region once with factory setting (120 kVp, 35 mA, ap tube position) and once with lowest selectable settings for the used CT scanner (80 kVp, 20 mA, pa tube position). The values represent the mean value of three Monte Carlo simulations with varying random numbers. The maximal standard deviation was 0.001 mSv. Effective dose in mSv am

af

Scan region

Factory setting

Lowest setting

Head Chest Abdomen-pelvis

0.027 0.227 0.176

0.002 0.013 0.012

Dose reduction 91% 94% 93%

Factory setting

Lowest setting

0.045 0.255 0.251

0.004 0.018 0.020

Dose reduction 92% 93% 92%

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Fig. 3. Effective dose of the male (am) and female (af) abdomen-pelvis localizer radiograph for different scan parameters (kVp, mA, and tube position). The point where the dotted vertical line hits the dotted horizontal line represents factory settings (120 kVp and 35 mA). The interpolated lines are truncated at the lowest tube current (20 mA) the CT scanner enables to select.

radiograph as a yellow rectangle (Fig. 4a,b). There is more noise visible in the optimized localizer (Fig. 4d) than in the localizer with factory settings (Fig. 4c); the pubic symphysis could, however, be defined in both localizer radiographs. The dome of the diaphragm was not clearly perceptible in the ART phantom when factory settings were used. However, this structure was clearly perceptible in the localizer radiograph of a real patient (Fig. 5) when factory settings or optimized acquisition parameters were used. This patient underwent a CT examination of the abdomen with a localizer radiograph performed with factory settings (Fig. 5a) and four months later with an optimized localizer radiograph (Fig. 5b). Although this patient had a higher cross sectional ap-dimension (initial study: 333 mm, follow-up study: 325 mm) than the ART phantom (∼245 mm), which matched the dimensions of a standard patient, the anatomical landmarks could be identified in the localizer performed with optimized acquisition parameters. In the chest localizer radiograph of the ART phantom with pa tube position (Fig. 6a–c), the lung apices, the heart, and the osseous structures were detectable in localizers performed with 120 kVp and 35 mA, 100 kVp and 20 mA, and 80 kVp and 20 mA. In all three localizers the costophrenic sulci was not clearly perceptible. However, it was clearly perceptible in the localizers in the lateral tube position (Fig. 6d–f). In the lateral chest localizer the osseous structures in the shoulder region did not allow a clear detection of the lung apices when

80 kVp and 20 mA were used (Fig. 6 f). The use of 100 kVp and 20 mA enabled the distinction of the lung apices in the lateral localizer of the ART phantom (Fig. 6e). 3.3 Influence of localizer settings to tube current modulation The exposure values (eff. mAs, CTDIvol , DLP) for the various helical CT scans based on localizer radiographs done with different acquisition parameters are listed in Tables 4 and 5. For the helical CT scans in Table 4, tube current modulation with CARE kV off was used and in Table 5 tube current modulation with CARE kV on. In the CARE kV off mode the helical CT scan was performed with 120 kVp and in the CARE kV on mode the software selected 100 kVp for the helical CT scan. A change from ap to lateral tube position for the localizer radiograph decreases the scanner-reported CTDIvol of the helical CT scan by about 9% (CARE kV off) and 10% (CARE kV on), respectively. The use of a pa tube position increases the CTDIvol of the helical CT scan by about 9% (CARE kV off) and 10% (CARE kV on) compared to an ap tube position of the localizer radiograph. A change of tube voltage and tube current from the factory settings to 80 kVp and 20 mA decreases the CTDIvol of the helical CT scan by about 1% for ap and 3% for the lateral and pa tube position in the CARE kV off scan mode and about 4% for pa and 2% for the lateral and ap tube position.

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Fig. 4. Abdomen-pelvis localizer radiograph of the ART phantom in the ap tube position with 120 kVp and 35 mA (a) and 80 kVp and 20 mA (b). The yellow rectangle represents a region of interest (ROI) that is pictured enlarged for 120 kVp and 35 mA (c) and for 80 kVp and 20 mA (d).

Fig. 5. Abdomen-pelvis localizer radiograph of a real patient (localizer of initial CT study and follow-up after four months): (a) factory settings of 120 kVp, 35 mA, and ap tube position and (b) optimized settings of 80 kVp, 20 mA, and pa tube position The effective dose for the male ICRP voxel phantom with a lower attenuation (maximal cross sectional ap- dimension in the scan field: 125 mm) than the real patient was 0.176 mSv for factory settings and 0.012 mSv for the optimized settings.

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Fig. 6. Chest localizer radiograph of the ART phantom in the ap tube position with (a) 120 kVp and 35 mA, (b) 100 kVp and 20 mA, and (c) 80 kVp and 20 mA and in the lateral tube position with (d) 120 kVp and 35 mA, (e) 100 kVp and 20 mA, and (f) 80 kVp and 20 mA.

Fig. 7 shows the mAs per slice of the helical CT scan (CARE kV off) for different localizer settings. The corresponding anatomical features and the slice positions are shown in Fig. 7a for orientation. The modulation curves of the helical CT scans based on ap localizers with different kVp and mA settings (Fig. 7b) were in good agreement (correlation coefficient between all curves: r = 0.99). The maximum standard deviation per slice of three repeated helical CT scans using a

localizer with 120 kVp and 35 mA was ± 5 mAs and using a localizer with 80 kVp and 20 mA was ± 7 mAs. Comparing the curves based on pa localizer radiographs with varied tube voltage and tube current (Fig. 7d) also resulted in a good agreement (r = 0.98). A higher variance of the modulation curves showed the helical CT scans using a lateral localizer (Fig. 7c). The three waveforms showed the highest difference between slice location −470 mm to

Table 4 Scanner-reported exposure values of abdomen CT scans (CARE kV off) of the ART phantom based on localizer radiograph done with different scan parameters. Helical CT scans were performed with 120 kVp. The effective dose E was estimated using a conversion factor of 0.017 mSv mGy−1 cm−1 . Localizer

Helical CT scan

Tube position

Tube voltage (kVp)

Tube current (mA)

Eff. mAs

CTDIvol (mGy)

DLP (mGy cm)

E (mSv)

ap

120*

35 20 20 35 20 20 35 20 20

111 (±1) 110 110 (±0) 100 102 99 122 120 118

9.09 (±0.04) 9.04 8.98 (±0) 8.23 8.36 8.09 10.00 9.86 9.66

410.1 (±1.8) 408.1 405 (±0) 371.3 377.3 365.2 451 444.8 435.5

6.97 6.94 6.89 6.31 6.41 6.21 7.67 7.56 7.40

lat

pa

*

100 80* 120 100 80 120 100 80

The mean value of repeated measurements with standard deviation in brackets.

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Table 5 Scanner-reported exposure values of abdomen CT scans (CARE kV on) of the ART phantom based on localizer radiograph done with different scan parameters. Helical CT scans were performed with 100 kVp. The effective dose E was estimated using a conversion factor of 0.017 mSv mGy−1 cm−1 . Localizer

Helical CT scan

Tube position

Tube voltage (kVp)

Tube current (mA)

Eff. mAs

CTDIvol (mGy)

DLP (mGy cm)

E (mSv)

ap

120*

35 20 35 20 35 20

147 (±0) 145 132 130 163 157

7.03 (±0) 6.91 6.32 6.2 7.79 7.51

317.3 (±0.1) 312 286.4 280.1 351.5 338.7

5.39 5.30 4.87 4.76 5.98 5.76

lat pa *

80 120 80 120 80

The mean value of repeated measurements with standard deviation in brackets.

Fig. 7. Orientation of slice position from the modulation curves in the ART phantom (a). The modulation curves are shown as mAs per slice of helical CT scans (CARE kV off) based on localizers with different acquisition parameters in ap (b), lateral (c), and pa tube position (d). The maximal standard deviation was ±5 mAs for the modulation curve (ap 120 kVp/35 mA) and ±7 mAs for the modulation curve (ap 80 kVp/20 mA). The gaps between the slices of the ART phantom could be located in all modulation curves.

about −360 mm. The correlation coefficient was 0.97 using a localizer with 120 kVp, 35 mA, and 100 kVp, 20 mA and 0.90 using a localizer with 120 kVp, 35 mA, and 80 kVp, 20 mA. The impact of the tube position of the localizer radiograph on tube current modulation is shown in Fig. 8. When just the localizer tube position was changed the modulated helical CT scans showed a correlation coefficient of 0.68 (ap and lateral

tube position) and 0.95 (ap and pa tube position). The waveform in the pelvic region of the helical CT scan (slice position −470 mm to about −260 mm) based on the lateral localizer differed considerably from the waveform of the helical CT scan based on an ap or pa localizer. The distinctions of modulation curves of helical CT scans using CARE kV on (not shown here) were similar to the results using CARE kV off.

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Fig. 8. The modulation curves as mAs per slice of helical CT scans (CARE kV off) based on localizers with different tube position and constant kVp and mA.

4 Discussion In this study the effective dose of localizer radiographs with different acquisition parameters were determined for head, chest, and abdomen-pelvis CT examinations to examine their dose-saving capabilities. To investigate the usability of optimized localizers, the image quality and impact on tube current modulation of the helical CT scan were also determined. The results presented here show that the localizer dose can be substantially reduced while preserving the visibility of anatomical landmarks with a negligible small influence on the subsequent helical CT scan. A dose reduction of more than 90% for the evaluated localizers was found when the lowest available acquisition parameters (kVp, mA, and pa tube position) were used instead of factory settings. This is in agreement with other studies [30,31]. The female voxel phantom showed higher effective doses (up to 50%) than the male voxel phantom. This is due mainly to the different size of the phantoms and their organs. While the female voxel phantom was shorter than the male voxel phantom more radiosensitive organs and tissues of the female than of the male phantom lay in the primary scan field of the localizer radiograph. The thyroid for example was mainly responsible for the higher effective dose in the female head localizer. In the head localizer the thyroid of the male voxel phantom was outside the primary scan field, while it was partly in the primary scan field of the female voxel phantom. The effective dose of the adult female ranges from 0.045 mSv (head) to 0.255 mSv (chest) when factory settings were used and from 0.004 mSv (head) to 0.020 mSv (abdomen-pelvis) when the lowest selectable settings were used. The effective dose for the male voxel phantom ranges from 0.027 mSv

(head) to 0.227 mSv (chest) when factory settings were used and from 0.002 mSv (head) to 0.013 mSv (chest) when the lowest selectable settings were used. Regarding the different tube positions, the results show that localizers with pa tube position yield the lowest dose for head, chest, and abdomen-pelvis examinations. The main reason is the position of the scanner table between the X-ray tube and the patient. For chest and abdomen-pelvis localizers, the dose of a right-lateral localizer is similar to that of a pa localizer. However, a left-lateral localizer shows a higher dose. This is due to the asymmetric location of some organs and their tissueweighting factor. Stomach, spleen, small intestine and heart induced the higher effective dose in the left-lateral localizer for the most part. When the CT scanner enables a selection of a left- and right-lateral tube position for the localizer, the right-lateral position should be preferred. The calculated effective dose in this study for localizers with factory settings was below the results of Schmidt et al. and Persinakis [24,31]. However, the results for the head localizers are similar to the values reported by Nauer et al. [30]. These differences could be explained by the different CT scanner models used, the resulting dose rate of these scanners, varied acquisition parameters, and different phantoms and calculation methods. The calculation of the effective dose compared to Schmidt et al. also differs, because the arms of the phantom were not removed in this study and lay within the scan region [24]. However, all these studies [24,30,31] recommend an optimization of acquisition parameters for the localizer radiograph. Optimizing the acquisition parameters for the CT localizer radiograph by using the lowest settings (80 kVp, 20 mA) and the ap or pa tube position preserves the detectability of

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anatomical landmarks used for CT scan-range adjustment. A clinical CT examination (Fig. 5) showed its feasibility for patients with a higher BMI. This is in good agreement with results of Persinakis et al. [31]. In contrast to the ap and pa position, the lateral localizer of the ART phantom required a higher tube voltage and tube current (100 kVp, 20 mA) because of the high attenuation in the shoulder region of this phantom. An exact localization of the lung apices was not possible using the lowest localizer settings. However, the use of a lateral localizer with 100 kVp and 20 mA instead an ap localizer with 120 kVp and 35 mA still resulted in a dose reduction of about 85% for the localizer. Additionally, the use of these setting may reduce the axial/helical CT scan dose because, according to Bang [18], the scan range could be minimized due to a better localization of the posterior costophrenic sulci. Comparing the optimized localizer radiograph of the real patient with the optimized localizer radiograph of the ART phantom, more details in the soft tissue and lungs were perceptible in the localizer radiograph of the real patient. Nevertheless, in most cases the ART phantom is a useful tool for the assessment of the perceptibility of anatomical landmarks in optimized CT localizer radiographs. In addition to the detectability of anatomical landmarks, the impact of localizer settings on tube-current modulation was examined. Optimized acquisition parameters for the ap and pa localizer affect the tube-current modulation of the helical CT scan just marginally. Due to the osseous structures in the pelvic region, there the optimization of the lateral localizer leads to different eff. mAs per slice compared to the factory settings. However, the optimization has no strong effect on the remaining slices (see Fig. 7). Furthermore, a change of the tube position in the localizer radiograph was investigated (see Fig. 8) to estimate its effect on tube current modulation. The higher effective dose of + 0.5 mSv to + 0.7 mSv in the helical CT scan planned with a pa localizer outweighs the dose saving of about −0.2 mSv from the use of a pa instead an ap localizer. It was observed that the influence of changed tube positions was essentially higher than that of optimizing localizer parameters. It is therefore assumed that optimizing tube current and tube voltage for localizers affects tube current modulation to a negligible degree. However, the influence of localizers performed with different tube positions on the image quality of the subsequent axial/helical CT scan should be carefully evaluated in further studies. Finally, regarding the influence of the CARE kV on and off mode on the tube current modulation, it was apparent that the waveforms of the mAs per slice diagrams are similar but have different mAs levels. This was due to the fact that the tube voltage of the helical CT scan was lower in the CARE kV on mode and hence the effective mAs and mAs per slice were higher. However, the resulting CTDIvol was lower in the CARE kV on mode (see Tables 4 and 5). Comparing the reported dose values of the helical CT scans subsequent to optimized localizers showed a discrepancy to

literature values. Brisse et al. [44], Lambert et al. [45] and Papadakis et al. [5] discovered that lowering the tube voltage for the localizer increases the dose of subsequent helical CT scans. A standard deviation of the dose values was not determined in these studies. Therefore, it could not be stated how much these evaluation parameters were affected by the system’s instability. Brisse et al. further report that the helical CT scan dose increases when lateral localizers were used instead of ap localizers. The discrepancy to Brisse et al. could be explained by the use of a different CT scanner (GE LightSpeed) and AEC technique. This CT scanner uses a Noise Index for AEC while Siemens uses Image Quality Reference mAs [46]. However, the dose relationship between the localizer tube position and the corresponding helical CT scan reported in this study is in good agreement with other studies [27,5]. The higher dose of a helical CT scan based on a pa instead of an ap localizer may be due to patient centering [45] or by the patient table, which should not be considered in patient centering. In the ap localizer, the patient table is closer to the detector than is the case in the pa localizer. Therefore in comparison to the ap localizer the table in the pa localizer is not completely recorded by the detector. This influences the determined attenuation information from the localizer. A slight reduction of quality reference mAs setting for the helical CT scan may compensate the increase in the dose when a pa localizer is used. Although this study is limited to one CT scanner model, the results for the impact of optimized localizers on automatic exposure control can be transferred to other scanner models using the CARE DOSE 4D software. The image quality of the helical CT scans was not examined, therefore it could not be stated whether a lateral or ap/pa localizer is more suitable for obtaining diagnostic axial plane images. Furthermore, the effect of different patient centering on tube current modulation was not investigated as well as different sized patients. For larger patients image quality of localizers performed with lowered tube voltage and tube current may be sufficient (see Fig. 5b) but it is unclear how far a change of acquisition parameters for the localizer will affect the helical CT scan with AEC for larger (more attenuating) patients. These localizer-AEC results are only valid for an abdomen-pelvis CT examination; the anatomy of the ART phantom restricted an assessment of the detectability of anatomical landmarks in the localizer radiograph.

5 Conclusion The localizer radiograph showed substantial dose-reduction potentials and should be optimized. For the used CT scanner an ap localizer performed with optimized settings (80 kVp and 20 mA) is recommended for an abdomen-pelvis CT examination. It preserved the detectability of anatomical landmarks used for CT scan-range adjustment and showed a negligible impact on dose and tube current modulation of the helical CT

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scan. This results in a dose reduction for the abdomen-plevis localizer of up to 85% for the CT scanner used. These findings confirm the potential for optimizing tube voltage and current for standard adults [43,47]. This work showed that optimizing localizier radiograph is not trivial and requires future work. A patient study done by several radiologists is preferable to assess the detectability of localizer radiograph for patients with different body size. This work should be extended to further exams like head and chest CT examinations. Furthermore several CT scanner with different AEC software should be evaluated as well as the image quality of axial/helical CT scans when localizer settings were changed.

[15]

[16]

[17]

[18]

[19]

References [1] Umweltradioaktivität und Strahlenbelastung: Jahresbericht 2013. Bundesamt für Strahlenschutz (BfS), Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit (BMUB); 2015 http://nbn-resolving.de/urn:nbn:de:0221-2015072112949. [2] National Council on Radiation Protection and Measurements. Ionizing Radiation Exposure of the Population of the United States. NCRP; 2009. Report No. 160. [3] Söderberg M, Gunnarsson M. Automatic exposure control in computed tomography – an evaluation of systems from different manufacturers. Acta Radiol 2010;51:625–34. [4] Mulkens TH, Bellinck P, Baeyaert M, Ghysen D, Van Dijck X, Mussen E, et al. Use of an automatic exposure control mechanism for dose optimization in multi-detector row CT examinations: clinical evaluation. Radiology 2005;237:213–23. [5] Papadakis AE, Persinakis K, Damilakis J. Automatic exposure control in pediatric and adult multidetector CT examinations: a phantom study on dose reduction and image quality. Med Phys 2008;35:4567–76. [6] Kalra MK, Rizzo SMR, Novelline RA. Reducing radiation dose in emergency computed tomography with automatic exposure control techniques. Emerg Radiol 2005;11:267–74. [7] Pickhardt JP, Lubner MG, Kim DH, Tang J, Ruma JA, del Rio AM, et al. Abdominal CT with model-based iterative reconstruction (MBIR): initial results of a prospective trial comparing ultralow-dose with standard-dose imaging. Am J Roentgenol 2012;199:1266–74. [8] Noël PB, Renger B, Fiebich M, Münzel D, Fingerle AA, Ernst J, et al. Does iterative reconstruction lower CT radiation dose: evaluation of 15,000 examinations. PLOS ONE 2013;8:1–7. [9] Noël PB, Köhler T, Fingerle AA, Brown KM, Zabic S, Münzel D, et al. Evaluation of an iterative model-based reconstruction algorithm for lowtube-voltage (80 kVp) computed tomography angiography. J Med Imag 2014;3, 033501-1-033501-7. [10] Christner JA, Zavaletta VA, Eusemann CD, Walz-Flannigan AI, McCollough CH. Dose reduction in helical CT: dynamically adjustable z-axis X-ray beam collimation. Am J Roentgenol 2010;194:W49–55. [11] Deak PD, Langner O, Lell M, Kalender WA. Effects of adaptive section collimation on patient radiation dose in multisection spiral CT. Radiology 2009;252:140–7. [12] Nijhof WH, Baltussen EJM, Kant IMJ, Jager GJ, Slump CH, Rutten MJCM. Low-dose CT angiography of the abdominal aorta and reduced contrast medium volume: assessment of image quality and radiation dose. Clin Radiol 2016;71:64–73. [13] Thot TL, Cesmeli E, Ikhlef A, Horiuchi T. Image quality and dose optimization using novel X-ray source filters tailored to patient size. Proc SPIE 2005;5745:283–91. [14] McCollough CH, Chen HG, Kalender W, Leng S, Samei E, Taguchi K, et al. Achieving routine submillisievert CT scanning: report from

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27] [28] [29] [30]

[31]

[32] [33]

[34]

13

the summit on management of radiation dose in CT. Radiology 2012;264:567–80. Kim Y, Kim YK, Lee BE, Lee SJ, Ryu YJ, Lee JH, et al. Ultra-lowdose CT of the thorax using iterative reconstruction: evaluation of image quality and radiation dose reduction. Am J Roentgenol 2015;204: 1197–202. Baumueller S, Winklehner A, Karlo C, Goetti R, Flohr T, Russi EW, et al. Low-dose CT of the lung: potential value of iterative reconstructions. Eur Radiol 2012;22:2597–606. Schuhbaeck A, Achenbach S, Layritz C, Eisentopf J, Hecker F, Pflederer T, et al. Image quality of ultra-low radiation exposure coronary CT angiography with an effective dose <0.1 mSv using high-pitch spiral acquisition and raw data-based iterative reconstruction. Eur Radiol 2013;23:597–606. Bang DH, Lim D, Hwang WS, Park SH, Jeong OM, Kang KW, et al. Lateral topography for reducing effective dose in low-dose chest CT. Am J Roentgenol 2013;200:1294–7. ICRP 87. Managing patient dose in multi-detector computed tomography (MDCT), ICRP Publication 87. Ann ICRP 2000;30(4). Huppertz A, Radmer S, Asbach P, Juran R, Schwenke C, Diederichs G, et al. Computed tomography for preoperative planning in minimalinvasive total hip arthroplasty: radiation exposure and cost analysis. Eur J Radiol 2011;78:406–13. Platon A, Jlassi H, Rutschmann OT, Becker CD, Verdun FR, Gervaz P, et al. Evaluation of a low-dose CT protocol with oral contrast for assessment of acute appendicitis. Eur Radiol 2009;19:446–54. Yang M, Mo XM, Jin JY, Zhang J, Wu M, Teng GJ. Image quality and radiation exposure in pediatric cardiovascular CT angiography from different injection sites. Am J Roentgenol 2011;196: W117–22. Park YJ, Kim YJ, Lee JW, Kim HY, Hong YJ, Lee HJ, et al. Automatic tube potential selection with tube current modulation (APSCM) in coronary CT angiography: comparison of image quality and radiation dose with conventional body mass index-based protocol. J Cardiovasc Comput Tomogr 2012;6:184–90. Schmidt B, Saltybaeva N, Kolditz D, Kalender WA. Assessment of patient dose from CT localizer radiographs. Med Phys 2013;40:084301–84311. Empfehlung der Strahlenschutzkommision. Orientierungshilfe für bildgebende Untersuchungen. Berichte der Strahlenschutzkommision (SSK) des Bundesministeriums für Umwelt, Naturschutz und Reaktorsicherheit 2012, 51. Lung Cancer Screening CT Protocols. AAPM 2015; Version 3.0. http:// www.aapm.org/pubs/CTProtocols/documents/LungCancerScreening CT.pdf. Cody DD. Management of auto exposure control during pediatric computed tomography. Pediatr Radiol 2014;44:427–30. Toth T, Ge Z, Daly MP. The influence of patient centering on CT dose and image noise. Med Phys 2007;34:3093–101. O’Daniel JC, Stevens DM, Cody DD. Reducing radiation exposure from CT localizer radiographs. Am J Roentgenol 2005;185:509–15. Nauer CB, Kellner-Weldon F, von Allmen G, Schaller D, Gralla J. Effective doses from scan projection radiographs of the head: impact of different scanning practices and comparison with conventional radiography. Am J Neuroradiol 2009;30:155–9. Persinakis K, Damilakis J, Voloudaki A, Papadakis A, Gourtsoyiannis N. Patient dose reduction in CT examinations by optimising scanogram acquisition. Radiat Prot Dosim 2000;93:173–8. Schmidt R, Wulff J, Zink K. GMctdospp: description and validation of a CT dose calculation system. Med Phys 2015;42:4260–70. Kawrakow I, Mainegra-Hing E, Rogers DWO, Tessier F, Walters BRB. The EGSnrc Code System: Monte Carlo Simulation of Electron and Photon Transport. NRCC; 2011. Report No. PIRS-701. Schmidt R, Wulff J, Kästner B, Jany D, Heverhagen JT, Fiebich M, et al. Monte Carlo based calculation of patient exposure in X-ray CTexaminations. 4th European conference of the international federation for medical and biological engineering, 22; 2009. p. 2487–90.

ARTICLE IN PRESS 14

E. Bohrer et al. / Z. Med. Phys. xxx (2016) xxx–xxx

[35] ICRP 110. Adult Reference Computational Phantoms, ICRP Publication 110. Ann ICRP 2009;39(2). [36] ICRP 89. Basic Anatomical and Physiological Data for Use in Radiological Protection: Reference Values, ICRP Publication 89. Ann ICRP 2002;32. [37] Bayer J, Pache G, Strohm PC, Zwingmann J, Blanke P, Baumann T, et al. Influence of arm positioning on radiation dose for whole body computed tomography in trauma patients. J Trauma 2011;70:900–5. [38] Brink M, de Lange F, Oostveen LJ, Dekker HM, Kool DR, Deunk J, et al. Arm raising at exposure-controlled multidetector trauma CT of thoracoabdominal region: higher image quality, lower radiation dose. Radiology 2008;249:661–70. [39] Lee C, Kim KP, Long D, Fisher R, Tien C, Simon SL, et al. Organ doses for reference adult male and female undergoing computed tomography estimated by Monte Carlo simulations. Med Phys 2011;38:1196–206. [40] ICRP 103. The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103. Ann ICRP 2007;37(2–4). [41] ACR–SPR Practice Parameter for the Performance of Computed Tomography (CT) of the Abdomen and Computed Tomography (CT) of the Pelvis Res. 32-2011, Amended 2014 (Res. 39). http://www.acr.

[42]

[43] [44]

[45]

[46] [47]

org/∼/media/ACR/Documents/PGTS/guidelines/CT Abdomen Pelvis. pdf [accessed 09.10.15]. ACR–SCBT-MR–SPR Practice Parameter for the Performance of Thoracic Computed Tomography (CT) Res. 10-2013, Amended 2014 (Res. 39). http://www.acr.org/∼/media/ACR/Documents/PGTS/guidelines/ CT Thoracic.pdf [accessed 09.10.15]. Leitlinie der Bundesärztekammer zur Qualitätssicherung in der Computertomographie. Bundesärztekammer 2007. Brisse HJ, Madec L, Gaboriaud G, Lemoine T, Savignoni A, Neuenschwander S, et al. Automatic exposure control in multichannel CT with tube current modulation to achieve a constant level of image noise: experimental assessment on pediatric phantoms. Med Phys 2007;34:3018–33. Lambert JW, Kumar S, Chen JS, Wang ZJ, Gould RG, Yeh BM. Investigating the CT localizer radiograph: acquisition parameters, patient centering and their combined influence on radiation dose. BJR 2015;88:1048. Keat N. CT scanner automatic exposure control systems. MHRA; 2005. Report 05016. Mayo-Smith WW, Hara AK, Mahesh M, Sahani DV, Pavlicek W. How I do it: managing radiation dose in CT. Radiology 2014;273:657–72.

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