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Published online before print July 20, 2006, 10.1148/radiol.2402050580
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(Radiology 2006;240:690-697.)
© RSNA, 2006

Experimental Studies

Pulmonary Embolism: Radiation Dose with Multi–Detector Row CT and Digital Angiography for Diagnosis1

Emmanuel Coche, MD, PhD, Stefaan Vynckier, PhD and Michelle Octave-Prignot, MS

1 From the Departments of Medical Imaging (E.C.) and Radiation Therapy (S.V., M.O.), Cliniques Universitaires St-Luc, Université Catholique de Louvain, Avenue Hippocrate, 10, 1200 Brussels, Belgium. Received April 7, 2005; revision requested June 7; revision received August 12; accepted September 13; final version accepted November 23. Address correspondence to E.C. (e-mail: coche{at}rdgn.ucl.ac.be).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To compare radiation dose delivered at four– and 16–detector row computed tomography (CT) with a dose-modulation program and that delivered at digital angiography for evaluation of pulmonary embolism (PE).

Materials and Methods: The part of the study involving patients (seven women, four men; mean age, 62 years ± 16 [standard deviation]; range, 41–85 years) was approved by the institutional review board. Patients gave written informed consent. Exposure was performed with an anthropomorphic phantom with thermoluminescent dosimeters for four–detector row CT without the dose-modulation program and 16–detector row CT without and with the dose-modulation program with standard protocols for pulmonary CT angiography (120 kV, 144 mAs, four and 16 detector rows with 1.00- and 0.75-mm section thickness, respectively). Digital angiograms were acquired with four standard projections at 80 kV. For digital angiography, radiation dose was calculated according to phantom measurements and adapted to acquisition and fluoroscopy times. Distribution of dose was compared for CT and digital angiography.

Results: During pulmonary CT angiography, mean radiation dose delivered at middle of chest was 21.5, 19.5, and 18.2 mGy for four–detector row CT and for 16–detector row CT without and with dose-modulation program, respectively. At the same level, a mean dose of 91 mGy was delivered with digital angiography. The dose adjusted to clinical conditions was 139.0 mGy for digital angiography and could be reduced after technical adjustment. Ratios of maximum dose to mean dose were 1.15 and 2.96 for CT and digital angiography, respectively. With application of the dose-modulation program at 16–detector row CT, radiation dose was reduced 15%–20% at the upper chest.

Conclusion: Multi–detector row CT delivers a lower radiation dose, with better spatial distribution of dose, than does pulmonary CT angiography. With 16–detector row CT and a dose-modulation program, radiation dose is decreased during PE work-up.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
The diagnostic work-up for pulmonary embolism (PE) has evolved during the past few years. Several strategies have been proposed to increase the use of multi–detector row computed tomography (CT) and to reduce the need for digital angiography in the diagnostic work-up for PE. Some authors have published data (1–3) concerning the accuracy of multi–detector row CT performed by using four detector rows with thin sections in patients who are suspected of having PE. They concluded that multi–detector row CT was very accurate for the diagnosis of acute PE even at the subsegmental level. In some publications (4–6), researchers have compared the radiation dose delivered at single–detector row CT and multi–detector row CT with that delivered at digital angiography during PE work-up. To our knowledge, however, no study to date has been published that included a comparison of the radiation dose delivered at thin-collimation four–detector row CT with that delivered at thin-collimation 16–detector row CT by using a comparable protocol for PE detection.

Thus, the purpose of our study was to compare the radiation dose delivered at four–detector row CT and 16–detector row CT with a dose-modulation program with that delivered at digital angiography for evaluation of PE.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Multi–Detector Row CT Scanners
Two multi–detector row helical CT scanners with four detector rows (MX 8000 Quad; Philips Medical Systems, Cleveland, Ohio) and 16 detector rows (MX 8000 IDT; Philips Medical Systems, Cleveland, Ohio) were compared in this study. These CT scanners have the same characteristics (eg, geometric features, tube, filtration) and differ only according to their detection system. The four–detector row CT scanner has eight asymmetric detectors with a total width of detector array of 20 mm in the craniocaudal direction, and the 16–detector row CT scanner has 32 asymmetric detectors with a total width of detector array of 24 mm.

At the time of experimentation, only the 16–detector row CT scanner was equipped with the dose-modulation program (Dose Right, version 2.5; Philips Medical Systems, Best, the Netherlands), a dedicated software that modulates the tube current according to the attenuation of the patient's body (7–10).

All protocols that were used are the standard CT protocols we used for PE work-up in our Department of Medical Imaging. First, a frontal scout view of the chest was obtained to plan the CT examination. Second, a bolus test was performed (11,12) to determine the circulation time. First, one unenhanced transverse CT image was obtained to define a region of interest at the level of the main pulmonary arteries. Thereafter, repetitive transverse CT images were obtained (Table 1) every second during 20 seconds at the selected level. The number of acquired images for the bolus test was therefore 21. The peak of enhancement in pulmonary arteries was measured by using dedicated software (Bolus Pro; Philips Medical Systems, Cleveland, Ohio).


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  		Table 1. Summary of Constant CT Parameters

 
Finally, pulmonary spiral CT angiography was performed to image the entire pulmonary circulation by using selected parameters (Table 1), and a time delay was calculated from the bolus test.

Digital Angiography
A digital angiographic examination, as usually performed in our department, was simulated (Table 2) with a monoplanar vascular x-ray system (Integris V3000; Philips Medical Systems, Best, the Netherlands). Posteroanterior (0°), left posterior oblique (+35°), right posterior oblique (–35°), and lateral right (90°) projections were systematically obtained after each selective catheterization. The acquisition was obtained with a delay of 4 seconds, a frame rate of 3 frames per second during 5 seconds and 2 frames per second during 10 seconds, with a 350-mm field of view, 80 kV, and 700 mA (half-value layer of 3.3 mm of aluminum). The values for total accumulated tube current–time product in our experimental conditions were, on average, 6.4, 29.7, 17.8, and 45.4 mAs for 0°, +35°, –35°, and 90° projections, respectively. Fluoroscopy was performed at 74 kV and 2.5 mA. This standard procedure was used to obtain baseline values of delivered radiation during digital angiography. Sequences performed during clinical work-up were recorded in the last 11 patients (seven women, four men; mean age, 62 years ± 16 [standard deviation]; range, 41–85 years) examined in our Department of Medical Imaging for PE work-up from February 2000 to October 2001. This part of our study was approved by the institutional review board of our institution. All patients gave appropriate written informed consent.


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  		Table 2. Summary of Constant Pulmonary Digital Angiographic Parameters

 
Dose Measurements
Measurements were performed in consensus by a physicist (M.O.) and a radiologist (E.C.) with an anthropomorphic phantom that represents the thorax of a male adult (13,14) (Alderson-Rando Phantom; Radiology Support Devices, Long Beach, Calif) (Fig 1). The phantom consists of 25-mm-thick transverse sections, with a grid of holes for dosimeter placement. The holes were filled with LiF thermoluminescent dosimeters (TLD 700; Harshaw Chemical, Cleveland, Ohio), which were extruded chips that were 3.2 x 3.2 x 0.89 mm. Only the sections numbered 10–20 were used for this study.


Figure 1
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  		Figure 1: Anthropomorphic phantom equipped with thermoluminescent dosimeters imaged at simulated digital angiography. The phantom was placed on the examination table, and a fixed reference was adjusted with laser and plumb line (arrow) in the angiographic suite to reproduce the same conditions of the experiment.

 
For CT, thermoluminescent dosimeters were positioned at four cross-sectional areas for scout view and pulmonary CT angiographic measurements as follows: (a) at the level of the thyroid (section 10, outside the field), (b) throughout the sternocostoclavicular joints (section 12, upper part of the field), (c) at the level of the middle part of the lungs (section 16, middle part of the field), and (d) at the level of the lower part of the chest (section 20, lower part of the field). Thirty-two thermoluminescent dosimeters were distributed at 16 organ positions (with two dosimeters at each position) as follows: Four dosimeters were at the thyroid gland, six were at the anterior chest wall, six were at the middle part of the lungs, two were at the axillae, six were at the posterior mediastinum, four were at the posterior chest wall, and four were at the periphery of the lungs.

For the bolus test, 20 thermoluminescent dosimeters, with eight at the periphery, were distributed on a section passing throughout the pulmonary arteries (section 15), and six dosimeters were placed on the adjacent section (section 16).

For digital angiography, special care was applied to keep the geometric parameters—focus-phantom distance, source-image distance, and phantom position—constant. Dose measurements were performed at four standard acquisition projections of 0°, +35°, –35°, and 90° and source-image distances of 100, 106, 106, and 117 cm, respectively (Fig 1). For each projection, the thermoluminescent dosimeters were positioned at 22 different places at the level of section 16. For fluoroscopy, accumulated doses during 4 minutes were measured at 0° and 2.5 mA. Thermoluminescent dosimeters were positioned at the anterior and the posterior part of the chest. Their relative response was in agreement with the ratio of extrapolated 0° digital angiographic data. Doses to the phantom were deduced, with the assumption of the same attenuation level and depth as were used for digital angiographic conditions. The LiF dosimeters were read by using a commercially available reader system with a nitrogen-heating gas flow (model 5500; Harshaw/Bicron, Solon, Ohio) (15) at least 24 hours after irradiation. The annealing cycle consisted of heating the thermoluminescent dosimeters for 1 hour at 400°C, followed by a controlled cooling phase at a temperature reduced to 100°C and held for 3 hours. A set of 100 thermoluminescent dosimeters, individually calibrated in a 6-MV photon therapy beam, has been used. Their response reproducibility expressed as a standard deviation of the mean was 2.6%. The set response was verified at each experiment with eight thermoluminescent dosimeters.

The doses are specified as doses to water at the level of the dosimeters. An increase in the LiF response of 25% and of 39% between the reference beam and the experimental beam is assumed for the measurements at the CT scanner (120 kV, half-value layer of 8.8 mm of aluminum) and at the monoplanar vascular x-ray system (80 kV, half-value layer of 3.3 mm of aluminum), respectively (16–18). As a control of the entire dosimetric procedure, a direct comparison of the computer dose index values in polymethylmethacrylate measured by a calibrated-pencil-ionization chamber (Radcal model 2025; Radcal, Monrovia, Calif) and by thermoluminescent dosimeters was performed at 16–detector row CT by a physicist.

At four–detector row CT, measurements were repeated six times; at 16–detector row CT with and without the dose-modulation program, measurements were repeated four times and twice, respectively. For pulmonary CT angiography, measurements were repeated three times at each projection. Adjustments of the angiographic suite for maintenance purposes were performed by the manufacturer's engineers after our experiment: For the same protocol and for the same geometric conditions, unchanged tube parameters (tube voltage, filtration, tube current) resulted in a substantial decrease in exposure time. The mean doses before and after adjustment were reported. The distribution of the dose was compared for CT and digital angiography.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Multi–Detector Row CT Scanners
Mean radiation dose.—For the scout view at four–detector row CT, the mean radiation dose delivered to sections 12, 16, and 20 during acquisition was 0.14, 0.17, and 0.18 mGy, respectively (range, 0.04–0.4 mGy). The dose at the level of the thyroid (section 10) was 0.07 mGy.

For the bolus test at four–detector row CT, the mean radiation dose delivered to section 15 was 120.4 mGy (range, 88.8–162 mGy). At 2.5 cm distance, the mean delivered radiation dose was reduced to 18.8 mGy.

For pulmonary CT angiography at four–detector row CT, the mean radiation dose to sections 12, 16, and 20 was 17.6 mGy ± 3.1, 21.5 mGy ± 1.9, and 21.5 mGy ± 2.8, respectively. The mean dose decreased to 5.7 mGy at the level of the thyroid gland (Table 3).


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  		Table 3. Radiation Dose with Multi–Detector Row CT during Pulmonary CT Angiography: Mean Doses and Dispersion as a Function of Position to Section

 
At 16–detector row CT, a dose reduction of 8% was observed. With the dose-modulation program, the dose was still reduced by 15%–20% at the level of section 12 and by approximately 5% at the level of sections 16 and 20.

Dose distributions.—Dose distribution observed at the middle part of the chest is relatively homogeneous, as can be seen in Figure 2. All the relative doses expressed as percentages of the total mean dose are included between 86% and 108% of the mean measured dose. The standard deviation of the relative dose to the section was 7%. The data also showed that the left side of the patient received a few percentage points less of the dose than did the right side: The greatest measured discrepancy was 10%. For the three selected sections, the maximal dose never exceeded 15% of the mean dose (ratio to the mean dose of 1.15). The same dose distribution was observed for the three acquisitions at the levels of sections 16 and 20. Dose dispersion in section 12 decreased from 18% (standard deviation of the mean) to 8% (standard deviation of the mean) when the dose-modulation program was applied.


Figure 2
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  		Figure 2: Transverse CT view in phantom at level of middle part of the chest (section 16) shows distribution of relative dose during multi–detector row CT. The phantom was cut into 25-mm-thick transverse sections, which were coated with a soft-tissue substitute to form smooth surfaces. The sections were drilled with a grid of holes for dosimeter placement. Relative doses, which are percentages of the total mean dose to the section, were reproduced.

 
Digital Angiography
Mean radiation dose for digital angiography.—The mean dose to section 16 was 7 mGy at 0°, 27 mGy at 35°, 15 mGy at –35°, and 42 mGy at 90°, with a total of 91 mGy for four projections. Mean delivered doses were 6.3, 9.1, 9.0, and 14.5 mGy for the same sequences after adjustment of the angiographic suite by the engineers of the manufacturer, which resulted in a total delivered dose of 38.9 mGy for the same examination. The mean radiation dose rate delivered during fluoroscopy at 2.5 mA was 12.5 mGy/min at the posterior part of the chest.

An average of 6.3 acquisitions (2.3 posteroanterior, 1.1 right posterior oblique, 2.2 left posterior oblique, and 0.7 lateral views) were performed in each patient during digital angiography in the last 11 consecutive patients who were referred for evaluation because they were suspected of having an acute PE; the mean time for fluoroscopy was 10 minutes, and the range was 3–18 minutes. Therefore, the mean radiation dose delivered to the middle part of the chest in a patient in the clinics was estimated at 139.0 mGy and 83.6 mGy before and after the adjustment performed by the engineers of the manufacturer, respectively (Table 4).


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  		Table 4. Estimation of Delivered Radiation Dose with Standard and Clinical Conditions at Middle Part of Chest or Section 16

 
Dose distribution.—Dose distribution for a standard sequence is highly inhomogeneous, as displayed in Figure 3 and Table 5. The standard deviation of the dispersion of the relative doses in the sections corresponding to the middle part of the chest is estimated to be 0.66. Maximal relative dose is 2.96 and minimal relative dose is 0.16. After adjustment, we observed a small reduction of the beam inhomogeneity. Standard deviation and maximal and minimal relative doses were 0.59, 2.46, and 0.17, respectively.


Figure 3
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  		Figure 3: Transverse CT view in phantom at level of middle part of the chest (section 16) shows distribution of relative dose during digital angiography (four projections). Relative doses are percentages of the total mean dose to the section.

 

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  		Table 5. Distribution of Mean Dose during Pulmonary Digital Angiography at Middle Part of Chest or Section 16

 
CT Dose Index Measurements
The doses estimated with the thermoluminescent dosimeter were systematically higher than those measured with the ionization chamber. On average, overestimation factors of 1.04 and 1.07 were obtained for a 16- and a 32-cm-diameter phantom, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
We believe that two findings are important in regard to our study: (a) Higher and more heterogeneous radiation doses are delivered at digital angiography compared with those delivered at multi–detector row CT. (b) The delivered radiation doses decrease with the number of detectors available with CT and with the dose–modulation program.

Angiography versus Multi–Detector Row CT
The mean radiation dose delivered to the middle part of the chest at digital angiography is more than four times the mean dose delivered during pulmonary CT angiography at four–detector row CT in standard conditions and seven times that delivered during this type of imaging in clinical practice. The scout view adds only a small amount of radiation dose that is equal to 0.17 mGy, which is equivalent to an increase of 0.7% of the acquisition dose (21.5 mGy) at the middle part of the chest. The radiation dose delivered during the bolus test was approximately equal to five times the mean radiation dose delivered with pulmonary CT angiography at four–detector row CT with an acquisition length of approximately 10 mm.

Resten et al (5) obtained total average doses of 28 mGy ± 7.6 and 6.4 mGy ± 1.5 for digital angiography and single–detector row CT, respectively. The effective dose reported by Kuiper et al (6) was 4.2 mSv for four–detector row CT and 7.1 mSv for digital subtraction angiography. No analysis of delivered radiation dose for the scout view and the bolus test was considered in previous articles. There are some differences between these results and ours, for the CT protocols and the equipment used were not the same. The values we obtained with thermoluminescent dosimeters were controlled with comparison of CT dose index values obtained during CT acquisition by using the same experimental conditions. Variations in patients' doses are large for digital angiography because the number of projections and duration of fluoroscopy vary according to the expertise of the radiologist, the patient's status, and the difficulty of the clinical case. Time of fluoroscopy and number of sequences in digital angiography were systematically recorded by Resten et al (5). The authors recorded four sequences per patient and a mean fluoroscopy time of 9 minutes 47 seconds (range, 1 minute 15 seconds to 26 minutes 20 seconds), and these data agree with our data.

A more homogeneous distribution of delivered radiation dose is observed with multi–detector row CT than with digital angiography. At CT, lower doses found on the posterior part of the chest were probably related to CT table absorption, but the higher doses on the right side remain unexplained. The heterogeneity at digital angiography is mainly related to the important attenuation of the x-rays (19,20) with depth and to an unbalanced distribution of the projections.

Effect of Multi–Detector Row Factor and Dose-Modulation Program
Analysis of our results shows that a decreased radiation dose is observed when the number of detectors increases. During our study, we observed an overall reduction of 8% of the total amount of delivered radiation dose with the 16–detector row CT system compared with that delivered with the four–detector row CT system. The x-ray tube and its filtration, the geometric features, and the collimator settings were identical for both CT units used in this study. With multi–detector row CT, only the plateau region of the dose profile may be used to ensure equal signal level for all detector sections. The penumbral region has to be discarded, with use of either postpatient collimation or intrinsic self-collimation of the multi–detector row CT unit and represents a "wasted dose." The relative contribution of the penumbral region decreases with an increasing number of simultaneously acquired sections and increases with a decreasing section width (20,21). This physical property explains the relative lower dose we obtained with 16–detector row CT compared with the dose we obtained with four–detector row CT.

The use of the dose-modulation program with the most recent multi–detector row CT unit can decrease radiation dose, particularly at the upper part of the chest. Attenuation-based, online tube-current control has great potential for the reduction of patient dose at CT. The mean value of the radiation dose was decreased by 15%–20% at the level of the upper chest and by approximately 5% at the level of the middle and lower parts of the chest with the dose-modulation program in our experiment. Despite that the images are noisier with this software, the diagnosis should not be affected. Tack et al (22) compared values for inter- and intrareader agreement for the diagnosis of PE on CT pulmonary angiographic images acquired with an effective tube current–time product of 90 mAs and with simulated values of 60, 40, 20, and 10 mAs. The authors showed that values for inter- and intrareader agreement remain stable when the tube current–time product is reduced from 90 to 10 mAs. A survey of practices and policies for pulmonary CT angiography (23) in pregnant women revealed that the method for reduction of the radiation dose that was most often used consisted of a reduction of the craniocaudal scan coverage.

Limitations
First, the doses were related to the shape of the phantom we used: The anthropomorphic phantom we used is a simulation of a slim patient (body weight, 70 kg). For obese patients, the automatic brightness control on the biplanar angiographic system would increase the tube current, and this increase would result in a higher delivered radiation dose, whereas at CT angiography, radiation dose is relatively much less dependent on patient size (24).

Second, the anthropomorphic phantom that we used is armless, and this structural difference between phantoms and humans probably contributes to an underestimation of the potential effect of online tube-current modulation.

The uncertainty about our determination of the absorbed doses to water is relatively important. First, the phantom (13,14) is not really equivalent to tissue because, within the energy range of 20–300 keV, the ratio of mass attenuation coefficients varies by approximately 5% and the ratio of absorption coefficients varies by approximately 7%, both acting in an opposite direction. Second, the thermoluminescent dosimeter response increases by approximately 40% with decreasing energy from 300 to 20 keV. Without the knowledge of the photon spectral distribution at the point of measurement, an "equivalent energy," which characterizes the quality of the photon beam, has to be selected.

As far as the overall uncertainty is concerned, the thermoluminescent dosimeter energy dependence induces a response variation with a depth of the order of 3% for depths that vary between of 0 and 5 cm (17). The beam perturbation caused by the presence of the thermoluminescent dosimeter is assessed to be approximately 1% (16).

Finally, the uncertainty about the dose in the reference beam can be estimated to be 2% at the level of 1 standard deviation (25). In spite of the reproducibility of the thermoluminescent dosimeter, the combined uncertainty about the absorbed dose, predominantly influenced by the estimation of the equivalent energy, could be as high as 15%.

Practical application: Even if bolus tracking concerns a narrow area of the chest, radiologists should be aware that this procedure locally delivers a relatively great amount of radiation. Reduction of the x-ray–tube current settings will have a major effect on radiation exposure. To date to our knowledge, however, there is no prospective experimental study in which the influence of reduced tube current–time product and tube voltage on timing values for the bolus test has been tested. Alternatively, reduction of repetition time during the bolus test also could be used to decrease radiation exposure. In our clinical practice, we have empirically decreased tube current–time product presets for the bolus test from 150 to 30 mAs.

Maintenance procedures are performed three times a year in our angiographic suite by the engineers of the manufacturer of the unit. These procedures consist of measurement of the tube voltage constancy, dose measurements at the level of the monitoring system, and image quality control. The results of our experiment with a phantom demonstrated a technical failure of the detection system of the angiographic unit that was not detected with the routine maintenance procedure. Image quality that resulted from increased tube current–time product was probably modified but not enough to be reported by clinicians. Therefore, we can suggest that the reproducibility of the relationship between the x-ray–tube current settings and image quality be systematically verified during the maintenance procedure by using a dedicated phantom.

From our data, we conclude that multi–detector row CT delivered a lower radiation dose, with a better spatial dose distribution, than did digital angiography. Multi–detector row CT with the higher number of detector rows that we tested is preferable for thin-section examinations, as recommended for PE work-up. Application of the dose-modulation program further helps to decrease the delivered radiation dose. The image quality, however, related to the detection of PE should be evaluated.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
The authors thank Pierre Goffette, MD, Frank Hammer, MD, and the nurses of the vascular unit for their assistance in completing this study. We are also grateful to technologists of Philips Medical Systems for their helpful comments in regard to the maintenance procedure performed in the angiographic suite of our Department of Medical Imaging.


   FOOTNOTES
 

Abbreviations: PE = pulmonary embolism

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, all authors; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, all authors; clinical and experimental studies, all authors; statistical analysis, all authors; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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