Original Research
Genitourinary Imaging
August 21, 2015

A Survey of Radiation Doses in CT Urography Before and After Implementation of Iterative Reconstruction

Abstract

OBJECTIVE. The purpose of this study was to survey the radiation dose used in CT urography (CTU) in routine clinical practice, both before and after implementation of a scanning protocol that uses iterative reconstruction (Adaptive Iterative Dose Reduction 3D [AIDR 3D]).
MATERIALS AND METHODS. We retrospectively surveyed dose reports from consecutive CTU examinations performed in 2011 with the use of 64- and 320-MDCT scanners that were reconstructed with filtered back projection (FBP) and from CTU examinations performed from May 2012 through November 2013 that were reconstructed with the use of AIDR 3D. Findings from these dose reports were then correlated with such patient characteristics as weight and body mass index (BMI; weight in kilograms divided by the square of height in meters). Only dose reports from single-bolus three-phase CTU examinations were included in the study. The volume CT dose index, dose-length product (DLP), and effective dose were surveyed both per examination and per phase by use of published effective dose DLP conversion factors. Image quality was evaluated subjectively for a subset of patients.
RESULTS. The two study cohorts included 82 patients (median patient weight, 75.0 kg; median BMI, 25.3) who underwent CTU with FBP and 85 patients (median patient weight, 78.0 kg; median BMI, 24.5) who underwent CTU with AIDR 3D. The median total DLP and median effective dose were 924 mGy·cm and 13.0 mSv, respectively, in the CTU with the FBP cohort and 433 mGy·cm and 6.1 mSv, respectively, in the CTU with the AIDR 3D cohort. The median DLP in the unenhanced, nephrogenic, and excretory phases was 218, 300, and 441 mGy·cm, respectively, in patients undergoing CTU with FBP and 114, 121, and 190 mGy·cm, respectively, in patients undergoing CTU with AIDR 3D. Image quality was diagnostic in both groups, with relatively fewer artifacts noted on scans obtained using CTU with AIDR 3D.
CONCLUSION. Our study presents detailed dose data from three-phase CTU examinations performed both before and after implementation of iterative reconstruction. Implementation of a CTU protocol using iterative reconstruction resulted in a mean effective dose of 6.1 mSv with preservation of clinical diagnostic image quality.
In recent years, CT urography (CTU) has become the primary diagnostic modality used in the evaluation of patients with hematuria [1]. CTU is a multiphase CT examination in which the clinical presentation will largely determine the protocol used. It includes an analysis of stones and can assess the process of contrast medium excretion at several time points, thereby providing both morphologic and functional information.
In theory, the number of phases in CTU can be highly variable, and once excretion commences, excretory phases may be repeated at multiple time points. Because of the multiphasic and functional nature of CTU, the radiation dose can reach high levels, and suggestions for optimized use of CTU have been published by scientific societies [2]. Continuous awareness of radiation dose reduction strategies is essential. This requires insight into the current CTU dose level, justification for the use of CTU, and optimization of CTU, both in terms of the number of phases and the technical CT parameters used.
In CTU, radiation dose mainly depends on the number of phases, the scanning parameters used for the individual phases, and the size and weight of the patient. The image quality achieved depends not only on acquisition but also on image reconstruction and noise reduction algorithms. Relatively few recent data on radiation dose levels in CTU have been published. In the early days of CTU, a three-phase CTU protocol could be associated with effective dose levels of 25–35 mSv [3]. A phantom analysis of single-bolus three-phase and split-bolus two-phase CTU protocols showed mean effective doses of 20–40 mSv, with individual effective doses as high as 66 mSv [4]. Proper stepwise protocol optimization toward use of a three-phase CTU protocol with unenhanced, corticomedullary, and excretory phases resulted in a reduction in CTU effective doses from 29.9 to 11.7 mSv for women and from 22.5 to 8.8 mSv for men [5]. A recent CT dose survey performed in The Netherlands showed that, for (split-bolus two-phase) CTU, comparable median effective doses would be 3.6 mSv for the unenhanced phase and 6.6 mSv for the concurrent nephrographic and excretory phases [6].
For decades, filtered back projection (FBP) was the main technique used for image reconstruction in CT. However, in recent years, iterative reconstruction techniques have have overtaken FBP in the CT world, and their use has already become mainstream in body CT examinations, including CTU. The current clinical implementations of iterative reconstruction can improve image quality and allow dose reductions of 40–50% in general abdominal CT examinations, whereas for a high-contrast examination such as CTU, this reduction may even be higher [79].
The purposes of this study were to survey the radiation dose associated with three-phase CTU performed in routine clinical practice, both before and after protocol changes were made in association with implementation of Adaptive Iterative Dose Reduction in 3D (AIDR 3D, Toshiba Medical Systems), and to evaluate image quality.

Materials and Methods

Patients

We identified consecutive examinations of all patients with the specific code “CTU” in their medical records. Before implementation of iterative reconstruction, patients underwent scanning performed using one of two MDCT scanners (Aquilion One or Aquilion 64, both Toshiba Medical Systems), both of which were operated in 64-MDCT mode. After implementation of iterative reconstruction, patients underwent MDCT performed with the Aquilion One scanner in 80-MDCT mode.
The following data were recorded for all patients: sex, age (measured in years), weight (measured in kilograms), and height (measured in meters). Patient weight and height were recorded at the time of the CTU examination. Body mass index (BMI; weight in kilograms divided by the square of height in meters) was also calculated. Only patients for whom complete data were available were included in the survey.

CT Urography Acquisition and Reconstruction Technique

At our medical center (Leiden University Medical Center in Leiden, The Netherlands), CTU is practiced in accordance with the CTU guidelines of the European Society of Urogenital Radiology and [1] and the Dutch Guideline on Hematuria [2]. Most patients who undergo CTU at our center are 50 years old or older with macroscopic hematuria for whom a single-bolus three-phase protocol is used. Inclusion in our study was limited to patients in this high-risk group.
The iterative reconstruction technique used in this study is AIDR 3D, an automated system that integrates raw data and image space noise reduction. By modeling noise statistics and scanner design, the AIDR 3D reduces noise in projection data in multiple iterative reconstruction steps, allowing the system to be manually set to three strengths. Because AIDR 3D is integrated, it directly influences the tube current modulation system of the scanner (SureExposure 3D, Toshiba Medical Systems) without the need for modification of tube current modulation parameters. On the basis of the parameters set in the image reconstruction (SureIQ, Toshiba Medical Systems), the system automatically adapts the iterative reconstruction technique (e.g., the number of iterations and the percentage of blending with FBP) to the type of examination, the slice width, and the image reconstruction kernel [10, 11].
Sixty minutes before the CTU examination was performed, patients were given 1000 mL of water to ingest. After the unenhanced phase, patients received furosemide, 0.1 mg/kg of body weight, 3–5 minutes before initiation of contrast medium injection. The dose and injection rate of the contrast medium were adapted to the body weight category of the patient, with a constant injection duration of 37.5 seconds maintained. In patients who weighed 65–80 kg, 115 mL of iopromide (370 mg I/mL; Ultravist, Bayer HealthCare) was injected at a flow of 3 mL/s and was flushed by a 10-second injection of 30 mL of saline given at a rate of 3 mL/s. The three-phase CTU protocol consisted of an unenhanced phase of the abdomen and pelvis; a nephrographic phase of the abdomen, with a scanning delay of 120 seconds after the start of contrast medium injection; low-dose test images obtained at the midureteral level beginning 7 minutes after injection and, if excretion was asymmetric, repeated at 9, 11, 15, and 20 minutes after injection [12]; and an excretory phase of the abdomen and pelvis, with the scanning delay individualized on the basis of findings on the test image(s) but occurring at least 8 minutes after injection.
In the FBP cohort, the scanning ranges for CTU were from the top of the highest kidney to just below the bladder base (for unenhanced and excretory phases) and from the top of the highest kidney to just below the lowest kidney (for nephrographic phase). To facilitate planning on the basis of bony landmarks, reduce the number of incomplete studies, and improve the quality of liver imaging in patients with urothelial cell carcinoma, the scanning range was extended in the cohort undergoing CTU with AIDR 3D, with scanning performed from just above the highest portion of the highest diaphragm to the lower end of the symphysis pubis for unenhanced and excretory phases and from just above the highest portion of the diaphragm to the upper edge of the os ilium for the nephrographic phase.
For the FBP protocol, the MDCT scanning parameters used for both scanners were beam collimation of 64 × 0.5 mm, pitch of 0.828, tube rotation time of 0.5 second, tube voltage of 120 kVp, and 3D tube current modulation using a noise index SD of 23 (as defined by a slice width of 5 mm, a medium smooth reconstruction kernel [FC12], and a noise reduction technique [QDS+, Toshiba Medical Systems]) for the unenhanced phase and a noise index SD of 15 for the nephrographic and excretory phases, for which tube current was allowed to modulate between minimum and maximum values of 10 and 500 mA. Test images at the midureteral level were obtained using a step-and-shoot four-slice technique with the following parameters: collimation, 4 × 4–5 mm; tube voltage, 120 kVp; fixed tube current, 60 mA; tube rotation time, 0.5 second, and image reconstruction using a smooth soft-tissue kernel (FC11).
Fig. 1A —Two patients with macroscopic hematuria who underwent CT urography (CTU). Representative examples of image quality of CTU scans are shown.
A, 67-year-old man who underwent CTU with filtered back projection.
Fig. 1B —Two patients with macroscopic hematuria who underwent CT urography (CTU). Representative examples of image quality of CTU scans are shown.
B, 49-year-old man with macroscopic hematuria who underwent CTU performed with Adaptive Iterative Dose Reduction 3D (Toshiba Medical Systems).
Images were reconstructed with an FBP algorithm using a medium-smooth soft-tissue kernel (FC12) in both thick-slice (slice width, 5 mm; reconstruction index, 2.5 mm), and thin-slice (slice width, 1 mm; reconstruction index for curved or multiplanar reformatting and volume rendering, 1 mm) datasets (Fig. 1A). Test images were reconstructed at 5 mm with use of a smooth soft-tissue kernel (FC11).
After new software with 80-MDCT scanning capability and AIDR 3D were implemented on the Aquilion One scanner, the scanning parameters for the CTU protocol changed as follows: collimation, 80 × 0.5 mm; pitch, 0.813; tube rotation time, 0.5 second; tube voltage, 120 kVp; and 3D tube current modulation with a noise index SD of 23 (as defined by a slice width of 5 mm, reconstruction with AIDR 3D, and a soft-tissue kernel [FC07]) for the unenhanced phase and a noise index SD of 15 (as defined by a slice width of 5 mm, reconstruction with AIDR 3D, and a soft-tissue kernel [FC07]) for the nephrographic and excretory phases, for which tube current was allowed to modulate between minimum and maximum values of 10 and 500 mA. AIDR 3D was set at the standard strength. Test images at the midureteral level were obtained using a step-and-shoot four-slice technique with the following parameters: collimation, 4 × 5 mm; tube voltage, 120 kVp; fixed tube current, 60 mA; tube rotation time, 0.5 second; and image reconstruction using a smooth soft-tissue kernel (FC11). As recommended by the manufacturer, images were reconstructed with an iterative reconstruction algorithm using a new sharper soft-tissue kernel (FC07) that was optimized for AIDR 3D in both thick-slice (slice width, 5 mm; reconstruction index, 2.5 mm) and thin-slice (slice width, 1 mm; reconstruction index for curved or multiplanar reformatting and volume rendering, 1 mm) datasets (Fig. 1B). Test images were reconstructed at 5 mm with the use of a smooth soft-tissue kernel (FC11).

Dose Reports

A list of CTU examinations was created using our hospital information system. The list was read using dedicated software for retrospective analysis of the radiation dose administered. For each patient, the software retrieved a dose report in the form of either a structured DICOM file or a secondary-capture DICOM image from our PACS. The dose reports stored as secondary-capture images were converted into structured DICOM files after optical character recognition. Finally, all structured DICOM files were read, and information on the scanning range (including the over-range), the volume CT dose index, the dose-length product (DLP) of each individual phase, and total DLP was extracted. The effective dose for CT acquisition was estimated by applying effective dose-DLP conversion factors dedicated for exposure to 120-kVp MDCT abdomen acquisitions and was calculated according to recommendations in International Commission on Radiological Protection Publication 103 [13].

Qualitative Image Quality Evaluation

Sixteen patients with BMI values near the study average (i.e., 23.5–25.5) were randomly selected from each of the FBP and AIDR 3D cohorts, and the quality of their CTU images was evaluated. This BMI range reflects average reported weight (77.0 kg), height (1.74 m), and BMI (25.4) values for individuals in The Netherlands in 2011. Axial excretory phase images were scored on a 5-point scale by two independent observers who had 20 and 4 years of experience in abdominal imaging. For overall image quality, a score of 1 was non-diagnostic; 2, poor; 3, average; 4, good; and 5, excellent. For intrarenal lower pole papillary sharpness, upper ureter wall sharpness, pelvic ureter wall sharpness, and lateral bladder wall sharpness, a score of 1 indicated the image was strongly blurred; 2, blurred; 3, moderately unsharp; 4, mildly unsharp; and 5, sharp. For streak artifacts in the kidneys and streak artifacts in the small pelvis, scoring was as follows: 1, very severe; 2, severe; 3, moderate; 4, limited; and 5, none.

Statistics

Box-and-whisker plots were used to analyze variations in the recorded patient, scanning, and radiation dose parameters. These box-and-whisker plots were characterized by five parameters. The central line in the box represented the median value; the edges of the box, the 25th and 75th percentiles; and the whiskers, the minimum and maximum values.
Comparison of patient and radiation dose parameters was done using unpaired two-tailed t tests. Comparison of the qualitative image quality scores of patient images was done using an unpaired nonparametric test.
All calculations were performed using Excel 2011 for Mac (Microsoft) or SPSS software (version 20, IBM). Statistical significance was denoted by p < 0.05.

Results

Patient Parameters

The FBP and AIDR 3D cohorts included 82 patients who underwent three-phase CTU from January 2, 2011, through January 1, 2012, and 85 patients who underwent three-phase CTU from January 5, 2012, through January 11, 2013, respectively. Patient characteristics are summarized in Table 1. Although the median age of patients in the AIDR 3D cohort was slighter higher than that of patients in the FBR cohort, statistically significant differences were not noted for other characteristics.
TABLE 1: Characteristics of Patients in the Two Study Cohorts Who Underwent CT Urography (CTU)
ParameterCohort Undergoing CTU With FBP (n = 82)Cohort Undergoing CTU With AIDR 3D (n = 85)pa
Sex, no. of patients   
 Male47570.19
 Female35280.19
Age (y)65(56-71)70 (60-75)0.03
Height (m)1.72 (1.65-1.80)1.75 (1.70-1.82)0.13
Weight (kg)75.0 (65.0-85.0)78.0 (67.0-86.0)0.26
BMI25.3 (22.8-27.7)24.5 (22.3-28.0)0.50

Note—Data are median (25th to 75th percentiles), unless indicated otherwise. FBP = filtered back projection, AIDR 3D = Adaptive Iterative Dose Reduction 3D (Toshiba Medical Systems), BMI = body mass index (weight in kilograms divided by the square of height in meters).

a
Two-tailed unpaired t test.

Dose Parameters

For the complete three-phase CTU examination, the median (25th and 75th percentiles) DLP and effective dose were 924 mGy·cm (653, 1382 mGy·cm) and 13.0 mSv (9.2, 19.5 mSv), respectively, with the use of FBP reconstruction, and 421 mGy·cm (311, 589 mGy·cm) and 5.9 mSv (4.4, 8.3 mSv), respectively, with the use of AIDR 3D. These data and the median (25th and 75th percentiles) values for scanning range, volume CT dose index, DLP, and calculated effective dose for individual phases are shown in Table 2.
TABLE 2: Radiation Doses During Three-Phase CT Urography (CTU)
ParameterCohort Undergoing CTU With FBPCohort Undergoing CTU With AIDR 3Dp
No. of scanograms, mean ± SD2.1 ± 0.42.3 ± 0.80.10
No. of test scans, mean ± SD1.2 ± 0.61.4 ± 0.80.09
Complete three-phase CTU examination   
 Total DLP (mGy-cm)924(653-1382)433 (327-581)< 0.0001
 Total effective dose (mSv)13.0 (9.2-19.5)6.1 (4.6-8.2)< 0.0001
Unenhanced phase   
 Scan length (mm)372 (341-424)495 (474-522)< 0.0001
 CTDIVol (mGy)5.8 (4.4-9.0)2.3 (2.1-2.8)< 0.0001
 DLP (mGy-cm)218 (170-332)114 (106-130)< 0.0001
 Effective dose (mSv)3.1 (2.4-3.7)1.6 (1.5-1.8)< 0.0001
Nephrographic phase   
 Scan length (mm)238 (210-274)322 (301-349)< 0.0001
 CTDIvol (mGy)11.7 (8.4-18.1)3.6 (2.6-4.9)< 0.0001
 DLP (mGy-cm)300 (200-439)121(81-185)< 0.0001
 Effective dose (mSv)4.6 (3.1-6.7)1.7 (1.1-2.6)< 0.0001
Test scans   
 Scan length (mm)16 (16-20)20 (20-20)< 0.0001
 CTDIw (mGy)4.8 (3.1-4.8)3.2 (3.2-3.4)< 0.0001
 DLP (mGy-cm)8 (6-8)6 (6-7)0.94
 Test effective dose (mSv)0.11 (0.09-0.12)0.09 (0.09-0.10)< 0.0001
Excretory phase   
 Scan length (mm)354(325-386)494 (459-520)< 0.0001
 CTDIvol (mGy)12.1 (8.5-17.9)3.8 (2.9-4.9)< 0.0001
 DLP (mGy-cm)441 (297-640)190 (140-255)< 0.0001
 Effective dose (mSv)6.2 (4.2-9.0)2.7 (2.0-3.6)< 0.0001

Note—Data are median (25th to 75th percentiles), unless indicated otherwise. FBP = filtered back projection, AIDR 3D = Adaptive Iterative Dose Reduction 3D (Toshiba Medical Systems), DLP = dose-length product, CTDIvol = volume CT dose index, CTDIw = weighted CT dose index.

Table 2 shows that even through the scan range for all individual phases increased by 33–38%, the median DLP and effective dose were reduced by 53% when the number of phases was kept constant between the FBP and the AIDR 3D cohorts. The 25th-to-75th percentile interval for the DLP and effective dose is wider in the FBP cohort, mainly because of the higher variability in scanning ranges in this cohort.

Qualitative Image Quality Scoring

On the basis of image quality scores for 16 patients in the FBP reconstruction cohort and 16 patients in the AIDR3D cohort, observers 1 and 2 calculated average (± SD) image quality scores for each technique used. For FBP reconstruction, the scores were as follows: overall image quality, 4.6 ± 0.5; lower pole papillary sharpness, 4.6 ± 0.4; renal streak artifacts, 4.3 ± 0.6; upper ureter sharpness, 4.6 ± 0.4; pelvic ureter sharpness, 4.2 ± 0.4; bladder wall sharpness, 4.6 ± 0.4; and pelvic streak artifacts, 3.9 ± 0.5. For AIDR 3D reconstruction, the scores were as follows: overall image quality, 4.5 ± 0.4; papillary sharpness, 4.6 ± 0.4; renal streak artifacts, 4.5 ± 0.3; upper ureter sharpness, 4.5 ± 0.5; pelvic ureter sharpness, 4.3 ± 0.5; bladder wall sharpness, 4.6 ± 0.4; and pelvic streak artifacts, 4.3 ± 0.4.
The only significant finding revealed by qualitative image quality scoring was the relative reduction in streak artifact in the pelvis noted for AIDR 3D reconstruction. No other significant differences between FBP and AIDR 3D reconstruction were found on review of qualitative image quality scoring.

Discussion

The results of this study, which represent a large inventory of CTU radiation doses, reveal a median DLP of 924 mGy·cm and a median effective dose of 13.0 mSv when a single-bolus three-phase CTU protocol with FBP reconstruction is used for a population at high risk for urothelial cell carcinoma. The scanning parameters and resulting median dose values for all individual phase images are well in line with the recommendations outlined by the European Society of Urogenital Radiology in 2008 [1]. Nevertheless, wide intervals for 25th to 75th percentiles still reflect considerable spread in the CTU findings, mainly because scanning was done relative to renal and bladder positions that are more difficult to judge on scout images.
Although scanning range increased as a result of the extended acquisition protocol, the introduction of iterative reconstruction reduced the radiation dose associated with the three-phase protocol by 53%, to a median DLP of 433 mGy·cm and a median effective dose of 6.1 mSv, with less variability noted in the scanned range. Introduction of iterative reconstruction into the CTU protocol led to this reduction in the mean effective dose while maintaining clinical diagnostic image quality. An added benefit of iterative reconstruction is a reduction in the number of streak artifacts, which has been found to be especially significant in imaging of the small pelvis. This finding has also been seen in other studies of iterative reconstruction techniques [10, 14].
When a modular approach is used in the design of CTU protocols, the median dose values for the individual phases allow CT users to gain a fair impression of the radiation dose that can be achieved with other CTU protocols used in daily practice. In the past, patients who were undergoing a three-phase CTU examination could regularly be exposed to effective doses of 40 mSv and higher. With advances in CT technology, iterative reconstruction, and protocol optimization, a complete three-phase CTU examination can now be performed with an effective dose as low as 6 mSv and a DLP as low as 430 mGy·cm while preserving the required diagnostic image quality. A split-bolus single-phase CTU examination (performed during nephrogenic-excretory combination phase) could have a median effective dose of 2.7 mSv and a DLP of 190 mGy·cm, and a split-bolus two-phase CTU examination (performed during unenhanced and concurrent nephrogenic and excretory phases) could have a median effective dose of 4.2 mSv and a DLP of 300 mGy·cm.
The radiation exposure risks associated with such low effective doses are very small. With the use of an effective dose of 6 mSv, and with a radiation-induced mortality rate of 4% per sievert of radiation exposure [15], the radiation-induced mortality rate associated with a three-phase CTU examination is 1 in 4000. The calculated risk will be even much lower when the advanced age of the typical patient undergoing CTU is taken into account. Our overall 53% reduction in the median DLP is similar to or better than previously published results obtained using similar iterative reconstruction techniques.
One retrospective study compared normal-dose to low-dose CTU protocols that used automatic selection of x-ray tube voltage in combination with iterative reconstruction (Sinogram Affirmed Iterative Reconstruction [SAFIRE] S3 setting, Siemens Healthcare). Either split-bolus two-phase (60%) or single bolus three-phase (40%) protocols were used with scanning parameter settings in the same range as ours. Although the iterative reconstruction settings are relatively difficult to compare, the median reduction in the radiation output of the scanner (i.e., the volume CT dose index) was 40% [7]. Another small study of single bolus three-phase CTU that used the same iterative reconstruction technique used in our study (AIDR 3D) but that operated at a different AIDR 3D strength and noise index (with an SD of 20) in the excretory phase showed that even higher reductions in scanner output (a 70% reduction in the volume CT dose index) may be feasible, but no DLP data were provided [8]. Previously, the same group showed a slightly lower reduction in the DLP in a CT examination with one portal phase acquisition with a lower noise index and comparable AIDR 3D strength setting [9].
Some limitations of our data warrant mention. First, as is routine in all dose surveys of clinical practice, the study was retrospective in nature. This results in inherent bias in patient selection. Second, the number of observers who participated in the qualitative imaging evaluation was small. Third, we evaluated an iterative reconstruction protocol with parameters based on suggestions made by the vendor at the time of installation. These parameters may not represent the most optimal settings to be used with the newest iterative reconstruction implementations. Furthermore, our parameters included not only a change in reconstruction technique but also a newer reconstruction kernel. Hypothetically, the benefit to image quality may therefore, to some extent, result from this kernel.
In conclusion, we present more recent and more detailed three-phase CTU dose data, obtained before and after iterative reconstruction implementation, that are lower than previously reported dose data. After implementation of iterative reconstruction, the effective dose reached a level of 6.1 mSv for a complete three-phase CTU examination in which clinical diagnostic image quality was preserved.

Footnote

Based on a presentation at the ARRS 2012 Annual Meeting, Vancouver, BC, Canada.

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 572 - 577
PubMed: 26295643

History

Submitted: September 17, 2014
Accepted: January 31, 2015

Keywords

  1. Adaptive Iterative Dose Reduction in 3D
  2. CT iterative reconstruction
  3. radiation dosage
  4. urography

Authors

Affiliations

Aart J. van der Molen
All authors: Department of Radiology, C-2S, Leiden University Medical Center, Albinusdreef 2, NL-2333 ZA Leiden, The Netherlands.
Razvan L. Miclea
All authors: Department of Radiology, C-2S, Leiden University Medical Center, Albinusdreef 2, NL-2333 ZA Leiden, The Netherlands.
Jacob Geleijns
All authors: Department of Radiology, C-2S, Leiden University Medical Center, Albinusdreef 2, NL-2333 ZA Leiden, The Netherlands.
Raoul M. S. Joemai
All authors: Department of Radiology, C-2S, Leiden University Medical Center, Albinusdreef 2, NL-2333 ZA Leiden, The Netherlands.

Notes

Address correspondence to A. J. van der Molen ([email protected]).

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