Comparison of Radiation Exposure Associated With Intraoperative Cone-Beam Computed Tomography and Follow-up Multidetector Computed Tomography Angiography for Evaluating Endovascular Aneurysm Repairs

Abstract

Purpose: To compare the radiation exposure associated with intraoperative contrast-enhanced cone-beam computed tomography (ceCBCT) acquisitions to standard 3-phase multidetector computed tomography (MDCT) angiography used for assessing technical success after endovascular aortic repair (EVAR). Methods: Effective doses (EDs) were calculated for 66 EVAR patients (mean age 71 years; 61 men) with a mean 27.7-kg/m² body mass index (range 17–49) who had both intraoperative ceCBCT and postoperative 3-phase MDCT angiography between November 2012 and April 2015. In addition, EDs were directly determined using thermoluminescent dosimeters (TLDs) embedded in anthropomorphic phantoms with body mass indexes of 22 and 30 kg/m². Effective doses were calculated by summing doses recorded by all TLDs corresponding to a specific tissue type before applying the International Commission on Radiological Protection (ICRP) 60 and 103 weighting factors. EDs were compared with each other for both imaging modalities as well as to TLD measurements. Results: Average EDs of the patient collective were 4.9±1.1 mSv for ceCBCT, 2.6±1.2 mSv for single-phase MDCT (46% decrease, covering solely the area of the implanted endograft), and 13.6±5.5 mSv for comprehensive 3-phase MDCT examinations (178% increase, anatomical coverage from the aortic arch to femoral artery bifurcation). EDs determined in phantom measurements ranged from 3.1 to 4.5 mSv for ceCBCT, amounting to 2.6 mSv for a single MDCT phase (15% to 40% decrease) using ICRP 60 conversion factors. Applying ICRP 103 factors resulted in higher values for ceCBCT and slightly lower ones for MDCT. Conclusion: ceCBCT offers the chance for immediate intraoperative revisions of endograft-related problems. Requiring only a single-phase acquisition, ceCBCT is associated with a considerable reduction in ED (50%–75%) compared to standard 3-phase MDCT angiography after EVAR. On the other hand, MDCT has a larger field of view and is associated with less radiation exposure for a single phase (reduction of 20%–60%) if only the stented region is covered; however, MDCT angiography also uses larger amounts of contrast.

Keywords

abdominal aortic aneurysm cone beam computed tomography computed tomography angiography DynaCT endovascular aneurysm repair effective dose anthropomorphic phantom surveillance imaging

Introduction

Endovascular aneurysm repair (EVAR) has become the primary treatment option in anatomically suitable patients with infrarenal abdominal aortic aneurysms (AAA).^1,2 While EVAR is associated with reduced perioperative mortality compared to open repair,^3,4 significant rates (34%) of endograft-related complications prompting early reinterventions (<30 days) in up to 10% of the patients have been reported.^3,5
–7 These early reinterventions for type I/III endoleaks, structural complications (limb stenosis/kinking), limb thrombosis, and accidental renal artery coverage were not detected on intraoperative completion digital subtraction angiography but were diagnosed with postoperative 3-phase computed tomography angiography.^1,5

There has been a recent increase in the use of intraoperative 3-dimensional (3D) flat-panel detector angiography systems that acquire contrast-enhanced cone-beam computed tomography (ceCBCT) for assessing the technical success of EVAR.^1,8,9 This imaging modality captures intraoperative 3D cross-sectional images of the implanted endograft, enabling immediate correction of complications. In contrast, MDCT is usually completed within the first week postsurgery, and complications detected at that time require secondary interventions for repair.^1,5,10,11

Currently, MDCT angiography is the gold standard for EVAR follow-up.¹ It consists of 3 phases: nonenhanced, arterial, and venous. Whereas the nonenhanced and arterial phases often cover the whole thorax, abdomen, and parts of the pelvis, the venous phase usually images only the stented region. Therefore, only the venous phase of the MDCT protocol corresponds to the ceCBCT acquisition over the region of the implanted graft. However, nonenhanced and arterial phases of the MDCT protocol might offer additional diagnostic information.

Both techniques seem equally suitable for assessment of EVAR outcome and complications, since initial studies have shown that ceCBCT can reliably detect all endograft-related complications seen on postoperative MDCT angiography.^1,5,8,9,12 Technically, the image acquisition with a flat-panel detector used in ceCBCT is different compared with helical MDCT acquisitions. A helical MDCT acquisition is started with a localizer for automatic determination of the appropriate tube potential and current, modulating the tube current–time product per body region and slice during the actual image acquisition to obtain image data of high diagnostic quality. During a ceCBCT acquisition, the flat-panel detector is irradiated until it reaches a certain preset exposure per frame associated with a signal threshold providing a good signal-to-noise ratio throughout the whole scan field of view (FOV). To this end, targeted tube potential (usually 90 kV_p for body protocols) and current are adjusted dependent on absorption per frame, resulting in the use of higher tube potentials of up to 125 kV_p for lateral projections, especially in larger patients.

Depending on image modality, patient dose data recorded automatically after completion of each imaging exam provide dose area products (DAPs) for ceCBCT and dose length products (DLPs) for CT. DAPs and DLPs are not directly comparable. For evaluating the radiation burden associated with the application of these different techniques, it is therefore necessary to determine a common denominator, which is effective dose, expressed in millisieverts (mSv). For MDCT examinations, effective dose is usually estimated from DLP by using body region–specific conversion factors. For ceCBCT, there are different approaches for determining conversion factors from DAP to effective dose.^13
–16 However, these conversion factors vary with technical implementation and additionally depend on body region and patient body mass index (BMI), potentially resulting in large errors of estimated effective dose.

To date, there are no valid data comparing the radiation exposure of patients examined with ceCBCT and postoperative MDCT angiography for assessing technical success after EVAR, despite the potentially high interest in the radiation burden associated with the use of each technique. Therefore, the aim of the present study was to compare the radiation exposure resulting from intraoperative flat-panel detector C-arm CT acquisition to a standard follow-up MDCT examination.

Methods

Study Design

This 2-part study was approved by the local ethics committee and conducted in accordance with the ethical standards of the Declaration of Helsinki. All patients gave written informed consent for participation in the study. The clinical component involved a retrospective analysis of imaging data from 66 EVAR patients (mean age 71 years, range 49–82; 61 men) with a mean BMI of 27.7 kg/m² (range 17–49) who had both intraoperative ceCBCT and postoperative 3-phase MDCT angiography examinations between November 2012 and April 2015. In the experimental component, an anthropomorphic Rando Alderson phantom was fitted with thermoluminescent dosimeters (TLDs) for acquiring quantitative dose values in a standardized manner for corresponding phantom measurements. Phantom acquisition protocols were matched to the patient collective by modality.

Patient Imaging

Intraoperative ceCBCT (Syngo DynaCT, Artis Zeego; Siemens Healthcare GmbH, Forchheim, Germany) was used to assess technical success of EVAR. Patients were imaged using the 5s DCT Body Care protocol by acquiring projections from the 90° left anterior oblique (LAO) to the 110° right anterior oblique (RAO) to cover 180° (+20° fan angle). After a delay of 2 seconds for injection of 45 mL iodinated contrast agent (UltraVist 300 mg I/mL; Bayer HealthCare AG, Berlin, Germany) diluted in 25 mL of saline with a flow of 10 mL/s, 248 frames (system dose 0.36 µGy/f) at intervals of 0.8° were acquired over 5 seconds. The detector (30×40 cm) was used in portrait mode without collimation, applying a diagonal zoom of 480 mm. (The portrait mode acquisition protocol was established by the manufacturer of the angiographic system at our institution as a new protocol in addition to the commercially available landscape mode protocols.) The change of detector orientation from landscape to portrait mode orientation increases the height of the acquired ceCBCT dataset from 185 to 237 mm thus reliably covering the abdominal aorta from above the renal arteries down to the hypogastric arteries. In the z-axis direction, collimation can be used if the region of interest is shorter. Average tube potential was 110.8±7.2 kV, and average tube current was 429.6±23.9 mA. Acquisitions were performed in apnea, covering a cylindrical 200-mm-diameter FOV and 237 mm of z-axis coverage. Radiation exposure (documented in terms of DAP), tube potential, and tube current values recorded for the ceCBCT image series were archived for each DynaCT examination.

Within 1 week after surgery, all patients had a 3-phase MDCT examination (Somatom Definition Flash; Siemens Healthcare GmbH) using 90 mL of nonionic iodinated contrast (Imeron 350 mg I/mL; Bracco Imaging Deutschland GmbH, Constance, Germany). The scan range for the nonenhanced and arterial phases covered the region from the aortic arch to the bifurcation of the femoral artery. The venous phase solely covered the stented region. Tube potential was automatically adapted according to patient constitution and contrast-enhancement phase (CARE kV with contrast settings of 11; Siemens Healthcare GmbH); the tube current–time product (in mAs) was also automatically modulated (CARE Dose 4D; Siemens Healthcare GmbH). Scans were acquired in full inspiration in the craniocaudal direction using a collimation of 2×64×0.6 mm, a gantry rotation time of 0.5 seconds, and a pitch of 1.2. Because of the use of CARE kV, the tube potential varied between 80 and 120 kV_p per patient. An overview of the obtained patient distribution, including DLPs and acquisition coverage per protocol phase, is displayed in Table 1.

Table 1.

Average Dose Length Products and Scan Lengths for Each Phase and per Tube Potential.

Variable	Nonenhanced Phase	Arterial Phase	Venous Phase	Venous Phase per Tube Voltage
MDCT acquisitions^a per tube potential, kV_p
80	34	34	43
100	30	30	21
120	2	2	2
DLP, mGy·cm^b	326.1±135.1	324.6±135.5	158.9±70.7	124.4±35.1 at 80 kV_p
				207.0±49.9 at 100 kV_p
				396.5±23.5 at 120 kV_p
Scan length, cm^b	56.2±10.8	56.0±10.2	24.5±6.9	25.1±7.1 at 80 kV_p
				23.8±5.0 at 100 kV_p
				20.9±1.9 at 120 kV_p

Abbreviations: DLP, dose length product; MDCT, multidetector computed tomography.

Acquisitions by automatically selected tube potential (CARE kV) for each phase.

Data are presented as the mean ± standard deviation.

The patient dose protocol (scan summary) of each of the MDCT acquisitions was used to extract the following acquisition parameters: DLP, tube potential, and tube current–time product values. Scan lengths were determined from the image datasets on a per-patient basis, thereby dividing the body into 3 anatomical regions: thorax (first imaged slice to the base of the heart), abdomen (heart apex to the sacrum), and pelvis (sacrum to the last imaged slice). This subdivision was in accordance with a classification recommended by an experienced radiologist (>10 years of experience) and used to calculate the percentage of the DLP belonging to the thorax, abdomen, and pelvis (Figure 1), respectively.

Phantom Measurements

For phantom measurements, an anthropomorphic phantom (Rando Alderson; The Phantom Laboratory, Salem, NY, USA) with a BMI of 22 kg/m² was equipped with lithium fluoride TLDs in the cardiac region, the cranium, liver, and vertebral column. In order to reflect overweight patients, a fat extension ring (QRM GmbH, Möhrendorf, Germany) was additionally fitted around the phantom (Figure 2) to increase the BMI to 30 kg/m². The phantoms underwent CBCT acquisition (DynaCT) using the aforementioned protocol but without contrast enhancement.

For the MDCT examination, a body angiography protocol (without the application of an iodinated contrast) was employed, corresponding to the venous phase of our aortic angiography protocol in order to make the MDCT and ceCBCT acquisition data comparable. Tube potential was 80 kV_p for the phantom without a fat extension ring and 100 kV_p with the fat extension ring in place. The scan protocol with a coverage of 24.0 cm had a DLP of 112.0 mGy·cm for the 22 BMI phantom and 195.8 mGy·cm for the 30 BMI phantom (Table 2). DLPs, scan coverage and lengths, and tube potentials were determined from the patient collective prior to the phantom acquisitions. In this way, tube potentials and DLPs of the phantom examinations were fitted to the tube potentials, DLPs, and BMIs of the real patient cohort in the best possible way (Table 2). No phantom acquisitions were performed with a tube potential of 120 kV_p as only 2 (3%) patients were scanned at this potential (Table 1).

Table 2.

Summary of MDCT Parameters for Venous Phase Acquisition Selected to Enable Direct Comparison to the ceCBCT Acquisition (DynaCT).^a

Parameter	Patients		Phantom
Tube potential, kV_p	80 (n=43)^b	100 (n=21)^b	80^c	100^c
BMI, kg/m²	26.6	31.1	22.0	30.0
Effective tube current–time product, mAs (reference value)	223.7±30.2 (239.0)	188.5±31.7 (138.0)	206.7^d (394.0)^c	171.7^d (212.0)^c
DLP, mGy·cm^e	124.4±35.1	207.0±49.9	112.0^d	195.8^d
Scan length, cm	25.1±7.1	23.8±5.0	24.0^c	24.0^c

Abbreviations: BMI, body mass index; ceCBCT, contrast-enhanced cone-beam computed tomography (DynaCT); DLP, dose length product; MDCT, multidetector computed tomography.

Data are presented as the mean ± standard deviation for the patient collective. Two patients were examined using a tube potential of 120 kV_p (not included in this table).

Automatic selection by CARE kV.

Manual selection (fixed).

Resulting from manual parameter selection (fixed).

Note that DLPs of phantom measurements were 5% to 10% lower than the DLPs of matching patient examinations due to the use of the automatic exposure control on the MDCT scanner.

For both modalities, all acquisitions were performed thrice, and resulting dose values were averaged. To this end, the TLDs were evaluated considering the calibration factor (determined prior to the phantom examinations) and zero effect. Effective doses to each exposed tissue were calculated by summing the doses recorded by all TLDs corresponding to a specific tissue type before applying the International Commission on Radiological Protection (ICRP) 60 weighting factors¹⁷ and 103 weighting factors.¹⁸

Radiation Exposure and Effective Dose

Conversion factors from DAP to effective dose are hardware- and BMI-dependent for flat-panel detectors, as other investigators have described.^15,16 No direct conversion factor was available for ceCBCT acquisitions of the system used in our study (Syngo DynaCT, Artis Zeego). In spite of similarities in hardware, a direct application of the conversion factors used by Suzuki et al¹⁵ was not possible, as the tube potential for their phantom acquisitions was considerably lower (72–79 kV_p, depending on phantom size). Furthermore, no comparison with Gosch et al¹⁶ was possible, as they used a different filtration and irradiated FOV. Therefore, in order to determine effective dose per patient from DAP of each respective ceCBCT acquisition, the following procedure was applied. The effective dose of each respective phantom TLD measurement at BMI 22 and BMI 30 (see above) was divided by its corresponding DAP associated with the phantom measurement. Subsequently, a DAP-to-effective-dose conversion factor for each specific BMI was interpolated on a per-patient basis using the conversion formula thus obtained for ICRP 60 and ICRP 103, respectively. For each patient’s ceCBCT acquisition, effective dose was then calculated by multiplication of the examination’s DAP with the thus determined BMI-specific conversion factor.

For MDCT examinations, the gold standard ICRP 60 conversion factors (CF) from DLP to effective dose for scans of thorax, abdomen, and pelvis were taken from the EU 16262 guidelines.¹⁹ The more recent ICRP 103 conversion factors were taken from a publication by Huda et al.²⁰ ICRP 60 conversion factors are 0.017 mSv/mGy·cm for the thorax, 0.015 mSv/mGy·cm for the abdomen, and 0.019 mSv/mGy·cm for the pelvis. ICRP 103 conversion factors are 0.0204 mSv/mGy·cm for the thorax, 0.0163 mSv/mGy·cm for the abdomen, and 0.0143 mSv/mGy·cm for the pelvis. The effective dose (D_eff) per body region (BR), for example, the thorax, was calculated as D_eff = CF_thorax × DLP × BR_thorax. In this equation, BR is used as a weighting factor allowing rescaling of the DLP of each MDCT phase acquired to the fraction of each body region covered relative to the total anatomic scan coverage of the acquired phase. For estimation of total effective dose, effective doses of each of the 3 body regions potentially covered by the MDCT acquisition were summed.

Statistical Analysis

Student t tests (2-sided, assuming equal variances) were applied to determine significant differences between results. A p<0.05 was considered significant. Comparisons were made between patient BMI subgroups (<25, 25–30, and >30 kg/m²) for DAP (ceCBCT), DLP (MDCT), and effective dose.

Results

The average effective doses associated with the ceCBCT acquisitions of the patient cohort for the intraoperative assessment of EVAR success were 4.9±1.1 mSv for ICRP 60 and 5.6±1.3 mSv for ICRP 103 (Table 3). The average effective doses (for ICRP 60) did not differ significantly for different BMI groups: 4.7±1.2, 5.1±1.0, and 4.8±1.1 mSv for the BMI groups <25, 25–30, and >30 kg/m², respectively (all p≥0.25), and 5.3±1.4, 5.8±1.2, and 5.4±1.3 mSv, respectively, for ICRP 103.

Table 3.

Effective Dose Data Averaged Over All BMI Groups.^a

Imaging Procedure	ceCBCT	MDCT Venous Phase	MDCT 3-Phase	Difference, %
Imaging Procedure	ceCBCT	MDCT Venous Phase	MDCT 3-Phase	ceCBCT vs Venous MDCT	ceCBCT vs 3-Phase MDCT
ICRP 60¹⁷
ED, mSv	4.9±1.1 (1.7, 7.8)	2.6±1.2 (0.9, 6.8)	13.6±5.5 (6.6, 37.3)	+88	–64
ICRP 103¹⁸
ED, mSv	5.6±1.3 (2.0, 8.8)	2.5±1.1 (0.9, 6.6)	13.4±5.6 (6.2, 37.5)	+124	–58

Abbreviations: BMI, body mass index; ceCBCT, contrast-enhanced cone-beam computed tomography (DynaCT); ED, effective dose; ICRP, International Commission on Radiological Protection; MDCT, multidetector computed tomography.

Mean ± standard deviation (minimum, maximum).

The average DAP of the ceCBCT in the patient collective was 4405±1053 µGy·m² (range 1948–7131). The mean DAP increased significantly with patient BMI: 3694.0±1021.4 µGy·m² for BMIs <25, 4451.8±950.9 µGy·m² for BMI 25–30, and 5112.0±722.0 µGy·m² for BMI >30 kg/m² (all p≤0.02).

The majority [43 (65%) patients] of the MDCT scans were acquired with a tube potential of 80 kV_p, while 21 (32%) patients were scanned with a tube potential of 100 kV_p. Only 2 (3%) patients had scans at 120 kV_p. The average scan length of the venous phase was considerably shorter than that of the arterial and nonenhanced phases (~24 cm compared with about 56 cm). The average DLP of the venous phase acquisition was therefore also considerably lower (~160 mGy·cm compared to about 325 mGy·cm).

The average effective doses calculated from the DLPs of the venous phase acquisition were 2.6±1.2 mSv (ICRP 60) and 2.5±1.1 mSv (ICRP 103). The complete MDCT acquisition consisting of 3 phases amounted to 13.6±5.5 mSv (ICRP 60) and 13.4±5.6 mSv (ICRP 103). Hence, the contribution of the venous phase to total effective dose of the 3-phase MDCT was about 20% (Table 3). The average effective dose (for ICRP 60) increased significantly with patient BMI (all p≤0.01): 1.8±0.6, 2.4±0.7, and 3.9±1.3 mSv for the BMI groups <25, 25–30 and >30 kg/m², respectively, and 1.8±0.6, 2.3±0.6, and 3.8±1.3 mSv for ICRP 103, respectively. Average DLP of the venous phase acquisition also increased significantly with patient BMI: 111.2±33.6 mGy·cm for BMIs <25, 145.4±41.3 mGy·cm for BMI 25–30, and 239.8±80.5 mGy·cm for BMI >30 kg/m² (all p<0.01).

Phantom

Using CBCT, the average effective doses determined with TLD measurements were 3.1 mSv for the “thin” phantom (BMI 22) and 4.5 mSv for the “large” phantom with the fat ring (BMI 30) for ICRP 60; for ICRP 103, the corresponding values were 3.5 and 5.1 mSv (Table 4). The average effective dose of the phantom measurements was about 20% lower than expected from the corresponding patient data (see above). The DAPs of the CBCT phantom acquisitions were 2350.0 µGy·m² (BMI 22) and 4258.0 µGy·m² (BMI 30).

Table 4.

Radiation Exposure Determined in Phantom TLD Measurements.

Imaging Procedure	CBCT	MDCT Venous Phase	MDCT 3-Phase^a	Difference, %
Imaging Procedure	CBCT	MDCT Venous Phase	MDCT 3-Phase^a	CBCT vs Venous MDCT	CBCT vs 3-Phase MDCT
ICRP 60
ED for BMI 22, mSv	3.1	2.6	12.5	+19	–75
ED for BMI 30, mSv	4.5	2.6	12.5	+73	–64
ICRP 103
ED for BMI 22, mSv	3.5	2.0	9.6	+75	–64
ED for BMI 30, mSv	5.1	2.1	10.1	+143	–50

Abbreviations: BMI, body mass index; CBCT, cone-beam computed tomography (DynaCT); ICRP, International Commission on Radiological Protection; MDCT, multidetector computed tomography; TLD, thermoluminescent dosimeter.

Effective dose of comprehensive MDCT = E_venous + 2 ∙ 1.9 ∙ E_venous (see main text).

For MDCT, the average effective doses determined from TLD measurements for a single (venous) phase acquisition were 2.6 mSv (BMI 22 acquired at 80 kV_p as well as BMI 30 acquired at 100 kV_p) for ICRP 60 and 2.0 mSv (BMI 22, 80 kV_p) and 2.1 mSv (BMI 30, 100 kV_p) for ICRP 103 (Table 4). Note that the DLPs of the 2 MDCT phantom acquisitions were matched to those of the average patient collective for each tube potential (Table 2). To approximate the corresponding radiation exposure of a 3-phase MDCT examination from the phantom measurements, effective doses were multiplied by a factor of 1.9 because in the patient collective the DLPs of the nonenhanced and arterial phase were about 1.9 times the DLP of the venous phase for both 80 kV_p and 100 kV_p acquisitions due to larger scan coverage. Thus, the effective dose of a 3-phase MDCT phantom acquisition = E_venous(kV_p) + 2 × 1.9 × E_venous(kV_p), with kV_p corresponding to the tube potential used for acquisitions at either low or high BMI (80 kV_p for BMI 22, 100 kV_p for BMI 30), and E(kV_p) therefore referring to the effective doses of TLD measurements at 80 kV_p or 100 kV_p. This resulted in effective dose estimates of 12.5 mSv (for both BMIs) for ICRP 60 and 9.6 mSv (BMI 22) and 10.1 mSv (BMI 30) for ICRP 103 (Table 4).

Discussion

This study compared the radiation exposure associated with intraoperative ceCBCT with that of follow-up MDCT angiography in a patient cohort, as well as to effective doses directly measured in an anthropomorphic phantom study using TLDs. For the phantom study, acquisition protocols matching the average acquisition parameters of the corresponding patient collective were applied for both ceCBCT and corresponding single venous phase MDCT acquisitions.

Patient data derived from our study population show that ceCBCT acquisition is associated with an average effective dose ~90% to 125% higher than that of a single venous phase MDCT acquisition covering the same body region (Table 3). However, when considering the MDCT protocol that has actually been used for examining these patients (3-phase, coverage from the aortic arch to femoral bifurcation), ceCBCT reduced the average effective dose by ~60% to 65% (Table 3). Corresponding phantom measurements indicated that the effective dose of an intraoperative ceCBCT acquisition was between ~20% and 140% higher than a standard venous phase follow-up MDCT examination covering the same body region, but between ~50% and 75% lower compared to a 3-phase MDCT examination (Table 4), depending on simulated patient BMI (lower relative differences for BMI 22) and the ICRP conversion factors used (ICRP 103 results in larger relative differences).

These differences in effective dose were due to the way the images are acquired. MDCT makes use of a localizer, thereby adjusting the tube current–time product per imaged slice, taking patient size and attenuation into account. The flat-panel detector of the CBCT system, however, is homogeneously irradiated until a certain exposure threshold per frame is reached, thereby ensuring a proper signal-to-noise ratio throughout each projection, but eventually leading to a higher exposure.

Although the same ceCBCT protocols for the patient and phantom study were used, the DAP of the patient cohort was about 20% higher than the measured DAP from the phantom acquisitions. The difference may be attributed to the different absorption characteristics of a human and the phantom. A phantom can model the human anatomy and structure characteristics only to a certain degree. Although the phantom does include a human skeleton, its overall absorption is lower than that of a human body, which results in a higher DAP for patients with the same BMI as the phantom. The same effect is also visible to a lesser degree for DLP of MDCT. Increasing absorption is also the reason why DAP and DLP increase with patient BMI.

In 2012, Bai et al²¹ published a comparison of effective doses resulting from CBCT and multislice CT (MSCT) examinations of the head, chest, and abdomen using the same anthropomorphic phantom as used in the current study. The effective doses they determined for routine abdomen examinations, calculated with ICRP 103 weighting factors, were considerably higher than those we determined. For ceCBCT, reported effective doses were 7.48 mSv (20-second protocol) and 7.04 mSv (8-second protocol); for MSCT, effective doses ranged from 8.23 to 8.42 mSv.²¹ Hence, the average effective doses the Bai group reported were about 100% and 320% higher for ceCBCT and MSCT, respectively, than our results for the “thin” phantom (BMI 22). In part, these large differences may be attributed to the use of different acquisition protocols. For CBCT, Bai et al²¹ reported lower average tube potential and current but a much larger number of projections (500 for the 20-second acquisition, factor of ~2). For MSCT, a tube potential of 120 kV_p was used together with a tube current–time product comparable to that of the 2 patients we examined at 120 kV_p (associated with an average effective dose of 6.25 mSv for the venous phase MDCT acquisition in our study); however, this differs from the 80 kV_p or 100 kV_p used most often in our patient cohort. Furthermore, Bai and colleagues give no information about the exact scan lengths of their acquisitions, making a direct comparison with our results difficult. In addition to using different acquisition protocols, Bai et al²¹ applied other imaging hardware that now can be considered as dated.

From a clinical viewpoint, the choice, usage, benefit, and limitations of ceCBCT as an immediate 3D control of an interventional procedure is well documented in the literature, though mainly with regards to image quality, added clinical value, and additional patient contrast dose.^{1,5,8,11,12,21
–23} A recent publication by Törnqvist et al¹ compares conventional angiography (CA), 3-phase MDCT, and ceCBCT with regard to the detection of endoleaks. They demonstrated that combining CA and CBCT provides the same relevant clinical information as follow-up MDCT examinations 1 month after intervention, making MDCT angiography “unnecessary in some patients.” As an alternative during or after EVAR, other modalities could be included, such as ultrasound or magnetic resonance imaging.^1,24

Determination of conversion factors from DAP to effective dose for interventional procedures employing ceCBCT is a topic of several further publications.^{13
–16,25
–30} To date, a general conversion factor is not available, as effective dose is not only dependent on the imaged body part but also on patient BMI¹⁴ and the technical design of the imaging system (including different filtration).^15,16,26 Furthermore, there is no general ceCBCT protocol among hospitals, which leads to highly varying DAPs (700–7000 µGy·m²) and published conversion coefficients.^{1,5,8
–10,12}

Off hand, it should be possible to compare the CBCT system used in the current study to the system described in the Suzuki study¹⁵ with comparable hardware and setup. They used the AXIOM Artis dTA (Siemens Healthcare GmbH) CBCT system with a 5-second protocol, acquiring images from 100° RAO to 100° LAO and tube potentials of 72 to 79 kV_p, depending on phantom size. Our automatically selected tube potentials were notably higher: 90 kV for the 22-BMI phantom and 103 kV for the 30-BMI phantom. For our patient cohort, the tube potential was even higher (average 110.8 kV). For this reason, we chose not to use the conversion factors published by Suzuki et al,¹⁵ which ranged from 0.13 to 0.15 mSv/Gy·cm², decreasing with increasing phantom BMI. The conversion factors in the current study ranged from 0.11 to 0.15 mSv/Gy·cm², showing the same decrease with increasing patient BMI. Despite the different tube potentials used, these conversion factors are of comparable order.

As MDCT angiography is the gold standard for EVAR follow-up,¹ several methods have been introduced to reduce the radiation exposure associated with this CT examination. One approach in reducing effective dose to the patients during follow-up was reported 2006 by Macari et al.³¹ They evaluated the necessity of the arterial phase in a standard 3-phase MDCT protocol and came to the conclusion that this phase does not contribute to the detection, classification, or determination of etiology of endoleaks.³¹ Stolzmann et al³² and Buffa et al³³ studied the use of dual-source, dual-energy CT (DS-DECT) in the late venous phase as an alternative to a 3-phase MDCT protocol. They claimed that one DECT acquisition in the delayed phase with reconstruction of virtual nonenhanced images is sufficient for detecting any endoleaks, thereby lowering the radiation exposure substantially.^32,33

Tube potential selection by automatic exposure control now implemented in modern CT scanners decreases the radiation dose by 18% compared to a standard 120 kV_p protocol.³⁴ Although our patient collective mainly consisted of overweight (25<BMI<30 kg/m²) or obese (BMI >30 kg/m²) patients (73% in total), only 2 patients received examinations with a tube potential of 120 kV_p. The optimal tube potential of 80 kV_p for iodine contrast–enhanced images is, however, not always used, eg, the CARE kV protocol we applied in our study chooses the potential depending on the size and shape of the patient to maintain a proper signal-to-noise ratio.

In this study, the conversion factors for the ceCBCT acquisitions were determined from phantom measurements. The factors were then interpolated to each patient’s BMI for calculating individual effective doses. This was done as no general conversion factors are available (see above), and published conversion coefficients as well as our phantom results show that effective dose to patients with higher BMI does not increase as much as the DAP (self-shielding). In our case, the DAP increases by 80% for the overweight phantom; however, effective dose increases by only 45%. Suzuki et al¹⁵ explained the effect well: For overweight patients, the percentage of the body volume that is directly exposed to radiation is lower than for thin or normal patients. Furthermore, the amount of subcutaneous fat shielding the organs at risk is greater in overweight patients.¹⁵

The conversion factors for the MDCT examinations used in this study originated from models, affecting accuracy.^19,20,30 Classifying the anatomic coverage of each patient acquisition by body region (thorax, abdomen, and pelvis) allows an individual dose calculation. Unfortunately, it is impossible to exactly separate body regions, as structures like the lung (thorax) and liver (abdomen) overlap on the axial slices. Hence, either the percentage of all tissues/organs belonging to the thorax and abdomen, for example, need to be exactly known to calculate the correct distribution on each slice, or oblique slices need to be used in the dose calculation. For this study, we assumed the chosen division to be sufficient. Furthermore, the use of general conversion factors for CT is arguable, as the dose and its distribution are again influenced by patient BMI. For example, the DLP of the MDCT acquisition of the high-BMI phantom increased by 70%, whereas measured effective dose determined by TLD measurements increased by only 5%. However, calculating the effective dose by using DLPs and appropriate conversion factors resulted in a 70% to 75% increase in the effective dose for the “large” phantom compared to the “thin” phantom (due to the higher DLP). If the 2 methods (TL dosimetry vs conversion coefficients) are compared, TL dosimetry leads to 20% to 35% higher effective doses for the “thin” phantom, but 20% to 40% lower ones for the “large” phantom. This is due to the effect of self-shielding intrinsically accounted for by TL dosimetry, as deposited doses in different tissue types and depths are directly measured. It is, however, not included if only the DLPs and region coverage are taken into account. Although the average DLPs determined for our patient collective were used as input values for the phantom acquisitions (after localizer acquisition), the DLPs for phantom acquisitions of venous phase MDCT differed by 5% to 10% from those of the patient collective (Table 2). These differences may be attributed to tube current modulation during CT acquisition, as phantom absorption differs from human absorption.

Limitations

Probably the largest limitation of this study is that the results are dependent on the imaging systems and acquisition protocols used and also on the patient collective. However, the general relations should also hold true for different hardware and patient cohorts.

In comparison with the gold standard of MDCT angiography after EVAR, ceCBCT acquisitions offer sufficient image quality to enable intraoperative evaluation of the placement of the stent or detection of any complications and technical issues, such as kinks and stent-graft compressions, that might not be visible on conventional angiography.^1,11 Immediate intraoperative correction is therefore possible.¹ The iodine contrast volume and concentration of fluoroscopy and angiography alone can already be problematic in patients with low glomerular filtration rates. A ceCBCT acquisition, whose image quality increases with increasing contrast dose, hence can be acquired only if enough contrast can be administered.¹¹ Over the entire hospital stay, however, iodine contrast dose and radiation exposure to the patient could be decreased (50% and 75%, respectively) if MDCT angiography became redundant in view of intraoperative ceCBCT.

Nonetheless, MDCT acquisition has clinical advantages over ceCBCT acquisition. With a 50-cm diameter, the FOV of MDCT is larger than the 20-cm diameter reconstructions of ceCBCT. MDCT thus also covers and depicts anatomy outside the FOV of ceCBCT, enabling clinical assessment of secondary findings possibly having consequences for further treatment. In addition, longitudinal coverage of MDCT is adaptable based on the requirements imposed by the diagnostic goal (as long as necessary, as short as possible), while it is fixed for ceCBCT. This may make thoracic and thoracoabdominal aneurysms difficult to image with the ceCBCT technique.¹¹

Furthermore, MDCT angiography is superior in view of its ability to differentiate low-contrast details¹¹ that might also be important for detecting secondary clinical findings. However, due to their inherently high contrast or overall good visibility, endoleaks, endograft kinks, stenosis, and intraluminal thrombosis can usually be detected using ceCBCT, such that superior low-contrast visualization of MDCT might not have any clinical impact for EVAR assessment.

By reducing the scan coverage of the nonenhanced phase of the MDCT acquisition to the necessary region and by omitting the arterial phase, effective doses lower than 5.5 mSv are achievable on average. The effective dose associated with such a tailored MDCT protocol (nonenhanced and venous phase) is then comparable to one ceCBCT acquisition (only venous phase), while possibly offering added clinical diagnostic value. This, however, is beyond the scope of the current study and should be the subject to further research on diagnostic value of the clinical application of different imaging modalities.

Conclusion

ceCBCT offers the chance for immediate intraoperative revisions of endograft-related problems. It is associated with an up to 75% reduction of effective dose and 50% reduction of contrast usage compared with a standard 3-phase MDCT examination after EVAR. MDCT enables using a larger FOV and is associated with less radiation exposure for a single phase (reduction of up to ~60%), if only the stented region is covered. A 2-phase MDCT acquisition, with nonenhanced and venous phases covering the stented region, would result in the same average radiation exposure as an intraoperative ceCBCT acquisition.

Footnotes

Acknowledgements

The authors thank M. von Roden and Dr B. Schmidt (both of Siemens Healthcare GmbH) for enabling access to and providing assistance with the operation of the imaging systems as well as for their comments and suggestions regarding experimental study design and setup. Furthermore, the authors thank R. Scholz (Siemens Healthcare GmbH) for organization, execution, and analysis of phantom measurements and TLD dosimetry.

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Philipp Geisbüsch received honoraria (speaker fees) and reimbursement of travel expenses from Siemens Healthcare GmbH. Christof J. Schulz received a research grant from Siemens Healthcare GmbH.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by a research grant from Siemens Healthcare GmbH, Forchheim, Germany.

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