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Dual-Source Dual-Energy CT Angiography of the Supra-Aortic Arteries with Tin Filter

Posted May 31, 2015 Updated Apr 10, 2023
By Andreas Korn MD , Benjamin Bender MD , Christoph Schabel MD , Malte Bongers MD , Ulrike Ernemann MD , Claus Claussen MD , Christoph Thomas MD
9 min read

Rationale and Objectives

Automatic bone and plaque subtraction (BPS) in computed tomographic angiographic (CTA) examinations using dual-energy CT (DECT) remains challenging because of beam-hardening artifacts in the shoulder region and close proximity of the internal carotid artery to the base of the skull. The selection of the tube voltage combination in dual-source CT influences the spectral separation and the susceptibility for artifacts. The purpose of this study was to assess which tube voltage combination leads to an optimal image quality of head and neck DECT angiograms after bone subtraction.

Materials and Methods

Fifty-one patients received tin-filter–enhanced DECT angiograms of the supra-aortic arteries using two voltage protocols: 24 patients were studied using 80/Sn140 kV and 27 using a 100/Sn140 kV protocol, both protocols with an additional tin filter. A commercially available DE-CTA BPS algorithm was used. Artificial vessel erosions in BPS maximum intensity projections (four-level Likert scale with CTA source data as reference) and vessel signal-to-noise ratio (SNR) were assessed in the level of the shoulders and the base of the skull in each patient and compared.

Results

At the level of the shoulder, 100/Sn140 kV achieved higher SNR (23.4 ± 6.4 at 80/Sn140 kV vs. 35.1 ± 11.8 at 100/Sn140 kV; P < .0001) with less erosions (erosion score 3.9 ± 0.4 in 80/Sn140 kV vs. 2.1 ± 1.3 in 100/Sn140 kV; P < .0001) than 80/Sn140 kV. At the level of the skull base, erosion scores and objective image quality of arterial segments were comparable with both protocols ( P = .14).

Conclusions

The 100/Sn140 kV protocol achieved more favorable results for BPS of the supra-aortic arteries than the 80/Sn140 kV protocol.

Computed tomography angiography (CTA) and magnetic resonance imaging (MRI) are the two most important imaging modalities for noninvasive angiographic studies of the supra-aortic vessels . A major advantage of CT over MRI is the detection of calcified plaques which is clinically relevant for risk stratification in patients with asymptomatic hemodynamically relevant internal carotid artery stenosis and for planning interventions or surgery . However, visibility of arterial segments is limited in regions with close contact to bones and extensive calcifications when using conventional CTA images. Furthermore, it is impossible to create angiographic-like maximum intensity projection (MIP) images from plain CT data sets because of superposition of bones and calcified plaques. Automatic bone and plaque subtraction (BPS) by dual-energy CTA (DE-CTA) has become an established method to suppress bones and calcified plaques in the final CTA image and to provide MIP reconstructions of the supra-aortic arteries similar to angiograms created by MRA . However, this technique is still limited in challenging anatomic regions such as the shoulders and at the base of the skull or in the presence of heavily calcified plaques where artificial vessel erosions may occur .

Second-generation dual-source CT systems (Somatom Definition Flash; Siemens) feature a tin filter which allows further hardening of the high energy spectrum. Thus, a combination of 100/140 kV with tin filter (100/Sn140 kV) achieves a comparable spectral separation as 80/140 kV without tin filter while providing lower noise levels in the low-energy spectrum , which may improve image quality in the shoulder region. On the other hand, a combination of 80/140 kV with tin filter (80/Sn140 kV) leads to a further improved spectral separation, which might improve the accuracy of BPS in the base of the skull because of a larger difference of the calcium and iodine slope. Thus, both protocols appear attractive for DE-CTA of the supra-aortic vessels, and it remains uncertain which voltage combination is optimal.

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Material and methods

Patients

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CT Data Acquisition

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CT Data Reconstruction

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Image Postprocessing

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Quantitative Image Analysis

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Qualitative Image Analysis

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Dose Measurements

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Statistical Analysis

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Results

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Quantitative Analysis

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Figure 1, Signal-to-noise ratio (SNR) for selected arterial segments. Although SNR of 80/Sn140 kV and 100/Sn140 kV dual-energy computed tomographic angiographic protocol was comparable at the level of skull base (SB), the latter protocol was associated with a substantially improved image quality in proximal supra-aortic segments. BT, brachiocephalic trunk.

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Qualitative Analysis

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Figure 2, Three-dimensional maximum intensity projection angiogram obtained with 100/Sn140 kV after bone subtraction (a) and after bone and plaque subtraction (b) with multiple calcium-containing high-grade calcifications obscuring the vessel lumen of the supra-aortic arteries (a) . Arrowheads show completely removed calcified plaques (b) . Arrows demonstrate artificial vessel gaps of the internal carotid artery, whereas the V3/4 segments show preserved and excellent vessel integrity.

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Figure 3, Average subjective erosion scores at the brachiocephalic trunk (BT), ostia and proximal supra-aortic segments of the subclavian arteries (SA), proximal vertebral arteries (V1/2), the common carotid arteries (CCA), the internal carotid artery (ICA), and the distal vertebral arteries (V3/4) with use of either 80/Sn140 kV or 100/Sn140 kV protocols. Increasing vessel erosion scores indicate increasing vessel erosions. Vessel erosion scores in the proximal supra-aortic segments (BT, SA, V1/2, and CCA) were clearly lower with the 100/Sn140 kV protocol and comparable in the distal segments (ICA and V3/4).

Figure 4, Comparison of image quality at the level of the arterial origin and proximal supra-aortic segments between 80/Sn140 kV (a) and 100/Sn140 kV dual-energy computed tomography angiography (b) in two different patients. All supra-aortic arteries are depicted with pseudo-occlusions using 80/Sn140 kV. There is significant improvement of image quality using the 100/Sn140 kV protocol without any extinction of the vessel lumen despite beam-hardening artifacts of the right subclavian artery.

Figure 5, Comparison of arterial image quality at the level of the skull base between 80/Sn140 kV (a) and 100/Sn140 kV dual-energy computed tomography angiography (b) in two different patients. Arrows point out partial extinctions of the internal carotid arteries in both protocols.

Table 1

Subjective Vessel Erosion Score (1–4) Per Vessel Segment

Location 80/Sn140 kV 100/Sn140 kV_P_ Value BT 2.92 ± 1.44 1.48 ± 0.94 <.05 SA 3.88 ± 0.61 1.93 ± 1.20 <.05 V1/2 3.48 ± 1.07 1.72 ± 1.20 <.05 CCA 2.10 ± 1.46 1.20 ± 0.57 <.05 ICA 2.45 ± 0.90 2.21 ± 0.55 .29 V3/4 1.81 ± 0.83 1.82 ± 0.91 .95

BT, brachiocephalic trunk; CCA, common carotid artery; ICA, internal carotid artery embedded in the bony canal of the scull base (C2–C6); SA, subclavian artery; V1/2 ostium, V1 and V2 segments of the vertebral artery; V3/4, V3 and V4 segments of the vertebral artery.

A lower score indicates good delineation of the vessel lumen. Data are presented as mean plus standard deviation.

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Radiation Dose

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Table 2

Radiation Dose as Reported by the Computed Tomography Scanner (CTDI vol and DLP) and Estimated Effective Dose (ED)

Feature CTDI vol (mGy) DLP (mGy × cm) ED (mSv) 80/Sn140 kV 8.1 ± 0.5 312.4 ± 24.5 1.50 ± 0.12 100/Sn140 kV 9.2 ± 0.5 350.0 ± 28.6 1.68 ± 0.14

CTDI vol , volume CT dose index; DLP, dose–length product.

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Discussion

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Conclusions

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Academic Radiology, Volume 22, Issue 6
Journals General Radiology
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