Dual-Energy CTA with Bone Removal for Transcranial Arteries

Rationale and Objectives

Dual-source computed tomography enables bone removal on computed tomographic angiographic data on the basis of simultaneous dual-energy (DE) acquisition. The aim of this study was to evaluate the impact of this technique for the assessment of transcranial arteries. Therefore, the degree of stenosis of the transcranial arteries on DE computed tomographic angiography (CTA) with bone removal was compared to those on standard CTA and time-of-flight (TOF) magnetic resonance angiography (MRA).

Materials and Methods

DE-CTA was performed using a dual-source computed tomographic scanner in 50 patients with suspected cerebrovascular disease. From the source images on DE-CTA, data sets with and without bone removal were reconstructed. TOF-MRA was performed on a 1.5-T scanner. Two blinded radiologists evaluated the segments of the internal carotid artery (C2–C7), the vertebral artery (V4), and the basilar artery for degree of stenosis. A five-step scale (0%–49%, 50%–69%, 70%–89%, 90%–99%, and 100% [occlusion]) for degree of stenosis was applied. Wilcoxon’s signed-rank test was used for statistical analysis.

Results

The degrees of stenosis on standard CTA were consistent with those on TOF-MRA in all segments. In contrast, DE-CTA showed significantly higher degrees of stenosis compared to standard CTA and TOF-MRA in both C2 segments ( P < .001). In addition, DE-CTA revealed a significantly higher degree of stenosis compared to standard CTA and TOF-MRA in the left C4 segment ( P < .01 and P < .005, respectively). All other segments showed no significant differences of stenosis among TOF-MRA, DE-CTA, and standard CTA.

Conclusions

Compared to TOF-MRA, standard CTA showed similar results. In contrast, DE-CTA revealed significant overestimation of stenosis for segments with close relations to bony structures as well as in calcified stenosis. Consequently, such findings on DE-CTA require confirmation with standard CTA or MRA to eliminate false-positive results.

Computed tomographic (CT) angiography (CTA) is a frequently used method for the evaluation of vessel pathologies of the supra-aortic and intracranial arteries. In particular for emergency assessment such as in acute stroke, computed tomography ( ) is used because of its wider availability, easier patient surveillance in the emergency setting, and fewer contraindications compared to magnetic resonance imaging (eg, metal implants, pacemakers) . Data from the literature emphasize the potential of noninvasive CTA for the assessment of acute and chronic cerebrovascular disease. The advantages of magnetic resonance angiography (MRA; eg, lack of radiation exposure) have already been described elsewhere , and this method is increasingly used in the evaluation of neurologic emergencies . Magnetic resonance imaging of the transcranial arteries is typically performed with time-of-flight (TOF) MRA .

In contrast to MRA, the delineation of vessels adjacent to bony structures (eg, the skull base) can be limited on CTA. Several techniques have been presented to eliminate bony structures from CT angiographic data sets on the basis of region-growth algorithms, section-by-section digital subtraction, and matched mask bone elimination . These bone-removal techniques have been assessed in several studies, which have shown partially correlative results . Therefore, none of these techniques has become standard in CTA of the transcranial arteries. The recent innovation of dual-source CT technology offers a new technical approach for bone removal in CTA . The two tubes of a dual-source scanner can be operated at different settings. This allows the simultaneous acquisition of two data sets at different tube voltages . Theoretically, bone and contrast-enhanced vessels can be identified by the different spectra of radiation absorption at two distinct tube voltages (eg, 80 and 140 kVp). Thereby, the selective elimination of bony structures and the preservation of contrast-enhanced vessels can be achieved .

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Materials and methods

Patient Characteristics

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Table 1

Patient Characteristics ( n = 50)

Characteristic Value Age (y) 63.4 ± 15.3 Gender Male 27 (54%) Female 23 (46%) Symptoms Aphasia/dysarthria 14 (28%) Hemiplegia 15 (30%) Hemihypesthesia 6 (12%) Vision Impairment 3 (6%) Diplopia 6 (12%) Vertigo 9 (18%) Coma 2 (4%) Cardiovascular risk factors Arterial hypertension 33 (66%) Diabetes mellitus 11 (22%) Hyperlipidemia 19 (38%) Smoking 11 (22%) None 9 (18%)

Data are expressed as mean ± standard deviation or as number (percentage).

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Examination Protocol

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Analysis

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Comparison of Stenosis Grading

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Statistical Analysis

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Results

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Table 2

Synopsis of Stenosis Scores ∗

Arterial Segment Standard CTA Bone-Removal DE-CTA TOF-MRA Right Left Right Left Right Left C2 1 44 42 40 40 44 45 2 0 2 4 4 0 0 3 0 0 0 0 0 0 4 0 0 0 0 0 0 5 2 2 2 2 2 1 C3 1 45 44 44 43 45 44 2 0 0 0 1 0 0 3 0 0 1 0 0 0 4 0 0 0 0 0 0 5 1 2 1 2 1 2 C4 1 45 41 45 38 45 44 2 0 2 0 3 0 0 3 0 1 0 1 0 0 4 0 0 0 1 0 0 5 1 2 1 3 1 2 C5 1 45 43 44 42 45 44 2 0 1 1 2 0 0 3 0 0 0 0 0 0 4 0 0 0 0 0 0 5 1 2 1 2 1 2 C6 1 45 44 44 43 45 44 2 0 0 0 0 0 0 3 0 0 0 0 0 0 4 0 0 0 0 0 0 5 1 2 2 3 1 2 C7 1 45 44 45 44 45 44 2 0 0 0 0 0 0 3 0 0 0 0 0 0 4 0 0 0 0 0 0 5 1 2 1 2 1 2 V4 1 46 46 45 45 46 45 2 0 0 0 1 0 1 3 0 0 1 0 0 0 Basilar artery 46 46 46

DE-CTA, dual-energy computed tomographic angiography; S-CTA, standard computed tomographic angiography; TOF-MRA, time-of-flight magnetic resonance angiography.

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Discussion

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Conclusion

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