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Seventy–Peak Kilovoltage High-Pitch Thoracic Aortic CT Angiography without ECG Gating

Posted Jun 30, 2015 Updated Apr 10, 2023
By Long Jiang Zhang PhD MD , Yan E. Zhao MS , U. Joseph Schoepf MD , Stefanie Mangold MD , Lloyd M. Felmly BS , Xie Li MS , Chun Xiang Tang MS , Changsheng Zhou BS , Li Qi MS , Guangming Lu MD
11 min read

Rationale and Objectives

To assess the feasibility of 70-kVp high-pitch non–ECG-gated thoracic aortic computed tomography angiography (CTA) with 40-mL contrast agent compared to 100-kVp standard-pitch CTA with 60-mL contrast agent.

Materials and Methods

Sixty-seven patients (51 men and 16 women; mean age, 55 ± 14 years) received non–ECG-gated aortic CTA at 70 kVp, high pitch of 3.4, and 40-mL contrast agent (group A, n = 31) or CTA at 100-kVp, pitch of 1.2, and 60-mL contrast agent (group B, n = 36). Iterative reconstruction was used in all patients. For image quality assessment, CTA images were evaluated on a three-point scale and signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated and compared. Furthermore, computed tomography (CT) dose index was recorded.

Results

Mean CT values and noise levels were higher in group A compared to group B (all P < .001), whereas SNR and CNR were lower than those in group B (all P < .001). Furthermore, the image quality of the aorta at the level of the diaphragm was lower in group A than that in group B ( P < .05). However, image quality was graded as diagnostic in all patients, and motion artifacts of the aortic arch were significantly decreased in group A ( P <.05). Interreader agreement was good or excellent for image quality assessment ( k = 0.625–0.835). The 70-kVp CTA protocol, which allows dose reduction of 85%, was considered diagnostic in all instances by two readers.

Conclusions

Our proposed thoracic aortic CTA protocol provides diagnostic information with substantial reduction of both radiation and contrast agent doses compared to standard-pitch CTA at 100 kVp.

Aortic disease accounts for significant cardiovascular morbidity and mortality worldwide . With high temporal and spatial resolution, versatile postprocessing techniques, and widespread availability, computed tomography angiography (CTA) has become the gold standard in the assessment of aortic diseases, especially in acute aortic syndrome . Furthermore, CTA is the preferred imaging modality for follow-up examinations in patients with thoracic aortic diseases . However, long-range scanning coverage of the aorta over multiple follow-up examinations results in a high cumulative radiation dose. Thus, reducing the radiation dose of aortic CTA is of great interest. Several CTA protocols that reduce radiation dose without compromising diagnostic image quality have been described in the literature. These include using lower tube voltage , automatic tube voltage selection , high-pitch or iterative reconstruction (IR) .

However, the potential to reduce radiation dose of aortic CTA examinations should be further explored by combining the previously mentioned radiation dose saving techniques. For example, the study by Farrelly et al. demonstrated a significant reduction of the effective dose (ED) from 26.2 ± 6.0 to 2.9 ± 0.5 mSv by performing a prospectively ECG-gated CTA of the thoracic aorta at 100 kVp instead of the standard retrospectively ECG-gated CTA at 120 kVp. In addition, CTA examinations at 70 kVp have recently been shown to provide diagnostic image quality with an ultralow radiation dose in selected patients undergoing imaging of peripheral, cerebral, and coronary arteries . However, to the best of our knowledge, there has been no published report on a 70-kVp CTA protocol of the thoracic aorta. Additionally, it is known that lower tube voltages allow the reduction of contrast agent as the attenuation of iodinated contrast agent increases at low tube voltages . This proves advantageous in certain situations, especially when scanning patients with preexisting renal dysfunction. Therefore, the aim of this study was to assess the feasibility of high-pitch 70-kVp CTA of the thoracic aorta with 40-mL contrast agent in comparison to the standard-pitch CTA of the aorta at 120 kVp with 60-mL contrast agent.

Materials and methods

Subject Population

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CT Imaging Protocol

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Image Reconstruction and Analysis

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Objective Analysis of Image Quality

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Subjective Analysis of Image Quality

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Estimation of Radiation Dose

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Statistical Analysis

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Results

Study Population

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Quantitative Image Analysis

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Table 1

Objective Measurements in Different Segments of the Aorta

Location Group A Group B_P_ Value Aortic root CT value (HU) 491.2 ± 124.5 352.7 ± 58.3 <.001 Noise (HU) 58.9 ± 10.9 24.9 ± 3.7 <.001 SNR 8.5 ± 2.3 14.3 ± 2.5 <.001 CNR 10.1 ± 4.6 17.3 ± 4.6 <.001 Ascending aorta CT value (HU) 508.4 ± 120.1 364.6 ± 55.0 <.001 Noise (HU) 49.7 ± 10.7 21.0 ± 5.0 <.001 SNR 10.5 ± 2.9 18.9 ± 9.0 <.001 CNR 9.3 ± 4.1 15.2 ± 4.1 <.001 Aortic arch CT value (HU) 510.0 ± 110.0 343.2 ± 67.4 <.001 Noise (HU) 51.0 ± 9.1 19.8 ± 3.9 <.001 SNR 10.3 ± 2.7 18.0 ± 5.0 <.001 CNR 10.6 ± 4.5 16.7 ± 4.7 <.001 Descending thoracic aorta CT value (HU) 508.3 ± 99.0 347.9 ± 49.9 <.001 Noise (HU) 59.7 ± 12.3 22.8 ± 4.6 <.001 SNR 8.9 ± 2.4 15.7 ± 3.1 <.001 CNR 9.3 ± 3.9 14.4 ± 3.8 <.001 Diaphragmatic aorta CT value (HU) 484.2 ± 92.4 337.6 ± 51.2 <.001 Noise (HU) 70.0 ± 13.4 22.7 ± 4.2 <.001 SNR 7.2 ± 2.0 15.4 ± 3.9 <.001 CNR 8.7 ± 3.6 13.9 ± 3.8 <.001 Paraspinal muscle CT value (HU) 102.4 ± 22.5 53.4 ± 8.3 <.001 Noise (HU) 48.0 ± 13.3 21.3 ± 4.7 <.001

CNR, contrast-to-noise ratio; CT, computed tomography; HU, Hounsfield unit; SNR, signal-to-noise ratio.

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Table 2

Subjective Image Quality of Aortic CTA in Different Groups

Location Group A Group B_P_ Value Reader 1 Aortic root 1.10 ± 0.40 1.39 ± 0.50 .010 Aortic arch 1.06 ± 0.25 1.03 ± 0.17 .476 Pulmonary trunk 1.06 ± 0.25 1.03 ± 0.17 .476 Diaphragmatic aorta 1.26 ± 0.44 1.00 ± 0.00 .001 Reader 2 Aortic root 1.06 ± 0.25 1.44 ± 0.50 <.001 Aortic arch 1.06 ± 0.25 1.03 ± 0.17 .476 Pulmonary trunk 1.06 ± 0.25 1.00 ± 0.00 .126 Diaphragmatic aorta 1.19 ± 0.40 1.03 ± 0.17 .027 Both readers Aortic root 1.09 ± 0.40 1.36 ± 0.49 .019 Aortic arch 1.06 ± 0.25 1.00 ± 0.00 .126 Pulmonary trunk 1.06 ± 0.25 1.00 ± 0.00 .126 Diaphragmatic aorta 1.19 ± 0.40 1.00 ± 0.00 .005

CTA, computed tomography angiography.

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Figure 1, Thoracic aorta CT angiography at 70 kVp in a 73-year-old woman with known aortic aneurysmal dilatation. (a) Curved reformatted CT image and (b) volume-rendered CT image of the thoracic aorta show the aneurysmal dilatation of the ascending aorta. (c–f) Contrast-enhanced axial CT images of the thoracic aorta at the levels of aortic arch, main pulmonary trunk, aortic root, and diaphragm show well-defined contours of the aorta in all levels with a lack of motion artifacts. CT, computed tomography.

Figure 2, Aortic CT angiography at 100 kVp in a 24-year-old man with suspected aortic dissection. (a) Curved reformatted CT image and (b) volume-rendered CT image of the thoracic aorta allow the exclusion of the suspected aortic dissection. However, contrast-enhanced CT images at the levels of aortic arch, main pulmonary trunk, aortic root, and diaphragm (c–f) show marked motion artifacts (arrows in panels a, b, d, e) of the ascending aorta and aortic root in images (d) and (e) . CT, computed tomography.

Figure 3, Thoracic CT angiography at 70 kVp in a 52-year-old woman with known aortic arch aneurysm. (a) Volume-rendered reformatted image shows a saccular aneurysm of the aortic arch ( arrow ). (b) Contrast-enhanced axial CT image and (c) maximum intensity projection reformatted image show thrombus formation within the aneurysm ( arrow ). CT, computed tomography.

Figure 4, Follow-up aortic CT angiography at 70 kVp in a 50-year-old man with Stanford type B aortic dissection. (a) Curved reformatted CT image and (b) volume-rendered CT image of the thoracic aorta show good postinterventional results without perfusion of the false lumen. CT, computed tomography.

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Radiation Dose

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Table 3

Radiation Dose Comparison Between Two Groups

Parameter Group A Group B_P_ Value CTDIvol (mGy) 0.8 ± 0.0 5.6 ± 1.0 ＜.001 DLP (mGy × cm) 31.3 ± 2.9 191.3 ± 42.4 ＜.001 ED (mSv) 0.4 ± 0.0 2.7 ± 0.6 ＜.001

CTDIvol, volume CT dose index; DLP, dose–length product; ED, effective dose.

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Discussion

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Academic Radiology, Volume 22, Issue 7
Journals General Radiology
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