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Dual-source CT Angiography in Aortic Stent Grafting

Posted Jun 30, 2010 Updated Apr 7, 2023
By Zhonghua Sun PhD , Curtise Ng PhD
10 min read

Rationale and Objectives

The aim of the study was to investigate the optimal protocols of dual-source computed tomography (CT) angiography in aortic stent grafting in terms of image noise and radiation dose, based on an in vitro phantom study.

Materials and Methods

A series of helical CT cans were performed on a human aorta phantom using a dual-source CT scanner with kVp of 100, 120, and 140, corresponding mAs of 180, 150, and 100; slice thickness of 1.0, 1.5, and 2.0 mm; and pitch value of 0.5, 1.0, and 1.5, respectively. Image quality was determined by measuring the standard deviation (SD) on three-dimensional virtual intravascular endoscopy (VIE) images. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured on two-dimensional (2D) axial images at superior mesenteric artery (SMA), renal arteries, and aneurysm. Effective dose was determined based on dose-length product.

Results

SD measured on VIE images was independent of kVp and pitch values but was determined by the slice thickness ( P < .05) at the SMA and renal arteries. SNR and CNR measured on 2D images showed significant differences between variable kVp values and slice thicknesses ( P < .05), but were independent of pitch values. The mean estimated effective dose for 120 kVp and 140 kVp protocols were 2.66 ± 0.21 mSv and 2.68 ± 0.18 mSv, respectively. The mean estimated effective dose for 100 kVp protocol was significantly lower (1.97 ± 0.07 mSv, P < .0001). This indicates a reduction of 26.5% radiation dose when the kVp was lowered from 140 to 100.

Conclusion

A scanning protocol of 1.5-mm slice thickness, pitch 1.5 with 100 kVp, and 180 mAs is recommended for a dual-source CT angiography in aortic stent grafting as it leads to significant reduction of radiation dose while achieving diagnostic images.

Helical computed tomography (CT) angiography has been recognized as the preferred imaging modality for both preoperative planning and postoperative follow-up of endovascular repair of abdominal aortic aneurysm (AAA) . The diagnostic value of CT angiography has been significantly enhanced with the recent development of multislice CT (MSCT) technique, beginning from initial 4-slice CT to 16-slice, and to the latest 64-slice, dual-source, or even more detector row CT scanners . Despite its advantages of MSCT angiography, the main concern is radiation exposure associated with the CT scans because radiation dose is gradually increased with the increased number of detector rows and reduction of detector size. Minimization of radiation exposure is of paramount importance for the patients treated with endovascular repair because these patients are normally followed with a series of CT scans during the rest of their life to check the position or integrity of aortic stent grafts and exclude any possible complications arising from the endovascular repair. Therefore, the purpose of this study was to investigate the strategies to reduce radiation dose while achieving diagnostic quality images in aortic stent grafting using dual-source CT scans, based on an in vitro aorta phantom study.

In particular, we were interested examining the relationship between image noise (defined as the standard deviation) visualized on three-dimensional (3D) surface rendered images, virtual intravascular endoscopy (VIE) of aortic ostium, as well as signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), which were measured on two-dimensional (2D) axial images acquired with variable scanning protocols. VIE has been previously reported valuable for assessment of the treatment outcomes of endovascular repair of patients with AAA . In this study, we applied the same methodology to perform a series of scans on the aorta phantom, but using the latest CT technique, dual-source CT. Because CT has become a routine imaging modality and more and more clinical centers are replacing older generation of CT scanners with the recent multislice CT models, it is expected that the study outcomes have practical applications for optimization of the MSCT scanning protocols in the daily clinical practice.

Materials and methods

Human Aorta Phantom and MSCT Scanning Protocols

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Figure 1, A human aorta phantom built with rapid medical prototyping with a suprarenal stent graft placed inside the phantom. Normal aortic branches and cephalad of the aneurysm can be clearly seen (a) and the uncovered suprarenal stent wires are visualized inside the phantom (b) . Arrows (a) refer to the fusion line in the middle of the phantom (as the phantom was initially constructed in two halves), whereas the arrow in (b) indicates that the suprarenal stent struts crossing the superior mesenteric artery.

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Generation of 3D Virtual Intravascular Endoscopy Images

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Quantitative Assessment of Image Quality Based on 2D Images

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Figure 2, Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured at the location of superior mesenteric artery. A circle with a minimal area of 20 mm 2 was drawn in the region of interest to measure the computed tomography attenuation and image noise.

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Quantitative Assessment of Image Quality Based on 3D VIE Images

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Figure 3, Measurement of the standard deviation (SD) using a line profile. A line is drawn on a virtual intravascular endoscopy (VIE) image viewing the superior mesenteric artery (SMA) and right renal ostia (a) with the distance of around 100 pixels being recorded. The corresponding line profile shows the degree of the stair-step artifacts (b) , with the SD measured 4.4.

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Subjective Analysis: Appearance of Aortic Ostia and Stent Wires

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Radiation Dose Measurements

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Statistical Analysis

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Results

Comparison of Radiation Dose

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Table 1

Dual source CT Angiography Scanning Protocols and Radiation Dose

Scans kVp mAs Slice thickness (mm) Pitch Reconstruction interval (mm) CTDIvol (mGy) DLP (mGycm) Effective dose (mSv) 1 100 180 1.0 0.5 0.5 7.60 133 2.00 2 100 180 1.5 0.5 0.7 7.60 125 1.88 3 100 180 2.0 0.5 1.0 7.60 136 2.04 4 100 180 1.0 1.0 0.5 7.60 133 2.00 5 100 180 1.5 1.0 0.7 7.60 125 1.88 6 100 180 2.0 1.0 1.0 7.60 136 2.04 7 100 180 1.0 1.5 0.5 7.60 133 2.00 8 100 180 1.5 1.5 0.7 7.60 125 1.88 9 100 180 2.0 1.5 1.0 7.60 136 2.04 10 120 150 1.0 0.5 0.5 10.82 161 2.42 11 120 150 1.5 0.5 0.7 10.82 177 2.66 12 120 150 2.0 0.5 1.0 10.82 194 2.91 13 120 150 1.0 1.0 0.5 10.82 161 2.42 14 120 150 1.5 1.0 0.7 10.82 177 2.66 15 120 150 2.0 1.0 1.0 10.82 194 2.91 16 120 150 1.0 1.5 0.5 10.82 161 2.42 17 120 150 1.5 1.5 0.7 10.82 177 2.66 18 120 150 2.0 1.5 1.0 10.82 194 2.91 19 140 100 1.0 0.5 0.5 10.84 165 2.48 20 140 100 1.5 0.5 0.7 10.84 178 2.67 21 140 100 2.0 0.5 1.0 10.84 194 2.91 22 140 100 1.0 1.0 0.5 10.84 165 2.48 23 140 100 1.5 1.0 0.7 10.84 178 2.67 24 140 100 2.0 1.0 1.0 10.84 194 2.91 25 140 100 1.0 1.5 0.5 10.84 165 2.48 26 140 100 1.5 1.5 0.7 10.84 178 2.67 27 140 100 2.0 1.5 1.0 10.84 194 2.91

CTDIvol, volume computed tomography dose index; CT, computed tomography; DLP, dose length product; mSv, millisievert.

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Comparison of Image Noise and SNR/CNR

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Figure 4, Relationship between standard deviation (SD) and kVp and pitch values. The mean SD measured on three-dimensional virtual intravascular endoscopy images of right renal artery ostium was independent of kVp and pitch values.

Figure 5, Relationship between virtual intravascular endoscopy (VIE) image quality and variable kVp ranges. VIE images of the superior mesenteric artery (SMA) and right renal ostia were acquired with the following scanning protocols: slice thickness 1.5 mm; pitch 1.0 with kVp of 100, 120 and 140; and corresponding mAs were 180, 150 and 100, respectively (a-c) . No significant difference was noticed among these three-dimensional images in terms of subjective visualization of aortic ostium and quantitative measurement of the degree of artifacts. Long arrows point to the SMA; short arrows refer to the right renal ostium, whereas arrowheads indicate the artifacts caused by air bubbles.

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Figure 6, Relationship between image quality (signal-to-noise ratio [SNR]) and variable kVp ranges. The mean SNR measured at the renal arteries reached significant difference with kVp changes, especially between 100 kVp and 140 kVp, but was independent of pitch changes.

Figure 7, Relationship between image quality (signal-to-noise ratio [SNR]) and variable slice thicknesses. The mean SNR measured at the renal arteries was significantly different when the slice thickness changed.

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Figure 8, Relationship between image quality (contrast-to-noise ratio [CNR]) and variable kVp ranges. The mean CNR measured at the aortic aneurysm was determined by the kVp changes, but was independent of pitch changes.

Figure 9, Relationship between image quality (contrast-to-noise ratio [CNR]) and variable slice thicknesses. The CNR measured at the renal arteries was determined by the slice thickness, but was independent of pitch changes.

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Figure 10, Relationship between virtual intravascular endoscopy (VIE) image quality and variable kVp ranges. VIE images of superior mesenteric artery (SMA) and right renal ostia were acquired with scanning protocols of slice thickness 1.5 mm; pitch 0.5, 1.0, and 1.5 with 100 kVp and 180 mAs. No significant difference was found among these three different pitch ranges with regard to the appearance and diameter of the suprarenal stent struts. Long and short arrows point to the SMA and right renal ostia, respectively; arrowheads refer to the suprarenal struts. The stent wire thickness was measured between 1.1 and 1.4 mm in all of these images. (a-c) Images correspond to the scanning protocols acquired with pitch value of 0.5, 1.0 and 1.5, respectively.

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Discussion

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Acknowledgment

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Academic Radiology, Volume 17, Issue 7
Journals General Radiology
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