Time-Resolved, Dynamic Computed Tomography Angiography for Characterization of Aortic Endoleaks and Treatment Guidance via 2D-3D Fusion-Imaging

Summary

Dynamic computed tomography angiography (CTA) imaging provides additional diagnostic value in characterizing aortic endoleaks. This protocol describes a qualitative and quantitative approach using time-attenuation curve analysis to characterize endoleaks. The technique of integrating dynamic CTA imaging with fluoroscopy using 2D-3D image fusion is illustrated for better image guidance during treatment.

Abstract

In the United States, more than 80% of all abdominal aortic aneurysms are treated by endovascular aortic aneurysm repair (EVAR). The endovascular approach warrants good early results, but adequate follow-up imaging after EVAR is imperative to maintain long-term positive outcomes. Potential graft-related complications are graft migration, infection, fraction, and endoleaks, with the last one being the most common. The most frequently used imaging after EVAR is computed tomography angiography (CTA) and duplex ultrasound. Dynamic, time-resolved computed tomography angiography (d-CTA) is a reasonably new technique to characterize the endoleaks. Multiple scans are done sequentially around the endograft during acquisition that grants good visualization of the contrast passage and graft-related complications. This high diagnostic accuracy of d-CTA can be implemented into therapy via image fusion and reduce additional radiation and contrast material exposure.

This protocol describes the technical aspects of this modality: patient selection, preliminary image review, d-CTA scan acquisition, image processing, qualitative and quantitative endoleak characterization. The steps of integrating dynamic CTA into intra-operative fluoroscopy using 2D-3D fusion-imaging to facilitate targeted embolization are also demonstrated. In conclusion, time-resolved, dynamic CTA is an ideal modality for endoleak characterization with additional quantitative analysis. It can reduce radiation and iodinated contrast material exposure during endoleak treatment by guiding interventions.

Introduction

Endovascular aortic aneurysm repair (EVAR) has shown superior early mortality results than open aortic repair¹. The approach is less invasive but may result in higher mid to long-term re-intervention rates due to endoleaks, graft migration, fracture². Hence better EVAR surveillance is critical to achieving good mid to long-term results.

Current guidelines suggest the routine use of duplex ultrasound and triphasic CTA³. Dynamic, time-resolved computed tomography angiography (d-CTA) is a relatively new modality used for EVAR surveillance⁴. During d-CTA, multiple scans are acquired in different time points along the time attenuation curve after contrast injection, hence the term time-resolved imaging. This approach has shown better accuracy in characterizing endoleaks after EVAR than conventional CTA⁵. An advantage of time-resolved acquisition is the ability to quantitatively analyze the Hounsfield unit changes in a selected region of interest (ROI)⁶.

The additional benefit of accurately characterizing endoleaks with d-CTA is that the scan can be used for image fusion during interventions, potentially minimizing the need for further diagnostic angiography. Image fusion is a method when previously acquired images are overlaid onto real-time fluoroscopy images to guide endovascular procedures and subsequently reduce contrast agent consumption and radiation exposure^7,8. Image fusion in the hybrid operating room (OR) using a 3D dynamic CTA scan can be achieved by two approaches: (1) 3D-3D image fusion: where 3D d-CTA is fused with intraoperatively acquired non-contrast cone-beam CT images, (2) 2D-3D image fusion, where 3D d-CTA is fused with biplanar (anteroposterior and lateral) fluoroscopic images. 2D-3D image fusion approach has been shown to significantly lower the radiation compared with 3D-3D technique⁹.

This protocol describes the technical and practical aspects of dynamic CTA imaging for endoleak characterization and introduces a 2D-3D image fusion approach with d-CTA for intra-operative image guidance.

Protocol

This protocol follows the ethical standards of the national research committee and with the 1964 Helsinki declaration. This protocol is approved by Houston Methodist Research Institute.

1. Patient selection and prior image review

NOTE: Dynamic CTA imaging should be considered as a follow-up imaging modality in patients with increasing aneurysm size and endoleak after stent-graft implantation, persistent endoleak after interventions, or in patients with increasing aneurysms sac size without demonstrable endoleak. Like conventional CT imaging, this technique involves iodinated contrast injection that may be relatively contraindicated in patients with severe renal failure.

1. Before starting the actual scan, review the prior imaging studies for the presence of endoleak and stent-graft type.
   ​NOTE: This can provide information to decide the scan range and temporal distribution during the image acquisition. The most commonly available imaging is the conventional CTA scans with bi-(non-contrast scan and arterial scan) or triple-phase (non-contrast scan, arterial scan, and delayed scan).

2. d-CTA Image acquisition

1. Position the patient in a supine position on the CT scanner table.
2. Gain peripheral venous access.
   NOTE: Ensure that access is gained by visualizing the venous back bleeding.
3. Perform Topogram and Non-Contrast CT Image Acquisition using Sn-100 Tin filter (see Table of Materials) to reduce the radiation exposure and for the region of interest selection in the d-CTA scan.
   NOTE: After the non-contrast scan, the location of the endograft will be visible. Place the region of interest just above the endograft.
4. Perform timing bolus⁶ to check the contrast arrival time by placing a region of interest above the stent graft in the abdominal aorta.
   1. Inject 10-20 mL of the contrast (see Table of Materials) through the peripheral venous access, followed by 50 mL of saline injection at a 3.5-4 mL/min flow rate. Acquire timing bolus scan.
      NOTE: Contrast arrival is recorded by the CT scanner (see Table of Materials) based on Hounsfield unit change inside the aorta⁶.
5. By selecting the DynMulti4D menu point in the pop-up "Cycle time window" plan the distribution and the number of scans based on the contrast arrival time from timing bolus and the findings from prior imaging studies.
   NOTE: If type I endoleak is suspected, perform more scans on the early phase of the contrast enhancement curve that is given by the timing bolus. If type II endoleak is suspected, perform more scans on the later phase.
   1. For type I endoleak, include more scans during the earlier phase of the time-attenuation curve (scan at every 1.5 s at the beginning and then every 3-4 s).
   2. For type II endoleak that appear later, include more scans during the later phase of the time-attenuation curve.
   3. If no prior imaging studies are available, distribute the scans equally around the peak of the time-attenuation curve.
6. Optimize imaging parameters, including kV, scan range, etc., to reduce radiation exposure. Use settings shown in Table 1 for acquiring a dynamic scan with the CT scanner (see Table of Materials) used in this work.
7. Inject the contrast for d-CTA acquisition: 70-80 mL of the contrast material, followed by 100 mL of saline injections at a 3.5-4 mL/min flow rate through the peripheral access.
8. Start d-CTA image acquisition using the delay time based on the timing bolus describedin step 2.4. Breath-hold is not necessary during acquisition, given that the duration of d-CTA image acquisition ranges from 30-40 s.
9. Send acquired, reconstructed images to Picture Archiving and Communication System (PACS) for qualitative and quantitative review of time-resolved angiographic images. To do this, select the data image and perform a mouse click on the bottom left side of the software.

3. Dynamic-CTA image analysis

1. Open the software (see Table of Materials) for reading the image. Search for the patient's name or identification number to find the acquired images. Select the acquired d-CTA images and process them using the CT dynamic angio workflow.
   NOTE: The layout is shown in Figure 1.
2. Minimize respiratory motion artifacts between d-CTA images by selecting the dedicated software's Align Body motion correction menu item (Figure 1).
3. Qualitative analysis: Check axial slices of CT images when maximum opacification of the aorta occurs to interpret any obvious endoleak.
   1. Then analyze scans in multiplanar reconstruction mode; if endoleak is suspected, focus on the endoleak and use the timescale shown in Figure 1 to watch time-resolved images and infer the source of endoleak.
4. Quantitative analysis: Click on the Time Attenuation Curve (TAC) function shown in Figure 1. Select a region above the stent-graft (ROI_aorta) and draw a circle using the TAC function, then select the endoleak (ROI_endoleak) region and draw a circle there as well.
   NOTE: Target vessels can be selected (ROItarget) to determine the role of the vessel to the endoleak (inflow or outflow).
   1. Analyze the acquired TAC (Figure 2) to determine the endoleak characteristics. Subtract the time to the peak value of the endoleak from the aortic ROI curves to get the Δ time to peak value. This value can be used for endoleak analysis⁶.
5. After qualitative and quantitative analysis, infer the type and source of endoleak.
   ​NOTE: Type I endoleaks appear as parallel contrast enhancement next to the graft, usually because of the inadequate sealing zone and have a shorter time difference between the aortic and endoleak enhancement curves (Δ time to peak value) between aortic and endoleak ROI. Type II endoleaks are related to an inflow vessel with retrograde filling through collateral and have prolonged Δ time to peak value between aortic and endoleak ROI. Based on experience, a Δ time-to-peak value of higher than 4 s was not recorded for type I endoleaks.

4. Intra-operative image fusion guidance

1. Position the patient supine on the hybrid operating room (OR) table.
2. Load the selected dynamic CTA scan that has the best visibility of the endoleak in the hybrid OR workstation. Manually annotate critical landmarks on the scan: renal arteries ostia, internal iliac arteries ostia, endoleak cavity, lumbar artery(ies), or inferior mesenteric artery.
3. Select 2D-3D image fusion in the workstation and acquire an anteroposterior and an oblique fluoroscopic image of the patient using the 2D-3D image fusion workflow. For this, move the C-arm to the required angle(s) with the joystick on the operating table and step on the CINE acquisition pedal.
4. Electronically align the stent graft with markers from the 3D dynamic CTA scan with the fluoroscopic images using automated image registration, followed by manual refinement if necessary (Figure 3) in the 3D post-processing workstation (Drag one image for manual alignment). Check and accept the 2D-3D Image Fusion and Overlay the markers from d-CTA on the real-time 2D fluoroscopic image (Figure 4).
5. Perform the endoleak embolization using the overlaid markers from d-CTA as guidance.

Results

The dynamic imaging workflow in two patients is illustrated here.

Patient I
An 82-year-old male patient with chronic obstructive pulmonary disease and hypertension had a previous infrarenal EVAR (2016). In 2020 the patient was referred from an outside hospital for a possible type I or type II endoleak based on conventional CTA. and an adjunctive endoanchor placement in 2020 for type Ia endoleak. Dynamic CTA was performed that diagnosed a type Ia endoleak, and the patient u...

Discussion

Dynamic, time-resolved CTA is an additional tool in the aortic imaging armamentarium. This technique can accurately diagnose endoleaks after EVAR, including identification of inflow/target vessels⁴.

Third-generation CT scanners with bidirectional table movement capability can provide dynamic acquisition mode with better temporal sampling along the time-attenuation curve⁶. To achieve the highest accuracy in the protocol it is critical to personali...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Siemens Artis Pheno	Siemens Healthcare	https://www.siemens-healthineers.com/en-us/angio/artis-interventional-angiography-systems/artis-pheno	Other commercially available C-arm systems can provide image fusion too
SOMATOM Force CT-scanner	Siemens Healthcare	https://www.siemens-healthineers.com/computed-tomography/dual-source-ct/somatom-force	Any commercially available third generation CT-scanner can perform such dynamic imaging
Syngo.via	Siemens Healthcare	https://www.siemens-healthineers.com/en-us/medical-imaging-it/advanced-visualization-solutions/syngovia	Any DICOM file viewer with 4D processing capabilities can review the acquired time-resolved images, TAC are software dependent.
Visipaque (Iodixanol)	GE Healthcare	#00407222317	Contrast material