What is the significance of Hounsfield units (HU) in detecting a Type I endoleak on Computed Tomography Angiography (CTA) after Endovascular Aneurysm Repair (EVAR)?

From the Guidelines

Hounsfield units (HU) are crucial for detecting Type I endoleaks on CTA after EVAR by allowing quantitative measurement of contrast enhancement within the aneurysm sac, with high-density contrast material (usually >100 HU) entering the aneurysm sac from the attachment zones, as recommended by the 2024 ESC guidelines for the management of peripheral arterial and aortic diseases 1.

Significance of Hounsfield Units in Detecting Type I Endoleaks

When evaluating for endoleaks, radiologists look for an increase in HU values within the aneurysm sac on post-contrast images compared to pre-contrast scans. For Type I endoleaks specifically, which occur at the proximal or distal attachment sites of the stent graft, HU measurements typically show high-density contrast material entering the aneurysm sac from these attachment zones. This contrasts with the lower HU values of unenhanced thrombus within the sac.

Importance of Timing and CTA Technique

The timing of HU measurements is also important, as Type I endoleaks appear in the arterial phase of imaging due to their direct communication with arterial flow, as described in the 2024 ESC guidelines 1. Serial CTA examinations comparing HU values over time can help distinguish persistent endoleaks from those that may resolve spontaneously. Accurate HU assessment requires proper CTA technique with pre-contrast, arterial, and delayed phase imaging to capture the dynamic nature of contrast flow in endoleaks, making Hounsfield unit analysis an essential component of post-EVAR surveillance, as supported by the ACR Appropriateness Criteria for abdominal aortic aneurysm: interventional planning and follow-up 1.

Key Points for Detection and Surveillance

Key points for detecting Type I endoleaks using HU include:

• High-density contrast material (usually >100 HU) entering the aneurysm sac from the attachment zones
• Increase in HU values within the aneurysm sac on post-contrast images compared to pre-contrast scans
• Arterial phase imaging to capture the direct communication with arterial flow
• Serial CTA examinations to distinguish persistent endoleaks from those that may resolve spontaneously
• Proper CTA technique with pre-contrast, arterial, and delayed phase imaging, as recommended by the 2024 ESC guidelines 1 and the ACR Appropriateness Criteria 1.

From the Research

Hounsfield Units and Type I Endoleak on CTA

• The significance of Hounsfield units (HU) in detecting a Type I endoleak on Computed Tomography Angiography (CTA) after Endovascular Aneurysm Repair (EVAR) is related to the characterization of endoleak type and identification of inflow vessels 2.
• Dynamic, time-resolved CT angiography can accurately characterize aortic endoleak type, including Type I endoleak, and identify inflow vessels as compared with digital subtraction angiography (DSA) imaging 2.
• Quantitative parameters such as time-to-peak enhancement (Δ TTP) can help better differentiate endoleak types, with mean Δ TTP values for Type I endoleak reported as 1.8 (±1.8) seconds 2.

Detection and Management of Type I Endoleak

• Type I endoleak is a high-pressure, high-risk leak that increases sac pressure up to or even above arterial pressure, and is associated with an increased frequency of open conversions or risk of aneurysm rupture 3, 4.
• Type I endoleak requires intervention, and management options include endograft explantation or custom-made fenestrated EVAR 5.
• The choice of surgical technique for managing Type I endoleak depends on morphological factors, sac growth rate, emergency presentation, and/or patient symptoms 5.

Role of CTA in Endoleak Detection

• CTA is a recommended imaging modality for detecting endoleaks after EVAR, with at least one contrast-enhanced computed tomographic angiogram or duplex ultrasound recommended every 5 years 4.
• CTA can detect endoleaks, including Type I endoleak, and provide information on sac size and growth, which is essential for managing patients after EVAR 4, 2.
• The presence of gas in the aneurysm sac on postoperative CTA is a frequent finding and is associated with the type of endograft and size of the perfused lumen, but is not related to infection or endoleak 6.