Experimental feasibility of spectral photon-counting computed tomography with two contrast agents for the detection of endoleaks following endovascular aortic repair

Abstract

Objectives: After endovascular aortic repair (EVAR), discrimination of endoleaks and intra-aneurysmatic calcifications within the aneurysm often requires multiphase computed tomography (CT). Spectral photon-counting CT (SPCCT) in combination with a two-contrast agent injection protocol may provide reliable detection of endoleaks with a single CT acquisition.

Methods: To evaluate the feasibility of SPCCT, the stent-lined compartment of an abdominal aortic aneurysm phantom was filled with a mixture of iodine and gadolinium mimicking enhanced blood. To represent endoleaks of different flow rates, the adjacent compartments contained either one of the contrast agents or calcium chloride to mimic intra-aneurysmatic calcifications. After data acquisition with a SPCCT prototype scanner with multi-energy bins, material decomposition was performed to generate iodine, gadolinium and calcium maps.

Results: In a conventional CT slice, Hounsfield units (HU) of the compartments were similar ranging from 147 to 168 HU. Material-specific maps differentiate the distributions within the compartments filled with iodine, gadolinium or calcium.

Conclusion: SPCCT may replace multiphase CT to detect endoleaks without sacrificing diagnostic accuracy. It is a unique feature of our method to capture endoleak dynamics and allow reliable distinction from intra-aneurysmatic calcifications in a single scan, thereby enabling a significant reduction of radiation exposure.

Key points: • SPCCT might enable advanced endoleak detection. • Material maps derived from SPCCT can differentiate iodine, gadolinium and calcium. • SPCCT may potentially reduce radiation burden for EVAR patients under post-interventional surveillance.

Keywords: Computed tomography; Endoleak; Gadolinium; Iodine; Photon counting.

Fig. 1

An 82-year-old EVAR patient (m) undergoing follow-up with a triphasic CT scan (level 100 HU, window 600 HU) 1 year after intervention. The arterial phase (a) and venous phase (b) show a low flow endoleak (arrowhead) apparent only in the venous phase (b). The native scan (c) identifies hyperdense material within the aneurysm sac as calcifications (arrows). These could be mistaken as an endoleak in the contrast-enhanced scan (d)

Fig. 2

Dual contrast agent injection protocol for endoleak detection by a single SPCCT scan. Peak enhancement by gadolinium, injected at T0, should be registered (T1) within the lumen of the stent-graft. The time period T1–T0 defines the time necessary for maximal enhancement in the arterial phase. Under consideration of T1–T0, iodine should be injected (T2) in order to meet a venous/delayed distribution for the initially applied gadolinium and an arterial distribution of iodine, when a single SPCCT scan is performed at T3 = T2 + T1–T0. (Blue dotted line, arterial distribution of gadolinium; solid blue line, venous/delayed distribution of gadolinium; red line, arterial distribution of iodine)

Fig. 3

Image-based separation of calcium and iodine: distribution of photoelectric effect and Compton effect image (a) and modelling of material distributions of iodine and calcium (b)

Fig. 4

Photograph of the aortic phantom (a), conventional CT scan (b), overlay of the three material maps (c) (green = calcium; red = iodine; blue = gadolinium), calcium map (d), gadolinium map (e) and iodine map (f) (level −153 HU, window 1120 HU). 1 = calcium; 2 = gadolinium; 3 = iodine; 4 = stent lined, iodine and gadolinium. 5 = oral contrast media

Fig. 5

a Normalized conventional greyscale image of the aortic aneurysm phantom obtained with SPCCT. b Line plots, indicated in a with red and green lines, are tracking through the compartments filled with iodine, gadolinium and calcium. Intensities measured in the conventional SPCCT image (blue line), measured in the gadolinium map (orange line), obtained from the iodine probability map (grey line) and the calcium probability map (yellow line) are displayed