# Spectral Computed Tomography for the Detection and Characterization of Communications Between the True and the False Lumen in Aortic Dissections

Antoine Janin-Manificat, Monica Sigovan, Eduardo Davila, Charles de Bourguignon, Salim Si-Mohamed, Loic Boussel, Benoit Cosset, Yoad Yagil, Antoine Millon, Philippe Douek, et al.

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Antoine Janin-Manificat, Monica Sigovan, Eduardo Davila, Charles de Bourguignon, Salim Si-Mohamed, et al. Spectral Computed Tomography for the Detection and Characterization of Communications Between the True and the False Lumen in Aortic Dissections. Investigative Radiology, 2025, 10.1097/RLI.0000000000001221. hal-05400863

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# Spectral Computed Tomography for the Detection and Characterization of Communications Between the True and the False Lumen in Aortic Dissections

Antoine Janin-Manificat, MD, Monica Sigovan, PhD, Eduardo Davila, MSc, Charles de Bourguignon, MSc, Salim Si-Mohamed, MD, PhD, Loic Boussel, MD, PhD, Benoit Cosset, MD, PhD, Yoad Yagil, PhD, Antoine Millon, MD, PhD, Philippe Douek, MD, PhD, and Sara Boccalini, MD, PhD

**Objectives:** To compare the performance of conventional CT (conv-CT) and spectral CT (spectral-CT) for the detection and characterization of communications between the true (TL) and false lumen (FL) in aortic dissections, using 4D flow MRI as the reference.

**Materials and Methods:** Eighteen patients with type A and type B aortic dissection who underwent 4D flow MRI, conv-CT and spectral-CT were retrospectively analyzed. For each patient, the examinations closest in time, without any intervention in-between, were retrieved and subjectively analyzed by 2 observers, independently for conv-CT and in consensus for MRI and spectral-CT. Communications between the two lumina were identified as: focal alterations in velocities corresponding to jet flows on 4D flow; both intimal tears and/or focal changes of contrast concentration corresponding to jet flows on conv-CT and spectral-CT. The number and location of communications were noted. The direction of the flow was assessed whenever the jet flow was visible. The performance of conventional and spectral-CT was calculated.

**Results:** Of the 176 communications detected with 4D flow, spectral-CT allowed visualization of 122(69%) compared with 58 (33%) for observer-1 and 38 (22%) for observer-2 for conv-CT, yielding an accuracy twice as high (63% vs 29% to 30%). Only jet flows, without visible intimal tears, were detected in 0 and 77 (63%) cases for conv-CT and spectral-CT, respectively. The flow was unidirectional FL-TL in 2 cases for MRI and spectral-CT and 1 case for conv-CT, bidirectional in 5 and 3 cases for MRI and spectral-CT, respectively. In all other cases, the direction was TL-FL.

**Conclusion:** Spectral-CT outperformed conv-CT for the detection of communications between TL and FL in aortic dissections. Spectral-CT

allowed for direct visualization of flow jets, and their direction, through intimal tears.

**Key Words:** aortic dissection, communications, jet-flow, spectral CT, 4D flow MRI

(Invest Radiol 2025;00:000–000)

Aortic dissection (AD) remains a threatening medical condition with high morbidity and mortality.¹ Even after successful initial treatment, these patients require lifelong surveillance due to the high rate of long-term complications, the most common being aneurysmal progression, and ultimately to aortic rupture, and true lumen (TL) compression.

Both are the results of complex morphologic and hemodynamic phenomena inside and between the two lumina as well as their equilibrium. In fact, during AD, the entry tear enables blood to penetrate the aortic wall triggering the formation of reentry tears due to the detachment of the ostium of aortic branches from the intima.² This results in several open communications between the TL and the false lumen (FL), many of which will remain patent even after initial treatment, either surgical or endovascular.

The importance of these communications is acknowledged by recent guidelines recommending that they be identified and reported.³ Indeed, several studies have shown that their localization, number and size are fundamental factors to understand hemodynamics of the dissection.⁴⁻⁸ Nevertheless, nowadays the only established anatomic risk factor to predict complications is the progression of aortic and false lumen diameters.⁹,¹⁰ Dilation of the aorta is surely an epiphenomenon of underlying changes that occur earlier than the morphologic remodeling. The incapacity to anticipate the dilation is due, among others, to the difficulty of identification of these tears on follow-up imaging examinations. In fact, the 2 available imaging modalities, CT and 4D flow MRI allow for the evaluation of distinct and complementary information: almost exclusively morphologic data with CT and prevalently hemodynamic data with 4D Flow MRI. Spectral-CT might help close this gap providing both types of data in a single examination.

Spectral CT (spectral-CT) has been part of daily practice for several years now and has demonstrated numerous advantages as compared with conventional CT (conv-CT).¹¹ These advantages derive mainly from improved iodinated contrast discrimination, visualization and quantification. We hypothesized that spectral-CT might enhance alterations in contrast media due to the presence of blood/contrast jets through communications between the two lumina of AD and thus to allow both direct

Received for publication April 16, 2025; and accepted for publication, after revision, June 3, 2025.

From the Department of Cardiovascular and Thoracic Radiology, Louis Pradel Hospital, Hospices Civils de Lyon, Bron, France (A.J.M., C.d.B., S.S.M., L.B., P.D., S.B.); University Lyon, INSA-Lyon, University Claude Bernard Lyon 1, UJM-Saint Etienne, CNRS, Inserm, Villeurbanne, France (M.S., E.D., S.S.M., L.B., P.D., S.B.); Department of Cardiac Surgery, Marie Lannelongue Hospital, Groupe Hospitalier Paris Saint Joseph, Le Plessis-Robinson, France (B.C.); Global Advanced Technologies, CT, Philips Medical Systems, Haifa, Israel (Y.Y.); Department of Vascular Surgery, Louis Pradel Hospital, Hospices Civils de Lyon, Bron, France (A.M.); University of Lyon, INSA-Lyon, University Claude Bernard Lyon 1, Villeurbanne, France (A.M.).

Conflicts of interest and sources of funding: Y.Y. is employee of Philips Healthcare.

Correspondence to: Sara Boccalini, MD, PhD, Department of Cardiovascular Radiology, Hôpital Pradel, 59 Boulevard Pinel, Bron 69500, France. E-mail: sara.boccalini@yahoo.com; sara.boccalini@chu-lyon.fr.

Copyright © 2025 The Author(s). Published by Wolters Kluwer Health, Inc. This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.

DOI: 10.1097/RLI.0000000000001221
(morphologic) and indirect (through the identification of flow jets) detection of intimal tears.

The aim of the present study is to compare the performance of spectral-CT versus conv-CT for the visualization and characterization of communications between the two lumina of aortic dissection using 4D flow MRI as a reference.

# MATERIALS AND METHODS

## Study Design

This was a monocentric retrospective analysis of patients with AD who had been prospectively included to undergo 4D flow MRI¹² and who also underwent a clinically indicated spectral-CT without intercurrent interventions.

## Patient Population

Between November 2018 and March 2023, 39 patients with aortic dissection underwent a 4D flow MRI of the aorta. These examinations were performed for a study that aimed at investigating hemodynamic changes after the implantation of an endoprosthesis of the aorta.¹² Thirty-five of these patients underwent a CT on a spectral system within 6 months of the MRI. CT scans were performed for follow-up as clinically indicated. All examinations were performed in a tertiary, university cardiovascular hospital.

Before the MRI, all patients signed informed consent (IRB 2018-A01742-53). Spectral-CTs were performed as part of standard care and need for informed consent was waived by the ethical committee of the institution (CNIL: 22_5688).

After exclusion of patients due to absent or corrupted data of 4D flow MRI (N = 12), bad image quality on one of the 2 examinations (N = 2) or the absence of residual aortic dissection (N = 3), 18 patients were finally included for analysis (Fig. 1).

## Examination Acquisition

Data about MRI and CT acquisitions and reconstructions are provided in Table 1.

## Magnetic Resonance Imaging

MRI examinations were performed on a 3T MRI system (Ingenia, Philips). 4D Flow was performed using a retrospective ECG-gated and respiratory navigated spoiled gradient echo sequence covering the aorta from the supra-aortic vessels to below the renal arteries in a sagittal oblique orientation.

## Computed Tomography

CT examinations were performed on a dual-energy dual-layer CT scanner (IQon or CT7500, Philips.). Eight CT scans were performed with ECG synchronization and 10 without. The timing of the acquisition was established based on bolus tracking with either manual or automatic triggering (Table 1). The delay between the triggering and the actual start of the acquisition was 6 seconds in all cases. Polychromatic (referred to as “conventional”) and spectral images were available for the same acquisition for all patients. Spectral images were reconstructed and stored in the proprietary “SBI” format (Philips) that allows for further visualization and analysis of all types of images.

## Assessment of Magnetic Resonance Imaging and Computed Tomography Examinations

### Preprocessing of Images

**Magnetic Resonance Imaging.** For each patient, the 4D Flow data set was analyzed using in-house developed tools.¹² Time-averaged speed was computed and used for manual segmentation of the entire volume of the aorta performed by a senior resident in radiology (AJM) and supervised by a radiologist with 10 years’ experience in cardiovascular imaging (SB). The aorta centerline was computed using a thinning algorithm applied to the aorta mask and then interpolated and smoothed to 1 mm spacing using 3D splines. Cross sections were automatically placed perpendicular to the centerline at 1 mm intervals, velocity vectors were rotated to the cross-sectional planes, and speed images were then computed for all cardiac phases. For each level, the observers had the cross-sectional speed images throughout the cardiac cycle displayed next to each other and could scroll along the centerline to establish the location of the findings (Fig. 2).

<figure><img src="image_3.png" /><figcaption>FIGURE 1. Flowchart of the study.</figcaption></figure>
<table><caption>TABLE 1. MR and CT Acquisition and Reconstruction Characteristics</caption><tbody><tr><td colspan="2"><strong>MR parameters</strong></td></tr><tr><td>TR (ms)</td><td>4-4.6</td></tr><tr><td>TE (ms)</td><td>2.2-2.7</td></tr><tr><td>Flip angle [degree]</td><td>8</td></tr><tr><td>Spatial resolution (mm³)</td><td>2.5-2.8 isotropic</td></tr><tr><td>VENC (cm/s)</td><td>70 (one patient) or 200 (all the others)</td></tr><tr><td colspan="2"><strong>CT parameters</strong></td></tr><tr><td colspan="2"><strong>Acquisition</strong></td></tr><tr><td>Retrospective ECG-gating; n (%)</td><td>8 (44)</td></tr><tr><td>Pitch</td><td>0.8 ± 0.5*</td></tr><tr><td>Rotation time (s)</td><td>0.27</td></tr><tr><td>Tube voltage (kV)</td><td>120</td></tr><tr><td>Tube current-time product (mAs)</td><td>154 ± 121</td></tr><tr><td>Dose modulation</td><td>Automatic (DoseRight)</td></tr><tr><td colspan="2"><strong>Injection</strong></td></tr><tr><td>Volume of contrast media (mL)</td><td>63 ± 18</td></tr><tr><td>Flow rate (mL/s)</td><td>3</td></tr><tr><td>Type of contrast</td><td>Iomeron 400 (Bracco, Italy)</td></tr><tr><td>Timing</td><td>Bolus-tracking; manual or automatic start†</td></tr><tr><td colspan="2"><strong>Reconstruction</strong></td></tr><tr><td>Phase of cardiac cycle (%)</td><td>75</td></tr><tr><td>Slice thickness (mm)</td><td>1</td></tr><tr><td>Kernel</td><td>Vascular (C or CB)</td></tr><tr><td>Iterative reconstruction</td><td>iDose level 3</td></tr><tr><td colspan="2"><strong>Dose</strong></td></tr><tr><td>CTDI (mGy)</td><td>13 ± 11</td></tr><tr><td>DLP (mGy.cm)</td><td>620 ± 460</td></tr></tbody></table>

*Average of 1.28 ± 0.08 and 0.28 ± 0.05 (ranges of 1.17-1.38 and 0.2-0.3) for acquisitions with and without ECG-gating, respectively.

†In 8 cases, the acquisition was started manually based on visual analysis. In the remaining cases, a region of interest (ROI) was placed in the proximal descending aorta with a 130 Hounsfield Unit of threshold. In 6 cases, the ROI had been placed correctly in the true lumen and the acquisition was started automatically. In 4 cases, the ROI had been wrongly placed in the false lumen and the acquisition was started manually by the radiographer.

CTDI, computed tomography dose index; DLP, dose length product; TE, time of echo; TR, time of repetition; VENC, velocity encoding.

**Computed Tomography.** For each patient, a centerline of the aorta was automatically created with the commercially available software (AVASpectral, Philips). The centerline was manually edited to fit the center of the entire aorta and then further modified whenever necessary as detailed in the following paragraph. The centerline was recorded and used for both conventional and spectral images.

**Alignment of Magnetic Resonance Imaging and Computed Tomography.** To ensure comparability of the location of the findings, the centerlines of CT and MRI were matched in length using specific anatomic landmarks. The distance between the left subclavian artery and the celiac trunk was measured on the 2 initial centerlines (one on MRI and one on CT) and compared. A maximum tolerance of 5 mm of difference was allowed. If the difference was greater, the centerline on the CT scan was modified appropriately to match on the 2 examinations. Since the length of the acquisition on MRI was limited at abdominal level, the analysis was interrupted at about 1 cm before the last slice on MRI examinations and at a corresponding level on CT examinations.

**Analysis of Magnetic Resonance Imaging and Computed Tomography Examinations**

Analysis was performed by a senior resident in radiology with 1 year of experience in cardiovascular imaging (AJM) and a

radiologist with 10 years' experience in cardiovascular imaging (SB). Both observers analyzed MRI, conv-CT and spectral-CT in random order, blinded to the results of the other imaging modalities and with a time difference of 1 week between different modalities. The analysis was performed independently by the 2 observers for conventional images and in consensus for spectral-CT and MRI.

Visualization of MRI images was performed as detailed previously. CT images were visualized on a multiplanar commercially available workstation (IntelliSpace Portal, Philips). The observers were free to change the plane, zoom, window level and width as desired. For spectral images, observers had conventional and spectral images shown next to each other. For the purpose of this study, color-coded Z-effective images (with or without overlay) were generally used. Nevertheless, the observers were free to use whatever spectral image and color-coding they preferred.

**Definition of the Communications and Their Location**

Each purported communication between the lumina was noted. This included all entry tears, all reentry tears, all communications along suture lines, all communications in the supra-aortic branches, and all type 1A endoleaks.

For different imaging methods, communications were identified as: focal alterations in velocities corresponding to jet flows on 4D flow; both intimal tears and/or focal changes of contrast concentration corresponding to jet flows on conv-CT and spectral-CT.

For each communication, the localization, the dimensions of the tear on the intimal flap, the direction of the flow (TL to FL, FL to TL or bidirectional) were noted. The assessment of the direction of the flow was based on the shape of the jet, in particular close to the flap and close to the aortic wall, if the jet reached it. To ensure the correct matching of the lesion, a morphologic description of the lesions was carried out, which included the relative position of the communication and the direction of the flow (Fig. 3). The localization of the lesion was defined along the centerline taking the celiac trunk as a reference.

For each finding, the observers scored their degree of confidence with a 3-point scale: 1-not very confident, 2-moderately confident, 3-very confident. Each observer scored spectral-CT and MRI independently even if the reading was consensual. Since 4D flow MRI was considered as the gold standard, only findings with a degree of confidence ≥2 were noted.

**Matching of Findings Between the Different Methods**

Upon completion of the analysis on all imaging methods, localization and morphology of the communications were compared. A tolerance of 5 mm was used for the localization to take into account for focal discordances of the centerlines. If the distance was more than 5 mm but the morphology was the same, the examinations were reevaluated to verify if the finding was indeed the same.

**Statistical Analysis**

Statistical analysis was carried out with SPSS, version 21 (IBM). Categorical variables are presented as frequencies and percentages. For continuous variables, normality was assessed with Shapiro-Wilk tests and QQ-plots. Normally distributed variables are expressed as average ± SD, and not-normally distributed variables are presented as median (interquartile range).

Sensitivity, specificity, negative predictive value (NPV), positive predictive value (PPV), and accuracy of conventional
<figure><img src="image_4.png" /></figure>

**FIGURE 2.** Image analysis and definition of the locations of the communications. On the left, an example of centerline on a volume rendering CT reconstruction, and on the right, the matching centerline on a 4D flow MRI maximum intensity projection reconstruction. The green line indicates the level of an example of intimal tear shown on cross-sectional images. The spectral image shown here is an overlay of effective-Z on a "iodine removed" background. The scale of colors used for the effective-Z is presented next to the image. For MRI analysis, the images of the different phases of the cardiac cycle were portrayed next to each other highlighting the bidirectional nature of this communication.

and spectral-CT were calculated. The Wilcoxon test was used to compare degrees of confidence. Correlation between ordinal and linear variables was calculated with the Spearman test. Intraclass correlation coefficient (ICC) with 2-way mixed-effect model was used to calculate interobserver agreement.

# RESULTS

## Patients' Characteristics

Out of the 18 included patients, 13 had a type A dissection that had been treated surgically at the time of the examinations and 5 had a type B dissection that had been treated in 3 cases at the time of the examinations. The time intercurred between the two examinations was of 65 ± 64 days (range: 0 to 123). Patients' characteristics are summarized in Table 2.

## Communications Between the True and False Lumen

### Identification

4D flow MRI allowed identification of 176 communications between the two lumina. Conv-CT allowed for visualization of 58 (33%) of these communications for one observer and 38 (22%) for the other observer, while spectral-CT showed 122 (69%) (Fig. 4). Communications that were detectable only as flow-jets without any detectable tears were 5 (11%), 2 (5%) and 89 (72%). To be noticed that 2 suture leakages of the ascending aorta, which resulted in a communication between the true and false lumen, were seen on MRI and on spectral-CT but not on conv-CT.

Overall, the accuracy of conv-CT was 29% and 30% for the 2 observers, respectively, and 63% for spectral-CT (with a per-patient range of 47% to 92%). Sensitivity, specificity, PPV, NPV and accuracy of the 3 methods are reported in Table 3.

Examples of communications well visible on spectral-CT and MRI and with various degrees of visibility on conv-CT are shown in Figure 5. The proportion of true positive (TP) and negative (TN) findings as well of the false positive and negative (FN) findings of the 3 methods for thoracic and abdominal communications is reported in Figure 6.

## Characterization of Flow Direction

On MRI, in only 2 cases (1%), the flow was only from the FL to the TL and in 5 cases (3%), it was bidirectional. In all other cases, the direction of the flow was mainly from the TL to the FL.

On spectral-CT images, the flow was from the FL to the TL in 2 cases (of which one FL-TL also on MRI and the other bidirectional on MRI), bidirectional in 3 cases (of which 2 bidirectional also on MRI) and from the TL to the FL in the remaining cases.

On conv-CT images, in only one case, the direction of the flow could be established with certitude (FL-TL).

## Dimensions of the Intimal Tears

The dimensions of the tears could not be measured on 4D flow MRI due to the low resolution of the morphologic examinations. The average dimensions of the laceration on the intimal flap visible on conv-CT images were 5.9 ± 6.1 mm and 5.5 ± 5.4 mm for observers 1 and 2, respectively. 89 communications were detectable on spectral-CT as jet flow but could not be measured since the intimal tear was not visible. The dimensions of the remaining tears were 4.9 ± 5.4 mm on spectral-CT.

## Confidence and Correlation With Other Parameters

Confidence was higher for MRI (2.8 ± 0.4 for both observers) as compared with conv-CT (2.2 ± 0.8 and 2.4 ± 0.7 for the 2 observers, respectively) and spectral-CT (2.3 ± 0.8 for both observers) with all *P* < 0.001. Only for one observer, the difference between conventional and spectral-CT was significant (*P* < 0.001).

In Figure 6, values of TP, TN, FP, and FN are reported for high (scores of 2 and 3) and low (score of 1) levels of confidence on spectral images.

Whereas there was a weak ($ρ$ = 0.4, *P* = 0.02 for one observer) to moderate ($ρ$ = 0.5, *P* = 0.001) correlation between the size of the tear and the degree of confidence on conv-CT, this was not true for spectral-CT (*P* = 0.4). There was no correlation between the degree of confidence and the time between
<figure><img src="image_5.png" /><figcaption>**FIGURE 3.** Definition of different types of jet flows through communications between the lumina. Two examples of different types of communications shown on the first and second column, respectively. Conv-CT (first line), spectral-CT (second line; overlay of Zeff on removed iodine), 4D flow MRI (third line) are shown. On the fourth line, schemes of the jet flows (morphology and direction). The first column shows a straight jet flow (on spectral-CT and 4D flow MI) originating from a central intimal tear barely visible only in retrospect on the conv-CT (first line). The second column shows a curvilinear blood jet flow originating from a lateral tear, invisible even in retrospect on conv-CT, and following the aortic wall. While on MRI images different shades of white and black translate different velocities (with white being the highest), on spectral-CT colors are discretional and depend on the Zeff, that is the concentration of contrast media. The scale used for each of the presented examples with some reference values is shown next to each spectral-CT panel.</figcaption></figure>

examinations nor the time between the dissection and the first examination (all *P* > 0.05).

The ICC was excellent for both MRI [ICC = 0.97, 95% interval of confidence (IC) = 0.96 to 0.98] and spectral-CT

<table><caption>**TABLE 2.** Patients' Characteristics</caption><thead><tr><th>Patients' Characteristics</th><th>N = 18</th></tr></thead><tbody><tr><td>Age at the moment of the examination</td><td>61 ± 13</td></tr><tr><td>Sex</td><td></td></tr><tr><td>M/F</td><td>16 (89)/2 (11)</td></tr><tr><td>BMI</td><td>27 ± 5 (21-39)</td></tr><tr><td>Dissection</td><td></td></tr><tr><td>Type A</td><td>13 (67)</td></tr><tr><td>Type B</td><td>5 (33)</td></tr><tr><td>Genetic diseases</td><td></td></tr><tr><td>Marfan</td><td>1 (6)</td></tr><tr><td>Other</td><td>0</td></tr><tr><td>Surgery/intervention</td><td></td></tr><tr><td>Ascending aorta replacement</td><td>9 (50)</td></tr><tr><td>Bentall</td><td>4 (22)</td></tr><tr><td>Elephant trunk/Evita</td><td>1 (6)</td></tr><tr><td>Endoprosthesis of the descending thoracic aorta</td><td>3 (17)</td></tr><tr><td>Age at the diagnosis of dissection (y)</td><td>57 ± 19</td></tr><tr><td>Time from dissection to the first examination (d)*</td><td>481 ± 724 (0-2931)</td></tr><tr><td>Time from surgery/intervention to the first examination (d)</td><td>495 ± 724 (0-2931)</td></tr><tr><td>Time between CT and MRI (d)</td><td>65 ± 64 (0-123)</td></tr></tbody></table>

Data is presented as frequencies (percentage) for categorical variables and as mean ± standard deviation (range) for ordinal variables.

*Data could not be retrieved for 1 patient.

(ICC = 0.94, 95% IC = 0.92 to 0.97) and very good for conv-CT (ICC = 0.81, 95% IC = 0.65 to 0.90)

## DISCUSSION

In the current study, we demonstrated that spectral-CT is more accurate than conv-CT for the detection of the communications between true and false lumen through the intimal flap of patients with aortic dissections. Spectral-CT combines the possibility to visualize the tears with the ability, similar to 4D flow MRI, to detect and characterize the focal hemodynamic variations of jet flows caused by the blood moving between true and false lumen. Most of these communications went unnoticed on conv-CT when an invisible or very small tear in the intimal flap was at their origin.

The important role of the characteristics of the entry and reentry tears in shaping the pulsatile blood flow in and out of the false lumen, and, therefore, the entire hemodynamic equilibrium of the aorta, has been demonstrated by in vitro and in vivo studies. The size of the communications has an important impact on hemodynamics since large tears (in the order of 25% of the aortic diameter in the study of Rudenick et al⁶) result in similar pressure in the 2 lumina.⁶,¹³ Also, the localization of the entry and reentry tears influences the pressure inside the false lumen.¹⁴,¹⁵ Moreover, in vivo, it has been shown that the ejection fraction of the entry tear correlates with the growth rate of the false lumen.¹⁶

In clinical practice, distal anastomotic new entries (DANE) are very important to detect as their presence prompts reflections on the necessity and feasibility of a new intervention.¹⁷ In addition, localizing the entry and reentry tears influences the extent and landing of stent-graft coverage.¹⁸ Based on our results, spectral-CT can already answer these questions. Other than this, what should be the fate of the communications
<figure><img src="image_6.png" /><figcaption>**FIGURE 4.** Comparison of performances of conventional and spectral CT. Graph showing the percentage of communications through the intimal flap visible on spectral and conv-CT images.</figcaption></figure>

between the TL and the FL remains open for debate with some authors advocating that more or bigger tears should be fabricated to allow equalization of pressures in the 2 lumina¹⁹ and a few authors proposing to close them all.²⁰,²¹

Part of this uncertainty derives from the lack of an established method to identify and characterize these tears in vivo due to the limitations of both 4D flow and CT, the only 2 noninvasive methods that are available.

First, 4D flow is limited by the prolonged examination time, its inappropriateness for emergency, the lack of information on structures surrounding the aorta, and its notably low spatial resolution.²² Indeed, its in-plane resolution is of 3x3x3 mm,²²,²³ which will make the morphologic identification of many tears impossible. Therefore, in most cases, only hemodynamic changes due to the jet flows through the communications are detectable. Since large tears have low-speed blood flow, they might be difficult to identify. Furthermore, the complexity of postprocessing as well as the differences of results between visualization tools and software should be taken into account.²⁴ For instance, this might explain the higher number of communications we reported in our study as compared with previously published papers.²⁵

In contrast, CT is the noninvasive imaging modality to diagnose and follow up AD par excellence. Nevertheless, the identification of the communications, relying on the direct visualization of the tears, can be arduous for several reasons. First, the spatial resolution might be insufficient to see very tiny tears, for instance, those resulting from the detachment of small branches such as the intercostal arteries. In addition, the intimal flap undergoes conspicuous morphologic changes during the cardiac cycle that might mask some of the tears.²⁶ Last but not least, it is commonly believed that CT examinations cannot provide hemodynamic information, or very limited and in specific situations. In this study, we show that spectral-CT can help move beyond this assumption.

Spectral-CT provides more detailed and specific information about material composition and characteristics as compared

with conv-CT.¹¹ For instance, Z-effective images allow for color-coded representation of the effective atomic number of the elements in the voxel.²⁷ We have shown that, thanks to these features of spectral imaging, hemodynamic changes occurring in the lumina of aortic dissection due to the jets of blood passing through the communications can be visualized. We believe that spectral-CT allows to detect small differences in iodine concentration, and, therefore, Z-effective values, which are otherwise invisible. In fact, it has been proven that spectral-CT allows for distinction of very small differences in iodine content.²⁸,²⁹ These small changes are made visible to the human eye by the use of contrasting colors or shades of colors with abrupt changes for very small differences of mg/mL of iodine. In the clinical scenario we focused on, these differences are due to the fact that the jet of blood passing from one lumen to the other has a concentration of contrast media that is different as compared with the lumen of destination. This phenomenon is already known and exploited to detect jet flows between cardiac chambers presenting a very high differential of densities.³⁰

Interestingly, looking at the cases presented in the figures of this paper, it is evident that the jets seen on spectral-CT have a morphology similar to that found on 4D flow images. Nevertheless, it should be kept in mind that images from CT and 4D flow MRI do not show the same phenomenon: 4D flow images represent the speed in a scale of greys while CT ones show differences in iodine concentration. This, as well as the fact that the 2 examinations were sometimes performed after a long interval, whereas hemodynamic changes can occur quite rapidly in aortic dissection (even during a single heartbeat), can explain some of the differences we encountered. Indeed, we cannot be entirely sure if the differences between MRI and spectral-CT are real or attributable to some changes in hemodynamics that occurred between the examinations.

It could be argued that if the communications are so small that they cannot be seen with conv-CT they cannot have important hemodynamic consequences. However, new data points in the opposite direction.³¹,³² Indeed, computer simulations performed with and without taking into account small aortic branches showed significant differences regarding several hemodynamic parameters, including velocities that differed of up to 46% between the two models.³¹

Spectral photon counting CT, allowing to combine very high spatial resolution, spectral data and reduced blooming artefacts, might allow for even better results.³³⁻³⁷ In addition, angio-K-edge and bicolor imaging open up the possibility for new scenarios.³⁸⁻⁴⁰

This study is subject to several limitations. The first is the small sample of patients included for analysis. The second is the already mentioned inconstant time between CT and MRI examinations. Thirdly, in most cases, we did not evaluate the entire aorta. Indeed, the analysis was not possible in the distal part of the abdominal aorta, as it often fell outside the field of 4D flow MRI. This is likely responsible for an underestimation of the number of bidirectional tears. In addition, the images obtained

<table><caption>**TABLE 3.** Sensitivity, Specificity, Positive Predictive Value, Negative Predictive Value, Accuracy for Conventional and Spectral-CT</caption><thead><tr><th></th><th>Sensitivity (%)</th><th>Specificity (%)</th><th>PPV (%)</th><th>NPV (%)</th><th>Accuracy (%)</th></tr></thead><tbody><tr><td>Conv-CT—observer 1</td><td>26</td><td>52</td><td>78</td><td>10</td><td>29</td></tr><tr><td>Conv-CT—observer 2</td><td>22</td><td>85</td><td>90</td><td>14</td><td>30</td></tr><tr><td>Spectral-CT</td><td>70</td><td>22</td><td>85</td><td>10</td><td>63</td></tr></tbody></table>

NPV, negative predictive value; PPV, positive predictive value.
<figure><img src="image_7.png" /><figcaption>**FIGURE 5.** Examples of communications between the two lumina and associated jet flows as visualized on the different imaging modalities. In the first column, an example of a small tear visible on conventional and spectral-CT with a corresponding jet flow passing through the intimal flap from the true to the false lumen with similar characteristics on spectral-CT and 4D flow MRI. In the second column, an example of a communication that was very doubtful on conv-CT due to the presence of a small area of intimal flap that is very thin yet continuous. In this case, spectral-CT shows a jet flow from the true to the false lumen. Notice how the part of the jet closest to the opposite wall of the aorta is larger, probably indicating some rebounding flow as confirmed by the 4D flow MRI that shows similar images. The third column shows a case where conv-CT showed a seemingly homogeneous intimal flap while spectral-CT and 4D flow MRI clearly show an eccentric jet that partially longs the aortic wall. For each of the 3 examples, conv-CT (first line), spectral-CT (second line; overlay of Zeff on removed iodine), and 4D flow MRI (third line) are shown. On the fourth line, schemes of the jet flows (morphology and direction). While on MRI images different shades of white and black translate different velocities (with white being the highest), on spectral-CT colors are discretional and depend on the Zeff, that is the concentration of contrast media. The scale used for each of the presented examples with some reference values is shown next to each spectral-CT panel.</figcaption></figure>

with CT scan (conventional and spectral) were mostly acquired without cardiac synchronization. Those obtained with ECG-gating were available only in the diastolic phase. Therefore, changes during the cardiac cycle could not be compared between MRI and CT. The observers were not blind to the type of imaging modality they were assessing, which might represent a bias, probably without a profound effect on the results considering that the findings on the different modalities were

not the same. Finally, we did not weight the results of our analysis against the image quality, injection and timing parameters as well as attenuation in the lumen, which could be the cause of some of the differences in accuracy between patients. While some of these parameters could be homogenized in further prospective studies, some others, such as the attenuation in the lumina, are very difficult or even impossible to control since they are the result of numerous and complex patient-specific factors.
<figure><img src="image_8.png" /><figcaption>**FIGURE 6.** Performances of conventional and spectral CT per degree of confidence and location of the communications. Graphs showing the percentage of true positives, true negatives, false positives and false negatives for the detection of communications through the intimal flap in the thoracic (A) and abdominal aorta (B) as well as divided by the degree of confidence on spectral-CT [the high degree of confidence corresponds to confidence scores of 2 and 3 (C), the low to a confidence score of 1(D)].</figcaption></figure>

## CONCLUSIONS

Spectral-CT scanner outperformed conv-CT for the detection of communications between the true and false lumen in aortic dissection while showing moderate accuracy compared with 4D flow MRI. These communications, most of which are undetectable with conv-CT scanner, are visible on spectral-CT images as jet flows, allowing also for the definition of the direction of the flow. Further studies are needed to derive further reliable and relevant hemodynamic parameters from spectral images as well as to assess the clinical implications of the findings of this study.

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