Bicuspid aortic valve (BAV) is the most common congenital heart defect in the general population, with a prevalence of 1–2% [1, 2]. Bicuspid aortic valve has a particular association with ascending aortic dilatation, which is present in up to 80% of patients [3]. As dilatation progresses, it confers an increased risk for aortic dissection [4].

A minority presents with an anatomical BAV (top row in Figure 1, from [5], i.e., two leaflets without a central commissure) [6].

From Schaefer, et al [5]: orientation of real BAV (upper row, without a raphe), and functional BAV (lower row, with raphe).

Most BAV patients demonstrate a functional BAV i.e., two unequally sized leaflets, one of which has a central raphe that results from fusion of two leaflets (bottom row in Figure 1, from [5]. Functional BAV most often (>70% of cases) presents with fusion of right (RCC) and left (LCC) coronary cusp (anteroposterior (AP) orientation), whilst fusion of the RCC and non-coronary cusp (NCC) (laterolateral [LL] orientation) is seen in only 10–20%; fusion of the NCC and LCC is rare (5–10%). Routine ultrasound imaging substantially underestimates the prevalence and severity of BAV, and sensitivity of CT for BAV is only 67% [7].

In computed tomography (CT) imaging studies where the focus is not on the aortic valve, the diagnosis of bicuspid aortic valve is even more unlikely. This is mainly due to the double oblique orientation of the aortic valve, which largely varies between individuals. It is therefore difficult to spot the two or three cusps of the aortic valve while scrolling through a stack of transverse images. It is time consuming to reconstruct the double oblique image plane for optimal depiction of the aortic valve leaflets and cusps.

Delayed diagnosis of a bicuspid aortic valve conveys a risk for complications of associated aortic disease, of which ascending aortic aneurysm is the most common [3].

In this study we investigated the use of the new ‘hammock sign’ on coronal and sagittal contrast enhanced CT images for quick ‘scroll-by’ detection of a bicuspid aortic valve. Computed tomographic imaging data is, by default, presented in transverse images, but coronal and sagittal image planes are reconstructed easily and routinely.

The hammock sign is inspired from the occasional depiction of a bicuspid aortic valve spanning the entire width of the aortic sinus on a coronal or sagittal reconstruction, its curve resembling a hammock (Figure 2).

Bicuspid aortic valve leaflet spanning the width of the sinus in a curve resembling a hammock.

Our main findings were that the hammock sign was not seen on coronal or sagittal images in any tricuspid valve or functional BAV, and that a positive hammock sign was always present on coronal images, often on both coronal and sagittal images (swinging hammock), but not on sagittal images alone. Inter-observer variability was very good.

The authors propose a new sign for rapid coronal scroll-by detection of a BAV. It solely relates to valve morphology, independent of eventual concomitant aberrant aortic morphology.

On transverse images, it is often difficult to discriminate bicuspid from tricuspid valve. Indeed, the aortic valve has a double oblique orientation in reference to any orthogonal plane with much interindividual variation. However, with the use of thin slice image datasets, image reconstruction in coronal and sagittal plane has become standard in daily routine.

Figure 2 is taken from a coronal stack of images (as seen in Figure 4A) reconstructed from a non-ECG-triggered CTA study of a patient with a bicuspid aortic valve. This bicuspid valve is, coincidentally, perfectly aligned with the coronal plane. The sagittal image stack of the same study shows the coaptation of the leaflets on every image (Figure 4B). In any other orientation of the bicuspid aortic valve the coaptation of the leaflets will show to be swinging to and fro in the sinus when scrolling through the coronal and the sagittal images (Figure 5). We referred to this image as ‘swinging hammock’ sign.

Upper Left (A): Scrolling through a coronal stack of images of a BAV which has a perfectly coronal coaptation plane. The hammock-like curve of the valve leaflets is present on every image: positive hammock sign. Lower Right (B): On the sagittal images there is no hammock-like curve at all.

Scrolling through a coronal image stack of a BAV which has an oblique orientation to the coronal plane: the valve coaptation swings from (the patient’s) right to left.

For comparison, coronal reconstruction of a tricuspid aortic valve is shown in Figure 6A. When scrolling through the image set, one coaptation of the leaflets moves to the middle of the sinus, one coaptation lies inconspicuously in the valve plane (*) and the third coaptation returns from the middle of the sinus to the same side of the aortic sinus.

A. Scrolling through an oronal stack of images of a tricuspid aortic valve: the coaption plane of right and posterior leaflet has a coronal orientation and is thus rather inconspicuous (indicated with *). The two other coaptation planes can be seen moving inwards from the (patient’s) left, down to the middle of the sinus, and then up to the left again. In B, an oblique reconstruction to show images perpendicular to the left-and-posterior coaptation plane: two coaptation planes move inwards from either side, and the third remains centered.

The same dataset was reconstructed obliquely, to show the situation where one of the three coaptation segments is perpendicular to the image reconstruction plane (Figure 6B).

Our aortic hammock sign should not be confused with the hammock sign of the coronary artery that has been proposed for the description of the exceptional course of the left main coronary artery dipping downwards into the interventricular septum and then sloping upwards before it bifurcates into left anterior descending and circumflex arteries [8].

The hammock mitral valve denotes a congenital abnormal morphology, and it is as such not an imaging sign [9].

This study aims to evaluate the aortic hammock sign on sagittal and coronal images combined in any contrast enhanced thoracic CT study, with or without ECG-triggering, in arterial or venous phase scans.

The hammock sign is evaluated at a glance, by quickly scrolling through the aortic valve, mainly in the coronal plane, maybe also in the sagittal plane.

The hammock sign is a dynamic sign, i.e., it can only be seen by scrolling through a stack of images. Indeed, in a tricuspid aortic valve, a single image may be found where one leaflet is depicted at its largest (see Figure 4B, upper row). The hammock sign can only be called positive if this pattern is present on all images across the aortic sinus. One cannot evaluate the hammock sign on a still image.

Finding a hammock sign on coronal or sagittal image stack, or both, implies that the aortic valve is bicuspid and should be further examined, if possible with a reconstruction in the aortic valve plane. In our population, all the hammock sign positive image datasets were subsequently confirmed to represent bicuspid aortic valves.

The absence of the hammock sign does not exempt the CTA reader from adequate reconstruction of the aortic valve plane image, especially if a possible bicuspid valve morphology was the reason for the CTA referral.

Whether the hammock sign is present on coronal or on sagittal images depends on the orientation of the aortic root, and the phenotype of the bicuspid valve. Aortic bicuspidy can result from the fusion of left and right cusp (73%), of right and posterior (non-coronary) cusp (24%), or of left and posterior cusp (3%) [10].

In our small population, there was no combination of a hammock sign presenting on sagittal images only, and not on coronal images, which seems consistent with the presentation of bicuspid valves without raphe according to Figure 1, from Schaefer et al [5], it would require an unusual rotation of a type 2 valve without raphe to show up as a hammock on sagittal images only. Still, it remains useful to inspect the aortic valve on sagittal images, to add to the confidence of the diagnosis, since a coaptation segment of a tricuspid valve lying in plane with the reconstructed image can be very inconspicuous.

Findings on ECG triggered CT studies provide far more certainty as to the aortic morphology. However, ECG triggered studies are nearly exclusively performed in the setting of a cardiologic clinical query, where the radiologist will indeed focus his attention on the heart and its valves. This report on the use of the hammock sign is aimed at a widespread application in any contrast enhanced CT study. The hammock sign represents bicuspidy on any contrast enhanced CT image, even though it may manifest far less clearly in venous phase.

We did not evaluate non-contrast images, as the aortic leaflets cannot be distinguished. Leaflet calcification will most probably impede recognition of the hammock sign.

The hammock sign cannot be seen when the aortic valve is open (scanned in systole). One CT study, scanned without ECG triggering, showed an open aortic valve: hammock sign was therefore absent (evaluated as ‘undecided’, ‘missing’ in statistics).

The spectrum of aortic bicuspidy was reflected in our results by the presence of two tricuspid sinuses with a bicuspid valve opening. Most probably, the hammock sign fails to detect those functional BAV’s as the method relies on valve leaflet morphology in the closed state and not on valve opening morphology. At three evaluations each, both functional BAV were categorized as ‘definitely tricuspid’ three times, ‘probably tricuspid’ twice, and ‘maybe bicuspid’ just once.

Other findings associated with bicuspid aortic valve are dilatation of the aortic root or ascending aorta, and coarctation. In the presence of these abnormalities, reconstruction of a double oblique aortic valve plane is mandatory.

The strengths of this study are the selection of the study population, the integrity of the gold standard, and the inter- and intra-observer evaluation of the sign. The sign is new and easily applicable for any radiologist in the clinical routine of any contrast enhanced thoracic CT study. Hardly any learning curve is needed to start looking for this sign. Evaluating the aorta for this sign on coronal and/or sagittal images requires no special software and only the time of a quick focused look. The sign is directly and exclusively related to the valve, independent of concomitant aortic pathology.

This study has a few limitations: there is but a small number of subjects. Especially the number of functional BAV, not being the focus of this study, is too low to determine the detection rate of functional BAV. This sign would probably not perform well in the larger part of the spectrum of BAV disease. In a static imaging method such as CT detection of a raphe in an aortic valve, even with dedicated valve plane reconstruction, will always be challenging. With this new sign, the authors hope to raise attention to the aortic valve in a way that is accessible for all radiologists. We hope that this sign be used as a fast tool for screening for aortic valve pathology.