Teaching Case

A 60-year-old male patient was referred for a follow-up carotid ultrasound after surgical treatment of a type A aortic dissection with Bentall procedure. The dissection was known to be extending to the right common and internal carotid artery.

Transverse B-mode could visualise the intimal flap as an echogenic linear structure floating within the lumen. However, when the ultrasound beam was parallel to the flap’s orientation, the latter was not visualised, potentially misleading the examiner. Similarly, due to overwriting artifact caused by inappropriately adjusted colour gain, the flap was occasionally missed, covered by colour flow signals, while with more careful adjustment of parameters, the intimal flap could be detected with this technique as well. The use of power Doppler imaging, colour B-Flow and B-Flow technique increased the technique’s sensitivity, accurately visualising the intimal flap in both transverse images and longitudinal images. (Fig.1, 2) The dissection could be seen extending to the internal carotid bulb. (Fig.2) The integration of various flow-detecting techniques like power Doppler technique and B-Flow has the potential to increase the ultrasonographic technique’s sensitivity for visualisation of dissection, in the setting of multiparametric ultrasound.

Dissection of the carotid and vertebral arteries is a rare entity with less than 5 new cases per 100.000 of the general population every year. However, it represents an important cause of cerebral ischaemia in young and middle-aged patients. Carotid dissection may either be associated with aortic dissection or may exclusively affect the carotid artery, either caused by trauma or occurring spontaneously. [1] US may be more than 90% sensitive in the diagnosis of carotid dissection, thus being a reliable technique to exclude the presence of this entity. Nevertheless, definite diagnosis and the extent of disease should always be evaluated with a cross-sectional imaging technique such as computed tomography angiography or magnetic resonance angiography. [2, 3] One characteristic but indirect finding of carotid dissection is the detection of a high resistance flow pattern at the initial part of the internal carotid artery, suggestive of distal occlusion or high-grade stenosis. This is explained by the tendency of dissection to affect the distal part of the internal carotid artery, differently to what happens with atherosclerosis. [1] Decreased or reversed diastolic flow can be visualised in 68% of internal carotid artery dissection. Finally, a normal waveform, low amplitude biphasic pattern or absence of flow in the occluded vessel can also be found during spectral Doppler interrogation. [2, 4, 5] Intramural haematomas of the carotid artery can be occasionally visualised as hypoechoic thickening of the vessel wall. The classic finding of carotid dissection is that of a visible intimal flap dividing the vessel lumen in two parts, but is only rarely found. [1, 6] Furthermore, the absence of calcifications, ulcerations and other findings of atherosclerosis helps differentiating dissection from atherosclerosis. [5] B-flow imaging is a newer vascular ultrasonographic technique which was reported to be more sensitive and specific than classic colour Doppler ultrasonography in the diagnosis of carotid dissection. It was found to more accurately identify intimal flaps and visualise flow within true and false lumens. B-Flow’s advantages include the lack of overwriting artifacts, angle dependence and increased sensitivity to slow flow. As a consequence, it can readily and accurately visualise fissures, intimal flaps and slow flow within false lumen in patients with carotid dissection. The intimal flap’s movement can also be better appreciated using the cine mode f B-Flow. [7, 8, 9] As a take home message, techniques like power Doppler or B-Flow can be used to increase ultrasound’s accuracy for the diagnosis and follow-up of carotid dissection.