Response to ‘Magnetic Resonance Imaging for Aortic Dissection’

We read with great interest the commentary by RE Clough, T Schaeffter and PR Taylor about the importance of MRA in aortic dissections. Nevertheless, controversy concerning the superiority of multidetector computed tomography (MD-CT) versus MRI still exists.

MRI allows swift 3 D high-resolution imaging, nevertheless, because of the closed bore design of the magnet and the need for patient monitoring devices MRI maybe less adapted than CT for unstable patients.

On the other hand, despite ionizing radiation hazards and nephrotoxicity of contrast agents, MD-CT optimized the balance between spatial and temporal resolution and invasiveness, hence propelling MD-CT to become the most widely used modality in current practice, thanks to its wide availability, speed, cost-effectiveness and efficiency. Imaging of all phases of contrast enhancement has also become possible using a single bolus of contrast agent, with delayed scans to visualize the parenchyma and the late opacification of false lumen. One significant drawback of MD-CT is a radiation dose, nevertheless, it is possible to reduce the radiation rate to a minimum by adequate parameter optimization.

As said by RE Clough, PR Taylor, it is true that movements of the dissected aortic intimal flap through the cardiac cycle are not clearly visualized without ECG-gating. On the other hand, CT without ECG-Gating is actually sufficient for the arch and the descending aorta, allowing a complete analysis of the thoracic and abdominal aorta in one single step. Moreover, ECG-gated CTA can accurately determine aortic distensibility.

In dissection, the selection of the “correct” Stent-graft dimensions is crucial. In acute dissections, the diameter of the non-dissected aortic segment immediately proximal to the entry tear is considered the reference. Inversely, for chronic dissection (>6 months) the intima becomes fibrotic and thus cannot expand. In such cases, a tapered stent-graft may be preferable and the distal diameter is easily measured, without major aortic variation during the cardiac phase.

RE Clough, PR Taylor said that “MRI generated data will allow the use of shorter endoluminal devices in patients requiring intervention”. We don't believe that short stent-graft should be used anymore for dissections in the future. It is well known that false-lumen thrombosis distal to the stent-graft, particularly in the distal descending aorta, is uncommon, longer stent-grafts than what is needed to simply cover the primary tear is the trend.

As a whole, considering the excellent accuracy of the two modalities, the imaging protocols for aortic diseases should be tailored to answer specific questions, taking into consideration the accessibility and the local expertise.