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A New Imaging Method for Assessment of Aortic Dissection Using Four-Dimensional Phase Contrast Magnetic Resonance Imaging
Study Questions:
Can four-dimensional phase contrast magnetic resonance imaging (4D PC-MRI) accurately characterize flow characteristics in type B aortic dissection, and do specific flow features relate to the rate of aortic expansion?
Methods:
4D PC-MRI was performed in 12 patients with medically treated type B dissection of the thoracic aorta. Subjects also underwent 3D MRI for anatomy and 2D PC-MRI as a standard for flow quantitation. 4D PC-MRI was acquired in the entire thoracic aorta, for quantitation of flow, and description of flow in the true and false lumen. Serial contrast-enhancement computed tomography was used to define the rate of aortic expansion over time.
Results:
4D PC-MRI flow measurements correlated well with 2D phase contrast MRI flow measurements (R2 = 0.98, p < 0.001) without significant bias. By computed tomography, the median aortic growth rate was 6.1 mm/yr, which correlated with the growth rate of false lumen. False lumen thrombosis (FLT) was seen in 7 of 12 subjects who had a growth rate of 11.4 mm/yr versus 9.9 mm/yr for those without FLT (p = 0.87). Flow in the true lumen was antigrade in 91.4% of instances, but was retrograde in 40.3% in the false lumen. Stroke volume was 54.3 ml in the true lumen compared to 31.6 ml in the false lumen (p = 0.0004). False lumen blood flow velocity was 7.1 cm/s compared to 18.0 cm/s in the true lumen (p < 0.0001). Differences in proximal and distal expansion rates correlated both with velocity and flow volume. Patients had 1-4 entry tears in the thoracic aorta (average 2), and a dominate tear was noted proximally in 9 and distally in 3 patients. The position of the dominant entry tear was associated with the area of greatest false lumen expansion in all cases. Flow was described as helical in the false lumen in 8 of 12 patients, and the degree of helical flow related to the rate of false lumen expansion.
Conclusions:
4D PC-MRI provides a high-resolution technique for describing flow characteristics in both the true and false lumen in patients with type B aortic dissection. Multiple parameters of flow in the false lumen as well as description of entry tears relate to the rate of subsequent aortic expansion.
Perspective:
Type B aortic dissection is typically treated medically unless complications arise necessitating a surgical or endovascular approach. One such complication is progressive expansion of the aneurysm, largely related to false lumen expansion. Although performed in a small number of subjects, this study nicely applies the sophisticated ability of 4D PC-MRI to describe flow patterns in both the false and true lumen, and quantify flow. In the context of a relatively small number of patients, several parameters related to a greater likelihood of expansion, which may necessitate either further surgical or endovascular approaches to the type B dissection. If confirmed in larger patient populations, these observations may allow prospective identification of patients at high risk of expansion for whom earlier intervention may be appropriate, rather than continued clinical observation.
Keywords: Aortic Aneurysm, Thoracic, Blood Flow Velocity, Thrombosis, Tomography, X-Ray Computed, Cardiovascular Diseases, Stroke Volume, Magnetic Resonance Imaging
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