False lumen pressure estimation in type B aortic dissection using 4D flow cardiovascular magnetic resonance: comparisons with aortic growth

Abstract

Background: Chronic type B aortic dissection (TBAD) is associated with poor long-term outcome, and accurate risk stratification tools remain lacking. Pressurization of the false lumen (FL) has been recognized as central in promoting aortic growth. Several surrogate imaging-based metrics have been proposed to assess FL hemodynamics; however, their relationship to enlarging aortic dimensions remains unclear. We investigated the association between aortic growth and three cardiovascular magnetic resonance (CMR)-derived metrics of FL pressurization: false lumen ejection fraction (FLEF), maximum systolic deceleration rate (MSDR), and FL relative pressure (FL ΔP_max).

Methods: CMR/CMR angiography was performed in 12 patients with chronic dissection of the descending thoracoabdominal aorta, including contrast-enhanced CMR angiography and time-resolved three-dimensional phase-contrast CMR (4D Flow CMR). Aortic growth rate was calculated as the change in maximal aortic diameter between baseline and follow-up imaging studies over the time interval, with patients categorized as having either 'stable' (< 3 mm/year) or 'enlarging' (≥ 3 mm/year) growth. Three metrics relating to FL pressurization were defined as: (1) FLEF: the ratio between retrograde and antegrade flow at the TBAD entry tear, (2) MSDR: the absolute difference between maximum and minimum systolic acceleration in the proximal FL, and (3) FL ΔP_max: the difference in absolute pressure between aortic root and distal FL.

Results: FLEF was higher in enlarging TBAD (49.0 ± 17.9% vs. 10.0 ± 11.9%, p = 0.002), whereas FL ΔP_max was lower (32.2 ± 10.8 vs. 57.2 ± 12.5 mmHg/m, p = 0.017). MSDR and conventional anatomic variables did not differ significantly between groups. FLEF showed positive (r = 0.78, p = 0.003) correlation with aortic growth rate whereas FL ΔP_max showed negative correlation (r = - 0.64, p = 0.026). FLEF and FL ΔP_max remained as independent predictors of aortic growth rate after adjusting for baseline aortic diameter.

Conclusion: Comparative analysis of three 4D flow CMR metrics of TBAD FL pressurization demonstrated that those that focusing on retrograde flow (FLEF) and relative pressure (FL ΔP_max) independently correlated with growth and differentiated patients with enlarging and stable descending aortic dissections. These results emphasize the highly variable nature of aortic hemodynamics in TBAD patients, and suggest that 4D Flow CMR derived metrics of FL pressurization may be useful to separate patients at highest and lowest risk for progressive aortic growth and complications.

Keywords: 4D flow MRI; 4D flow magnetic resonance imaging; Aortic growth rate; False lumen; False lumen ejection fraction; Maximum systolic deceleration rate; Relative pressure; Type B aortic dissection.

Fig. 1

a Overview of the cardiovascular magnetic resonance (CMR) post-processing including contrast-enhanced magnetic resonance angiography (CE-MRA) segmentation of the true lumen (TL) and the false lumen (FL) and registration with 4D Flow CMR data. Subsequently, three different flow-based metrics relating to FL pressurization are derived: b False lumen ejection fraction (FLEF), calculated as the ratio between retrograde and antegrade flow through the dissection entry tear; c Maximum systolic deceleration rate (MSDR), calculated as the absolute difference between peak systolic acceleration and peak systolic deceleration in the TL; d Maximal and minimum relative pressures (ΔP_max & ΔP_min), or the difference in absolute pressure between the aortic root and level of the diaphragm, computed for both TL and FL using the image-based virtual Work-energy relative pressure (vWERP) approach

Fig. 2

Representative patient examples of FL hemodynamic evaluation, provided for two subjects with slow aortic growth (top and middle row), and one with rapid aortic growth (bottom row). In all instances, extraction of FLEF (a), MSDR (b), and relative pressure by vWERP (c) is shown together with associated output (VF = virtual field)

Fig. 3

Scatter plots depicting the correlation between aortic growth rate and a baseline maximal aortic diameter, b false lumen ejection fraction, c maximum systolic deceleration rate, and d false lumen maximum relative pressure

Fig. 4

Conceptual model depicting the proposed relations between growth and 4D Flow-derived markers of FL pressurization. False lumen ejection fraction (FLEF): The top row depicts increased retrograde flow (light blue) relative to antegrade flow (light red) with increasing aortic growth rate, hypothesized to be related to increased FL pressurization. Maximum systolic deceleration rate (MSDR): The middle row depicts the acceleration of blood through a proximal portion of the FL (the light purple shaded area), with MSDR representing the mean rate of change between peak acceleration and peak deceleration (i.e. the downward slope between peaks). With increasing FL pressurization, higher resistance FL flow leads to faster flow deceleration (i.e. a more pronounced, steeper slope). FL maximal relative pressure (FL ΔP_max): The bottom row depicts the observed trend between decreased relative pressure between the aortic root and the distal FL (the striped region in the TBAD to the left) and increasing aortic growth rate. Increased FL pressurization leads to increased resistance to flow, and thus dampening of relative pressure gradients