Noninvasive Morphologic and Hemodynamic Evaluation of Type B Aortic Dissection: State of the Art and Future Perspectives

Abstract

Stanford type B aortic dissection (TBAD) is associated with relatively high rates of morbidity and mortality, and appropriate treatment selection is important for optimizing patient outcomes. Depending on individualized risk factors, clinical presentation, and imaging findings, patients are generally stratified to optimal medical therapy anchored by antihypertensives or thoracic endovascular aortic repair (TEVAR). Using standard anatomic imaging with CT or MRI, several high-risk features including aortic diameter, false lumen (FL) features, size of entry tears, involvement of major aortic branch vessels, or evidence of visceral malperfusion have been used to select patients likely to benefit from TEVAR. However, even with these measures, the number needed to treat for TEVAR remains, and improved risk stratification is needed. Increasingly, the relationship between FL hemodynamics and adverse aortic remodeling in TBAD has been studied, and evolving noninvasive techniques can measure numerous FL hemodynamic parameters that may improve risk stratification. In addition to summarizing the current clinical state of the art for morphologic TBAD evaluation, this review provides a detailed overview of noninvasive methods for TBAD hemodynamics characterization, including computational fluid dynamics and four-dimensional flow MRI. Keywords: CT, Image Postprocessing, MRI, Cardiac, Vascular, Aorta, Dissection © RSNA, 2021.

Keywords: Aorta; CT; Cardiac; Dissection; Image Postprocessing; MRI; Vascular.

Figure 1:

Kaplan-Meier survival analysis in the matched patients (with uncomplicated type B aortic dissection) who were treated with TEVAR or BMT. The differences between the two groups were assessed with log-rank test. (A) Freedom from all-cause death between the two groups. Freedom from all-cause death in the TEVAR group was significantly greater than that of the BMT group (P = .028). (B) Freedom from aortic-related death between the two groups. Freedom from aortic-related death in the TEVAR group was significantly greater than that of the BMT group (P = .044). (Reprinted, under a CC BY license, from reference 11). Note that best medical treatment (BMT) terminology is equivalent to optimal medical therapy. TEVAR = thoracic endovascular aortic repair.

Figure 2:

(A–D) Images from standard CT evaluation of rTAAD show multiple planar reformatted imaging with (D) double oblique measurement of descending aorta diameter. This patient has repaired TAAD and a large fenestration between the true lumen (TL) and the false lumen (FL) in the proximal aortic arch (red arrow). rTAAD = residual type B aortic dissection following repair of type A aortic dissection.

Figure 3:

Representative MR angiographic image of a type B aortic dissection in cross section utilizing a respiratory navigator–gated, T2-prepared balanced steady-state free precession sequence. In this patient, the false lumen (FL) circumferential extent is approximately 215° of the total circumference of the aorta. According to Sailer et al, an FL angular extent greater than 249° is considered a high-risk feature predictive of adverse events (20). TL = true lumen.

Figure 4:

Boxplot shows the spatial variation and mean values of lumen pressure difference between the false lumen (FL) and the true lumen (TL) of an ex vivo porcine model after propagation with and without distal re-entry tear. Mean values are represented by diamond symbol within the boxplot. (Reprinted, under a CC BY license, from reference 26).

Figure 5:

Left: Patient-specific TBAD reconstructed from CT image. Right: TAWSS shows areas of high wall shear stress proximally at entry tear (right). (Reprinted, under a CC BY license, from reference 27). TAWSS = time-averaged wall shear stress, TBAD = type B aortic dissection.

Figure 6:

Images in a 69-year-old man with repaired type A aortic dissection. Color-coded streamline visualizations during (A) midsystole, (B) late systole, and (C) early diastole show the differences in flow patterns in the TL (solid arrow) and FL. Open arrow indicates a local region of flow acceleration. (Reprinted, under a CC BY license, from reference 37). FL = false lumen, TL = true lumen.

Figure 7:

Blood flow rate per aortic lumen area (mL/s/cm²) curves standardized by one cardiac cycle. (A–D) Control participants. (E–H) TL of patients with CDTAD. (I–K) FL of patients with CDTAD. * = P < .01 versus control (peak systole), † = P < .05 versus control (peak systole), ‡ = P < .05 versus CDTAD TL (peak systole). (Reprinted, with permission, from reference 39). CTDAD = chronic descending thoracic aortic dissection, FL = false lumen, TL = true lumen.

Figure 8:

Forward flow, reverse flow, kinetic energy (KE), and stasis maps in three example subjects. Top row: A 55-year-old medically managed patient with TBAD; middle row: a 63-year-old patient with rTAAD after open AAo replacement with aortic valve replacement; and bottom row: a 54-year-old control. Arrows show regions of elevated forward flow, reverse flow, and KE (patient with AAo repair), as well as elevated stasis (patient with TBAD). (Reprinted, under a CC BY license, from reference 42). AAo = ascending aorta, rTAAD = residual type B aortic dissection following repair of type A aortic dissection, TBAD = type B aortic dissection.

Figure 9:

Hemodynamic characterization of the FL in patients with TBAD. Boxplot is shown with red line = median, large box = interquartile range, 25%–75% of data. Each data point represents the average ROI value for one subject along the FL (for patients) or entire aorta (for controls). * = P < .05, ** = P< .001. (Reprinted, under a CC BY license, from reference 42). Red + indicates outliers, values that fall below the boxplot-defined minimum and above the maximum. AAo Repair = repaired type A (ascending aorta [AAo]) aortic dissection, ET = open elephant trunk repair, FL = false lumen, KE = kinetic energy, ROI = region of interest, TBAD = type B aortic dissection.

Figure 10:

Four-dimensional (4D) flow hemodynamic assessment. (A) The FL EF was measured in the plane of the dominant entry tear and was defined as the proportion of retrograde flow (L/min) exiting the FL during diastole over the systolic antegrade flow volume (L/min) at the dominant entry tear. (B) Three-dimensional visualization of 4D flow MRI data in a patient with TBAD demonstrating the flow analysis plane for the entry tear FL EF measurement (red line) and the TL and FL analysis planes (gray line) measured 3 cm distal to the tear. Antegrade flow is depicted in the TL and FL during systole (black arrow), with retrograde flow being “ejected” from the FL during diastole (yellow arrow) in a representative case with measured FL EF of 49%. (Reprinted, under a CC BY license, from reference 44). FL EF = false lumen ejection fraction, LSC = left subclavian artery, TBAD = type B aortic dissection, TL = true lumen.

Figure 11:

Scatterplots demonstrate the correlations between aortic growth rate and (A) baseline maximal aortic diameter, (B) false lumen ejection fraction, (C) dominant entry tear size, and (D) distance from the LSC to the entry tear. (Reprinted, under a CC BY license, from reference 44). LSC = left subclavian artery.

Figure 12:

(A) CTA, (B) 4D flow MRI velocity MIP overlaid on magnitude images in an oblique sagittal view, and (C) CEMRA. The CTA and MRA clearly demonstrate dissection anatomy (TL [yellow arrows] and FL), however, both are often limited by blurring artifact related to pulsatile flap motion. The 4D flow velocity MIP reveals four discrete fenestrations from their associated flow jets into the FL during systole (white arrows). (Reprinted, under CC BY 4.0 license, from reference 49). AAo = ascending aorta, CEMRA = contrast material–enhanced MR angiogram, CTA = CT angiogram, DAo = descending aorta, FL = false lumen, MIP = maximum intensity projection, TL = true lumen, 4D = four dimensional.

Figure 13:

(A) Image from CEMRA demonstrates a complex type B dissection. (B) Image from 4D flow MRI velocity MIP during systole reveals several large, relatively high-velocity jets entering an aneurysmal segment of the false lumen (*) at the proximal DAo (white arrows) and impinging on the false lumen wall. (C) In diastole, there is retrograde flow into the true lumen at these sites (white arrows), with an additional fenestration in the distal DAo seen only due to diastolic flow (red arrow). This fenestration was not seen at CT. (Reprinted, with permission, from reference 49). AAo = ascending aorta, CEMRA = contrast material–enhanced MR angiogram, DAo = descending aorta, MIP = maximum intensity projection, 4D = four dimensional.