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Parametric Hemodynamic 4D flow MRI maps for the Characterization of Chronic Thoracic Descending Aortic Dissection

Kelly Jarvis¹, Judith T Pruijssen², Andre Y Son³, Bradley D Allen¹, Gilles Soulat¹, Alireza Vali¹, Alex J Barker⁴, Andrew W Hoel⁵, Mark K Eskandari⁵, S. Chris Malaisrie³, James C Carr¹, Jeremy D Collins⁶, and Michael Markl¹
¹Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, ²Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen, Netherlands, ³Division of Cardiac Surgery, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, ⁴Department of Radiology, University of Colorado, Denver, CO, United States, ⁵Division of Vascular Surgery, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, ⁶6. Department of Radiology, Mayo Clinic, Rochester, MN, United States

Synopsis

Systematic evaluation of complex flow in descending aortic dissection (DAD) is needed to better understand which patients are predisposed to complications. Our goal was to utilize quantitative maps from 4D flow MRI for monitoring true and false lumen (TL, FL) flow characteristics. 4D flow was acquired in 20 DAD patients (6 medically managed, 14 with surgical repair), and 21 age-matched controls. 4D flow-derived quantitative maps demonstrated global and regional hemodynamic differences between DAD patients and controls. DAD patients with and without repair showed significantly altered TL and FL aortic hemodynamics, indicating this technique’s potential to characterize flow dynamics in DAD.

INTRODUCTION

Aortic dissection is a life-threatening vascular disease that occurs when blood flows through entry tears in the intima of the native aorta, or true lumen (TL), generating a false lumen (FL). Descending aortic dissection (DAD) can be isolated (Stanford type-B) or associated with ascending aortic dissection (Stanford type-A). One of the main questions in chronic DAD is which patients should have thoracic endovascular aortic repair to prevent complications (1,2). We hypothesized that comprehensive evaluation of aortic hemodynamics will help to better understand the disease and classify patients. 4D flow MRI is uniquely poised to evaluate complex flow patterns in DAD. This technique has detected flow alterations in aortic dissection related to aortic dilatation (e.g., helical flow, FL velocity), found increased TL flow with FL thrombus, and been used to identify small dissection flap fenestrations (3-7). Previous studies are promising but were limited by either utilization of qualitative assessment or localized quantification of hemodynamics and thus systematic parameter mapping of flow in the TL and FL is needed. We have developed 4D flow MRI-derived parametric maps of aortic dissection hemodynamics (forward flow, reverse flow, flow stasis, and kinetic energy). Our goal was to analyze these parametric maps across different subtypes of chronic descending aorta dissection and compare to controls.

METHODS

Free-breathing 4D flow MRI was acquired in 20 DAD patients (age=60±11 years; 12 male), including 6 medically managed type B (TBAD) and 14 repaired type-A (rTAAD) with ascending aortic (AAo) graft or elephant trunk (ET1) repair, and 21 age-matched controls (age=59±10 years; 13 male). Scan parameters were spatial resolution=2.7-5.0 x 2.0-3.1 x 2.2-5.0 mm3, temporal resolution=36.8-40.0 ms, and venc=150-270 cm/s (1.5T, 3T, Avanto, Aera, Skyra: Siemens Healthcare, Erlangen, Germany). The 4D flow data was preprocessed including calculation of time-averaged magnitude images and a time-averaged 3D phase contrast angiogram (PC-MRA). These data were used to segment the aorta (controls) and TL and FL (patients) (Figure 1a-e). Maps of aortic hemodynamics were derived based on home-built analysis tools similar to recently reported workflow (8,9). Briefly, 4D flow data were regridded to 1 mm3 voxels. A 3D aortic centerline was calculated to determine the direction of forward and reverse flow. Voxel-wise net forward flow (FF) and reverse flow (RF) were calculated as the sum over the cardiac cycle (Figure 1f-h). In addition, velocity magnitude was determined for each voxel at each cardiac time-frame, v(t), and voxel-wise flow stasis was calculated as the percentage of cardiac time-frames with v(t)<0.10 m/s. Also, voxel-wise kinetic energy (KE) was determined by KE=0.5*ρ*dV*v(t)2 with ρ=blood density (1060 kg/m3) and dV=unit voxel volume (1 mm3) (10) and summed over the cardiac cycle. 3D voxel-wise data for each parameter was collapsed into an average intensity projection and evaluated in 5 ROIs (Figure 1h).

RESULTS

Patient characteristics were TBAD (n=6, age=62±8 [54-76] years, 33% male), rTAAD with AAo repair (n=11, age=60±13 [35-86] years, 64% male, time since last surgery=34±51 [4-156] months), and rTAAD with ET1 (n=3, 55±1 [54-56], 100% male, 7±6 [0.1-13] months). Results for example subjects are shown in Figure 2. Elevated voxel-wise forward flow (TL), reverse flow (TL, FL) and kinetic energy (TL) are shown in the patient with AAo repair, compared to the patient with TBAD and the control. Additionally, elevated levels of voxel-wise stasis are shown in the patient with TBAD, compared to the patient with AAo repair and the control. Overall, patients with rTAAD presented with elevated TL reverse flow (AAo repair: p=.004, ET1: p=.018) and increased TL kinetic energy (AAo repair: p=.0002, ET1: p=.011) compared to controls. In addition, TL kinetic energy was increased vs. patients with TBAD (AAo repair: p=.021, ET1: p=.048). (Figure 3, Figure 5). rTAAD was associated with higher FL kinetic energy and lower FL stasis compared to patients with TBAD (AAo repair: p=.002, ET1: p=.024 and AAo repair: p=.003, ET1: p=.048, respectively) (Figure 4, Figure 5).

CONCLUSIONS

This study demonstrates the utility of hemodynamic mapping from 4D flow MRI as a quantitative technique for the characterization of chronic DAD showing differences between rTAAD and TBAD in descending aorta TL and FL flow patterns. These results indicate the potential for parametric mapping of underlying hemodynamics in the TL and FL (directional flow, flow stasis, kinetic energy) to play an important role in the understanding of aortic dissection. Future studies are warranted to determine key metrics related to outcome, help to plan invasive procedures and monitor asymptomatic patients over time.

Acknowledgements

No acknowledgement found.

References

Please see the full paper at: Jarvis K, Pruijssen JT, Son AY, et al. Parametric Hemodynamic 4D Flow MRI Maps for the Characterization of Chronic Thoracic Descending Aortic Dissection. J Magn Reson Imaging 2019.

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Figures

Figure 1: Data analysis workflow. (a) Preprocessing of the original 4D flow data included calc. of time-avg'd magnitude images & a 3D phase-contrast MR angiogram enabling (b) 3D segmentation of the thoracic aorta (ie, extending from the aortic trunk to celiac arteries), & (c–e) separation of TL (ie, depicted by 3D PC-MRA) & FL (ie, depicted by time-avgd magnitude images). (f) Automatic calculation of 3D centerline allowed for (g) voxelwise def.of flow direction, ie, forward & reverse flow, based on closest orthogonal plane. (h) Avg intensity map with 5 ROIs defined for quantification.

Figure 2: Forward flow, reverse flow, kinetic energy and stasis maps in three example subjects. Top: A 55-year-old medically managed TBAD patient; middle: a 63-year-old patient with rTAAD after open AAo replacement with aortic valve replacement; and bottom: a 54-year-old control. White arrows show regions of elevated forward flow, reverse flow, and kinetic energy (patient with AAo repair) as well as elevated stasis (patient with TBAD).

Figure 3: Hemodynamic characterization of the true lumen in patients with aortic dissection. Boxplot is shown with red line = median, large box = [25, 75]% of data. Each datapoint represents the average ROI value for one subject along the TL (for patients) or entire aorta (for controls). *P < 0.05, **P < 0.001. KE: kinetic energy.

Figure 4: Hemodynamic characterization of the false lumen in patients with aortic dissection. Boxplot is shown with red line = median, large box = [25, 75]% of data. Each datapoint represents the average ROI value for one subject along the FL (for patients) or entire aorta (for controls). *P < 0.05, **P < 0.001. KE: kinetic energy.

Figure 5: Regional analysis results. Schematic of aortic hemodynamics for ROIs in the TL and FL for patients with medically managed TBAD and rTAAD (ie, after AAo repair or ET stage 1). For comparison, results in aorta of healthy controls are shown on the left. Note, color-coding illustrates elevated values of reverse flow, KE, and stasis. Symbols indicate significant P-values: † = P < 0.05 compared to controls, ▲= P < 0.05 compared to TBAD.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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