Three-dimensional Growth Analysis of Thoracic Aortic Aneurysm with Vascular Deformation Mapping (VDM)

Patients with thoracic aortic aneurysm (TAA) undergo regular imaging surveillance, commonly with computed tomography angiography (CTA). Aortic diameter measurements are the standard metric for assessing aortic growth and risk for adverse events, but are subject to significant measurement variability, on the order of 1-5 mm, a problem that is compounded when serial studies are compared ^{1, 2}. Vascular Deformation Mapping (VDM) is a recently developed technique that utilizes serial CTA examinations to quantify interval aortic growth in a three-dimensional manner ³. VDM measures the local change in aortic wall dimensions between two time points through registration of clinical computed tomography angiography (CTA) data followed by quantification of local aortic wall deformation (i.e., growth) between studies using a spatial Jacobian analysis. The determinant of the spatial Jacobian at each voxel is normalized by the time interval to yield a deformation rate (|J|/year) and this data is superimposed on a 3D model of the aorta to allow for topographical depiction of aortic growth. The spatial Jacobian tensor is a dimensionless parameter, thus paired luminal circumference measurements are used to calculate aortic growth rate (mm/year). We present the case of a young woman with aortitis and a rapidly enlarging ascending aortic aneurysm undergoing pre-surgical evaluation, studied as part of a HIPPA-compliant and IRB-approved study at the University of Michigan. While the tubular ascending aorta met size criteria for surgical repair, an accurate assessment of growth in adjacent segments was desired to determine the extent of repair (i.e., how much aorta to resect), as replacement of the aortic root and arch carry added technical challenges and patient risk. Maximal diameter measurements were performed on clinical CTA studies spanning a 2-year period acquired on the same dual-source CT scanner (Siemens SOMATOM Force; Siemens Healthcare AG, Erlangen, Germany) but acquired at difference centers (Figure 1). Rapid growth of the mid-ascending level was clearly detected by diameter measurements with a calculated growth rate of approximately 9 mm/year. Although there was approximately 1 mm of increase in the maximal aortic diameter at the level of the sinuses, proximal arch, and distal arch, the conclusion of clinical diameter assessment was that these segments were stable within the limits of measurement variability (i.e., ± 2mm)⁴. Subsequently, VDM analysis was performed on the same CTA studies (Figure 2, Video 1) and results were validated by comparison with paired luminal circumference measurements (Figure 3). In agreement with diameter measurements, rapid growth (9.2 mm/year) was noted at the ascending aorta by VDM analysis and the aortic root dimension were stable over the 2-year interval. VDM analysis demonstrated that growth of the ascending aorta extended proximally to involve the sinotubular junction, from which both the right and left coronary ostia arose, implying the need for coronary reimplantation. In disagreement with diameter assessment low intensity (1.0 mm/year), eccentric growth was noted at the proximal arch, with a higher degree of growth along the greater curvature (yellow arrow) than the lesser curvature (purple arrow). Lastly, an area of low intensity growth (0.8 mm/year) was detected in at the mid-descending level which was not clinically suspected but consistent with the patient’s aortitis (blue arrowhead). Using VDM results, a surgical repair strategy was devised that maximized resection of diseased aortic tissue while balancing surgical risk (Figure 2, grey dotted line). Aortic growth occurs as a result of failing aortic wall structural integrity, however, diameter-based assessments are often limited for confident detection of slow due to measurement variability and do not depict growth in a three-dimensional manner. Vascular Deformation Mapping is a new imaging analysis technique that overcomes these limitations while harnessing the high-resolution, volumetric data produced by modern CTA techniques, allowing for a more comprehensive depiction of aortic growth that can be applied to inform surgical planning and advance understanding of thoracic aortic aneurysm disease progression.

Figure 1.

A 3D volume rendering of a the most recent CTA examination (left) demonstrating standard locations for clinical aortic diameter measurements including: sinuses of Valsalva (A), mid-ascending (B), proximal arch (C), and distal arch (D). Maximum aortic diameter measurements (in mm) at each location are shown at baseline (2016) and follow-up (2018) CTA examinations at the matched locations, using double-oblique technique.

Figure 2.

Result of Vascular Deformation Mapping (VDM) shown in left anterior oblique (left) and right posterior oblique (right) projections. The color scale represents the degree of measured aortic wall deformation by spatial Jacobian analysis and is normalized by the interval between CTA examinations to yield a growth rate (|J|/year). Based on VDM analysis, growth of the ascending aorta involved the coronary ostia (white arrows), and eccentric growth was noted in the proximal arch, which was higher in degree along the greater curvature (yellow arrow) than the lesser curvature (purple arrow). A focal region of growth was detected in at the mid-descending level (blue arrowhead). An aberrant origin of the right subclavian artery was incidentally noted.

Figure 3.

VDM results were validated though measurement of change in aortic perimeter measurements (in mm) between baseline (2016) and follow-up (2018) CTA studies at five standard locations: sinuses of Valsalva (A), mid ascending (B), proximal arch (C), distal arch (D), mid descending (E). Aortic circumference was used to calculate a derived diameter for growth rate assessment [circumference (in mm)/π].