Preoperative Computed Tomography Angiography Reveals Leaflet-Specific Calcification and Excursion Patterns in Aortic Stenosis

Computed tomography (CT)–based scoring of aortic valve calcification (AVC) is gaining popularity as an alternative to echocardiography for adjudicating the severity of aortic stenosis (AS) and guiding the timing of valve replacement.¹ The utility of assessing AVC is well established by previous studies demonstrating strong correlations between CT-determined AVC load (AVC_CT) and AS patient survival.^2,3 Although AVC_CT load as a single measurement confers simplicity for clinical use, it masks distinct contributions from individual valve leaflets. The importance of examining valvular calcification in a leaflet-specific manner is further supported by the association of regional leaflet calcification with transcatheter aortic valve replacement (TAVR)–related complications (eg, advanced heart block).⁴

Although previous studies have noted significant differences in the calcification loads of individual valve leaflets, little is known about their functional contributions to AS.^4,5 In this study, we aimed to (1) retrospectively analyze pre-TAVR CT angiograms of patients with AS to characterize the interleaflet differences in calcification load, appearance, systolic excursion, and systolic excursion-calcification load relationships, (2) conduct computer simulations to examine how interleaflet differences in systolic excursion affect aortic valve area (AVA), and (3) identify factors that may affect calcification in a leaflet-specific manner.

Methods

All supporting data are available upon reasonable request to the corresponding author.

Study Population

This cross-sectional study involving patients from the Veterans Affairs Palo Alto Health Care System was conducted with prior approval from the Stanford-Veterans Affairs Institutional Review Board. Informed consent was waived by the Institutional Review Board. Two hundred consecutive male patients with degenerative (tricuspid) calcific AS who underwent retrospective electrocardiography–gated pre-TAVR computed tomography angiography (CTA) from July 2011 to October 2019 were enrolled (Figure S1 for study cohort flow chart). The only female patient who underwent TAVR during this period was not included due to known sex differences in AVC_CT,⁶ which could confound data analysis. Twenty consecutive male patients with normal aortic valves on echocardiography who underwent clinically indicated retrospective electrocardiography-gated coronary CTA with maximum tube current during both systole and diastole between December 2017 and October 2019 were enrolled as controls. Patients with bicuspid aortic valves, prosthetic aortic valves, inadequate image quality, insufficient image data, and history of endocarditis, rheumatic valve disease, and mediastinal radiation were excluded.

Echocardiography

Transthoracic echocardiography was performed per clinical guidelines.⁷ All examinations involved the inspection of the aortic valve for morphology, leaflet characteristics, and leaflet mobility. For patients with AS, measurements of AVA, mean gradient, peak jet velocity, left ventricular ejection fraction, and stroke volume index were obtained to assess AS disease severity as described.¹ For control patients, normal tricuspid aortic valves were confirmed by noting thin leaflets with normal mobility.

Pre-TAVR CT Angiography

Pre-TAVR CTA was performed using either the GE Lightspeed VCT (GE Healthcare, Chicago, IL) or the Siemens SOMATOM Force CT System (Siemens Healthineers AG, Munich, Germany). Retrospective electrocardiography-gated image acquisition was performed through the heart after intravenous administration of 120 to 150 mL of iohexol (Omnipaque 350, GE Healthcare) at 4 to 5 mL/s for 30 seconds followed by 80 to 100 mL of saline at the same rate as the contrast. This was immediately followed by a standard-pitch nongated (GE) or high-pitch electrocardiography-triggered (Siemens) acquisition through the chest, abdomen, and pelvis. Either a fixed x-ray tube energy of 120 kilovoltage peak (kVp; GE) or an automated x-ray tube energy selection tool (70–120 kVp; Siemens, CARE kV) was used. Tube current modulation was performed from 30% to 70% of the R-wave to R-wave interval. Images were reconstructed with a slice thickness of either 0.625 mm (GE) or 0.750 mm (Siemens), with further reconstruction to 2.5 mm for calcium scoring. Retrospective electrocardiography-gated coronary CTA of control patients was similarly performed as the electrocardiography-gated portion of the TAVR protocol except with a triphasic contrast protocol.

CTA Image Analyses

The Aquarius iNtuition Edition Ver.4.4.13.P3A viewer (TeraRecon, Foster City, CA) was used to analyze the CTA images of AS and control cohorts and assess the calcification loads, leaflet scores, and systolic excursions of individual valve leaflets (Figure 1, yellow path).

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Calcium scoring of the aortic valve and individual leaflets was performed in midsystole (20%–30% R-wave to R-wave) as previously described.⁸ Briefly, a 1-cm² region of interest was first placed in the ascending aorta to obtain the mean Hounsfield unit (HU_aorta) and SD. A calcium threshold of HU_aorta+2 SD was set before the calcified regions within each leaflet were quantified in terms of Agatston units and summed to obtain the CTA-derived aortic valve leaflet calcification load (AVLC_CTA) of that leaflet. The summation of AVLC_CTA of the noncoronary (NCC), right coronary (RCC), and left coronary (LCC) cusps then resulted in the CTA-derived aortic valve calcification load (AVC_CTA). The fractional AVLC_CTA of each leaflet was calculated as the quotient of AVLC_CTA of that leaflet divided by AVC_CTA.

The leaflet appearance was assessed in diastole in two views using a visual leaflet score (VLS): (1) a short-axis maximal intensity projection of the aortic valve (with a slab thickness large enough to cover all the aortic valve calcium based on the coronal left ventricular outflow tract view) showing the distribution of leaflet calcification and (2) an oblique view orthogonal to the annular plane and cutting through the midline and opposing commissure of each leaflet showing leaflet thickness. A VLS score of one to four (VLS-1 to VLS-4) was assigned for a normal thin leaflet, a predominantly thickened leaflet (<1/3 area with calcification), a leaflet of significant mixed features of calcification (between 1/3 and 2/3 area with calcification) and leaflet thickening, and a predominantly calcified leaflet (>2/3 area with calcification), respectively.

The systolic leaflet excursion (LE_sys) for each leaflet was assessed in the same plane as for VLS scoring and obtained by measuring the angle between the annular plane and the longitudinal length of the leaflet body during maximal valve opening (20%–30% R-wave to R-wave).

The ostial heights (OH) of the right coronary artery (RCA) and the left main coronary artery measured from the annular plane and the sinus of Valsalva diameter at each cusp measured by the sinus-to-commissure method were assessed in diastole as previously described.⁹

Computer Simulation of the Impact of Individual Leaflet Excursions on AVA

Computer simulations were performed to examine the effects of individual leaflet excursions on AVA using a customized workflow (Figure 1, blue path). A more detailed description of the simulation method can be found in Supplemental Methods. Briefly, for each control patient, we used the SimVascular¹⁰ and MeshMixer (Autodesk, San Rafael, CA) software to create a three-dimensional valve model in two cardiac phases from the mid-systolic and diastolic CTA images. We used Coherent Point Drift algorithm¹¹ to map the corresponding valve surfaces in the 2 phases to obtain a displacement field, from which the maximal systolic displacement within each leaflet was derived. We then assessed the contribution of each leaflet to AVA by initializing the valve model in midsystole and computing the AVA as we incrementally closed one leaflet at a time.

Statistical Analyses

Stata/SE Version 15 (StataCorp LLC, College Station, TX) was used for all statistical analyses except for sample size calculations which were performed with G*Power 3.1.9.7.¹² This study was powered to (1) detect interleaflet differences in both AVLC_CTA and LE_sys and identify their predictors in patients with AS and (2) detect interleaflet differences in LE_sys in control patients (Supplemental Methods for sample size calculations). Data were presented as mean±SD for normally distributed variables, median (interquartile range) for non-normally distributed variables, and frequency/percentage for nominal variables. All continuous variables were tested for normality using the Shapiro-Wilk test. Student t test, Wilcoxon rank-sum test, and Fisher exact test were used to compare the means of normally distributed variables, the mean ranks of non-normally distributed variables, and the frequencies of nominal variables, respectively, between 2 independent groups. For comparison of multiple related groups, one-way repeated measures ANOVA followed by pairwise paired t tests were used to compare the means of normally distributed variables, whereas the Friedman test followed by pairwise Wilcoxon signed-rank tests were used to compare the mean ranks of non-normally distributed variables.

Multivariable linear regression analyses were performed to assess the LE_sys-AVLC_CTA relationship of each leaflet and to identify predictors of leaflet-specific calcification (Figure 1, red path). All regression analyses were performed after square root transformation of the dependent variables to satisfy the linear regression model assumptions/conditions (Figures S2 and S3). Method for selecting adjustment variables and handling outliers can be found in Supplemental Methods (also Tables S1 through S3). Multinomial logistic regression was used to assess the dependence of VLS on individual leaflets (Figure 1, red path).

In general, statistical significance was defined as P<0.05 and adjusted as P<0.05/n for n group comparisons (Bonferroni correction).

Results

Clinical Profile of Study Population

Our study population consisted of 200 male patients with degenerative tricuspid AS undergoing TAVR evaluation and 20 male control patients with normal aortic valves (Table 1). The median age of the AS cohort was older than that of the control cohort (78.0 [71.0–84.0] years versus 59.5 [54.3–64.8] years). A large majority (80.5%) of the patients with AS had high-gradient severe AS. Nearly half (40.5%) had ≥3 clinical risk factors for AS. Approximately three-quarters (77.0%) had concurrent coronary artery disease and about one-quarter (22.5%) with left ventricular systolic dysfunction. One-quarter (25.5%) had coronary artery bypass grafting, compared with 15.0% with ostial chronic total/subtotal occlusion (CTO/STO) of RCA, 2.0% with ostial CTO/STO of left main coronary artery, and 11.5% with small nondominant RCA. The AS and control cohorts were similar in terms of risk factors/comorbidities, despite significantly greater AVC_CTA in the former (912.4 [590.4–1453.3] Agatston unit versus 17.2 [0.95–41.2] AU; P<0.001).

Leaflet-Specific Calcification Loads

The AVC_CTA of our patients with AS was highly heterogeneous and for the most part continuous from 116.0 AU to 3219 AU (Figure 2A) in a right-skewed distribution (Figure 2B). The AVLC_CTA varied greatly among the leaflets such that AVLC_{CTA, NCC} (365.9 [237.3–595.4] AU) was significantly greater than AVLC_{CTA, RCC} (278.5 [169.2–478.8] AU) and AVLC_{CTA, LCC} (240.6 [137.3–439.0] AU; P<0.001 comparing all leaflets; P<0.001 comparing NCC and RCC, P=0.055 comparing RCC and LCC, P<0.001 comparing NCC and LCC; Figure 2C). More than two-thirds (71.0%) of the patients with AS had fractional AVLC_{CTA, NCC} >0.333 (ie, NCC-dominant), the expected fraction if the entire AVC_CTA was evenly distributed among the 3 leaflets (Figure 2D). The AVLC_CTA of all leaflets in control patients were trivial, with no significant differences among them (NCC 7.9 [0.15–12.2] AU, RCC 3.5 [0–13.0] AU, LCC 2.8 [0–13.6] AU; P=0.270).

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Interleaflet Differences in VLSs

All leaflets in AS and control patients had abnormal (>VLS-1) and normal (VLS-1) appearances by VLS scores, respectively (Figure 3A and 3B for conventions). In patients with AS, compared with NCC, LCC was descriptively associated with greater proportions of VLS-2 (thickened; 23.5% versus 12.5%) and VLS-3 (mixed; 25.0% versus 18.5%), and a lesser proportion of VLS-4 (calcified; 51.5% versus 69%; Figure 3C). The proportions of visual scores for RCC were intermediate to those of NCC and LCC (16.5%, 20.5%, and 63% with VLS-2, VLS-3, and VLS-4, respectively). Multinomial logistic regression further found LCC to differ significantly from NCC in association with VLS-2 (thickened; relative risk ratio=3.071, P<0.001) and VLS-3 (mixed; relative risk ratio=1.964, P=0.014) relative to VLS-4 (calcified; baseline outcome; Table S4). LCC differed significantly from RCC in association with only VLS-2 (relative risk ratio=1.983, P=0.016) but not VLS-3 (relative risk ratio=1.575, P=0.068) relative to VLS-4. There was no significant difference between RCC and NCC in association with either VLS-2 (P=0.169) or VLS-3 (P=0.420) relative to VLS-4.

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Leaflet-Specific Excursion Patterns

We observed small interleaflet differences in the LE_sys of our patients with AS (P<0.001) such that LE_{sys, RCC} was the greatest (47.7° [42.0°–52.3°]), followed by LE_{sys, NCC} (44.8° [41.1°–49.78°]), and LE_{sys, LCC} (42.8° [38.8°–49.0°]; P=0.027 comparing NCC and RCC, P<0.001 comparing RCC and LCC, P=0.001 comparing NCC and LCC; Figure 4A, 4B). This pattern of interleaflet differences was similar but even more pronounced in control patients: LE_{sys, RCC} 84.3° (80.6°–89.9°), LE_{sys, NCC} 79.4° (°71.9–83.0°), LE_{sys, LCC} 72.6° (69.9°–76.0°; P<0.001 comparing all leaflets; P=0.002 comparing NCC and RCC, P<0.001 comparing RCC and LCC, P<0.001 comparing NCC and LCC; Figure 4C and 4D).

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Leaflet-Specific Excursion-Calcification Load Relationships

Conceptually, LE_sys should correlate well with AVLC_CTA if valvular calcification were to be the predominant mechanism of restricted leaflet motion in AS. Because this relationship seemed to depend heavily on AVC_CTA (Figure S4), we included AVC_CTA as a covariate in our subsequent regression analysis to examine the LE_sys-AVLC_CTA relationship for each leaflet. After adjusting for AVC_CTA and other variables that could influence either LE_sys (left ventricular systolic dysfunction [left ventricular ejection fraction<50%]) or AVLC_CTA (age, body mass index, hypertension, dyslipidemia, diabetes, chronic kidney disease, smoking) or its measurement (scanner type), we found the relationship between LE_sys (transformed) and AVLC_CTA to be significant for both NCC (β=−0.0012, P<0.001) and RCC (β=−0.0013, P<0.001), but not LCC (β=-0.0006, P=0.084; Table 2).

Impact of Leaflet-Specific Excursion on AVA

To examine the functional implications of asymmetrical LE_sys observed in control patients, we performed computer simulation by initializing the aortic valve model of each patient in maximally opened configuration and incrementally closing one leaflet at a time while assessing AVA (Figure 5A). With the valve fully opened, the maximal displacement value within each leaflet from its diastolic configuration was significantly greater for RCC (10.4 [9.5–11.7] mm) and NCC (9.1 [8.5–10.6] mm) than LCC (7.9 [6.3–10.1] mm; P<0.001 comparing all leaflets; P=0.349 comparing NCC and RCC, P<0.001 comparing RCC and LCC, P=0.008 comparing NCC and LCC; Figure 5B). With incremental closure of each leaflet, the degree of reduction in percentage AVA was greater for NCC (–32.2% [–38.4% to –25.8%]) and RCC (−35.7% [−40.2% to −32.9%]) than LCC (−24.5% [−28.5% to −18.3%]; P<0.001 comparing all leaflets; P<0.001 comparing RCC and LCC, P<0.004 comparing NCC and LCC, P=0.055 comparing NCC and RCC; Figure 5C).

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Relationships Between Leaflet Calcification Load and Local Coronary Factors

To identify local factors that may influence calcification load in a leaflet-specific manner, we considered factors that could influence coronary flow because the lack thereof had been thought to predispose NCC to disease.¹³ We performed multivariable linear regression to examine the relationship between the AVLC_CTA of each leaflet (dependent variable) and the different coronary factors, including ostial RCA CTO/STO, RCA OH, sinus of Valsalva diameter at RCC (SOVd_RCC). small nondominant RCA, left main coronary artery OH, and sinus of Valsalva diameter at LCC (SOVd_LCC). Ostial left main coronary artery CTO/STO was not included due to its rare prevalence in our patients with AS (2%). For the corresponding regression model of each leaflet, we adjusted for AVC_CTA to account for its significant interindividual differences and factors that could influence either valvular calcification (age, body mass index, hypertension, dyslipidemia, diabetes, chronic kidney disease, smoking) or its measurement (scanner type). We found both ostial RCA CTO/STO and RCA OH to be significant positive predictors of AVLC_{CTA, RCC} (β=1.8792 and P=0.005 for ostial RCA CTO/STO; β=0.3437 and P=0.001 for RCA OH; Table 3). Additionally, ostial RCA CTO/STO and RCA OH were found to be negative predictors of AVLC_{CTA, NCC} (β=−2.1196, P=0.002) and AVLC_{CTA, LCC} (β=−0.2223, P=0.032), respectively.

Discussion

This study addresses a significant knowledge gap regarding interleaflet differences in contributions to AS. The main findings were: (1) In male patients with AS being considered for TAVR, significant heterogeneity was observed for their AVC_CTA and AVLC_CTA; (2) AVLC_CTA was the greatest for NCC compared to other leaflets, whereas leaflet thickening, as the predominant disease feature, was more often visualized in LCC than other leaflets; (3) LE_sys was the least for LCC compared to other leaflets, and the same pattern was seen in control patients; (4) The LE_sys-AVLC_CTA relationship was significant for both NCC and RCC but not LCC; (5) In male control patients, closures of both NCC and RCC were associated with greater reductions in AVA than that of LCC, suggesting their greater functional contributions to AVA; (6) Both ostial CTO/STO and OH of RCA were significant positive predictors of AVLC_{CTA, RCC}. Collectively, these findings support the notion that there are significant interleaflet differences in contribution to AS, some of which may be related to local coronary flow.

This work was motivated by the clinical observation that patients with AS often have heterogeneous distribution of leaflet calcification that does not always correlate with reduced leaflet motion. Here, we sought to address these discrepancies by examining the interleaflet differences in AVLC_CTA, VLS, LE_sys, and LE_sys-AVLC_CTA relationships. We first observed significant interleaflet differences in AVLC_CTA in patients with AS, with NCC carrying the most load as previously described.⁴ However, it was LCC, rather than NCC, that had the least LE_sys. Although noncalcific leaflet thickening/fibrosis could have significantly affected LCC at least in part to underlie these findings, the fact that a similar pattern of interleaflet differences in LE_sys was also observed in control patients suggests baseline functional asymmetry among the leaflets. As the flow profile across the left ventricular outflow tract could be skewed, with maximal velocity biasing towards the anteromedial sector leading to NCC/RCC,¹⁴ the asymmetrical fluid forces impinging on the leaflets could have contributed to their differential LE_sys under normal (and diseased) conditions. The implication of asymmetrical leaflet opening is that NCC and RCC contribute more to AVA than LCC and their protection from disease would be paramount for retarding AS progression.

Noncalcific leaflet thickening (fibrosis) is increasingly recognized as a significant contributor of AS and was found in this study to be the predominant disease feature more frequently in LCC than other leaflets. This explains why a significant LE_sys-AVLC_CTA relationship was found for only NCC and RCC but not LCC. These results caution against the sole use of AVC_CT to gauge AS severity, as it does not factor in leaflet thickening. A clinical dilemma would arise when an AS patient with discordant echocardiographic findings is found to have a subthreshold AVC_CT composed of considerable AVLC_CTA, _NCC and AVLC_{CTA, RCC} but low AVLC_CTA, _LCC. The patient could have either truly nonsevere AS to justify continued clinical monitoring if LCC is mildly thickened with mildly restricted systolic motion or severe AS warranting valve replacement if LCC is severely thickened with severely limited excursion. Although careful examination of individual leaflet characteristics/motion can assist in deciding whether to fully rely on AVC_CT to adjudicate AS severity, a more comprehensive, quantitative method for assessing noncalcific leaflet thickening is needed to complement calcium scoring. The recent development of an image analysis platform that allows combined assessment of noncalcific and calcific volumes can potentially overcome the shortcomings of AVC_CT load and better predict AS severity.¹⁵

To date, the reasons for interleaflet differences in calcification have remained elusive, with the complex hemodynamic milieu of the aortic root likely playing a role. Previous studies have implicated flow disruption in the aortic sinuses as the cause of abnormal shear stress on the aortic valve that leads to endothelial dysfunction and other sequelae of AS.¹⁶ Although differences in root dimensions and leaflet sizes can also influence the flow patterns of the aortic root to result in differential leaflet calcification, coronary flow (or lack thereof) is more commonly hypothesized as a primary cause of interleaflet differences in calcification.¹³ Our results support this hypothesis by showing that ostial CTO/STO and greater OH of RCA (factors affecting RCA drainage) were associated with greater AVLC_{CTA, RCC} (Figure S5). These same coronary factors were also found to have negative and distant associations with AVLC_CTA, _NCC and AVLC_{CTA, LCC.}, respectively. Nevertheless, the nature of this study does not lend itself to establishing causality for these interesting relationships between leaflet calcification and coronary factors. Further investigation using computational modeling will be needed to understand how each coronary factor influences leaflet-specific calcification.

Study Limitations

The main limitation of this study concerns the generalizability of our findings to females, as only males were enrolled. Although similar interleaflet patterns of AVLC_CTA have been reported in females,⁵ our results pertaining to the leaflet-specific excursion, thickening, excursion-calcification relationship, and coronary factors may not apply and will need further validation. Another study limitation concerns our assessment of LE_sys and leaflet thickening at the midline of each leaflet, which may not capture their heterogeneity elsewhere in the leaflet. To address this limitation, we have developed a robust algorithm to map local leaflet displacement as a part of the computer simulation presented herein. Further refinement of this technique with additional superimposed mapping of regional leaflet calcification and thickness should enable a more thorough evaluation of the interactions among leaflet calcification, thickness, and motion.

Conclusions

This study contributes to a better understanding of the interleaflet differences in calcification load, appearance, leaflet excursion, and their interrelationships. The results presented herein suggest that calcification contributes more to AS via NCC and RCC, rather than LCC. When assessing AS severity, AVLC_{CTA, NCC} and AVLC_{CTA, RCC} should be given unreserved consideration because of their significant values, predictability for reduced LE_sys, functional contribution to AVA, and the latter’s potential relation with local coronary alterations. However, because noncalcific leaflet thickening can be a predominant disease feature in LCC more often than in other leaflets, careful interpretation of AVC_CT in the setting of low AVLC_{CTA, LCC} is needed to avoid underestimation of AS severity. The development of new techniques to simultaneously assess valvular calcification and leaflet thickening/fibrosis would be highly desirable for the comprehensive evaluation of AS.

ARTICLE INFORMATION

Acknowledgements

We thank Blake Wu and Dr Pei-Yu Lee for their assistance with article preparation.

Supplemental Materials

Supplemental Methods

Figures S1–S5

Tables S1–S4

Footnotes