Aortic stenosis assessment from the 3-chamber cine: Ratio of balanced steady-state-free-precession (bSSFP) blood signal between the aorta and left ventricle predicts severity

Abstract

Background: Cardiovascular magnetic resonance (CMR) imaging is an important tool for evaluating the severity of aortic stenosis (AS), co-existing aortic disease, and concurrent myocardial abnormalities. Acquiring this additional information requires protocol adaptations and additional scanner time, but is not necessary for the majority of patients who do not have AS. We observed that the relative signal intensity of blood in the ascending aorta on a balanced steady state free precession (bSSFP) 3-chamber cine was often reduced in those with significant aortic stenosis. We investigated whether this effect could be quantified and used to predict AS severity in comparison to existing gold-standard measurements.

Methods: Multi-centre, multi-vendor retrospective analysis of patients with AS undergoing CMR and transthoracic echocardiography (TTE). Blood signal intensity was measured in a ∼1 cm² region of interest (ROI) in the aorta and left ventricle (LV) in the 3-chamber bSSFP cine. Because signal intensity varied across patients and scanner vendors, a ratio of the mean signal intensity in the aorta ROI to the LV ROI (Ao:LV) was used. This ratio was compared using Pearson correlations against TTE parameters of AS severity: aortic valve peak velocity, mean pressure gradient and the dimensionless index. The study also assessed whether field strength (1.5 T vs. 3 T) and patient characteristics (presence of bicuspid aortic valves (BAV), dilated aortic root and low flow states) altered this signal relationship.

Results: 314 patients (median age 69 [IQR 57-77], 64% male) who had undergone both CMR and TTE were studied; 84 had severe AS, 78 had moderate AS, 66 had mild AS and 86 without AS were studied as a comparator group. The median time between CMR and TTE was 12 weeks (IQR 4-26). The Ao:LV ratio at 1.5 T strongly correlated with peak velocity (r = -0.796, p = 0.001), peak gradient (r = -0.772, p = 0.001) and dimensionless index (r = 0.743, p = 0.001). An Ao:LV ratio of < 0.86 was 84% sensitive and 82% specific for detecting AS of any severity and a ratio of 0.58 was 83% sensitive and 92% specific for severe AS. The ability of Ao:LV ratio to predict AS severity remained for patients with bicuspid aortic valves, dilated aortic root or low indexed stroke volume. The relationship between Ao:LV ratio and AS severity was weaker at 3 T.

Conclusions: The Ao:LV ratio, derived from bSSFP 3-chamber cine images, shows a good correlation with existing measures of AS severity. It demonstrates utility at 1.5 T and offers an easily calculable metric that can be used at the time of scanning or automated to identify on an adaptive basis which patients benefit from dedicated imaging to assess which patients should have additional sequences to assess AS.

Keywords: Aortic Stenosis; Aortic Valve; Balanced Steady-State Free Precession; Left Ventricle; Magnetic Field Strength; Valvular heart disease.

Fig. 1

Example of a 3-chamber cine in a patient with aortic stenosis. A region of interest (ROI) in the aorta and left ventricle is manually measured in the end-systolic cardiac phase. The ratio of Ao:LV bSSFP blood signal in this patient: 67.5/186.2 = 0.36. Ao: aorta; LV: left ventricle; bSSFP: balanced steady state free precession.

Fig. 2

A. Dot plots for Ao:LV ratios according to aortic stenosis severity. Adjusted P values for Tukey’s multiple comparisons test. B. Example of four cases with varying degrees of aortic stenosis in the 3-chamber view with their corresponding Ao:LV ROI annotations. Ao:LV ratios are: Normal = 1.29, Mild AS = 0.70, Moderate AS = 0.61, and Severe AS = 0.41. ROI, region of interest; AS, aortic stenosis.

Fig. 3

Results of comparisons between Ao:LV ratio, echocardiography, and phase contrast CMR. Scatterplots with Pearson correlation coefficients comparing Ao:LV and echo measures of peak velocity (A), dimensionless index (B), mean pressure gradient (C), peak pressure gradient (D), aortic valve area index (E) and peak velocity derived from PC-CMR (F). The trend suggested by Figs. 3C and 3D is of a gently decaying exponential, and not strictly linear; we have used a linear fit for consistency with the analysis for the remaining variables. PC, phase contrast; CMR, cardiovascular magnetic resonance; Ao: aorta; LV: left ventricle

Fig. 4

A. ROC curve showing the diagnostic accuracy for Ao:LV ratio in predicting AS of any severity. B. ROC curve showing the diagnostic accuracy for Ao:LV ratio in predicting severe AS. AS, aortic stenosis. ROC: receiver operating characteristic curve; Ao: aorta; LV: left ventricle.

Fig. 5

: Dot plots for Ao:LV ratios according to aortic stenosis severity and valve morphology. BAV, bicuspid aortic valve; TAV, trileaflet aortic valve; AS: aortic stenosis; Ao: aorta; LV: left ventricle.

Fig. 6

Dot plots for Ao:LV ratios according to aortic stenosis severity and differentiated into low (<35 ml/m2) and normal (>35 ml/m2) stroke volume index categories. LF, low flow; NF, normal flow; Ao: aorta; LV: left ventricle

Fig. 7

Dot plots for Ao:LV ratios according to aortic stenosis severity and differentiated into non-dilated (<40 mm) or dilated (>40 mm) aortic root dimensions. Ao: aorta; LV: left ventricle; AS: aortic stenosis.

Fig. 8

Dot plots for Ao:LV ratios according to aortic stenosis severity at 3 Tesla magnetic field strength. Adjusted P values for Tukey’s multiple comparisons test. Ao: aorta; LV: left ventricle; AS: aortic stenosis