Surface–length index: a novel index for rapid detection of right ventricles with abnormal ejection fraction using cardiac MRI

Abstract

Objective

To validate a new index, the surface–length index (SLI) based on area change in a short-axis view and length reduction in the horizontal long-axis view, which is used to quickly (<1 min) detect right ventricles with an abnormal ejection fraction (EF) during a cardiac MRI examination. SLI can be used to avoid a complete delineation of the endocardial contours of normal right ventricles.

Methods

Sixty patients (group A) were retrospectively included to calibrate the SLI formula by optimisation of the area under the ROC curves and SLI thresholds were chosen to obtain 100 % sensitivity. Another 340 patients (group B) were prospectively recruited to test SLI’s capacity to detect right ventricles (RVs) with an abnormal EF (<0.5).

Results

The appropriate threshold to obtain 100 % sensitivity in group A was 0.58. In group B, with the 0.58 threshold, SLI yielded a sensitivity of 100 % and specificity of 51 %. SLI would have saved 35 % of the RV studies in our population, without inducing any diagnostic error. SLI and EF correlation was good (r ² = 0.64).

Conclusion

SLI combines two simple RV measures, and brings significant improvement in post-processing efficiency by preselecting RVs that require a complete study.

Key Points

• Assessment of right ventricle ejection fraction (RVEF) with cine-MRI is time consuming.

• Therefore, RVEF is not always assessed during cardiac MRI.

• Surface–length index (SLI) allows rapid detection of abnormal RVEF during cardiac MRI.

• SLI saves one third of the operator time.

• Every cardiac MRI could include RVEF assessment by means of SLI.

Appendix A

The RV volume has often been approximated by area–length methods [13, 21–24]. These methods postulate that there is a proportional ratio (usually close to two thirds) among RV volume, its length (L) and its basal short­axis area (A).

$$ V=\mathrm{c}\times A\times L $$

(1)

Let X _d and X _s be the value of X (X being one of the variables V, L or A) in end-diastole and in end-systole.

By definition,

$$ \mathrm{EF}={{{\varDelta V}} \left/ {V} \right.}=1-{{{{V_{\mathrm{s}}}}} \left/ {{{V_{\mathrm{d}}}}} \right.} $$

(2)

$$ \mathrm{FAC}={{{\varDelta A}} \left/ {A} \right.}=1-{{{{A_{\mathrm{s}}}}} \left/ {{{A_{\mathrm{d}}}}} \right.} $$

(3)

$$ \mathrm{SF}={{{\varDelta L}} \left/ {L} \right.}=1-{{{{L_{\mathrm{s}}}}} \left/ {{{L_{\mathrm{d}}}}} \right.} $$

(4)

$$ \begin{array}{*{20}c} {1+2\Leftrightarrow } \hfill & {\mathrm{EF}=1-{{{\left( {c\times {A_{\mathrm{s}}}\times {L_{\mathrm{s}}}} \right)}} \left/ {{\left( {c\times {A_{\mathrm{d}}}\times {L_{\mathrm{d}}}} \right)}} \right.}=1-\left( {{{{{A_{\mathrm{s}}}}} \left/ {{{A_{\mathrm{d}}}}} \right.}} \right)\times \left( {{{{{L_{\mathrm{s}}}}} \left/ {{{L_{\mathrm{d}}}}} \right.}} \right)} \hfill \\ \end{array} $$

(5)

$$ \begin{array}{*{20}c} {\left( {3+4+5} \right)\ \Leftrightarrow } \hfill & {\mathrm{EF}=1-\left( {1-\mathrm{FAC}} \right)\times \left( {1-\mathrm{SF}} \right)} \hfill \\ \end{array} $$

(6)

However, the area–length models are based on the hypothesis of isotropic contraction: The shape of the ventricle is not modified by the contraction, only its size is. It has been proven that this hypothesis is not verified and many studies have shown the existence of a longitudinal contraction gradient from the basal segments (which contract less) to the apical segments (which contract more).

We applied the calculus presented above to a more complex model proposed in 1989 by Aebischer and Czegledy [13] called the crescentic shell model and introduced the notion of a contraction gradient from base to apex. This led to the introduction of a correction factor α in the longitudinal member of the formula with α slightly greater than 1:

$$ \mathrm{EF}=1-\left( {1-\mathrm{FAC}} \right)\times \left( {1-\alpha \mathrm{SF}} \right) $$