损失不变性决定概念层编码的内容：超声心动图中的容积定位

中文摘要 AI 辅助

目的：概念瓶颈模型通过可解释的中间变量进行预测，其有效性通常由这些变量的预测准确性来判断。我们以左心室容积作为超声心动图视频中射血分数估计的基础概念，探讨这种判断是否足够。方法：在一个公开可用的超声心动图数据集上训练视频变压器编码器。收缩末期和舒张末期容积形成一个概念层，通过解析计算射血分数，且没有到输出的残差路径。我们比较了仅在射血分数目标下的训练与在毫升容积的额外监督下的训练，并在1276项验证研究中进行评估。结果：与直接回归相比，概念瓶颈并未增加射血分数误差，平均绝对误差分别为6.89和7.13。然而，在没有容积监督的情况下，预测容积的范围相对于参考范围35.7和45.7毫升缩小到0.1毫升，同时相关性部分保留。我们表明这源于目标的不变性：射血分数是一个比率，当两个容积都重新缩放时不变，所以损失仅在比例上确定概念层。以射血分数误差增加0.4为代价，绝对单位的监督将容积误差从89.8毫升降低到25.8毫升。结论：仅概念准确性可能掩盖一个没有物理尺度的概念层。意义：临床模型中的可解释中间变量不仅应根据预测准确性，还应根据训练目标不变性结构进行验证。

英文摘要

Objective: Concept bottleneck models route prediction through interpretable intermediate variables, and their validity is normally judged by how accurately those variables are predicted. We ask whether that judgement is sufficient, using left ventricular volumes as the concepts underlying ejection fraction estimation from echocardiographic video. Methods: A video transformer encoder was trained on a publicly available echocardiography dataset. End-systolic and end-diastolic volumes formed a concept layer from which ejection fraction was computed analytically, with no residual path to the output. We compared training under an ejection fraction objective alone against training with additional supervision of the volumes in millilitres, and evaluated both on 1276 held-out studies. Results: The concept bottleneck did not increase ejection fraction error relative to direct regression, at 6.89 against 7.13 mean absolute error. Without volume supervision, however, the spread of predicted volumes collapsed to 0.1 millilitres against reference spreads of 35.7 and 45.7 millilitres, while correlation was partly preserved. We show that this follows from an invariance property of the objective: ejection fraction is a ratio and is unchanged when both volumes are rescaled, so the loss determines the concept layer only up to scale. Supervision in absolute units reduced volume error from 89.8 to 25.8 millilitres at a cost of 0.4 in ejection fraction error. Conclusion: Concept accuracy alone can conceal a concept layer that carries no physical scale. Significance: Interpretable intermediate variables in clinical models should be validated against the invariance structure of the training objective, not only against prediction accuracy.

发表机构

• Yonsei University College of Medicine（延世大学医学院）
• Research Institute of Radiologic Science（放射科学研究所）

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