  Deep neural networks (DNNs) for medical images are extremely vulnerable to
adversarial examples (AEs), which poses security concerns on clinical decision
making. Luckily, medical AEs are also easy to detect in hierarchical feature
space per our study herein. To better understand this phenomenon, we thoroughly
investigate the intrinsic characteristic of medical AEs in feature space,
providing both empirical evidence and theoretical explanations for the
question: why are medical adversarial attacks easy to detect? We first perform
a stress test to reveal the vulnerability of deep representations of medical
images, in contrast to natural images. We then theoretically prove that typical
adversarial attacks to binary disease diagnosis network manipulate the
prediction by continuously optimizing the vulnerable representations in a fixed
direction, resulting in outlier features that make medical AEs easy to detect.
However, this vulnerability can also be exploited to hide the AEs in the
feature space. We propose a novel hierarchical feature constraint (HFC) as an
add-on to existing adversarial attacks, which encourages the hiding of the
adversarial representation within the normal feature distribution. We evaluate
the proposed method on two public medical image datasets, namely {Fundoscopy}
and {Chest X-Ray}. Experimental results demonstrate the superiority of our
adversarial attack method as it bypasses an array of state-of-the-art
adversarial detectors more easily than competing attack methods, supporting
that the great vulnerability of medical features allows an attacker more room
to manipulate the adversarial representations.
