  Multiple reflections between transducer and imaged object can naturally occur
in ultrasound imaging and other acoustic sensing applications such as sonar.
The repeated interaction of the emitted wavefront with the imaged object is
traditionally regarded an undesired reverberation artifact, often
misinterpreted as fictitious acoustic boundaries. Here we introduce high order
reflection pulse-echo (HOPE)-ultrasound, a novel method that leverages high
order reflections to improve on several aspects of conventional ultrasound
imaging. HOPE is experimentally demonstrated to resolve sub-micrometer features
by breaking through the Nyquist resolution limit. The major contrast
enhancement of the high reflection orders allowed to reveal defects within
materials invisible to conventional scanning acoustic microscopy. The technique
is further shown to improve accuracy of frequency-dependent ultrasound
attenuation measurements from biological tissues. HOPE ultrasound requires no
additional hardware and is easy to implement, underscoring its potential to
boost imaging performance in a broad range of biomedical imaging,
nondestructive testing, and other acoustic sensing applications.
