title: High-resolution harmonic motion imaging (HR-HMI) for tissue biomechanical property characterization.
abstract: BACKGROUND
  Elastography, capable of mapping the biomechanical properties of biological tissues, serves as a useful technique for clinicians to perform disease diagnosis and determine stages of many diseases. Many acoustic radiation force (ARF) based elastography, including acoustic radiation force impulse (ARFI) imaging and harmonic motion imaging (HMI), have been developed to remotely assess the elastic properties of tissues. However, due to the lower operating frequencies of these approaches, their spatial resolutions are insufficient for revealing stiffness distribution on small scale applications, such as cancerous tumor margin detection, atherosclerotic plaque composition analysis and ophthalmologic tissue characterization. Though recently developed ARF-based optical coherence elastography (OCE) methods open a new window for the high resolution elastography, shallow imaging depths significantly limit their usefulness in clinics.
  
  
  METHODS
  The aim of this study is to develop a high-resolution HMI method to assess the tissue biomechanical properties with acceptable field of view (FOV) using a 4 MHz ring transducer for efficient excitation and a 40 MHz needle transducer for accurate detection. Under precise alignment of two confocal transducers, the high-resolution HMI system has a lateral resolution of 314 µm and an axial resolution of  147 µm with an effective FOV of 2 mm in depth.
  
  
  RESULTS
  The performance of this high resolution imaging system was validated on the agar-based tissue mimicking phantoms with different stiffness distributions. These data demonstrated the imaging system's improved resolution and sensitivity on differentiating materials with varying stiffness. In addition, ex vivo imaging of a human atherosclerosis coronary artery demonstrated the capability of high resolution HMI in identifying layer-specific structures and characterizing atherosclerotic plaques based on their stiffness differences.
  
  
  CONCLUSIONS
  All together high resolution HMI appears to be a promising ultrasound-only technology for characterizing tissue biomechanical properties at the microstructural level to improve the image-based diseases diagnosis in multiple clinical applications.
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Teng
authors.initials: T
authors.lastname: Ma
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Xuejun
authors.initials: X
authors.lastname: Qian
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Chi Tat
authors.initials: CT
authors.lastname: Chiu
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Mingyue
authors.initials: M
authors.lastname: Yu
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Hayong
authors.initials: H
authors.lastname: Jung
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Yao-Sheng
authors.initials: YS
authors.lastname: Tung
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: K Kirk
authors.initials: KK
authors.lastname: Shung
authors.affiliation: Department of Biomedical Engineering, NIH Recourse Center on Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California, USA.
authors.forename: Qifa
authors.initials: Q
authors.lastname: Zhou
keywords: Elastography; acoustic radiation force (ARF); biomechanical properties; harmonic motion imaging (HMI); high frequency ultrasound; medical imaging