title: Dual-frequency tissue harmonic suppression using phase-coded pulse sequence: proof of concept using a phantom.
abstract: BACKGROUND
  The presence of tissue harmonic generation during acoustic propagation is one major limitation in nonlinear detection of microbubble contrast agents. However, conventional solutions for tissue harmonic suppression are not applicable in dual-frequency (DF) harmonic imaging. In DF harmonic imaging, the second harmonic signal at second harmonic (2f(0)) frequency and the inter-modulation harmonic signal at fundamental (f(0)) frequency are simultaneously generated for imaging and both need to be suppressed to improve contrast-to-tissue ratio (CTR). In this study, a novel phase-coded pulse sequence is developed to accomplish DF tissue harmonic suppression.
  
  
  METHODS
  Phase-coded pulse sequence utilizes multiple firings with equidistant transmit phase for harmonic cancellation in the sum of respective echoes. For the f(0) transmit component, the transmit phase comes from the equidistant set of {-2π/3, 0, 2π/3} to suppress the second harmonic signal at 2f(0) frequency. Moreover, in order to provide the inter-modulation harmonic suppression at f(0) frequency, the 2f(0) transmit phase has to be particularly manipulated for the corresponding f(0) transmit phase.
  
  
  RESULTS
  The proposed three-pulse sequence can remove not only the second-order harmonic signal but also other higher-order counterparts at both f(0) and 2f(0) frequencies. Measurements were performed at f(0) equal to 2.25 MHz and using hydrophone in water and contrast agents in tissue phantom. Experimental results indicate that the sequence reduces the tissue harmonic magnitude by about 20 dB along the entire axial depths and the corresponding CTR improves at both frequencies.
  
  
  CONCLUSIONS
  In DF harmonic imaging, the proposed phase-coded sequence can effectively remove the tissue harmonic background at both f(0) and 2f(0) frequencies for improvement of contrast detection.
authors.affiliation: Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. choushen@mail.ntust.edu.tw
authors.forename: Che-Chou
authors.initials: CC
authors.lastname: Shen
authors.forename: Hui-Ting
authors.initials: HT
authors.lastname: Wang
mesh_terms: D003287:Contrast Media / Q000737:chemistry*; D004867:Equipment Design; D007089:Image Enhancement / Q000379:methods*; D045423:Microbubbles; D019047:Phantoms, Imaging; D010743:Phospholipids / Q000737:chemistry*; D012815:Signal Processing, Computer-Assisted; D013459:Sulfur Hexafluoride / Q000737:chemistry*; D014159:Transducers; D014463:Ultrasonography / Q000379:methods*
chemical_list: D003287:Contrast Media; D010743:Phospholipids; C420843:contrast agent BR1; D013459:Sulfur Hexafluoride