Konofagou E E, Ottensmeyer M, Agabian S, Dawson S L, Hynynen K
Department of Biomedical Engineering, Columbia University, New York, NY 10027, USA.
Ultrasonics. 2004 Apr;42(1-9):951-6. doi: 10.1016/j.ultras.2003.12.001.
The technique of harmonic motion imaging (HMI) uses the localized stimulus of the oscillatory ultrasonic radiation force as produced by two overlapping beams of distinct frequencies, and estimates the resulting harmonic displacement in the tissue in order to assess its underlying mechanical properties. In this paper, we studied the relationship between measured displacement and stiffness in gels and tissues in vitro. Two focused ultrasound transducers with a 100 mm focal length were used at frequencies of 3.7500 MHz and either 3.7502 (or 3.7508 MHz), respectively, in order to produce an oscillatory motion at 200 Hz in the gel or tissue. A 1.1 MHz diagnostic transducer (Imasonics, Inc.) was also focused at 100 mm and acquired 5 ms RF signals (pulse repetition frequency (PRF)=3.5 kHz) at 100 MHz sampling frequency during radiation force application. First, three 50x50 mm(2) acrylamide gels were prepared at concentrations of 4%, 8% and 16%. The resulting displacement was estimated using crosscorrelation techniques between successively acquired RF signals with a 2 mm window and 80% window overlap at 1260 W/cm(2). A normal 1-D indentation instrument (TeMPeST) applied oscillatory loads at 0.1-200 Hz with a 5 mm-diameter flat indenter. Then, 12 displacement measurements in 6 porcine muscle specimens (two measurements/case, as above) were made in vitro, before and after ablation which was performed for 10 s at 1260 W/cm(2). In all gel cases, the harmonic displacement was found to linearly increase with intensity and exponentially decrease with gel concentration. The TeMPeST measurements showed that the elastic moduli for the 4%, 8% and 16% gels equaled 3.93+/-0.06, 17.1+/-0.2 and 75+/-2 kPa, respectively, demonstrating that the HMI displacement estimate depends directly on the gel stiffness. Finally, in the tissues samples, the mean displacement amplitude showed a twofold decrease between non-ablated and ablated tissue, demonstrating a correspondence between the HMI response and an increase in stiffness measured with the TeMPeST instrument.
谐波运动成像（HMI）技术利用由两个不同频率的重叠波束产生的振荡超声辐射力的局部刺激，并估计组织中产生的谐波位移，以评估其潜在的力学性能。在本文中，我们研究了体外凝胶和组织中测量位移与硬度之间的关系。使用两个焦距为100mm的聚焦超声换能器，频率分别为3.7500MHz和3.7502（或3.7508MHz），以便在凝胶或组织中产生200Hz的振荡运动。一个1.1MHz的诊断换能器（Imasonics公司）也聚焦在100mm处，并在施加辐射力期间以100MHz采样频率采集5ms的射频信号（脉冲重复频率（PRF）=3.5kHz）。首先，制备了浓度为4%、8%和16%的三种50x50mm(2)的丙烯酰胺凝胶。使用互相关技术，在1260W/cm(2)的条件下，在2mm窗口且窗口重叠80%的连续采集的射频信号之间估计产生的位移。一台普通的一维压痕仪（TeMPeST）使用直径为5mm的扁平压头，在0.1 - 200Hz施加振荡载荷。然后，在6个猪肌肉标本中进行了12次位移测量（每个标本测量两次，如上所述），在1260W/cm(2)下进行10s消融前后各测量一次。在所有凝胶情况下，发现谐波位移随强度线性增加，随凝胶浓度指数下降。TeMPeST测量表明，4%、8%和16%凝胶的弹性模量分别等于3.93±0.06、17.1±0.2和75±2kPa，表明HMI位移估计直接取决于凝胶硬度。最后，在组织样本中，非消融组织和消融组织之间的平均位移幅度下降了两倍，表明HMI响应与TeMPeST仪器测量的硬度增加之间存在对应关系。
