Ultrasound and Underwater Acoustics Group, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK.
Department of Medical Physics and Biomedical Engineering, University College London, Malet Place Engineering Building, Gower Street, London WC1E 6BT, UK.
Ultrasonics. 2020 Dec;108:106218. doi: 10.1016/j.ultras.2020.106218. Epub 2020 Jul 12.
High-frequency calibration of hydrophones is becoming increasingly important, both for clinical and scientific applications of ultrasound, and user safety. At present, the calibrations available routinely to the user community extend to 60 MHz. However, hydrophones that can measure beyond this are available, and ultrasonic fields often contain energy at higher frequencies, e.g., generated through nonlinear propagation of high-amplitude ultrasound used for therapeutic applications, and the increasing use of higher frequencies in imaging. Therefore, there is a need for calibrations up to at least 100 MHz, to allow ultrasonic fields to be accurately characterized, and the risk of harmful bioeffects to be properly assessed. Currently, sets of focused piezoelectric transducers are used to meet the pressure amplitude and bandwidth requirements of Primary Standard calibration facilities. However, when the frequency is high enough such that the size of the ultrasound focus becomes less than the hydrophone element's diameter, the uncertainty due to spatial averaging becomes significant, and can be as high as 20% at 100 MHz. As an alternate to piezoelectric transducers, a laser-generated ultrasound calibration source was designed, fabricated, and characterized. The source consists of an optically absorbing carbon-polymer nanocomposite excited by a large-diameter 1064 nm laser pulse of 2.6 ns duration. Peak pressure amplitudes of several Mega-Pascal were readily achievable, and the signal contained measurable frequency components up to 100 MHz. The variation in the pressure amplitudes was less than 2% from its mean over a three-hour test period. The ultrasound beam was sufficiently broad that the uncertainties due to spatial averaging were negligible.
水听器的高频校准对于超声的临床和科学应用以及用户安全变得越来越重要。目前，用户社区通常可获得的校准范围扩展到 60MHz。但是，能够测量更高频率的水听器已经可用，并且超声场通常包含更高频率的能量，例如，通过用于治疗应用的高振幅超声的非线性传播产生，以及在成像中越来越多地使用更高的频率。因此，需要进行至少 100MHz 的校准，以准确描述超声场，并适当评估有害生物效应的风险。目前，使用聚焦压电换能器组来满足主要标准校准设备的压力幅度和带宽要求。然而，当频率足够高以至于超声焦点的尺寸小于水听器元件的直径时，由于空间平均引起的不确定性变得显著，并且在 100MHz 时可高达 20%。作为压电换能器的替代方案，设计、制造和表征了激光产生的超声校准源。该源由大直径 1064nm 激光脉冲激发的吸收光的碳聚合物纳米复合材料组成，激光脉冲持续时间为 2.6ns。很容易实现数兆帕斯卡的峰值压力幅度，并且信号包含可测量的频率分量，最高可达 100MHz。在三个小时的测试期间，压力幅度的变化小于其平均值的 2%。超声束足够宽，因此空间平均引起的不确定性可以忽略不计。
