Future ultrasound technologies for the perioperative physician

Commercial ultrasound systems lack sufficient resolution to differentiate exactly between tissue planes - A 10 MHz transducer has a resolution to only 300 µm. Incorporation of reflectors has improved needle echogenicity but visibility is still impaired out-of-plane to the transducer and with increasing depths. Active needles using electro-magnetic guidance or peizo-electric vibration promise better needle direction. Optical spectroscopy needles needles are able to differentiate between blood, lipid and protein and will be available in practice soon. However, a fundamental physical problem remains - the greater the depth and the higher the transducer frequency, the greater the attenuation of energy, and the poorer tissue visibility. High frequency ultrasound using frequencies between 30 MHz and 50 MHz provide resolution to between 100 µm and 60 µm respectively. Micro-ultrasound is already being used to visualize and measure the properties of tissue in laboratories. Miniature ultrasound transducers integrated at the tip of needles will enable clinicians to image regions such as the epidural space or perineural spaces and identify tissue morphology in colour using features such as strain and shear wave elastography.