Ultrasound transmission at reduced frequencies in human lungs: a novelty for pulmonary diagnosis and monitoring

Conventional sonography cannot permeate the chest. Aerated lung tissue strongly scatters ultrasound and thus inhibits a significant penetration. Even thin slices of in vitro lung samples are opaque for any ultrasound from 10kHz to several MHz. As a common consensus, alternative technologies (X-ray, magnetic resonance imaging) are required for thoracic imaging. Interestingly, human lungs in vivo were never explicitly probed with ultrasound at lower frequencies below one MHz. In contrary to the expected opacity, this study reports a noticeable transparency: Apparently, ultrasound below one MHz can pervade the normal lungs and thorax. To explain this novel observation we suggest that the vascular filling, which was absent in previous in vitro studies, permits ultrasound propagation in the lungs. In normal lungs, blood filling is surprisingly high and can amount to 50% of its mass in basal parts. Ultrasound probing of healthy humans and a group of patients with various pulmonary diseases clearly demonstrate a lung-specific signal. Our observations strongly suggest efficient ultrasound propagation at moderately reduced frequencies. We propose that “transpulmonary“ ultrasound is a novel non-invasive approach for continuous real time imaging of the lungs and the thorax interior. This method appears also applicable to continuous pulmonary monitoring in mechanically ventilated patients and in patients with left heart failure (lung oedema).

Keywords: thorax, lung, pulmonary, ultrasound