Figure 4. M-mode images with the linear transducer. Left, the thoracostomy tube appears in transverse view as an anechoic circular structure with shadowing (double white arrow). With the cursor to the lateral aspect of the tube a stratosphere sign is present (indicating pneumothorax). The visceral pleura is also seen (thin white line). A pleural effusion can be seen just above the visceral pleura. Right, when the cursor is placed on top of the tube there is a distinct absence of waveform. We have named this the black-out sign.