This week’s images show us the use of M-mode for the diagnosis of pneumothorax. We have a negative study performed by Dr. Houry and a positive study performed by Drs. Pitts and Lamm for comparison. Remember air in pneumothorax will typically rise to the least dependent portion of the hemithorax. This is where we want to look with our ultrasound. In the supine trauma patient, this will typically be the anterior chest wall lateral to the sternum in the second intercostal space. The probe is positioned with the indicator towards the head. Adjust the probe so you see the pleura (a bright hyperechoic line) outlined on both sides by a rib with shadowing beneath. Orient the probe as perpendicular to the chest wall as possible. This will make the pleura standout. Once this view is achieved you can look for evidence of lung sliding, or switch to M-mode.
M-mode takes the depth from your 2-D B-mode image, and plots this against time. If you think about the picture as a graph, the x-axis is time and the y-axis is depth. The screen will look different depending on the machine you use. The normal study here was obtained on the trauma bay M-turbo. This machine shows your B-mode image at the top with the M-mode tracing beneath. The gray line through the center of the B-mode image corresponds to the y-axis of the M-mode image. The second pneumothorax positive image was obtained on the Phillips machine. Here M-mode and B-mode are displayed side by side. The dotted line through the center of the B-mode image corresponds to the Y-axis of the M-mode image.
M-mode of the normal lung has been described as having a “Sand on the Beach” appearance. Motion within the lung changes the lung artifacts that return to the machine, creating a speckled appearance like grains of sand beneath the bright pleural line. The soft tissues above do not move and thus have a linear appearance (the echoes do not change with time). With the presence of pneumothorax we see what is called the “Stratosphere Sign.” Everything looks like the stillness of outer space. None of the images varies with time, creating a linear appearance over the entire image.
This is best seen between the purple lines on the second image. Outside the purple lines, we see a speckled appearance in some areas. You might be fooled into thinking this was a normal lung. If you look closely this specking is seen above and below the pleural line and indicates that either the patient or the probe was moving. Patients with pneumothorax may have significant air hunger and lots of chest wall movement creating this appearance. Be aware of this and try to keep your probe a still as possible in order to obtain good images.
Date: August 2011