M-mode of the Lung

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 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 above was obtained using the Sonosite 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 positive pneumothorax 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 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 a pneumothorax, we see what is called the “Stratosphere Sign”. Everything looks like the stillness of outerspace.  None of the image varies with time creating a linear appearance over the entire image.  This is best seen between the purple lines on the second image. Outside of the purple lines we see a specked appearance in some areas.  You might be fooled into thinking this was normal lung.  If you look closely this specking is seen above and below pleural line and indicates that either the patient or the probe was moving. Patients with a 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 accurate images.