Figure 9: M-mode doppler ultrasound demonstrating discrimination of a pleural effusion (left side) from a lung mass (right side). This ultrasound was taken with a 10 MHz probe. On the left, note the presence of the pleural effusion, appearing as a dark or anechoic fluid collection, with the lung seen moving in the background. On the right side, note the linear repeating lines of both the chest wall and the lung indicating an absence of motion of the lung due to the presence of the solid mass.