Measurement:
(1) For a chest X-ray, the patient is in a lateral decubitus position, with the side being evaluated dependent.
(2) For ultrasound the patient is supine.
(3) The maximum width of the fluid lamella is measured, from inner aspect of chest wall to the air-filled lung.
For a chest X-ray:
volume of effusion in mL =
= (45.4 * (width of effusion in mm)) - 403
For ultrasonography:
volume of effusion in mL =
= (47.6 * (width of effusion in mm)) - 837
where:
• The results for ultrasonography were nonlinear when the volume was very small, corresponding to a measurement of < 20 mm.
• An X-ray width less than 8.9 mm gives a negative result.
• An US width less than 17.6 mm gives negative result.
Variables affecting accuracy:
(1) size of thoracic cavity: A tall person may have the volume underestimated, since a given volume of fluid will be distributed over a larger area and the vertical measurement will be lower.
(2) position of diaphragm: An elevated diaphragm will have a greater thickness while a flattened diaphragm will have a lesser thickness.
(3) collapse of lower lobe: A collapsed lung will compress the fluid, reducing the thickness.
Performance:
• The mean prediction error was 465 mL with linear radiographic equation.
• The mean prediction error was 264 mL with linear US equation.
• The mean prediction error was 224 mL with a nonlinear US equation.
