Quad sign | Radiology Signs

Answer:

The quad sign is the roughly rectangular, four-sided anechoic space seen in an intercostal view when a pleural effusion is present: the pleural line above, the two rib shadows at the sides and the regular lung line below bound the collection. Its companion on M-mode is the sinusoid sign, in which the lung line moves rhythmically toward the pleural line with inspiration, indicating a mobile, low-viscosity collection. Together they give a bedside diagnosis of effusion with high sensitivity and specificity, well beyond what a supine chest radiograph achieves, and they are used to guide thoracentesis by identifying the largest pocket, measuring the distance from skin to pleura and locating the diaphragm. The appearance of the fluid also carries information: anechoic fluid is usually a transudate, while echogenic fluid, floating debris, septations, loculation and a thickened pleura suggest an exudate, empyema or haemothorax, and complex septated collections often need drainage with a larger catheter or fibrinolysis. A hyperechoic, non-mobile appearance with a preserved quad configuration should prompt consideration of pleural thickening or a solid pleural process rather than fluid. Correlation with the curtain sign, the spine sign, which is visibility of the vertebral bodies above the diaphragm, and with lung sliding completes the basic pleural assessment.

Why is it called so?

The collection is bounded on four sides, giving a quadrangular outline on the image.

Pathophysiology

Fluid conducts ultrasound well and reflects little, so a pleural collection appears anechoic and provides an acoustic window through which the underlying visceral pleura and lung surface become visible as a regular line. The ribs block the beam and cast shadows that form the lateral boundaries, and the parietal pleura forms the upper boundary. Because the collapsed or partially collapsed lung floats within the fluid and moves with respiration, the lower boundary shifts cyclically, which is what generates the sinusoid sign and confirms that the space contains free fluid rather than solid tissue. Loculation by fibrinous septae immobilises the lung line and abolishes the sinusoid, which is itself diagnostically useful.

Alternative names: Quadrangle sign; quad sign of effusion

Other associated named signs: Sinusoid sign, the curtain sign, the spine sign and the plankton sign of an exudative effusion

References

1. Lichtenstein DA. Lung ultrasound in the critically ill. Ann Intensive Care. 2014;4(1):1. PMID: 24401163.