ABSTRACT
Interpretation The finding of free fluid during FAST in any location is considered a positive examination; however, one exception may be the visualization of small amounts of free fluid within the pelvis of women of childbearing age and
children (Fig. 3). One study demonstrated a higher rate of intra-abdominal injury in female blunt trauma patients with pelvic free fluid, regardless of volume detected (14). It may thus be unwise to disregard the finding of minimal pelvic free fluid in the setting of trauma. The quantification of minimal pelvic free fluid has been studied. An average of 100 mL of pelvic free fluid was required to be visualized by FAST in one report (15). Quantification of free fluid has not been standardized and may be objectively measured in centimeters at its greatest width or subjectively described as small, moderate, and large. Free fluid will usually appear homogeneously hypoechoic but may be hypoechoic with a few internal echoes. At the site of the injured solid organ, there is often echogenic blood, which may be in the form of a subcapsular hematoma (Fig. 4). The echogenic fluid may be less obvious than the hypoechoic fluid but should not be overlooked, since it may pinpoint the site of injury. Free intraperitoneal fluid may also have mixed, swirling echogenicity representing active hemorrhage and clots (Fig. 5). It is otherwise difficult to distinguish ascites from intraperitoneal blood, and this is a common reason for false-positive scans. With regard to solid organs, aberrations of normally homogenous parenchymal architecture may be visualized, providing potential injury localization. Lacerations, hematomas, and contusions may be hyper-or hypoechoic, and the initial sonographic appearance may change over time.
