FAST is performed with a 3- to 5-MHz transducer placed sequentially in the following locations: Subxiphoid (to image the pericardium), right upper quadrant, left upper quadrant, suprapubic. In preparation for the examination, make sure that the patient has a full bladder by clamping the Foley catheter, if present. The purpose of the examination is simply to determine if fluid is, or is not, present in the locations examined. Fluid may be blood, gastric contents, bile, or succus. The examination is not designed to yield a definitive diagnosis. Interpretation of the FAST examination must be done in conjunction with clinical picture and other imaging studies. In some circumstances, a
repeat FAST examination may be helpful, because it takes time for blood or fluid to accumulate in these locations.

These four locations are chosen for two reasons: First of all, they provide good ultrasound “windows” into the peritoneal cavity; second, they are regions were fluid accumulation is likely to occur in trauma.

The concept of an acoustic window is quite simple. Ultrasound is strongly reflected by interfaces between liquid/tissue and air (i.e., the lungs) or bone, and this reflection obscures the visualization of deeper structures. A good window avoids these interfaces. Thus the subxiphoid approach to the pericardium avoids potential overlap of ribs or lung and takes advantage of the anatomy illustrated in Chapter 20.

Free fluid most commonly results from bleeding from the spleen or liver. Initially this blood may accumulate under these organs, where it is detectable by examination of Morrison’s pouch or in the splenorenal space. Blood also tends to pool in the pelvis, where it may be picked up on suprapubic examination. Although it is true that blood will also be found around loops of small bowel or in the paracolic gutters, these regions are more difficult for the nonradiologist to interpret.