Male with blunt abdominal trauma, walked into the Emergency Department.
In both the longitudinal and transverse views free fluid can be seen deep to the bladder. In the longitudinal view the superior bladder wall is well defined between the intravesical fluid (urine) and the pelvic free fluid.
This is a positive scan, however is only a small amount of pelvic free fluid.
Free fluid in the abdomen gathers in the most dependent portions of the abdomen.
This patient walked into the department and fluid was only seen in the pelvic views. It could easily have been missed if the bladder was empty. The full bladder acts as an acoustic window and fluid lying deep to it can be seen through it. If the bladder is empty gas filled bowel will obscure the view. In some hospitals filling the bladder is advocated to ensure pelvic free fluid is not missed.
Putting the patient 10 degrees head down tends to redistribute fluid back into the upper quadrants often making its detection easier.
In this image the gain behind the bladder is a little too high (there is posterior acoustic enhancement). this makes the free fluid appear grey rather than black, and again is another reason it can be missed.