INTRODUCTION TO FAST ULTRASONOGRAPHY
- The term FAST is an acronym that comes from the english word Focused Assessment with Sonography for Trauma.
- It was first used by Rozycki et al. in 1995, and is nowadays the most widely used term in the world when it comes to ultrasonography in trauma.
- FAST is an important tool in the emergency department for the initial management of patients with blunt abdominal trauma.
- Its primary purpose is the detection of free fluid in the abdominal cavity, pericardial sac and chest.
- Although ultrasound has a good sensitivity to detect small amounts of peritoneal free fluid, it is inaccurate in the differentiation of types of free fluid (blood, bile, pus) as well as in the detection of organ damage.
- Reports suggests that the minimum amount of peritoneal fluid detected by ultrasonography ranges from 100-620 ml.
- Ultrasound has become the method of choice in the detection of free abdominal fluid for several reasons. First because it does not pose risks to the patient. It is worth remembering that prior to the advent of ultrasonography, the diagnosis of free abdominal fluid was performed by diagnostic peritoneal lavage (DPL), which carries a risk of up to 10% of bowel perforation as well as vascular injury. Second it is a portable and accessible method, therefore, in unstable patients, it can be easily used in emergency rooms. Finally, the detection of free fluid through FAST ultrasound can be performed in less than 5 minutes by a suitably trained person, unlike other diagnostic methods such as peritoneal lavage and computed tomography.
In a trauma setting, the presence of free peritoneal fluid visualized on ultrasonography is presumptive of solid organ injury (most commonly liver or spleen).
PRINCIPLES AND ANATOMY
FAST ultrasound is based on the principle that injuries to the abdominal organs are commonly associated with the presence of free abdominal fluid (hemoperitoneum), which appear as anechoic areas on ultrasound. Hemoperitoneum usually arises from injury to the spleen or liver in a context of abdominal trauma. Therefore, recognizing the abdominal cavity as well as its peritoneal recessess, we are able to use sonography looking for free fluid.
The peritoneal cavity is subdivided into supracolic and infracolic compartments by the transverse mesocolon. When fluid accumulates in the peritoneal cavity, it has free access between these two compartments via the right and left paracolic gutters. If we consider the peritoneal cavity above the pelvic inlet, the most gravitationally dependent space is the hepatorenal recess, also known as the Morrison pouch. If the source of fluid is above the pelvic inlet and the patient remains in a supine position, fluid will flow to this recess for two reasons: First, the hepatorenal recess is the most posterior peritoneal reflection in comparison with the others. Second, the peritoneal reflection of the phrenicocolic ligament in the upper left quadrant favors fluid to gravitates into the upper right quadrant. On the other hand, in a patient who has already been in a stand position, fluid will run to the pelvic peritoneal spaces, which are the rectouterine (pouch of Douglas) in women or the rectovesical in men.
TECHNIQUE AND IMAGE ACQUISITION
Abdominopelvic cavity should be scanned using a lower frequency (3.5 - 5.0 MHz) sector or curved-array transducer. The original FAST scan includes the evaluation of four potential areas looking for free fluid: the right upper quadrant, the left upper quadrant, the suprapubic view and the subxiphoid pericardial view (Fig. 1). We should use the liver as an acoustic window in the upper right quadrant; the spleen as acoustic window in the left upper quadrant and a full bladder in the pelvic region.
FIG. 1 - Diagram depicting the four views originally reported to the FAST scan. 1: right upper quadrant; 2: left upper quadrant; 3: suprapubic view ; 4: subxiphoid view.
RIGHT UPPER QUADRANT
In a supine patient, free fluid will accumulate in the most dependent area of the abdominal cavity, the hepatorenal recess (fig. 2). Because of its posterior location, this peritoneal reflection is the most susceptible area to fluid accumulation and the least obstructed to fluid flow above the pelvic inlet.
To adequate visualization of this quadrant, place the probe in a coronal plane in the midaxillary line between the 9th and 11th intercostal spaces with the probe marker pointing towards the patient’s head. Angle the transducer from anterior to posterior through the hepatorenal recess searching for free fluid.
FIG. 2 - DIagram depicting the right upper quadrant window. In this quadrant four potential spaces should be visualized: above the diaphragm (right pleural space), below the diaphragm (subphrenic space), in the hepatorenal recess (Morrison's pouch) and below the inferior pole of right kidney in the paracolic gutter.
FIG. 3 - Free fluid in the Morrison’s pouch depicted as an anechoic area between the liver and right kidney.
LEFT UPPER QUADRANT
In the left upper quadrant the following spaces should be visualized: above the diaphragm (left pleural space), below the diaphragm (left subphrenic space), in the splenorenal recess. and below the inferior pole of the left kidney in the paracolic gutter. To adequate visualization of the first three spaces, we must use the probe in a more posterior and superior position compared to the right upper quadrant. This is due to the location, shape and size of the spleen.
FIG. 4 - Diagram depicting the left upper quadrant window. Place the transducer in a coronal plane on the left posterior axillary line, between the 6th and 9th intercostal spaces. Also angle and fan the probe searching for free fluid in the subphrenic and splenorenal spaces.
FIG. 5 - Ultrasound of the left upper quadrant demonstrating free fluid in the perisplenic area. On the left, fluid flows preferentially into the subphrenic space than to the splenorenal space.
On the pelvis, the peritoneal free fluid will accumulates in the rectovesical space in men and in the rectouterine (pouch of Douglas) space in women. Pelvic scan must be done with a full bladder visualized in the top 1/3 to 1/2 of the screen. In a longitudinal plane, fan the probe from left to right and vice versa scanning the entire bladder looking for free fluid on rectovesical or rectouterine spaces. After that, rotate the transducer 90o counterclockwise to obtain transverse planes and scan the entire bladder form fundus to neck.
FIG. 6- Male pelvis ultrasound. Note a minimum amount of free fluid on the peritoneal reflection between the bladder and rectum. In male pelvis, free fluid will accumulates on rectovesical space. The bladder should be full when evaluating the pelvis looking for free fluid, otherwise the chance of a false negative exam is increased.
FIG. 7 - Female pelvis ultrasound. Note the moderate amount of free fluid in the Douglas pouch. In women free pelvic fluid will accumulates on rectouterine space. The bladder should be full when evaluating the pelvis looking for free fluid, otherwise the chance of a false negative exam is increased.
The subxiphoid window allows for visualization of free fluid in the pericardium. Subxiphoid images of the heart are obtained by placing the probe on upper abdomen pointing its footprint superiorly toward the patients left shoulder.
FIG. 8 - Subxiphoid window. Subxiphoid images of the heart are obtained by placing the probe on upper abdomen pointing superiorly toward the patients left shoulder. The left liver lobe allows for an adequate visualization of he pericardium.
FIG. 9 - Pericardial fluid depicted as an anechoic space surrounding the myocardium.
IMAGE INTERPRETATION, FALSE POSITIVES AND PITFALLS
Free peritoneal fluid will accumulate in those most gravitationally dependent spaces. In the abdominopelvic cavity, these peritoneal recesses are located in the pelvis, which are the retouterine and rectovesical spaces. Normally free liquid should not be found in the peritoneal cavity of men. A pitfall that can mimic fluid in the rectovesical space of the male pelvis are the seminal vesicles (fig. 10). In females of reproductive age, it is common to characterize a small amounts of fluid - up to 50 ml - in the pouch of Douglas (fig.11). False positive scans may result from:
- Peritoneal dialysate
- Ventriculperitoneal shunt outflow
- Ovarian hyperstimulation
- Ovarian cyst rupture
FIG. 10 - A - Male pelvis image depicting the seminal vesicles (arrow) lying posteriorly to the bladder. It’s important to recognize those structures not to confuse them with free fluid. B - Longitudinal section shows the seminal vesicle (arrow) entering the posterior aspect of the prostate.
FIG. 11 - Longitudinal view of a female pelvis in reproductive age depicting minimal amount of physiological fluid on pouch of Douglas.
Although abdominal free liquid detected in the setting of trauma is assumed to be hemoperitoneum, it can also represent injury-related urine, bile and bowel contens. In summary, FAST exam should be performed correctly, not forgetting to interpretate and correlate the findings with the mechanism of trauma and the type of injury suffered by the patient.
1.Richards, John R., and John P. McGahan. "Focused assessment with sonography in trauma (FAST) in 2017: what radiologists can learn." Radiology 283.1 (2017): 30-48.
2. Christie-Large, M., D. Michaelides, and S. L. J. James. "Focused assessment with sonography for trauma: the FAST scan." Trauma 10.2 (2008): 93-101.