Background and Indications for Examination
Bedside ultrasound is an optimal first-line imaging modality for the evaluation of peritoneal free fluid. The “focused assessment with sonography in trauma” (FAST) examination is one of the common applications used in evaluating the patient for free peritoneal fluid from hemorrhage. Ultrasound has been shown to be sensitive and specific for significant hemoperitoneum. Ultrasound has long been accepted as a life-saving tool in the hypotensive trauma patient, but its benefits have expanded beyond this. Bedside ultrasound is now readily used to search for free fluid in the non-traumatic patient and in other disease processes. It can be used to evaluate for free peritoneal fluid in patients with end-stage liver disease, renal disease, or congestive heart failure.
In addition, the evaluation for peritoneal fluid is invaluable in other patients who may present with abdominal or flank pain. For example, intraperitoneal free fluid in a pregnant patient with abdominal or pelvic pain usually indicates a ruptured ectopic pregnancy that will require operative intervention. The rapid diagnosis of free fluid at the bedside can expedite patient care and definitive treatment.
- The acute trauma patient with blunt or penetrating torso injuries
- The pregnant or pediatric trauma patient
- The subacute trauma patient with a delayed presentation who complains of worsening abdominal pain
- The elderly patient with undifferentiated abdominal or flank pain
- The pregnant patient with abdominal or pelvic pain
- The patient with undifferentiated shock or dyspnea
Probe Selection and Technical Considerations
- The abdomen and pelvis are best imaged using a wide footprint curvilinear probe with an operating frequency between 3.5 and 5.0 MHz. Lower frequencies may help with more obese patients while higher frequencies can be used in thin or pediatric patients.
- The phased-array or microconvex transducer in the 2.0–5.0 MHz range can also be used to evaluate the abdomen and pelvis for free fluid. The small footprint allows imaging between the ribs, which is particularly helpful in young thin patients.
The total gain can be increased to brighten the signal returning from the structure of interest and improve the image on the machine monitor. Often times, the near field has adequate gain, and therefore does not need to be adjusted. In order to compensate for signal loss in the obese patient, time-gain compensation (TGC, far gain) may be increased to enhance the attenuated signal reflecting from deeper tissues in the patient. Decreasing the far gain is often required to adjust for the posterior acoustic enhancement artifact that appears behind the bladder in the pelvic window. The TGC controls should be equalized prior to beginning an exam in order to avoid a false-positive “stripe” that can result when the TGC is set too low.
The sonographer should start with a depth of 15 cm or more and scan through the entire far field before focusing in on the more superficial structures of interest. This ensures that abnormal findings are not missed in the deeper fields, and helps to orient the sonographer. Once the deeper areas are interrogated, the depth can be decreased resulting in the structure of interest using approximately three-fourths of the screen.
Normal Ultrasound Anatomy
There are three views of the peritoneal cavity that should be evaluated when searching for free fluid. These are the perihepatic [right upper quadrant (RUQ), hepatorenal, “Morison’s pouch”], perisplenic [left upper quadrant (LUQ), splenorenal], and suprapubic views. While Morison’s pouch is the most sensitive single view for significant free fluid, acquiring multiple views increases overall sensitivity. Evaluation of the paracolic gutters was initially advocated in some descriptions of the FAST exam, but have not been found to improve sensitivity.
The perihepatic view examines the RUQ and is typically the first abdominal view obtained, as it is the most sensitive for free peritoneal fluid and the most reliably imaged. This view visualizes Morison’s pouch, the potential space between the liver and the right kidney, which is an extension of the right paracolic gutter. This view should also include evaluation of the right hemithorax above the diaphragm for pleural effusion or hemothorax.
The main imaging plane for the RUQ is the right coronal view. The transducer is placed on the right flank with the probe indicator pointing toward the patient’s head. It should be positioned in the midaxillary line over the eighth to eleventh intercostal rib spaces (Fig. 9-1). The resulting image is a coronal plane with the left side of the screen toward the head, the feet toward the right, more superficial structures at the top of the screen, and deeper structures at the bottom (Fig. 9-2). The exam should focus on the potential space between the liver and kidney (Morison’s pouch). Tilting the probe superiorly should allow visualization of the right hemithorax above the diaphragm. An adequate examination is obtained when the liver, including the inferior liver tip, is visualized in the same plane as the inferior pole of the right kidney. Interrogation of the inferior liver tip is important, as this is the most dependent region, and may contain a subtle collection of free fluid.
A normal coronal view of the RUQ with landmarks. The left side of the image is toward the patient’s head. The bright white hyperechoic diaphragm can be seen with a mirror image artifact of the liver above it, which signifies a normal pleural space without fluid. A normal Morison’s pouch is seen between the liver and kidney. The patient’s feet would be directed toward the right side of the screen. Note the inferior tip of the liver and kidney are both imaged on the right where fluid often collects first. K: kidney, L: liver.
A normal Morison’s pouch will be visualized as a lack of anechoic or hypoechoic fluid in between the liver and right kidney interface. A normal right hemithorax will be visualized as a mirror image artifact in which the liver appears to be on both sides of the diaphragm (see Fig. 9-2). This artifact is normal and assures the sonographer that there is no free fluid in the pleural space. Free fluid above the diaphragm will appear as an anechoic (black) or hypoechoic space and represents a hemothorax or pleural effusion depending on the clinical scenario.
It is sometimes difficult for the sonographer to fully interrogate the RUQ due to rib shadowing. In this situation, the probe should be angled in between the ribs. This right intercostal oblique view is obtained by orienting the probe in the midaxillary line between the lower rib spaces angling slightly toward the patient’s posterior body surface. The transducer can be subtly rotated both anteriorly and posteriorly to optimize the image and reduce artifacts from rib shadowing. A smaller footprint or phased-array probe may also help in this situation.
The LUQ view is used to assess for left pleural effusion, and for free fluid in the subphrenic and splenorenal recesses, which communicate with the left paracolic gutter. The probe can then be tilted superiorly to image the diaphragm and lower chest. This view is typically more difficult than the RUQ view to obtain for a few reasons. The kidney is anatomically more superior and posterior on the left, and the spleen does not provide as large of a sonographic window as the liver. This more posterior position can add additional rib shadows on the left that obscure the anatomy. In addition, the operator stands on the patient’s right side and must lean over the patient in order to place the transducer in the LUQ. “Knuckles on the bed” is a phrase often used to describe the LUQ view as it emphasizes the need to reach all the way around the patient posteriorly in order to obtain an adequate window.
As on the right, start with a coronal view. The probe should be placed on the left flank in the posterior axillary line over the seventh to ninth intercostal spaces with the indicator pointing toward the patient’s head. Once again, the positioning will need to be more superior and posterior than the RUQ and the sonographer will typically have their hand against the bed to obtain the best view. If the patient is cooperative, inspiration may help the view. The patient may have to be placed in the lateral decubitus position, if tolerated, in order to optimize the image (Fig. 9-3). If rib shadows are obscuring important anatomy, an intercostal oblique view should be attempted by rotating the transducer slightly and directing it posteriorly. Sometimes a more anterior view through the rib cage can visualize the structures needed.
The orientation is similar to the RUQ view, with the left side of the screen toward the patient’s head and the right side of the screen toward the patient’s feet. An adequate LUQ view should visualize the spleen, left kidney, and the subphrenic space (Fig. 9-4). As opposed to the right side, free fluid will often collect in the subphrenic space first before collecting in the splenorenal recess. Therefore, adequate imaging should always include good views of the interface between the spleen and diaphragm. As with the perihepatic view, the operator should make sure to visualize the inferior tip of the spleen lying adjacent to the inferior pole of the left kidney in order to be complete. The inferior thorax can be visualized by angling the probe superiorly, and having the patient take a breath in if they are able to.
A normal coronal view of the LUQ with landmarks. The bright white hyperechoic diaphragm can be seen with a mirror image artifact of the spleen above it, which signifies a normal pleural space without fluid. A normal subphrenic space (SPS) is visualized between the spleen and diaphragm. A normal splenorenal recess (SRR) is visualized between the spleen and left kidney. Note the inferior tip of the spleen and kidney are both imaged on the right. K: kidney, L: liver.
A normal subphrenic space and splenorenal recess will be visualized as a lack of anechoic or hypoechoic fluid in between the spleen and both the diaphragm and left kidney. A normal left hemithorax will be visualized as a mirror image artifact above the diaphragm that gives the illusion of spleen appearing on both sides (see Fig. 9-4). This artifact assures the sonographer that there is no free fluid in the pleural space.
The suprapubic view is the most dependent potential space for free fluid to collect. The posterior cul-de-sac or pouch of Douglas, also called the rectouterine space, is the dependent continuation of the peritoneal cavity between the rectum and the back wall of the uterus in the female patient. In a male patient, the most dependent area is a similar space posterior to the bladder and anterior to the rectum, known as the rectovesicular space.
The suprapubic view is easier to obtain when the bladder is full because it provides a large sonographic window and views should be obtained prior to Foley catheter placement if possible. The suprapubic view looks for free fluid in the pelvis. While fluid in blunt trauma is assumed to be blood, it may also be urine or ascites. In young women, a small amount of physiologic fluid may be a result of the normal menstrual cycle.
The suprapubic view is obtained by positioning the transducer on the lower abdomen just above the pubic symphysis. In the sagittal view, the transducer should be oriented longitudinally with the indicator pointing toward the patient’s head and angled inferiorly. When full, the bladder is easily found due to the large sonographic window it provides and because there are no ribs or other bony structures obstructing the view. In a female patient, the uterus can be seen superiorly and posteriorly relative to the bladder (Fig. 9-5). There may be bowel gas, which can scatter the ultrasound signal, but usually, sliding and/or angling the probe more inferiorly will reduce this impediment. The imaging plane should be swept through the entire volume of the bladder in the left-to-right directions while the operator looks for a black anechoic stripe behind the bladder or uterus.